Experimental Investigation of Porous Structures in Splash Zone
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- Institutt for marin teknikk 
First, the KC number (or porous KC number) varying hydrodynamic coefficients for porous plates are determined from forced oscillations in deep water. Then water-entry drag coefficient is determined from the force impulse observed during constant velocity water-entry for both types of plates. Finally, the rod screen type porous plate is subjected to incident waves, while it is held fixed and fully submerged at a small water depth. Results indicate that damping can be as high as 1.5 to 3.0 times the added mass for a porous structure in forced oscillation case whereas it can be, surprisingly, 10 to 100 times when subjected to incident waves. In case of constant velocity water-entry, it is observed that the slamming impact peak has negligible contribution to the force impulse and thus, the total force is drag dominated. This indicates that a strong emphasis should be placed on estimation of damping or drag loads on such structures. Further, the empirical method suggested by DNV-GL to estimate zero amplitude (KC=0) added mass and the added mass reduction factor curve to obtain conservative estimates of added mass for a porous structure were compared with the results from current work. In case of first a validity range, in terms of KC number, is defined where it can be applicable to obtain conservative estimates of added mass. Additionally, linearized damping model was discussed for deep water forced oscillation tests. The water-entry drag was found to be significantly influenced by free surface phenomenon like attachment of air bubbles and delayed wetting of top surface. Negative added mass was observed in some cases of wave tests.