## Fluids

#### Why am I getting different flow field for different Supersonic Initial Gauge Pressure?

• KSD
Subscriber

In theory it is mentioned :

Supersonic/Initial Gauge Pressure  sets the static pressure on the boundary when the flow becomes (locally) supersonic.

Now I am solving a CD nozzle problem with Inlet Stagnation pressure and temperature and outlet is ambient conditions.

I have read papers where they have used either Method-1 or Method-2

Method-1

• Pressure Inlet = Total Pressure = Po
• Supersonic/Initial Gauge Pressure  same as nozzle exit Ambient Pressure

Method-2

• Pressure Inlet = Total Pressure = Po
• Supersonic/Initial Gauge Pressure   = Same as Po or slightly lesser value than Po

Now I am confused which solution should I actually go with, for me Method-2 seems more logical.

So I tried running all cases but I am surprised that for different initial gauge pressure I tried, I am getting different flow field.

I don't know why but only for Method-1 (as shown in Case-1 Image), I am getting same Total Pressure as I had mentioned in Pressure Inlet but for Method-2 (as shown in Case-2 Image) The Total Pressure I am getting is slightly higher.

### Case-1 Supersonic/Initial Gauge Pressure  = Ambient Pressure

• DrAmine
Ansys Employee
The gauge pressure is a user input and is required when you have a supersonic inlet: so how can we then answer to something you need to know on your side.
If you know the Mach number then calculating the required pressure inputs is straightforward.
• KSD
Subscriber
Sir, In Pressure Inlet BC we have to fill 2 things,
One is Total gauge pressure where I am specifying the given Po.
But in supersonic/Inlet gauge pressure I don't know what I need to specify.
Also, I am specifying Ambient pressure at Pressure Outlet BC for sides outside of nozzle wall.
Because at Nozzle Inlet the M = 0 so logically Static Pressure is same as Stagnation pressure at nozzle inlet.
But I have seen in some papers where they specify Ambient pressure in Supersonic/Inlet Gauge Pressure itself, as well as in Pressure outlet.
Could you please tell me what might be the reason for that?
• DrAmine
Ansys Employee
You need to specify the gauge pressure: that is a required user input when supersonic Inlet.
If at the nozzle inlet there is no flow then stagnation pressure is the same as static pressure so you can leave that gauge pressure zero as it won't be used at all (if really no supersonic flow coming from there).
• KSD
Subscriber
Sir, as you have mentioned to specify Supersonic Initial Gauge Pressure = 0 (if really no supersonic flow coming from there)
Sir, this problem is similar to like Nozzle Chamber at given stagnation pressure and temperature. So I don't think so there will be any flow at nozzle Inlet, because of pressure difference created by Chamber Pressure and Ambient Pressure the flow will pass through nozzle.
Sir, my doubt is why 0 instead of specifying Supersonic Initial Gauge Pressure = Po.
As static pressure is same as stagnation pressure at nozzle chamber inlet.
Or is it because as there is no supersonic flow at nozzle chamber inlet, hence specify Supersonic Initial Gauge Pressure = 0.
Sir, I hope my understanding is correct.
• DrAmine
Ansys Employee
That supersonic Gauge pressure will only be used if the flow is supersonic at inlet. That value can be used at initialization if you decide to initialize from that inlet. It is not used at all if the flow is subsonic.
With no flow I was referring to "supersonic Flow".
• KSD
Subscriber
Ok Sir, Now it is more clear, that if flow is not supersonic at inlet then the whatever value you mention in Supersonic/Inlet Gauge Pressure it doesn't matter.
It will only be used as initialization purpose, if you use standard initialization from nozzle inlet, nothing more nothing less.
But Sir, if it is so then, why I am getting different flow field solution (as shown in above three images) for different Supersonic/Inlet Gauge Pressure, when I am not having supersonic flow at nozzle chamber inlet.

