On the Measurement of Wall-Normal Velocity Derivative in a Turbulent Boundary Layer

The wall-normal velocity derivative of ∂u/∂y is measured in a turbulent boundary layer, down to 2 wall units from the wall, using two parallel hot-wires. The momentum-thickness- and friction-velocity-based Reynolds numbers are 1450 and 584, respectively. The experimental results indicate that ∂u/∂y may be captured with an adequate accuracy given a separation (∆y) of 2 ~ 5 Kolmogorov length scales η between the two parallel hot-wires, slightly different from that (2η ~ 4η) required for a turbulent channel flow. The surrogate \( \overline{{\left(\Delta u/\Delta y\right)}^2} \) for \( \overline{{\left(\partial u/\partial y\right)}^2} \) displays a significant departure in the buffer layer from that in the turbulent channel flow, due to a difference in the large-scale coherent structures between the two flows. The skewness and kurtosis of ∂u/∂y differ markedly from their counterparts in a turbulent channel flow, which reflects a difference in the small-scale turbulent structures of the outer region between the two flows.