Aeronautical Engineering/wind tunnel
how to calculate the boundary layer in test section of wind tunnel,i heard that o.8 and 0.2 are the useful and boundary layer areas...in wind tunnel,i am doing my engineering,i will be thank full to you ,if you reply me with an derivation or the mathematical form or it,i mean ,how one can know that it is 0.8 and 0.2
Margini - A wind tunnel test section boundary-layer thickness is difficult to predict because it is so dependent on the shape of the inlet contraction, the type of transition from upstream duct to test section shape, length of the inlet and test section, Reynolds number, inflow conditions, test section wall angle (not always zero), and more. This is why many wind tunnel designers will build a scale model of the facility and test it before building the final design. However, as a rough estimate of the boundary-layer thickness you can use the classic equation for a turbulent boundary layer:
delta = 0.37 L/Re^0.2
where delta = perpendicular distance from the surface to where the velocity in the boundary layer is 0.99 times the free stream, L = distance from the starting point, and Re = Reynolds number. L is not easy to define, but you could use the start of the constant area section as a first approximation. The Reynolds number is based on the same dimension L.