ALBERT

Beschreibung: Thrust performance and wake structure were investigated for a rigid rectangular panel pitching about its leading edge in a free stream. For ReC = O(104), thrust coefficient was found to depend primarily on Strouhal number St and the aspect ratio of the panel AR. Propulsive efficiency was sensitive to aspect ratio only for AR less than 0.83; however, the magnitude of the peak efficiency of a given panel with variation in Strouhal number varied inversely with the amplitude to span ratio A/S, while the Strouhal number of optimum efficiency increased with increasing A/S. Peak efficiencies between 9% and 21% were measured. Wake structures corresponding to a subset of the thrust measurements were investigated using dye visualization and digital particle image velocimetry. In general, the wakes divided into two oblique jets; however, when operating at or near peak efficiency, the near wake in many cases represented a Kármán vortex street with the signs of the vortices reversed. The three-dimensional structure of the wakes was investigated in detail for AR = 0.54, A/S = 0.31 and ReC = 640. Three distinct wake structures were observed with variation in Strouhal number. For approximately 0.20 〈 St 〈 0.25, the main constituent of the wake was a horseshoe vortex shed by the tips and trailing edge of the panel. Streamwise variation in the circulation of the streamwise horseshoe legs was consistent with a spanwise shear layer bridging them. For St 〉 0.25, a reorganization of some of the spanwise vorticity yielded a bifurcating wake formed by trains of vortex rings connected to the tips of the horseshoes. For St 〉 0.5, an additional structure formed from a perturbation of the streamwise leg which caused a spanwise expansion. The wake model paradigm established here is robust with variation in Reynolds number and is consistent with structures observed for a wide variety of unsteady flows. Movies are available with the online version of the paper. © 2008 Cambridge University Press.

Beschreibung: Turbulence measurements were obtained in a three-dimensional supersonic turbulent boundary layer. A 20° curved fin was used to generate a three-dimensional compression of a boundary layer at Mach 3 in the absence of shock waves. Data include hot-wire measurements of five components of the Reynolds stress tensor. The results are interpreted in terms of the mean flow field history of the turbulence. It is demonstrated that in-plane curvature can have a strong stabilizing effect on the turbulence.

Beschreibung: This paper reports an experimental investigation of the instantaneous structure of a supersonic turbulent boundary layer (M = 2.86, Red= 82000) over a short region of longitudinal concave surface curvature. The radius of curvature was 12 initial boundary-layer thicknesses and the turning angle was 16°. Severe distortion of the boundary layer occurred, as evidenced by an alteration of the mean velocity profiles and an increase in wall shear stress of 125%. The large-scale organized motions in the boundary layer were significantly altered as illustrated by changes in the character of the mass flux ‘fronts’ (large gradients in the fluctuating streamwise mass flux). © 1994, Cambridge University Press. All rights reserved.

Beschreibung: Flow visualization is used to interrogate the wake structure produced by a rigid flat panel of aspect ratio (span/chord) 0.54 pitching in a free stream at a Strouhal number of 0.23. At such a low aspect ratio, the streamwise vorticity generated by the plate tends to dominate the formation of the wake. Nevertheless, the wake has the appearance of a three-dimensional Kármán vortex street, as observed in a wide range of other experiments, and consists of horseshoe vortices of alternating sign shed twice per flapping cycle. The legs of each horseshoe interact with the two subsequent horseshoes in an opposite-sign, then like-sign interaction in which they become entrained. A detailed vortex skeleton model is proposed for the wake formation. © 2005 Cambridge University Press.

Beschreibung: Streamwise and wall-normal turbulence components are obtained in fully developed turbulent pipe over a Reynolds number range from 1.1 × 105 to 9.8 × 106. The stream-wise intensity data are consistent with previous measurements in the same facility. For the wall-normal turbulence intensity, a constant region in v'r.m.s. is found for the region 200 ≤ y+ ≤ 0.1R+ for Reynolds numbers up to 1.0 × 106. An increase in v'r.m.s. is observed below about y+ ∼ 100, although additional measurements will be required to establish its generality. The wall-normal spectra collapse in the energy-containing region with inner scaling, but for the low-wavenumber region a y/R dependence is observed, which also indicates a continuing influence from the outer flow on the near-wall motions. © 2007 Cambridge University Press.

Beschreibung: Experimental and computational results are presented on an aerofoil undergoing pitch oscillations in ground effect, that is, close to a solid boundary. The time-averaged thrust is found to increase monotonically as the mean position of the aerofoil approaches the boundary while the propulsive efficiency stays relatively constant, showing that ground effect can enhance thrust at little extra cost for a pitching aerofoil. Vortices shed into the wake form pairs rather than vortex streets, so that in the mean a momentum jet is formed that angles away from the boundary. The time-averaged lift production is found to have two distinct regimes. When the pitching aerofoil is between 0.4 and 1 chord lengths from the ground, the lift force pulls the aerofoil towards the ground. In contrast, for wall proximities between 0.25 and 0.4 chord lengths, the lift force pushes the aerofoil away from the ground. Between these two regimes there is a stable equilibrium point where the time-averaged lift is zero and thrust is enhanced by approximately 40Â %. © 2014 Cambridge University Press.

