Moderate subsonic open cavity flows are simulated using the DNS and LES techniques. The experimentally observed pattern of vortices moving along the cavity’s upper side is produced. The Rossiter modes are also produced, where it is found that the first mode is more sensitive to numerical constraints such as inflow disturbance, spanwise symmetry and LES modelling than the dominant second mode for Re D = 1000 – 3000. Increasing Re D to 10000 reduces the sensitivity to the spanwise symmetry. Three dimensional flow analysis shows a 3D motion that involves fluid entering from the cavity side walls, impinging on the cavity flow and then rising up towards the cavity’s rear wall.