Coherent Explanation of Aerodynamic Lift and Threshold Condition of Drag Divergence Using the Concept of Sanal Flow Choking

Abstract

Explanations of aerodynamic lift based on Bernoulli’s theorem and Newton’s third law of motion are correct but not conclusive. Aerodynamic lift is a force generated by a solid object creating a flow-turning situation. Theories on the generation of lift and the prediction of drag divergence Mach number have become a source of great controversy and a topic for heated arguments over the decades. Herein, after invoking the concept of Streamtube compression and Sanal flow choking [Global Challenges (2020), Scientific Reports (2021), Physics of Fluids (2022)], we are presenting a cogent explanation of aerodynamic lift and the condition at which the wave-drag begins. When a solid object moves through a fluid and if the geometry creates a flow-turning condition the physical situation of streamlines compression occurs due to the viscous effect. Gas viscosity increases at the stagnation zone. If the streamline compression is more at the upper surface of the airfoil the flow passing between the streamlines gets accelerated faster than the lower surface for meeting the continuity condition set by the law of conservation mass. It creates a low-pressure region at the upper surface of the airfoil for meeting the condition set by the law of conservation of energy. This results in aerodynamic lift towards the low-pressure region of the solid object due to the generation of momentum thrust as dictated by Newton's third law, which requires the air to exert an upward force on the airfoil for meeting the law of conservation of momentum. In silico simulation results are presented to demonstrate the streamlines compression and Sanal flow choking on the upper surface of the airfoil. We concluded that the critical pressure ratio for Sanal flow choking due to Streamtube compression is the threshold condition of developing drag divergence. The phenomenon of Sanal flow choking can be negated by increasing the drag divergent Mach number by injecting fluid with a high heat capacity ratio at the Streamtube pinching zone or a little ahead. The coherent explanation of aerodynamic lift and the threshold condition of drag divergence presented herein, after invoking all the conservation laws of nature, are pointers for the design optimization of subsonic, transonic, supersonic, and ground effect vehicles lucratively.