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Title: An experimental study of drag forces and other performance criteria of baffle blocks in hydraulic jumps
Authors: Lehigh University. Department of Civil Engineering.
United States. Assistant Secretary of the Army (R & D)
Basco, David R.
Keywords: Hydraulic jumps
Baffle blocks
Drag forces
Hydraulic structures
Hydraulic models
Open channel flow
Publisher: Hydraulics Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; H-70-4.
Description: Miscellaneous Paper
Abstract: Hydraulic jumps are commonly used to dissipate the energy of high velocity open channel flow. Large bluff-shaped objects (baffle blocks) placed in the jump improve the performance characteristics. A purely theoretical determination of the drag force of the baffle blocks was impossible. The experimental approach employed was similar to wind tunnel tests. Specially developed equipment incorporating load cells directly measured the drag force on the blocks. Because of the spatially varying velocity in the jump, selection of a reference velocity became crucial to the calculation of a drag coefficient. Four types of force coefficients were studied. Jump length, wave height, block pressure and water surface profile were the other performance criteria of interest. Resulting tailwater depths were also measured and used to check the theoretical values computed from a linear momentum balance which incorporated the measured drag force. Block location, height, spacing, width, second row location, and shape were the geometrical variables investigated. The problem was limited to nonsubmerged, forced jumps that occured on a horizontal floor in a wide rectangular channel. The drag force generally was found to increase as: (1.) the block height increased; (2.) the blocks moved near the jump toe; (3.) the flow blockage increased; and (4.) the second row moved near the first. The maximum drag force in the region of best water surface profile was employed as the optimization criterion and generally resulted in near optimum jump length and wave heights. Optimum block height ratio increased and the location ratio decreased as the Froude number increased. Both trends were shown to be experimentally and physically plausible. The optimum flow blockage was near 0.50 and little evidence was uncovered in support of a second block row to improve those performance variables of interest. A force ratio based on the downstream, free jump, hydrostatic pressure force proved to be more practical than the other coefficients studied. Use of the free jump, mean, local velocity as reference did not eliminate the block location as a geometry variable. Tailwater depths computed using the measured drag force in the momentum balance were slightly conservative if the bed shear forces were neglected. The results of the investigation will enable the designer to determine the optimum baffle block geometry for Froude numbers between 3 and 10. The effects on those performance criteria studied can also be determined for other block geometries. The study was sponsored by the U. S. Army Corps of Engineers, Waterways Experiment Station. NOTE: This file is very large. Allow your browser several minutes to download the file.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Miscellaneous Paper

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