AN EXPERIMENTAL STUDY OF THE FORCING EFFECT ON THE FLOW AND HEAT TRANSFER IN A TURBULENT WALL JET

The effect of the exit wall jet flow excitation on the flow and thermal behaviors of the turbulent wall jet is experimentally. investigated. Various forcing amplitude's and frequencies are used in the presence and absence of a free stream flow. Forcing the flow showed to have a major impact on the fluid mechanics of the turbulent wall jet which was clearly shown in the velocity fields and the associated time-averaged quantities such as the wall jet spread and the maximum velocity decay. The normal direction at which the local maximum velocity occurs, also known as the wall jet spreading, is shown to move further away from the wall and is increased by more than 20% under some forcing conditions. The local maximum velocity decay with the downstream direction is reduced by more than 2.5% at further downstream locations. At a given location, the increase in the wall jet spreading together with the reduction in the mean velocity results in a decrease in the wall skin friction calculated using the slope of the mean velocity in the viscous sublayer, a behavior consistent with the literature. Due to its importance in enhancing heat transfer phenomena, the effect of the forcing on the streamwise velocity fluctuations is also investigated under the various forcing conditions. The profiles of the fluctuating component of the velocity, u', are measured at various downstream locations since they are essential in understanding the growth of the disturbances. Forcing the wall jet increased u' in the inner and outer regions and revealed the two peaks corresponding to the inner and outer shear layers respectively. This phenomenon is attributed to the added disturbance at the jet exit in addition to the disturbance growth with the downstream direction. The introduction of wall jet flow forcing at various amplitudes and frequencies showed a significant effect on the thermal behavior of the wall jet and was more pronounced in the absence of a free stream flow, a fact related to the evolution of the mixing layer with the downstream direction. In the absence of a free stream flow, Nusselt number decreases with increasing forcing amplitude and frequency in the region close to the jet exit. The decay of Nusselt number in the downstream direction showed an inflection point at further downstream locations which leads to a larger Nusselt number value than the one observed in the unforced case. This behavior is related to the enhanced mixing between the wall jet flow and the free stream due to forcing, which results in a reduction in the wall skin friction and consequently a decrease in the heat transfer rate from the wall.