Direct Numerical Simulations Of Gas-solid Two-Phase Turbulent Boundary Layer Flow Over Rough Walls

It is well known that the wall roughness extensively exists in energy and chemical fields etc.The existence of roughness not only destroys the original structure in the viscous sublayer,but also significantly affects the flow characteristics and particle dispersion in the two-phase flow.Therefore,there is a strong engineering and scientific value to investigate the effects of roughness on the turbulent boundary layer.In this paper,a combination of direct numerical simulation,immersed boundary and point-particle methods was used to study the single-and two-phase turbulent boundary layer(TBL)flow over the roughened surface in order to reveal the relationship and interaction mechanism among the turbulence,particle and wall-roughness.Firstly,the numerical studies of flow pass around a circular cylinder and a sphere were performed to evaluate the correction and accuracy of the immersed boundary method under the moderate/high Reynold number and the stretched grid system.Then,based on those methods,a series of direct numerical simulations were carried out to study the TBL flow over the hemisphere-roughened walls.The effects of different wall-roughness on the flow field,coherent structure and anisotropy were investigated in detail.The evolution of the vortex structure induced by the roughness element and the interaction mechanism between the inner and outer vortex structures in the boundary layer were discussed.It is found that the hairpin vortex,which periodically sheds from the top of the roughness element,interacts with the turbulent vortex structures in the boundary layer strongly,destroys the turbulent structure in the near wall region and then lead to a stronger momentum exchange between inner and outer layers and a shorter size of coherent structures.The existence of the roughness element also increases the flow resistance which makes the fluid velocity significantly reduced.The blocking effect of the roughness element makes the streamwise energy spread to the spanwise and normal directions resulting in a reduction of streamwise fluctuation velocity and an increment of spanwise and wall-normal fluctuation velocity.Moreover,it's also confirmed that the blocking effect of the roughness element is correlated to the shape of roughness element.Secondly,the effects of Gaussian random rough wall on the laminar and turbulent boundary layer were also investigated.The growth mode of perturbation induced by the Gaussian random rough wall was studied.The effects of different correlation lengths,skewness and kurtosis on the growth mode,flow field and coherent structures were analyzed.The results show that the random rough wall,which is similar to the regular arranged roughness elements,will cause a transient growth.The skewness and kurtosis of the rough wall affected not only the energy distribution among the different wavelengths,but also the subsequent growth of disturbance in each wavelength.And the harmonic wavelength of the disturbance energy was related to the correlation lengths in the streamwise and spanwise directions.In the TBL,the skewness and kurtosis of the rough wall surface,and the relationship between the crests also affected the flow field,vortex structures and turbulent Reynolds stresses.Generally,the formation of turbulent vorticity and Reynolds stress will be more obvious when the crest of roughness is higher.The effect of mutual inhibition will appear when the two peaks are close to each other.Finally,based on the studies of single-phase TBL,a series of direct numerical simulations were also performed to investigate the two-phase TBL flow over the hemisphere-roughened walls.The interaction mechanism between turbulence-particle-roughness was investigated.It is found that the addition of particles suppresses the generation of the vortices at the top of the roughness elements,changes the length of the backflow region behind the roughness elements and weakens the effect of the roughness elements on the TBL.Compared with the TBL over the smooth wall,the existence of the roughness elements made the fluid and particle velocity in the near wall region lower than that in the single-phase flow,and the peak positions of the Reynolds stresses were shifted upward.In addition,the roughness-induced ejection-and sweep-events transported the particles from the near wall into the outer layer which reduced the preferential accumulation of particles in the near wall region,and its effects were enhanced with the decrease of the spacing.For small-scale particles,due to their small particle inertia,a number of the particles followed the fluid into backflow region before or after the roughness elements.Compared with the large-scale particles,its distribution in the normal direction is more uniform.