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Cipherflak October 22, 2011 08:47

First post and first project; seeking hints and advice!
Greetings, I'm quite new to CFD, except for some simplified ocean models earlier programmed from scratch (barotropic 2D models etc).

What I'm attempting now is something completely different. What I whish to do is to analyse the air resistance on a solid object i drew in Inventor. Something like this (I might simplify the geometry a bit of course).

So, I have some basic questions;
1) Atm I have access to Ansys products. Should I use CXF or fluent for this problem? Or is openfoam "just as good" for me? A friend suggested CFX, so that's why I posted in this subforum. Let's hope I chose right.

2) I guess I have to specify a computational fluid domain around my little trailer. How do I go about that? Do I do that in Design Modeler?

3) Any other advice or heads-up for me as I plunder my way throug this jungle of applications and settings?

Thanks a lot in advance, appreciate all answers :)

mauricio October 22, 2011 11:13

First of all, welcome to this interesting field. Second, i'd agree on your friend that if you're starting at CFD ansys CFX is nice choice against Openfoam; even though learning openfoam is great in the long run since it's open and etc., as far as i know you need some skills in c++ to understand what to do in opefoam and the learning curve might be steeper than with cfx... Or at least that's my point of view as a cfx user.

Assuming you're sold on ansys cfx, the first thing to do is obviously the tutorials. You're in luck with your first project since there's a tutorial on it, i believe the name is flow around a blunt body. Yes, you'd have to set a domain around your little trailer; Design modeler has some useful tools, as you'll be able to read in the tutorial you can split the truck in two and use a simmetry to model just one side. How to make the domain in design modeler? well, after splitting the truck in two and importing it, create a rectangular box that includes your truck and then use something like a body operation to "cut" your truck into the box. Remember that the box needs be the geometry of where the fluid the fluid is passing through, so you'd have to cut your part into it. Please check the tutorials.

my advice is also to simplify that geometry to the very minimum (One solid block), mesh and simulate as the tutorial explained, then start adding feature by feature and see how it goes. The more features you include the more the mesh resolution and complexity near the truck will have to be, so do be careful with that.

Good luck, have patience most of all.

Cipherflak October 22, 2011 15:15

Thanks :)

You suggest splitting the truck into two, that is simply to lower the computation time by (reasonably) assuming the flow is uniform on both sides, right?

Also, I see that there is a lot more activity on the Fluent forum than on the CFX forum. Is the former more used? Is it because it is older and more familiar, or is it 'better' in some ways?

I will definitely try to keep my patience, as I hope you will have patience with my silly questions! :o

ghorrocks October 22, 2011 23:34

Splitting the truck in two will halve the mesh size and make the problem more tractable, but be aware the end fluid flow result is unlikely to be symmetric, even though the geometry is symmetric. For bluff bodies like this you are almost certainly going to get large scale vortex shedding and that is inherently 3D and asymmetric.

Initially I would use RANS turbulence modelling approaches as they are an easy place to start but be aware these models are not very good on bluff bodies. Your body looks like it has a very sharp trailing edge so the separation point is obvious at the back - RANS models are generally poor at getting separation points but your geometry might be OK. The alternative is to use LES, DES or SAS, but these are not techniques for beginners, but they can correctly capture vortex shedding.

As for which is better, try CFX, Fluent and openfoam and use which ever you like the best. They all can do this sort of thing so it is a personal preference.

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