HYDRODYNAMIC FORCE CHARACTERISTICS AND COEFFICIENTS IN THE SPLASH ZONE REGION OF REGULAR WAVES

A comprehensive "experimental" and "analytical" study of the "overall" and ''local'' hydrodynamic force characteristics of both "rigid" and "compliant" surface piercing segmented cylinders with "reguk" and "irregular" waves in "the splash zone" had been performed at the University of Melbourne for the last four years (Daliri, 1992-1996). Two segmented cylinders with dSerent diameters were tested with a variety of compliancy bnditiom and natural frequencies. Conditions chosen produced variations in the test cylinder's natural fkquency in the range 0.7,2.5,3,5, 10 and 12 Hz which was considered to be reasonably representative. This paper concentrates on a portion of this study associated with the hydrodynamic force characteristics and the respective inline force coeficients of "rigid" tests (&&>fmve) under "regular" waves. "Regular" long-crested gravity and &lagravity waves, at frequency range between 0.2 to 2 Hz and wave steepness's (Ill)in the range 0.0005-0.1580 were synthesised. Reynolds numbers of up to 12000 were used and the KC numbers were in the range 0.5-35 which encompasses inertia force dominant (KCd) to drag force dominant conditions (KC>20). Observed response signals from two wave surface elevation probes q(t), an acoustic Doppler water particle velocimeter u(z,t), the along and across wave forces from instrumented segments f(z,t) in the wave run-up region, wave crest to trough region (the traditional splash zone), immediately below the splash zone and fully submerged zone plus along and across wave reactions of the cylinder's tip restraint force were simultaneously collected for subsequent analysis. In the rigid tests, the measured wave forces and the respective values of the in-line force coefficients CD and CM at different elevations have been intapkd using the traditional form of the "Morison equation" which was modified to consider the intmnittency effects at the splash - zone region. (A major focus was given on the "local" nature of the hydrodynamic force chanxtaistics as ; realised in the splash zone and in the Illy submerged 3 zone immediately below this region.) 3 3 In regular wares, it has been found that a correct application of the Morison equation (which is modified for the intermittency effects) in conjunction with the stream function wave theory (Fenton, 1988) can provide a reasonable description of the flow field within the crest to trough region and immediately below the splash zone. In addition, the results suggest that both CD and CM values in the splash zone region are "higher" and exhibit a mild ''frequency dependence" in comparison with their corresponding counterpa& for the llly submerged segments. For weakly nonlinear waves (Wl.4.1) only wave fluctuation is found to be important and any mild nonlkarities do not significantly affect the test model force response and consequently the force coefficient values. However, for relatively nonlinear waves ( O . l ~ ~ d ) . l 6b)o,th the free surface intermittency effect and the non-linearity in the wave kinematics severely affect the test model force response, producing ringing effects in conducive conditions.