Investigation of Streamtube Flow Choking and Unchoking on the Aerodynamic Performance of Transonic Aircraft

Abstract

The diagnostic investigation of fluid flow characteristics of transonic aircraft during the streamtube compression (pinching) at ground effect (V.R.S. Kumar et al., Physics of Fluids, 2021) and cruise condition is of topical interest. V.R.S. Kumar et al., (Physics of Fluids, 2022) reported conclusively the phenomenon of streamtube compression and sonic-fluid-throat effect due to Sanal flow choking in both internal and external fluid flow systems. The Sanal flow choking and/or streamtube flow choking model offers the luxury to the scientific community to solve numerous unresolved real-world fluid flow problems (V.R.S. Kumar et al., Scientific Reports, 2021). Herein, comprehensive in silico studies have been carried out to examine the causes and effects of streamtube compression during the transonic flight regime. Streamtube compression leads to Sanal flow choking and normal shock wave generation causing an enhancement of entropy. An increase in entropy increases the gas temperature and viscosity. An increase in viscosity leads to the subsequent streamtube compression due to the flow stickiness. This cascade effect leads to the formation of convergent-divergent shaped streamtubes over the transonic aircraft creating a series of oblique shock waves until the flow reaches subsonic flow condition. We have noticed streamtube compression and sonic-fluid-throat effect leading to normal shock at a critical total-to-static pressure ratio creating high entropy and turbulence level. We have comprehended that the sonic-fluid-throat effect and undesirable shock wave can be negated by injecting suitable fluid into the Sanal flow choking region with a high heat capacity ratio. Comprehensive in silico studies have been carried out with high heat capacity fluid injection to the streamtube flow choking region of transonic airfoils and found that flow choking can be negated. The 2D and 3D in silico simulations are carried out for stationary airfoils in ground effect and cruise conditions with and without fluid injection. Different types of airfoils are chosen for the diagnostic investigation to establish the phenomenon of streamtube flow choking during the transonic flying conditions of aircraft. We observed that the flow choking is more susceptible to the low-wing aircraft flying close to the ground and/or sea with a relatively high subsonic Mach number (M > 0.56) and a low angle of attack. At this flying condition, the underneath of the transonic aircraft (wing and/or fuselage) and the ground creates the convergent-divergent (CD) channel flow effect leading to Sanal flow choking at the critical total-to-static pressure ratio. We observed that streamtube compression and flow choking are more prone in regions where turbulent viscosity is relatively high. We observed that injecting microfluid jets with a high heat capacity ratio at the region ahead of streamtube flow choking can delay or negate the shock wave generation and can improve the aerodynamic performance of transonic aircraft. This diagnostic investigation is a pointer towards increasing the drag divergence Mach number for the lucrative design optimization of high-performance transonic vehicles with improved propulsion and aerodynamic performances. We concluded that injecting fluid with a high heat capacity ratio is an effective technique for unchoking the streamtube for increasing the critical Mach number and delaying the drag divergence (see animation results <https://youtu.be/V2rRhJW92R4>; https://youtu.be/0K-8NwXhH00). This study is a pointer towards the design optimization of transonic aircraft with a high critical Mach number lucratively for various industrial applications.