ALBERT

Description: Research Articles Graça Ermida Polar Record , FirstView Article(s) , 6 pages Abstract

Description: Research Articles Torill Nyseth, Arvid Viken Polar Record , FirstView Article(s) , 10 pages Abstract

Description: Book Reviews William Barr Polar Record , FirstView Article(s) , 2 pages Abstract

Description: Book Reviews Arthur G. Credland Polar Record , FirstView Article(s) , 1 page Abstract

Description: Review Articles Łukasz Kaczmarek, Natalia Jakubowska, Sofia Celewicz-Gołdyn, Krzysztof Zawierucha Polar Record , FirstView Article(s) , 28 pages Abstract

Description: Research Articles Iselin Stensdal Polar Record , FirstView Article(s) , 10 pages Abstract

Description: Research Articles Stephen Pax Leonard Polar Record , FirstView Article(s) , 11 pages Abstract

Description: Annual Review of Earth and Planetary Sciences Volume 43, Page 139-166, May 2015, ISSN 0084-6597, eISSN 1545-4495.

Description: Annual Review of Earth and Planetary Sciences Volume 43, Page 477-507, May 2015, ISSN 0084-6597, eISSN 1545-4495.

Description: Annual Review of Earth and Planetary Sciences Volume 43, Page 167-205, May 2015, ISSN 0084-6597, eISSN 1545-4495.

Description: Research Articles Douglas R. Stenton, Anne Keenleyside, Diana P. Trepkov, Robert W. Park Polar Record , FirstView Article(s) , 6 pages Abstract

Description: Annual Review of Earth and Planetary Sciences Volume 43, Page 207-231, May 2015, ISSN 0084-6597, eISSN 1545-4495.

Description: Article Commentary JONATHAN S. LEFCHECK, VINICIUS A. G. BASTAZINI, JOHN N. GRIFFIN, Environmental Conservation , Volume 42 Issue 02 , pp 104-107 Abstract

Description: Research Articles RUTH L. KAMROWSKI, STEPHEN G. SUTTON, RENAE C. TOBIN, MARK HAMANN, Environmental Conservation , Volume 42 Issue 02 , pp 171-181 Abstract

Description: Article Commentary PETER F. SALE, Environmental Conservation , Volume 42 Issue 02 , pp 97-101 Abstract

Description: Research Articles J. Chela-Flores, A. Cicuttin, M.L. Crespo, C. Tuniz, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 427-434 Abstract

Description: Research Articles Tianzhi Li, De Chang, Huiwen Xu, Jiapeng Chen, Longxiang Su, Yinghua Guo, Zhenhong Chen, Yajuan Wang, Li Wang, Junfeng Wang, Xiangqun Fang, Changting Liu, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 435-444 Abstract

Description: Research Articles Jacopo Farinato, Carlo Baffa, Andrea Baruffolo, Maria Bergomi, Luca Carbonaro, Alexis Carlotti, Mauro Centrone, Johanan Codona, Marco Dima, Simone Esposito, Daniela Fantinel, Giancarlo Farisato, Wolfgang Gaessler, Emanuele Giallongo, Davide Greggio, Philip Hinz, Franco Lisi, Demetrio Magrin, Luca Marafatto, Fernando Pedichini, Enrico Pinna, Alfio Puglisi, Roberto Ragazzoni, Bernardo Salasnich, Marco Stangalini, Christophe Verinaud, Valentina Viotto, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 365-373 Abstract

Description: Research Articles Paul A. Mason, Jorge I. Zuluaga, Pablo A. Cuartas-Restrepo, Joni M. Clark, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 391-400 Abstract

Description: Review Articles Amri Wandel, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 511-516 Abstract

Description: Research Articles Carlo Canepa, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 497-504 Abstract

Description: Research Articles Jeff R. Kuhn, Svetlana V. Berdyugina, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 401-410 Abstract

Description: Research Articles Wesley A. Traub, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 359-363 Abstract

Description: Research Articles Duncan H. Forgan, Alexander Mead, Charles S. Cockell, John A. Raven, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 465-478 Abstract

Description: Research Articles Marina G. Martynova, Sergej V. Shabelnikov, Olga A. Bystrova, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 489-495 Abstract

Description: Research Articles Yu Komatsu, Masayuki Umemura, Mitsuo Shoji, Megumi Kayanuma, Kazuhiro Yabana, Kenji Shiraishi, International Journal of Astrobiology , Volume 14 Special Issue 03 , pp 505-510 Abstract

Description: Book Reviews Nikolas Sellheim Polar Record , FirstView Article(s) , 1 page Abstract

Description: Research Articles GILLIAN B. AINSWORTH, HEATHER J. ASLIN, MICHAEL A. WESTON, STEPHEN T. GARNETT, Environmental Conservation , Volume 43 Issue 03 , pp 294-305 Abstract

Description: Research Articles RAFAEL URBINA-CASANOVA, FEDERICO LUEBERT, PATRICIO PLISCOFF, ROSA A. SCHERSON, Environmental Conservation , Volume 43 Issue 03 , pp 199-207 Abstract

