Fully Resolved Direct Numerical Simulation Of The Particle-laden Turbulent Channel Flow

Particle-laden flows are widely encountered in industrial and agricultural settings. However, the present understanding of the mechanisms in such flows are still poor due to the complexity of the nonlinear hydrodynamic interactions between particles and fluids. A direct-forcing fictitious domain method was employed to perform fully-resolved numerical simulations of turbulent channel flow laden with large particles. The effects of the particles on the turbulence (including the fluid-phase average velocity, the root-mean-square (rms) of the velocity fluctuation, the probability density function of the velocity and the vortex structures) at the friction Reynolds number of180and395were investigated.For the neutrally buoyant case, the particles enhance the fluid-phase average streamwise velocity at sufficiently low particle volume fractions, which indicates that the presence of the particles reduces the drag. In contrast, the particles have the opposite effect at relatively high particle volume fractions. The presence of particles decreases the maximum rms of streamwise velocity fluctuation near wall via weakening the large-scale streamwise vortices, and on the other hand increases the rms of transverse and spanwise fluctuating velocities in vicinity of the wall via inducing smaller-scale vortices. The probability density function (PDF) of the fluctuating velocities near wall is significantly modified by the addition of the particles:firstly, the skewness of the PDF of the streamwise and transverse velocities is reduced, and secondly, the probability for large fluctuating velocities is decreased in the streamwise direction but increased in other two directions. However, the effect of the particles on the velocity PDF normalized with the rms velocity is small. As the friction Reynolds number increases to395, the effect of particles on the turbulence is weakened.When considering the particle settling effect, the particles settle to the wall and form a particle-thickness sediment layer at low Shields numbers, which plays a role of rough wall. The rms of streamwise velocity fluctuation near wall is weakened significantly, but the rms of spanwise and transverse velocity fluctuations are enhanced. Unlike the neutrally buoyant case, in the vicinity of the particle sediment, the skewness of the PDF of the streamwise velocity is decreased, and the velocity PDF normalized with the rms velocity is significantly affected by the particles. Nevertheless, the effects of the particles on the velocity PDF normalized with the rms velocity remain insignificant in the region away from the particle sediment layer.