Passive control of the aerodynamic forces acting on a circular cylinder

Abstract

Abstract The passive control of the unsteady loads induced on a circular cylinder has been studied. For this purpose, an aerodynamic balance allowing the simultaneous measurement of fluctuating lift and drag forces has been developed. The balance, designed for a cylinder of 25.4 mm in diameter, was used in the subcritical regime (15 000 ⩽ Re⩽ 40 000). This balance is characterized by its sensitive element which forms a small central part of the cylinder. The design steps and the calibration procedure – static and dynamic – are first presented. A set of experiments carried out with a single cylinder confirms the appropriateness of the present design. Both the mean and rms values of the lift and drag coefficients are in good agreement with those found in the literature. Section 3 presents the passive control performed by means of the wake of a small cylinder (ds=d/8) interacting with the larger one. The small cylinder was located in various places at a spacing S/d=2.55 and covering a range of stagger angles 0∘⩽α⩽90∘. A mean drag reduction of 32% was obtained for the tandem arrangement (α=0∘). At α=4.5∘, one observes a peak of mean lift coefficient and a 41% reduction of the rms lift fluctuations. This leads to an instantaneous force vector which exhibits more steadiness both in angle and amplitude. Moreover, at this stagger angle, the energy of the lift fluctuations at the shedding frequency is reduced by 80% compared with the single cylinder case. Smoke-wire flow visualizations were used to provide more insight into the analysis.