
[Sponsors] 
how many BC's should be assigned physically in navier stokes runs? 

LinkBack  Thread Tools  Search this Thread  Display Modes 
August 9, 2013, 17:26 
how many BC's should be assigned physically in navier stokes runs?

#1 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 25 
how many BC's on inlet and outlet have to be assigned in navier stokes equations?and what are they?
There is a table for euler and navier stokes but isn't clear enough for inlet and outlet.
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. 

August 10, 2013, 09:36 

#2 
Senior Member
Jonas T. Holdeman, Jr.
Join Date: Mar 2009
Location: Knoxville, Tennessee
Posts: 128
Rep Power: 17 
It can be as few as none with the pressurefree modifiedHermite finite element method.


August 10, 2013, 12:42 

#3 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 25 
I'm not familiar with this method dear Jonas.I want to know about ordinary cases with navierstokes equations.
I know we need p,T and direction of U for inlet and only p for outlet.is it true? then whats the difference between navierstokes and Euler equations? 

August 10, 2013, 14:39 

#4  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,290
Rep Power: 67 
Quote:
see http://adsabs.harvard.edu/abs/1992JCoPh.101..104P 

August 10, 2013, 17:17 

#5  
Senior Member
Jonas T. Holdeman, Jr.
Join Date: Mar 2009
Location: Knoxville, Tennessee
Posts: 128
Rep Power: 17 
Quote:
In principle, one could form exactly divfree functions from non divfree functions by subtracting off the gradient of an irrotational function. The problem is that one cannot do this with a FE and still preserve the element BCs. So it is common to satisfy the divfree condition only weakly. Here it gets complicated with infsup conditions and such. This function to be subtracted out satisfies an equation just like the pressure equation, but with different boundary conditions. But here it gets confusing because people use the symbol p to represent this projection function, and worse yet, they call it the pressure. And, they apply physical arguments about the pressure to this function. So you are really asking about conditions on this projection function. I don't know if a firm mathematical answer exists. The practical answers are based on lore and are part of the mystique of CFD. Including a coupled energy equation does not change the argument above, nor does inclusion of magnetic effects with conducting fluids. And there are some interesting possibilities using a fully FE method with moving domains. While this applies to the Euler equation, that subject is treated differently because of an additional simplification. 

August 10, 2013, 17:33 

#6 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 25 
thanks Jonas for this fundamental argument you did.when you speak about FE do you use it in contradiction of finite volume method(FV)?
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. 

August 10, 2013, 23:29 

#7 
Senior Member
Jonas T. Holdeman, Jr.
Join Date: Mar 2009
Location: Knoxville, Tennessee
Posts: 128
Rep Power: 17 
My interest is in FEM for incompressible flow by the method described using modified Hermite finite elements. I have also applied the method to magnetohydrodynamics and am presently extending the work to moving fluid domains. I have placed a couple of educational codes using the method on the CFD Wiki.


August 18, 2013, 10:29 

#8  
Senior Member
Join Date: Aug 2011
Posts: 270
Rep Power: 14 
Quote:
These features are very important especially for outlet boundaries treatment where non reflective BC is repquired especially for Euler equations. 

August 25, 2013, 09:01 

#9 
New Member
JAY
Join Date: Jul 2013
Posts: 10
Rep Power: 11 
I have some problem with my boundary condition that how i utilize the neumann condition(∂φ/∂x = 0) at outlet. Can i select pressure outlet at outlet?
my inlet condition is : flat velocity profile =40m/s Re=1.916*10^5 turbulent intensity=4.5% And Inlet height of duct=5mm I select velocity inlet at inlet because my flow is incompresible. And axis boundary condition applied along the centerline. and no slip condition at wall. pls help me. 

August 25, 2013, 09:09 

#10  
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 25 
Quote:
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. 

August 25, 2013, 09:14 

#11  
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,208
Rep Power: 25 
Quote:
I have to say one thing,NavierStokes in steadystate mode is elliptic but in unsteady form,is parabolic.and why you say its necessarily 2nd order in space?we can descritize space in 1st order methods like upwind. and in related to Euler,I think it depends on Mach number,if Mach<.7 its elliptic,if M>1.4 its hyperbolic and in .7<M<1.4 its a combination of elliptic and hyperbolic.
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. 

August 25, 2013, 09:42 

#12  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,290
Rep Power: 67 
Quote:
Note also that the timedependent Euler equations are always hyperbolics, whatever the Mach numeber is, since the characteristic lines (or surface) are always real. 

August 25, 2013, 13:01 

#13  
Senior Member
Lefteris
Join Date: Oct 2011
Location: UK
Posts: 305
Rep Power: 14 
Hi immortality!
Quote:
Quote:
__________________
Lefteris 

August 30, 2013, 20:08 

#14  
Senior Member
Join Date: Aug 2011
Posts: 270
Rep Power: 14 
Hi Immortality
Quote:
NS equation is second order derivative for the diffusive term: div(mu (grad(u) + grad(u) T) ) term with d²u/dx², d²u/dy² etc... Quote:
I agree with Filippo 

Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
NAvier Stokes and Newton's law coupling  archeoptyrx  Main CFD Forum  10  March 6, 2014 02:42 
Adding a term to Navier Stokes Equation  ashtonJ  CFX  3  January 15, 2011 06:32 
Navier stokes compresible viscid flow fea, somebody can help?  Jose Choy  Main CFD Forum  3  October 24, 2003 02:28 
Newbie:Viscoelasticity and Navier stokes equation  Rajil Saraswat  Main CFD Forum  2  June 9, 2003 07:21 
help: I am trying to solve Navier Stokes compressible and viscid flow  Jose Choy  Main CFD Forum  2  May 18, 2000 05:45 