SimScale CAE Forum

Hybrid airship

Hi I am trying to simulate a trilobed airship model and my force plot for Pressure force y is oscillating.(Please check run 5) In Run 5 the mesh is the most refined and still the force plot is oscillating. Please help ,e out with mesh settings as the model is very large also it would be great if you could run it yourself and help me out with the settings.
https://www.simscale.com/workbench/?pid=7790746680417188468&mi=run%3A16%2Csimulation%3A8&mt=SIMULATION_RUN

Thank you
Pranshul

1 Like

Hi, thanks for reaching out.

The linked project seems to be set to private - could you please double check on your end? Optimally, other users should be able to inspect the project before making comments on the setup.

Cheers

yes its private do i need too change anything?

https://www.simscale.com/workbench/?pid=7790746680417188468&mi=run%3A17%2Csimulation%3A8&mt=SIMULATION_RUN
This is the link to the project i am having an unstable force plot although the residuals are fine

1 Like

Hi, thanks for the link.

Some comments from my end:

  • First, to address your main question, residuals are not the only thing to keep track of, when assessing convergence. Naturally they are important, but I have already seen cases that:
    - Have “high” residuals but the parameters of interest have not converged and,
    - Cases where the residuals are very low (e.g. <1e-6 for all parameters), but the quantities of interest have not yet converged.

If the residuals plot looks good, that’s definitely a good start, but it’s no guarantee of convergence just by itself. Sometimes you just have to let the simulation run for longer until your parameters of interest have converged.

In any case, some tips:

  • For external aerodynamics cases, it’s always a good idea to lower the absolute tolerance of the turbulence solvers. The residuals from the turbulence parameters tend to drop very quickly, and we should ensure that the solvers continue to iterate:

  • I’d also highly recommend manually estimating the turbulence parameters (e.g. using this calculator, there is also a very interesting discussion here).

PS: in the calculation, for external aerodynamics, the turbulence intensity is often very small (0.5%-1%). The second link contains insights about the eddy viscosity ratio as well.

Cheers

2 Likes

Hi thanks for manual. Its help me :nerd_face:

Hi I have tried various mesh settings and have also increased my number of iterations for the simulation but still my force plot is not converging can you please have a look?
https://www.simscale.com/workbench/?pid=567586720218461166&mi=spec%3A572fe7f8-9438-44d2-9a2e-4a7ccad3a1e0%2Cservice%3AMESHING%2Cstrategy%3A9&sh=8