Suppression Of Vortex Shedding From Stationary And Oscillating Cylinders

The control of vortex shedding from bluff bodies is of theoreticalsignificance and important applications in ocean, aerospace and civil engineering.This paper contains two parts, the first part describes the suppression ofvortex shedding from a stationary two-dimensional rectangular cylinder bythe method of small element; and the second part shows the suppression ofvortex shedding from a two-dimensional circular cylinder oscillating instream-wise direction by three different methods.In the first part, circular, square, triangular and thin-strip elements areused respectively and set downstream of a rectangular cylinder ofstream-wise to transverse scale ratio B/H=3.0. The test range of Reynoldsnumber Re=V H/=75~130, the transverse dimension of element b/H isfixed at0.2. The results of numerical simulation and experiment show thatvortex shedding from both sides of the cylinder can be suppressed andfluctuating drag and lift of the cylinder can be greatly reduced, if an elementis placed in a certain region called effective zone. Comparisons at a Reynoldsnumber show that, the square element is of the largest, and the triangularelement of the smallest size of effective zone. The effective zone of squareelement shrinks with the increase of Re number and disappears at Re>130.The mechanism of suppression is discussed and a physical model of vortexformation is proposed in this paper.In the second part, three different methods are used to suppress vortexshedding from a two-dimensional circular cylinder oscillating in stream-wisedirection:1) Base blowing: air is blown out of the hollow cylinder through aspan-wise slit of width h/D=0.015and length nearly equal to the axial scaleof the cylinder. The test range of blow speed is Vb/V_∞=0~18. The test rangesof Reynolds number Re=V_∞D/=2700~9800, amplitude and frequency ofcylinder oscillation A/D=0.2and f_eD/V_∞=0~0.36, where D is diameter of cylinder, V_∞is velocity of on-coming stream.In case without control, with the increase of oscillation frequency f_eD/V_∞from0to0.36, there are four patterns of vortex shedding: a) f_eD/V_∞<0.08,the frequency of vortex shedding is the same as that shedding from astationary cylinder. b) f_eD/V_∞=0.08~0.10, vortex shedding frequency is2times the frequency of cylinder oscillation. c) f_eD/V_∞=0.10~0.16, vortexshedding frequency is the same as the frequency of cylinder oscillation. d)f_eD/V_∞=0.16~0.32, vortex shedding frequency is half the frequency ofcylinder oscillation. Results of smoke-wire visualization and hot-wiremeasurement in wind tunnel show that, all the four patterns of vortexshedding can be suppressed by base blowing, if the blow speed is in a certainrange called effective zone. The effective zone is found out in the plane ofVb/V_∞and f_eD/V_∞coordinates. The effect of Reynolds number on the effectivezone is investigated, the influence of blow angle was studied, and thesuppression mechanism is discussed.2) Single element set downstream of the oscillating cylinder: the mainsurface of strip element is perpendicular to the oncoming stream, the stri p ofwidth b/D=0.2, thickness h/D=0.05. The test ranges of Reynolds numberRe=2700~9800, amplitude and frequency of cylinder oscillation A/D=0~0.3,and f_eD/V_∞=0~0.36. Visualization pictures, power spectra of fluctuatingvelocity and fluctuating lift of cylinder show that, the first, second, and forthpatterns of vortex shedding can be suppressed if the element is set in a certainregion called effective zone. The effect of element on mean and fluctuatinglift and drag of cylinder is studied. The influences of Reynolds number,amplitude and frequency of cylinder oscillation on the effective zone areinvestigated.Double elements set downstream of the oscillating cylinder:the mainsurface of each strip element is perpendicular to the oncoming stream.Thestrip width is b/D=0.2, the strip thickness is h/D=0.05. The test Reynoldsnumber Re=6000，the ranges of oscillation frequency0<f_eD/V_∞<0.064. Two typesof geometrical arrangements of double elements are investigated:side-by-side and non side-by-side. In side-by-side arrangement, effective zone is found out, and the influences of distance between the elements, andamplitude and frequency of oscillation on the effective zone are studied. Innon side-by-side arrangement, the position of the first element is fixed, andthe position of the second element is adjustable. Two cases are studied: i n onecase vortex shedding from the oscillating cylinder cannot be suppressed bysolely the first element applied at a certain place, but it can be suppressed ifthe second element is also applied and set in an effective zone. In the othercase vortex shedding from the oscillating cylinder can be suppressed bysolely the first element applied at a favorable place, but vortex sheddingrecovers if the second element is also applied and set in a certain region. Theeffective zones of second element are found out at different frequencies ofcylinder oscillation, and the influence of position of first element on effectivezone of second element is investigated.