Two- and Three-Dimensional Simulations of the Flow Around a Cylinder Fitted With Strakes

Abstract

Two- and three-dimensional simulations of the flow around straked cylinders are presented. For the two-dimensional simulations we used the Spectral/hp Element Method, and carried out simulations for five different angles of rotation of the cylinder with respect to the free stream. Fixed and elastically-mounted cylinders were tested, and the Reynolds number was kept constant and equal to 150. The results were compared to those obtained from the simulation of the flow around a bare cylinder under the same conditions. We observed that the two-dimensional strakes are not effective in suppressing the vibration of the cylinders, but also noticed that the responses were completely different even with a slight change in the angle of rotation of the body. The three-dimensional results showed that there are two mechanisms of suppression: the main one is the decrease in the vortex shedding correlation along the span, whilst a secondary one is the vortex wake formation farther downstream.

Stability analysis of the flow around a cylinder fitted with helical strakes

“…It is also remarkable that the Strouhal number changes with the phase angle, so the change in vortex shedding frequency with the phase angle could explain the decrease in vortex shedding correlation along the spanwise. These effects of the straight strakes homogeneous in the spanwise spatial direction on the cylinder flow features are in perfect qualitative agreement with the ones described using two-dimensional VIV analysis by Carmo et al 14 Complete fully three-dimensional time-resolved Direct Numerical Simulations have been performed from Re = 20 to Re = 100 employing the solver and meshes described in the previous section. Comparisons have been made against the flow of a bare cylinder with the same pitch p = 5d as the straked cylinder.…”

“…This least-squares fitting permits the identification of the linear regime of the growth of the leading eigenmodes before the corresponding linear saturation. As a consequence of the linearization carried out in equation (5) and the ansatz (6), the residuals from the temporal integration of the drag coefficient defined as C L = C L (t + dt) − C L (t) can be fitted to the sought solution as C L = Ĉ L e λrt cos(2πSt + φ) (14) for the particular case of an unique oscillatory leading eigenvalue. The fitting is iteratively carried out until the the sum of the squared differences of the function χ 2 is less than 10 −5 and the asymptotic standard error of each parameter Ĉ L , λ r , St and φ are less than 1%.…”

VIV response of a long flexible riser fitted with strakes in uniform and linearly sheared currents

“…Hence, the problem can be reframed as finding a riser with a specified strake configuration that exhibits the least fatigue damage. As shown in Table 1, this problem has been studied experimentally [21][22][23][24][25][26][27][28][29][30] and numerically [31][32][33][34] by many researchers in last decade.…”

A review on flow-induced vibration of offshore circular cylinders

“…(3) Helical strakes can destroy regular vortex shedding in the streamwise direction and prevent the vortex structure from becoming correlated in the span wise direction and elongate the formation length of wake [132][133][134][135] , as shown in Fig. 12 [132] demonstrating the flow visualization for both cylinders.…”

“…Fig. 12 (Color online) Flow visualization of the bare and straked cylinder wakes on different planes [132] (4) Helical strakes increase the mean drag coefficient d C [135][136][137] , which is found to be independent of Re [138] and result in a non-zero mean lift coefficient l C [137]. (5) Helical strakes can change the high order harmonics and multi-mode response to a single mode or at least fewer mode response with lower orders [139] .…”