# SIMPLE SIMPLER convergence problem

 Register Blogs Members List Search Today's Posts Mark Forums Read February 17, 2016, 10:39 SIMPLE SIMPLER convergence problem #1 New Member   Che-An Lee Join Date: Feb 2016 Posts: 1 Rep Power: 0 Hi everyone, I am a PhD student in Taiwan. My background is electronic engineering, which means I am totally new to CFD. Because my research demands, I need to write a CFD simulator. Now I am following the book written by Versteeg "An Introduction to Computational Fluid Dynamics" to implement my own simulator. I only need to simulate laminar flow for electro-osmosis behavior, so a simplified NS equation is good enough for me. η∇^2(μ) = -∇p -qψ η : viscosity μ : velocity p : pressure qψ : coulomb force It is a incompressible steady navier stokes equation, and only diffusion problem need to be considered. So I think FVM central difference is enough for this problem. Now I am facing the following problems : 1. When I am using SIMPLE algorithm with coarse mesh, the simulator will converge very easily. But when I starting to refine the mesh, the simulator seems going to start oscillating and diverge. I need to lower the value of under relaxation coefficient for P' (AlphaP) to make the simulator stable. And of course, it will slower the converge speed. For example, simulation area 10nm x 10nm (my length unit is nm.) inlet flow = 1nm/s at left bounardary. Coordinate (0,4)~(0,6) outlet flow = 1nm/s at right bounardary. Coordinate (10,4)~(10,6) noslip boundary is applied. 20x20 nodes, AlphaP=0.01 diverge, AlphaP=0.005 converge 40x40 nodes, AlphaP=0.005 diverge, AlphaP=0.002 converge 50x50 nodes, AlphaP=0.002 diverge, AlphaP=0.001 converge (When it converge, the simulation result seems reasonable and all of them looks identical.) Is normal? Is there any way to find out a appropriate AlphaP without try and error? I need to simulate about 100um * 100um area with several nm meshing step. This will take me forever..... 2. By tracing the simulation process, I found it seems because the velocity is not satisfied continuity equation, so the pressure will try the correct the velocity field. But when I using fine mesh, pressure field will over correct the velocity field and start oscillating. Is this normal that the influence of pressure will increase as nodes increase? 3. Same problem also happens in SIMPLER algorithm, but this time I have no way to stabilize the simulator, because there is no under relaxation coefficient in SIMPLER algorithm. After calculate pseudo-v and solve P by using pseudo-v, I found that this pressure field P will generate irrational velocity field and makes it diverge. For example, initial guess inlet flow and outlet flow is 1 nm/s. Afer the calculation, somehow the maximum velocity will higher than 1. But If I simulate different cases, sometimes it will still converge. (even faster than SIMPLE) I have already check my code many times, but still no clue about this. 4. Is there any recommended code/solver on the internet for incompressible steady navier stokes equation with finite volume method? Last edited by wizard8426; February 17, 2016 at 12:27.  Tags convergence problem, simple algorithm, simpler algorithm Thread Tools Search this Thread Show Printable Version Email this Page Search this Thread: Advanced Search Display Modes Linear Mode Switch to Hybrid Mode Switch to Threaded 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 Off Pingbacks are On Refbacks are On Forum Rules Similar Threads Thread Thread Starter Forum Replies Last Post cfd seeker CFX 2 July 2, 2015 12:12 sarah l FLUENT 0 July 1, 2015 16:47 shiyun ANSYS Meshing & Geometry 4 May 9, 2012 20:55 gradstudent2430 FLUENT 1 April 19, 2012 06:23 Trushar Phoenics 5 August 28, 2002 00:40

All times are GMT -4. The time now is 10:39.