| Much attention has been paid to the development of marine resources due to the growingscarce of land resources. Wave and current flow causes periodic vortex shedding in the wakeof the riser under certain flow rate, and vortex-induced vibration (VIV) may occur when thenatural frequency of the riser is close to the vortex shedding frequency. Sustained VIV maycause fatigue failure of the riser, thereby affecting marine operations, and even lead to seriousengineering and environmental accidents. Therefore, the basic investigation of different VIVsuppression devices for riser is of important practical and theoretical significance.Wind tunnel experiments were used as an important method for studying the flow controland vortex shedding of riser in the paper. Comparison among the wind tunnel experiments,water tunnel experiment and CFD calculation results verified the feasibility of this method.Wind and water tunnel results of the fixed splitter plates and axial-rod shrouds show that, theflow similarity of the two methods is very good when only considering dynamiccharacteristics and vortex shedding behavious, namely without considering vortex inducedvibration. While CFD calculation results of axial-rod shrouds are also close to that of the windtunnel, which indicates the feasibility of this method in another aspect.Characteristics of fluctuating forces and spectrum of fixed/rotatable splitter plates wereexperimentally studied. The results show that: Rotatable splitter plates oscillate slightly withvery small amplitude in some certain equilibrium position when L/D<4.0. But it wouldoscillate slightly at=180°instead of deflecting to a stable angle on one side when L/D4.0.Base pressure of the riser with fixed splitter plates are much higher than that of the bare riser.The time-averaged pressure distribution of the short rotatable splitter plates is no longersymmetrical to=180°. While the results of long rotatable splitter plates are similar to thefixed ones. Within the studied range of0.5L/D6.0, a maximum time-averaged drag Cdand fluctuating lift Cl’ reduction up to30.3%and96.4%was obtained for fixed splitter plates when L/D=6.0, and31.2%and91.4%for rotatable ones also when L/D=6.0. St of fixed splitter plates are all lower than that of bare riser. But St of rotatable ones are higher than that of bare riser when L/D≤2and get lower than that when L/D>2.Characteristics of fluctuating forces and spectrum of fixed/rotatable fairings were experimentally studied. The results show that:Rotatable fairing rotates to an equilibrium position (on either side of the wake with equal probability) rather than align itself with free stream due to the integrated effect of the pressure difference along the either side of the fairing. Near wake pressure of the riser is relative stable and higher than that of the bare riser when fixed fairings attached, but it is higher on only one side when rotatable fairings attached. Further, it is concluded that both of the time-averaged drag coefficients and the fluctuating lift coefficients are less than the corresponding bare riser. Within the studied range of30°≤α≤90°, a maximum time-averaged drag Cd and fluctuating lift Cl’ reduction up to36.1%and74.5%was obtained for fixed fairings when α=30°and α=45°, respectively; and43.5%and67.0%for rotatable ones when α=30°and α=75°, respectively. No matter fixed or rotatable fairings, there is only one main frequency in each power spectrum of lift, and the peak values are all much lower than that of the bare riser. St gets smaller with the decreasing of α when fixed fairings attached, but the trend is opposite for rotatable ones.Characteristics of fluctuating forces and spectrum of tail-fairings were experimentally studied. The results show that:Tail-fairing also rotates to an off-axis equilibrium position (on either side of the wake with equal probability) rather than align itself with free stream. But compared to the corresponding rotatable fairing, the existence of flexible tail-plate plays a role in reducing off-axis angle δ. Within the studied range of45°≤α≤90°, a maximum time-averaged drag Cd reduction up to16.5%was obtained for tail-fairings when α=90°, L=1.5D; and a maximum fluctuating lift Cl’ reduction up to25.7%when α=60°, L=1.0D. There is no obvious main frequency in each power spectrum of lift for tail-fairings, and the peak values are extremely lower than that of the bare riser.Characteristics of fluctuating forces and spectrum of axial-rod shrouds were experimentally studied. The results show that:For axial-rod shrouds with two rods diameters4mm and8mm, there are great differences between the pressure distributions of all shrouds. More rods cases (N>8) are less affected by inflow angle than less rods cases (N<6), and the more rods the less sensitive to inflow angle. Within the studied range of3≤N≤12, N≥8shrouds are least sensitive to inflow angle, over30%and80%reductions of time-averaged drag Cd and fluctuating lift Cl’ were obtained for these shrouds, respectively. The peak values of few cases, including (N=3, α=0°(N=4, α=45°) and (N=5, α=36°), are higher than that of the bare riser, while the peak values are lower for most cases. It illustrates that the alternating Karman type vortex is effectively suppressed with axial-rod shrouds attached.The advantages and disadvantages of these devices were summarized after a comparative analysis. First, fixed splitter plate and fixed fairing have the best effects of drag and lift reduction and vortex shedding suppression. But the shortcoming of not having flow direction adaptive ability limits their engineering application. Second, rotatable splitter plate develops a time-averaged transverse force, but due to flow direction adaptive ability and fine fluctuating lift and vortex shedding suppression effects, it shows certain practical value. Third, time-averaged drag, fluctuating lift and vortex shedding suppression effects of rotatable fairing are not less than the corresponding fixed fairing when α=30°and α=45°Considering its flow direction adaptive ability, rotatable fairing also have good practical value. Forth, although fluctuating lift suppression effect of tail-fairing is worse than that of rotatable fairing, the better performances on time-averaged lift and vortex shedding ensure its potential research value. Fifth, axial-rod shroud has advantages in maneuverability and economy. It shall have broad application prospects in case of optimal geometric parameters selected.Optimized selection of all parameters of the tested devices has been made through a comprehensive analysis:(1) Although suppression effect of L/D=4.0cases for fixed and rotatable splitter plates are the best, L=0.5D～1.5D fixed and L=1.0D～2.0D rotatable splitter plates also can achieve good effect and possess higher practical value. As a result, the latter ones were recommend as optimized selection in all splitter plates;(2) Finally, α=45°fixed fairing and α=60°rotatable fairing were recommend as optimized selection in all fairings;(3) Finally,(α=60°, L=1.5D) tail-fairing was recommend as optimized selection in all tail-fairings;(4) Finally,(d=4mm, N=10) and (d=8mm, N=12) axial-rod shrouds were recommend as optimized selection in all shrouds. |
PDF Full Text Request