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.
A wave tank requires the following:
- Rectangular tank
- Wave generator
- Test cell.
- Wave absorber.
- Data recording section.
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.
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 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 or leg power. The amplitude and period may not be perfect, but they may be good enough to get the job done.
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.
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.
The idea of using a split vision system comes from some robotics vision work presented by Ingolf Sander at the Homebrew Robotics Club on January 26, 2005. Additional information can be found at Ingolf's Robotics Web Page.
An aquarium tank can be pressed into duty for a wave tank. Petco sells aquariums and one of them is a reasonable choice for a wave tank. The rectangular 125 gallon 72 inch × 18 inch × 22 inch, might be usable as a wave tank. Petco sells the tank for $420 (as of September 2008.) This tank weighs 1250 pounds when completely filled with water. However, the wave tank will not fill the tank to capacity.
The clear sides make it very easy to see what is going on inside the tank. The 72 inch length provides just enough space for a wave absorber and a wave generator.
Many people own swimming pools. While a swimming pool is not an optimal shape, they tend to be larger than an aquarium.
Most people do not want anything scraping up the inside of their swimming pool. Thus, extra care is required to design a wave generator and wave absorber that can not scuff the pool surface.
In order to get straight wave propagation, it is necessary to have two sides to guide the waves down. This can be with some long pieces of plywood that span the length of the pool (carefully built to avoid scratching anything.) Some clear plastic can be placed near the test cell so that cameras can see what is happening to the model under water. Either an underwater periscope or an under water camera can be used to view what is happening under the water.
Dig a Ditch
You can rent equipment for a day that will let you dig a good sized ditch in dirt for a reasonable amount of money. You can also hire someone with a backhoe. Hard and rocky ground could be difficult but good dirt is easy for equipment to dig quickly. Digging the ditch so that one end slopes up to make the wave absorber is easy. Lining this with something to make it waterproof is easy and cheap. Since the ditch is down in the ground draining it when not in use would require a pump. You could also leave it full and chlorinate just like a pool. Would probably want to build up a bump of dirt around the ditch so that rain water did not wash other stuff into the ditch. Do this before putting down the liner. You could almost have part of your ditch tank above ground level. However, you want straight sides on your wave tank so the waves propagate well, which is not going to work with the fresh dirt above ground level.
One minor disadvantage of the ditch is that it is not easy to put a clear side on the ditch for viewing what is going on under water. Instead of doing that, it is possible to use a water periscope to obtain a view of what is happening below the water surface.
Using housing studs and plywood, it is possible to build a water trough of pretty much any reasonable dimension. For example, 20 feet × 3 feet × 3 feet is quite possible. The plywood does not have to be much thicker than 3/8 inch or 1/2 inch. The studs would basically be assembled into 3 foot × 3 foot squares with plywood lining three sides of the interior of each square. One or two of the plywood sides could be replaced with either a clear piece of plastic or glass (whichever costs less.) The wood would not be water proof, but an inexpensive clear plastic sheet purchased to line the trough and make it water proof. The water trough is probably significantly cheaper and larger than the large aquarium solution.