Small Wiggles in Hydrofoil Pressure Distribution

James Date

Hi everyone,

Here's something I have noticed recently when looking at the pressure distribution around a hydrofoil I'm investigating. I have created a high quality hex mesh around a NACA profile using ICEM 5.1. The points which describe the NACA section have be generated with an accuracy of 6 decimal places. The curves and hence surfaces created within ICEM 5.1 have been produced using these points. A total of 50 points have been used to describe the section.

The mesh created around the section has been refined so that the first node near wall normalised wall distance (Y+) is < 1.0. The section has a chord of 1m. The near wall first node distance is 0.00000146m.

I pass this mesh to CFX-5.7.1 and use the k-e SST turbulence model to solve the flow over the section. I manage to obtain a well converged solution in 500 iterations down to a MAX residual of 1.0e-05.

The computed CL, CD and CM values compare favourably with experimental data and potential flow solutions obtained using a panel code.

Now, when I produce a Cp plot for the section, I notice that although the Cp distribution is close to those found by experiment and using a panel methods, there are small wiggles in the Cp at various locations around the section. Although these are not significant, just slightly annoying!, I'm trying to ascertain their origin. Has anyone else come across the phenomena when solving aero/hydrofoil flows using CFX-5.7.1.

I have a few ideas what might be causing these wiggle's, but as yet haven't had a chance to investigate them fully. Is it because the points describing the section have not been specified with enough accuracy, since the near wall spacing is of a similar order?

Would specifying points with an accuracy of say 10 decimal places over come this problem? Or possibly using section defined using fewer points with 6 decimal place accuracy?

Any pointers would be gratefully received.

James