Description: Research Articles MERSHA GEBREHIWOT, MARINE ELBAKIDZE, GUN LIDESTAV, MATS SANDEWALL, PER ANGELSTAM, HABTEMARIAM KASSA, Environmental Conservation , Volume 43 Issue 03 , pp 263-272 Abstract

Description: Review Articles Elif Koeksoy, Maximilian Halama, Kurt O. Konhauser, Andreas Kappler, International Journal of Astrobiology , Volume 15 Issue 03 , pp 205-217 Abstract

Description: Research Articles Bettina E. Schirrmeister, Patricia Sanchez-Baracaldo, David Wacey, International Journal of Astrobiology , Volume 15 Issue 03 , pp 187-204 Abstract

Description: Research Articles Chaitanya Giri, Christopher P. McKay, Fred Goesmann, Nadine Schäfer, Xiang Li, Harald Steininger, William B. Brinckerhoff, Thomas Gautier, Joachim Reitner, Uwe J. Meierhenrich, International Journal of Astrobiology , Volume 15 Issue 03 , pp 231-238 Abstract

Description: Obituaries Dan Small Polar Record , FirstView Article(s) , 2 pages Abstract

Description: Research Articles KAERA L. COETZER-HANACK, E.T.F. WITKOWSKI, BAREND F.N. ERASMUS, Environmental Conservation , Volume 43 Issue 03 , pp 253-262 Abstract

Description: Research Articles LINCOLN R. LARSON, APRIL L. CONWAY, KATHLEEN E. KRAFTE, SONIA M. HERNANDEZ, JOHN P. CARROLL, Environmental Conservation , Volume 43 Issue 03 , pp 242-252 Abstract

Description: Research Articles ALAINE A. BALL, PEDRO H.S. BRANCALION, Environmental Conservation , Volume 43 Issue 03 , pp 208-220 Abstract

Description: Research Articles KIM E. REUTER, HALEY RANDELL, ABIGAIL R. WILLS, BRENT J. SEWALL, Environmental Conservation , Volume 43 Issue 03 , pp 273-283 Abstract

Description: Research Articles DOUGLAS SHEIL, BRENTON LADD, LUCAS C. R. SILVA, SHAWN W. LAFFAN, MIRIAM VAN HEIST, Environmental Conservation , Volume 43 Issue 03 , pp 231-241 Abstract

Description: Research Articles DANICA SCHAFFER-SMITH, JENNIFER J. SWENSON, ANTONIO J. BÓVEDA-PENALBA, Environmental Conservation , Volume 43 Issue 03 , pp 221-230 Abstract

Description: Research Articles CLARA THERVILLE, LIVIO CASELLA-COLOMBEAU, RAPHAËL MATHEVET, FREDERIC BIORET, Environmental Conservation , Volume 43 Issue 03 , pp 284-293 Abstract

Description: Research Articles INAOYOM IMONG, HJALMAR S. KÜHL, MARTHA M. ROBBINS, ROGER MUNDRY, Environmental Conservation , Volume 43 Issue 02 , pp 161-171 Abstract

Description: Book Reviews Nikolas Sellheim Polar Record , FirstView Article(s) , 2 pages Abstract

Description: Research Articles KIM E. REUTER, DANIEL JUHN, HEDLEY S. GRANTHAM, Environmental Conservation , Volume 43 Issue 02 , pp 181-198 Abstract

Description: Research Articles EMMANUEL MWAKIWA, JOHN W. HEARNE, JOHANNES D. STIGTER, WILLEM F. DE BOER, MICHELLE HENLEY, ROB SLOTOW, FRANK VAN LANGEVELDE, MIKE PEEL, CORNELIA C. GRANT, HERBERT H.T. PRINS, Environmental Conservation , Volume 43 Issue 02 , pp 128-139 Abstract

Description: Research Articles AURELIE BOUVET, YOAN PAILLET, FREDERIC ARCHAUX, LAURENT TILLON, PASCAL DENIS, OLIVIER GILG, FREDERIC GOSSELIN, Environmental Conservation , Volume 43 Issue 02 , pp 148-160 Abstract

Description: Research Articles GUSTAVO M. MARTINS, STUART R. JENKINS, ANA I. NETO, STEPHEN J. HAWKINS, RICHARD C. THOMPSON, Environmental Conservation , Volume 43 Issue 02 , pp 109-116 Abstract

Description: Research Articles Mia M. Bennett, Wilfrid Greaves, Rudolf Riedlsperger, Alberic Botella Polar Record , FirstView Article(s) , 15 pages Abstract

Description: Research Articles Jim Mayer Polar Record , FirstView Article(s) , 9 pages Abstract

Description: Research Articles E. Tammiksaar Polar Record , FirstView Article(s) , 23 pages Abstract

Description: Editorial Jan-Peter Duda, Joachim Reitner, International Journal of Astrobiology , Volume 15 Issue 03 , pp 161-163 Abstract

