# Experimental Investigation on Jet Vector Deflection Jumping Phenomenon of Coanda Effect Nozzle
###### Abstract  
The Coanda effect nozzle is a fluid thrust vectoring technology that uses the Coanda effect to control jet vector deflection. The jumping phenomenon often occurs in the process of controlling jet vector deflection. This phenomenon leads to the nonlinearity of thrust vector control. It destroys the control performance of the aircraft and brings potential dangers to the safety of the aircraft. The jumping phenomenon occurs in an unsteady flow field different from the traditional flow phenomenon. The flow structure in an unsteady flow field changes with time, so it is not easy to control by the traditional active flow control method. This paper explains the reasons for the jumping phenomenon from two aspects: flow field stability and flow structure. Secondly, the unsteady flow field with the jumping phenomenon is studied and analyzed by a flow visualization experiment and dynamic force measurement. Furthermore, the dynamic modal decomposition (DMD) method is used to extract the characteristic frequencies of the critical vortices causing jets to jump in unsteady flow fields. Finally, a pulsed jet with the same characteristic frequency is used to control the varying vortices in the unsteady flow field. The experimental results show that the active flow control method, which extracts the characteristic frequency of the critical flow field structure by DMD, effectively suppresses the jumping phenomenon in the unsteady flow field. It also linearizes the process of jet nonlinear vector deflection. Coanda effect; fluid thrust vectoring nozzle; active flow control; the jumping phenomenon; DMD +
Footnote †: journal: Applied  
mathematical model of the Coanda effect nozzle based on the ACHEN project in the form of the integral equation, proposed a new aircraft architecture based on this model, and verified the model based on this nozzle through numerical simulation. In 1960, [PERSON] [9] proposed the inclined wall jet model, and suggested that the dimensionless reattachment distance is only a function of the wall deflection angle. Subsequently, the model has been widely studied. The position of the inclined wall and the outlet can be divided into two cases: no potential difference and potential difference. The main structural characteristics of the flow model are that the jet attaches at a certain distance downstream of the outlet and that there is a recirculation zone upstream of the reattachment point, with separation bubbles, reattachment points, and other flow structures. In 2000, [PERSON] et al. [10] obtained consistent results of reattachment positions using pressure measurement and flow display, pointing out that the reattachment positions increase with the increase of wall deflection angle. In 2013, [PERSON], et al. [11] of the Iranian university of science and technology studied the vortex position, size, velocity, average turbulence intensity, Reynolds stress, and reattachment length (attachment point) in the recirculation zone under different wall inclination angles, with a potential difference using experiments and calculations, and obtained the near\(-\)wall velocity field and pressure field in the steady\(-\)state. In 2014, [PERSON] et al. [12] used a numerical simulation method to study the turbulent wall jet flow field structure under different deflection angles, including downstream velocity field, Reynolds stress, and wall static pressure distribution. The influence of the deflection angle on the flow field structure was studied in the range of small deflection angles. The above literature research results show that the main near\(-\)wall flow structure characteristics of the Coanda flow model in steady\(-\)state wall attachment include the separation bubble and reattachment structure. For the wall\(-\)attached deflection jet's flow structure, scholars' research mainly focuses on the near\(-\)wall flow structure and flow characteristics when the wall is stable. The transverse pressure gradient between the space environment and the separation area determines jet detachment and attachment.