Studies on complex oceanographic flows, such as those governed by nonlinear and multiscale physics, have been conducted in relevant experimental studies to date. For the study of wavy oceanographic flows, most studies have used traditional techniques of wave generation with solid boundary movement, involving a moving wall boundary. The movement is based on the velocity profile of the wave to be created. In long wave generation, for instance, the vertical wave-maker is commonly driven by a piston.
For deep water waves, the hinge of a paddle mounted on the bottom of the tank mimics how orbital particle motion decays with water depth. When the shape of the wave-maker does not perfectly match the vertical kinematic profile of the targeted wave, evanescent modes result. In cases of poor wave-maker wave matching, spurious and undesirable free waves are generated. Wave-maker theories encompass dispersive and shallow water theories and linear to weakly nonlinear waves. In general, these theories are well-established [6].
In laboratory studies on internal waves, a wide range of generation approaches have been used, and they are similar to free surface wave studies. The most common types of wave-makers are hinged flap and plunger. The flap type wave-maker [7], [12], and [17] as well as the plunger type wave-maker, where a solid object vertically oscillates near the interface, have been researched [8], [11], and [15].
Wave-makers for internal wave generation that force a stratified current over a sill [1], such as the vertically-segmented, dual-plunge wave-maker [10], and dual-piston wave-maker [18] have been introduced. These wave-makers can successfully generate internal waves by targeting a narrow frequency range. The equipment is designed to isolate a particular element of a challenging physical issue; multiscale and nonlinear wave interactions often present too complex a problem to tackle using these existing laboratory devices.
Experimentally, studies with respect to wave-current interactions have been performed by a traditional wave generator coupled with a pump-driven current generator. These studies have had difficulty in generating a steady current and a wave train in tandem. Several authors have considered wave-current interactions in the near bed region[3], [2], [9], and [16]. For distributing adverse current in the direction of wave propagation over the whole water depth, Thomas [14] added a rotating louvre blade. Swan et al. [13] used a complicated current-apparatus to simultaneously generate depth-varying currents and waves.
The true complexity of oceanographic flows can only be observed in the field; but, of course, field experiments suffer from the enormity of scales that must be covered. Most field instruments can capture good time resolution data at a limited number of points. To map a flow field, for instance, velocity profiles can be collected along a network of tracks using a bottom-tracking Acoustic Doppler Current Profiler (ADCP). The ADCP records short-term average velocity in several bins below the towing platform. Though each instantaneous profile has high resolution, the data along a track are not synoptic, and the tracks are very sparse when a large region must be mapped, such as an inlet or headland. Turbulence data can be collected at individual points using Acoustic Doppler Velocimeters (ADVs), which can be moored at key locations to obtain a complete view of the velocity structure of the water column at a point. Yet, to understand the spatial transformation of a directional wave spectrum, multiple moorings are required at a high expense. Moreover, all of these acoustic methods are strictly limited to weakly stratified flows, and none give an accurate measure of turbulent mixing over large scales. Conductivity, temperature, and depth (CTD) profilers equipped with fluorometers can be used to track a natural or injected dye tracer to better understand mixing [4]. However, these results are still limited to non-synoptic profile data. To capture the dynamics of an internal breaking wave, laboratory experiments are required [15].
Figure 1 shows the schematic of a HCW with three baffles. Both the inlet and outlet flume boundaries are composed of an adjustable set of vertical baffles. Each baffle is connected to an individual flow control system, such that vertical distribution of the flow is entirely controllable. In such a system, any arbitrary flow can be reasonably created as shown in Fig. 2; for long and short wave generations, for instance, each flow control system can make each target flow at each position mimic the shape of a quadratic and parabolic velocity profile, respectively. If different sets of baffles can be connected to different reservoirs, an internal wave with multiphase profiles can be created. Thus, the HCW system can be applied to experimental studies owing to its wide spectrum of waves. In this study, a small-scale prototype of the HCW system is developed by a combination motor and screw jack, and then sinusoidal waves generated by a calibration between the motor speed and flow rate are obtained. A scaled-up physical model of the HCW is built, and solitary waves with various wave amplitudes are generated and verified.