Description: Research Articles Blanca Rincón-Tomás, Bahar Khonsari, Dominik Mühlen, Christian Wickbold, Nadine Schäfer, Dorothea Hause-Reitner, Michael Hoppert, Joachim Reitner, International Journal of Astrobiology , Volume 15 Issue 03 , pp 219-229 Abstract

Description: Research Articles W. Goetz, W. B. Brinckerhoff, R. Arevalo, C. Freissinet, S. Getty, D. P. Glavin, S. Siljeström, A. Buch, F. Stalport, A. Grubisic, X. Li, V. Pinnick, R. Danell, F. H. W. van Amerom, F. Goesmann, H. Steininger, N. Grand, F. Raulin, C. Szopa, U. Meierhenrich, J. R. Brucato, International Journal of Astrobiology , Volume 15 Issue 03 , pp 239-250 Abstract

Description: Research Articles H. Mißbach, J.-P. Duda, N.K. Lünsdorf, B.C. Schmidt, V. Thiel, International Journal of Astrobiology , Volume 15 Issue 03 , pp 165-175 Abstract

Description: Research Articles A. Montinaro, H. Strauss, International Journal of Astrobiology , Volume 15 Issue 03 , pp 177-185 Abstract

Description: Rapid Communication Nikolas Sellheim Polar Record , FirstView Article(s) , 5 pages Abstract

Description: Book Reviews Beau Riffenburgh Polar Record , FirstView Article(s) , 2 pages Abstract

Description: Research Articles Rip Bulkeley Polar Record , FirstView Article(s) , 14 pages Abstract

Description: Book Reviews Philip Sidney Polar Record , FirstView Article(s) , 2 pages Abstract

Description: Book Reviews Beau Riffenburgh Polar Record , FirstView Article(s) , 1 page Abstract

Description: Book Reviews Nikolas Sellheim Polar Record , FirstView Article(s) , 2 pages Abstract

Description: Book Reviews Hanna Lempinen Polar Record , FirstView Article(s) , 2 pages Abstract

Description: Book Reviews Ian R. Stone Polar Record , FirstView Article(s) , 2 pages Abstract

Description: Book Reviews Nikolas Sellheim Polar Record , FirstView Article(s) , 2 pages Abstract

Description: Research Articles W. Barr Polar Record , FirstView Article(s) , 25 pages Abstract

Description: Research Articles Keith Millar, Adrian W. Bowman, William Battersby, Richard R. Welbury Polar Record , FirstView Article(s) , 19 pages Abstract

Description: Research Articles Steve Heavens Polar Record , FirstView Article(s) , 13 pages Abstract

Description: Research Articles Carol Brown-Leonardi Polar Record , FirstView Article(s) , 14 pages Abstract

Description: Research Articles Emmanuel Guy, Frédéric Lasserre Polar Record , FirstView Article(s) , 11 pages Abstract

Description: Research Articles Purnima Singh, Utpal Roy, Masaharu Tsuji Polar Record , FirstView Article(s) , 8 pages Abstract

Description: Research Articles Joshua D. Reuther, Jason S. Rogers Polar Record , FirstView Article(s) , 10 pages Abstract

Description: Book Reviews Nikolas Sellheim Polar Record , FirstView Article(s) , 1 page Abstract

Description: Research Articles C. PAXTON RAMSDELL, MICHAEL G. SORICE, ANGELA M. DWYER, Environmental Conservation , Volume 43 Issue 01 , pp 34-44 Abstract

Description: Research Articles D. I. URZEDO, E. VIDAL, E. O. SILLS, F. C. M. PIÑA-RODRIGUES, R. G. P. JUNQUEIRA, Environmental Conservation , Volume 43 Issue 01 , pp 13-23 Abstract

Description: Research Articles SERGI HERRANDO, LLUÍS BROTONS, MARC ANTON, FERRAN PÁRAMO, DANI VILLERO, NICOLAS TITEUX, JAVIER QUESADA, CONSTANTÍ STEFANESCU, Environmental Conservation , Volume 43 Issue 01 , pp 69-78 Abstract

Description: Article Commentary GILBERT PROULX, RYAN K. BROOK, MARC CATTET, CHRIS DARIMONT, PAUL C. PAQUET, Environmental Conservation , Volume 43 Issue 01 , pp 1-2 Abstract

Description: Research Articles OMAR OHRENS, ADRIAN TREVES, CRISTIÁN BONACIC, Environmental Conservation , Volume 43 Issue 01 , pp 24-33 Abstract

Description: Research Articles JAMIE HOGBERG, ADRIAN TREVES, BRET SHAW, LISA NAUGHTON-TREVES, Environmental Conservation , Volume 43 Issue 01 , pp 45-55 Abstract

Description: Research Articles MATTHEW A. L. YOUNG, SIMON FOALE, DAVID R. BELLWOOD, Environmental Conservation , Volume 43 Issue 01 , pp 90-95 Abstract

