It is well known that a floating body drifts in the direction of wave propagation. This drift motion is caused by the 2nd -order steady horizontal force (or so-called mean drift force or ). I saw many calculated results of mean drift force by numerical models or measured results from physical experiments, but I did not know how the results are given.
If they are obtained from averaging the maximum positive horizontal wave forces and the minimum negative horizontal wave forces (in other words, calculating the mean horizontal wave forces)?
Thank you very much.
Typically you just take the average of the horizontal load time signal. You should however bear in mind that you should take a sufficiently long time series so that the steady drift result has converged to a fixed value. You should also make sure you exclude from the the time series the transient part of the series and use only the part where a steady state has been achieved. This may be tricky since by the time the steady state has been settled there are usually parasitic wave energy coming back to the model location (this depends of course on your model set up and on the ability to disperse the incident wave energy on the end of your wave tank or wave flume). Second order loads are very small when compared to the linear loads, so may have problems calibrating your measuring equipment to capture both loads, specially in large (wave length/ body characteristic length) ratios where the horizontal steady drift load is smaller.
Make sure you record for some time the zero wave signal to set a zero for your horizontal load.
You should also consider that there may also be some contribution from viscous effects to the steady drift loads, which are in fact a third order effect. If you are considering just the potential flow model you may see some discrepancy between numerical and experimental results on objects with sharp corners, for instance.
Typically you just take the average of the horizontal load time signal. You should however bear in mind that you should take a sufficiently long time series so that the steady drift result has converged to a fixed value. You should also make sure you exclude from the the time series the transient part of the series and use only the part where a steady state has been achieved. This may be tricky since by the time the steady state has been settled there are usually parasitic wave energy coming back to the model location (this depends of course on your model set up and on the ability to disperse the incident wave energy on the end of your wave tank or wave flume). Second order loads are very small when compared to the linear loads, so may have problems calibrating your measuring equipment to capture both loads, specially in large (wave length/ body characteristic length) ratios where the horizontal steady drift load is smaller.
Make sure you record for some time the zero wave signal to set a zero for your horizontal load.
You should also consider that there may also be some contribution from viscous effects to the steady drift loads, which are in fact a third order effect. If you are considering just the potential flow model you may see some discrepancy between numerical and experimental results on objects with sharp corners, for instance.
Thanks a lot Pessoa.
Just as you said, I found it necessary to absorb the wave energy propagating to the end of the wave tank and to prevent the multiple reflections of waves between the floating body and wave maker.
It is a strict test of the wave tank both in experimental measurements or in the numerical calculations.
the procedure is quite similar to the engine indicator diagram. In this case take readings of the fluctuating forces, say over a period , then measure the area under the curve , then divide the area by the time base and you will come up with mean force
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