Vortex Induce Vibration Essay Sample free essay sample

1. 1. Background of Study Vortex induced gesture ( VIM ) or vortex induced quiver ( VIV ) is an object quiver influenced by the whirl casting. When fluid flow across the blunt organic structure. aftermath formed behind the bluff object and ensuing in whirl sloughing. Due to the long periods of gestures. the whirl induced quiver will normally mention as whirl induced gesture. Vortex-induced gesture is an of import beginning of fatigue harm for blunt cylindrical organic structure underwater particularly for production risers. When the casting frequence matches the organic structure Eigen frequence. the organic structure will get down to vibrate and the body’s motion becomes self-sufficient. Continuing vibrating will take to material fatigue and the stuffs tend to fracture or tire. Vortex casting was one of the causes proposed for the failure of the Tacoma Narrows Bridge in 1940. During the winter of 2001. a bang drive â€Å"Vertigo† in Ohio suffered the whirl casting consequence one of the three towers collapsed. We will write a custom essay sample on Vortex Induce Vibration Essay Sample or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page On 1968. whirl casting due to high air currents caused the prostration of three towers at Ferrybridge power station. Through countless of surveies and researches. several vortex suppression methods developed designed to cut down the effects of whirl induced gesture on blunt organic structure. Normally the fairing is used in cut downing vortex casting consequence for cylindrical organic structure. Fairing will efficaciously cut down the retarding force force and aftermath generated by fluid flow. This research begins with the understanding on rules. parametric quantities and effect of whirl induced gesture ( VIM ) or vortex induced quiver ( VIV ) so follow by carry oning simulation. The analysis is simulated by CFD package which is ANSYS Fluent. The consequences obtained will be compared with the experimental consequences conducted by other research workers. 1. 2. Problem Statement In transporting out the research. several issues need to be clarified:I. The effects of moving ridges and current on Energytwo. The effects of cylinder dimension on VIMthree. The effects fairing on whirl suppressionfour. Any suppression method is more effectual than fairing 1. 3. Objective of Study The aim of survey as follow:I. To look into the influences of moving ridges and current on VIM two. To look into the influences of cylinder dimension on VIM three. To place the effects of fairing on vortex suppression four. To develop an effectual method in vortex suppression 1. 4. Significant of StudyThe of import of this research is to develop an effectual method for vortex suppression. The method will be able to stamp down the whirl casting more efficaciously compare to the other methods. Besides. this method will cut down the retarding force force and lift force generated by the whirl casting. The forces are the chief part to the material break. 1. 5. Scope of Study The Scopess of survey of this research are listed as follows:I. Investigate and understand the basic rules of VIM and VIV on round cylinder two. Analysiss VIM utilizing CFD simulationthree. Develop an effectual whirl suppression method1. 6. Research Flow Chart1. 7. Research Gantt ChartLITERATURE REVIEW ( 1st Draft )2. 1. Introduction Vortex-induced gesture ( VIM ) or vortex-induced quiver ( VIV ) is a phenomenon happens when fluid flow across a cylindrical organic structure. When a fluid flow across a cylindrical organic structure. an unsteady flow with hovering gesture formed behind the organic structure is called casting frequence. This casting frequence will associated with formation of whirls. When the whirls are non formed symmetrically around the organic structure. a clip changing non-uniform force per unit area distribution will bring forth. ensuing lift force moving on each side of organic structure. As the clip changing lift force continues moving on the organic structure. the organic structure will vibrate in inline and transverse to the flow. When the casting frequence is close or equal to the Eigen frequence of the organic structure. resonance occur and the quiver amplitude of the organic structure is maximized. This phenomenon is called lock-in and fatigue tends to go on. 2. 2. Whirls Sheding Formation As the fluid approaches the front side of the tubing. the unstable force per unit area rises from the free watercourse value to the stagnancy point value. The high force per unit area forces the fluid to travel along the tubing surface and boundary beds develop on both sides. The force per unit area force is counteracted by syrupy forces and the fluid can non follow the tubing surface to the rear side but separates from both sides of the tubing and signifier two shear beds. The inmost portion of the shear beds are in contact with the tubing surface and moves slower than the outmost portion. As a consequence. the shear beds roll up. [ 1 ] A whirl is in the procedure of formation near the top of the cylinder surface. Below and to the right of the first whirl is another whirl which was formed and shed a short period before. Thus. the flow procedure in the aftermath of a cylinder or tubing involves the formation and sloughing of whirls alternately from one side and so the other. This phe nomenon is of major importance in technology design because the surrogate formation and sloughing of whirls besides creates jumping forces. which occur more often as the speed of the flow additions. [ 2 ] Figure 2. 