GB2232934A

GB2232934A - An artificial beach wave absorber

Info

Publication number: GB2232934A
Application number: GB8913526A
Authority: GB
Inventors: D K Fryer
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1989-06-13
Filing date: 1989-06-13
Publication date: 1991-01-02
Anticipated expiration: 2009-06-13
Also published as: GB8913526D0; GB2232934B

Abstract

A tunable wave absorber (beach) particularly for use in model ship tanks for siting at the end of a testing tank remote from the wave generator comprises three vertically slatted screens which are adjustably placed so as to act as a tunable absorber. The generator is controlled such that low frequency parts of the frequency spectrum are started first and the resonant absorber is tuned to absorb these frequencies since these long wavelength waves travel quickest and after reflection interfere with the outgoing waves from the generator and hence with the model. The first screen is placed at about 1 DIVIDED 2 wavelength in front of the tank end and has c 25% transparency, the other two screens are 15% transparent and are located at 1 DIVIDED 4 and 1 DIVIDED 6 wavelength respectively in front of the end. The beach also has application to fixed sites such as harbours, marinas, shore protection, swimming pools etc.

Description

AAn Artificial heach Wave Absorber The e invention relates to beaches for absorbing waves in a body of water and is particularly, though not exclusively related to a beach for use in a model ship tank to prevent reflection of waves from the end of the tank.

Studies of seakeeping experiments have highlighted the importance of beaches which are generally installed at the opposite end of ship test tanks to wavemaking machinery. In general, tests are started when generated waves reach a model under test and end when reflections from the beach reach the model from the opposite direction.

One of the constraints on the design of a ship tank beach is that part of it must be removable so that models can be rigged on to the towing carriage in a dock at the end of the tank, and then brought out through the position of the beach, which is then replaced prior to testing. Simple, passive, beaches are therefore preferred. In one form they consist of a sloping, slatted surface extending through the water surface, but not to the bottom of the tank. This is fairly easy to remove, and is often made from materials which have nearly neutral buoyancy so that it can be simply floated to one side as models are brought out of the dock. The design objective is to minimize the reflection coefficient over the full range of waveheights and periods.Variables which may be adjusted to achieve this include slat size and spacing, slope, length, and intercept with the still water level. Actual beaches, however, rarely seem to have been designed at all. They are often a collection of various surfaces and structures, both curved and plane, built up as a succession of 'ad hoc' improvements to what was already there.

In regular waves, low reflection coefficients are achieved by ensuring that waves break as they run up the beach. If the beach is steep, this may not happen because the wave does not travel a sufficient distance on the beach to break, and very high reflection coefficients result. If the waves are small, but the beach is long, they may surge on and off the beach without actually breaking. This results in higher reflection coefficients than if the wave breaks.

A further problem associated with ship test tanks arises because a spectrum of waves generated by the wavemaker is not the same as the spectrum at the model at any one instant of time. At any given frequency, a tank cannot be filled with waves of equal amplitude. When the wavemaker is started, various frequency canponents are produced. These interfere with each other and the components of differing wavelength travel at different velocities, complicating the interpretation of model results.

Waves are a source of erosion of river banks etc and there are also areas such as marinas where it is desirable to reduce or eliminate wave motions.

The e object of the invention is to provide an improved passive beach which could be used in model tanks or in docks or harbours etc to absorb wave motions or to protect vulnerable river banks, for example.

The invention provides a tunable resonance effect beach for reducing waves of wavelength L1 in a body of water comprising: a support means; a first porous screen held on the support means a distance of LV6 from one edge of the body of water; a second porous screen held on the support means a distanceL1/4 from the edge of the water; and a third porous screen held on the support means a distance LV2 form the edge of the water; the screens being supported vertically and transversely of the tank such that they are partly out of the water.

In a test tank, /\ is selected to be the wavelength corresponding to the longest wave in the spectrum of waves on the water, because the longest waves travel fastest and it is reflection of these waves which first interferes with the incident wave train.

Preferably the transverse porous screens are slatted.

Advantageously it has been found that the first and second screens have 15% porosity and the third screen has 25% porosity.

When applied to a test tank beach reflections of shorter wavelength waves can be made less significant by suitable timing of the introduction of each frequency component to the wavemaker. Thus, short period reflections can be delayed until after the test has been terminated due to the arrival of long wavelength reflected waves.

Thus in a preferred arrangement the invention provides in combination a tunable resonance effect beach and a wavemaker for a model ship test tank comprising: a wave-generating mechanism for location at one end of a ship test tank; and means to control the oscillation of the wave-generating mechanism; the control mechanism being arranged such that a pre-determined wave-train at the location of the ship model is generated by starting the faster travelling low frequency wave components before the slower travelling high frequency components and the resonance effect beach is arranged for location at the end of the tank remote fran the wavemaker and for absorption of the longest wave component.

