I and some colleagues of mine are actually studying the cl, cd, and cm coefficients for a public geometry of a helicopter’s rotorcraft blade profile (the so-called SC1095 - the geometry can be downloaded from the website UIUC Airfoil Data Site). In view of testing the capabilities of the PyFR code, we are trying to reproduce the experimental results of the paper AIAA Aerospace Research Central, studying the lift, drag, and moment coefficients at several angles of attack, adopting a 2D O-grid of our production (the mesh has a wall-spacing of 4.08e-5, 40 points per side of the airfoil and 2 points only in the tab – those choices were made in order to adopt polynomial order 4 of the PyFR solver).
In particular, if you consider Fig. 3 of the reference above, we are actually able to catch the values of the coefficients for all the angles of attack between the negative and positive stall points (i.e. in the range [-11°; 15°]) but we cannot do that for angles outside this range (stall points included).
We have some questions regarding the described issue:
- Could it be related to the fact that we are using one too-coarse resolution in the airfoil profile direction for the O-grid? To be honest, I personally doubt that because I verified that repeating the simulations with 80 points per airfoil side (instead of 40), the results do not vary significantly;
- Is some refinement of the mesh required behind the airfoil with respect to the flow direction? Actually, we decided to use an O-grid in order to avoid re-meshing for different angles but doing that via the far-field condition (basically, modeling the inlet direction at the far-field via the angles); this approach works pretty well with RANS modeling but I am not completely sure that this can be done straightforwardly with DNS;
- Does the PyFR code rely on the realistic effects in 3D in order to get a correct estimation of the stall?
- To be more precise, does the code inherit any of the key aspects of 2D turbulence phenomena (namely the inverse energy cascade) when simulating on such a topology, therefore making it necessary to perform a 3D simulation of a finite wing?
Thank you for every hint or comment that you could share.