Thermophysics and Aeromechanics

Yakovenko, S. N.;Chang, K. C.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S0869864308020017

Abstract The broken dam problem flow is tested to check accuracy of different procedures for gas-liquid interface resolution based on solution of the additional equation for the volume fraction of liquid phase. The study is focused on the numerical schemes used to approximate advection fluxes of this equation. In particular, the MUSCL scheme with QUICK interpolants and compressive minmod TVD limiters with the slope modification technique for the volume fraction fluxes is applied, as well as the upwind-downwind donor acceptor procedure designed in the VOF method. As the first stage, the quite simple and explicit procedure adopting the artificial compressibility method is used to solve the velocity and pressure equations. Computations are initially performed with a careful grid and time step independence studies. Importance of the wall boundary condition is also discussed. To present free surface motion, results of numerical investigation are shown in terms of contour plots for the volume fraction at successive times, as well as surge front and column height positions versus time.

Kashinsky, O. N.;Kaipova, E. V.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S0869864308020029

Abstract Results of experimental investigation of a bubbly gas-liquid flow in horizontal and weakly inclined (from −20° to +20°) flat channel are presented. These measurements were carried out within the 0.2–1 m/s range of superficial velocities and volumetric gas flow rate ratio of up to 0.2. The hydrodynamic structure was measured by the electrochemical method with application of wall shear stress and conductivity microprobes. During the experiments signals of shear stress on the upper channel wall and local gas flow rate ratio were recorded completely. After numerical treatment of recorded signals the profiles of local gas flow rate ratio were obtained, average shear stress and its relative mean square pulsations on the upper channel wall were determined. It is shown that under the studied regimes the bubbles are grouped into clusters, and the bubbly flow is presented by alternation of bubbly clusters and single-phase liquid with separate bubbles and without them. Average wall shear stress and absolute shear stress pulsations in the range of bubbly clusters and beyond them were determined. Histograms of probability density distribution were obtained for the wall shear stress on the upper wall. It is shown that average shear stress and absolute pulsations in clusters are significantly higher than those in the flow zone free from bubbles.

Vetlutsky, V. N.;Ganimedov, V. L.;Muchnaya, M. I.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S0869864308020030

Abstract The ideal gas exhaustion from an infinite volume into a gas at rest through a supersonic conical Laval nozzle is considered. The problem was solved numerically by steadying in time in a unified formulation for the regions inside the nozzle and in the ambient environment. In such a statement, the nozzle outlet section is no internal boundary of the region under consideration, and there is no need of specifying the boundary conditions here. Local subsonic zones arising in the flow lie inside the region under consideration, which eliminates the possibility of using a marching technique along one of the coordinates. The numerical solution is constructed by a unified algorithm for the entire flow region, which gives a possibility of obtaining a higher accuracy. The computations are carried out in the jet initial interval, where, according to monograph [1], the wave phenomena predominate over the viscous effects. The exhaustion process is described by the system of gas dynamics equations. Their solution is constructed with the aid of a finite difference Harten’s TVD (Total Variation Diminishing) scheme [2], which has the second approximation order in space. The second approximation order in time is achieved with the aid of a five-stage Runge-Kutta method. The solution algorithm has been parallelized in space and implemented on the multi-processor computer systems of the ITAM SB RAS and the MVS-128 of the Siberian Supercomputer Center of SB RAS. The influence of the semi-apex angle of the nozzle supersonic part and the pressure jump between the nozzle outlet section and the ambient environment on the flow in the initial interval of a non-isobaric jet is investigated in the work. A comparison with experimental data is presented. The computations are carried out for the semi-apex angles of the nozzle supersonic part from 0 (parallel flow) to 20 degrees. For all considered nozzles, the Mach number in the nozzle outlet section, which was computed from the one-dimensional theory, equaled three. Computations showed that in the case of flow acceleration in a conical supersonic nozzle, its geometry is one of the main factors determining the formation of the jet initial interval in ambient environment.

