Numerical Simulation Of Friction Drag Reduction By Micro-bubbles For The Non-smooth Surfaces

The under-water vehicle has a fixed volume of container to carry the air for which can reduce the drag when voyaging under the water.It is important to study how to increase the drag reduction ratio under a low air volume fraction at the inlet region for it can make the underwater vehicle has a lower energy consumption.Here,we main studied the effect of the surface structure on the microbubble drag reduction.We used the Weierstrass-Mandelbrot(W-M)fractal function to generate the surface profile curves.The large eddy simulation model and Mixture multiphase flow model were used to simulate the turbulent boundary layer flow with different roughness and rough surface in different positions respectively,and the Reynolds number Re? = 1430.We used a set of three-dimensional velocity perturbation equations at the inlet region of the computational domain to reduce the distance of fluid flow in the boundary layer from laminar to turbulent.And the disturbance parameters of perturbation equation here we refer to the literature values of smooth plate microbubbles numerical simulation under the same Reynolds number.The results show that the resistance of a flat plate with a rough surface is decreasing with the increase of the inlet air volume fraction.It is the same as the smooth plate under the same inlet air volume fraction.Comparing with the drag reduction rate of the smooth plate under the same inlet air volume fraction,the drag reduction rate of the plate with the rough surface decreases first and then increases with the increase of the inlet air volume fraction at the inlet region.With the increase of the content of microbubbles,the influence of the rough surface on the flow field decreases gradually.The drag reduction rate of rough surface is 15.9%,when ?v,in is 0.1,which is 1.5% higher than that of smooth plate under the same inlet air volume fraction.The drag reduction rate of the rough surface is less than that of the smooth plate when ?v,in = 0.2,0.3.We found that the drag reduction of the plate for the rough surface at the downstream is better than that at the upstream when the microbubble injected into the boundary layer.For the rough surface at 7? from the inlet region,the drag reduction can reach to 51.9%,which is 12.1% higher than that for the rough surface at 4?.Furthermore,for ?v,in = 0.3,the drag reduction rate is higher than that for the other two kinds of small roughness surface at the case Ra = 75,and the drag reduction rate tends to increase with the increase of roughness,but the increment of drag reduction rate is becoming smaller.