Thanks a lot..! But since prandtl lifting line theory itself is derived based on an inviscid model I thought inviscid model would give me a proper means for validation. I am confused at the moment though.

I have some results from an inviscid analysis which I did a few days back. The main problem I am facing is a higher value of Cl than predicted by theory. And a total Cd value lower than the induced drag itself. For the theory I used a formula for the lift curve slope of the wing which I got from a website.

I have attached the formula to this post. And after finding out the lift curve slope of the wing I used the ratio formula to find out the value of Cl of the wing. I do not know if this method of validating the Cl value is correct or not.

The comparison of the lifting line theories with inviscid solutions is reasonable, however comparing your results with the formula you should have points in mind, the formula, is based on below assumption, ofcourse additional to that of lifting line theory:

1. Low Aspect Ratio

2. Straight Wing (No sweep or dihedral angle)

3. section lift slope of 2*π (flat plane curve slope)

if you have fundamentals of aerodynamics by Anderson, you may want to look at page 442 and specially Figure 5.37.

Thanks a lot!!Yes.. I am right now analysing only a plain rectangular wing of aspect ratio 4. There is no sweep or dihedral and no aerodynamic or geometric twist. Is it then proper to use that formula?

I think so, just note your airfoil, the formula is based on "section lift slope of 2*π (flat plane curve slope)"

you may change 2*π to your airfoil lift slop, and evaluating "k" in following:

http://www.desktop.aero/appliedaero/compress3d/images/image474.gif

following may help

http://www.desktop.aero/appliedaero/compress3d/subsoniccompress.html

If I understand your problem correctly, you want to use Fluent to identify the induced drag for your wing. Your simulation would then need to follow a certain path.

First you need to make sure that the viscous model you are using in your simulation is suitable for the type of flow you are going to simulate. Generally for external flow over wings, a Spalart-Almaras based RANS would generally give you reasonably good results with a computationally cheap simulation. You could opt for a more grid-intensive LES if you are interested in capturing better recirculation and separation. In any case, you would need to use a model that has already been established as a viable model for your flow.

Secondly, you would need to run the simulation over a range of angles of attack to establish the variation of CD with CL; to determine the drag polar for your wing. Once you have done that, you would be able to identify the CD0 value which corresponds to the zero lift condition. The drag polar would then provide the induced drag or more accurately, Lift-Induced Drag, for the wing you are studying.

For a quick solution, you could even try out a few computationally cheap RANS models over a well-resolved grid to develop more than one drag polars and establish the accuracy of your results through comparison of the results.

I hope this helps your study.

Thank you everyone. But one thing I cannot understand is why I cannot used inviscid model for finding out induce drag.

Inviscid model does not predict boundary layer development which is vital for correct drag prediction.

Dear Qazi, I don't think so! the problem is calculating "induced" drag and as I mentioned, one can predict that part of drag as long as "lift" prediction is done! nothing more is needed! AVL, VLM, inviscid, potential flow, ... with such assumption of no boundary layer and viscosity effect can do the job of predicting "induced drag", "drag due to lift"!

I agree with you on the prediction part. However, as I mentioned in my earlier post, if the aim is to get an accurate assessment, it may matter. The wing in question is AR 4 only, and depending on the aifoil section and AOA of interest, BL development may not be trivial and BL interaction with induce elements due to interference. While this does not fall categorically into induced drag, the overall value of the predicted drag may differ substantially.

Thanks Mr Hamdaoui,

Sometimes your perspective makes you overlook the basics. Thanks for your input.

Thank you everyone. I will try to run first on the inviscid model and then on the viscous model. Maybe Spalart-Allamaras model. But thanks a lot everyone.

My boundary conditions are as follows.

The domain is a rectangular domain with one surface as inlet, one as outlet, one as symmetry plane and the remaining three as wall.

The inlet, outlet and walls are pressure far fields with gauge pressure 101325Pa and Mach number 0.21. To define the flow direction which is towards the y axis I have used the cosine and sine of the angles of attack that I would be investigating.

The operating pressure is 0Pa. As far as the monitors are concerned.

For Cl,

X=0; Y= -sin(alpha) and z= cos(alpha)

For Cd

X=0, Y= cos(alpha) and Z= sin(alpha)

The aerofoil of the wing is on the Y-Z plane.

Hope I am correct with the values that I have given.

Hi everyone, I analysed the wing using euler as well as N-S models. I used Spalart Allmaras model for viscous flow. The main problem I am facing is a very high value of coefficient of lift and a very low value of coefficient of drag. For instance I am getting a total value of Cd which is by far less than the theoretical Induced drag coefficient.For example at angle of attack 2 deg, I am getting a Cl value of 0.3238 but a Cd value of 0.0068 which is less that induced drag predicted by the formula, Cl^2/(3.14xexA.R). I am not able to find out the reason behind this discrepancy. As I am involved in a time bound project, it would be really helpful if anyone shares their views as soon as possible.

Hallo Guys,

I have planned to calculate the Induced drag by using the wake integral method (Far-Field-Method). I have made a flow simulation for NACA 2412 wing with a reynoldnumber of 200000. I have placed a plane on the back of the trailing edge (perpendicular to the freef low direction which is 100 m/s.)

This is the formula i need to place in the User defined section . the formula and the model , are in the diagramm.

My question is hhow can i implment this formula inside the fluent solver?

can anybody help me with this?