Description: Research Articles C. BELLARD, C. LECLERC, B. D. HOFFMANN, F. COURCHAMP, Environmental Conservation , Volume 43 Issue 01 , pp 79-89 Abstract

Description: Research Articles KELSEY C. JAMES, REBECCA L. LEWISON, PETER W. DILLINGHAM, K. ALEXANDRA CURTIS, JEFFREY E. MOORE, Environmental Conservation , Volume 43 Issue 01 , pp 3-12 Abstract

Description: Research Articles JÉRÔME CIMON-MORIN, MARCEL DARVEAU, MONIQUE POULIN, Environmental Conservation , Volume 43 Issue 01 , pp 56-68 Abstract

Description: Book Reviews Nikolas Sellheim Polar Record , FirstView Article(s) , 2 pages Abstract

Description: 〈div data-abstract-type="normal"〉〈p〉The effectiveness of streak modes in controlling the oblique-type breakdown in a supersonic boundary-layer at Mach 2.0 is investigated using direct numerical simulations. Investigations in the literature have shown the effectiveness of streak modes in delaying the onset of transition dominated by two-dimensional waves, but in oblique breakdown, three-dimensional waves and a strong streak mode dominate the transition process. Paredes 〈span〉et al.〈/span〉 (〈span〉J. Fluid Mech.〈/span〉, vol. 831, 2017, pp. 524–553) discussed the possible stabilization of supersonic boundary layers by optimally growing streaks using parabolized stability equations. However, no study has as yet been reported regarding direct nonlinear control of oblique breakdown. This study deals with the effects of large-amplitude decaying streak modes generated by a blowing–suction strip at the wall to control full breakdown in a reference case. Modes with four to five times the fundamental wavenumber are found to be beneficial for controlling the transition. In the first region after the control-mode forcing, the beneficial mean-flow distortion (MFD), generated by inducing the control mode, is solely responsible for hampering the growth of the fundamental-mode. On the whole, the MFD and the three-dimensional part of the control contribute equally towards controlling the oblique breakdown. The results show significant suppression of transition, and substantial improvements have been observed in the levels of the skin-friction coefficient and wall-temperature in comparison to the uncontrolled case. Moreover, refreshing the control using an additional downstream control strip increases the gain. However, the forcing amplitude must be carefully chosen in order not to introduce a generalized inflection point in the spanwise averaged mean flow invoking enhanced disturbance growth.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉The problem of interaction between disturbances and shock waves was solved by a theoretical approach called linear interaction analysis in the mid-twentieth century. More recently, great progress has been made in analysing shock–turbulence interactions by direct numerical simulation. However, an unsolved theoretical problem remains: What happens when no acoustic waves are stimulated behind the shock wave? The concept of a damped wave is introduced, which is a type of excited plane wave. Based on this, the dispersion and amplitude relationships between any incident plane wave and resulting stimulated waves are constructed analytically, systematically and comprehensively. The physical essence of damped waves and the existence of critical angles are clarified. It is demonstrated that a damped wave is a complex number space solution to the acoustic dispersion relationship under certain conditions. It acts as a bridge connecting fast and slow acoustic waves at the position where the 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628130807083-0762:S0022112019004385:S0022112019004385_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 component of the group velocity is zero. There are two critical angles that can excite fast and slow acoustic waves, which determine the conditions that stimulate a damped wave. Our results show good agreement with theoretical and simulation results. The contribution of each excited wave to the transmission coefficient is evaluated, the distribution of the transmission coefficient is analysed and application to an engineering wedge model is performed.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉The onset of thermal convection in a rapidly rotating spherical shell is studied by linear stability analysis based on the fully compressible Navier–Stokes equations. Compressibility is quantified by the number of density scale heights 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, which measures the intensity of density stratification of the motionless, polytropic base state. The nearly adiabatic flow with polytropic index 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline2.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is considered, where 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline3.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the adiabatic polytropic index. By investigating the stability of the base state with respect to the disturbance of specified wavenumber, the instability process is found to be sensitive to the Prandtl number 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline4.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and to 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline5.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. For large 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline6.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and small 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline7.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, the quasi-geostrophic columnar mode loses stability first; while for relatively small 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline8.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 a new quasi-geostrophic compressible mode is identified, which becomes unstable first under strong density stratification. The inertial mode can also occur first for relatively small 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline9.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and a certain intensity of density stratification in the parameter range considered. Although the Rayleigh numbers 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline10.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 for the onsets of the quasi-geostrophic compressible mode and columnar mode are different by several orders of magnitude, we find that they follow very similar scaling laws with the Taylor number. The critical 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline11.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 for convection onset is found to be always positive, in contrast with previous results based on the widely used anelastic model that convection can occur at negative 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline12.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. By evaluating the relative magnitude of the time derivative of density perturbation in the continuity equation, we show that the anelastic approximation in the present system cannot be applied in the small-〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline13.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and large-〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628092857401-0357:S0022112019004361:S0022112019004361_inline14.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 regime.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉We study the breakup of confined fluid threads at low flow rates to understand instability mechanisms. To determine the critical conditions between the earlier quasi-stable necking stage and the later unstable collapse stage, simulations and experiments are designed to operate at an extremely low flow rate. The critical mean radii at the neck centres are identified by the stop-flow method for elementary microfluidic configurations. Two distinct origins of capillary instabilities are revealed for different confinement situations. One is the gradient of capillary pressure induced by the confinements of geometry and external flow, whereas the other is the competition between the capillary pressure and internal pressure determined by the confinements.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉We report on a combined experimental and numerical study of convective heat transfer along ratchet surfaces in vertical natural convection (VC). Due to the asymmetry of the convection system caused by the asymmetric ratchet-like wall roughness, two distinct states exist, with markedly different orientations of the large-scale circulation roll (LSCR) and different heat transport efficiencies. Statistical analysis shows that the heat transport efficiency depends on the strength of the LSCR. When a large-scale wind flows along the ratchets in the direction of their smaller slopes, the convection roll is stronger and the heat transport is larger than the case in which the large-scale wind is directed towards the steeper slope side of the ratchets. Further analysis of the time-averaged temperature profiles indicates that the stronger LSCR in the former case triggers the formation of a secondary vortex inside the roughness cavity, which promotes fluid mixing and results in a higher heat transport efficiency. Remarkably, this result differs from classical Rayleigh–Bénard convection (RBC) with asymmetric ratchets (Jiang 〈span〉et al.