ABSTRACT

Whole-field kinematic measurements are presented for steep, steady waves travelling on strongly sheared currents, concentrating on the region between trough and crest level For slab currents, the crest kinematics and wave parameters were found to be correctly obtained by using Doppler shifting and standard wave theories When the current was sheared, the crest kinematics were found to be well predicted by adding the results of an irrotational model to the stretched current profile, although this approach did not yield good predictions in the trough The average shear was found to be reduced, in the presence of waves, from the undisturbed value, in agreement with the findings and predictions of other authors

INTRODUCTION

It has recently been accepted that large waves require the use of high order theories for the prediction of their internal kinematics, and such predictions, in the absence of current, have been extensively verified by laboratory studies On a uniform current the combined wave-current kinematics can be calculated by changing reference frame to one moving with the current and using a high-order model, after Doppler shifting the wave frequency Several authors have recently tackled the problem of steep waves on currents with linear and bi-linear profiles [Dalrymple and Heideman, 1989, Eastwood and Watson, 1989], and programs are now being developed for the prediction of the kinematics in the presence of currents with arbitrary profiles [Chaplin, 1990, Thomas, 1990]

Whilst numerical solutions are available from several sources, experimental studies are st111 comparatively rare Swan [1990] produced moderate amplitude waves on strongly sheared currents and addressed some of the difficulties posed by Dalrymple and Heideman in their cautionary note on tank testing Other notable experiments include those of Thomas [1990], where good agreement was obtained between laboratory measurements and numerical predictions for weakly sheared currents, and Kemp and Simons [I9831 whose study concentrated on the effects near the bed

EXPERIMENTAL MEASUREMENTS

In the present study, experiments were performed in a purpose built wave flume, capable of producing forward or reverse currents with various profiles The experimental arrangement is shown in figure 1 The man current condition selected for the experimental programme was strongly sheared in the direction of wave propagation, and was generated by introducing a flow opposed to the waves along the bottom half of the flume In the subsequent analysis, the bulk value of the current was altered to yield the desired profile in the chosen reference frame, and the wave frequency modified with the appropriate Doppler shift

Figure 1 Wave flume used for the kinematic measurements of wave/current interaction, showing a reverse current with forward shear(Available in full paper)

In the experimental test sequence, steep waves were run onto the current and kinematic measurements made once the waves were steady and before reflections came back The combined kinematics were measured by the method of Particle Image Velocimetry (PIV), in which small seeding particles are multiply exposed and recorded onto film Further details of the use of the technique in this and other hydrodynamic studies are described in [Greated, 1992]

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