Limitations on the Creation of Continuously Surfable Waves Generated by a Pressure Source Moving in a Circular Path

Abstract

In this paper, a novel idea to produce continuous breaking waves is discussed, whereby a pressure source is rotated within an annular wave pool. The concept was that the pressure source generates non-breaking waves that propagate inward to the inner ring of the annulus, where a sloping bathymetry (beach) triggers wave breaking. In order to refine the technique, research was conducted to better understand the mechanics of waves generated by a pressure source moving in a circular track in a constrained waterway, the transformation of these waves as they travel across the channel and the effect of the sloping beach on the wave quality for surfing. The quality of the waves was defined in terms of wave height, speed and shape, with the desired aim to create plunging waves, known as “barrels”, that are highly desired by surfers. Surfers also require a long steep crestline or “wall”, to allow a full range of manoeuvres to be performed. Finally, the pool needed to be able to create waves suitable for surfers from beginner to expert level, defined in terms of both the wave height and angle between the wave break point angle and the beach, known a peel angle. The primary novel outcome of the research conducted was to be able to design a pressure source that most efficiently imparted wave making energy into the water, and thus generated the largest possible waves whilst travelling at the required speed for surfing. The major finding was that the design parameters are generally in competition, and to determine a balance of limiting values, the design parameters cannot be considered in isolation. Therefore, a set of empirical relationships between the design parameters were developed to allow the pool to be designed for a combination of desired wave height at the breakpoint, wave shape and given pool radius. The limiting values for the parameters were determined experimentally, with the wave life-cycle from generation through transformation to wave breaking and dissipation used to focus the investigation. Scale model experiments were conducted in both linear and circular tracks. In addition to taking quantitative measurement of wave height and current formation, a method of qualitatively scoring the waves was developed to allow various pressure source shapes, operating conditions and bathymetries to be compared in terms of their suitability for surfing. The best quality waves were produced by a wedge-shaped wavedozer pressure source, such as the device detailed in Driscoll and Renilson [1]. Blockage, defined as the pressure source cross sectional area to channel cross-sectional area, was found to have a significant limitation on the generation of high quality waves suitable for surfing in a constrained waterway. Lateral wave decay, length and depth Froude Numbers also strongly influenced the waves during their life-cycle. Fundamentally, it was determined that only a very small range of design parameter values produce the desired high and shapely waves in the extremely constrained waterway under consideration.