The e transverse screens are preferably wooden with the slats arranged vertically. In an advantageous arrangement the transverse screens are moveable so as to provide a means to adjust the resonance frequency of the beach.

Longitudinal screens may also be provided so as to absorb transverse waves and provide additional structural support for the transverse screens.

Preferably two longitudinal screens are provided, each of 15% porosity, and located one quarter of the width of the tank from each respective tank side wall.

The e invention will now be described with reference to the accompanying Drawings of which: Figures 1 and 2 show a known sloping beach in side and end section for absorbing ship test tank waves; Figure 3 shows a graph of reflection coefficient against wavelength for the sloping beach; Figure 4 shows a plan view of a known 2-screen resonant beach; Figure 5 shows a graph of reflection coefficient against wavelength for the Figure 4 beach; Figure 6 is a graphical representation of the standing waves present at the end of the test tank; Figure 7 is similar to Figure 5 for the beach of the current invention; Figure 8 shows a 3-screen beach adapted for absorbing transverse as well as longitudinal waves in a ship test tank.

Conventional beaches in ship tanks and wave flumes have used sloping elements as shown in Figures 1 and 2. Waves 10, generated by awavemaker, travel along the ship tank 11 in the direction 12 towards the end wall 13 and are then reflected back towards the wavemaker. In order to experiment, for example, on the seakeeping performance of a ship, the behaviour of a scaled model in known ship tank wave conditions is recorded. Sea conditions can be simulated by driving the wavemaker by an oscillating input signal. It is not possible to start a wave train where all waves, including the first have the same frequency and amplitude. Thus there will be higher frequency wave components even when the wavemaker is driven in simple sinusoidal motion.

Thus there will be interference effects in the tank between the various frequency components and further interference effects once the waves 10 are reflected back from the end wall 13 of the tank remote from the wavemaker.

The e beach 14 is camnonly made of wooden slats 15. Incoming waves break on the beach 14 and the wave absorption is determined by the size and separation of the slats 15. This type of beach is most effective for the fairly short, steep, waves which occur at or near the peak of the incident spectrum, and very ineffective for the long, low waves which travel fastest.

As an example, measurements of the reflection coefficient, measured in a ship test tank having a sloping beach component, are shown in Figure 3. These results show that this beach is an effective absorber for both regular and random waves at frequencies above 0.5 Hz (corresponding to a wavelength of about 6 metres), but a very ineffective absorber at lower frequencies. It is also generally less effective as an absorber of random waves than of regular waves, and the regular wave reflection coefficient showed a strong dependence on wave amplitude. Because of these characteristics it would be necessary to scale this beach for use at lower frequencies. The e steepness of the sloping beach elements would need to be scaled in proportion of the ratio of the steepness of the long waves which it was required to absorb to the steepness of the short waves which the original beach was able to absorb effectively.

This would result in an unreasonably long, cumbersome structure. These results are typical of existing sloping beach structures.

The inventor was aware that it is the effect of faster travelling long wave components which invariably leads to the termination of ship testing in tanks. Making the assumption that it will be practically possible to ignore the effects of short wavelength waves it is then desirable to provide a beach of low reflection coefficient over a limited range of frequencies. This suggests a resonant structure to absorb the waves. If, in addition, the resonator could be simply tuned, the frequency of maximum absorption could be optimised for any given wave spectrum which was to be generated in the ship tank. A further benefit was also envisaged if a tuneable resonant absorber could be developed.The e absorber could be tuned to absorb the waves generated by a moving ship model in calmrwaterexperiments, thus reducing the waiting time between test runs - necessary to allow the surface to calm down after being disturbed.

Figure 4 shows a known resonantabsorberarrangement consisting of two porous screens 41,42 mounted transversely across the ship tank 43 at distances of Z\o/4 and lilo/8 from the reflecting end wall 44 where lilo is the wavelength at which maximum absorption is required. The inventor has appraised this known arrangement and has concluded that the optimum 2-screen resonance beach would require one screen 41 to be located art #o/2 and the second screen 42 at Llo/4.

Then at resonance, a standing wave would form with antinodes at the end wall 44 and theLlo/2 screen 41 and a node (where the vertical movement of the surface is a minimum) at the #o/4 screen 42. A low-porosity screen at the L oM position causes little reflection because the vertical movement is very small, but can absorb energy by friction as the water flows horizontally backwards and forwards through the screen. The e #o/2 screen was tested at the same porosity (15%) and at a higher porosity (25%) than theZ\o/4 screen, and it was concluded that the higher porosity produced better results - the bandwidth over which effective absorption occurred was slightly greater.

