[shariq12]

Hello,
I am currently working on a project related to an axisymmetric 2D aircraft diffuser.
We aim to study how AOA and various geometric parameters would impact pressure recovery at subsonic speeds.
Currently, I am trying to simulate an engine requiring 30kg/s mass flow flying at about 0.6 Mach.
For this case, I have set up boundary conditions to be
Inlet velocity,
Mass flow inlet at diffuser exit,
pressure outlet(gauge pressure=0)

However, with these results, I am not able to achieve much of a difference in total pressure inside the diffuser.
Moreover, I also tried changing boundary conditions from
Inlet velocity to Inlet pressure(Gauge pressure=0),
And changed mass flow inlet at diffuser exit to pressure inlet and kept the desired value of gauge pressure obtained from isentropic relations for M=0.6.
But with this setup I believe I am simulating the conditions for zero flight speed, Please do help out.

Thanking you for anticipation

[LoGaL]

You sure you need to check the total pressure difference? I actually find it is quite reasonable that it is low

[shariq12]

Originally Posted by LoGaL

You sure you need to check the total pressure difference? I actually find it is quite reasonable that it is low

Thankyou for your reply,
Actually thats what the project is all about, even 2 per of loss in total pressure can lead to about 2% of reduction of thrust.
It would be great if you could advice me with the proper boundary conditions for the case I am trying to simulate.

[LoGaL]

But 2% of pressure loss is a reasonable amount, the result is correct. I dunno why you expect it to be higher

[shariq12]

Originally Posted by LoGaL

But 2% of pressure loss is a reasonable amount, the result is correct. I dunno why you expect it to be higher

Indeed 2per of loss is fine and expected but the problem is I am not getting any losses ,
Instead I'm getting a increase in total pressure along the centre line of the diffuser, moreover is this comparison method valid, should I even measure the pressure along the centre line or should I check it section wise( parallel to y axis)?
Setting boundary conditions as
Inlet velocity:200m/s
Mass flow inlet at the diffuser end:30kg/s

Thankyou

[LoGaL]

You should check area weighted averages at inlet and outlet. So section wise if that is what you mean.

It makes no sense to check the centerline, in subsonic flow most of the pressure loss comes from the boundary layer

[LuckyTran]

You absolutely have to check the bulk averages over the cross section. The (angled) walls of diffusers have a tendency to generate thrust and the the total pressure rise in the centerline is accentuated if you have boundary layers accelerating the centerline flow.

Moreover you can calculate the loss coefficient and thrust coefficient of your diffuser with either of the upstream boundary condition types. If you were building a diffuser map for example, you would use many flowrates and many pressure ratios to get many many loss coefficients. So if you're unhappy with the current results, better to deep dive it than wonder what is the correct upstream boundary condition.