Passive Control of Flow-Induced Vibration(FIV)by Helical Strakes for Two Staggered Flexible Cylinders

Helical strake is a widely-used device for passive flow-induced vibration(FIV)control of cylindrical structures.It is omnidirectional and can effectively reduce FIV response amplitude.Studies on the passive FIV control for cylindrical structures are mainly concerned with a single isolated cylinder,while the influence of wake interference between multiple cylinders on FIV suppression devices is less considered up to now.In engineering applications,multiple flexible cylinders with large aspect ratios can be subjected to complex flow forces,and the effects of wake interference are obvious.The FIV suppression effect of helical strake of a common configuration(17.5D pitch and 0.25D height,where D is the cylinder diameter)in two staggered cylinders system is still unknown.This paper systematically studied the FIV response of multiple cylinders system fitted with the helical strakes by model tests.The relative spatial position of the two cylinders is fixed at S=3.0D and T=8.0D,which ensures the cylindrical structures in the flow interference region.The experimental results show that the helical strakes effectively reduce the FIV response on staggered upstream cylinder,and the suppression efficiency is barely affected by the smooth or straked downstream cylinder.The corresponding FIV suppression efficiency on the downstream cylinder is remarkably reduced by the influence of the upstream wake flow.The wake-induced vibration(WIV)phenomenon is not observed on the staggered downstream cylinder,which normally occurs on the downstream straked cylinder in a tandem arrangement.

Flow-Induced Vibration Fatigue Damage of A Pair of Flexible Cylinders in A Staggered Array

Flow-induced vibration(FIV)of a group of long,flexible cylinders involves a complex interaction between fluid and structures.Although a substantial number of studies have been devoted to assessing FIV response behaviours,fatigue damage features of staggered flexible cylinders are not fully understood.Moreover,the wake-induced flutter constitutes an intricate hydrodynamic behaviour that frequently occurs when one cylinder is in the vicinity of another one.Unfortunately,existing studies on the fatigue damage caused by wake-induced flutter are incapable of achieving better results.This paper,therefore,estimates the FIV fatigue damage of two staggered flexible cylinders with an aspect ratio of 350 and a mass ratio of 1.90 based on normal S−N curves according to Det Norske Veritas(DNV)regulations.Twelve staggered cases(cross-flow spacing ratios of 2.0,3.0,4.0,and 6.0 and in-line spacing ratios of 4.0,6.0,and 8.0)are discussed for comparison,and fatigue damage caused by wake-induced flutter is thoroughly considered.Fatigue damage results indicate that the variation of the cross-flow(CF)spacing ratio has a greater influence than that of the in-line(IL)spacing ratio on the CF fatigue damage of the upstream cylinder.Lower IL fatigue damages of the upstream cylinder are observed when reduced velocity V_(r)≥15.03 due to the wake flow patterns with different IL spacing ratios.Moreover,wake interference,especially wake-induced flutter,predominates the fatigue damage characteristics of the downstream cylinder.When V_(r)=8.77−11.27,wake-induced flutter enhances the IL fatigue damage of the downstream cylinder and slightly affects that of the upstream body.Furthermore,wake-induced flutter causes considerable IL fatigue damage disparity between the two staggered cylinders by suppressing the IL fatigue damage of the upstream cylinder when V_(r)≥20.04.

Investigation of wake-induced vibration interference between two staggered wave-cone cylinders at subcritical Reynolds number

The wake induced vibration(WIV)of a one-and two-degree-of-freedom(1DOF,2DOF)downstream wave-cone cylinder(WCC)behind a stationary equal-size upstream wave-cone cylinder in the staggered arrangement is numerically investigated at subcritical Reynolds number of 3900 by using shear stressed transfer(SST)k-ωturbulence model.The streamwise pitch ratios(P/Dm)vary from 4 to 6 with a fixed incident angleα=8°.Experimental measurements were also performed for the validation of the present numerical models.It is found that the largest vibration amplitude in crossflow direction occurred at P/Dm=4,Ur=8 with small difference of streamwise vibration at P/Dm=4,6.Different from single wavy-cone cylinder(SWCC),the downstream flexible one of a pair staggered WCCs got larger vibration amplitude during phase switching stage instead of in-phase stage.The upstream wake will suppress the triple frequency of main frequency in the power spectra density(PSD)functions of Cl but stimulate the double one of that.An intriguing vibration mechanism happened in all 2DOF cases where the trajectory of downstream WCC is a significant ellipse rather than a figure of 8.The transformation of phase switching and the variation of the main frequency of drag coefficient(Cd)can be explained by the vortex-shedding modes of downstream WCC.

Local buckling analysis of biological nanocomposites based on a beam-spring model

Biological materials such as bone, tooth, and nacre are load-bearing nanocomposites composed of mineral and protein. Since the mineral crystals often have slender geometry, the nanocomposites are susceptible to buckle under the compressive load. In this paper, we analyze the local buckling behaviors of the nanocomposite structure of the biological materials using a beam-spring model by which we can consider plenty of mineral crystals and their interaction in our analysis compared with existing studies. We show that there is a transition of the buckling behaviors from a local buckling mode to a global one when we continuously increase the aspect ratio of mineral, leading to an increase of the buckling strength which levels off to the strength of the composites reinforced with continuous crystals. We find that the contact condition at the mineral tips has a striking effect on the local buckling mode at small aspect ratio, but the effect diminishes when the aspect ratio is large. Our analyses also show that the staggered arrangement of mineral plays a central role in the stability of the biological nanocomposites.

Simulation Study on Flow and Heat Transfer of Cross-Arrangement Tube Bundle

The air precooler of SABRE engine(Synergetic Air-Breathing Rocket Engine)is a tube bundle type of heat exchanger consisting of thousands of small tubes.The cold helium gas flows in the small tube bundle,and the high-speed hot air circulates outside the tubes and exchanges heat with the helium in the tubes.In this study,the cross-arrangement tube bundle consisting of small tubes with outer diameter of 1mm was taken as the research object.The system simulation calculation and data sorting were carried out.It is found that the empirical correlation of the conventional tube bundle is no longer applicable to the small tube and the reasons for inapplicability are explained by analysis of flow field.In addition,the results show that the average heat transfer coefficient of cross small/micro tube bundle isn’t affected by heat flux,but increases with the increase of Reynolds number,increases with the increase of transverse spacing,and decreases with the increase of longitudinal spacing.The flow resistance coefficient isn’t affected by heat flux and longitudinal spacing,but decreases with the increase of Reynolds number,and decreases with the increase of transverse spacing.

“3-Section” Technology of Full-Dimension Mining in Flat Thick-Seam

To raise coal recovery ratio and solve the key problem of full-dimension mining, a new “3-section” technology of full-dimension mining is put forward in flat thick-seam according to the theory and mining practice. This technology is proved to be rational and feasible in rodway layout system of stagger arrangement.