# Linear wave force on a cylinder

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January 13, 2013, 18:24
Linear wave force on a cylinder
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Tim
Join Date: Jan 2013
Location: Germany
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Hello,
I am new to ANSYS CFX but I am familiar with the linear wave theory.
I have to calculate wave forces on a based cylinder in a wave filed with ANSYS CFX 14.0 and validate the results with the Morison equation.
My problem is that I don't get a proper wave to calculate the forces.

My model is a transient simulation of 10 sec(timesteps 0.04 sec)
I used a laminar homogenous model (air and water have the same velocity field) with a free surface.
A Picture of the meshed model is attached (pre model.png).

My Inlet conditions are:
speed u : wavu*watinlvf+wavspd
v: 0
w: wavw*watinlvf
Expressions (extract):
watinlvf: step(1.0[m^-1]*(waveh-z))
both velocities from linear theory
wavspd: womeg/wavnum (more or less fixed value, depends on the wavelenght 5 m)

Initial conditions are:
u: wavspd
v:0
w:0
The Outlet is defined as an opening. The sides, bottom and the top are free slip walls with no velocity.
The cylinder is also defined as a not moving free slip wall.

Now, the main problem is to get rid of the flow velocity. You can see on the picture (bad results.png and bad results2.png) that the current influence the wave(form).
How can I get a model where only the wave reacts with the cylinder (with no wave speed at the Inlet)?
I tried to omit wavspd at Inlet and/or initial condition but then the wave won’t stay linear or so.....

Is it possible to show to the particle path or just with dispersed fluid? I used continuous fluid model.
My numerical results differ from my results with Morisons equation from round about 800 N (cylinder diameter 0.5m, submerged height 6m) after I subtracted the force from the wave speed.
Is there someone who has done a similar work?
Attached Images
 pre model.jpg (41.2 KB, 42 views) bad results.jpg (33.7 KB, 46 views) bad results 2.jpg (43.3 KB, 39 views)

 Tags cylinder, force, linear, morison, wave

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