〈/span〉, 〈span〉Phys. Rev. Lett.〈/span〉, vol. 120, 2018, 044501), wherein the heat transfer is stronger when the large-scale wind faces the steeper side of the ratchets. We reveal that the reason for the reversed trend for VC as compared to RBC is that the flow is less turbulent in VC at the same 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628072522266-0966:S0022112019004464:S0022112019004464_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. Thus, in VC the heat transport is driven primarily by the coherent LSCR, while in RBC the ejected thermal plumes aided by gravity are the essential carrier of heat. The present work provides opportunities for control of heat transport in engineering and geophysical flows.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉We explore the dynamics of inclined temporal gravity currents using direct numerical simulation, and find that the current creates an environment in which the flux Richardson number 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, gradient Richardson number 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline2.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and turbulent flux coefficient 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline3.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 are constant across a large portion of the depth. Changing the slope angle 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline4.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 modifies these mixing parameters, and the flow approaches a maximum Richardson number 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline5.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 as 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline6.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 at which the entrainment coefficient 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline7.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. The turbulent Prandtl number remains 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline8.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 for all slope angles, demonstrating that 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline9.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is not caused by a switch-off of the turbulent buoyancy flux as conjectured by Ellison (〈span〉J. Fluid Mech.〈/span〉, vol. 2, 1957, pp. 456–466). Instead, 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline10.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 occurs as the result of the turbulence intensity going to zero as 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline11.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, due to the flow requiring larger and larger shear to maintain the same level of turbulence. We develop an approximate model valid for small 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline12.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 which is able to predict accurately 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline13.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline14.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline15.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 as a function of 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline16.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and their maximum attainable values. The model predicts an entrainment law of the form 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190627132031350-0413:S0022112019004300:S0022112019004300_inline17.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, which is in good agreement with the simulation data. The simulations and model presented here contribute to a growing body of evidence that an approach to a marginally or critically stable, relatively weakly stratified equilibrium for stratified shear flows may well be a generic property of turbulent stratified flows.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉Ice scallops are a small-scale (5–20 cm) quasi-periodic ripple pattern that occurs at the ice–water interface. Previous work has suggested that scallops form due to a self-reinforcing interaction between an evolving ice-surface geometry, an adjacent turbulent flow field and the resulting differential melt rates that occur along the interface. In this study, we perform a series of laboratory experiments in a refrigerated flume to quantitatively investigate the mechanisms of scallop formation and evolution in high resolution. Using particle image velocimetry, we probe an evolving ice–water boundary layer at sub-millimetre scales and 15 Hz frequency. Our data reveal three distinct regimes of ice–water interface evolution: a transition from flat to scalloped ice; an equilibrium scallop geometry; and an adjusting scallop interface. We find that scalloped-ice geometry produces a clear modification to the ice–water boundary layer, characterized by a time-mean recirculating eddy feature that forms in the scallop trough. Our primary finding is that scallops form due to a self-reinforcing feedback between the ice-interface geometry and shear production of turbulent kinetic energy in the flow interior. The length of this shear production zone is therefore hypothesized to set the scallop wavelength.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉Direct numerical simulation (DNS) is performed for two wall-bounded flow configurations: laminar Couette flow at 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and turbulent channel flow at 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline2.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, where 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline3.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the shear stress at the wall. The top wall is smooth and the bottom wall is a realistically rough superhydrophobic surface (SHS), generated from a three-dimensional surface profile measurement. The air–water interface, which is assumed to be flat, is simulated using the volume-of-fluid (VOF) approach. The two flow cases are studied with varying interface heights 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline4.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 to understand its effect on slip and drag reduction (〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline5.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉). For the laminar Couette flow case, the presence of the surface roughness is felt up to 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline6.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 of the channel height in the wall-normal direction. Nonlinear dependence of 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline7.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 on 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline8.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is observed with three distinct regions. A nonlinear curve fit is obtained for gas fraction 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline9.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 as a function of 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline10.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, where 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline11.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 determines the amount of slip area exposed to the flow. A power law fit is obtained from the data for the effective slip length as a function of 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline12.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and is compared to those derived for structured geometry. For the turbulent channel flow, statistics of the flow field are compared to that of a smooth wall to understand the effects of roughness and 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline13.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. Four cases are simulated ranging from fully wetted to fully covered and two intermediate regions in between. Scaling laws for slip length, slip velocity, roughness function and 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline14.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 are obtained for different penetration depths and are compared to past work for structured geometry. 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190628070320625-0031:S0022112019004191:S0022112019004191_inline15.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is shown to depend on a competing effect between slip velocity and turbulent losses due to the Reynolds shear stress contribution. Presence of trapped air in the cavities significantly alters near-wall flow physics where we examine near-wall structures and propose a physical mechanism for their behaviour. The fully wetted roughness increases the peak value of turbulent intensities, whereas the presence of the interface suppresses them. The pressure fluctuations have competing contributions between turbulent pressure fluctuations and stagnation due to asperities, the near-wall structure is altered and breaks down with increasing slip. Overall, there exists a competing effect between the interface and the asperities, the interface suppresses turbulence whereas the asperities enhance them. The present work demonstrates DNS over a realistic multiphase SHS for the first time, to the best of our knowledge.