Learning Forum

[Ruben Joosten]

I’m currently simulating an enclosed boat in water.
I want to find the drag coefficient of the boat. I’m currently achieving values of 0.09 for the drag coefficient which seems unrealistic.
I have attached the drag coefficient and lift coefficient plots below over a specific time frame.

It would be preferred to have the boat moving up and down and implement waves at some point, however currently the drag coefficient is all I need. Which is why I feel this setup is good enough for the low speeds. Correct me if i'm wrong.

I’m currently using the following settings:

Mesh-> Around 3 million elements containing-
BOI at the back of the boat,
BOI around the boat and a long BOI at the waterline to refine mesh in these sections
General: Gravity , transient flow, pressure based solver, VOF open channel flow
Multiphase flow
K-Omega SST turbulence model
BC:
Pressure inlet and outlet
Open channel BC’s
Velocity 1.5m/s
Walls stationary
Waterline at 0.25m

Solution settings:
PISO scheme
Gradient = Green- gauss cell based
Pressure = PRESTO
Remaining sections have Second order upwind
2500 time steps, time step size is 0.003
Transient formulation= First order implicit

Can someone help me identify why I get such low drag coefficients or help me understand what I can do to ensure a reliable simulation for the drag forces. I would really appreciate a quick video call if possible.

[Ahmed Hussien]

Hello Ruben, I recommend examining the calculation of the drag coefficient. The drag coefficient can exhibit significant variations based on the chosen reference values. To illustrate, certain approaches utilize the wetted area (the portion covered by water), while others use the cross-sectional area. Additionally, ensure that you appropriately assign the accurate reference values in your model, such as density and area. To conclude, make sure that the same reference values are used in your fluent model.

[Ruben Joosten]

Hello Ahmed, thank you for taking the time to read the post.
I changed the reference values to the ones shown in the image, as these values correspond to that of sea water and the wetted surface area. This time I received values of 0.28 for the drag coefficient, which still seems quite small.
I was wondering whether there is another way to simulate as I assume the drag coefficient is calculated using the drag force over thole body and using the reference values to find the drag coefficient for the wetted surface area. 
Is there a way to only obtain the drag force of the wetted surface area to split the hydrodynamic and aerodynamic drag?
I trie splitting the object into two sections. However, this doesn't work as you subtract both in the Fluid domain, which makes it one structure again.