FLUID FORCES ON SHORT-SPINED SEA-URCHIN A ND NORTHERN SEA-URCHIN

The drag coefficients and the added mass coefficients of short-spined sea urchin Strongylocentrotus intermedius and northern sea urchin Stro ngylocentrotus nudus were determind. Meanwhile adhesive forces exerted by the tube feet were also measured. The drag forces were measured in steady flow and the added masses were measured by torsion pendulum co ating the sea urchins with paint. The drag coefficient C(d) was greate r for the northern sea urchin, while the added mass coefficient C(a) w as greater for the short-spined sea urchin. The average values of C(d) , C(a) in the direction of the test diameter was 0.66, 0.64 for the sh ort-spined sea urchin and 1.06, 0.34 for the northern sea urchin respe ctively. Adhesive forces on smooth glass substrates were 0.3 approxima tely 2.8 kg for the short-spined sea urchin and 0.2 approximately 2.4 kg for the northern sea urchin when they were more than 40 mm in test diameter. The adhesive forces of both species were much larger than th e calculated maximum wave forces using determined C(d), C(a) on the sa me size of sea urchin in an estimated rough sea condition.