diverge with 2nd order upwind scheme for momentum

- August 8, 2019 at 10:39 am
  
  zhengxuanwu
  Subscriber
  
  Dear Ansys Community,
  
  Hi everyone, i am going on with the simulation of a melting process now, during which a phase change material(liquid phase at the beginning) is heated up by fluid flowing in a u-pipe heat exchanger and begin to melting. The geomrtry is like in the figure
  
  It is a transient simulation and what i want to study is the heat transfer phenomenon between the fluid and phase change material. for simulation, i used the solidification and melting model and k-omega for turbulent.
  
  it was all good when i used first order upwind schema for discretization of momentum equation and i managed to simulate the system. However, when i switched it to second order schema for higher accuency. It starts to diverge after several times steps like the following pic
  
  Then i tried to solve it with the method which i can find online including :
  
  1. improve the mesh quality. The min. orthogonal quality is now 0.15 and i am not able to improving it any more.
  
  2. start the simulation with 1st order schema till a stable converge and then switch to 2nd order
  
  3. decreasing the time step size
  
  4. with coupled solver (used SIMPLE at first).
  
  But sadly all the methods don´t really help.
  
  So i am back here and hope anyone can help or give some suggestion.
  
  Best Regards
  
  Zhengxuan
- August 8, 2019 at 11:03 am
  
  Amine Ben Hadj Ali
  Ansys Employee
  
  1/Try using actual version
  
  2/Mesh quality is not only orthogonal quality. Check values of skewness, aspect ratio and cell volume change. (increase mesh check verbosity in TUI to 2)
  
  3/Enable HOTR
  
  4/Save residuals for post-processing (/solve expert..). Check residuals of the equations to know where they accumulate.