ALBERT

Description: The stability of a uniformly rotating and infinitely conducting jet with twisted magnetic field is discussed. The dispersion relation is obtained for the non-axisymmetric perturbations; this involves a complex and implicit function of the growth rate. The complex growth rate implies that instability develops as an overstability. The effect of axial current is destabilizing for both the instabilities m = 0 and m = 1 discussed here. The effect of rotation on the stability of the jet is as follows: (a) for the m = 0 mode, rotation always has a destabilizing effect; (b) for the m = 1 mode, rotation has a stabilizing effect for very long-wave perturbations, but for short-wave perturbations it destabilizes the jet. In particular there exists a disturbance of a particular wavelength which is unaffected by the rotation of the jet. © 1965, Cambridge University Press. All rights reserved.

Description: Agriculture in the Central Himalayan Region depends on the availability of suitable germplasm as well as natural conditions. Due to extreme weather conditions, food and nutrition security is a major issue for communities inhabiting these remote and inaccessible areas. Millets are common crops grown in these areas. Foxtail millet (Setaria italica (L.) P. Beauv) is an important crop and forms a considerable part of the diet in this region. The aim of the present study was to explore, collect, conserve and evaluate the untapped genetic diversity of foxtail millet at the molecular level and discover variability in their nutritional traits. A total of 30 accessions having unique traits of agronomic importance were collected and molecular profiling was performed. A total of 63 alleles were generated with an average of 2.52 alleles per locus and average expected heterozygosity of 0.37 ± 0.231. Significant genetic variability was revealed through the genetic differentiation (Fst) and gene flow (Nm) values. Structure-based analysis divided whole germplasm into three sub-groups. Rich variability was found in nutritional traits such as dietary fibre in husked grains, carbohydrate, protein, lysine and thiamine content. The collected germplasm may be useful for developing nutritionally rich and agronomically beneficial varieties of foxtail millet and also designing strategies for utilization of unexploited genetic diversity for food and nutrition security in this and other similar agro-ecological regions.

Description: We present a numerical study of the reversals and reorientations of the large-scale circulation (LSC) of convective fluid in a cylindrical container of aspect ratio one. We take Prandtl number to be 0.7 and Rayleigh numbers in the range from 6×105 to 3×107. It is observed that the reversals of the LSC are induced by its reorientation along the azimuthal direction, which are quantified using the phases of the first Fourier mode of the vertical velocity measured near the lateral surface in the midplane. During a complete reversal, the above phase changes by around 180°, leading to reversals of the vertical velocity at all the probes. On the contrary, the vertical velocity reverses only at some of the probes during a partial reversal with phase change other than 180°. Numerically, we observe rotation-led and cessation-led reorientations, in agreement with earlier experimental results. The ratio of the amplitude of the second Fourier mode and the first Fourier mode rises sharply during the cessation-led reorientations. This observation is consistent with the quadrupolar dominant temperature profile observed during the cessations. We also observe reorientations involving double cessation. © 2010 Cambridge University Press.

Description: A flow-induced instability in a tube with flexible walls is studied experimentally. Tubes of diameter 0.8 and 1.2 mm are cast in polydimethylsiloxane (PDMS) polymer gels, and the catalyst concentration in these gels is varied to obtain shear modulus in the range 17-550 kPa. A pressure drop between the inlet and outlet of the tube is used to drive fluid flow, and the friction factor f is measured as a function of the Reynolds number Re. From these measurements, it is found that the laminar flow becomes unstable, and there is a transition to a more complicated flow profile, for Reynolds numbers as low as 500 for the softest gels used here. The nature of the f-Re curves is also qualitatively different from that in the flow past rigid tubes; in contrast to the discontinuous increase in the friction factor at transition in a rigid tube, it is found that there is a continuous increase in the friction factor from the laminar value of 16/Re in a flexible tube. The onset of transition is also detected by a dye-stream method, where a stream of dye is injected into the centre of the tube. It is found that there is a continuous increase of the amplitude of perturbations at the onset of transition in a flexible tube, in contrast to the abrupt disruption of the dye stream at transition in a rigid tube. There are oscillations in the wall of the tube at the onset of transition, which is detected from the laser scattering off the walls of the tube. This indicates that the coupling between the fluid stresses and the elastic stresses in the wall results in an instability of the laminar flow. © 2011 Cambridge University Press.

Description: Propeller crashback is an off-design operating condition where a propeller rotates in the reverse direction. Experiments (Bridges 2004, Tech Rep. MSSU-ASE-04-1, Department of Aerospace Engineering, Mississippi State University) have shown that the presence of an upstream hull significantly increases the side force on a propeller in crashback below an advance ratio of J= ensuremath-0. 7. Large-eddy simulation (LES) is performed for a propeller with and without a hull at two advance ratios, J= ensuremath-1. 0 and J= ensuremath-0. 5. LES reproduces the experimentally observed behaviour and shows good quantitative agreement. Time-averaged flow fields are investigated for a qualitative understanding of the complex flow resulting from the interaction of the upstream hull with the propeller blades. At J= ensuremath-1. 0, two noticeable flow features are found for the case with the hull a recirculation zone upstream in the vicinity of the propeller and a vortex ring much closer to the propeller. In contrast, at J= ensuremath-0. 5, there is a much smaller recirculation zone which is further upstream due to the increased reverse flow. As a result, the hull does not make much difference in the immediate vicinity of the propeller at J= ensuremath-0. 5. For both advance ratios, side force is mainly generated from the leading-edge separation on the suction side. However, high levels of side force are also generated from trailing-edge separation on the suction side at J= ensuremath-1. 0. © 2012 Cambridge University Press.

