Effect of pulsation on the wall jet flow in the near region of an impinging jet

In the present work, Particle Image Velocimetry technique is used to investigate the effect of jet pulsation in the wall jet region of a turbulent impinging jet. A water jet exits a 15.6-mm diameter (D) nozzle into quiescent water and impinges on a target plate kept at a distance of 4D from the nozzle exit. The PIV measurements are taken at a constant Reynolds number (Re =2600) based on the nozzle diameter and jet exit velocity. The frequency of pulsation varies from f= 0 to 9 Hz (corresponding to Strouhal number, St =0 to 0.99), while the amplitude of the pulsation is kept constant at 18% of the average velocity. The results show that the flow dynamics in the wall jet region begin to change with pulsation. For example, as the pulsed frequency increases, vortices, first observed nearer to the stagnation point, shift away from the stagnation point along the impinging wall. We observe that the potential core length decreases with pulsation. The velocity profile, when scaled with outer scaling parameters, shows a good collapse in the outer shear region. One expects that the stream-wise mean velocity approaches to zero in the direction normal to the plate. Indeed, a negative value is observed exhibiting that the surrounding travels opposite to the wall jet flow for the case St =0.11, 0.22 and 0.44. The evolution of the velocity rms and shear stress profiles show that the wall jet in the case of pulsed jet interacts with the ambient fluid earlier than when the jet is not pulsing. The results further suggest the existence of an optimal frequency of the early development of coherent structures which, from a practical point of view, can be exploited for mixing enhancement and heat transfer. [GRAPHICS] .