Commentary on Dam-Atoll made by John Isaacsat the First Symposium on Wave Energy Utilization, held at Gothenburgh, Sweden.
John Isaac On Dam-Atoll
Commentary on Dam-Atoll made by in a talk given by Professor John Isaac, then Director of Scripps Institution of Oceanography.
Note: What follow are comments about Dam-Atoll made by John Isaac at the First Symposium on Wave Energy Utilization, held at Gothenburg, Sweden, in 1979. The full article can be found in Karl-Gustav Jansson, and Johannes K. Lunde, and Thomas Rindby, eds: FIRST SYMPOSIUM ON WAVE ENERGY UTILIZATION 30 OCTOBER-1 NOVEMBER 1979: PROCEEDINGS (Gothenburg, Sweden, Chalmers University of Technology, 1980, pp. 204-221). These extracts from the article comply with the standards of Fair Use.
At the time, Professor Isaac was Director of Scripps Institution of Oceanography. He died unexpectedly shortly after giving this talk in Sweden.
In the following passage, taken from the article's abstract, John Isaac mentions Arthur's Island. "Arthur's Island" refers to the precise shape (a particular dome-shape) of a submerged artificial atoll that is optimal for converting wave energy to mechanical work. The name of this shape stems from Professor Isaac himself, who dubbed it "Arthur's Point Island" in honor of Robert S. Arthur, his colleague for many years at Scripps Institute. It was Professor Arthur who first explored using a geometical optics method the mathematical properties of wave refraction around circular atolls.
Professor Isaac also refers in the following passage to the "sequential concentration of wave power through both linear and nonlinear processes." A linear process is describable by a simple equation. By contrast, a nonlinear process requires complex mathematical monkey motion to come up with even an approximation of the process.
(From the Abstract of the Article) ....The second [device discussed] is the ... "Dam-Atoll" device. This is a bottom-founded system suitable for nearshore shallow waters. Waves are focussed on the center of a large dome configured after "Arthur's Island." At or near their breaking point the waves are directed tangentially into a vertical well in the structure. This "fluid flywheel" then acts through a turbine located at the bottom of the well. The advantages of this approach include: a considerable sequential concentration of wave power through both linear and nonlinear processes; omnidirectional effectiveness; rather broad spectral response; constant unidirectional output; and excellent efficiency (ca. 50 percent).
In the next excerpt, taken from the body of his article, Isaac describes how the wave, as it progresses into the very shallow water submerging the artificial atoll, is governed by nonlinear processes. This is the region where the wave will crest and break. It is in this region that the guide vanes take over.
(next)(From the Body of the Article) ... The next of the systems that I want to discuss is the "Dam-Atoll", conceived and designed by ... engineer Les Wirt. The system employs a bottom-founded, lenticular dome of revolution, probably in prototype a prestressed concrete shell, designed on the principle of Arthur's Island, which has the following properties: Waves of the design frequency and infinitesimal amplitude are totally focused at the center. Finite waves, of course, break at some depth on the dome in a roughly circular zone about the center. Crests of the design-wave frequency arrive continuously at the breaking point at some point in the circle. Waves of somewhat shorter or longer wave lengths are also quite well focused. The performance of the island is unaffected by direction of the waves. The next step in this system for the sequential concentration of wave energy is affected by a series of converging radial vanes commencing at a somewhat greater depth than the breaking point. These vanes take over at the non-linear portion of the wave trajectory and act to capture the bores that form, in a way roughly analogous to shock wave interaction in compressible gases. The flow from these bores is then introduced tangentially into a central cylindrical well, which is set into continuous rotation by the tangential flow and acts as a "fluid flywheel" that smooths out variations in incident wave energy over short time periods. There is also a super-elevation of water within the well which aids in driving the column of rotating water through a specially designed turbine at the bottom of the well. It is planned that the water discharged from the turbine will flow thorugh a diffuser to recover the residual angular and linear moments. Total estimated efficiency in the model is about 50 percent.
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Models of this system have operated very well. It is probable, however, that a full-scale installation will be troubled by biological fouling, especially on the converging vanes and in the diffuser,. Fouling on the vanes can perhaps be controlled by nylon whips, which I will mention later in connection with the dynamic breakwater. It is unlikely that the diffuser can be designed both for hydrodynamic efficiency and for protection from biofouling. I will show a brief motion picture of a model of this system later [see the video on this website].
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