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Anders Olofsson August 30, 2000 08:25

Newbie questions
Hello all!

I'm a complete newbie at CFD. I'm interested in CFD in the field of aerodynamics. What I'd like to do is to calculate lift and drag for an arbitrary shaped body. (Generally it will look like an aircraft!)

So how to do this? :)

The body will be modelled up of polygons. Known facts are the velocity vector for the whole body, normal vectors and the area of each polygon.

I thought something like this: Calculate lift/drag for each polygon. Sum lift and drag from each polygon and use that for lift/drag for the whole body. Compare lift/drag infront and back of center of gravity to determine rotation stuff for the body.

So the important thing is how to calculate lift/drag for one polygon?. I can see that surrounding polygons affect the calculations somehow.

Some booktips on books that really teach the basics of CFD and aerodynamics would be great.

John C. Chien August 30, 2000 09:37

Re: Newbie questions
(1). You can check out the book section of this forum for cfd related books. (2). For aerodynamics, any standard aerodynamics book should be all right. (3). To calculate the lift and drag, you need to know the static pressure distribution on the body of the vehicle, as well as the surface shear force (viscous force). The overall contribution in the vertical direction is the lift, and the contribution in the flow direction is the drag. (4). To obtain the surface pressure and the shear stress, you will have to solve the flow field equations around the vehicle. The idea is very simple, if you know how to solve these equations. (5). If you are not interested in solving the flow field equations, then you can look for existing codes which are capable of solving these flow field equations.

Daniel Jaeggi August 31, 2000 06:56

Re: Newbie questions - Book tip
"Low Speed Aerodynamics" by Plotkin & Katz is an excellent text.

Anders Olofsson August 31, 2000 08:54

Re: Newbie questions
Ok, thanks for your answer.

Guess I've learn what flow field equations is! :)

I checked the book section and "Computational Fluid Dynamics : The Basics With Applications" seems to be introductionary.

Anders Olofsson August 31, 2000 08:55

Re: Newbie questions - Book tip
Ok. I'll go check it out!.

clifford bradford September 1, 2000 09:09

Re: Newbie questions
Anders, John gave youa partial answer to what i think is your question. you want to be able to calculate the lift and drag for a body assuming you already know the flowfield, right? well as John said the you only need to know the surface pressure and skin friction. the pressure is always mormal to the surface and the skin friction is always along the surface. so suppose we're dealing with a aircraft flying in your CFD model in the negative x direction and the lift force is in the z direction for each polygon the contribution of the pressure to the lift is the projection of the local pressure*the polygon area onto the positive z axis. ie: P*A*n projected onto the z axis, where P is the pressure, A is the polygon area and n is the unit normal perpendicular to the polygon (I assume you are familiar with the projection operator from vector analysis). similarly the contribution of the pressure to the drag is the projection onto the x axis. Assuming you've calculated the shear force on the surface you can do something similar to calculate its contributions to the lift and drag. calculating the shear force isn't easy though and without a book beside me I couldn't go into the details

Anders Olofsson September 1, 2000 14:40

Re: Newbie questions
Ah, insightful!. I sort of figured out that the polygon area and normal would be needed.

But no, I dont have the flowfield. Infact I hardly know what it is!.. I assume its some information on how the air is moving around the body?. Sort of streamlines?. I guess I'll need to calculate the airflow in order to get the pressure?


clifford bradford September 1, 2000 16:19

Re: Newbie questions
well usually your code output would have the pressure density and velocity as well as specific energy. and you can do the lift and drag calculations as a stricly postprocessing step. but many codes including the 2-D code i have do it inside the code because for the surface shear force you need the viscosity and the calculation is more involved. many codes are inviscid and so don't calculate skin friction drag so it might be easier for you to start off with an Euler or even potential flow code where the force calculations can be done totally external to the code. that way you gain experience without too much tears.

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