Timestep becomes very small

[Santanu Das]

Dear all,

I am currently simulating the case of rising bubbles, specifically using multiple bubbles while avoiding coalescence by incorporating the no-coalescence.h file. For this simulation, I have implemented a periodic boundary condition with a fixed grid size of 1024 x 1024 using a box size of 20. Additionally, I have set the maximum timestep (DTMAX) to 1e-3.

The code runs smoothly until t ~ 70 with a DT of 1e-3. However, beyond this point, I have noticed that the DT consistently decreases. I have attached the logfile that records the time versus DT for your reference. The bubbles reach a maximum velocity of 1.5. Given the CFL condition of 0.5, along with the defined cell size and velocity, I would expect the timestep to be around 5e-3.

Could anyone provide insight into why the timestep drops so significantly and suggest how I might address this issue?

Best,

Santanu Das

[Patrick Fischer]

Hi,
I observe the same behavior in my simulations (see my post from October 29). But the program eventually crashes after a certain time. I have studied the flow and found out that the values are blowing up at the exit of the channel due to the Neumann boundary condition. How do the velocity/pressure fields look like ?

Regards