# Convergence and backflow problem in cavitation simulation

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 August 9, 2016, 07:38 Convergence and backflow problem in cavitation simulation #1 New Member   Burak Altıntaş Join Date: Apr 2016 Posts: 6 Rep Power: 3 Hi, I am running steady, periodic cavitation case for the out-design parameters of a Francis runner. it has 4 million boundary layer mesh (unstructured), all y+ values on the blade are lower than 2. Max aspect ratios in layers are lower than 10000( I read that this is acceptable for boundary layer meshes). I have two problem. Firstly, my single phase simulations,which are used as initial guess, were converged to 1e-5. However, the cavitation simulations have not converged to 1e-5. Secondly, some runs give backflow in both inlet and outlet, is it normal? if it is not normal, how can i cope with it. Any help will make me happy! A wall has been placed at portion(s) of an INLET | | boundary condition (at 15.8% of the faces, 0.1% of the area) | | to prevent fluid from flowing out of the domain. | | The boundary condition name is: R1 Inlet. | | The fluid name is: Water. | | If this situation persists, consider switching | | to an Opening type boundary condition instead. | +--------------------------------------------------------------------+ +--------------------------------------------------------------------+ | ****** Notice ****** | | A wall has been placed at portion(s) of an INLET | | boundary condition (at 15.8% of the faces, 0.1% of the area) | | to prevent fluid from flowing out of the domain. | | The boundary condition name is: R1 Inlet. | | The fluid name is: Vapour. | | If this situation persists, consider switching | | to an Opening type boundary condition instead. | +--------------------------------------------------------------------+ +--------------------------------------------------------------------+ | ****** Notice ****** | | A wall has been placed at portion(s) of an OUTLET | | boundary condition (at 8.1% of the faces, 0.3% of the area) | | to prevent fluid from flowing into the domain. | | The boundary condition name is: R1 Outlet. | | The fluid name is: Water. | | If this situation persists, consider switching | | to an Opening type boundary condition instead. | +--------------------------------------------------------------------+ +--------------------------------------------------------------------+ | ****** Notice ****** | | A wall has been placed at portion(s) of an OUTLET | | boundary condition (at 8.1% of the faces, 0.3% of the area) | | to prevent fluid from flowing into the domain. | | The boundary condition name is: R1 Outlet. | | The fluid name is: Vapour. | | If this situation persists, consider switching | | to an Opening type boundary condition instead.

 August 9, 2016, 12:51 #2 Member   Join Date: Jun 2009 Posts: 53 Rep Power: 10 Most of the time it depends on the cavitation number. but I would say cavitation is not a steady state phenomenon so you have to switch to transient simulation. The error is not normal at all.

 August 9, 2016, 20:03 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,853 Rep Power: 107 It is not an error. It is a notice. FAQ: http://www.cfd-online.com/Wiki/Ansys...f_an_OUTLET.22 mortazavi is correct, most cavitation models I have done require transient simulations to converge.

 August 11, 2016, 04:08 #4 New Member   Burak Altıntaş Join Date: Apr 2016 Posts: 6 Rep Power: 3 Thanks mortazavi and ghorrocks, i know that cavitation simulation consist of 3 runs. 1. steady-state run without cavitation 2. steady-state run with cavitation, used 1 as initial guess 3. transient run with cavitation, used 2 as initial guess is this wrong? Additionally, I want to use Entrainment with opening pressure type boundary condition instead of outlet type boundary condition because it provides more convergent results and a run without backflow. However, I am not sure how it resolve the system. should i use it?

 August 11, 2016, 06:02 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 13,853 Rep Power: 107 You can do it that way. But I would skip 2 and just go straight to 3. If entrainment converges better and is a good representation of what you are modelling then it sounds like a good choice of boundary condition. The difference between outlet and opening is openings allow back flow. The entrainment option allows flow pulled into the domain to enter at a angle if the flow wants to - the default option only allows flow perpendicular to the boundary.

 August 11, 2016, 06:21 #6 New Member   Burak Altıntaş Join Date: Apr 2016 Posts: 6 Rep Power: 3 Thanks ghorrocks

 August 11, 2016, 10:00 #7 Member   Join Date: Jun 2009 Posts: 53 Rep Power: 10 please let us know if you have any progress in convergance.

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