Affiliations: Laboratoire de Mécanique des Fluides,
Département de génie mécanique, Université Laval,
Ste-Foy (Québec) G1K 7P4, Canada.
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.