1: Vortex formation behind a round cylinder. [ 2 ]2. 3. Reynolds Number dependance By and large the flow form around a round cylinder can be characterized by the Reynolds figure of the incident flow and by the location of points at which the flow separates from the cylinder surface which in bend depend on the province of the boundary bed ( laminar or turbulent ) . [ 3 ] For syrupy fluids the flow form is much more complicated and the balance between inertia forces and syrupy forces is of import. [ 3 ] The comparative importance is expressed by the Reynolds figure Re defined as Re =U?Dinertial effectsviscous effects where U? is the free watercourse speed. D is the tubing diameter and ? the kinematic viscousness of the fluid. Figure 2. 2 shows the chief description of whirl casting from a smooth round cylinder in unvarying flow for the major Reynolds figure governments. Figure 2. 2 Regimes of fluid flow across a smooth tubing. [ 3. 4 ] At Reynolds Numberss below 1. no separation occurs. The form of the streamlines is different from those in an inviscid fluid. The syrupy forces cause the streamlines to travel further apart on the downstream side than on the upstream side of the tubing. [ 1 ] In the Reynolds figure scope of 5 ? Re ? 45. the flow separates from the rear side of the tubing and a symmetric brace of whirls is formed in the close aftermath. [ 1 ] As the Reynolds figure is further increased the aftermath becomes unstable and Vortex Shedding is initiated. At first. one of the two whirls breaks off and so the second is shed because of the nonsymmetrical force per unit area in the aftermath. The intermittently shed whirls form a laminar periodic aftermath of staggered whirls of opposite mark. This phenomenon is frequently called the Karman vortex street. [ 1 ] In the Reynolds figure scope 150 lt ; Re lt ; 300. periodic irregular perturbation s are found in the aftermath. The flow is transitional and bit by bit becomes disruptive as the Reynolds figure is increased. [ 1 ] The Reynolds figure scope 300 lt ; Re lt ; 1. 5 ·105 is called subcritical ( the upper bound is sometimes given as 2 ·105 ) . The laminar boundary bed offprints at about 80 grades downstream of the front stagnancy point and the whirl sloughing is strong and periodic. [ 1. 3 ] With a farther addition of Re. the flow enters the critical government. The laminal boundary bed offprints on the front side of the tubing. forms a separation bubble and subsequently reattaches on the tubing surface. Reattachment is followed by a disruptive boundary bed and the separation point is moved to the rear side. to about 140 grades downstream the front stagnancy point. As an consequence the retarding force coefficient is decreased aggressively. [ 1 ] The scope 1. 5 ·105 Re3. 5 ·106. referred to the literature as the transitional part. includes the critical part ( 1. 5 ·105 Re3. 5 ·105 ) and the supercritical part ( 3. 5 ·105 Re3. 5 ·106 ) . In these parts. the cylinder boundary bed becomes disruptive. the separation points move aft to 140 grades. and the cylinder retarding force coefficient drops suddenly. [ 3 ] La minar separation bubbles and 3-dimensional effects disrupt the regular sloughing procedure and broaden the spectrum of casting frequences for smooth surface cylinders. [ 3. 5 ] In the post-critical Reynolds figure scope ( Re3. 5 ·106 ) . regular whirl sloughing is re-established with a disruptive cylinder boundary bed. The whirl casting persists at Reynolds figure every bit high as 1011. [ 3. 6 ] 2. 4. Strouhal figure dependance When the casting frequence is near the Eigen-frequency of the construction. the resonance will happen and the construction appears to sing. A dimensionless figure. the Strouhal figure Sr. is normally used as a step of the prevailing sloughing frequence degree Fahrenheit. The definition is Sr= fsDU? where D is the diameter of a round cylinder or tubing in cross flow and U? is the free watercourse speed. The Strouhal figure of a stationary tubing or round cylinder is a map of Reynolds figure but less of surface raggedness and free watercourse turbulency as shown in Figure 2. 3. Figure 2. 3: Strouhal figure versus Reynolds figure for round cylinders. [ 4 ] Most of the Strouhal figure informations were derived from the measurings of the speed fluctuations in the aftermath. while fewer informations were derived from the lift force spectra. However. that lift force spectra are a more direct step of the force features than wake speed measurings. [ 3 ] The behaviour of the Strouhal figure is stable for a broad scope of Reynolds Numberss. except around 106 ( transitional part ) where important spread occurs in the trial information. [ 3 ] Whirls are often shed in this part and the Strouhal figure is near to 0. 2. In the transitional part. the Strouhal figure becomes scattered changing from 0. 05-0. 5. Delany A ; Sorensen ( 1953 ) found a sudden addition of their values of Strouhal figure to 0. 45 and so a lessening to 0. 3 at about the same Reynolds figure of 2 ·106. This indicates the passage to postcritical flow conditions. Bearman ( 1969 ) measured a similar value of S=0. 