Forces from CFD simulation ridiculously high

Project Support
1

I am trying to run a multiphase simulation to get resistance forces for a small trimaran boat (24 ft long by 8 ft wide overall).

I have tried to copy the approach used by dheiny ( SimScale in terms of domain setup, boundary conditions, etc.

The problem is this - for a boat of this size I would expect resistance numbers on the order of 400 to 1000 pounds at 20 knots. The results from Simscale are a good two orders of magnitude higher, in the 10’s and 100’s of thousands of pounds or more. Somehow, despite specifying inlet and outlet velocities of 20 knots / 10 m/s, the program is generating much higher velocities. Can somebody tell me why this might be?

So far I have tried switching to metric units, adjusting the mesh size, reducing the time step, and changing to laminar flow.

2

Hi Tom.

when looking at your Simulation I’m noticing something:
Warning of high Courant number, and very small times steps.
This results in the fact that your simulation only runs for 8.1e-8 seconds. Which is not enough to represent a realistic Simulation.
Please have a look at these articles to understand and solve the problem:
https://www.simscale.com/knowledge-base/time-step-transient-cfd-simulation/
https://www.simscale.com/knowledge-base/why-do-multiphase-simulations-take-so-long/

Multiphase simulations do always require a transient simulation and a very good mesh. therefore please have a look at the simulation and mesh control parameter.

Best regards
Sebastian