Resolving the 3D velocity field inside a Roughness Sublayer in a turbulent channel flow using HPIV

Microscopic holographic PIV is used to measure the 3D velocity field within the roughness sublayer of a turbulent channel flow at Re$_{\tau }$ of 3400. Recording holograms through a rough surface is facilitated by matching the optical refractive index of the rough wall with that of the working fluid, a concentrated solution of NaI in water. The pyramidal roughness height is $k$=0.45mm, the sample volume size is 3.2$\times $1.8$\times $1.8mm$^{3}$, the long dimension being in the streamwise direction, and the wall-normal range is -0.33$