Terekhov, V. I.;Yarygina, N. I.;Smulsky, Ya. I.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S0869864308020042

Abstract Results of an experimental study of turbulent flow past a flat rib installed at an angle to the free-stream direction are reported. In the experiments, external flows with two different turbulence numbers were used, and the angle of rib inclination to the free stream was varied from 50 to 90°. The experiments were performed for ribs of various heights under conditions with natural and high (13.4 %) free-stream turbulence levels. Visualization tests were performed to elucidate the vortex formation pattern and the direction of flow streamlines. Deformations of the recirculation region and secondary-vortex zone as well as enhanced effects due to 3D flow structure observed on decreasing the angle ϕ, and also notable restructuring of the flow at a high free-stream turbulence intensity, were identified. A comparison between pressure coefficients in different longitudinal sections of the channel is reported for ribs of various heights installed at various angles ϕ. The influence of rib inclination angle, rib height, and free-stream turbulence number on local heat-transfer coefficients and heat-transfer intensification is analysed.

Davletshin, I. A.;Mikheev, N. I.;Molochnikov, V. M.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S0869864308020054

Abstract Experimental data on heat transfer in turbulent separation region behind obstacle in a broad frequency range of superimposed free-stream pulsations are reported. The heat-transfer coefficient was determined by solving an inverse non-stationary heat conduction problem based on experimentally measured wall transient temperature. Substantial heat-transfer intensification in the separation region of the pulsating flow was identified.

Kondaurova, L. P.;Nemirovsky, S. K.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S0869864308020066

Abstract Results of simulation study of evolution of solitary intensive second-sound waves spreading in superfluid helium are presented. Quantitative description was carried out on the basis of equations of hydrodynamics of superfluid turbulence (HST). HST equations with second-order accuracy (relative parameter deviation from equilibrium) were written for the cases of planar, cylindrical, and spherical geometries. The system of equations was solved using the disruption decay technique. Calculations were carried out for the temperature of undisturbed helium T 0 = 1.4 K. Simulation results were compared with experimental data.

Sagitov, R. V.;Sharifulin, A. N.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S0869864308020078

Abstract The model system of ordinary differential equations [1, 2] governing the behavior of a non-uniformly heated fluid in a tilted cavity is used for studying the stability of steady regimes of thermal convection at arbitrary (not small) tilting of the rectangular cavity. The bifurcation curve is constructed, which separates the region of parameters (the Rayleigh number — the cavity tilting angle) into two regions — the internal and external ones. In the external region, the system has one stable steady solution, and in the internal region, it has three steady solutions. One of them is always unstable in a monotone way, and two others may be both stable and unstable. The neutral curves are constructed, which determine the boundaries of the incipience of the oscillatory and monotone instabilities.

Demekhin, E. A.;Shapar, E. M.;Selin, A. S.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S086986430802008X

Abstract The reaction of the film interface to low-amplitude waviness of the wall was studied. A linearized version of the problem described by the Orr — Sommerfeld equation was considered; the solution was sought by asymptotic expansion in small parameter 1/Re, and usual spectral problem concerning stability to perturbations of exp[iα(x-ct)] type was solved. According to calculations, for some specially chosen wave numbers α the drift and dispersion effects balance each other, providing zero resulting velocity c R = 0. If we assume that a rigid wall is corrugated with the same α, we can say that stationary waves caused by the wavy wall are in resonance with intrinsic perturbations of the second kind.

Shilyaev, M. I.;Bogomolov, A. R.;Petrik, P. T.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S0869864308020091

Abstract The results of theoretical and experimental studies of heat exchange at condensation of motionless steam on a vertical tube placed in a granular layer with different wetting angle are presented. Theoretical dependencies are obtained for the estimate of heat-exchange intensity, which account for the condensate slip on the surfaces of grains, and their satisfactory agreement with experimental data of the authors is shown.

Avksentyuk, B. P.;Ovchinnikov, V. V.

2008 Thermophysics and Aeromechanics

doi: 10.1134/S0869864308020108

Abstract The effect of liquid subcooling below the saturation temperature on the third heat transfer crisis was studied experimentally at pool boiling. Experimental data on the threshold values of superheating and heat fluxes, above which the evaporation front and third heat transfer crisis for acetone at subcooling from 0.3 to 10 K are formed, were shown. Formation of evaporation fronts is the necessary, but not sufficient condition for the third heat transfer crisis at subcooling. It was found that formation of a stable vapor film after propagation of condensation fronts over the heater surface is possible at heat fluxes considerably lower than the first critical one.