Figure 4.

Wall shear stress measurements based on UVFI for nonlaminar flow; ${\overrightarrow{v}}_1,{\overrightarrow{v}}_2,{\overrightarrow{v}}_3,\text{and}{\overrightarrow{v}}_4$, denoted by the black solid arrows, are the vector (true) velocities, which can be obtained by UVFI; v ₁, v ₂, v ₃, and v ₄, denoted by the red dashed arrows, can be derived from the vector velocities and are the velocities that should be used in the WSS estimations.