Wall shear stress measurements and parametric analysis of impinging wall jets

This paper investigates the influence of some governing parameters on the near wall characteristics of a circular impinging jet onto a smooth flat plate. Laser Doppler anemometry (LDA) is used to characterize the mean and turbulent fields including the wall shear stress. The experiments were conducted at one nozzle-to-plate space (H/D = 2) and Reynolds number of 47,100. The work makes a parametric analysis of impinging jets based on (i) conventional parameters that include the nozzle diameter, the nozzle-to-plate distance and the bulk velocity of the jet and (ii) gross parameters like the jet momentum flux. Parametrization schemes based on conventional quantities are shown to be very sensitive to the particular choice of reference quantity, resulting in functional behaviours that can be represented through either power law or linear expressions. On the other hand, it is shown that the jet momentum flux and the kinematic viscosity suffice to determine the mean and fluctuating flow parameters, even in the initial region of wall jet development (1 < r/D < 5). With the latter choice, the streamwise variation of the maximum mean velocity and maximum Reynolds longitudinal stress are shown to decay according to power law expressions. A particular near wall parametrization scheme for the mean velocity profile that resorts to a scaling procedure based on the stream-wise evolution of the flow characterized by its maximum velocity is also presented. Higher-order moments of the velocity fluctuations are discussed.