where, L = average submerged pile length, ft 



For a symmetric pile such as a circular cylinder, Cp£ = Cj- 



is no net wave force. But a net force will be produced by a non-symmetric 

 pile in waves. Its magnitude and direction depends upon its orientation 

 with respect to the direction of wave propagation. For a semi-circular 

 pile, C^iT ~ 1.16 and Ct^ ~ 1-5, when it is positioned with the curved wall 

 facing the waves. For this case, a net force will be produced opposite 

 to the wave direction. A similar condition results for a crescent shaped 

 pile which will have C^^ Si 1.20 and Cp ss 2.3 when the convex side faces 

 the waves. If these piles are rotated 180°, net forces in the direction 

 of wave travel would be generated. 



PRELIMINARY TESTS - DRIFT OBSERVATIONS 



The objective of the preliminary test was to determine whether a 

 quantitative model test with more elaborate measurements was necessary to 

 verify or to contradict the hypothesis of producing net wave forces by 

 non-symmetric cylinders. 



The preliminary model test contained a series of drift speed observations 

 of small scale model platforms in waves of various heights and frequencies. 

 Circular and non-symmetric leg cross-sections were tested. By observing 

 the speed and direction of the drift of these models, the relative magnitude 

 of net wave forces and their direction could be determined quantitatively. 



Three models were built. Each consisted of three vertical cylinders 

 8 inches long connected by metal bars forming an equilateral triangle 

 with 6 inch sides. The cross-sections of the three models were circular, 

 semicircular and crescent in shape (Figures 1, 2, and 3). Each leg was 

 carved out of a 1-inch diameter wood dowel. The legs were ballasted to 

 give about 2 inches of free board (Figure 4). The wave tank was 18 inches 

 wide, 36 inches deep, and about 100 feet long. A wave generator was located 

 at one end of the tank and created waves of small amplitude and low frequency. 

 A solid beach, about 70 feet from the wave generator, absorbed the waves. 

 Wave profile observation was possible through a glass wall near the middle 

 of the wave tank. A point gauge was used to measure the wave amplitude. 

 The drifting velocity of the platform was calculated from the time recorded 

 for the platform to traverse a prescribed distance in the tank. Five geome- 

 tries were tested for each wave as shown in Figure 5. The test wave 

 properties and measured drift velocities were tabulated in Table 1. 



Since the floating model is in a dynamic flow condition, the test 

 results can be presented in the form of dynamic response curves. The wave 

 frequency, f, is non-dimensionalized by the natural pitching frequency of 

 the model, f^^. The drift velocity, V(j, is divided by the maximum peak 

 particle velocity, V^, to form another non-dimensional quantity. 



The natural pitching frequencies of the three platform models were 

 measured in still water by inducing free pitching oscillations. The measured 

 values are approximately 0.8 Hz for the circular legged model with least 

 damping, 0.83 Hz for the semicircular model and 0.62 Hz for the crescent 

 shaped model with heaviest fluid damping. 



