How to test different inlet velocities in the same CFX simulation?
 

Hi everybody,

I am running a two-ways FSI simulation of a wing with Transient Structural and CFX. I need to find the velocity at which the equivalent stresses are higher than the UTS of the material. So far I have been running a bunch of simulations, each with a different velocity, but this approach is extremely inefficient and time consuming.

Is there a way to define a set of velocities and compute the equivalent stress for each velocity within the same simulation? I need to test velocities ranging from 100 m/s to 130 m/s. Is it possible to test this velocity range in the same simulation?

Also, what does it mean when it asks for the "Initialization" velocity components and pressure? I leave them as 0 but I don't understand what they are.



Please let me know and thank you for your help :)

Why do you need to do this as FSI? Can't you get the lift versus speed from a stationary CFD model, then apply a pressure load on the wing to get the stress? As the scaling of pressure and stresses is well known you really only need to do one CFD simulation and one FEA and you can extrapolate it to failure from there.

Glenn,

Thank you for answering. The reason why I opted for a FSI is to include the full spectrum of aerodynamic forces acting on the wing, including drag, moment, and ambient pressure, so that I can consider all the loads that can bring the wing to failure.

If I understand what you suggest, you say to run a single CFD simulation (what do you mean with stationary? Does it need to be transient or static?), compute the lift, and apply that lift in the form of a pressure load in the FEA. Is this what you mean? If not, can you please explain in more detail.

A few questions:
1. shouldn't lift be applied as a force in the FEA?

2. Following your suggestion, how does that include the other aerodynamic forces acting on the wing, like drag, moment, and ambient pressure?

3. Is it possible to define a set of velocities for a single CFD (for example from 100 to 130 m/s), and obtain the aerodynamic loads for each velocity?



Thank you for your help, I appreciate a lot.

Force acts at a point. Pressure acts over an area.

To model a proper wing loading profile then do a 1-way FSI. Get the aerodynamic forces from a CFD model and impose them as loads on a FEA simulation. This is much simpler than 2-way FSI.

Use parametric modelling in ANSYS workbench to do this. It is very easy. Or a batch file with a series of CCL files. That's easy as well.

Glenn has already suggested the best way to do it: paramteric modelling.

But a "quick and dirty" solution could be the following:

Your velocities (I suppose inlet BC) can be defined as a list or even better as function. There you would be able to define, that the during certain periods of time a certain velocity is applied. Meaning you would do a transient simulation, let the velocity at the inlet be applied for a duration long enough for you to determine the values you're interested in and then switch to the next one.
Smth. like:
0s-100s inlet velocity=100m/s
100s-200s inlet velocity = 110m/s
etc...

Doing it as a series of transient steps is much longer than some steady state simulations, more complex and introduces new errors (did the flow converge before it moved to the new flow rate?). I do not recommend it.

By far the easiest and quickest way is to do it as a sequence of steady state simulations.

Thank you both for your answers. I will do like Glenn suggested. But I also want to learn parametric modelling, I am sure I will need it in the future.

So I am first looking at the two-ways FSI, because I have it already available. I set three velocities (110, 120, and 130 m/s) as the workbench input parameters (see pictures). But how do I get the equivalent stress for each velocity? I believe I have to set an output parameter, but I don't know how. In fact, when I look on the CFX "Results", Equivalent Stress does not appear in the parameters list (see picture). Could you please list the steps I have to make?

Thanks you.