Learning Forum

[SRI VENKATA VATHSALA MUSUNURI]

Hello,

I am an Ansys Workbench user, I am struggling to correctly predicit data with a fluent external subsonic aerodynamic flow case of a mini-jet engine. 

Below is the grid of the domain around (10m long by 20m wide) 500 times the diamter of the nozzle, i am trying to simulate with 

Total number of elements: 268,038

Orthogonality: 0.49

Skewness: 0.89  (even if i had reduced the skeweness to 0.6 i had not seen any difference with the behaiour of the output paramters post simulation) 

As you can see, I am running a steady, 2D-axisymmetric; density based solver, for ideal gas with energy equation, the turbulnce model being Realizable, k-e, scalable wall function.

I tried the case where i would set the ambient pressure value at operating pressure and the gauge pressures are set to zero. this gives me results of the following .....  It is anticipated to have subsonic flow of Mach 0.7894 but fluent gives only 0.59, same with velocity, supposed to give around 480 m/s but we get back 372m/s, given all these parametrs are pressure dependent, i checked the Static and Total Pressure values, which also seemed to be extremely low and deviated. Ihad expereimented various combination of boundary conditions its either 20% less or 20% more prediction for the values but no where close to the sensible physics. 

When i set the operating pressure to zero and set gauge pressure to the ambient pressure values which we are usually supposed to do, both hybrid and standard initialization diverge unfortunately. the cfl number is also not too high, I usually run it with 1-2.

 

I would really need some feasible inputs to go forward. 

[SRI VENKATA VATHSALA MUSUNURI]

I forgot to mention, there would be heavy re-circulation while running for the case where values are underpredicted. I want to avoid the re-circulation as well. 

[SRI VENKATA VATHSALA MUSUNURI]

To add, sorry about adding couple of things later, just want to list down what all I have been trying, I have changed the ff_top to a slip wall from a pressure outlet, could not see any benefit in terms of the results from it as well. and the re-circulation would not go away even then.  

[Essence]

Hello,

What is the y+ and the growth rate of the mesh in the entire domain? Please have y+ less than or equal to 1. Growth rate not more than 1.2. Use k-w SST. Did you initialize using FMG? Try using pressure-farfield BC and apply the appropriate Mach number over there.

[SRI VENKATA VATHSALA MUSUNURI]

Thanks for responding , Y+ is around 3.3, after i added boundary layers, i will reduce it down to less than 1.

I tried fmg initialization after you had mentioned, it says something like this: 

temperature limited to 1.000000e+00 in 208 cells on zone 2

temperature limited to 1.000000e+00 in 20 cells on zone 2

temperature limited to 1.000000e+00 in 6 cells on zone 2

temperature limited to 1.000000e+00 in 4 cells on zone 2

FMG: Converge FAS on level 5

FMG: Converge FAS on level 4

FMG: Converge FAS on level 3

FMG: Converge FAS on level 2

FMG: Converge FAS on level 1

0.

Reversed flow on 18 faces (100.0% area) of pressure-outlet 5.

Reversed flow on 32 faces (100.0% area) of pressure-outlet 6.

time step reduced in 66 cells due to excessive temperature change

for the pressure-farfiled BC, I can only give mach 0, would that be a logical thing to do? Because, i only have a subsonic flow at the nozzle with a mach of 0.59. but the Pressure-farfield BC sets the entire domain to a mach, i am not sure about that. 

 

[Essence]

But you mentioned you are carrying out an external flow simulation. That's why I recommended pressure-farfield BC. Could you indicate which is the nozzle in your model? It would be helpful if you share the picture, where the inlet, outlet, walls and other BCs are displayed properly. And as the FMG is concerned, you need to bring down the Courant number to ensure the shock wave travels till the outlet.

[Essence]

Please refer to the Ansys guide links for more information:

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v232/en/flu_tg/flu_tg_oneram6_wing.html

https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v232/en/flu_tg/flu_tg_transient_compressible.html