Absorbing beaches
Absorbing the wave after it has passed the model is extremely important if testing is to be repeatable. Edinburgh Designs have designed and installed many beaches and can design a beach for any testing requirement.
There are a wide variety of beach designs and the best summary is given in a paper “A survey of wave absorbers”[1]. This survey assessed the performance of about 48 wave absorbers and a revue of research papers. One factor that is common to many of the sloping beach designs is some form of innovative porosity mechanism, usually to channel the water flow caused by the wave advancing up the beach to be transferred back without affecting the wave. Similarly surface roughness is often used with the intention of tripping the wave over. The significant conclusions of the survey report are:-
- A reflection of up to 10% is to be expected even for well designed beaches and that the % reflection tends to increase with reduced wave height.
- It does not appear possible to attain reflection coefficients below 10% for absorbers shorter than 0.5 to 0.75 of a wavelength
- A porosity of 70% in one case was shown to decease reflection coefficient by 2%
- Most beaches surveyed have a steepness of between 1:6 and 1:10 at the waterline
Absorption, especially in a wide tank, is surprisingly difficult to define. It is dependent on amplitude, angle, and frequency. Many of the mechanisms that ultimately dissipate the energy rely on Reynolds number so similar beaches will have different characteristics as the scale is altered. Another difficult with beaches is that they appear, in a tank, to be less effective than they are. A wave reflected from a beach that absorbs 90% of the energy will be 31% the height of the original wave.
Sloping beaches do not have to run the full depth of the tank and can be sloped at up to 30 degrees without degrading performance. A typical wave tank beach will have a steep underwater section with a curved transition to a very gentle 6 degree slope meeting the waterline. Very little structure is required above the waterline as by this stage the wave has broken and it’s energy is dissipated. It is useful to allow water to run over the end of the beach so it does not cause back waves by surging back down the slope. Ripples caused by the wave breaking can affect smaller tanks. These can be reduced by covering the surface with an absorbent layer of foam or mesh material.
The loads on a beach can be high and it is particularly important to design for the up-thrust which can be just as high as the down-thrust. Beaches are also subjected to fully reversing cyclic loading so can fail in fatigue rather than by direct loading. It is very important to consider the mounting points where the entire structural load is transferred to the body of the tank.
Sloping beaches do not work so well in variable depth tanks. An alternative is to use mesh filled wedges. Multiple layers of plastic mesh dissipate the waves as they flow past and create eddies on the millions of sharp edges and ideally present the same impedance to waves as would unobstructed water in an infinitely long tank. The flow velocity varies for different waves so the foam density should increase progressively with depth and with distance down wave. There is a full description of the method of construction in “The Edinburgh Curved tank”[2].
References:
- Ouslett & I Datta A survey of wave absorbers, Journal of Hydraulic Research vol 24 p265 -279 [1986]
- Taylor J., Rea M., Rogers. D., The Edinburgh Curved Tank , Fifth Wave Conference, HRML Cork Sept 17-20 [2003]