Steel trusses were mounted across rails on either side of the tank to support the tops of the two screens, the bottans of the screens being located by wedges hammered into the small gap between the screen and the tank floor. Figure 5 shows the reflection coefficient for the 2-screen beach. This shows a minimum at about 0.3 Hz - an improvement over the non-resonant beach (Figure 3) where the minimum was at about 0.6 Hz. At lower frequencies, the new beach was better at frequencies dbwn to almost 0.1 Hz; where the wavelength is so long compared with the depth of the tank, that the validity of any experiment designed to model deepwater conditions would be very questionable.At higher frequencies, the new beach is better up to about 0.45 Hz, above which there were signs of resonant reflection rather than absorption.

Interpretation of the results was complicated by the presence of depth changes within the resonator, but it was postulated that the performance was governed by the principles illustrated in Figure 6. The situation in (B) corresponds to the minimum reflection coefficient when the node of the standing wave set up in the resonator coincides with the inner (15%) screen 42i i.e. when the screens are at A lilo/4 and /\o/2 metres from the end of the tank. If this is the case, then the high reflection coefficients at very low frequencies correspond with situations where the node is at or beyond the 25% screen 41 as shown in (A).It follows that the resonant reflection which occurred at about 0.5 Hz must correspond with the situation shown in (C), where conditions at the end wall and the two screens are very similar to the conditions in (A), although conditions are not similar elsewhere in the resonator.

Since the resonant absorption was so pronounced in (B) when the node was located at the position of the 15% screen, the placing of a second 15% screen at the position of the node in (C) was examined. A 3-screen absorber was tested with the screens placed at distance ratios of 1:1/2:1/3 from the end wall of the ship tank.

The results obtained with this configuration are shown in Figure 7. The resonant reflection at 0.5 Hz was markedly reduced but also (somewhat surprisingly) the minimum reflection coefficient was even lower than for the 2-screen configuration.

One disadvantage of the test absorber was that, because all parts of the structure were vertical and mounted transversely across the tank, it provided little suppression of energy which leaked into transverse modes of oscillation of the water surface. Unfortunately, the structure also assisted the leakage of energy fran longitudinal to transverse modes by flexing under the oscillatory loads which were imposed on it. During the tests, it was found that the only transverse mode which actually become a problem was at 0.5 Hz where the wavelength equalled the width of the tank.

The oscillation was set up with antinodes at the sides and centre of the tank and nodes one-quarter of the tank width fran each side.

The e success of the 15% screens placed transversely at nodes in the longitudinal standing waves suggests that similar screens should be placed longitudinally at nodes in the transverse standing waves. If, in addition, the longitudinally placed screens were used to support the transverse ones, a strong, stiff cage-like structure would be produced. The e greater stiffness would result in less flexure and hence lower leakage of energy into the transverse modes, and the longitudinal screens would absorb most of the leakage which took place.

The longitudinal screens would need to be at least half the longest wavelength ever generated in the ship tank to accortite the full range of adjustment of the transverse screens.

The resonant absorber could in principle be set up to absorb waves generated by moving models in calm water (i.e. the ship's surface wake). In this case, the screen separation would need to be adjusted for maximum absorption of waves having the same celerity as the ship model's forward speed. Because speed changes occur frequently in this type of test programmer adjustment of the beach needs to be easy and quick.

If the absorber is installed at the opposite end of the tank from the wavemaker, then it would be necessary to conduct calmrwater experiments with the model running towards the beach. This would minimise the time necessary for the water surface to settle between test runs, but is opposite to current practice and would require modifications to the carriage speed control and braking systems.

However, the transverse screens need to be removable to allow access to the docks, so there is no reason why absorbers should not be mounted at both ends of the tank. For seakeeping experiments, the transverse screens would the be removed from the absorber nearest the wavemaker.

Figure 8 shcrws the resonant absorber of the invention adapted for use in a ship test tank. The e tank 81 has three docks 82 -84 closed by dock gates 85 during ship testing. Longitudinal wave motion is absorbed by a 3-screen resonant absorber. A 25% porosity screen 86 is located at a distance L1 from the dock gates 85 and 15% porosity screens 87 and 88 are located at distances L2 and L3 from the dock gates 85. The e distances L1 - L3 are set in the ratios 1:1/2:1/3 from the end wall (gates 85) of the tank where L1 = /\o/2. Transverse waves are absorbed by providing longitudinal screens 89, 810, each 15% porosity, extending 8m from the end wall of the tank.The e screens 89, 810 are placed l.9m from the side walls of the tank which is 6.2m wide. Preferably the longitudinal screens would be placed at a quarter of the tank width from the side wall of the tank (i.e. 1.55m) however this would have caused some obstruction to the docks. For access to the docks the transverse screens 8688 are removable as well as being adjustable longitudinally so as to adjust the resonant frequency. The e arrangement selected allowed L1 to be varied within the range im - 8m (maximum permitted by the length of the longitudinal screens) while maintaining the transverse screen position ratios as above.