Description: According to the celebrated Bolgiano–Obukhov (Bolgiano, J. Geophys. Res., vol. 64 (12), 1959, pp. 2226–2229; Obukhov, Dokl. Akad. Nauk SSSR, vol. 125, 1959, p. 1246) phenomenology for moderately stably stratified turbulence, the energy spectrum in the inertial range shows a dual scaling: the kinetic energy follows (i)  ${sim}k^{-11/5}$ for $kk_{B}$ , where $k_{B}$ is the Bolgiano wavenumber. The $k^{-5/3}$ scaling, akin to passive scalar turbulence, is a direct consequence of the assumption that buoyancy is insignificant for $k〉k_{B}$ . We revisit this assumption, and using the constancy of kinetic and potential energy fluxes and simple theoretical analysis, we find that the $k^{-5/3}$ spectrum is absent. This is because the velocity field at small scales is too weak to establish a constant kinetic energy flux as in passive scalar turbulence. A quantitative condition for the existence of the second regime is also derived in the paper.

Description: A dynamical instability is observed in experimental studies on micro-channels of rectangular cross-section with smallest dimension 100 and 160 μm in which one of the walls is made of soft gel. There is a spontaneous transition from an ordered, laminar flow to a chaotic and highly mixed flow state when the Reynolds number increases beyond a critical value. The critical Reynolds number, which decreases as the elasticity modulus of the soft wall is reduced, is as low as 200 for the softest wall used here (in contrast to 1200 for a rigid-walled channel). The instability onset is observed by the breakup of a dye-stream introduced in the centre of the micro-channel, as well as the onset of wall oscillations due to laser scattering from fluorescent beads embedded in the wall of the channel. The mixing time across a channel of width 1.5 mm, measured by dye-stream and outlet conductance experiments, is smaller by a factor of 105 than that for a laminar flow. The increased mixing rate comes at very little cost, because the pressure drop (energy requirement to drive the flow) increases continuously and modestly at transition. The deformed shape is reconstructed numerically, and computational fluid dynamics (CFD) simulations are carried out to obtain the pressure gradient and the velocity fields for different flow rates. The pressure difference across the channel predicted by simulations is in agreement with the experiments (within experimental errors) for flow rates where the dye stream is laminar, but the experimental pressure difference is higher than the simulation prediction after dye-stream breakup. A linear stability analysis is carried out using the parallel-flow approximation, in which the wall is modelled as a neo-Hookean elastic solid, and the simulation results for the mean velocity and pressure gradient from the CFD simulations are used as inputs. The stability analysis accurately predicts the Reynolds number (based on flow rate) at which an instability is observed in the dye stream, and it also predicts that the instability first takes place at the downstream converging section of the channel, and not at the upstream diverging section. The stability analysis also indicates that the destabilization is due to the modification of the flow and the local pressure gradient due to the wall deformation; if we assume a parabolic velocity profile with the pressure gradient given by the plane Poiseuille law, the flow is always found to be stable. © 2013 Cambridge University Press.

Description: The thermal response of hot-film anemometers to fluid velocity fluctuations is investigated for low-Prandtl-number fluids. A quartz-coated hot-film probe with a diameter of 0⋅002 inches and an aspect ratio of 20 was oscillated in a horizontal plane while immersed in a steadily rotating tank of mercury. The probe was oscillated sinusoidally from 2 to 1200 Hz with a vibrator. The amplitude of velocity fluctuation was regulated to about 20 % of the mean flow within a Peclet-number range of 0⋅1–1⋅0. The findings concur with the theoretical results obtained numerically and published earlier by Malcolm & Verma (1973) and compare well with some results of other researchers. The results confirm that the sensitivity of the hot-film probes is inhibited at low Peclet numbers, even at quite low frequencies in liquid metals owing to their very low Prandtl numbers. Two important effects are noticed: (a)The amplitude of fluctuation is attenuated and the degree of attenuation depends upon a non-dimentional quantity αf/U2, for the range of Peclet numbers considered, whereL α is the thermal diffusivity, f is the frequency of the fluctuations and is the free-stream velocity, in compatible units. The amplitude is attenuated by 10 % and 90 % at αf/U2 values of 0⋅02 and 4⋅0 respectively. (b)There is a phase lag in the hot-film probe signal with respect to the true velocity of the fluctuation which is somewhat the same as that in potential flow at low frequencies, but is considerably higher than that in potential flow at higher frequencies. The measured lag does not level off asympotically at high frequencies as noted in the numerically obtained results for potential flow. Corrections may be made to unsteady velocity measurements in low Prandtl number fluids to account for the above effects with some confidence depending upon the value of the αf/U2 quantity. The results of the investigation are of more general interest, in that the hot-film sensor can be considered as a model of a long circular cylinder in a flow at low to moderate Peclet numbers. © 1981, Cambridge University Press. All rights reserved.