Influence of actuation waveshape on flow field and heat transfer performances of an impinging synthetic jet

An experimental investigation of impinging synthetic jets (SJs) driven by signals with different waveshapes has been conducted by using Infrared Thermography coupled with heated thin foil as a heat flux sensor and phase-locked Particle Image Velocimetry. Experiments have been performed at a fixed Reynolds number equal to 4,100 and two Strouhal numbers St = 0.056 and 0.11, at a constant nozzle-to-plate distance equal to 2 nozzle exit diameters and by varying the duty cycle of the input sinusoidal voltage signal from 0.3 to 0.7 with 0.1 increments. These variations allow us to understand how the waveshape input influences the SJ flow field and heat transfer capabilities. This research aims to unveil the fundamental fluid dynamic mechanisms explaining the enhanced heat transfer performances of impinging SJs, providing concrete physical explorations of their flow features as they evolve over the plate. The impinging SJs exhibit axisymmetric heat transfer patterns similarly to those observed for continuous jets. Local maxima appear in a ring-shaped distribution around the stagnation point due to the dominance of the ring vortices. At St = 0.056, the increase of the ejection duty cycle improves the heat transfer performances compared to the baseline sinusoidal case (i.e. d = 0.5). An enhancement of 8 % is reached, for d = 0.7. Conversely, at St = 0.11, this improvement is less pronounced (4.3 %) and it is reached for d = 0.3. Such improvements are strictly correlated to the SJ formation mechanisms and dynamics of the coherent structures. The flow patterns forming under the effects of the ejection duty cycle feature an energized growth of coherent flow structures compared to that observed in the baseline case. An earlier development of the SJ shear layer is achieved for d to 0.7 and 0.3 for St values of 0.056 and 0.11, respectively. It is found that the variation of d alters the growing rate of the axial exit velocity, i.e., the axial acceleration, affecting the vortex ring characteristics and, consequently, the heat transfer performances. Furthermore, a correlation law between the heat transfer enhancement and the synthetic jet axial acceleration at the nozzle exit is presented.