Fluid–structure interaction of a square cylinder at different angles of attack

Abstract

This study investigates the free transverse flow-induced vibration (FIV) of an elastically mounted low-mass-ratio square cylinder in a free stream, at three different incidence angles: α = 0 • , 20 • and 45 • . This geometric setup presents a body with an angle of attack, sharp corners and some afterbody, and therefore is a generic body that can be used to investigate a wide range of FIV phenomena. A recent study by Nemes et al. (J. Fluid Mech., vol. 710, 2012, pp. 102-130) provided a broad overview of the flow regimes present as a function of both the angle of attack α and reduced flow velocity U * . Here, the focus is on the three aforementioned representative angles of attack: α = 0 • , where the FIV is dominated by transverse galloping; α = 45 • , where the FIV is dominated by vortex-induced vibration (VIV); and an intermediate value of α = 20 • , where the underlying FIV phenomenon has previously been difficult to determine. For the α = 0 • case, the amplitude of oscillation increases linearly with the flow speed except for a series of regimes that occur when the vortex shedding frequency is in the vicinity of an odd-integer multiple of the galloping oscillation frequency, and the vortex shedding synchronizes to this multiple of the oscillation frequency. It is shown that only odd-integer multiple synchronizations should occur. These synchronizations explain the 'kinks' in the galloping amplitude response for light bodies first observed by Bearman et al. (J. Fluids Struct., vol. 1, 1987, pp. 19-34). For the α = 45 • case, the VIV response consists of a number of subtle, but distinctly different regimes, with five regimes of high-amplitude oscillations, compared to two found in the classic VIV studies of a circular cylinder. For the intermediate α = 20 • case, a typical VIV 'upper branch' occurs followed by a 'higher branch' of very large-amplitude response. The higher branch is caused by a subharmonic synchronization between the vortex shedding and the body oscillation frequency, where two cycles of vortex shedding occur over one cycle of oscillation. It appears that this subharmonic synchronization is a direct result of the asymmetric body. Overall, the FIV of the square cylinder is shown to be very rich, with a number of distinct regimes, controlled by both α and U * . Importantly, α controls the underlying FIV phenomenon, as well as controlling the types of possible synchronization between the oscillation and vortex shedding.

Experimental investigation of flow-induced vibration of a rotating circular cylinder

“…At m * = 5.78, the upper branch covers the range U * = 4.8-6.4; the lower response branch lies within the range 6.4 U * 9.5; and the desynchronised region occurs for U * > 9.5. Previous studies by Khalak & Williamson (1997) and Zhao et al (2014b) at m * = 2.4 showed a peak amplitude response of A * ≈ 0.95. The mean of the highest 10 % of peak amplitude response (A * 10 ) of the current system is A * 10 = 0.82.…”

“…The vibration response of a non-rotating circular cylinder undergoing VIV is compared against previous work by Khalak & Williamson (1997& Williamson ( , 1999 and Zhao et al (2014b) in figure 4 . F igure 4 (a) p resents t he a mplitude r esponse a s a f unction o f reduced velocity and shows the current experimental configuration p roduces n on-rotating VIV results in good agreement with previous VIV studies.…”

“…VIV and galloping need not occur mutually exclusively; their occurrence depends on the flow and structural properties. In fact, the two FIV phenomena may occur in the same flow regimes, as shown in Corless & Parkinson (1988), Nemes et al (2012) and Zhao et al (2014b). Nemes et al (2012) and Zhao et al (2014b) have shown that large amplitude response regions can result from the interaction between VIV and galloping.…”

“…galloping), it should be noted that VIV can still occur under certain conditions (e.g. mass and damping ratios) and dominate over a range of reduced velocity when the axial symmetry is broken, as demonstrated by Corless & Parkinson (1988), Nemes et al (2012) and Zhao et al (2014b). In such cases, however, the body may also be subjected to transverse galloping.…”

Experimental Investigation on Soft Galloping and Hard Galloping of Triangular Prisms

“…In addition to the circular cylinder, other prisms such as square prism and triangular prism also presented VIV responses. Nemes et al [10,11] reported that the incident angle had a significant influence on the performance of square prisms. When the incident angle was 45 degrees (diamond cross-section), the square prism presented a self-limited motion (which was VIV).…”