Hello, I am working on a flow over a flat plate in a uniform flow based on SIMPLE scheme and I have used Gauss Seidel for solving system of equations which is explicit. But the rate of convergency is very slow. Can anyone tell me what can cause this problem? Also the residuals are very oscillatory.

This looks like a relatively easy flow to solve and SIMPLE should work very well. Try playing around with the under-relaxation factors on pressure and velocity and/or tighten the pressure solution tolerance a bit (say 0.01 of the initial residual). For my money, you could do with a better linear equation solver...

Hello, I just work on subject 'flow around a square cylinder'. To my point, you should check your pressure boundary condition on the wall of the flat plate, especially the sharp corners. By the way, Do you make any comparison with the case for steady flow around a flat plate(Re<50), Can you get good agreement?