Receptivity of an axi-symmetric shear layer to a localized vortical disturbance

Active flow control has demonstrated its potential as an effective means for manipulating flows in the laboratory; for example, thrust vectoring, enhanced mixing, higher lift, etc. However, recent interest in transitioning laboratory experiments to engineering applications has highlighted the lack of the knowledge base necessary to develop effective design tools for control applications. An important component of this knowledge base is the understanding of the response (receptivity) of the flows to different types of actuation. In the present work, the receptivity of a free shear layer (generated from an axi-symmetric 2.54-cm-diameter jet) to mechanical excitation (produced by a cantilever piezoelectric actuator) was investigated over a range of parameters in a well-controlled flow environment. The forcing was limited to low excitation levels that did not exceed the linear amplification threshold, and all hot-wire measurements of the streamwise velocity fluctuation in the shear layer were conducted to be within one forcing wavelength from the actuator. This allowed the development and use of a simple linear model in order to decouple the mechanical and acoustic effects produced by the actuator. The results of the model showed that the acoustic effects were insignificant compared to the mechanical influences.; The effect of the following parameters on the shear-layer flow transfer function to mechanical effects was investigated: first, the effect of, different streamwise actuator locations from the nozzle lip, covering a range of x _A/λ_N= 0.1 to 5; second, different actuator radial locations (r_off/Θ_o), covering a range of –1 to +3 from the shear layer centerline; finally, different actuator levels of forcing, as measured by actuator amplitude of oscillation (13 to 23 μm). This parameter range was covered for two jet Reynolds numbers of 27,000 and 50,000. All the measured results concerning the spatial development of the disturbances agreed well with linear stability theory. The flow transfer function results demostrated that the actuation phase (with respect to the naturally existing shear layer structure) and streamwise location had a significant effect on the response of the shear layer, while the radial location of the forcing was not as important. Furthermore, unlike acoustic receptivity, a clear dependence of the flow transfer function (defined as the ratio of the disturbance amplitude at the location of the actuator to the input mechanical forcing amplitude) on the excitation level was found, which suggested the existence of a non-linear receptivity process.