# Acoustic and Flow Aspects of Synthetic Jet Actuators with Chevron Orifices
## 1 Introduction  
The synthetic jet (SJ) is generated by periodic expulsion and injection of fluid to a closed cavity through an orifice. This is achieved by replacing one of the walls of the synthetic jet actuator (SJA) with movable or deformable elements such as piezoelectric, acoustic (loudspeaker), or mechanical (piston) drivers. The SJAs can be used in electronic cooling [1, 2, 3], flow control [4, 5, 6], or mixing [7, 8, 9]. The use of SJs on a larger scale in the industry requires compliance with many standards. Among them are the ones regarding the level of generated noise. [PERSON] and [PERSON] [10] investigated the noise level generated by single-chamber and double-chamber actuators with lobed and circle orifices. Double-chamber actuators generated a lower noise level than single chambers at all tested frequency ranges. A lobed orifice was quieter than a circular orifice at frequencies below 2.1 kHz. The impact of orifice shape on the noise level generated by SJAs has also been investigated [11, 12, 13]. [PERSON] had the idea of closing the SJA in a small box with a muffler [14]. Such a solution reduces the generated noise but is impractical. Another popular way for decreasing the sound pressure level (SPL) is the use of a chevron nozzle [15, 16]. Chevron nozzles are used in medium- and high-bypass turbofan engines, and they represent one of the most popular noise reduction technologies [17, 18]. The shape of the chevron nozzle increases mixing between the adjacent streams and reduces the velocity gradient. This reduces the generated noise. Chevron nozzles are also used in other active and passive applications, for example, in continuous impinging jets [19]. At this point, the difference between a nozzle and an orifice must be pointed out. A nozzle is a device that is used to increase stream velocity. Therefore, in the case of subsonic flows (like in the case of SJs), the cross-section area of the nozzle must decrease in thedirection of flow. The orifice's cross-section area is invariable all along of length. In the case of SJs, the term \"nozzle\" is sometimes misused in relation to the orifices. Such abuse of the word \"nozzle\" can be observed, for example, in [20; 21; 22]. [PERSON] et al. [15] compared the velocity field of SJAs with a circular nozzle and a chevron nozzle with six chevron nozzles. The parameters were investigated in the apex and notch planes of the chevron nozzle. The velocity time-averaged component and time- and phase-averaged turbulent kinetic energy were measured with the stereoscopic particle image velocimetry (Stereo-PIV) method. The velocity profiles in the apex and notch planes were different from each other. The axial velocity component was smaller in the apex plane than the circular SJ; in the notch plane, it was higher. However, the most important thing was to show the difference in the shape of the vortex topology. The actuator with a chevron nozzle generates a more complex flow field, with streamwise vortices that change the shape of the vortex ring. The streamwise vortices disappear very quickly, and the vortex topology is similar to the circular shape for \(x/d>3\) (where \(x\) is the axis distance from the nozzle [m], and d is the nozzle diameter [m]). [PERSON] at al. [21] investigated piston-driven SJAs with chevron orifices. They showed that the use of a chevron nozzle and a chevron orifice provides similar results in terms of Nusselt number distribution. It can, therefore, be assumed that the chevron nozzle and the chevron orifice can be used equally in continuous impinging jets. The chevron orifice was compared to the chevron nozzle presented in [23]. The same nozzle was used by [PERSON] et al. [15]. [PERSON] at al. [21] focused on the impact of the orifice-to-surface distance on the radial distribution of circumferentially-averaged Nusselt numbers. The impact of the orifice length was investigated as well. The benefits of SJ heat transfer enhancement were confirmed for the chevron orifice, and the negative impact of orifice length on the Nusselt number was described.