# Experimental Investigation on Jet Vector Deflection Jumping Phenomenon of Coanda Effect Nozzle
## 3 Results and Discussion  
### Jumping Phenomenon  
Figure 7a shows the measured values at diffusion angle \(\alpha\) = 12-22\({}^{\circ}\) in the experiment, with the jet vector deflection angle \(\theta\) and the secondary flow valve opening \(\delta\) change curve. It is defined in the figure that when the jet is in the neutral condition, the jet vector deflection angle \(\theta\) = 0\({}^{\circ}\); when the jet is attached to the wall, \(\theta\) = \(\alpha\). Defining the secondary flow valve opening as \(\delta\) = 0, the secondary flow valve is fully opened, and the jet maintains a neutral working condition; when the opening of the secondary flow valve is 1, the secondary flow valve is completely closed, and the jet is attached to the Coanda wall at the closing side.  
On the condition of a diffusion angle of \(\alpha\) = 12\({}^{\circ}\), the vectoring jet attaching to the wall is stable. This phenomenon shows that when the diffusion angle of the vectoring nozzle is too small, the Coanda effect will cause the jet to attach to the wall. When the diffusion angle is \(\alpha\) = 13-18\({}^{\circ}\), there is an evident jumping phenomenon in the change curve of the jet vector angle, and the vector angle does not have a linear relationship with the valve opening of secondary flow. When the secondary flow valve is gradually opened, the jet will remain attached to the wall for a while until the secondary flow valve is opened to a critical opening, and the jet will then suddenly leave the wall, resulting in a sudden jump. The vector change angle of the jumping phenomenon, \(\Delta\theta\), with the diffusion angle within a specific range, \(\alpha\), gradually increases. When \(\alpha\) > 20\({}^{\circ}\), the jet remains neutral and will not produce vector deflection when the Coanda effect occurs. Therefore, in the Coanda effect nozzle, the Coanda effect has a specific effective range \(\theta_{\rm C}\). When the distance between the vector jet and Coanda wall \(\leq\theta_{\rm C}\), the jet will have the Coanda effect with the wall, resulting in the jet jumping to the Coanda wall; when the vector jet is attached to the Coanda wall, sufficient secondary flow will make the vector jet leave the Coanda wall and jump at the vector angle \(\Delta\theta=\theta_{\rm C}\).  
Figure 7: The jumping phenomenon of jet vector deflection in the Coanda effect nozzle. (**a**) Jet vector deflection angle \(\theta\) with secondary flow valve opening \(\delta\) variation curve (diffusion angle \(\alpha\) = 12–22\({}^{\circ}\)); (**b**) force variation curve of vectoring nozzle during jet dynamic detachment (diffusion angle \(\alpha\) = 18\({}^{\circ}\)).  
Due to the spatial distance between the jet and the wall when the jet deflects towards the wall, it is not easy to install the pulsed jet exciter. Therefore, the phenomenon of the jet jumping off of the wall is studied in this paper. In order to better study the jump phenomenon in the Coanda effect nozzle, the expansion angle with the most apparent jump phenomenon is selected (\(\alpha=18^{\circ}\)) as the working condition and is taken as the research object. Figure 7b shows the expansion angle and time\(-\)dependent force curve of the vectoring nozzle during jet dynamic wall separation at \(\alpha=18^{\circ}\). It can be seen from the figure that the nozzle will jump instantaneously after the force lags for a while, until the secondary flow reaches a certain level.