We investigate the turbulence modulation by particles in a turbulent two-phase channel flow via an analysis of turbulence anisotropy-invariants. The fluid turbulence is calculated by a large eddy simulation with a point-force two-way coupling model and particles are tracked by the Lagrangian trajectory method. The channel turbulence follows the two-component turbulence state within the viscous sub-layer region and outside the region the turbulence tends to follow the right curve of the anisotropy-invariant. The channel turbulence, interacting with heavy particles, is modulated to the two-component turbulence limit state near the wall and is separate from the axisymmetric turbulence state in the turbulence anisotropy-invariants map. The fluctuations of streamwise component are transferred to the other two components and hence the anisotropy decreases due to particle modulation. The study has deepened the understanding of the turbulence modulation mechanism in two-phase turbulent flows.
A three-dimensional large eddy simulation(LES) of a spatially developing round jet is carried out in cylindrical coordinates using a dynamic subgrid model with strong inflow instability.Evolutions of large-scale vortex structures represented by tangential vortices are obtained and compared with flow visualization.Also presented are three-dimensional spatial evolutions of coherent structure,which are of quasi two-dimensional Kelvin-Helmholtz instability and vortex rings as well as breaking up of the vortex rings with fully three-dimensional characteristics.Predicted results of mean velocity and turbulent intensity agree well with experiments.They are also compared with the results predicted by LES using standard Smagorinsky model and show good self-similarity.Turbulence spectrum of the predicted velocity shows the 5/3 decay for higher wave number,as expected for turbulent round jet flows.In addition,-test and-test are carried out for the instantaneous velocity,showing that the present LES method can successfully predict the hierarchical structure of round jet.
Turbulent two-phase reacting flow in the chamber of LOX/RP-1 bipropellant liquid rocket engine is numerically investigated in this paper. The predicted pressure and mean axial velocity are qualitatively consistent with the experimental measurements. The self-excited pressure oscillations are obtained without any disturbance introduced through the initial and boundary conditions. It is found that amount of abrupt pressure peaks appear frequently and stochastically in the head regions of the chamber, which are the important sources to drive and strengthen combustion instability. Such abrupt pressures are induced by local constant volume combustion, because local combustible gas mixtures with high temperature are formed and burnt out suddenly due to some fuel droplets reaching their critical state in a rich oxygen surrounding. A third Damkhler number is defined as the ratio of the characteristic time of a chemical reaction to the characteristic time of a pressure wave expansion to measure the relative intensity of acoustic propagation and combustion process in thrusters. The analysis of the third Damkhler number distributions in the whole thrust chamber shows that local constant volume combustion happens in the head regions, while constant pressure combustion presents in the downstream regions. It is found that the combustion instability occurs in the head regions within about 30 mm from the thruster head.
The micro-and macro-time scales in two-phaseturbulent channel flows are investigated using the direct numerical simulation and the Lagrangian particle trajectorymethods for the fluid-and the particle-phases,respectively.Lagrangian and Eulerian time scales of both phases are calculated using velocity correlation functions.Due to flowanisotropy,micro-time scales are not the same with the theoretical estimations in large Reynolds number(isotropic) turbulence.Lagrangian macro-time scales of particle-phaseand of fluid-phase seen by particles are both dependent onparticle Stokes number.The fluid-phase Lagrangian integral time scales increase with distance from the wall,longerthan those time scales seen by particles.The Eulerian integral macro-time scales increase in near-wall regions but decrease in out-layer regions.The moving Eulerian time scalesare also investigated and compared with Lagrangian integraltime scales,and in good agreement with previous measurements and numerical predictions.For the fluid particles themicro Eulerian time scales are longer than the Lagrangianones in the near wall regions,while away from the walls themicro Lagrangian time scales are longer.The Lagrangianintegral time scales are longer than the Eulerian ones.Theresults are useful for further understanding two-phase flowphysics and especially for constructing accurate predictionmodels of inertial particle dispersion.
The particle modulations to turbulence in roundjets were experimentally studied by means of two-phasevelocity measurements with Phase Doppler Anemometer(PDA). Laden with very large particles, no significant attenuationsof turbulence intensities were measured in the far-fields,due to small two-phase slip velocities and particleReynolds number. The gas-phase turbulence is enhanced byparticles in the near-fields, but it is significantly attenuatedby the small particles in the far-fields. The smaller particleshave a more profound effect on the attenuation of turbulenceintensities. The enhancements or attenuations of turbulenceintensities in the far-fields depends on the energyproduction, transport and dissipation mechanisms betweenthe two phases, which are determined by the particle propertiesand two-phase velocity slips. The non-dimensionalparameter CTI is introduced to represent the change of turbulenceintensity.
Bing Wang Huiqiang Zhang Yi Liu Xiaofen Yan Xilin Wang School of Aerospace, Tsinghua University, 100084 Beijing, China
