An Experimental Investigation of the Flow Around Straked Cylinders

Abstract

This paper presents experimental results concerning the response of circular cylinders with and without strakes. The longitudinal and transverse fluid forces (drag and lift), amplitude response and wake structures of plain and helically straked cylinders are compared. Six different configurations of straked cylinders with pitches (p) equal to 5D, 10D and 15D and heights (h) equal to 0.1D and 0.2D are investigated. Measurements on the dynamic response oscillations of an isolated plain and straked cylinders and flow visualization employing a PIV system are shown. Fixed cylinder drag measurements are also shown. The models are mounted on an elastic base fitted with flexor blades and instrumented with strain gauges or in an air bearing base. The base is fixed on the test-section of a water channel facility. The flexor blades possess a low-damping and the flexor blades base an the air bearing base are free to oscillate only in the cross-flow direction. The Reynolds number of the experiments ranges from 2000 to 10000, and reduced velocities, based on natural frequency in still water, vary up to 13. The drag coefficient is increased by 20% for the h = 0.1D cylinder, and 60% for the h = 0.2D cylinder, comparing both with the plain cylinder. The smaller height strokes (h = 0.1D) do not prevent vortex formation in the region very close to the body, resulting in a decrease of about 50% of the amplitude response compared with the plain cylinder. Lowest amplitude response was found to the p = 10D and h = 0.2D case. The analysis of the vorticity contours shows that the shear layer does not roll close to the body (same result for the other cases with h = 0.2D).

Investigation on the effectiveness of helical strakes in suppressing VIV of flexible riser

“…However, the findings can be directly applied to real world riser at much higher Reynolds numbers as the increase of Reynolds number does not affect the performance of helical strakes. This is due to the fact that the separation points of the flow remain the same for various Reynolds number [ 27 ]. In addition, it was proved that the drag coefficient of a cylinder with strakes was practically unchanged over sub-critical, critical and post-critical flow condition, as cited by Simantiras and Willis [ 21 ].…”

Suppression of wake-induced vibration of tandem cylinders with free-to-rotate control plates

“…These means might not only be ineffective for the interference-induced oscillations [WIV] but even detrimental.'' To cite an example, Korkischko et al (2007) showed that helical strakes typically effective in reducing VIV for an isolated cylinder are no longer successful if the body is immersed in the wake interference region.…”

Experimental investigation of flow-induced vibration on isolated and tandem circular cylinders fitted with strakes

“…Zdravkovich (1984) showed the adverse effect of interference in the effectiveness of strakes in suppressing VIV. More recently, Korkischko et al (2007a) demonstrated that strakes lose their effectiveness when a downstream straked cylinder elastically mounted is immersed in the wake of an upstream plain fixed cylinder at s/d = 3.4 centre-to-centre distance.…”

“…Among the various geometrical forms of devices for passive suppression, helical strakes are some of the most common in both air and water flows, as can be seen in chimneys and offshore structures, such as risers and spars. Experimental studies about the effectiveness of strakes were performed by Simantiras and Willis (1999), Bearman and Brankovi´c (2004), Trim et al (2005), Brankovi´c and Bearman (2006) and Korkischko et al (2007b). Strakes can effectively suppress VIV, depending on pitch and height parameters at the expense of drag increase.…”