J. Fluid Mech. (2006), vol. 551, pp. 1–17. c  2006 Cambridge University Press doi:10.1017/S0022112005008268 Printed in the United Kingdom 1 Creeping flow around a finite row of slender bodies in close proximity By EFRATH BARTA AND DANIEL WEIHS Faculty of Aerospace Engineering, Technion, Haifa 32000, Israel (Received 27 January 2004 and in revised form 23 March 2005) The flow through and around a finite row of parallel slender bodies in close proximity moving in a viscous incompressible fluid is studied. The motion occurs under creeping flow (Re ≪ 1) conditions. This row is a model of a comb-wing configuration found in insects of the Thrips family and being developed for use for flying vehicles of mm size, operating in the creeping flow regime. We show here that such wings utilize viscous effects to carry along enough fluid to approximate continuous surfaces. The comb is described as a row of rod-like ellipsoids of slenderness ratio smaller than 0.01 at distances apart of order 10 times the minor axis and the flow field is computed by distributing singularities along the major axes of the ellipsoids. Results for the drag on the individual rods, as well as for the full row are presented. It is shown that above a certain number of rods, dependent on the geometric parameters of the comb, the row acts very much like a continuous surface, with over 95% of the flow moving around, and not through the comb. This allows a potential saving of tens of percents in wing weight. Parametric results for number of rods, rod density (ratio of inter-rod distance to rod length) and slenderness ratio are presented demonstrating the dependence of the flow field on the configuration. It is found that 50–80 rods are required to approach the asymptotic limit of large number of rods, for various combinations of rod parameters with inter-rod distances of order of the cross-section diameter. 1. Introduction The Stokes approximation for low-Reynolds-number flows (creeping flow) has been very successful and useful in describing the flow phenomena occurring under these conditions, which in air or water usually mean very small bodies moving at low speeds (Happel & Brenner 1973 for example). However, the slow decay of boundary effects has resulted in serious difficulties in analysing problems in which multiple bodies are involved. Recent developments in microelectronics and power supply technology enable the design of extremely small flying vehicles, with wingspan of O(10 -3 m). Such vehicles move at speeds of up to O(10 -1 ms -1 ), thus approaching the Stokes flow regime. One of the cardinal problems in designing such minuscule vehicles is weight (mass). This constraint caused us to look for weight-reducing options, leading to the idea of utilizing the slow decay of boundary effects in Stokes flow by building non-continuous comb-like structures as aerodynamic surfaces. Such comb-like surfaces will act as full wings as the fluid in the spaces between the solid parts will presumably be dragged along, and oncoming flow will be deflected around the structure. Comb-wings of this