• DrAmine
Ansys Employee
You have different flow regimes in each case. You have two case where Mach Number is larger than one and the input will be used. It might be all related to the initial pressure field which drives the whole thing into a wrong direction (hybrid initialization or standard will use your input as gauge pressure). What about ramping the pressure drop: start with high pressure at outlet and then reduce it to the real one?
• KSD
Subscriber
Ok Sir, I will try using Patch initialization and check how flow is being developed.
• DrAmine
Ansys Employee
:)
• KSD
Subscriber
Sir, I have tried using patch initialization by patching static pressure on all domain, same as inlet stagnation pressure but still I am getting different flow feild using different initialization of supersonic inlet gauge pressure.
So Sir, for safe side I am giving the same nozzle inlet stagnation pressure in supersonic inlet gauge pressure, even if fluent is considering the flow at inlet locally supersonic, then also it will not be a problem, as stag and static pressure are same at chamber nozzle inlet. I guess I am right by doing so.
• KSD
Subscriber
Sir, do you have any idea why it is happening like this even after patch initialization in order to ramp down the pressure drop happening across the nozzle.
• Rob
Ansys Employee
Given 18th was a Friday & 20th a Sunday and bothand I work in Europe you may find we take time to respond: especially if you post after we finish for the day/week. From skimming the thread, you're setting a supersonic pressure on a model that's supersonic: altering the inlet condition will alter the results.
• KSD
Subscriber
,Sir, I am very sorry I didn't mean to hurt anyone, as I was so desperate that I forgot that Ansys employees have holidays on weekends.
I am still not understanding, For a chamber nozzle (Where flow across the nozzle is driven by pressure difference) , it shouldn't matter what I specify at supersonic/inlet gauge pressure.
As in chamber nozzle the M = 0 at nozzle inlet.

But DrAmine Sir told It might be all related to the initial pressure field which drives the whole thing into a wrong direction (hybrid initialization or standard will use your input as gauge pressure). What about ramping the pressure drop: start with high pressure at outlet and then reduce it to the real one?

For which I tried using patch initialization by giving static pressure same as nozzle chamber stagnation pressure throughout the domain, but still I am getting different flow field at different initialization.

Sir, Supersonic/Inlet gauge pressure will only be used if flow is locally supersonic right?
By locally it means at chamber nozzle inlet the flow has to be supersonic (As at Chamber nozzle inlet I am specifying the Pressure Inlet BC)
Or does it means if flow evolves to be supersonic after some iterations, then supersonic/inlet gauge pressure will be used (which actually doesn't seems to be logical)
• ibmb2020
Subscriber
If there are cells near your inlet and those cells go supersonic, then all of the variables (ie. pressure, velocity, temperature) should be set since solver uses those variables at boundary. Moreover, you set total pressure and total temperature, and also giving supersonic initial gauge pressure (which is basically static pressure) solver can get velocity and other variables at inlet cell. If you know your flow at inlet is supersonic you must know your static pressure there otherwise it is not necessary since it will not be used.
Normally one should not expect supersonic flow at inlet of nozzle, you should check your setup (materials, models etc.), or slowly increase your inlet pressure as flow settle.
• KSD
Subscriber
I have used ideal gas with properties of air, Operating Pressure = 0, Energy Turned on, k-w sst model, Tried Both Density/Pressure Based Solver, And using 2nd order scheme and default values as it is. I don't think so because of settings, I am getting different solution.
Both of you told that try to slowly increase pressure (or ramp the pressure drop) at inlet. Means how should I do that?
Should I start specifying stagnation pressure same as outlet static pressure and then after certain number of iterations stop the calculation then go back to the Boundaries and edit the pressure inlet to increase stagnation pressure value by some factor and again run the solution, Is this what you means by slowly increasing pressure at pressure inlet?
• ibmb2020
Subscriber
Yes, I mean what you summarize above. You can do this by define_profile udf according to iteration you can increase your pressure to final value ie. at the end of 20000 iteration. You can do this with expressions i guess (but never used).
Also you can try density based implicit (or explicit but it will definitely take more time) transient simulation with atmosphere initial conditions and your final stagnation inlet conditions.
• KSD
Subscriber
Ok Sir, I will try using udf and check the results. Thanks a lot for the suggestion.
• DrAmine
Ansys Employee
Ramping pressure up (on outlet) or down at the Inlet is always a method to help solver convergence. I agree with suggestions.