I am trying to find the lift force to investigate Magnus force. However, my project does not give me reasonable results and even gives me negative pressure at some points. I do not know which part of my project is wrong. Could you help me to fix my problem?

You are using a steady-state approach. Hence, the results you are seeing do not represent a variation with physical time but with iterations. The first results represent the initial attempts of the solver to come out with a solution to the discretized system of diff. equations. Naturally, the first attempts are not accurate and produce those non-physical oscillations in the solution. It might be a good idea for SimScale to modify the post-processing x-axis name to â€śiteration #â€ť for steady-state cases.