Vortex shedding in oscillatory flow

• Main Author: Al-Asmi, Khalfan
• Published: University of Surrey 1992
• Subjects: 532; Fluid mechanics
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305535

Measurements of the response to inline flow oscillations of vortex shedding from certain bluff bodies have been made. Four cylinders with fixed separation points were exposed to a mean stream with controlled sinusoidal oscillations at defined frequency ratios and amplitudes. Attention was concentrated on the highly sensitive reduced velocity regime around the inverse of twice the Strouhal number, 1/2S. Synchronisation of vortex formation was first established; then, for conditions at which synchronisation occured, threshold amplitude was measured. In order to determine the common and distinguishing features of bluff body shape the response from cylinders having 'zero' and finite afterbody were investigated. Finally, for a selected cylinder, the influence, in the presence of oscillations, of turbulence intensity (of defined scale), solid blockage and aspect ratio to the synchronous range were examined. Of particular interest were the changes that occur in the characteristic period of vortex formation and base pressure, relevant to the design and application of vortex flowmeters and self induced excitations of structures in general. The experiments were carried out in two separate blower tunnels under various oscillatory flow conditions in the Reynolds number range (0.5-5.0) X 104 and amplitudes of velocity fluctuation (+/-AU/U) of up to 0.3. It was found that amplitudes of oscillations of the order of 0.025 were sufficient to induce frequency lock-in when the reduced velocity was close to 1/2S and, provided the amplitude was sufficiently large, limited synchronisation could also be induced near the upper and lower reduced velocities of 1/S and 1/4S. Synchronisation is accompanied by enhanced vortex shedding which, in turn, can lead to increased base suction (and therefore increase in drag force). Flow visualisation revealed that the near wake vortex arrangement can vary a great deal depending on the ratio of Strouhal numbers at the forced and self-excited frequencies (N/no). It was concluded that the behaviour of the base pressure reflected this situation and that the range of synchronisation depended strongly on the stability of the prevailing mode of vortex shedding. The precise details of the flow were found to be highly dependent on body geometry and the frequency ratio N/no. The production of oscillatory flow in the wind tunnels did not proved to be an easy task. A review of the various methods that have been used in the past is presented. The advantages and disadvantages of different techniques are highlighted and details are given of a further method developed for use with the present open circuit blower tunnels of differing sizes. In the smaller tunnel, having a working section size of 0.3x0.3m, it was possible to produce sinusoidal variations of the working section flow, having peak to peak amplitude of up to 60% of the mean flow speed and frequencies up to, typically, that corresponding to the acoustic quarter-wavelength frequency determined by the tunnel length. Over the viable working range, the device is shown to produce high quality periodic flow with negligible harmonic distortion or acoustic noise difficulties.