Abstract
When a cylinder is subject to a flow, vortices will be shed that can lead to strong tonal noise. The modification of the cylinder with soft, flexible flaps made of silicone rubber has been shown to affect the vortex shedding cycle in a way that the Strouhal number associated with the vortex shedding suddenly jumps to a higher value at a certain Reynolds number. In the present study, the effect of the flexible flaps on the vortex shedding is further examined by subsequently reducing the number of flaps and additionally shortening their length. Acoustic measurements and camera recordings of the flap motion, performed in an aeroacoustic wind tunnel, suggest that the sudden jump of the Reynolds number is caused by the movement of the outer flaps. A comparison with the eigenfrequencies obtained from a numerical modal analysis of the different flap rings revealed that the cause of the Strouhal number jump is most likely a lock-in of the natural vortex shedding cycle with the next higher eigenfrequency of the outer flaps.
Highlights
Periodic vortex shedding from cylinders is a classical problem in aerodynamics and an important source of noise [1, 2]
Results from Acoustic Measurements The sound pressure level spectra measured for the cylinders from Figure 2 as well as for the reference cylinder are shown in Figure 8 as a function of Strouhal number based on the outer diameter
The above discussed effect that the vortex shedding peak obtained for the flap cylinders suddenly jumps towards a higher Strouhal number at Reynolds numbers between 23,300 and 26,000 is visible, while the peak of the reference cylinder stays at a constant value just above 0.2
Summary
Periodic vortex shedding from cylinders is a classical problem in aerodynamics and an important source of noise [1, 2]. In a water tunnel study by Kunze and Brücker [15] it has been shown that the presence of flexible flaps at the aft part of a circular cylinder strongly affects the vortex shedding behavior. This led to the fact that at a certain specific Reynolds number Re (based on cylinder diameter d) the Strouhal number Sr associated with vortex shedding quite suddenly increased from a value of about 0.23 to a value of 0.29. The result was a jump in the corresponding Reynolds-Strouhal number diagram (when instead of the cylinder diameter the streamwise length of the separation bubble was used to calculate the Strouhal number, no jump was visible). The reason for this effect was found to Received 16 February 2018, accepted 4 December 2018
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