Hydrodynamic forces on bridge structures

mbn1454 · January 29, 2012, 04:38

I am trying to calculate hydrodynamic forces on a bridge structure subject to waves.
My results are ok for bigger waves but for small waves I am over predicting significantly (the vertical force). I can see that air gets trapped under the bridge.
I am using mesh size in vicinity of bridge and free surface of order of 2cm and time step size of order of dt=0.004s. The simulation under predicts big wave forces and grossly over predicts small wave forces. It also shows a lot of oscillation. I am using number of internal iteration up to 200. I have modeled air as compressible. Can you please let me know what I can do to improve results specially for small wave amplitudes?

Josh · January 31, 2012, 18:24

We need WAY more details of your problem. Your mesh and timestep size mean nothing without a domain size.

ping · February 13, 2012, 02:30

cant see any reason to not use constant density for both fluids.
would expect that 10-20 inner iterations are enough, but keep an eye on a range of monitors eg forces at a few locations plotted version iteration rather than timestep - force convergence dependent on timestep and inner iterations - more of one means less of the other in simplistic terms.
would get it going using timesteps of around 0.01s, then test smaller ones.
need about 20 cells vertically to capture a wave and about 40 per wave length.
sometimes need second order time to retain wave height.

January 29, 2012, 04:38	Hydrodynamic forces on bridge structures	#1
mbn1454 New Member Mehrdad Join Date: Apr 2009 Posts: 11 Rep Power: 17	I am trying to calculate hydrodynamic forces on a bridge structure subject to waves. My results are ok for bigger waves but for small waves I am over predicting significantly (the vertical force). I can see that air gets trapped under the bridge. I am using mesh size in vicinity of bridge and free surface of order of 2cm and time step size of order of dt=0.004s. The simulation under predicts big wave forces and grossly over predicts small wave forces. It also shows a lot of oscillation. I am using number of internal iteration up to 200. I have modeled air as compressible. Can you please let me know what I can do to improve results specially for small wave amplitudes?

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January 31, 2012, 18:24		#2
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 17	We need WAY more details of your problem. Your mesh and timestep size mean nothing without a domain size.

February 13, 2012, 02:30		#3
ping Senior Member Ping Join Date: Mar 2009 Posts: 556 Rep Power: 20	cant see any reason to not use constant density for both fluids. would expect that 10-20 inner iterations are enough, but keep an eye on a range of monitors eg forces at a few locations plotted version iteration rather than timestep - force convergence dependent on timestep and inner iterations - more of one means less of the other in simplistic terms. would get it going using timesteps of around 0.01s, then test smaller ones. need about 20 cells vertically to capture a wave and about 40 per wave length. sometimes need second order time to retain wave height.