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉We developed a numerical method for the set of equations governing fully compressible convection in the limit of infinite Prandtl numbers. Reduced models have also been analysed, such as the anelastic approximation and the anelastic liquid approximation. The tests of our numerical schemes against self-consistent criteria have shown that our numerical simulations are consistent from the point of view of energy dissipation, heat transfer and entropy budget. The equation of state of an ideal gas has been considered in this work. Specific effects arising because of the compressibility of the fluid are studied, like the scaling of viscous dissipation and the scaling of the heat flux contribution due to the mechanical power exerted by viscous forces. We analysed the solutions obtained with each model (fully compressible model, anelastic and anelastic liquid approximations) in a wide range of dimensionless parameters and determined the errors induced by each approximation with respect to the fully compressible solutions. Based on a rationale on the development of the thermal boundary layers, we can explain reasonably well the differences between the fully compressible and anelastic models, in terms of both the heat transfer and viscous dissipation dependence on compressibility. This could be mostly an effect of density variations on thermal diffusivity. Based on the different forms of entropy balance between exact and anelastic models, we find that a necessary condition for convergence of the anelastic results to the exact solutions is that the product 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190625082702251-0920:S0022112019004208:S0022112019004208_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 must be small compared to unity, where 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190625082702251-0920:S0022112019004208:S0022112019004208_inline2.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the ratio of the superadiabatic temperature difference to the adiabatic difference, and 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190625082702251-0920:S0022112019004208:S0022112019004208_inline3.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the ratio of the superadiabatic heat flux to the heat flux conducted along the adiabat. The same condition seems also to be associated with a convergence of the computed heat fluxes. Concerning the anelastic liquid approximation, we confirm previous estimates by Anufriev 〈span〉et al.〈/span〉 (〈span〉Phys. Earth Planet. Inter.〈/span〉, vol. 152, 2005, pp. 163–190) and find that its results become generally close to those of the fully compressible model when 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190625082702251-0920:S0022112019004208:S0022112019004208_inline4.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is small compared to unity, where 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190625082702251-0920:S0022112019004208:S0022112019004208_inline5.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the isobaric thermal expansion coefficient, 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190625082702251-0920:S0022112019004208:S0022112019004208_inline6.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the temperature (here 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190625082702251-0920:S0022112019004208:S0022112019004208_inline7.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 for an ideal gas) and 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190625082702251-0920:S0022112019004208:S0022112019004208_inline8.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the dissipation number.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉Progress in roughness research, mapping any given roughness geometry to its fluid dynamic behaviour, has been hampered by the lack of accurate and direct measurements of skin-friction drag, especially in open systems. The Taylor–Couette (TC) system has the benefit of being a closed system, but its potential for characterizing irregular, realistic, three-dimensional (3-D) roughness has not been previously considered in depth. Here, we present direct numerical simulations (DNSs) of TC turbulence with sand grain roughness mounted on the inner cylinder. The model proposed by Scotti (〈span〉Phys. Fluids〈/span〉, vol. 18, 031701, 2006) has been modified to simulate a random rough surface of monodisperse sand grains. Taylor numbers range from 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉(corresponding to 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline2.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉) to 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline3.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 (〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline4.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉). We focus on the influence of the roughness height 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline5.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 in the transitionally rough regime, through simulations of TC with rough surfaces, ranging from 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline6.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 up to 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline7.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. We analyse the global response of the system, expressed both by the dimensionless angular velocity transport 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline8.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and by the friction factor 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline9.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. An increase in friction with increasing roughness height is accompanied with enhanced plume ejection from the inner cylinder. Subsequently, we investigate the local response of the fluid flow over the rough surface. The equivalent sand grain roughness 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline10.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is calculated to be 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline11.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, where 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline12.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the size of the sand grains. We find that the downwards shift of the logarithmic layer, due to transitionally rough sand grains exhibits remarkably similar behaviour to that of the Nikuradse (〈span〉VDI-Forsch.〈/span〉, vol. 361, 1933) data of sand grain roughness in pipe flow, regardless of the Taylor number dependent constants of the logarithmic layer. Furthermore, we find that the dynamical effects of the sand grains are contained to the roughness sublayer 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline13.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 with 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621104521411-0582:S0022112019003768:S0022112019003768_inline14.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉Geometric, kinematic and dynamic properties of focusing deep-water surface gravity wave packets are examined in a simplified model with the intent of deriving a wave breaking threshold parameter. The model is based on the spatial modified nonlinear Schrödinger equation of Dysthe (〈span〉Proc. R. Soc. Lond.〈/span〉 A, vol. 369 (1736), 1979, pp. 105–114). The evolution of initially narrow-banded and weakly nonlinear chirped Gaussian wave packets are examined, by means of a trial function and a variational procedure, yielding analytic solutions describing the approximate evolution of the packet width, amplitude, asymmetry and phase during focusing. A model for the maximum free surface gradient, as a function of 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621105221417-0357:S0022112019004282:S0022112019004282_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621105221417-0357:S0022112019004282:S0022112019004282_inline2.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉, for 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621105221417-0357:S0022112019004282:S0022112019004282_inline3.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 the linear prediction of the maximum slope at focusing and 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621105221417-0357:S0022112019004282:S0022112019004282_inline4.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 the non-dimensional packet bandwidth, is proposed and numerically examined, indicating a quasi-self-similarity of these focusing events. The equations of motion for the fully nonlinear potential flow equations are then integrated to further investigate these predictions. It is found that a model of this form can characterize the bulk partitioning of 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621105221417-0357:S0022112019004282:S0022112019004282_inline5.