PIV and Water Waves

The following sections are included:

• Preface

• Contents

Quantitative imaging (QI) techniques are a general class of optically based laboratory measurement techniques used in the field of experimental fluid mechanics, which have seen rapid growth over the last two decades. They are particularly well suited for the study of wavy fluid flows which are characterized by unsteady free surfaces and internal motions. This paper presents an overview of QI techniques in general, with a particular focus on particle image velocimetry (PIV). We present QI methods in the context of the broader fields of pattern recognition and image processing techniques, which are currently used in a wide range of fields. In this review QI methods and their fundamentals are described in detail and recent developments, targeted at increasing accuracy and resolution, are described and put into perspective. More specifically we identify QI techniques as a digital data analysis (through software) set of issues built upon general principles of pattern matching. Throughout the paper we address the aspects that are particular to wavy flows although these issues can be argued to be important for any unsteady fluid flow of interest, e.g. turbulence. This review article thus serves as a general reference for the neophyte and experienced fluid mechanics experimentalist.

This chapter catalogs the development of a submersible PIV system at The Johns Hopkins University for the measurement of turbulence in the bottom boundary layer of the coastal ocean. Currently, this system acquires data simultaneously in two independently oriented sample areas, while operating continuously for several hours. A profiling platform enables sampling from the seabed, up to 10m above the bottom. This system has been deployed on several occasions at different locations on the east coast of the United States. Analysis methods for characterizing the turbulence and determining the Reynolds stress and dissipation rates are presented, along with some sample results.

Using Digital Particle Tracking Velocimetry (DPTV), we measured the oscillatory boundary-layer flow motions induced by surface gravity waves propagating over a fixed ripple bed. Spatial and temporal variations of the velocity field were recorded. Two sets of wave parameters, yielding two different Keulegan-Carpenter and Reynolds numbers, were used. The phase-dependant mean velocity and vorticity fields are presented in detail, illustrating the vortex generation and ejection processes. The bootstrap percentile technique is used to quantify the random error in the measurement. The wave-induced streaming profile is determined from the experimental data and its implications are discussed. The evolution of the Reynolds stress and turbulent kinetic energy are also presented. An estimate of the production and dissipation in the turbulent kinetic energy budget calculated within the turbulent boundary layer is illustrated.

A review is made of application of PIV to ship velocity fields. An overview is given of PIV techniques, including measurement systems, tracer particles, laser, camera and recording medium, and evaluation methods along with recent advances. PIV data post processing and uncertainty assessment methods and procedures are described. Investigations from 1992-2002 are reviewed with application to boundary layer and wake without and with free surface, bow flow, wave hydrodynamics, propulsor hydrodynamics, appendages, and cavitation and bubbly flows in support of design through explication of physics, model development, and CFD validation. Discussions include PIV system; physics, geometry, and conditions; facilities; data acquisition, reduction, and locations; uncertainty assessment; and results. Overall conclusions are given as well as comments on future directions.

Turbulence and scalar exchange at air-water interfaces have been investigated in the laboratory, in field studies, and with numerical simulations. Parameterizations and phenomena are reviewed, in particular surface divergence and renewal concepts for scalar exchange predictions. Micro-breaking is seen to be important over a wide range of conditions but is not well understood. The effect of heat losses on near-interface transport processes also requires further study.

We review recent laboratory measurents of internal waves using Particle Tracking Velocimetry and Particle Image Velocimetry. The methods enable accurate recordings of the wave induced velocity and vorticity fields. Global properties like wave length and wave speed are obtained from the measurements. Solitary waves and dispersive wave trains in two-layer fluids with an upper layer that is either homogeneous or linearly stratified, and a lower layer that is homogeneous, are investigated. The measurements provide references for mathematical models and are used to judge their applicability. Particular focus is paid to solitary waves that exhibit convective breaking, shear instability, how wave breaking changes the global wave properties, how solitary waves break at a submerged ridge or shelf, and how the wave breaking induces transport of suspended particles. Run-up of very long internal waves at a shelf-slope is found to introduce strong fluid velocities where the pycnocline meets the slope. The experiments are used to interpret observations in large scale.

The following sections are included:

• PIV measurements of accelerations in water waves

• PIV measurements of the velocity field in steep water wave events

• LDA laboratory measurements in the surf zone for the RANS depth-averaged shallow-water equations

• Wave acceleration measurement using PTV technique

• Two-color LIF-PIV for gas exchange at air-water interface

• Run-up of solitary waves

• DPIV measurements for unsteady deep-water wave breaking on following currents

The following sections are included:

• A digital high speed camera system applied to PIV measurements in water waves

• Three-dimensional stereoscopic particle tracking velocimetry for a large domain (3DSPTV-LD) and its application

• Enhanced PIV technique to investigate bottom boundary layer dynamics

• The turbulent boundary layer structure beneath a gravity wave and its proper decomposition

• Intermittency in surf zone turbulence and observations of large-scale eddies using PIV

• Turbulence distributions and flow structure in the coastal ocean from PIV data

The following sections are included:

• Blocking effects in supercritical flows over topography

• Study of the flow pattern in a perforated breakwater using PIV measurements

• Pseudo image velocimetry of a regular wave flow near a submerged breakwater

• Laboratory modelling of the motion of an internal solitary wave over a ridge in a stratified fluid

• Measurements of velocities during run-up of long internal waves on a gently sloping beach

• Comparison of modelling of strong internal wave events with PIV studies of the phenomenon