tra at all spatial locations showed the same peak frequency in a given wave and that this frequency decreased with the wavelength of the breaker. The wavelength of these fluctuations increased with the wavelength of the breaker. These disturbances were found to propagate downstream. More recently, Walker, Lyzenga, Ericson and Lund  studied surface ripples on waves generated with a submerged hydrofoil in a recirculating water channel. They used a single flow speed of 108 cm/sec and studied three waves of different breaking strengths by varying the angle of attack of the foil. Surface shape measurements in the breaking region and in the wake were made photographically with a laser light sheet (aligned with the flow direction) and a video camera. The temporal sampling rate of this system was 30 images per second. Radar returns from these breakers were also measured. It was found that the variance in surface height fluctuations was largest at the breaking region and decreased rapidly with distance behind the breaking wave crest. The temporal frequency of these fluctuations was low at the toe of the breaker and about 8 Hz further back on the breaker. This frequency remained constant with distance downstream in the wake. The variation of these frequencies with breaker strength was not reported, presumably because of the rather low temporal sampling frequency used in this work. It was also found that the wavenumber of the ripples decreased with distance downstream. The fact that the frequency was constant with distance downstream while the amplitude and wavenumber decreased lead the authors to speculate that the ripples were behaving like surface waves propagating on a spatially varying current, which has speeds of 0 at the breaking region and 108 cm/sec in the far wake.
In the present work, both the surface ripples and flow fields in steady, hydrofoil-produced breaking waves are investigated. The waves are generated by three NACA 0012 hydrofoils with chords (c) of 15, 20 and 30 cm. Waves were generated with with one towing speed (U) and three depths of submergence (d) for each foil. The conditions were chosen with a single Froude number and three values of d/c that were used for all the experiments. In this way, the effect of experimental scale was investigated. The surface ripple measurements were taken from high-speed movies of the breakers. Both streamwise and cross-stream profiles were taken. The flow field in the breaking region was measured with particle image velocimetry for the weaker breaking conditions. Unlike the studies of Lin and Rockwell [6, 7] many realizations of the flow field were measured in order to obtain average statistics of the flow fields. In addition, the flow field in the wake of the breakers was also measured to obtain the drag due to breaking. At the time of this writing the analysis of the data is incomplete. In the following the data presented is primarily from the experiments with the 15- and 20-cm foils and is centered on results from streamwise profiles. Some data from cross-stream profiles and limited PIV measurements in the breaking region is also presented.
The experiments were performed in a tank that is 15 m long, 1 m deep, and 1.2 m wide, see Fig. 1. For visualization purposes, the side walls of the tank are composed of tempered glass. The tank includes an underwater towing system for the hydrofoils and an above-surface instrument carriage that moves with the breaking wave. The tracks for the subsurface towing system are 14 meters long and are made of 3.8 cm stainless steel angle. These tracks are positioned at a nominal distance of 48 cm from the tank bottom. Measurements showed that the rail depth variation did not exceed ±0.75 mm in the center 8 m of the tank where the wave measurements were taken. The force for moving the foil through the water is supplied by a cable and pulley towing system that is powered by an electric motor and gear box located below the tank at one end (see Figure 1). The foil can be towed at speeds up to 1.2 m/s. The same motor that tows the hydrofoils also tows the above-surface instrument carriage. The carriage travels on two precision tracks, one on each side of the tank, and is supported by hydrostatic oil-film bearings. These bearings result in a very low vibration level for the carriage which serves as a platform for the camera and optical equipment. Reproducibility checks on the towing speed speed, made by timing the foil and carriage over a 4-m distance, showed less than 1% error in speed from run to run.
Three hydrofoils were used in the present study. Each hydrofoil has a NACA 0012 shape and is made of anodized aluminum. This foil shape is symmetric