Use of the wake of a small cylinder to Control unsteady loads on a circular cylinder

Abstract

Simultaneous measurements of lift and drag forces have been performed in order to study passive control of unsteady loads induced on a circular cylinder. For this purpose, an aerodynamic balance has been developed. The balance, developed for a cylinder of 25.4 mm in diameter, was designed to operate in the subcritical regime (Re = 32000). This instrument is characterized by its sensitive element that forms a small central part of the cylinder. The static and dynamic calibrations of the balance show the appropriateness of the present design. Moreover, qualification experiments carried out with a single cylinder gave results (mean and rms values of the lift and drag coefficients) that are in good agreement with those found in the literature. The purpose of this paper is to present a passive control experiment performed by means of the wake of a smaller cylinder interacting with a larger one. Firstly, a parametric study was performed by varying the following: i) the diameter d_s of the small cylinder for one large cylinder diameter d (7 values in the range 0.047 ≤ d_s/d ≤ 0.125); ii) the center-to-center spacing S/d (11 values in the range 1.375 ≤ S/d ≤ 2.5); and iii) the stagger angle a (0 ≤ a ≤ 90 with a fine angular step Da for a ≤ 15 .). A maximum mean drag reduction of about 48% is achieved. At a = 4· to 8·, one can observe a peak of mean lift coefficient. Then unsteady fluid forces, vortex shedding frequency and flow pattern were systematically investigated for the small cylinder having a diameter d_s = 2.4 mm (0.094 d). Reductions of 78% and 56% of the rms lift and drag fluctuations respectively were obtained with the small cylinder placed at a slight stagger angle in the range 6· < a < 9·. This leads to an instantaneous force vector that exhibits more steadiness both in angle and amplitude. Moreover, at these stagger angles, the energy of the lift fluctuations at the shedding frequency is significantly reduced compared to the single cylinder case.