# mass balance over a cell _ urgent help

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 August 4, 2007, 08:28 mass balance over a cell _ urgent help #1 taw Guest   Posts: n/a I am a little bit confused due to the result i am getting using a finite volume SIMPLE like code. Considering an incompressible code, for a fully converged and (simplifying assuming steady state condition) I think the continuity equation should be fulfilled at each cell, either as the finite difference or finite volume form. so i expect (u(i+1,j)-u(i,j))/(0.5*(dx(i+1)+dx(i)) + (v(i,j+1)-vg(i,j))/(0.5*(dy(i+1)+dy(i)) = 0 at each cell; But my code gives me this value for the corrected u and v only. I used non staggered grid, but even if I use the staggered I think that wouldn't be different. I am just solving the flat plate laminar flow problem, so there is no complex flow here. For instance I decreased the residual criterion but not helped. I also increased the no iteration for the momentum the pressure routines still no difference. I APPRECIATE SOME ONE GIVE ME LIGHT ON MY PROBLEM. taw

 August 4, 2007, 10:12 Re: mass balance over a cell _ urgent help #2 otd Guest   Posts: n/a My understanding is that SIMPLE works sort of like a predictor-corrector for each iteration. In the first step, an improved estimate for the velocities is calculated - 'improved' meaning that the answers are a bit closer to those that satisfy the momentum balances. In the second step, the pressures and velocities are adjusted so that mass conservation is solved. But, since the second step changes the momentum-satisfying velocities, the momentum equations are a bit out of balance (but hopefully closer than before the first step). This dance is repeated as needed until both mass and momenta balances are within tolerance. I haven't checked your continuity equation in detail, but I think what you describe is to be expected. You'll have agreement in momentum and continuity equations only after you've cycled through both sets of equations many times.

 August 5, 2007, 00:32 Re: mass balance over a cell _ urgent help #3 taw Guest   Posts: n/a Thankyou sir, your quick valued response helped me to look thru what I did, the following form: (u(i+1,j)-u(i,j))/(0.5*(dx(i+1)+dx(i)) + (v(i,j+1)-v(i,j))/(0.5*(dy(i+1)+dy(i)) = 0 should have better been written as (for my case): (u(i+1,j)-u(i-1,j))*(0.25*(y(i+1)-y(i-1)) + (v(i,j+1)-v(i,j-1))*(0.25*(y(i+1)-y(i-1)) = 0 i.e. FV form and taking as averages. Best regards, Taw

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Purushothama Siemens 2 May 31, 2010 21:58 Lindel Phoenics 2 October 23, 2008 19:18 novice FLUENT 0 March 21, 2006 03:33 michele OpenFOAM Other Meshers: ICEM, Star, Ansys, Pointwise, GridPro, Ansa, ... 2 July 15, 2005 04:15 Laurent FLUENT 2 October 15, 2002 10:52

All times are GMT -4. The time now is 09:04.

 Contact Us - CFD Online - Privacy Statement - Top