# Synthetic Jet Actuators with the Same Cross-Sectional Area Orifices-Flow and Acoustic Aspects
## 1 Introduction  
The synthetic jet actuator (SJA) is a simple device consisting of only three parts: cavity, the orifice or nozzle (one or many), and the movable or deformable element. The modifications of shape, the number, or the dimension of cavity or orifice may significantly change the parameters of the generated synthetic jet (SJ). A specific case may limit the ability to change these parameters or oblige their specific shape or type [1; 2; 3]. The use of more than one movable or deformable element is also possible and beneficial [3; 4], but this can cause a significant increase in the actuator size especially in the case of mechanics and acoustic actuators, and limit their applicability. The modification of SJA cavity dimensions [5] and shape [6] both have an impact on the produced SJ, but the modification of the orifice is much easier and more important in terms of SJA application and design than the other two possibilities. The modification and increasing of the SJA parameters are important, especially for their application. The SJA can be used for enhanced heat transfer [7; 8; 9], mixing [10; 11; 12], or broadly defined active flow control [13; 14; 15]. [PERSON] et al. [16] investigated how the orifice shape impacts the heat transfer from an impinging SJ. Authors investigate SJAs with a slot orifice with different aspect ratios and hydraulic diameters, and SJAs with different orifice shapes. The comparison of Nusselt numbers for different orifice types shows that the square orifice gives a maximum heat transfer coefficient for the axial distance \(x/d>5\), where \(x\) is the orifice-wall distance and \(d\) is the hydraulic diameter of the orifice. The slot (rectangular) orifice was better for smaller axial distances, \(x/d<5\). The circular orifice generates the lowest heat transfer coefficient. This phenomenon was explained in [17], where the topology of SJ generated by SJAs with the slot and the circular orifice were compared. The impact of orifice shape on the heattransfer coefficient was also investigated by [PERSON] and [PERSON] [18] for oval and diamond shapes. However, these shapes of the orifice did not give better results than the rectangular and square orifices. The other authors investigated not only the impact of shape on SJ parameters but also the number of orifices [19; 20; 21]. The change of orifice dimension is also significant. The aspect ratio [16] in the case of the rectangular orifice and the diameter [22] in the case of the circular orifice are as important as its shape. [PERSON] et al. [22] showed that there exists an optimal diameter for which the velocity of SJ is maximal, the method of determination of which was presented by [PERSON] and [PERSON] [23]. However, [PERSON] et al. [22] also investigated orifices with the walls inclined at different angles. In other words, they changed the orifices into nozzles with different tapering. The parameters of SJs, the Reynolds number and the mass flow rate, were dependent on the nozzle angle, but the effect of orifice geometry in synthetic jets was not clear. [PERSON] and [PERSON] [24] investigated SJAs with five different types of nozzle. They designed nozzles by adding other plates with profiled nozzles and measured the time-mean reaction force generated by each SJ. The results were related to the reference nozzle (the sharp-edged nozzle-- essentially a circular orifice). The actuators with all tested nozzle types generated higher time-mean reaction forces than the actuator with the sharp-edged nozzle. This means that for these nozzles, the SJ velocity, the Reynolds number [25], and the energetic efficiency [26] were also higher. The replacement of the orifice with the nozzle may also be more practical; for example, for noise reduction chevron nozzles are used [27; 28], although the effect can also be obtained for chevron orifices [29]. The orifice with oscillating cross-section area [30; 31], the virtual orifice [32], or using the confinement effect [33; 34] are the other methods of changing the SJ parameters by the change of the orifice.