Couple of questions: please help
 

Hi everyone, new here and fairly new to cfd analysis. My intention is to analyse the flow around a half fuselage and then seperately wing.

My project is not centred around cfd but analysis is required to prove my design is feasible. I am looking for advise and ideas, to get general results:

What Clmax, Cp, total pressure, and density to expect?
Mesh, what type of mesh?
Size of mesh?
Angle of attack to set wing?
Do I set my density and temperature at sea level or ceiling
Do I use Vmax?

Try to do the tutorial 5: Flow Around a Blunt Body. Maybe this will help you.

Flow over the fuselage and wing of an aircraft is no minor simulation. I hope you are going to give this the attention it requires otherwise your results will be rubbish.

Your questions are little surprising - surely you have to design a craft which cruises efficiently but still has good margins during takeoff and landing, can handle off-design conditions (eg cross winds, wind shear etc).... This means you need to consider ALL these scenarios and check your aircraft won't fall out of the sky just because the wind is not exactly how you modelled it.

Unless you are expecting big separations (and for most aircraft this is carefully avoided) then I would do the blunt body tutorial but I don't think it is a good guide for your analysis. You would be looking at more like attached turbulent flow, possibly with transition modelling.

There are millions of papers published in the literature on this topic. Time to look a few of them up to find what the state-of-the-art is in modelling for this area.

Thanks Mario, I will study tutorial 5.

ghorrocks, I just want to view the flow over my wing and then half fuselage to possibly optimise my design. It is not a cfd assignment, just a tool to justify my design..?

What should my boundary condition be?

If you are thinking that you will just do a quick CFD analysis of it to check it is OK - well I suggest you shouldn't bother. If you don't put in enough work to make sure it is accurate you will get results which are massively wrong and the CFD analysis will send you in totally the wrong direction. CFD analysis of wing/fuselage flows require massive grids and big computers to run them.

The only thing I suggest you can do quickly is an inviscid analysis. That is pretty quick and easy and you should be able to get that accurate (as far as the inviscid assumption allows) without too much effort. Of course you won't get any results for separations or drag but to get those will require much more than a quick analysis.

thanks for your help ghorrocks !

Hi thanks for your help! My project is going well thanks to you guys!

I have searched the forum for on how to plot cd and cl and I have been successful, one issue I am having is plotting the pressure distribution over an airfoil.

Can a member please point me in the right direction please??

Thanks

Post processing is covered in the tutorials as well. It is really easy to plot pressure profiles, just select the surface, set it to variable and select pressure.

Sorry, not to sound ungrateful, but I meant this..?

http://i26.tinypic.com/10hopjp.jpg

create a plane in CFX Post ...
 

... that intersects with your wing in the desired location.
Then create a "polyline" with method "boundary intersection" at the intersection between your wing surface and the plane you have created before (you can pick both from a list)
Then you can create a chart, with a data series taken from that polyline. For the two axes of you chart you can choose which variable to plot, so you can choose your chordwise coordinate as x-variable and your cP or pressure or whatever as y-variable.

You should end up with something quite similar to what you wanted.

Cheers,

Jules

thanks Jules, appreciate that!!

-
 

You're welcome.
Did it work?

Thanks for you reply and help Jules!

I am on the right track, need to make some modifications some where, not sure...??

http://i30.tinypic.com/jh8vux.jpg

http://i25.tinypic.com/2eyfj47.jpg

Ok, looks like you have got it they way I meant. :) The cP profile looks a little weird though. Is there separation on your wing?
And how did you define cP ?

Cheers,
Jules

Hi Jules, thanks again for relying!

1. created a poly line with method "boundary intersection"
2. created a new variable "Cp"
http://i32.tinypic.com/2ug0x8x.jpg
3.Created new chart
X axis - X variable
Y axis - Cp
data series polyline 1 (location polyline1)

Any recommendations please?

That procedure looks allright to me.
To comment on your plot again:
From what I see I'd think the flow is coming from the left (-X)., and you have compressible flow (air ideal gas) so that cP is greater than 1 in the stagnation point.
You seem to have your wing/airfoil at 0° angle of attack because the pressure is the same on either side of the airfoil, for the first 3/4 of the chord.
But what happens in the back? There's some lift (or downforce) generated there, but nowhere else on the airfoil. Also there's no stagnation point on your TE, not even nearly. Is there a Gurney flap on your wing? Or is there a vortex that comes from some other part of the aircraft or wing(let)?
Or is this a non-converged solution after only very few iterations?

That's what comes to my mind for now. I guess you're on the right track, and you know better than I do what you want to do in your simulation. I can't to do all the thinking for you.
So unless you have some specific questions there's not really anything else I could tell you.

Cheers, Jules

thanks jules!!

Hi guys, due to processor and memory limitations, I have scaled my model down to 5% of the original model.

What considerations do I have to take in the pre and post?

Reynolds number? Velocity?

What did you scale? Did you scale the geometry (ie made it smaller) or use a coarser mesh?

Thanks for your reply, I scaled just the geometry.