# Experimental Study of Coaxial Jets Mixing Enhancement Using Synthetic Jets
## 1 Introduction  
Jet mixing enhancement is widely used in many industries, such as the energy industry, chemical reactions, and especially in the aerospace field. For example, enhancing the mixing of air and fuel in the combustion chamber to improve emission quality [1]. Jet mixing enhancement is applied to aero engines to achieve thrust augmentation [2] and adjusting infrared radiation visibility [3]. Jet mixing enhancement methods can be divided into two categories: Active flow control technology and passive flow control technology. Solid tabs [4], chevron nozzles [5], lobe nozzles [6], and other methods to modify the nozzle geometry to control the flow are called passive flow control technology. Synthetic jets [7; 8; 9; 10], plasma actuators [11; 12], acoustic actuation [13], control jets [14; 15], and pulsed jets [16] generally input additional or auxiliary energy into the flow belonging to active flow control technology. Active flow control technology has many significant advantages. It can adjust control parameters according to flow conditions and turn off when it is not needed. Therefore, active flow control technology can achieve higher performance on jet mixing enhancement. [PERSON] et al. [17] numerically studied jet mixing enhancement with pulsed jet. The simulations were in good agreement with the experimental data, showing that the jet potential core lengths at several conditions were significantly shortened by pulsed jet. [PERSON] et al. [18] used two steady minijets to enhance a turbulent jet mixing. The flow field was measured by the flow visualization and hot wire technique to explore the physical mechanism of mixing enhancement. They found that periodical structures generated by minijets significantly affected the development of the main jet. [PERSON] et al. [19] studied the mixing enhancement of a transonic jet using steady and pulsed control jets separately. They found that the control jet can effectively enhance the mixing and the potential core length can be shortened by 50% in the experiment. The interaction area between the jet and ambient was a key factor in the physical processes of mixing. The unsteady pulsed jet had a better effect on mixing than the steady jet. The synthetic jet is one of the most promising applications in active flow control technology and has received widespread attention [20; 21; 22; 23]. Synthetic jets do not require additional air and pipelines, and are also known as zero-mass jets. Scholars have used synthetic jets to study jet mixing enhancement in many application fields, proposing the main mechanisms of synthetic jet mixing enhancement in their respective research fields. [PERSON] et al. [24] investigated the synthetic jet to improve fuel mixing in combustion chambers. The temperature and velocity distributions were measured to indicate that mixing was significantly enhanced by synthetic jets. [PERSON] and [PERSON] [25] performed a series of numerical simulations on fuel and air mixing enhancement using synthetic jets. They analyzed the fuel velocity and species distribution, finding that synthetic jets perpendicular to the fuel jet enhanced jet mixing more significantly than synthetic jets parallel to the fuel jet. [PERSON] et al. [26] experimentally studied the control of the diffuser flow with two synthetic jets placed at the entrance. They found that a high vorticity zone was generated when only one synthetic jet worked, which enhanced mixing and increased the jet deflection angle. When two symmetrically placed synthetic jets worked, the mixing enhancement was more obvious, and the flow section significantly increased. [PERSON] et al. [27] experimentally studied the interaction between a synthetic jet and a free jet (Re = 6600). They analyzed the control mechanism by changing parameters such as Strouhal numbers, the direction angle of synthetic jet, and the momentum coefficients. Results showed that mixing was enhanced appreciably by the synthetic jet.