Numerical study on the placement of vortex generator in a serpentine air intake duct

Abstract

In order to avoid radar detection of the aircraft engine in a combat aircraft, serpentine duct is used in the air intake system. Due to change in geometry, the core flow path undergoes change in direction resulting in secondary flows and possible separation. The main requirements of a combat aircraft air intake is to deliver design airflow to the engine with minimal distortion and pressure loss. While flow distortion can increase the chances of engine stalling, large pressure loss will adversely affect engine overall performance. To address these requirements, both active and passive flow control techniques have been used in practice. Mechanical vortex generator is one of the passive flow control technique and it is attractive due to lesser complexity and the cost involvement. The formation of vortex and its interaction with the core flow is to be carefully analysed to have better design. Co-flow and counter flow vortex generators are reported in the literature. In the present work, effect of co-flow vortex generator on the flow field in an elliptic to circular cross section serpentine duct has been considered. The curvature of the bends is high, and the flow in the lower surface of the first bend interacts with upper surface of the second band in the mid-plane. The vortex generators are placed at the lower surface of the first bend and also at the upper surface of the second bend.The flow field created by the vortex generators placed at the lower surface of the first bend interacts slightly more with the upper surface of the second bend than the baseline case due to slight reduction in separation length. It alters the internal pressure field and improves flow uniformity. In the case of placement of vortex generator at the upper surface of the second bend alone did not show any significant variation on flow uniformity. Vortex generator placed at the lower surface of the first bend and the upper surface of the second bend helps to achieve better flow uniformity except at the upper portion of the aerodynamic inlet plane of the engine. The flow uniformity is improved with reduction in DC-60 by 11% and the pressure recovery is reduced by 6% as compared to the base design without the vortex generators. The trade-off between flow uniformity and pressure recovery clearly indicates placing of vortex generator at the lower surface of first bend is better for this serpentine duct having 2.6% reduction in pressure recovery and improvement in flow uniformity having 9% reduction in DC-60 as compared with baseline duct.