SEAVAX - WAVE MAKING MACHINE

WATER TANK TESTING FACILITIES FOR SEAVAX and SEANET

As part of our SeaVax testing program we will be creating artificially high seas and force 10 hurricane conditions in our water testing tank. Waves may be created quite simply using any device to generate a surge. A wave tank is a laboratory setup for observing the behavior of surface waves. The typical wave tank is a box filled with liquid, usually water, leaving open or air-filled space on top. At one end of the tank an actuator generates waves; the other end usually has a wave-absorbing surface. A similar device is the ripple tank, which is flat and shallow and used for observing patterns of surface waves from above.

One of the reasons for building our own facility is the significant costs of hiring academic facilities. Another reason is convenience, but the main reason is the ability to tailor our experiments to ocean cleaning and plastic recovery - and the special considerations for a ship powered by energy from nature.

AUTONAUT - This fabulous little boat is driven by waves, here shown being tested in QinetiQ's water basin in modest waves, one of the largest and most expensive testing facilities in Europe.

ECONOMICS 

The Berkeley Wave Tank (circa 1930's), which is basically a long narrow tank designed for pull testing of ship hulls costs about $1500/day to hire. A more modern wave tank costs around $10,000/day. As we will be conducting tests on our robot vessel routinely we could be looking at $300,000 dollars for 30 days of testing in 2016 - if we were to go the hire route. Our DIY tank is costing a little more than at first estimated, but not by much in the grand scheme of things.

The question many will be asking is: "can we build our own wave tank economically and still get useful results?" Our consultant engineer explained to us that science is about observation and observation absorbs time. Measurement is important, but humans are particularly good at noticing unusual phenomena and effects, learning from experimenting (playing) and then applying such discoveries. If time costs $10k a day, observation goes out the window, because it would simply cost too much to waste on observation.

We are crowd funded and this project is not-for-profit, so we have not included a lot of the skilled labour it is taking to build our tank, with a present estimate of cost being: £9,782 ($13,000). See our development timetable for details.

GENERATING A WAVE

A sophisticated wave generator is capable of producing just about any wave form imaginable. For a low cost wave tank, being able to generate sinusoidal waves with a known period and amplitude is sufficient for our purposes. Our wave generator is basically a paddle attached to the bottom of the sluice gate frame on a hinge. A motor attached to a crank can be used to generate waves. The larger the crank radius, the higher the wave amplitude. The wave period will depend upon motor speed.

Having said all that, an alternative to a motor and crank is to simply have a person use a lever attached to the paddle and generate waves by hand power establishing a rhythm to the second hand on a clock. The amplitude and period may not be as regular, but may be good enough to get the job done. 

WAVE TABLE - The speed of all ocean waves is controlled by gravity, wavelength, and water depth. Most characteristics of ocean waves depend on the relationship between their wavelength and water depth. Wavelength determines the size of the orbits of water molecules within a wave, but water depth determines the shape of the orbits. The paths of water molecules in a wind wave are circular only when the wave is traveling in deep water. A wave cannot "feel" the bottom when it moves through water deeper than half its wavelength because too little wave energy is contained in the small circles below that depth. Waves moving through water deeper than half their wavelength are known as deep-water waves. On the other hand, the orbits of water molecules in waves moving through shallow water are flattened by the proximity of the sea surface bottom. Waves in water shallower than 1/20 their original wavelength are known as shallow-water waves. Transitional waves travel through water deeper than 1/20 their original wavelength but shallower than half their original wavelength. In general, the longer the wavelength, the faster the wave energy will move through the water.

WIND WAVES

In fluid dynamics, wind waves, or wind-generated waves, are surface waves that occur on the free surface of oceans, seas, lakes, rivers, and canals or even on small puddles and ponds. They result from the wind blowing over an area of fluid surface. Waves in the oceans can travel thousands of miles before reaching land. Wind waves range in size from small ripples, to waves over 100 ft (30 m) high.

When directly generated and affected by local winds, a wind wave system is called a wind sea. After the wind ceases to blow, wind waves are called swells. More generally, a swell consists of wind-generated waves that are not significantly affected by the local wind at that time. They have been generated elsewhere or some time ago. Wind waves in the ocean are called ocean surface waves.

Wind waves have a certain amount of randomness: subsequent waves differ in height, duration, and shape with limited predictability. They can be described as a stochastic process, in combination with the physics governing their generation, growth, propagation and decay—as well as governing the interdependence between flow quantities such as: the water surface movements, flow velocities and water pressure. The key statistics of wind waves (both seas and swells) in evolving sea states can be predicted with wind wave models.

