Flow around a circular cylinder: Aspects of fluctuating lift

The paper is concerned with some aspects of the fluctuating lift acting on a stationary circular cylinder in cross flow, in particular effects of Reyn olds number in the nominal case of a large aspect-ratio cylinder at small t o vanishing blockage and free-stream turbulence, respectively. The Reynolds number range covered is from about Re = 47-22 x 10(5), i.e., from the onse t of vortex shedding up to the point where a subsequent increase in Re give s 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 coeffic ient (C-L ') within the laminar shedding regime can be approximated as root epsilon /30+epsilon (2)/90, where epsilon = (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 correla ted along the span. The lift fluctuations on a finite (spanwise) length of the cylinder are then not only dependent on the sectional lift variations b ut 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-si ded spanwise correlation length (Lambda) is about twice the wavelength of t he most unstable mode A instability, Lambda /d approximate to 7. Up to Re = 260-300 the spanwise correlation increases dramatically, the indicated pea k value being Lambda /d approximate to 30. From 3-D numerical simulations, the corresponding C-L ' is approximately 0.5, which coincidentally is about the same value as found experimentally just before the rapid fall when ent ering the critical regime. Dramatic variations of both sectional C-L ' and Lambda /d occur within the range Re similar or equal to 0.3 x 10(3)-2.2 x 1 05. For instance, at around Re = 1.6 x 10(3) a local minimum of about C-L ' = 0.045 is indicated, at Re - 16 x 103 the corresponding C-L '-value is te n times higher. At Re = 5.1 x 103 there is a peak in spanwise correlation, Lambda /d approximate to 15. (C) 2001 Academic Press.