outlet pressure drop

Elias Alekseeff · August 9, 2019, 06:57

Hello for everyone,I have the body (Fig 1) with sharp narrowing of the middle part (Fig 2).Basic settings: Solver Pressure-Based, SteadyFluid is a superheated steam (model water vapor -- real gas soave redlich kwong), Boundary conditions -at the inlet (cylinder) use “pressure inlet” (912 000 Pa), at the outlet (rectangle) use “mass flow outlet” (12 kg/s), I use the equations Energy, viscous -- k-epsilon -- standard -- standard wall functions.
Solution Methods - SIMPLE (I was select for all parameters “second order upwind”).
In Solution Controls Under-Relaxation Factors for Momentum, Turbulent Kinetic Energy, Energy set 0.1, as there were problems with the error "Divergence detected in AMG solver: pressure correction" and "Divergence detected in AMG solver temperature". These settings helped solve the problems. Standard Initialization of the inlet is used.
My problem is the pressure in the output region drops to values lower boundary pressure, which is set in the Solution Controls Limits, default is 1 Pa, and the result is 1 Pa, I increase the lower boundary Solution Controls Limits for pressure to 100 000 Pa, in result I have 100 000 Pa, I increase the lower boundary to 400 000 Pa, in result I have 400 000 Pa, the real result should be around 550 000 Pa. I used different versions of the grid such as Tetra, Hexa (CutCell), broke into separate parts of the body and created for each of its own, used a different size for grid’s elements, the result is the same, I was getting lower pressure limit from Solution Controls Limits .Does anyone know how to solve this? Thanks.
https://drive.google.com/open?id=1ws...8xS3FBFgHKpNbe

LuckyTran · August 9, 2019, 12:52

It's a general divergence error, you just have to debug it as everyone does. The solver calculates that the pressure should be much much lower and then gets limited by the setting. That's why you always get the pressure of the minimum setting. If you turned off the limiter, you might even get negative absolute pressures. It is quite meaningless regardless cause your case is diverging.

If your mesh is not crap, I would tinker with the initial conditions and/or urf's. I would start with using a pressure outlet BC (instead of a mass flow BC) because it is much much more robust. At least run this until you get some reasonable looking flow field, and then you can turn it back to a mass-flow outlet.

Elias Alekseeff · August 13, 2019, 04:02

Thanks for answer. How may i check my mesh quality?
https://drive.google.com/open?id=1Us...R_tv7p6O6CZOwv

August 9, 2019, 06:57	outlet pressure drop	#1
Elias Alekseeff New Member Elias Join Date: Aug 2019 Posts: 2 Rep Power: 0	Hello for everyone,I have the body (Fig 1) with sharp narrowing of the middle part (Fig 2).Basic settings: Solver Pressure-Based, SteadyFluid is a superheated steam (model water vapor -- real gas soave redlich kwong), Boundary conditions -at the inlet (cylinder) use “pressure inlet” (912 000 Pa), at the outlet (rectangle) use “mass flow outlet” (12 kg/s), I use the equations Energy, viscous -- k-epsilon -- standard -- standard wall functions. Solution Methods - SIMPLE (I was select for all parameters “second order upwind”). In Solution Controls Under-Relaxation Factors for Momentum, Turbulent Kinetic Energy, Energy set 0.1, as there were problems with the error "Divergence detected in AMG solver: pressure correction" and "Divergence detected in AMG solver temperature". These settings helped solve the problems. Standard Initialization of the inlet is used. My problem is the pressure in the output region drops to values lower boundary pressure, which is set in the Solution Controls Limits, default is 1 Pa, and the result is 1 Pa, I increase the lower boundary Solution Controls Limits for pressure to 100 000 Pa, in result I have 100 000 Pa, I increase the lower boundary to 400 000 Pa, in result I have 400 000 Pa, the real result should be around 550 000 Pa. I used different versions of the grid such as Tetra, Hexa (CutCell), broke into separate parts of the body and created for each of its own, used a different size for grid’s elements, the result is the same, I was getting lower pressure limit from Solution Controls Limits .Does anyone know how to solve this? Thanks. https://drive.google.com/open?id=1ws...8xS3FBFgHKpNbe

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August 9, 2019, 12:52		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,747 Rep Power: 66	It's a general divergence error, you just have to debug it as everyone does. The solver calculates that the pressure should be much much lower and then gets limited by the setting. That's why you always get the pressure of the minimum setting. If you turned off the limiter, you might even get negative absolute pressures. It is quite meaningless regardless cause your case is diverging. If your mesh is not crap, I would tinker with the initial conditions and/or urf's. I would start with using a pressure outlet BC (instead of a mass flow BC) because it is much much more robust. At least run this until you get some reasonable looking flow field, and then you can turn it back to a mass-flow outlet.

August 13, 2019, 04:02		#3
Elias Alekseeff New Member Elias Join Date: Aug 2019 Posts: 2 Rep Power: 0	Thanks for answer. How may i check my mesh quality? https://drive.google.com/open?id=1Us...R_tv7p6O6CZOwv