# Experimental Study of Coaxial Jets Mixing Enhancement Using Synthetic Jets
## 1 Introduction
The increase of the synthetic jet momentum coefficient can increase its penetration depth and enhance the strength of vertical structures. The synthetic jet achieved the most significant effect on mixing when the Strouhal number was 0.32 under low momentum coefficient condition. However, the effect of the Strouhal number on mixing was not obvious with high momentum coefficient. [PERSON] et al. [28] investigated the mixing enhancement of subsonic round jet using high-momentum synthetic jets. They numerically simulated the temperature reduction of the mainstream affected by synthetic jet penetration depth, vorticity of vortices pair, and interaction area. Results showed that synthetic jets reduced the length of the high temperature region by 75%. The streamwise vortices were the major mixing mechanism, which improved heat and mass transfer between the mainstream and ambient. Mainstream flapping was the second mechanism. [PERSON] et al. [29] used numerical simulation methods to study the mixing enhancement of heat temperature jet using piston-type synthetic jets. They found that the primary jet flapping caused by the synthetic jet accelerated the spread of potential core fluid to the environment. The streamwise vortex increased the quality of ambient and primary jet potential core transportation. The low-temperature zone in the potential core changed positions under the action of the synthetic jet, which promoted the mixing of ambient and primary jet. They named this phenomena as geometric axis transformation. Based on their results, it can be seen that jet flapping, streamwise vortex, and geometric axis transformation are the main mechanisms of jet mixing enhancement. Coaxial jets mixing enhancement is more complicated and has a wide range of applications. Especially in the aerospace field, mixing enhancement of combustion gas and air from a rear variable area bypass injector in the variable cycle engine can adjust infrared radiation visibility and augment thrust. Mixing enhancement of fuel air and oxygen in air-breathing rocket engine or ramjet engine can significantly improve its combustion efficiency. Coaxial jets are not only mixed with the ambient air, but the two jets inside and outside are also mixed. [PERSON], [PERSON] [30], and [PERSON] et al. [31] used nine synthetic jet actuators parallel to the mainstream to investigate mixing processes in the shear layers of coaxial jets with experimental methods. The hot wire anemometry and planar laser-induced fluorescence were used to measure the structure and scalar concentration of mixing fields under three velocity ratio conditions. Results showed that synthetic jets caused the mainstream to expand in the radial direction and the vortical structure between inner and outer jet increased. The continuous high frequency synthetic jets resulted in the enhancement of small-scale mixing in the flow field near outlet, which greatly increased the radial velocity of mainstream. The low frequency amplitude modulation of synthetic jets generated large scale vortical structures which enhanced the radial flow toward the mainstream centerline. [PERSON] et al. (2018) numerically studied the coaxial jets mixing enhancement with synthetic jets. The effects of different phases between two synthetic jets were studied, and it was found that mixing enhancement was significant when the phase angle was greater than zero and less than a maximum value. There have been many studies on the mixing enhancement of free jets and ambient air, but few have specialized in coaxial jets mixing enhancement. In this paper, experimental methods were used to study the distribution of parameters such as velocity, Reynolds shear stress, and turbulence intensity in the coaxial jet with and without synthetic jets perpendicular to the mainstream. Then a series of comparative experiments were carried out to explore the mechanism and key factors of coaxial jets mixing enhancement with synthetic jets.