Beschreibung: Recent investigations by Hellström have uncovered certain errors in the papers by Hellström & Smits (2014) and Hellström, Ganapathisubramani & Smits (2015), which are corrected below. Hellström & Smits (2014) estimated the convective displacement between two consecutive data snapshots, based on the bulk velocity, to be 0:48R and 0:96R for Reynolds numbers 47 000 and 93 000, respectively. These displacements should be corrected to 2:20R and 4:40R. The convective length scales were not used as a part of the analysis and so the errors have no further impact on this work and do not affect its conclusions. The incorrect estimation of the convective displacement propagated to Hellström et al. (2015), where the displacement should be corrected from 0:96R to 4:77R for ReD D 104 000. This correction implies that the large-scale structures remain spatiotemporally correlated for longer than the reported 1-2R. Figure 1 shows the corrected temporal autocorrelation of the first proper orthogonal decomposition (POD) mode and third azimuthal mode number, φn.mI r/Dφ 1.3I r/, for Reynolds numbers 52 000 and 104 000. The decay to the e-1 point suggests that the streamwise spatio-temporal length of the coherent structures is approximately 3R, which is in good agreement with the streamwise extent of the corresponding conditional mode, ψ(3,1). The corrections to the estimated spatio-temporal length scale do not further impact the work, including the conclusion that these structures of O.3R/ in length are a basic building block that line up to create longer structures similar to the very large-scale motions (VLSMs). (Figure Presented). © Cambridge University Press 2016.

Beschreibung: Townsend's attached eddy hypothesis assumes the existence of a set of energetic and geometrically self-similar eddies in the logarithmic layer in wall-bounded turbulent flows, which can be scaled with their distance to the wall. To examine the possible self-similarity of the energetic eddies in fully developed turbulent pipe flow, we performed stereo particle image velocimetry measurements together with a proper orthogonal decomposition analysis. For two Reynolds numbers, Reτ = 1330 and 2460, the resulting modes/eddies were shown to exhibit self-similar behaviour for eddies with wall-normal length scales spanning a decade. This single length scale provides a complete description of the cross-sectional shape of the self-similar eddies. © 2016 Cambridge University Press.

Beschreibung: This investigation describes the effects of an adverse pressure gradient on a flat plate supersonic turbulent boundary layer (Mref≈2.9, βmax≈5.8, Reθref≈75600). Single normal hot wires and crossed wires were used to study the Reynolds stress behaviour, and the features of the large-scale structures in the boundary layer were investigated by measuring space-time correlations in the normal and spanwise directions. Both the mean flow and the turbulence were strongly affected by the pressure gradient. However, the turbulent stress ratios showed much less variation than the stresses, and the essential nature of the large-scale structures was unaffected by the pressure gradient. The wall pressure distribution in the current experiment was designed to match the pressure distribution on a previously studied curved-wall model where streamline curvature acted in combination with bulk compression. The addition of streamline curvature affects the turbulence strongly, although its influence on the mean velocity field is less pronounced and the modifications to the skin-friction distribution seem to follow the empirical correlations developed by Bradshaw (1974) reasonably well. © 1990, Cambridge University Press. All rights reserved.

Beschreibung: A study was undertaken to examine the flat plate relaxation behaviour of a turbulent boundary layer recovering from 90° of strong convex curvature (δ0/R = 0.08), for a length of ≈ 90=0 after the end of curvature, where δ0is the boundary layer thickness at the start of the curvature. The results show that the relaxation behaviour of the mean flow and the turbulence are quite different. The mean velocity profile and skin friction coefficient asymptotically approach the unperturbed state and at the last measuring station appear to be fully recovered. The turbulence relaxation, however, occurs in several stages over a much longer distance. In the first stage, a stress ‘ bore’ (a region of elevated stress) is generated near the wall, and the bore thickens with distance downstream. Eventually it fills the whole boundary layer, but the stress levels continue to rise beyond their self-preserving values. Finally the stresses begin a gradual decline, but at the last measuring station they are still well above the unperturbed levels, and the ratios of the Reynolds stresses are distorted. These results imply a reorganization of the large-scale structure into a new quasi-stable state. The long-lasting effects of curvature highlight the sensitivity of a boundary layer to its Condition of formation. © 1990, Cambridge University Press. All rights reserved.