compressible flow with acusolve

Matthias Conjaerts_21193

Hi, I am working on a case, where a pressure wave hits a structure. I only have a measured pressure profile of the static pressure, the maximum is at about 1bar overpressure. 

At first I only want to simulate this pressure wave in an simple rectangular pipe. I am struggeling a bit to find the right bc's and model settings. Is there any documentation with general advice and guidelines for compressible flow with acusolve? (I already found ACU-T: 2400)

 

acupro

Not sure on all the details here.  Maybe you initialize the pressure to 1 bar, and outlet at 1 bar, then use a pressure BC at the inlet.  Assign a multiplier function to the pressure value to get the pressure shape you want.  Not sure if that will make a pressure wave.

Matthias Conjaerts_21193

Hi acupro,

thanks for the quick response.

I used your suggestions and bulit a simple model. The results on Inlet and Outlet seem to be plausible.

But there are two things which I cant explain.
1. Why does the staic pressure on the outlet not reach the pressure peak of the pressure wave.

2. I calculated the dynamic pressure using two ways. Theoretically it should make no difference if I subtract the static pressure from total pressure or use p_dyn=1/2*rho*(vel)^2. But I did not get the same results. (I used z-velocity for the formula, but in my case x- and y-velocity is very low)

 

Maybe you have an explanation.

 

I attached some diagramms and the model file.

acupro

Matthias Conjaerts_21193 said:

Hi acupro,

thanks for the quick response.

I used your suggestions and bulit a simple model. The results on Inlet and Outlet seem to be plausible.

But there are two things which I cant explain.
1. Why does the staic pressure on the outlet not reach the pressure peak of the pressure wave.

2. I calculated the dynamic pressure using two ways. Theoretically it should make no difference if I subtract the static pressure from total pressure or use p_dyn=1/2*rho*(vel)^2. But I did not get the same results. (I used z-velocity for the formula, but in my case x- and y-velocity is very low)

 

Maybe you have an explanation.

 

I attached some diagramms and the model file.

1.  Try using outflow_type = free_pressure on the Simple BC for the outlet surface (type = outflow).  This seems to correct the main issue, but then you may also get reverse flow.  Don't know if having the Inlet BC as pressure instead of stagnation_pressure will make a difference.

2. I wonder if using the averaged values for the dynamic pressure calculations is the best way.  Do the contour plots make sense?

Matthias Conjaerts_21193

If I use outflow_type = free_pressure I get heavy reverse flow, as you assumed.

If I use a static-pressure Inlet for the pressure wave, I get an error and the solver will not run:

 

 

Regarding the dynamic pressure, I did the comparison again with nodal values. The results are still different.

Matthias Conjaerts_21193

I also noticed, that the pressure wave propagates with far more than sound speed (if I compare the maximum of the total pressure at inlet and outlet). I calculated 2000 m/s, which seems to be a bit high.

Matthias Conjaerts_21193

any updates on this case?