A description is given of the development of a particle image velocimetry (PIV) illumination system which uses photographic flash units. The errors are estimated and shown to be relatively small. The usefulness of the system is demonstrated in a practical application involving the measurement of velocities within steep impacting waves. These measurements enable the calculation of impact pressures using both Bernoullis" equation and Taylors" model of the oblique impact of a jet on a moving plane surface. Both methods are shown to under predict. The advantages and disadvantages of the flash system relative to the more usual laser based system are discussed. It is concluded that the system is suitable for many applications due to its low cost and versatility.
Breaking wave impact loads are usually significant to the design of offshore and coastal structures but are difficult to predict. One of the principle reasons for this is the difficulty associated with measuring or predicting breaker kinematics. The complex deformations of the wave and the high velocities which occur during breaking mean that the performance of traditional instruments such as hot wire anemometers and propeller meters is poor. The need for whole flow field measurements and the difficulties associated with obtaining repeatability of wave impacts mean that laser Doppler anemometry (LDA) is time consuming to apply. Particle Image Velocimetry (PlY, see Grey et ai, 1987) is a technique for measuring fluid velocities which provides the whole flow field in an "instant". NeutralIy buoyant opaque tracer particles are introduced to the fluid and illuminated with a pulsating light source. A multiple image of the measurement region is recorded as the tracer particles move with the flow (see Figure I). This provides the orientation of motion and the distance between successive images of each particle and therefore the flow velocities.
