Reversed flow and turbulent viscosity

BHAVESHNAGAR98 · May 10, 2022, 08:15

Hi Fellow members,
I am trying to simulate leakage of H2 gas from a nozzle in a garage with vent outlet and pressure outlet(door), I have a high turbulent flow(Re = 108000) releasing from nozzle and using sst k omega model for that. Problem is I am having reversed flow at my pressure outlet (covering ~60-70 % area), also I am getting warning "turbulant viscosity limited to 10^5 in x cells" . I tried reducing mesh sizing (also making it finer at appropriate location), reducing URFs, moving the outlet further downstream, changing boundary conditions, reducing the timestep, changing the discretization methods to first order. But the problems persists, I am out if ideas.
kindly help me if any with my problem.

NOTE: I have attached a pdf containing my geometry, mesh and solution approach.
link: https://drive.google.com/file/d/1Jpe...ew?usp=sharing

Vestdam · May 10, 2022, 09:14

The warnings that you get are just warnings. Reversed flow on the outlet might be expected from the massflow from the nozzle and massflow through the outlet vent? You should probably monitor the massflows on the inlet and outlets.

I have not done this type of simulation before, but my guess would be that the divergence is due to the gas velocity of 1300 m/s and a timestep of 0.005 s, which I assume would give you a very high Courant number? A Courant number of 1 would require cell lengths of 6.5 meters.
Again, I have not done this before, but you might need to rethink how you define your nozzle inlet conditions.

BHAVESHNAGAR98 · May 10, 2022, 14:20

Yes, I think because I have given higher mass flow rate at outlet vent than inlet flow rate, that's what causing flow reversal. thanks for pointing that out. I also reduced time step size to 1e-6 so turbulent viscosity warning is gone.

May 10, 2022, 08:15	Reversed flow and turbulent viscosity	#1
BHAVESHNAGAR98 New Member Bhavesh Join Date: Apr 2022 Location: Delhi, India Posts: 4 Rep Power: 4	Hi Fellow members, I am trying to simulate leakage of H2 gas from a nozzle in a garage with vent outlet and pressure outlet(door), I have a high turbulent flow(Re = 108000) releasing from nozzle and using sst k omega model for that. Problem is I am having reversed flow at my pressure outlet (covering ~60-70 % area), also I am getting warning "turbulant viscosity limited to 10^5 in x cells" . I tried reducing mesh sizing (also making it finer at appropriate location), reducing URFs, moving the outlet further downstream, changing boundary conditions, reducing the timestep, changing the discretization methods to first order. But the problems persists, I am out if ideas. kindly help me if any with my problem. NOTE: I have attached a pdf containing my geometry, mesh and solution approach. link: https://drive.google.com/file/d/1Jpe...ew?usp=sharing

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May 10, 2022, 09:14		#2
Vestdam Member Vestdam Join Date: Nov 2017 Posts: 44 Rep Power: 8	The warnings that you get are just warnings. Reversed flow on the outlet might be expected from the massflow from the nozzle and massflow through the outlet vent? You should probably monitor the massflows on the inlet and outlets. I have not done this type of simulation before, but my guess would be that the divergence is due to the gas velocity of 1300 m/s and a timestep of 0.005 s, which I assume would give you a very high Courant number? A Courant number of 1 would require cell lengths of 6.5 meters. Again, I have not done this before, but you might need to rethink how you define your nozzle inlet conditions.

May 10, 2022, 14:20		#3
BHAVESHNAGAR98 New Member Bhavesh Join Date: Apr 2022 Location: Delhi, India Posts: 4 Rep Power: 4	Yes, I think because I have given higher mass flow rate at outlet vent than inlet flow rate, that's what causing flow reversal. thanks for pointing that out. I also reduced time step size to 1e-6 so turbulent viscosity warning is gone.