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|Title:||The damping of oscillatory waves by laminar boundary layers|
|Authors:||Massachusetts Institute of Technology. Hydrodynamics Laboratory|
United States. Army. Office of the Chief of Engineers
Eagleson, Peter S.
|Publisher:||United States, Beach Erosion Board|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Technical memorandum (United States. Beach Erosion Board) ; no. 117.|
Abstract: This report presents the results of an analytical and experimental investigation of shearing stresses exerted on a smooth bottom by the passage of oscillatory water waves. The experimental facilities consisted of a wave channel 90 feet long, 3 feet deep and 2 and 1/2 feet wide. Test waves of various periods and heights were generated at one end of this tank by means of a horizontally reciprocating vertical bulkhead and were dissipated at the other end on a plane, smooth beach of 1:15 slope. All tests were conducted using a smooth bottom and a stillwater depth of 1.31 feet. Force measurements were made by setting an isolated test panel in a false bottom placed in the wave channel and recording the time-history of instantaneous force on the panel during the passage of waves. Simultaneous measurements of instantaneous wave characteristics at the test panel were also obtained. The force measurements were corrected for pressure and inertia forces to obtain net tangential forces. Average resistance coefficients and damping coefficients were derived in terms of the pertinent physical properties of the waves using existing small amplitude wave theory and assuming non-separating flow in a laminar boundary layer. Analysis of the experimental results on the basis of these coefficients consistently showed the experimental bottom shearing stresses to greatly exceed those predicted by theory for the range of waves tested. The boundary layer was then assumed to be disrupted each half wave cycle due to flow separation and the periodic regrowth of the layer was calculated by the approximate momentum technique. Resistance and damping coefficients calculated on this basis show, for the most part, excellent agreement with experiment.
|Appears in Collections:||Technical Memorandum|