46. [ 3 ] Besides in the transitional scope. Achenbach and Heine cke ( 1981 ) found that smooth stationary cylinders had a helter-skelter. disorganized. high-frequency aftermath and Strouhal figure every bit high as 0. 5. Cylinders with some raggedness ( surface raggedness e/D=3 ·10-3 or greater. where vitamin E is the characteristic surface raggedness ) had organized. periodic aftermaths with Strouhal Numberss S=0. 25. [ 3 ] In the Reynolds figure scope 250 lt ; Re lt ; 2?105 the empirical expression Sr=0. 1981-19. 7Re is sometimes recommended for appraisal of the Strouhal figure. [ 2 ] It has been suggested to present a cosmopolitan Strouhal figure based on the distance between the shear beds. Over a big Reynolds figure scope a Strouhal figure of approximately 0. 2 is so valid regardless of the organic structure geometry. [ 2 ] Vortex casting from a stationary cylinder in the post-critical part does non happen at a individual distinguishable frequence. but instead wanders over a narrow set of frequences and it is non changeless along the span. An mean Strouhal figure value of 0. 25 is suggested. [ 3 ] 2. 5. Reduced Speed Reduced speed is a finding of the speed ranges where the whirl casting will be in resonance with Eigen frequence of the object. VR=U?fiD where U? free watercourse speed. fi is the ith natural frequence of the member and D is member diameter. For low decreased speeds. there exists on initial subdivision associated with a 2S whirl casting manner ( two individual whirls shed per rhythm ) and the agencies forces and cylinder response are in stage. For intermediate and larger reduced speeds there exists an upper and a lower subdivision associated with a 2P whirl casting manner ( two braces of whirls per rhythm ) . [ 7 ] Figure 2. 4: Sketch of the â€Å"three-branch† response theoretical accountHowever. really few 3-dimensional numerical consequences have been able to accurately reproduce the three-branch response theoretical account obtained from experiments. Some have successfully predicted the 2P casting manner in the lower subdivision [ 8 ] . but this consequence has merely been observed at big mass-damping parametric quantities. little facet ratio and moderate Reynolds figure. In general. the undertaking of capturing numerically the big amplitude response of the upper subdivision for low mass-damping systems with big facet ratio has remained out of range. [ 7 ] 2. 6. Raise CoefficientLift force is sinusoidal constituent and residuary force. Parameter of lift force usually is used to find the lift coefficient. CL. CL= FL12?DLV2 where FL is the time-average of the retarding force force. FL ( T ) . ? is the unstable mass denseness. D is the cylinder diameter. L is the cylinder length and V is the flow speed. Lift is the cross constituent of force happening at the whirl casting frequence. Lift will be influenced by organic structure gesture. and there is considerable grounds showing the influence of organic structure on lift force frequence and correlativity. But it is an applied force which owes its being to the character and strength of the hydrodynamic aftermath formed by flow around the organic structure. [ 3 ] Lift force besides can be expressed in the signifier of gesture as equation stated below FL=m+az+2mzz+Kz where ? is the muffling factor of the cylinder. ?z is the cylinder round natural frequence and K is the spring invariable. The clip changing lift force on the hovering cylinder may hold a stage. ? difference. Then raising amplitude will go. z=Azsin? ( 2?fst+? ) where Az is maximal gesture amplitude.Refering the fluid kineticss portion of the job the Reynolds figure Re can play an of import function here because flow separations are frequently Reynolds figure dependant. even if the organic structures have crisp borders. This has already been observed and explained for the attack span cross subdivision of the Great Belt Bridge ( Schewe A ; Larsen 1998 ) . High Reynolds figure dependance is observed from numberless of experiments and researches. The ground for this dependance is that. the province of the boundary bed has a far-reaching influence on the full flow field about a organic structure. Both the province of the boundary bed and the location of passage are frequently responsible for the formation. length. and form of separation bubbles. [ 9 ] In peculiar when the symmetricalness is broken. i. e. the cross subdivision is asymmetric or the angle of incidence ? is non zero. the behaviour of the separated flow that depends on the Reynolds figure can be different on the upper- and the lower side of the subdivision. Therefore planetary values. like the lift coefficient CL. for illustration. can be affected by the Reynolds figure. This in bend can hold a big influence on the derived functions. which are differential values and therefore sensitive to little fluctuation of the implicit in CL ( ? ) curve. the latter are. in add-on. typically nonlinear in instance of bluff organic structures. In general the derived functions and the nonlinearities are finding the type and strength of possible flow induced quivers. [ 9 ] Over countless experiments and researches. a big figure of consequences had been published to try the relationship between the lift coefficient and Reynolds figure.