Multi-element Wave makers Geometry The CSIR has added to its research and development capabilities by acquiring multi-element dynamic absorption wave generators used in the 2D flumes and 3D wave basins. These wave makers comprise rack and pinion paddles (made by HR Wallingford). The 2D flumes have a single paddle (0.75m wide) with wave absorption capabilities. For the 3D basins, the wave generators are made up of narrow paddle multi-element wave makers. Each paddle is 50 cm wide, and each module has a width of 4 m. A total of six modules are available for model tests making up 24 m. The wave paddles are mounted on a linear slide assembly that is supported by a common framework. All the bearing and precision components are situated above water level. Long-crested and short-crested waves can be generated with this wave maker system. The angle of incidence of the waves can be varied depending on the frequency distribution of the wave condition. The maximum wave height generated in 80 cm water depth is approximately 47 cm between frequencies of 0.4 Hz and 0.6 Hz. Both irregular and regular waves can be generated. The wave makers are equipped with a dynamic wave absorption feature. Software The wavemaker program generates regular (sinusoidal) waves and random waves using two methods. These are digitally filtering white noise in real time and summation of sine waves in both real time and using an offline playback method. The option to generate user defined spectral shapes is also available. Waveforms can be long or shortcrested and the mean direction of wave propagation can be set at any angle to the face of the wave machine. Wave fields from two or more directions can also be defined. The random waves produced by wavemaker conform to one of two standard spectral shapes, JONSWAP and Pierson Moskowitz. Many other less often used spectral definitions can be input using the user-defined option as a series of spectral densities separated by a constant frequency increment. Set-down The second order effect of setdown beneath wave groups is important in shallow water harbour studies where resonances and ranging of moored vessels can be a problem. The same is true for some coastal studies where longperiod disturbances may be of interest. As they move into shallow water groups of waves of high and low amplitude produce changes in the mean water level which induces a long period disturbance not dissimilar to a real long wave but different in that it is tied to the wave groups. Set-down derives its name from the fact that a depression in the mean water surface occurs beneath groups of large waves. This wave-like perturbation of the water surface may induce a response in the system being studied and must be accurately represented in physical models. The software includes the option to modify the motion of the pistontype wave machines in real time to ensure that the set-down propagates realistically without contamination by other unwanted second order effects. Directional spreading functions include cos2θ, cos6θ and the Mitsuyasu type among others. Contact: Kishan Tulsi Email: ktulsi@csir.co.za Ph: +27(0)21 888 2455 Website: www.csir.co.za Contact: Kishan Tulsi Email: ktulsi@csir.co.za Ph: +27(0)21 888 2455 Website: www.csir.co.za