### Meeting Abstract

**113.2** Saturday, Jan. 7 **Swimming at intermediate Reynolds numbers in water boatmen** *NGO, V.*; MCHENRY, M.J.; Univ. of California, Irvine; Univ. of California, Irvine* vngo3@uci.edu

A wide diversity of aquatic animals swim at intermediate Reynolds numbers, where both inertial or viscous forces may contribute to propulsion. Here we studied the relative importance of these forces during the powerstroke of the lesser water boatmen (*Corixa punctata*) through a novel approach for estimating force coefficients in a freely swimming animal. Due to a large range in body length (1 mm to 10mm), these animals span a 100-fold difference in Reynolds numbers (200 < Re < 30,000). We used high-speed video of forward swimming to measure the kinematics of the propulsive appendage. We modeled the thrust generated by this motion using a quasi-steady, blade-element approach. For each swimming sequence recorded, this model provided the basis for an optimization algorithm that iteratively determined the thrust coefficient of the swimmer during a powerstroke. Using this method to calculate the coefficients of individuals of different sizes, we determined that the thrust coefficient varies little with Reynolds number. This finding suggests that the thrust generated by the appendage is dominated by inertial fluid forces throughout the large range in Reynolds numbers spanned by these animals during swimming. Therefore, boatmen of very different sizes swim with similar hydrodynamics.