The study was conducted for the purpose of describing lift and drag forces exerted upon a structure with plane vertical sides and a horizontal bottom by shallow water waves which approach the structure with normal incidence and pass beneath it. Results of the study are presented in graphical form. Some degree of generalization is achieved through the relation of dimensionless parameters, which contain physical quantities pertinent to the problem. The graphs show maximum and minimum relative values of lift force and values of drag force for various values of relative wave height, relative clearance, relative width of structure, and relative length of structure. This study made use of the same structural models as were used in a previous study to determine pressure intensities on the bottom surfaces under similar conditions of clearance and wave action.
It has been observed that severe structural damage can result from shallow water wave action on the underside or bottom of a raised platform with vertical sides. In order to gain some knowledge as to pressure intensities that might be anticipated on the horizontal bottom surface of such a structure, a laboratory study was conducted and reported by the authors1 A follow-up study of lift and drag forces on structural models used in the previous study, and for the same relative conditions of wave action and clearance above the stillwater surface, was conducted and is reported herein.
Essentially, the structural models were rectangular boxes, suspended in such a way as to have a horizontal bottom and vertical sides, which were oriented for normal wave incidence, and to permit passage of the waves beneath the structures. The separate effects of wave height, clearance above the stillwater surface, width of structure, length of structure, and stillwater depth on the lift and drag forces were investigated. This was done by varying each quantity, except depth, from the value associated with a set of common or reference conditions. The results have been generalized insofar as possible through the use of dimensionless parameters, with stillwater depth as the common quantity.
As long period waves move from deep water into shallow water, they appear to behave more and more as individuals, rather than as parts of an oscillatory wave train. This means that such quantities as wave period and wave length appear to have little, if any, significance in a study such as that reported herein. Consequently, the waves generated for this study were sufficiently far apart that they were considered to be independent of one another.
The results of the study are presented in graphical form and are generalized, insofar as practicable, through the arrangement of pertinent physical quantities into dimensionless parameters. The lift and drag forces are time dependent. That is, they vary in magnitude with the passage of the waves. For practical reasons, the graphical presentation of this paper is limited to maximum and minimum conditions. The relation of these results to those previously reported by the authors1 on pressure intensities will be noted.
