HI
I am looking to model hydrodynamic forces in a slender cofferdam dynamic analysis.
Rather than use an added mass approach, I was considering running the model and extracting the +/- xdir velocities at various points in the structure. I can then use this to calculate the additional or reduced hydrodynamic forces at each time step (using the calculated force per time step as the load multiplier) such that the hydrodynamic effect of the structure moving into a body of water is captured by an increased opposing pressure acting as resistance (damping the system), and moving away from the water body results in a reduced pressure.
Has anybody tried this, or can suggest improvements to this proposal? Are there any obvious pitfalls that I am not seeing. (note, this is purely for academic purposes, not a real-world problem).
Thanks
Scot
Hi Scot,
You can find some background here for pseudo-static and full dynamic calculations dealing with water: https://www.plaxis.com/support/tips-and-tricks/how-does-plaxis-deal-with-water-pressures-during-dynamic-calculations/
As for your question, if you want to add the hydrodynamic pressure as a time-dependent input, you could try to use dynamic loads: define a certain load profile combined with the dynamic load multipliers to define the load as a function of time.
Many thanks Micha,
You are correct, I am looking to model the dynamic effects of the water. I had previously considered the cluster approach as defined in your link (very small Su and equivalent 2.2GPa Bulk modulus). I rejected this some time ago as I was experiencing difficulties with generating accurate pore pressures below bed level (as the water column had disappeared above 'sea bed level'). I will revisit to see if it can be made to work.
I think this tip / trick is quite old - Is it equally valid in Plaxis 2019? Are there more accurate UDSM's to model water clusters available?
Dear Scott,
Although the article is indeed not so recent, the methodology is still valid, although maybe not optimal. We do get more requests on this kind of behaviour to add this water inertia effect into the dynamics calculations (e.g. Westergaard, Add mass techniques or a special user-defined soil model to model the water inertia). We are looking into this, but I have no clear timeline yet if and when this will be available.
Kind regards,
Micha
Thanks Micha, We have undertaken some work on these hydrodynamic effects and would routinely make use of Westergaard and WAM (as very dense soil clusters at an appropriate place in the geometry) for large port design, but I still feel the tendency is to over design.
I look forward to reading about progress on a water inertia model. I would be happy to trial these as and when they may become available in a beta test capacity.
Regards
Not sure if this is currently still relevant for you w.r.t. your academic interests, but in the PLAXIS CONNECT Edition Update 1 release we have added a technology preview (i.e. roughly similar to a beta) for Westergaard added mass.
We have also published a simple case study explaining how to use it in PLAXIS here. In case you are interested to have a look at it, we would be interested to receive your feedback.
Cheers,
Vincent