  In this paper, we explore methods for computing wall-normal derivatives used
for calculating wall skin friction and heat transfer over a solid wall in
unstructured simplex-element (triangular/tetrahedral) grids generated by
anisotropic grid adaptation. Simplex-element grids are considered as efficient
and suitable for automatic grid generation and adaptation, but present a
challenge to accurately predict wall-normal derivatives. For example,
wall-normal derivatives computed by a simple finite-difference approximation,
as typically done in practical fluid-dynamics simulation codes, are often
contaminated with numerical noise. To address this issue, we propose an
improved method based on a common step-length for the finite-difference
approximation, which is otherwise random due to grid irregularity and thus
expected to smooth the wall-normal derivative distribution over a boundary.
Also, we consider using least-squares gradients to compute the wall-normal
derivatives and discuss their possible improvements. Numerical results show
that the improved methods greatly reduce the noise in the wall-normal
derivatives for irregular simplex-element grids.
