I'm running an external flow case over a vehicle model similar to the Ahmed body and calculating, among other things, the

**skin friction coefficient**at certain measurement points on the surface (to compare with experimental results). The drag values are coming out very well (~4% error), but when comparing the skin friction coefficient at the measurement points, I'm detecting a significant error (~33% below experimental results). One possibility is that I might not be calculating this coefficient correctly.

Regarding this, I calculate it within cs_user_extra_operations.c, based on the

*boundary_forces*field. What I do is identify the closest faces to the measurement points and extract the net force. Then, I subtract the normal component to obtain the tangential force and divide it by the area to obtain the tangential stress on the face (τ_f). Subsequently, I calculate Cf=τ_f/q∞ (where q∞ is the dynamic pressure, known). Lines 449-526 from attached file.

My question is,

**is it correct to calculate the numerator of the skin friction coefficient by using the tangential component of**?

*boundary_forces*? Am I forgetting contributions from turbulent quantities or the symmetric part of the viscous stress tensorNote: Since the surface on which the measurement points lie is flat, I attempted to compare results by simulating a 2D case of a flat plate with the same setup, and it fits much better with the experimental/theoretical values, with an error of about 5 times less, but still below the experimental/theoretical value.