Interaction of inner and outer layers in plane and radial wall jets

Large eddy simulations of turbulent radial and plane wall jets were performed at different Reynolds numbers using the Lagrangian dynamic eddy viscosity subgrid-scale model. The results were validated with experimental data available in the literature. Compared to the plane ones, the radial wall jets have an extra direction for expansion, which causes faster decay rates. Thus, the resulting pressure gradient distributions are different. However, the comparison of the results with the turbulent boundary layers under adverse and favourable pressure gradients reveals that these pressure gradients are not strong enough to cause any fundamental physical difference between plane and radial wall jets. In both cases, the local Reynolds number is an important determining factor in characterisation of the flow. The joint probability density function analysis shows that the local Reynolds number determines the level of intrusion of the outer layer into the inner layer: the lower the local Reynolds number, the stronger is the interaction of the inner and outer layers. These results can be used to clarify the scatter of the reported log-law constants in the literature.     

Fluctuating Thermal Boundary Layers and Heat Transfer in Turbulent Rayleigh–Bénard Convection

We investigate the effect of fluctuations in thermal boundary layer on heat transfer in turbulent Rayleigh–Bénard convection for Prandtl number greater than one in the regime where the thermal dissipation rate is dominated by boundary layer contribution and in the presence of a large-scale circulating flow.     

On the appearance of a system of ring vortices in the mixing layer of axially symmetric turbulent jets under acoustic action

The shadow visualization method is applied to study the process of loss of stability of the mixing layer of a subsonic axially symmetric turbulent jet under longitudinal internal action of saw-tooth sound waves of finite amplitude. Such action leads to the formation of a system of ring vortices in the mixing layer at the frequency of its intrinsic instability. The interaction of the vortices can be accompanied by sound emission. A similar phenomenon is also observed in turbulent jets for small supercritical pressure fluctuations on a nozzle.     

Experimental studies of flow noise around a surfacing device

The wall pressure fluctuations in turbulent boundary layers play an important role in acoustic measurements carried out in moving media. Results of measuring the frequency spectra of wall pressure fluctuations around a surfacing device are presented. The spatial resolution achieved in measuring the wall pressure fluctuations is investigated. It is demonstrated that the results of hydrodynamic flow noise measurements strongly depend on the aperture size of the measuring acoustic transducer and its orientation in the turbulent boundary layer. The pseudosound pressure fluctuation spectra observed in a series of experiments with surfacing devices show that the resolution of the pressure receivers operating in the turbulent boundary layers considerably varies. On the basis of systematic measurements of wall pressure fluctuations by miniature and distributed receivers at high Reynolds numbers, the effect of the geometric dimensions of a pressure receiver on its resolution in the flow noise measurements is studied. An experimental method is proposed for estimating the receiver-induced distortions.     

On the stability of certain collisional configurations of hydromagnetic flows

We study the flow of a hydromagnetic fluid toward an obstacle in two different cases: when this is a rigid wall or when two plasma masses collide with each other. The magnetic field far from the obstacle is assumed to be aligned with the flow. The diffusivity is taken as low, and a boundary layer approach for the stationary MHD system is considered. The relevant equations turn out to be a generalization of the Falkner-Skan ones, and while analytical solutions are impossible to obtain, a qualitative analysis shows that whenever the size of the Alfvén speed far from the interface exceeds the size of the fluid velocity, the system has no nontrivial solutions. The interpretation of this is that in this case disturbances occurring in the boundary layer travel upstream and disturb the boundary conditions at the outer layers.     

Flowfield features on hypersonic flow over rectangular obstacles

The complex separated flows induced by shock wave/boundary layer interaction were studied at hypersonic speed of Mach number 5. The experimental results on hypersonic flow over a set of rectangular cylinders are presented in this paper. The rectangular cylinder mounted on a flat plate worked as a typical model to simulate the obstacle on the vehicle surface. The effects of flow interaction on the aerodynamic characteristics have to be predicted for various geometrical parameters. So the static pressure distributions on the model surface were measured, the complex shock wave system was shown by schlieren photos, and the separated flow patterns around the obstacle were visualized by oil flow technique. All of the results describe the interactive flowfield features including peak pressure levels and their locations as well as separated boundaries associated with influence regions.     

