An Experimental Study of Wave Filters

Wave filters may be employed in laboratory facilities to attenuate oscillatory surface waves reflected from a test model, or to improve the quality of generated waves. When used for these purposes, they are placed between the generator and the test model, usually quite close to the generator. The studies reported herein consisted of experimental tests of the attenuation and reflection characteristics of plate and wire-mesh filters. The results indicated that high attenuations can be achieved with either type filter without serious reflections. The attenuation and reflection depend, in addition to other factors, upon the plate spacing of plate type filters and the porosity of wire-mesh filters. Very dense or closely spaced filters produced high attenuations and high reflections. The best over-all performance was obtained by plate spacings or porosities selected to utilizee a filter length equal to or in excess of the wave length. Reflection produced by plate filters depended, to a considerable extent, on the blockage or reduction in cross-section of the channel, as well as on the ratio of wave length to filter length. Reflections from plate filters were reduced when the filter was constructed of plates of various lengths. The experimental results were compared with available theory. Very good agreement was obtained with the plate-type filter when the spacing was not too small. Brief comparisons between the data on wire-mesh filters and two suggested theories were not successful. The majority of the filter tests were conducted in a relatively small wave channel. limited tests were conducted on wire-mesh filters in a large channel; a comparison of data from the two sources indicated that porosity was not the sole factor affecting the attenuation and that the resistance of the wire mesh probably varied as a function of Reynolds number.