Johan Kristian Sveen

Properties of large-amplitude internal waves

Properties of solitary waves propagating in a two-layer fluid are investigated comparing experiments and theory. In the experiments the velocity eld induced by the waves, the propagation speed and the wave shape are quite accurately measured using particle tracking velocimetry (PTV) and image analysis. The experiments are calibrated with a layer of fresh water above a layer of brine. The depth of the brine is 4.13 times the depth of the fresh water. Theoretical results are given for this depth ratio, and, in addition, in a few examples for larger ratios, up to 100:1. The wave amplitudes in the experiments range from a small value up to almost maximal amplitude. The thickness of the pycnocline is in the range of approximately 0.13{0.26 times the depth of the thinner layer. Solitary waves are generated by releasing a volume of fresh water trapped behind a gate. By careful adjustment of the length and depth of the initial volume we always generate a single solitary wave, even for very large volumes. The experiments are very repeatable and the recording technique is very accurate. The error in the measured velocities non-dimensionalized by the linear long wave speed is less than about 7{8% in all cases. The experiments are compared with a fully nonlinear interface model and weakly nonlinear Korteweg{de Vries (KdV) theory. The fully nonlinear model compares excellently with the experiments for all quantities measured. This is true for the whole amplitude range, even for a pycnocline which is not very sharp. The KdV theory is relevant for small wave amplitude but exhibit a systematic deviation from the experiments and the fully nonlinear theory for wave amplitudes exceeding about 0.4 times the depth of the thinner layer. In the experiments with the largest waves, rolls develop behind the maximal displacement of the wave due to the Kelvin{Helmholtz instability. The recordings enable evaluation of the local Richardson number due to the flow in the pycnocline. We nd that stability or instability of the flow occurs in approximate agreement with the theorem of Miles and Howard.

On the realm of validity of strongly nonlinear asymptotic approximations for internal waves

Analytical and numerical results from recently developed strongly nonlinear asymptotic models are compared and validated with experimental observations of internal gravity waves and results from the numerical integrations of Euler equations for solitary waves at the interface of two-fluid systems. The focus of this investigation is on regimes where large amplitudes are attained, where the classical weakly nonlinear theories prove inadequate. Two asymptotically different regimes are examined in detail: shallow fluids, in which the typical wavelengths of the interface displacement are long with respect to the depths of both fluids, and deep fluids, where the wavelengths are comparable to, or less than, the depth of one of the two fluids. With the aim of illustrating the breakdown of the asymptotic assumptions, the transition from a shallow to a deep regime is examined through numerical computation of Euler systems solutions and by comparisons with solution to models.

A dynamic masking technique for combined measurements of PIV and synthetic schlieren applied to internal gravity waves

We present a novel technique for dynamically masking images for synthetic schlieren, a recently developed technique for measuring density gradients in fluids. Synthetic schlieren uses pattern matching algorithms to determine the apparent displacement of a background pattern. In the present case this pattern is displayed on a regular LCD monitor. By changing what is displayed on the monitor, foreground features (such as particles or optical defects in the apparatus) can be identified separately from the background pattern. The influence of these features can then be locally masked out from the synthetic schlieren calculations. The technique also has the benefit of enabling the simultaneous use of particle image velocimetry or particle tracking velocimetry, and hence it may produce combined measurements of both density gradients and velocity. The masking techniques introduced here significantly reduce the errors in the measured density gradient that would otherwise be introduced by the presence of particles in the flow. The mask itself will introduce errors in the pattern matching, but we show that these are smaller than the benefits.

Laser Doppler anemometry (LDA) and particle image velocimetry (PIV) for marine environments

Abstract: This chapter will review the most common optical measurement techniques applied in fluid mechanics in general. None of these techniques is limited to subsea applications, but we have placed an emphasis on particular aspects from the underwater world where applicable. Subsea application of these techniques is a highly challenging task, but it does not alter the intrinsic nature of the various measurement techniques. We have also chosen to briefly include a few techniques that have not yet been applied subsea for future reference.

Experimental Study on Interaction Between Waves and Netting: Damping Effects and Forces

One of the possibilities to expand sea-based fish farming is to move the aquaculture installations away from the conflicts of the coastal zone, and into more open ocean locations. However, open ocean aquaculture puts other demands on the structures than aquaculture in sheltered locations, and in this context it is necessary to understand the behaviour of the aquaculture structures as they are exposed to large sea-loads from waves and current. Flexible netting is a main part of most sea-based aquaculture structures, and in this paper the interaction between waves and netting is studied. Experiments were conducted at the narrow wave flume facility at the University of Oslo, Norway, where several different regular wave cases were run through netting with different solidity. The wave energy was measured after the wave had passed through the net and compared with the energy of an undisturbed wave to assess the wave damping properties of the net. The vertical and horizontal forces were also measured. The findings show that the damping effects of the netting are not necessarily correlated with the wave forces, indicated complex nonlinear processes contributing to the fluid-net interaction. The amount of nonlinear energy in the wave and force waveforms is also investigated, and it is shown that the nonlinear energy in the incoming wave results in an even higher level of nonlinear components in the forces experienced by the net.Copyright