MRI: Development of Pneumatic Water Wave Genesis, a versatile wavemaker for the UNC Joint Fluids Lab

The goal of this project is to develop a potentially transformative new surface and internal wave-making method, based on a novel pneumatic concept with no mechanical actuators, and use it to dramatically extend the range of scientific research activities taking place in the UNC Joint Fluids Lab modular wavetank. The investigators intend to demonstrate feasibility of a method for excitation of a general family of gravity waves in non-uniform incompressible fluids. The method uses an array of independently controlled monopole flux sources arranged to provide a flux boundary condition over an arbitrary 2D manifold. The tangible results of the development will be a wave generating apparatus for use in an existing 0.75[m] wide by 36[m] long wave tank containing water with prescribed salt-stratified density profiles. The generator will excite a 1[m] by 0.75[m] rectangular boundary comprising N independent rectangular flux sources, each being 0.75[m] wide by 1/N[m] high, stacked up vertically. This boundary is inserted into a wave tank to form one end of an active flume. Each source will drive a near-spatially-uniform volume velocity over its aperture. Wave modes of zeroth horizontal order and up to (N-1)th vertical order can thereby be excited. The proposed design was inspired by a recently developed mechanical actuator system designed at ENS-Lyon (Gostiaux et al. 2007) to produce monochromatic plane waves. However, the proposed apparatus will go well beyond this design, by doing away with the mechanical moving parts, and substituting displacement chambers driven with air pressures generated by feedback-controlled pneumatic exciters capable of following arbitrary band-limited control signals. This allows generation of arbitrary vertical excitation profiles, including but not limited to superpositions of monochromatic plane waves of differing periods and directions. This design will further enable other interesting excitations, including non-periodic or even non repetitive impulse profiles. In the future, this general excitation method can in principle extend these capabilities to fully 2D boundary flux conditions, by tessellating an arbitrarily shaped boundary surface.

Both surface waves and their less widely known internal counterparts, which can occur for example in layers between fresh and salt water, have properties of more significant consequences than are generally appreciated. To name a few, the spontaneous development of a rogue wave can sink a ship; the formation and development of a Tsunami as it propagates in open water and upon landfall such as in a harbor can wreak havoc in some places while leaving others relatively unscathed; and the effects of internal waves on mixing stratified salt and fresh water layers contributes to the ecological health of an estuary and the well being of the species inhabiting it. A substantial body of theory continues to be developed to understand these so-called gravity wave behaviors and to render them predictable, by turning guesswork into solid physical and engineering mathematical models useful in naval architecture, disaster prediction, environmental impact, among many other areas of application. But precisely how good are these theories? Fundamental to the Scientific Method is experimental verification, and gravity wave mechanics is no exception. The problem addressed by this project is that of finding a method to repeatably generate and controllably vary the conditions in a laboratory environment equivalent to natural phenomena such as rogue waves. Often, theories pertaining to such phenomena undergo only limited testing, leaving the naval, environmental and ecological engineering decisions based thereon vulnerable to error. Making waves is easy: drop a stone in a wavetank. Creating the exact conditions to excite a rogue wave in a tank, and exploring how varying parameters affects the behavior or even the existence of such a wave requires an exquisite degree of finesse in controlling these conditions. The aim of this project is to develop a wavemaker for excitation of a general family of gravity waves in non-uniform incompressible fluids, thereby enabling experimental exploration in a controlled laboratory environment of real world phenomena having social and engineering relevance.