Learning Forum

[killian153]

Hello everyone,

 

I try to simulate a convergent-divergent nozzle flow based on this subject https://ntrs.nasa.gov/search.jsp?R=19820006179

I can represent quite well the experimental results from the technical document below but I noticed a problem, which I can't solve :

The boundary condition at the nozzle inlet is a pressure-inlet, with a Gauge Total Pressure of 253 312 Pa (2.5 atm, the pressure I need in my thrust chamber) and a Supersonic/Initial Gauge Pressure of 250 000 Pa. My operating condition is 0 Pa and my domain outlet is a Pressure-outlet at 101 325 Pa.

When I initialize my solution (Hybrid Initialization), the maximum pressure is at the right value (253 312 Pa) and this pressure is located at the inlet. But when I run my solution and look at the results, the maximum pressure dropped a little bit and the pressure at inlet is not the right one anymore (246 472 Pa i.e. a drop of 6840 Pa). I plotted the pressure at inlet to get the divergence (picture attached), it shows that the pressure drop happens at first iteration then stabilize at the unwanted value after 25-30 iterations.

The thing is I can't explain this phenomenon. I should be able to keep a fixed pressure value at inlet since this is the pressure-inlet.

Input parameters:

Solver: Steady - Pressure-based - Axisymmetric

Material: Air (ideal-gas)

Model: SST k-omega

Boundary conditions: 1 pressure inlet (2.5 atm), 1 pressure outlet (1 atm)

Solution method: ROE-FDS - Least square - 2nd order

Initialization method: Hybrid

Pictures

Domain:

 

Evolution of pressure at inlet:

 

Thanks!

Killian

[RK]

 Hello, 

Can you please try with Standard initialization and reference the quantities to the inlet? 

Regards,

Rahul 

[killian153]

Hello Rahul,

Here's what I get with standard initialization:

 

Thank you for your quick reply!

Killian

[DrAmine]

You are specifying 2.5 atm at the inlet as total pressure and assume as static gauge pressure which is used only for initialization unless flow is supersonic. Report the value of total pressure at inlet and check if fits your input.

[killian153]

You are specifying 2.5 atm at the inlet as total pressure and assume as static gauge pressure which is used only for initialization unless flow is supersonic. Report the value of total pressure at inlet and check if fits your input.

I'm really sorry for the late reply, but indeed when I plot the total pressure, I get a right and constant value. Thank you!