Study of boundary layer parameters on rough surfaces

Abstract

When real fluid flows past a solid body or a solid wall, the fluid particles adhere to the boundary and condition of no slip occurs. This means that the velocity of fluid close to the boundary will be same as that of boundary.If the boundary is stationary, the velocity of fluid at the boundary will be zero. Further away from the boundary,the velocity will be higher and as a result of this variation of velocity, the velocity gradient will exist. The velocity of fluid increases from zero velocity on the stationary boundary to the free stream velocity of the fluid in the direction normal to the boundary. This variation of velocity from zero to free stream velocity in the direction normal to the boundary takes place in a narrow region in the vicinity of solid boundary. This narrow region of fluid is called Boundary Layer.Three main parameters (described below) that are used to characterize the size and shape of a boundary layer are the boundary layer thickness, the displacement thickness, and the momentum thickness. The displacement thickness is the distance a streamline just outside the boundary layer is displaced away from the wall compared to the inviscid solution. Another way to describe it is the distance the wall would have to be displaced to yield the same solution for flow outside the boundary layer as the boundary layer equations yield.The momentum thickness, is the distance that, when multiplied by the square of the free-stream velocity, equals the integral of the momentum defect across the boundary layer. In our thesis, we have obtained the boundary layer parameters and velocity ratio on smooth and different types of rough surfaces by using Air Flow Bench. The sand papers of different grain sizes have been considered as rough surfaces.