Abstract:Structures in crossflow are susceptible to vortex-induced vibrations (VIV) when the vortex-shedding becomes synchronised with the structural vibration. Strategies to control VIV often include modifying the separation point on the cylinder surface, such as adding helical strakes, although there remains disagreement regarding the mechanism by which these work. We explore the role of the separation point on VIV acting in the streamwise (drag) direction, by performing high-speed Particle-Image Velocimetry (PIV) me… Show more
“…They argued that the ability of the separation points to move was a requirement for the various wake modes that are capable of exciting VIV, and that the absence of VIV for the triangular prism was a result of the sharp corners and the fixed nature of the separation points. Although the above conclusion is made for the streamwise VIV by Cagney & Balabani (2019), we believe the same mechanism also applies to the transverse VIV. Figure 26.Time histories of the displacement and velocity gradient () at the upper side intersection point of a D-section prism for and .…”
Section: Discussionsupporting
confidence: 57%
“…Mei & Currie (1969) stated that for a vibrating circular cylinder, the vortex shedding frequency significantly depends on the angle displacement range θ sp of the separation points. This is further proved by the recent high-speed particle image velocimetry observations of Cagney & Balabani (2019). It was observed that when the vibration amplitude is low, the separation points show very little variation for a circular cylinder while they are fixed for a triangular prism, and correspondingly, the vortex shedding frequency closely follows the Strouhal frequency.…”
Section: The Role Of the Separation Pointmentioning
confidence: 68%
“…As shown in figure 26, in a vibration period, the velocity gradient () at the upper intersection point of the D-section prism is maintained positive, which suggests that the boundary layer always separates from the prism at the sharp corner. However, for a circular cylinder, the separation points move along the cylinder surface throughout the vortex shedding cycle (Mei & Currie 1969; Jukes & Choi 2009; Cagney & Balabani 2019). Mei & Currie (1969) stated that for a vibrating circular cylinder, the vortex shedding frequency significantly depends on the angle displacement range of the separation points.…”
Section: Discussionmentioning
confidence: 99%
“…For these categories, the circular cylinder, the square or triangular prism and the semicircular (D-section) prism are the typical ones, respectively. Cagney & Balabani (2019) found that although the flow separation point of a vibrating circular cylinder oscillates back and forth along a small segment of the cylinder surface, it significantly affects the vibration responses in the lock-in region by modifying the added mass and the vortex shedding frequency. As a result, the vibration frequency of the circular cylinder locks on its real natural frequency (considering the added mass effects) in a wide reduced velocity range, instead of achieving the resonance in a narrow velocity band for a linear dynamic system.…”
This paper presents the response and the wake modes of a freely vibrating D-section prism with varying angles of attack (
$\alpha = 0^\circ \text {--}180^\circ$
) and reduced velocity (
$U^* = 2\text {--}20$
) by a numerical investigation. The Reynolds number, based on the effective diameter, is fixed at 100. The vibration of the prism is allowed only in the transverse direction. We found six types of response with increasing angle of attack: typical vortex-induced vibration (VIV) at
$\alpha = 0^\circ \text {--}35^\circ$
; extended VIV at
$\alpha = 40^\circ \text {--}65^\circ$
; combined VIV and galloping at
$\alpha = 70^\circ \text {--}80^\circ$
; narrowed VIV at
$\alpha = 85^\circ \text {--}150^\circ$
; transition response, from narrowed VIV to pure galloping, at
$\alpha = 155^\circ \text {--}160^\circ$
; and pure galloping at
$\alpha = 165^\circ \text {--}180^\circ$
. The typical and narrowed VIVs are characterized by linearly increasing normalized vibration frequency with increasing
$U^*$
, which is attributed to the stationary separation points of the boundary layer. On the other hand, in the extended VIV, the vortex shedding frequency matches the natural frequency in a large
$U^*$
range with increasing
$\alpha$
generally. The galloping is characterized by monotonically increasing amplitude with enlarging
$U^*$
, with the largest amplitude being
$A^* = 3.2$
. For the combined VIV and galloping, the vibration amplitude is marginal in the VIV branch while it significantly increases with
$U^*$
in the galloping branch. In the transition from narrowed VIV to pure galloping, the vibration frequency shows a galloping-like feature, but the amplitude does not monotonically increase with increasing
$U^*$
. Moreover, a partition of the wake modes in the
$U^*$
–
$\alpha$
parametric plane is presented, and the flow physics is elucidated through time variations of the displacement, drag and lift coefficients and vortex dynamics. The angle-of-attack range of galloping is largely predicted by performing a quasi-steady analysis of the galloping instability. Finally, the effects of
$m^*$
and
${\textit {Re}}$
, the roles of afterbody and the roles of separation point in determining vibration responses and vortex shedding frequency are further discussed.
