# Experimental Investigation on Jet Vector Deflection Jumping Phenomenon of Coanda Effect Nozzle
## 3 Results and Discussion
### Analysis of the Attachment Flow Structure  
The PIV image at Z/W = 0.5 inside the Coanda effect fluid thrust vectoring nozzle is shown in Figures 8 and 9. The region between the jet and the Coanda wall is region A. The velocity nephogram of the jet under neutral working conditions is shown in Figure 9a. Under this working condition, the secondary flow control valves open between the upper and lower sides of the jet. As shown in Figure 9b, when the main jet flow deflects downward, the upper control valve is open, and the lower control valve is closed. Only one passive secondary flow is injected in the same direction as the main jet into the nozzle flow field. The main jet remains separated from the side of the injection passive secondary flow and attached to the side of the closed valve. When the main jet flows to the lower wall, the internal flow field structure of the nozzle is divided into the main jet, the shear layer outside of the main jet, the separation bubble, and the return basin. The velocity difference between the main jet and the flow field in the nozzle causes the shear layer, and the shear layer leads to a large velocity gradient in the nozzle. The \(K\)-\(H\) instability caused by the high\(-\)speed shear sucks up many small vortex structures. There are apparent separation bubble structures on the wall side of the main jet, and several small and stable vortex structures in the separation bubble. In the POD method, the flow field is sorted according to the energy series, and the first\(-\)order mode is generally regarded as the time\(-\)average flow result of the flow field. The PIV results of jet attachment and jet detachment are usable to construct the snapshot set (snapshot = 100). As shown in Figure 10a, the first\(-\)order energy level in the wall\(-\)attached state is lower than in the neutral state. The first\(-\)order modal energy in the wall\(-\)attached state is 48.08%, while the first\(-\)order modal energy in the neutral state is 71.506%. It can be seen that the stability of the jet in a neutral state is higher than that in an attachment state. Therefore, when the jet deflects to the wall, the jumping phenomenon will occur after  
Figure 8: Detachment condition PIV visualization. (**a**) Velocity vector and velocity nephogram of the whole flow field under the jet detachment condition; (**b**) velocity vector and velocity nephogram of the Region A under the jet detachment condition. the jet vector deflects a certain angle, and when the jet deflects away from the wall, the jumping phenomenon will occur suddenly. DMD is used to arrange the frequencies based on time series so that DMD can identify the stability of the flow, to a certain extent. When the modal curve is a straight line, the flow field is stable; when the modal curve is periodic, the flow field is periodic and stable; when the modal curve diverges, it indicates that the flow field is unstable; when the modal curve converges, it indicates that the flow field will be stable at a specific time. As shown in Figure 10b, the modal coefficient curve of the jet in the neutral state rapidly changes to a zero returning straight line, and the jet presents a completely stable flow state. This phenomenon indicates that the steady velocity of jet neutrality is very rapid. The modal coefficient curve of the jet attached to the wall is a pulsating curve with periodic frequency attenuation. It can be considered that the wall attachment flow is stable under steady flow and periodically stable under unsteady flow. The periodic frequency of the separation bubble structure affects the attachment flow, and changes the characteristic frequency of the flow structure under the wall -attached condition. Figure 10: The POD results and DMD results of jet attachment and detachment conditions. (**a**) The modal energy distribution of the POD under attachment and detachment conditions; (**b**) the modal coefficient curve of the DMD under attachment and detachment conditions. Figure 9: Attachment condition PIV visualization.