topic Re: Properly setting up simulation for supersonic flow in CFD Forum

Properly setting up simulation for supersonic flow

Anonymous — Tue, 09 Jan 2018 17:17:39 GMT

I am trying to simulate this body going at about Mach 8 at 86km and I am confused on the inputs needed.

I currently have the inlet as "Pressure (0.37338 Pa), and Velocity (2744m/s)" and outlet as "Velocity (2744m/s), Pressure (0.37338 Pa), and Unkown). How can I also make sure that the air stream speed is Mach 8 when it reaches the model?

I know something is wrong here but I can't put my finger on it. I have tried to follow this tutorial but A) It's 2015 and B) I got lost in here.

One of the problems I had previously was that near the inlet, the velocity was about Mach 8 but by the time the air stream reached the model the speed was significantly slow.

Re: Properly setting up simulation for supersonic flow

matt.bemis — Tue, 09 Jan 2018 18:41:27 GMT

Hi @Anonymous,

Thanks for reaching out to us on the forums! It looks to me like your boundary conditions are wrong. I suggest checking out the external compressible flow tutorial model and look at how BC's are done there.

Take a good look at all the help documentation on compressible flow here. FYI the cfdst file is just a pointer file and contains almost nothing. If you'd like to share your project, just attach the .cfz file. FYI I can't personally recall doing a compressible flow analysis at mach 8, but we can try.

Thanks,

Re: Properly setting up simulation for supersonic flow

Anonymous — Tue, 09 Jan 2018 19:32:22 GMT

Thanks. I have also tried to follow those tutorials but I ran into a few issues.

In this part of the tutorial, it says to change coordinate system to Axisymmetric in X. What exactly does this mean and how can I know if that applies to my model? If it is needed, I can't find the option to change it

I have set the CFDCreatedVolume to Air and everything else to Titanium.

I have set the inlet BC for Velocity at 2744m/s, Pressure =0.

Outlet= Unknown. Seems to be correct so far? Will the air stream still be around Mach 8 by the time it reaches the model? I had a issue in the past where the air stream slowed down significantly by the time it reached the model...

Now here is the tricky part...

Here it wants me to assign Free-Stream Velocity Boundary Condition. I am wondering why it would want me to do this and do I select this part of my model to properly assign this (Selected in red)? Or do I select all four faces of the rectangular box with "Vz Enabled" instead of "Vx Enabled" as shown in the tutorial?

Thanks for taking the time to answer these!

P.S

"FYI I can't personally recall doing a compressible flow analysis at mach 8, but we can try" What exactly do you mean by this? Are you suggesting that the results might be inaccurate?

Re: Properly setting up simulation for supersonic flow

matt.bemis — Tue, 09 Jan 2018 19:49:59 GMT

Hi @Anonymous,

Don't worry about the "asymmetric in x" part. The tutorial model is 2D and is a half model. Your model is 3D. For now I think your inlet/outlet boundary conditions look good (based on the images). Let's not change them until we test.

The free stream boundary conditions just defines the velocity at the wall to be the velocity at the inlet. You will want to apply this to your 4 sides. Alternatively you could use a slip boundary condition instead.

It is important to think of what physical set up you are trying to model. In the tutorial model adding the free stream velocity BC's just ensures that the side walls have the same velocity as the inlet. It is telling the solver that the bullet is not actually surrounded by 4 walls which has a no-slip boundary condition. Instead, the bullet is flowing by itself in air, as if it was shot horizontal to ground and several meters off the ground (no boundary layer from ground).

In "real life" what is around your titanium volume? Are there really 4 solid walls, at that exact distance away? Or are you modeling this object to be flowing freely in air? If the latter, add the velocity BC's.

Re: Properly setting up simulation for supersonic flow

Anonymous — Tue, 09 Jan 2018 23:42:14 GMT

I am modeling this as if it was flowing freely in the air.

I ran the simulation with BC:

Inlet: Velocity (2744 m/s), Pressure (0 Pa)

Outlet: Unkown

The 4 sides of the CFDCreatedModel are defined as "Vz Enabled" with Magnitude of 2744 m/s.

How do I properly set the pressure of the air to be 0.37338 Pa? I keep getting "the density ratio is greater than the density ratio threshold" when I right click to edit the environment on the Material tab.

Re: Properly setting up simulation for supersonic flow

Anonymous — Wed, 10 Jan 2018 00:57:01 GMT

So I ran the model at 50km and it worked. It seems that 86km would be too high for CFD?

And why would the velocity be accelerating after the inlet? The red/orange area is so much higher than 2744m/s.

Re: Properly setting up simulation for supersonic flow

Anonymous — Wed, 10 Jan 2018 02:10:15 GMT

Excuse me for the excessive posting (Edit times out by the time I find new questions), but when I decided to look at the total Force in Newtons of the grid fin, I got a negative number, which is really odd. The air stream is going upwards and the Wall Calculator indicates the force is going against the stream?