Low Cost Wave Tank

Wave tank time costs money. The Berkeley Wave Tank (cerca 1930's), which is basically a long narrow tank designed for pull testing of ship hulls costs about $1500/day. A more modern wave tank costs $10,000/day. At these expense rates, the question arises "can we build our own wave tank, get useful results and still achieve significant cost savings?" This article explores several low cost wave tank alternatives that are geared toward testing seastead desgins.

To date, four basic low cost concepts have emerged:

• Large Aquarium Tank. A large 125 gallon aquarium tank is long and narrow and might be suitable for some wave tank testing.
• Swimming Pool. A swimming pool of the right shape and depth might be adaptable to wave tank usage.
• Dig a Ditch. A ditch can be dug in the ground and lined with plastic.
• Water Trough. An above ground trough is filled with water.

These four designs are explored futher below.

Requirements

A wave tank requires the following:

• Rectangular tank
• Wave generator
• Test cell.
• Wave absorber.
• Data recording section.

Rectangular Tank

A rectangular tank is optimized for analyzing waves in one dimension. Waves are generated at one end, propagate down the tank and are absorbed at the other end. A square tank allows for the generation of waves in 2 dimensions, but probably costs too much.

Wave Generator

A sophisticated wave generator is capable of producing just about and wave form imaginable. For a low cost wave tank, being able to generate sinusoidal waves with a known period and amplitude is probably sufficient. The wave generator is basically a paddle attached to the bottom of the tank 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 depends upon motor speed.

Having said all of that, an alternative to a motor and crank is to simply have a human grab the paddle and generate waves as by hand. The amplitude and period may not be perfect, but they may be good enough to get the job done.

Test Cell

The test cell is the location where the model is inserted. This is where any visual observation window is placed. Any test equipment is placed here as well.

Wave Absorber

When the wave hits the other end of the tank, it has a tendency to be reflected back. The back reflections can mess up the test run. To avoid the back reflections, a shallow ramp is placed at the end of the wave tank to break the waves. The shallow ramp typically has some "bumps" in it to help break the waves up more.

Data Recording Section

The purpose of a wave tank is to measure the behavior a model is placed in the test cell. While looking at the model behavior provides a subjective understanding, collecting quantitative data provides a more object understanding of what is going on.

All of the low cost designs under consideration use the same basic data collection system. A low cost color video camera is set up to record a video stream. Three LED's (red, green and blue) are attached to the top of the model under test and illuminated. The video camera is arranged to have a split view via some mirrors. The split views provide a stereoscopic view of the model under test. The left and right views are both recorded in the camera frame so that there is no issue with frame synchronization. Software digitizes each frame to identify the X/Y location of each LED in each view. Stereoscopic math is used to identify the model position and orientation for each frame. Using adjacent frames it is possible to compute velocities and accelerations.

Aquarium Tank

{Discuss aquarium tank}

Swimming Pool

{Discuss Swimming Pool here}

Dig a Ditch

{Discuss a ditch}

Water Trough

{Discuss a water trough}