“…They argued that the ability of the separation points to move was a requirement for the various wake modes that are capable of exciting VIV, and that the absence of VIV for the triangular prism was a result of the sharp corners and the fixed nature of the separation points. Although the above conclusion is made for the streamwise VIV by Cagney & Balabani (2019), we believe the same mechanism also applies to the transverse VIV. Figure 26.Time histories of the displacement and velocity gradient () at the upper side intersection point of a D-section prism for and .…”
Section: Discussionsupporting
confidence: 57%
“…Mei & Currie (1969) stated that for a vibrating circular cylinder, the vortex shedding frequency significantly depends on the angle displacement range θ sp of the separation points. This is further proved by the recent high-speed particle image velocimetry observations of Cagney & Balabani (2019). It was observed that when the vibration amplitude is low, the separation points show very little variation for a circular cylinder while they are fixed for a triangular prism, and correspondingly, the vortex shedding frequency closely follows the Strouhal frequency.…”
Section: The Role Of the Separation Pointmentioning
confidence: 68%
“…As shown in figure 26, in a vibration period, the velocity gradient () at the upper intersection point of the D-section prism is maintained positive, which suggests that the boundary layer always separates from the prism at the sharp corner. However, for a circular cylinder, the separation points move along the cylinder surface throughout the vortex shedding cycle (Mei & Currie 1969; Jukes & Choi 2009; Cagney & Balabani 2019). Mei & Currie (1969) stated that for a vibrating circular cylinder, the vortex shedding frequency significantly depends on the angle displacement range of the separation points.…”
Section: Discussionmentioning
confidence: 99%
“…For these categories, the circular cylinder, the square or triangular prism and the semicircular (D-section) prism are the typical ones, respectively. Cagney & Balabani (2019) found that although the flow separation point of a vibrating circular cylinder oscillates back and forth along a small segment of the cylinder surface, it significantly affects the vibration responses in the lock-in region by modifying the added mass and the vortex shedding frequency. As a result, the vibration frequency of the circular cylinder locks on its real natural frequency (considering the added mass effects) in a wide reduced velocity range, instead of achieving the resonance in a narrow velocity band for a linear dynamic system.…”
This paper presents the response and the wake modes of a freely vibrating D-section prism with varying angles of attack (
$\alpha = 0^\circ \text {--}180^\circ$
) and reduced velocity (
$U^* = 2\text {--}20$
) by a numerical investigation. The Reynolds number, based on the effective diameter, is fixed at 100. The vibration of the prism is allowed only in the transverse direction. We found six types of response with increasing angle of attack: typical vortex-induced vibration (VIV) at
$\alpha = 0^\circ \text {--}35^\circ$
; extended VIV at
$\alpha = 40^\circ \text {--}65^\circ$
; combined VIV and galloping at
$\alpha = 70^\circ \text {--}80^\circ$
; narrowed VIV at
$\alpha = 85^\circ \text {--}150^\circ$
; transition response, from narrowed VIV to pure galloping, at
$\alpha = 155^\circ \text {--}160^\circ$
; and pure galloping at
$\alpha = 165^\circ \text {--}180^\circ$
. The typical and narrowed VIVs are characterized by linearly increasing normalized vibration frequency with increasing
$U^*$
, which is attributed to the stationary separation points of the boundary layer. On the other hand, in the extended VIV, the vortex shedding frequency matches the natural frequency in a large
$U^*$
range with increasing
$\alpha$
generally. The galloping is characterized by monotonically increasing amplitude with enlarging
$U^*$
, with the largest amplitude being
$A^* = 3.2$
. For the combined VIV and galloping, the vibration amplitude is marginal in the VIV branch while it significantly increases with
$U^*$
in the galloping branch. In the transition from narrowed VIV to pure galloping, the vibration frequency shows a galloping-like feature, but the amplitude does not monotonically increase with increasing
$U^*$
. Moreover, a partition of the wake modes in the
$U^*$
–
$\alpha$
parametric plane is presented, and the flow physics is elucidated through time variations of the displacement, drag and lift coefficients and vortex dynamics. The angle-of-attack range of galloping is largely predicted by performing a quasi-steady analysis of the galloping instability. Finally, the effects of
$m^*$
and
${\textit {Re}}$
, the roles of afterbody and the roles of separation point in determining vibration responses and vortex shedding frequency are further discussed.