The e design has met the requirements necessary for accurate seakeeping experiments: viz low reflection coefficient for longperiod waves.

Furthermore, the absorber is tuneable so that the wave period corresponding to minimum reflection coefficient can be optimised for tests in different seastates.

The design provides access to the end docks of the ship tank.

It can also be used to minimise the time necessary between ship test runs in calm water, by tuning the absorber to selectively absorb the waves generated by the moving model.

Although described with reference to ship test tanks, the 3-screen resonant beach has application to fixed sites such as harbours, marinas, shore protection, swimning pools etc.

Claims

1. A tunable resonance effect beach for reducing waves of wavelength /\ in a body of water comprising: a support means; a first porous screen held on the support means a distance of LV6 from one edge of the body of water; a second porous screen held on the support means a distanceLV4 from the edge of the water; and a third porous screen held on the support means a distance LV2 form the edge of the water; the screens being supported vertically and transversely of the tank such that they are partly out of the water.

2. A tunable resonance effect beach as claimed in claim 1 wherein the transverse porous screens are slatted.

3. A tunable resonance effect beach as claimed in claim 1 or 2 wherein the first and second screens have 15% porosity and the third screen has 25% porosity.

4. A tunable resonance effect beach as claimed in any one preceding claim wherein the transverse screens are wooden.

5. A tunable resonance effect beach as claimed in any one preceding claim wherein the slats are arranged vertically.

6. A tunable resonance effect beach as claimed in any one preceding claim wherein the transverse screens are moveable so as to provide a means to adjust the resonance frequency of the beach.

7. A tunable resonance effect beach as claimed in any one preceding claim for use in a model ship test tank wherein /\ is selected to be the wavelength corresponding to the longest wave in the spectrum of waves on the water.

8. A ship test tank wavemaking apparatus including a tunable resonance effect beach as claimed in claim 7 and a wavemaker wherein reflections of shorter wavelength waves are made less significant by suitable timing of the introduction of each frequency component to the wavemaker.

9. A ship test tank wavemaking apparatus including a tunable resonance effect beach and a wavemaker as claimed in claim 8 wherein: the wave-generating mechanism is located at one end of the ship test tank; there is provided means to control the oscillation of the wave-generating mechanism, the control mechanism being arranged such that a pre-determined wave-train at the location of the ship model is generated by starting the faster travelling low frequency wave components before the slower travelling high frequency components; and the resonance effect beach is arranged for location at the end of the tank remote from the wavemaker and for absorption of the longest wave component.

10. A ship test tank wavemaking apparatus as claimed in claim 8 or 9 wherein longitudinal screens are provided to absorb transverse waves.

11. A ship test tank wavemaking apparatus as claimed in claim 10 wherein two longitudinal screens are provided, each of 15% porosity, and located one quarter of the width of the tank fran each respective tank side wall.

11. A ship test tank wavemaking apparatus as claimed in claim 10 wherein two longitudinal screens are provided, each of 15% porosity, and located one quarter of the width of the tank from each respective tank side wall.

Claims Amendments to the claims have been filed as follows 1. A tunable resonance effect beach for reducing waves of wavelength in a body of water composing: a support means; a first porous screen held on the support means a distance of Z y 6 fran one edge of the body of water; a second porous screen held on the support means a distanceZZJ4 from the edge of the water; and a third porous screen held on the support means a distance /V2 form the edge of the water; the screens being supported vertically and transversely of the wave direction such that they are partly out of the water.

7. A tunable resonance effect beach as claimed in any one preceding claim for use in a model ship test tank wherein iN is selected to be the wavelength corresponding to the longest wave in the spectrum of waves on the water.

8. A ship test tank wavemaking apparatus including a tunable resonance effect beach as claimed in claim 7 and a wavemaker wherein reflections of shorter wavelength waves are made less significant by suitable timing of the introduction of each frequency component to the wavenaker.

9. A ship test tank wavemaking apparatus including a tunable resonance effect beach and a wavemaker as claimed in claim 8 wherein: the wave-generating mechanism is located at one end of the ship test tank; there is provided means to control the oscillation of the wave-generating mechanism, the control mechanism being arranged such that a pre-determined wave-train at the location of the ship model is generated by starting the faster travelling low frequency wave components before the slower travelling high frequency components; and the resonance effect beach is arranged for location at the end of the tank remote fran the wavemaker and for absorption of the longest wave component.