Question

What is lift? What causes it? Does wall shear contribute to the lift?

Short answer

Answer: Wall shear does not directly contribute to lift, as it acts in the direction parallel to the object's surface and fluid flow. However, it affects the fluid flow around the object, influencing the overall lift-generating mechanism, primarily by generating drag forces and affecting flow separation.

Step-by-step solution

Definition of Lift

Lift is the aerodynamic force exerted on an object moving through a fluid (usually air) that is perpendicular to the direction of motion, supporting the object against the force of gravity and keeping it airborne. In general terms, lift is created by generating a pressure difference above and below an object, such as an aircraft wing or airfoil, producing an upward force.

Factors Causing Lift

There are several factors contributing to the generation of lift, such as the shape of the object, its angle of attack, the fluid's speed and density, and surface roughness. The two primary theories responsible for lift are the Bernoulli's Principle and the Newton's Third Law: 1. Bernoulli's Principle: According to this principle, as the fluid's speed increases, its pressure decreases. When applied to an airfoil, the air velocities above the wings are higher than the velocities below, creating a pressure difference which generates the lift force. 2. Newton's Third Law: This law states that for every action, there is an equal and opposite reaction. In the case of lift, the airfoil shape and its angle of attack deflect the air downwards, and as a result, the air pushes the airfoil upwards, generating lift.

Wall Shear Contribution to Lift

Wall shear refers to the tangential friction force exerted by a fluid on the surface of an object, primarily resulting from the fluid's viscosity. It acts in the direction parallel to the object's surface and the fluid flow. Since lift is a force acting perpendicular to the direction of motion, wall shear does not directly contribute to lift, but it affects the flow of fluid around the object, influencing the overall lift-generating mechanism, primarily by generating drag forces and affecting flow separation.