In order to investigate the influence of surface roughness on turbulent flow and examine the wall-similarity hypothesis of Townsend, three-dimensional numerical study of turbulent channel flow over smooth and cube-rough walls with different roughness height has been carried out by using large eddy simulation(LES) coupled with immersed boundary method(IBM). The effects of surface roughness array on mean and fluctuating velocity profiles, Reynolds shear stress, and typical coherent structures such as quasi-streamwise vortices(QSV) in turbulent channel flow are obtained. The significant influences on turbulent fluctuations and structures are observed in roughness sub-layer(five times of roughness height).However, no dramatic modification of the log-law of the mean flow velocity and turbulence fluctuations can be found by surface cube roughness in the outer layer. Therefore, the results support the wall-similarity hypothesis. Moreover, the von Karman constant decreases with the increase of roughness height in the present simulation results. Besides, the larger size of QSV and more intense ejections are induced by the roughness elements, which is crucial for heat and mass transfer enhancement.
A particle-laden turbulent channel flow is investigated to study particle clusters in large-scale turbulent coherent structures.The fluid phase is calculated by large eddy simulation and particles are tracked using Lagrangian trajectory method.The flow Reynolds number is 180 based on the friction velocity and half-width of the channel.The particle is lycopodium with St=0.93which may well follow the fluid phase.The mean and fluctuating velocities of both two phases are obtained,which are in good agreement with previous data.The strongest accumulations of particles in low-speed streak structures are observed at y+=11.3.Moreover,once particles are captured in low-speed streaks,most of them will reside there for a long period.Particles clustered in low-speed streaks obtain smaller instantaneous wall-normal and spanwise velocities than those out of there,which induce a larger particle flux into low-speed streaks than that out of there.The study is important for understanding particle dispersion mechanisms in gas-particle turbulent channel flows.
LU HaoWANG BingZHANG HuiQiangQIN JianXiuWANG XiLin
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