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 phase space, between non-breaking and breaking waves, serving as a breaking criterion. Application of this result to better understanding air–sea interaction processes is discussed.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉We conduct direct numerical simulations (DNS) of the Cahn–Hilliard–Navier–Stokes (CHNS) equations to investigate the statistical properties of a turbulent phase-separating symmetric binary-fluid mixture. Turbulence causes an arrest of the phase separation which leads to the formation of a statistically steady emulsion. We characterise turbulent velocity fluctuations in an emulsion for different values of the Reynolds number and the Weber number. Our scale-by-scale kinetic energy budget analysis shows that the interfacial terms in the CHNS equations provide an alternative route for the kinetic energy transfer. By studying the probability distribution function (p.d.f.) of the energy dissipation rate, the vorticity magnitude and the joint-p.d.f. of the velocity-gradient invariants we show that the statistics of the turbulent fluctuations do not change with the Weber number.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉With the aim to characterize the near-wall flow structures and their interaction with large-scale motions in the log-law region, time-resolved planar and volumetric flow field measurements were performed in the near-wall and log-law region of an adverse pressure gradient turbulent boundary layer following a zero pressure gradient turbulent boundary layer at a friction Reynolds number 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621113954055-0112:S0022112019004087:S0022112019004087_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. Due to the high spatial and temporal resolution of the measurements, it was possible to resolve and identify uniform-momentum zones in the region 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621113954055-0112:S0022112019004087:S0022112019004087_inline2.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 or 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621113954055-0112:S0022112019004087:S0022112019004087_inline3.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and to relate them with well known coherent flow motions near the wall. The space–time results confirm that the turbulent superstructures have a strong impact even on the very near-wall flow motion and also their alternating appearance in time and intensity could be quantified over long time sequences. Using the time record of the velocity field, rare localized separation events appearing in the viscous sublayer were also analysed. By means of volumetric particle tracking velocimetry their three-dimensional topology and dynamics could be resolved. Based on the results, a conceptual model was deduced that explains their rare occurrence, topology and dynamics by means of a complex interaction process between low-momentum turbulent superstructures, near-wall low-speed streaks and tilted longitudinal and spanwise vortices located in the near-wall region.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉We experimentally investigate the extensional flow of a sheet – or curtain – of viscoelastic liquid falling freely from a slot at constant flow rate under gravity. Extruded liquids are aqueous solutions of flexible polyethylene oxide (PEO) and of semi-rigid partially hydrolysed polyacrylamide (HPAM) with low shear viscosities. Velocimetry measurements reveal that the mean velocity field 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 (where 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline2.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the distance from the slot exit) does not reduce to a free fall. More precisely, we show that the liquid falls initially with sub-gravitational accelerations up to a distance from the slot which scales as 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline3.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 (where 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline4.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is gravity and 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline5.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 is the extensional relaxation time of the liquid) due to the stretching of polymer molecules. Beyond this elastic length, inertia dominates and the local acceleration reaches the asymptotic free-fall value 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline6.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. The length of the sub-gravitational part of the curtain is shown to be much larger than the equivalent viscous length 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline7.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 for Newtonian liquids of density 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline8.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 and dynamic viscosity 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline9.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 which is usually small compared to the curtain length. By analogy with Newtonian curtains, we show that the velocity field 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190621102720940-0917:S0022112019003896:S0022112019003896_inline10.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉 rescales on a master curve. Besides, the flow is shown to be only weakly affected by the history of polymer deformations in the die upstream of the curtain. Furthermore, investigations on the curtain stability reveal that polymer addition reduces the minimum flow rate required to maintain a continuous sheet of liquid.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉Previous experiments have revealed that shock waves driven through dissipative media may become unstable, for example, in granular gases, and in molecular gases undergoing strong relaxation effects. The current paper addresses this problem of shock stability at the Euler and Navier–Stokes continuum levels in a system of disks (two-dimensional) undergoing activated inelastic collisions. The dynamics of shock formation and stability is found to be in very good agreement with earlier molecular dynamic simulations (Sirmas & Radulescu, 〈span〉Phys. Rev.〈/span〉 E, vol. 91, 2015, 023003). It was found that the modelling of shock instability requires the introduction of molecular noise for its development and sustenance. This is confirmed in two stability problems. In the first, the evolution of shock formation dynamics is monitored without noise, with only initial noise and with continuous molecular noise. Only the latter reproduces the results of shock instability of molecular dynamics simulations. In the second problem, the steady travelling wave solution is obtained for the shock structure in the inviscid and viscous limits and its nonlinear stability is studied with and without molecular fluctuations, again showing that instability can be sustained only in the presence of fluctuations. The continuum results show that instability takes the form of a rippled front of a wavelength comparable with the relaxation thickness of the steady shock wave, at scales at which molecular fluctuations become important, in excellent agreement with the molecular dynamic simulations.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉We performed two-way coupled direct numerical simulations of turbulent channel flow with Lagrangian tracking of small, heavy spheres at a dimensionless gravitational acceleration of 0.077 in wall units, which is based on the flow condition in the experiment by Gerashchenko 〈span〉et al.〈/span〉 (〈span〉J. Fluid Mech.〈/span〉, vol. 617, 2008, pp. 255–281). We removed deposited particles after several collisions with the lower wall and then released new particles near the upper wall to observe direct interactions between particles and coherent structures of near-wall turbulence during gravitational settling through the mean shear. The results indicate that when the Stokes number is approximately 1 on the basis of the Kolmogorov time scale of the flow (〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190624134045701-0463:S0022112019004002:S0022112019004002_inline1.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉), the so-called preferential sweeping occurs in association with coherent streamwise vortices, while the effect of crossing trajectories becomes significant for 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190624134045701-0463:S0022112019004002:S0022112019004002_inline2.gif"〉 〈span data-mathjax-type="texmath"〉 〈/span〉 〈/span〉〈/span〉. Consequently, in either case, the settling particles deposit on the wall without strong accumulation in low-speed streaks in the viscous sublayer. When particles settle through near-wall turbulence from the upper wall, more small-scale vortical structures are generated in the outer layer as low-speed fluid is pulled farther in the direction of gravity, while the opposite is true near the lower wall.〈/p〉〈/div〉

