Showing papers by "Sanjoy Banerjee published in 1999"

Numerical investigation of the entrainment and mixing processes in neutral and stably-stratified mixing layers

Abstract: The direct numerical simulation (DNS) of a temporally-growing mixing layer has been carried out, for a variety of initial conditions at various Richardson and Prandtl numbers, by means of a pseudo-spectral technique; the main objective being to elucidate how the entrainment and mixing processes in mixing-layer turbulence are altered under the combined influence of stable stratification and thermal conductivity. Stratification is seen to significantly modify the way by which entrainment and mixing occur by introducing highly-localized, convective instabilities, which in turn cause a substantially different three-dimensionalization of the flow compared to the unstratified situation. Fluid which was able to cross the braid region mainly undisturbed (unmixed) in the unstratified case, pumped by the action of rib pairs and giving rise to well-formed mushroom structures, is not available with stratified flow. This is because of the large number of ribs which efficiently mix the fluid crossing the braid region. More efficient entrainment and mixing has been noticed for high Prandtl number computations, where vorticity is significantly reinforced by the baroclinic torque. In liquid sodium, however, for which the Prandtl number is very low, the generation of vorticity is very effectively suppressed by the large thermal conduction, since only small temperature gradients, and thus negligible baroclinic vorticity reinforcement, are then available to counterbalance the effects of buoyancy. This is then reflected in less efficient entrainment and mixing. The influence of the stratification and the thermal conductivity can also be clearly identified from the calculated entrainment coefficients and turbulent Prandtl numbers, which were seen to accurately match experimental data. The turbulent Prandtl number increases rapidly with increasing stratification in liquid sodium, whereas for air and water the stratification effect is less significant. A general law for the entrainment coefficient as a function of the Richardson and Prandtl numbers is proposed, and critically assessed against experimental data.

Turbulence modulation by an array of large-scale streamwise structures of ehd origin

Abstract: Recently, it was shown (Schoppa and Hussain, Phys. Fluids, 10, 1049) that superimposing large-scale, synthetic, streamwise vortical flow structures onto a turbulent Poiseuille flow led to suppression of the low-speed streak instability mechanism, which, in the end, appears to be responsible for drag enhancement in turbulent flows. In this work, we use large-scale ElectroHydroDynamic flow structures to control turbulent transfer mechanisms. We consider a channel with two different flow control configurations: E-control, in which streamwise wireelectrodes are embedded into one of the walls and C-control, in which streamwise wire-electrodes are placed in the central-plane of the channel. In all cases, the wires are maintained at a potential sufficient to ensure ionic discharge. Ions are driven by the applied electrostatic field and generate plane, streamwise jets, which impinge on the opposite (grounded) wall and, by continuity, generate two-dimensional vortical flows. Control flows have a spanwise periodicity of 340 wall units, in order to encompass three low-speed streaky structures. Results indicate that, after flow control actuation, the flow field undergoes an initial steep transient of about 600 shearbased time units with a moderate drag decrease (about 6 7%), followed by a steady-state in which the overall drag is only slightly modified. All cases with E-control indicated drag increase. Higher intensity C-control cases led to drag reduction.

The Role of Visualization in Clarifying Interfacial Heat and Mass Transfer Mechanisms

Abstract: Mechanisms governing heat and mass transfer at air-water interfaces may be studied experimentally and by mean of Direct Numerical Simulations (DNS). Flow visualizations play a central role in unraveling the mechanisms that govern these transfer rates. In particular visualizations show that the flow is organized in large structures. These are sweeps, high-speed (relative to the interface velocity) fluid traveling toward the interface, and ejections, low speed fluid moving away from the interface region. It is the frequency with which these large flow structures refresh the interface that controls mass transfer. On the liquid side, flow fluctuations in the near-interface region are relatively unimpeded, so fluid poor in solute can be transported by sweeps to the interface, and then pick up solute through diffusion before being carried away again. On the gas side, flow fluctuations in the near-interface region are strongly impeded, so mass transfer is controlled by sweeps and ejections, i.e., any events with significant interface-normal velocity. The frequency, with which these large flow structures are generated, can be computed from the DNS. Simple parameterizations, based on the mechanisms discussed above, can be developed and appear to predict mass transfer velocities in excellent agreement with experimental and numerical results. The parameterizations capture the effect of capillary waves.