A Slotted-Wall Test Section for a Water Tunnel

Abstract

Studies have been under way at the St. Anthony Falls Hydraulic Laboratory to investigate the possibility of including an alternate slotted-wall test section in a proposed 36-in. water tunnel for the David Taylor Model Basin. The experimental program was conducted in a 6-in. model water tunnel previously used in design studies for the 36-in. tunnel, and in a 10-in. free-jet tunnel at the Laboratory. Tests were run on slotted-wall test sections 2.18 and 2.38 test-section diameters in length and indicated that perhaps 2.5 diameters would be the maximum length possible without altering the diffuser following the test section. Axial pressures uniform within 1/2 per cent of the free stream dynamic pressure were obtained for a length of 2 test-section diameters. Velocity profiles in the test section compared favorably with those for a closed-jet test section, being flat within 1 per cent over 90 per cent of the diameter in the upstream portion, and over 80 per cent in the downstream portion of the useful test-section length. It was estimated that the energy losses for this slotted-wall test section would be about 10 per cent greater than for the shorter open-jet test section which will be included in the 36-in. tunnel. The cavitation characteristics were not as good as those for an open-jet tunnel, incipient cavitation occurring at an index: of about 0.6 to 0.9; below 0.5 the reservoir chamber became cloudy. A minimum of optical distortion is expected to result if slots are omitted from the Slotted-wall cylinder near the axis on the viewing side and if flat windows are used for the reservoir barrel. Tests on a hemispherical-nosed body indicated that the 2.4-diameter length was shorter than would be desired for testing bodies whose diameters are one-third the test-section diameter and more than two or three body-diameters in length.