WAVE TABLE - Conditions Necessary for a Fully Developed Sea at Given Wind Speeds, and the Parameters of the Resulting Waves. (NOTE: Most of the wave speeds calculated from the wave length divided by the period are proportional to sqrt (length). Thus, except for the shortest wave length, the waves follow deep water theory. The 28 ft long wave must be either in shallow water or between deep and shallow.)

TYPES OF WAVE

Three different types of wind waves develop over time:

1. Capillary waves, or ripples
2. Seas
3. Swells

Ripples appear on smooth water when the wind blows, but will die quickly if the wind stops. The restoring force that allows them to propagate is surface tension. Sea waves are larger-scale, often irregular motions that form under sustained winds. These waves tend to last much longer, even after the wind has died, and the restoring force that allows them to propagate is gravity. As waves propagate away from their area of origin, they naturally separate into groups of common direction and wavelength. The sets of waves formed in this way are known as swells.

Individual "rogue waves" (also called "freak waves", "monster waves", "killer waves", and "king waves") much higher than the other waves in the sea state can occur. In the case of the Draupner wave, its 25 m (82 ft) height was 2.2 times the significant wave height. Such waves are distinct from tides, caused by the Moon and Sun's gravitational pull, tsunamis that are caused by underwater earthquakes or landslides, and waves generated by underwater explosions or the fall of meteorites—all having far longer wavelengths than wind waves.

WAVE RIDER - Large waves are generally hazardous to shipping, but great fun in the right circumstances for experienced surfers.

Yet, the largest ever recorded wind waves are common — not rogue — waves in extreme sea states. For example: 29.1 m (95 ft) high waves have been recorded on the RRS Discovery in a sea with 18.5 m (61 ft) significant wave height, so the highest wave is only 1.6 times the significant wave height. The biggest recorded by a buoy (as of 2011) was 32.3 m (106 ft) high during the 2007 typhoon Krosa near Taiwan.

Ocean waves can be classified based on: the disturbing force(s) that create(s) them; the extent to which the disturbing force(s) continue(s) to influence them after formation; the extent to which the restoring force(s) weaken(s) (or flatten) them; and their wavelength or period. Seismic Sea waves have a period of ~20 minutes, and speeds of 760 km/h (470 mph). Wind waves (deep-water waves) have a period of about 20 seconds.

DESCRIPTION - The red circles are the present positions of mass-less particles, moving with the flow velocity. The light-blue line gives the path of these particles, and the light-blue circles the particle position after each wave period. The white dots are fluid particles, also followed in time. In the case shown here, the mean Eulerian horizontal velocity below the wave trough is zero. Observe that the wave period, experienced by a fluid particle near the free surface, is different from the wave period at a fixed horizontal position (as indicated by the light-blue circles). This is due to the Doppler shift.

WAVE FLUMES

In fluid dynamics, a wave flume (or wave channel) is a laboratory facility for the physical modelling of water waves, in order to study their properties and their effects on coastal structures, offshore structures, sediment transport and other transport phenomena.

A wave flume is a special sort of wave tank: the width of the flume is much less than its length. The generated waves are therefore – more or less – two-dimensional in a vertical plane (2DV), meaning that the orbital flow velocity component in the direction perpendicular to the flume side wall is much smaller than the other two components of the three-dimensional velocity vector. This makes a wave flume a well-suited facility to study near-2DV structures, like cross-sections of a breakwater. Also (3D) constructions providing little blockage to the flow may be tested, e.g. measuring wave forces on vertical cylinders with a diameter much less than the flume width.

The waves are most often generated with a mechanical wavemaker, although there are also wind–wave flumes with (additional) wave generation by an air flow over the water – with the flume closed above by a roof above the free surface. The wavemaker frequently consists of a translating or rotating rigid wave board. Modern wavemakers are computer controlled, and can generate besides periodic waves also random waves, solitary waves, wave groups or even tsunami-like wave motion. The wavemaker is at one end of the wave flume, and at the other end is the construction being tested, or a wave absorber (a beach or special wave absorbing constructions).

Often, the side walls contain glass windows, or are completely made of glass, allowing for a clear visual observation of the experiment, and the easy deployment of optical instruments (e.g. by Laser Doppler velocimetry or particle image velocimetry).

 

 

RECIRCULATING WAVE - Without re-modeling the steps that were in the lower left hand corner of this diagram, we would not have been able to install the turning vanes to be able to use the tank in recirculating mode. In this mode we can measure the drag of a hull, or any other object, even those submerged, such as a submarine - and for a fraction of the cost of other facilities.

 

LINKS & REFERENCE

https://en.wikipedia.org/wiki/Wind_wave

https://avaaz.org/

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