Direct Numerical Simulation Of Flow Characteristics Around A Particle In Proximity To A Wall

Particle-laden turbulent boundary layer flows are widely encountered in a host of industrial applications,especially in the fields of energy,powering and chemical engineering,etc.The presence of particles has a great influence on coherent structures and on turbulent properties.On the other hand,the wake structures behind a particle and the forces acting on a particle are also affected by the presence of turbulent boundary layer.In the present study,direct numerical simulations(DNSs)have been performed to investigate the effect of a stationary sphere adjacent to the wall on the fluid turbulence.Firstly,we have studied the effect of a proximal wall on the wake of a stationary particle.The present results have also been compared with previous experimental data and good agreement is observed.The development and formation mechanism of the particle wake are studied.It is found that a jet flow between the particle and the wall suppresses the vortex separation on the bottom of the particle and stretches to the recirculation region.As a result,the recirculation region shapes and the wake structures are significantly changed.The small-scale vortex structures concentrate on the near wake region,and become larger and spread transversely along the downstream direction.Moreover,the vortex shedding process has also been illustrated with time series.The processes of instability,amplification,roll up,break,stretch and secondary structures generation are demonstrated in detail.Besides the vortex shedding patterns,the coherent structures induced by the particle in the turbulent boundary layer is also analyzed.Secondly,DNSs of turbulent boundary layers laden with a stationary particle adjacent to the wall have also been carried out with a realistic turbulent inflow condition,in order to investigate the influences of different parameters,including the gap ratio between the bottom of the particle and the wall and the particle Reynolds number,on the flow field.The present simulation results indicate that the particle-wall distances and particle Reynolds number have a significant effect on the mechanism of the formation of the backflow pattern and on the turbulent coherent structures.Based on the difference in the formation mechanism,the particle recirculation region can be divided into two parts,one is the region affected by the particle and the other is the wall recirculation region due to presence of the solid wall.The combination of these two different mechanisms leads to the formation of the particle recirculation region.Moreover,the present results show that the formation of particle recirculation region is substantially affected by the particle-wall distances.In addition,it is also found that the particle Reynolds number has a significant influence on the coherent structures in the turbulent boundary layer.Furthermore,the outer region of the boundary layer was also affected by the presence of particle,and the effect is enhanced with increasing particle Reynolds number.The present study has investigated the effect of different gap ratios and different particle Reynolds numbers on the fluid turbulence and on coherent structures.The variations of vortex dynamics,vortex shedding patterns and statistical property of the flow field around a particle adjacent to wall are also studied.The numerical results will be expected to provide a basis for the further study on the characteristics of particle clusters in the vicinity of the wall.