Learning Forum

[EliasAlekseeff96]

Hello everyone, I have problem with droped pressure at outlet boundary zone

[EliasAlekseeff96]

I have the body (Fig 1) with sharp narrowing of the middle part (Fig 2). 

Basic settings: 

Solver Pressure-Based, Steady

Fluid 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&rdquo.

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.

[EliasAlekseeff96]

I'm sorry that everything turned out so crookedly, I had problems with the publication of this post

[DrAmine]

Can you formulate your question once again? Thanks!

[EliasAlekseeff96]

Outlet pressure drops to the lower pressure limit set in Solution Controls Limits

[DrAmine]

Can you add some more details about mesh quality?

[EliasAlekseeff96]

I used many different type of mesh. Last used mesh had this characteristic

[Rob]

I suspect you need a lot more mesh in that region: I think there are only 2-3 cells across the gap? You've also got several discontinuities in the mesh suggesting you're using mesh interfaces: that's not ideal and combined with a large jump in cell size won't help convergence or accuracy. 

[EliasAlekseeff96]

Thanks for answer, yeah, in this case used ~3 cells, early i used cutcell mesh with 9 cells. "You've also got several discontinuities in the mesh suggesting you're using mesh interfaces" - how may i fix it?

[DrAmine]

Through Sensible sizings

[EliasAlekseeff96]

"Through Sensible sizings" Do You mean manually select ratio (proportions)

of size elements? What remarks do You have for my settings?

[Rob]

Use a share topology operation (SpaceClaim) or multibody part (DesignModeler).  Mesh quality is a lot more than just getting good cells: there's resolution & growth rate to consider too. 

[EliasAlekseeff96]

Unfortantli I dont find this "multibody part (DesignModeler)", may I use Share Topology in DesignModeler for it? And for now I use CutCell mesh in my problem, but it dont solve my trouble with dropped pressure.Thank you for you attention.

I get kind of this

[Raef.Kobeissi]

Is there a possibility to upload the case?

[EliasAlekseeff96]

Sorry, I can't do it.

[DrAmine]

Multibody part is when you put all bodies into a single part and the mesh will be then conformal.

Check mesh quality again.