Description: 〈div data-abstract-type="normal"〉〈p〉Streaks have been found to be an important part of wall-turbulence dynamics. In this paper, we extend the analysis for unbounded shear flows, in particular a Mach 0.4 round jet, using measurements taken using dual-plane, time-resolved, stereoscopic particle image velocimetry (PIV) taken at pairs of jet cross-sections, allowing the evaluation of the cross-spectral density of streamwise velocity fluctuations resolved into azimuthal Fourier modes. From the streamwise velocity results, two analyses are performed: the evaluation of wavenumber spectra (assuming Taylor’s hypothesis for the streamwise coordinate) and a spectral proper orthogonal decomposition (SPOD) of the velocity field using PIV planes in several axial stations. The methods complement each other, leading to the conclusion that large-scale streaky structures are also present in turbulent jets where they experience large growth in the streamwise direction, energetic structures extending up to eight diameters from the nozzle exit. Leading SPOD modes highlight the large-scale, streaky shape of the structures, whose aspect ratio (streamwise over azimuthal length) is approximately 15. The data were further analysed using SPOD, resolvent and transient growth analyses, good agreement being observed between the models and the leading SPOD mode for the wavenumbers considered. The models also indicate that the lift-up mechanism is active in turbulent jets, with streamwise vortices leading to streaks. The results show that large-scale streaks are a relevant part of the jet dynamics.〈/p〉〈/div〉