Double layers and ion phase-space holes in the auroral upward-current region

The dynamic evolution of the boundary between the ionosphere and auroral cavity is studied using 1D and 2D kinetic Vlasov simulations. The initial distributions of three singly ionized species (H+, O+, e-) are determined from space-based observations on both sides of an inferred strong double layer. The kinetic simulations reproduce features of parallel electric fields, electron distributions, ion distributions, and wave turbulence seen in satellite observations in the auroral upward-current region and, for the first time, demonstrate that auroral acceleration can be driven by a parallel electric field supported, in part, by a quasistable, strong double layer. In addition, the simulations verify that the streaming interaction between accelerated O+ and H+ populations continuously replenished by the double layer provides the free energy for the persistent formation of ion phase-space holes.     

Measurements of Reynolds stress and turbulence in the boundary plasma of the HT-7 tokamak

Measurements of electric field fluctuations, Reynolds stress and poloidal flow have been performed in the boundary region of the HT-7 tokamak using a Langmuir probe array.Sheared radial electric field and poloidal flow have been found in the vicinity of the limiter and the turbulence has been clearly modified in this region. Furthermore,the electrostatic Reynolds stress component shows a radial gradient close to the velocity shear layer location.All results here indicate that the radial gradient of Reynolds stress may play an important role in the driving of poloidal flows in the plasma boundary region.     

Direct numerical simulation of the flow around a wall-mounted square cylinder under various inflow conditions

The flow around a wall-mounted square cylinder of side d is investigated by means of direct numerical simulation (DNS). The effect of inflow conditions is assessed by considering two different cases with matching momentum-thickness Reynolds numbers Re_θ ? 1000 at the obstacle: the first case is a fullyturbulent zero pressure gradient boundary layer, and the second one is a laminar boundary layer with prescribed Blasius inflow profile further upstream. An auxiliary simulation carried out with the pseudo-spectral Fourier–Chebyshev code SIMSON is used to obtain the turbulent time-dependent inflow conditions which are then fed into the main simulation where the actual flow around the cylinder is computed. This main simulation is performed, for both laminar and turbulent-inflows, with the spectral-element method code Nek5000. In both cases the wake is completely turbulent, and we find the same Strouhal number St ? 0.1, although the two wakes exhibit structural differences for x > 3d downstream of the cylinder. Transition to turbulence is observed in the laminar-inflow case, induced by the recirculation bubble produced upstream of the obstacle, and in the turbulent-inflow simulation the streamwise fluctuations modulate the horseshoe vortex. The wake obtained in our laminar-inflow case is in closer agreement with reference particle image velocimetry measurements of the same geometry, revealing that the experimental boundary layer was not fully turbulent in that dataset, and highlighting the usefulness of DNS to assess the quality of experimental inflow conditions.     

Diagnostics of picosecond laser pulse absorption in preformed plasma

We present results where highly supersonic plasma jets and accelerated plasma fragments are generated by interaction of an intense picosecond laser pulse with a metallic target (Al, Cu, W, and Ta) in gas atmosphere. The formation of jets and well-localized massive plasma fragments occurs when a strong forward shock from a main laser pulse and a reverse shock from a pre-pulse meet to. Interferometric and shadow graphic measurements with high temporal (100 ps) and spatial (1 μm) resolution yield information about the formation and evolution of plasma jets and plasma fragments. The excitation of the electric and self-generated magnetic field by ponderomotive force during propagation of the laser pulse in a gas atmosphere was investigated as well. It had been shown previously that under certain conditions a hollow current channel can be generated in laser-produced plasma. The azimuthal magnetic field in such a micro-channel was determined by Faraday rotation of a probing laser beam to be 7.6 MGauss (MG). Ion acceleration in a pinched annular current channel up to 8 MeV analogous to micro-“plasma focus” conditions, may be realized at lengths of 100 μm. Self-generated magnetic fields of 4-7 MG have also been measured in thin skin layers in front of shock waves, where well-collimated plasma blocks were separated and accelerated away from the plasma body. The velocity of dense plasma blocks reaches values of order of 3 × 10⁸ cm/s and they are stable during acceleration and propagation in gas.