Effect Of The Surface Roughness And Other Factors To The Boundary Layer Flow

Using rough surface for drag reduction is a common phenomenon in nature.It is a typical example that sharks use the shield scale structure which is on the skin surface to reduce drag.Because drag reduction is of great significant in practical engineering,the research on drag reduction by using rough surface has become extensive around the world.Based on the numerical simulation requirements for simulating the flow field of the riblet surface,after detailed analyzed the computational fluid dynamics principles,the flow control equations which are suitable for this type of simulation are given.Using FLUENT software,based on the k-? turbulence model,simulation and data analysis of the flow field of riblets surface under specific initial and boundary conditions are performed.Research shows:1.Within subsonic and supersonic flows,the transverse riblets will largely increase the thickness of the boundary layer for several times,small vortices will appeal between the riblets.The drag can be reduced by the vortices changing the sliding friction between the fluid and the wall to rolling friction.However,the transverse riblets will bring the pressure drag to the surface,and the reduced of friction resistance cannot offset the increased of the pressure drag,so the total drag coefficient of the riblet surface is still increased.2.Within subsonic and supersonic flow,the longitudinal riblets will slightly increase the thickness of the boundary layer of the surface,the fluid moves slowly at the bottom of the riblets,this phenomenon will reduce the gradient of normal velocity and the turbulent kinetic energy near the wall surface area.By reducing the wall surface shear stress,the frictional drag of the surface is reduced.Since the riblets increase the surface area,only the longitudinal riblets in a specific size can make the drag coefficient close to smooth surface.3.The riblet of different directions will change the shape of the boundary layer.The boundary layer of the transverse riblets is thicker than the riblets,the boundary layer is a flat air layer covering the riblets.The boundary layer of the longitudinal riblets is thinner than the riblets,which is zigzagged along with riblet surface geometric shape.