Equations in the MRFsimpleFOAM

waynezw0618 · June 5, 2008, 02:24

Hi

in the MRFsimpleFOAM,the basic equation seems in the Absolute Velocity Formulation,which have the relative velocity and abolute velocity both in the momentum eqution.that is why ?is there any
advantage in that way ?

hjasak · June 5, 2008, 03:15

Yes: you do not have to do explicit coordinate transformation for the MRF interface, which makes the code MUSH simpler.

Enjoy,

Hrv

waynezw0618 · June 29, 2008, 04:12

Hi Hrv!
thank you very much!but i still cannot understand it very much for the question that,when you use absolut formulation,the "U" in the kepsilon model is relative velocity or absolute veloctiy?for example the production term in the k equation there is a Grad(U).what is this "U" is it relative velocity or abosolut velocity?

thanks

wayne

hjasak · June 30, 2008, 04:25

The "relative" or "absolute" velocity is for the moving part of the multiple reference frames - in that region you have solid-body rotation + local flow velocity, giving you a choice of formulation.

Enjoy,

Hrv

waynezw0618 · June 30, 2008, 13:32

Hi Hrv!
thanks!,but my equestion is when in the rotation region,what velocity is used to calculate the "k","epsilon" and "omega" for KEpsilon or KOmega turbulence model.although the equation of k","epsilon" and "omega"have the same form in the both rotation and fixed frame,they have convection term and production term which is related to the velocity,for this velocity,i think there will be different from the rotation frame to the fixed one.or i think in the rotation frame the turbulence kinetic--k is convected with the "relative velociy" and produced by the gradient of "relative velociy" ,but for the station part that is "abosulte velocity". is that right?how about the MRFsimpleFoam? could you please tell me more

thanks
wayne

be_inspired · May 7, 2015, 05:43

Same doubt here.
I am using OF2.3.x and very strange results are obtained at first cell around blade. I am simulating a wind turbine.
Even if the first cell height has been calculated for a y+=100 taken into account that the blade is rotating.
pressure distribution is non-real and so epsilon field at that location. The simulation crashes quickly

June 5, 2008, 02:24	Hi in the MRFsimpleFOAM,the	#1
waynezw0618 Senior Member wayne.zhang Join Date: Mar 2009 Location: Shanghai, Shanghai, P.R.China Posts: 309 Rep Power: 18	Hi in the MRFsimpleFOAM,the basic equation seems in the Absolute Velocity Formulation,which have the relative velocity and abolute velocity both in the momentum eqution.that is why ?is there any advantage in that way ?

June 5, 2008, 03:15	Yes: you do not have to do exp	#2
hjasak Senior Member Hrvoje Jasak Join Date: Mar 2009 Location: London, England Posts: 1,907 Rep Power: 33	Yes: you do not have to do explicit coordinate transformation for the MRF interface, which makes the code MUSH simpler. Enjoy, Hrv __________________ Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk

June 29, 2008, 04:12	Hi Hrv! thank you very much!b	#3
waynezw0618 Senior Member wayne.zhang Join Date: Mar 2009 Location: Shanghai, Shanghai, P.R.China Posts: 309 Rep Power: 18	Hi Hrv! thank you very much!but i still cannot understand it very much for the question that,when you use absolut formulation,the "U" in the kepsilon model is relative velocity or absolute veloctiy?for example the production term in the k equation there is a Grad(U).what is this "U" is it relative velocity or abosolut velocity? thanks wayne

June 30, 2008, 04:25	The "relative" or "absolute" v	#4
hjasak Senior Member Hrvoje Jasak Join Date: Mar 2009 Location: London, England Posts: 1,907 Rep Power: 33	The "relative" or "absolute" velocity is for the moving part of the multiple reference frames - in that region you have solid-body rotation + local flow velocity, giving you a choice of formulation. Enjoy, Hrv __________________ Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk

June 30, 2008, 13:32	Hi Hrv! thanks!,but my equest	#5
waynezw0618 Senior Member wayne.zhang Join Date: Mar 2009 Location: Shanghai, Shanghai, P.R.China Posts: 309 Rep Power: 18	Hi Hrv! thanks!,but my equestion is when in the rotation region,what velocity is used to calculate the "k","epsilon" and "omega" for KEpsilon or KOmega turbulence model.although the equation of k","epsilon" and "omega"have the same form in the both rotation and fixed frame,they have convection term and production term which is related to the velocity,for this velocity,i think there will be different from the rotation frame to the fixed one.or i think in the rotation frame the turbulence kinetic--k is convected with the "relative velociy" and produced by the gradient of "relative velociy" ,but for the station part that is "abosulte velocity". is that right?how about the MRFsimpleFoam? could you please tell me more thanks wayne

May 7, 2015, 05:43		#6
be_inspired Senior Member M. Montero Join Date: Mar 2009 Location: Madrid Posts: 155 Rep Power: 17	Same doubt here. I am using OF2.3.x and very strange results are obtained at first cell around blade. I am simulating a wind turbine. Even if the first cell height has been calculated for a y+=100 taken into account that the blade is rotating. pressure distribution is non-real and so epsilon field at that location. The simulation crashes quickly

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
CellZons and MRFSimpleFoam	vinz	OpenFOAM Running, Solving & CFD	19	December 1, 2016 16:18
MRFSimpleFoam Tutorial	bastil	OpenFOAM Running, Solving & CFD	48	August 1, 2012 11:00
MRFSimpleFoam ERROR	waynezw0618	OpenFOAM Running, Solving & CFD	5	July 28, 2011 11:34
MRFSimpleFoam command to run	shyam	OpenFOAM Running, Solving & CFD	2	February 5, 2009 06:38
MRFSimpleFoam	xdanielx	OpenFOAM Running, Solving & CFD	0	December 17, 2008 02:28