Aerodynamic drag coefficients of circular cylinders of finite elongation at low subsonic speeds

Аuthors

Golovkin M. A.^*, Grudinin M. V., Efremov A. A., Mirgazov R. M.

Central Aerohydrodynamic Institute named after N.E. Zhukovsky (TsAGI), 1, Zhukovsky str., Zhukovsky, Moscow Region, 140180, Russia

*e-mail: spintest@tsagi.ru

Abstract

The introduction presents a review of the literature on the considered subject. It is noted that in accordance with the previously obtained data for the c_x(Re), namely the dependence of the drag coefficient of a circular cylinder of relatively large aspect ratio λ = 17.8 (λ = L/d, L – cylinder length, d – its diameter) on the Reynolds number (Re = V·d/ν, V – flow velocity, ν – coefficient of kinematic viscosity of air), the transition mode from the laminar separation to the turbulent one is close to that of cylinder with infinite aspect ratio, and for the cylinder with λ = 9.2 it is noticeably lagged by the speed value, or Re number.

This is of practical importance, in particularl for the spin simulation in a wind tunnel (WT) with aircraft models, to know whether this trend holds for the cylinders of lower aspect ratio, which are usually used in typical fuselages of modern long-haul aircraft (λ = 6 ÷ 8). In this regard, this study considered the aspect ratio of λ = 6.15.

The tests were conducted in a vertical WT with a sufficiently low degree of the flow turbulence, within the speed range of V = 3 ÷ 33 m/s by increasing and decelerating speed with a pitch of 2 m/s.

This presented study confirmed a significant lag by the Re number for the laminar separation mode of relatively low aspect ratio, compared to an infinite cylinder.

The article demonstrates that transition to the turbulent separation at the cylinders of relatively low aspect ratio occurs in a much narrower range of the Re numbers than with an infinite cylinder.

A hysteresis in the drag coefficient while increasing and decelerating speed exists in the region of Re numbers corresponding to the transition mode from laminar separation to the turbulent one.

The drag coefficients values of the finite aspect ration circular cylinders in the ranges of Re numbers corresponding to laminar separation obtained in the vertical WT agree satisfactorily with the data available in the known literature.

The detected effects and the presented results should be accounted for when developing and conducting studies with aircraft models in the WTs.

Keywords:

circular cylinder, relative elongation, drag coefficient, small subsonic speeds, Reynolds number

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