FLOW AROUND A CIRCULAR CYLINDER: ASPECTS OF FLUCTUATING LIFT

Abstract

The paper is concerned with some aspects of the fluctuating lift acting on a stationary circular cylinder in cross flow, in particular effects of Reynolds number in the nominal case of a large-aspect-ratio cylinder at small to vanishing blockage and free-stream turbulence, respectively. The Reynolds number range covered is from about Re=47–2·2×105, i.e., from the onset of vortex shedding up to the point where a subsequent increase in Re gives a rapid fall in the mean drag coefficient, the all-familiar drag crisis. A review of 2-D numerical simulations suggests that the r.m.s. lift coefficient (CL′) within the laminar shedding regime can be approximated asε/30+ε2/90, whereε =(Re−47)/47. For all Reynolds numbers above the inception of three-dimensional flow instabilities, i.e., for Re>(160–190), the near-wake flow is supposed to be partially correlated along the span. The lift fluctuations on a finite (spanwise) length of the cylinder are then not only dependent on the sectional lift variations but also on the spanwise correlation of the lift-related flow. At around Re=230, which is the approximate onset Re for mode B instability, the one-sided spanwise correlation length (Λ) is about twice the wavelength of the most unstable mode A instability,Λ /d≈7. Up to Re=260–300 the spanwise correlation increases dramatically, the indicated peak value being Λ/d≈30. From 3-D numerical simulations, the correspondingCL′ is approximately 0·5, which coincidentally is about the same value as found experimentally just before the rapid fall when entering the critical regime. Dramatic variations of both sectionalCL′ and Λ/d occur within the range Re≃0·3×103–2·2×105. For instance, at around Re=1·6×103a local minimum of about CL′=0·045 is indicated, at Re≃16×103the corresponding CL′-value is ten times higher. At Re=5·1×103there is a peak in spanwise correlation, Λ/d≈15.