Étude expérimentale systématique en vue de l'utilisation des modèles mathématiques pour l'étude de la diffraction pure de la Houle

A systematic experimental study of the use of mathematical models for pure wave diffraction research

Ingénieur à la SOGREAH.

Résumé

I.-Subject of Investigation. In a paper to the IXth. Coastal Engineering Congress in Lisbon in 1964, two SOGREAH engineers, Mr. Barailler and Mr. Gaillard, described a number of applications of electronic computers for research on waves in coastal areas. lnvestigation with a physical scale model yields both a validity criterion for calculation and a interesting illustration. The present article covers the full results of a systematic pure wave diffraction study on a model and compares them with the corresponding computer-calculated data. It is in two parts, which discuss the following subjects: (i) Pure diffraction in a body of water in which there are no wave-reflecting obstacles. (ii) Pure wave diffraction in a body of water containing totally or partly reflecting obstacles. II.-Test Conditions. The tests were conducted in a constant depth of wate rand with the waves heading straight for the "harbour entrance," which was determined in the first part of the investigation by the following characteristic breakwater head configurations: Thin breakwater heads; Thick vertical-fronted breakwater heads; Thick conical-section breakwater heads. The second part of the investigation considered wave conditions in a body of water bounded by wave reflecting obstacles. III.-Results. Only the general findings of the investigation are given, as available space does not allow full sets of test results to be included and compared with calculated curves. Calculations allows neither for the thickness of the breakwaters to either side of the opening nor for the embankment slope at the considered point. The position and dimension of the "theoretical" opening therefore require very careful examination. Only position need be considered for a vertical-fronted brea1ewater. Comparison of various solutions shows that closest agreement is obtained for the opening coinciding with the breakwater centerline (case 2). Embankment slope has to be allowed for in the conical-section breakwater lead case, because of its "converging duct" effect. The "equivalent opening" is shorter than that measured at datum level, which is shown up by a shift in the measured maximum amplitude values towards the centerline. Provision has been made for the determination of this "equivalent length" on a computer. In the tests with reflecting obstacles the theoretical opening was assumed to lie in the plane of the inside breakwater face, as the calculation of wave diffraction with reflection requires basing on clearly-defined boundaries. In this case, breakwater head thickness could be allowed for by further subdivision of the considered region; computer tests are now in progress on this point. Conclusion. a) Diffraction without reflection: Comparison between physical and mathematical model results under the above conditions shows very satisfactory agreement for vertical-fronted breakwater heads. In the case of conical-section breakwater heads, agreement is satisfactory beyond a limit at about 1 1/2 wave lengths from the opening. Though absolute measured and computed amplitudes also agree well up to this limit, a shift in the maximum value positions is observe d, which it should be possible to correct by establishing the position and length of the "equivalent opening". b) Diffraction with reflection: Satisfactory agreement is achieved with both methods.