Active stabilization of thrust axis in serpentine convergent-divergent nozzles via suction control

The serpentine convergent-divergent nozzle meets the requirements for super-stealth and super-flight in advanced fighter aircraft. However, under over-expanded conditions, it may cause thrust deflection, threatening aircraft stability, thus requiring methods to control this deflection. This paper aims to stabilize the thrust axis by controlling of shock waves and flow separation in serpentine convergent-divergent nozzle through suction. A separation criterion suitable for symmetric separation in two-dimensional convergent-divergent nozzles was established based on the Schmucker's separation criterion. Using this criterion as a reference, the effects of different suction positions and mass flow rates on thrust axis stability were analyzed under varying nozzle pressure ratios (NPRs). The results indicate that achieving thrust axis stability requires adjusting the suction position and mass flow rate to confine incident shock at the suction slot, thereby attaining symmetric wave system and symmetric flow separation, with the thrust vector angle reduced to nearly 0°. For high NPRs, the thrust axis can be stabilized by increasing suction mass flow rates near the predicted suction position or shifting the suction position downstream to counteract upstream flow non-uniformity. As the NPR increases, the suction range required to stabilize the thrust axis becomes larger, and the loss in axial thrust performance required to achieve stability decreases. For low NPRs, the suction position must be shifted further downstream relative to the predicted location, accompanied by significantly increased suction mass flow rates. In this case, achieving the desired thrust axis stability requires sacrificing approximately 9% of the axial thrust performance.