
[Sponsors] 
Karman vortex street: boundary conditions for velocity, pressure 

LinkBack  Thread Tools  Display Modes 
September 12, 2013, 17:07 
Karman vortex street: boundary conditions for velocity, pressure

#1 
New Member
Join Date: Sep 2013
Posts: 2
Rep Power: 0 
Hi all,
with incompressible NavierStokes, I'd like to build a PDE for Kármán's vortrex street and I wonder what consistent boundary conditions for the velocity u and the pressure p would be. While on the obstacle and the tube walls I'd employ noslipnopenetetration for u and free boundary conditions for p, i.e., u = 0, n.grad(p) = 0, (where n is the outer normal), the situation for the inlet and outlet are less clear to me. To force a inflow on the left, one could certainly use something along the lines of u = ((ytube_lower)*(tube_uppery)) (0.0 ) How about the pressure here, though? What about the outlet? 

September 12, 2013, 17:21 

#2  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,518
Rep Power: 31 
Quote:
For incompressible flows, the "pressure" is not a thermodinamic function but it enforces the divergencefree velocity field by means of the Poisson equation. Said that, BC.s for velocity and pressure can not be prescribed independently. If you fix the velocity, the pressure BC for the Poisson equation is constrained by the congruent Neumann condition. 

September 12, 2013, 17:40 

#3 
New Member
Join Date: Sep 2013
Posts: 2
Rep Power: 0 
That is my understanding as well.
If I see things correctly, I can use n.grad(p)=0 everywhere if n.u is specified everywhere, and in this case I have to make sure that \int_{boundary} n.u = 0 to have a consistent PDE.  That corresponds with the incompressibility assumption. In the case of Karman's vortex street, though, I don't want to prescribe n.u at the outlet. What is the consequence for the pressure boundary conditions? Will I have prescribe the pressure somewhere? 

September 13, 2013, 06:45 

#4 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,518
Rep Power: 31 
outlet can be managed in terms of some freestream condition, e.g. n.grad (u) = 0. That leads to change the source term and the coefficients of the pressure matrix.
It is quite easy to find in literature papers about the outlet bcs. 

September 15, 2013, 15:05 

#5  
Senior Member
Join Date: Aug 2011
Posts: 251
Rep Power: 8 
I don't understand what you mean.
NavierStokes equations are the PDE which rule the Karman's vortex street as any flow!! Quote:
du/dt + ubarre du/dx =0 dv/dt + ubarre dv/dx =0 dp/dx=0 if you use SIMPLE like algorithm or projection method for velocitypressure coupling algorithm ubarre is the mean longitudinal velocity in the domain here d /dt or d/dx are the partial derivative. 

Tags 
boundary conditions, navier stokes, vortex street 
Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
An error has occurred in cfx5solve:  volo87  CFX  5  June 14, 2013 17:44 
CFX13 Post Periodic interface  EtaEta  CFX  7  December 8, 2011 18:15 
Boundary Conditions : Total Pressure or Velocity  Gearb0x  OpenFOAM Running, Solving & CFD  2  February 28, 2011 22:18 
RPM in Wind Turbine  Pankaj  CFX  9  November 23, 2009 05:05 