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February 9, 2000, 09:00 
Physical difference between airfoils flows

#1 
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Hello,
I have a general question concerning airfoil flows. With different airfoils at the same angle of attack one gets different lift performance. Can anyone give a physical explanation what flow features change if you take an airfoil with a different profile to have higher lift ? Thanks, Tim 

February 9, 2000, 10:58 
Re: Physical difference between airfoils flows

#2 
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(1). Flow around a body will create pressure force on the surface of the body. (2). When the fluid flow is not moving, the pressure force on the surface will be the same. (gravitational force not included) Thus no net force is produced. (3). As the fluid flow moves around an airfoil, it will create variable pressure force on the different part of the surface. The resultant force from the integration of the surface pressure force will have a component normal to the flow direction, the socalled lift. (4). This is how a bird can fly , without learning the "physics". (5). The physical explanation is that in order to satisfy the conservation laws (mass, momentum, and energy conservations), different airfoil shape will produce different surface pressure distribution. The resultant lift force is a linear function of the angle of attack for small angles. At high angle of attack, the flow will separate from the upper surface of the airfoil. When that happens, the lift force will drop and the airfoil will stall. (6). So, the surface pressure distribution is a nonlinear function of the airfoil shape and the angle of attack, as it should be, because the governing equations (conservation laws) are the highly nonlinear NavierStokes equations. (7). Sometimes, for limited range of airfoil shapes and angles of attack, it is possible to use simplified equations or linearized equation to predict the pressure distribution and the lift. And thus some simple physical explanations are possible. But in general, we are not much smarter than the birds in understanding and predicting the lift. (8). My interpretation of "physical explanation" is "mathematical equations and solutions based on fluid dynamics". (similar question has been asked related to how the lift is created, check the old messages)


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