NAUEN, J.C.; LAUDER, G.V.: Wake structure and force production during steady swimming by the chub mackerel, Scomber japonicus

We quantified patterns of water motion in the wakes of chub mackerel (Scomber japonicus) swimming steadily in a recirculating flow tank at speeds of 1.2 and 2.2 lengths/s. Using Digital Particle Image Velocimetry methods, thrust was calculated from independent measurements of flow velocity made in the horizontal and vertical planes; these thrust values were compared to measurements of drag obtained by towing the same individuals. The pattern of flow in the wake is consistent with that of a series of linked vortex rings, each with a central jet of flow. The rings were elliptical in shape. Ring height was approximately equal to caudal fin height; ring diameter was dependent on swimming speed and up to twice the magnitude of ring height. Profiles of wake velocity components were very similar to theoretical profiles for isolated vortex rings, supporting the conclusion that the wake is composed of a series of elliptical vortex rings, or vortex loops. Drag estimates for the two smallest fish based on DPIV analysis were not significantly different from drag measurements made by towing those animals (comparison of median values using the Kruskal-Wallis test, P = 0.34 and 0.31 for the 20 cm and 22 cm fork length fishes, respectively). At the speed of 1.2 l/s the magnitude of thrust was on average 14+7 mN (mean+s.d, N=40); lateral forces were approximately twice the magnitude of thrust (28+8 mN, mean+s.d., N=20); lift was negligible (-1+1 mN, mean+s.d., N=20). The relatively high lateral forces may aid in stabilizing the fish against rolling. A speed increase speed by a factor of 1.8 resulted in an average increase in thrust by a factor of 4.4, an average increase in lateral forces by a factor of 3, and no change in the magnitude of lift produced.