Learning Forum

[q9999]

Hi there,

I've encountered a serious problem with Fluent during LES of the non-reacting jet.

Params:
Re=23 750
nozzle diam=4.6 mm
velocity profile defined at the inlet (Umax=100 m/s, Umin=3.5 m/s)
inlet perturbation - spectral synthesizer with Ti=0.1%
solver: press-vel coupling - SIMPLE, momentum - central differencing, time-integration - bounded second order implicit

I expect to get RMSE velocity at the inlet: 0.1% * 75 m/s (Umean) approx. 0.075 m/s however I get ~3 m/s!!!

I decided to turn off the inlet perturbation and I get the same RMSE value ~3 m/s!

It seems like Fluent calculates Ti using some fraction of inlet velocity apart from specified Ti value... This is completely unphysical and leads to false results for the whole class of flow problems!!! What the hell? Could someone explain it to me please?

Best regards,
Jakub

 

PS some results:

https://ibb.co/Mg1M2W0
https://ibb.co/G2gwP6g
https://ibb.co/gdRQjTq

[Amine Ben Hadj Ali]

First of all you need to add screenshots instead of attaching them if you want that someone from ANSYS Staff has a look into them.

Do not use spectral synthesizer.

[q9999]

 

Please:

1) axial velocity field:

 

2) axial velocity at inlet plane:

 

3) RMSE z-velocity at the inlet plane:

 

 

"Do not use spectral synthesizer." - Why?

As you can see from my post I also tried no perturbation option. What I didn't show is that I also repeated calculation for Vortex Method. All the 3 tries gave me the same result i.e., Ti at inlet ~3% of mean velocity apart from specified value 0.1%.

What I found in FLUENT manual is: "The turbulence intensity value specified at a velocity inlet for LES, as described in Section 12.20.4, is used to randomly perturb the instantaneous velocity field at the inlet. It does not specify a modeled turbulence quantity."

So, my questions are: why Ti is modelled at the inlet instead of taken explicitly? How explicitly specify Ti at the inlet in LES similarly as in case of RANS?

 

Regards,

Jakub

 

 

[Amine Ben Hadj Ali]

Based on some investigation we do not recommend the spectral synthesizer.
Random fluctuations will depend on TI as states in the documentation.

Regarding your case with no perturbations please create custom field function and calculate the quantity you want to quantify. RMSE only refer to resolved stresses. To build up TI one requires the whole stress.

[q9999]

"Regarding your case with no perturbations please create custom field function and calculate the quantity you want to quantify. RMSE only refer to resolved stresses." - Doesn't RMSE z-velocity refer to u'=sqrt(1/N sum{(U-U_mean)^2}) ? If the Reynolds stresses are variances and covariances of fluctuating velocities, won’t "the whole stresses" be greater than resolved? If so, Ti will be greater then too. What for to calculate whole stresses if the current value is too high?

Would be kindly grateful for answering my questions:

"So, my questions are: why Ti is modelled at the inlet instead of taken explicitly? How explicitly specify Ti at the inlet in LES similarly as in case of RANS?"

BR,

Jakub

 

PS

as the Fluent calculates false Ti (or u') at the inlet plane, it seems as a big problem for me, it has serious consequnces for the whole flow further downstream...

[Amine Ben Hadj Ali]

Let me check and come back to you. Please be patient a bit.

RMSE is defined as you wrote. With full stress i was meaning to add the modeled fluctuations  and  I am also only referring to fluctuations. We will update here soon.

[Amine Ben Hadj Ali]

The too high velocity fluctuations may result from the numerical oscillation of the central difference scheme for momentum. For compressible flows it may be worse due to the pressure wave reflections, which may even cause instability. What happens with BCD?

[q9999]

That may be, thank you for suggestion I'll check it.

BR,

Jakub

[q9999]

Now it works like a charm. Thank you!

BR,

Jakub