Question

If simple CFD softwear like XFoil and OpenVSP use the panel method to find an estimate of the drags and lifts/pressure distributions. How do they find drag and lift if the panel method works based on invicid flow?

Answer 1

CFD software like XFoil and OpenVSP use the panel method for lift and drag estimations based on inviscid flow, but the drag coefficient, which is empirically determined, helps in accurately computing the drag force.

Step-by-step explanation:

In computational fluid dynamics (CFD) software like XFoil and OpenVSP, lift and drag estimations are made using the panel method despite the assumption of inviscid flow. The panel method simplifies the problem by considering the effect of air around the object without the viscous effects, which is a good approximation at high Reynolds numbers where the flow tends to be less affected by viscosity.

However, to calculate a more accurate drag value, the software relies on the drag coefficient (C), which is empirically determined—often through wind tunnel experiments. The drag force (FD) can be expressed as FD = CpAv², where A is the reference area and p is the fluid density.

Fundamentally, drag in such simulations accounts for air resistance and the force that this imparts on the object in question. Though the panel method does not provide a direct measure of viscous drag, the drag coefficient incorporates it, and thus, is an essential aspect in calculating the total drag force experienced by the object.