 ## Fluids

#### What should be the appropriate boundary conditions for doing crossflow analysis of Nozzle jet?

• KSD
Subscriber

This is the picture of simulation I want to do, where external flow (supersonic : Mach = 2) is interacting with scarfed nozzle jet. For now I am trying to do steady flow simulation. In below image red arrow represents external flow and blue arrow represents nozzle jet.

• SPielmeier
Subscriber
If you know the ambient pressure you could calculate the total pressure at your desired Ma No. using the isentropic relations. You can then used this calculated pressure as your BC at Inlet 2.
Keep in mind that these relations assume isentropy, so they can┬┤t be used over shocks.
• DrAmine
Ansys Employee
You might use Far Field BC at your Inflow or pressure inlet with appropriate Total and Gauge/Supersonic Static Pressure. You have all input you require given the isentropic relationships.
• KSD
Subscriber
Thanks a lot for responding.
When I tried to give Pressure Inlet at Inlet 2 with stagnation condition corresponding to the Mach No. and Ambient Pressure using isentropic relations, it is working fine But If I want to use pressure far field with M = 2 and Static pressure/temp = Ambient Pressure/temp (In Ansys Fluent Pressure Far field BC, I think we have to specify static pressure/temp). But it shows stabilizing error. Could you tell me where I am going wrong.
• DrAmine
Ansys Employee
With your pressure Inlet inputs are you getting good results? Are you getting the expected Mach number? If yes then you should focus on your next steps.
With Far Field yes you provide static pressure which you can get from stagnation pressure and isentropic relationships. I won't use the whole boundary as Pressure Farfield and leave the other two edges or at least one of them as pressure outlet
• KSD
Subscriber
Sir, I think using pressure inlet BCs is giving me correct results because when I used isentropic relation corresponding to M = 3.75 and ambient pressure = 101325 Pa at pressure Inlet (Not Nozzle Pressure Inlet but Outer domain one). I am getting these results  As far as ambient pressure is concerned it is matching.
But the Contour of Mach no. is showing 3.65 at the entry of flow which is not same as M = 3.75, What would be the reason for it ?
I have considered gamma = 1.2 for air, and in Ansys fluent I am defining Cp corresponding to the gamma = 1.2 using the relation gamma*R/(gamma-1).
But for far field even after defining only one or two boundary as far field like these as shown in below images  I am getting error like floating point, AMG, stabilizing etc...
• KSD
Subscriber
Sir, Could you please confirm whether I did anything wrong?
• DrAmine
Ansys Employee
far field might be sensitive to initialization. However your input is wrong: Stagnation pressure is the ambient pressure. You need to provide the static pressure corresponding to that ambient pressure and Mach number. But again if you have good results with pressure inlet then keep using it and post your input for the BC here: Again the same input will lead to same results. Upstream pressure / farfield pressure is in your case the 1 atm.
• KSD
Subscriber
Sir, I had given stagnation pressure corresponding to desire external flow Mach No. and ambient pressure using isentropic relations.
Means I am treating ambient pressure as static pressure and then finding stagnation pressure according to the desired Mach number I want , when I am using Pressure Inlet as BC, for e.g.
If ambient conditions are as follows Pressure (Pa) = 101325 Pa , (Ta) = 300 K and if I want an external flow of M = 3.75, so by using isentropic relation I am getting (Po) = 19668073.9555 Pa, (To) = 721.875009 K
So this is how I am specifying in BCs section of Fluent:
(1) Pressure Inlet BC = Po, To , (2) At Pressure Outlet BC = Pa, Ta
For this set of BCs inputs I am getting these as results, which I think is correct.  But Sir, as in above answer you are mentioning to treat ambient as stagnation pressure and calculate static pressure corresponding to that Mach No. using isentropic relations, then my simulation is showing floating point error for both cases for e.g.
Po = 101325 Pa, To = 300 K, corresponding to M = 3.75 the static pressure I am getting (P) = 522.0011 Pa
If I give above inputs then for Pressure Inlet or Far field BCs floating point error is occurring.
• KSD
Subscriber
Sir, can you tell me what is going wrong now?
• KSD
Subscriber 