“…The movement of the separation point can also alter the vortex formation. This is why we find that, in the VIV of a circular cylinder, the lock-in occurs where the separation points on both sides of the circular cylinder oscillate along the surface significantly while in the desynchronization region the oscillation of the separation points is relatively small (Cagney & Balabani 2019). Together with the alternate vortex shedding, the separation points on the two sides of the prism move back and forth and, as a result, the pressure distribution on the prism surface displays significant changes during the vibration, which is responsible for the sustenance of the response.…”
This paper presents a comprehensive study of flow-induced vibrations of a D-section prism with various angles of attack
$\alpha$
(
$= 0^{\circ }\unicode{x2013}180^{\circ }$
) and reduced velocity
$U^*$
(= 2–20) via direct numerical simulations at a Reynolds number
${Re} = 100$
. The prism is allowed to vibrate in both streamwise and transverse directions. Based on the characteristics of vibration amplitudes and frequencies, the responses are classified into nine different regimes: typical VIV regime (
$\alpha = 0^{\circ }\unicode{x2013}30^{\circ }$
), hysteretic VIV regime (
$\alpha = 35^{\circ }\unicode{x2013}45^{\circ }$
), extended VIV regime (
$\alpha = 50^{\circ }\unicode{x2013}55^{\circ }$
), first transition response regime (
$\alpha = 60^{\circ }\unicode{x2013}65^{\circ }$
), dual galloping regime (
$\alpha = 70^{\circ }$
), combined VIV and galloping regime (
$\alpha = 75^{\circ }\unicode{x2013}80^{\circ }$
), narrowed VIV regime (
$\alpha = 85^{\circ }\unicode{x2013}145^{\circ }$
), second transition response regime (
$\alpha = 150^{\circ }\unicode{x2013}160^{\circ }$
) and transverse-only galloping regime (
${\alpha = 165^{\circ }\unicode{x2013}180^{\circ }}$
). In the typical and narrowed VIV regimes, the vibration frequencies linearly increase with increasing
$U^*$
. In the hysteretic and extended VIV regimes, the vibration amplitudes are large in a wider range of
$U^*$
as a result of the closeness of the vortex shedding frequency to the natural frequency of the prism because of the shear layer reattachment and separation point movement. In the two galloping regimes, the transverse amplitude keeps increasing with
$U^*$
while the streamwise amplitude stays small or monotonically increases with increasing
$U^*$
. In the combined VIV and galloping regime, the vibration amplitude is relatively small in the VIV region while drastically increasing with increasing
$U^*$
in the galloping region. In the transition response regimes, the vibration frequencies are galloping-like but the divergent amplitude cannot persist at high
$U^*$
. Furthermore, a wake mode map in the examined parametric space is offered. Particular attention is paid to physical mechanisms for hysteresis, dual galloping and flow intermittency. Finally, we probe the dependence of the responses on Reynolds numbers, mass ratios and degrees of freedom, and analyse the roles of the shear layer reattachment and separation point movement in the appearance of multiple responses.
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