Velocity Profiles of Synthetic Jets using Piezoelectric Circular Actuators

The performance of systems involving a body in a fluid flow can be enhanced by manipulation of the body shape through flow control. Active flow control, the focus of this work, is achieved by energizing low-momentum regions at the wall by tangential blowing using air. One mechanism for blowing air into this region is through synthetic jets. They are devices that push air out from a fixed orifice or a slit in the form of a jet. The jets are usually created using compressed air or an electromechanically driven vibrating platform. To diminish complexity, reduce weight, and increase time response, piezoelectric actuators used as oscillating diaphragms are often used. This study concentrates on characterizing a synthetic jet using three types of piezoelectric actuators as circular mechanical diaphragms: (1) pre-stressed curved metallic Unimorph or Thunder, (2) Bimorphs, and (3) Radial Field Diaphragms, RFD. The diaphragm is clamped around its perimeter, so that when voltage is applied to the device, it oscillates and a jet is pushed out an orifice perpendicular to the actuator. Maximum synthetic jet velocity into quiescent flow was monitored when varying frequencies, and driving waveforms of the diaphragms, as well as varying physical cavity characteristics. Results show that maximum velocity magnitude is markedly different with the different waveforms especially a sine waveform which produces the weakest jet. Changing the jet exit from a rectangular slot to a circular orifice produces a more uniform velocity profile independently of actuator and waveform type. Maximum velocity is recorded at a fixed distance from the orifice (z-axis) at various positions along the orifice (x-axis). Regardless of the orifice shape used, the velocity profiles of the jets are bell shaped. To identify these factors a fractional factorial design of experiments was performed individually on each diaphragm. The results obtained were further verified using regression analysis. Results indicate that depending on the diaphragm utilized these factors may be different. This type of study can be extended to analyze other parameters that are significant to the successful implementation of these devices such as back pressure and cavity shape.