Viv Abhirop Jayanthi

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    Vortex Induced Vibrations

    By:Abhiroop J ayanthi

    Indian Institute of Technology , Delhi

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    Some Questions ! What is VIV?

    What are the details of a steady approachflow past a stationary cylinder?

    How and why does VIV occur? What kind of body shapes experience

    VIV? What kinds of VIV are there?

    How do you eliminate VIV?17/12/2007 Abhiroop J ayanthi IIT Delhi

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    17/12/2007 Abhiroop J ayanthi IIT Delhi

    Contents

    Introduction to Fluid flows

    (Instabilities and Bifurcations)1

    Bifurcation of flow around a cylinder(Karman Vortex Street)2

    Galloping Vs VIV

    3 Vortex Induced Vibrations.

    4

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    Flow around a circular cylinder?

    At very low Reynolds numbers (based on

    the diameter of the circular member) thestreamlines of the resulting flow isperfectly symmetric as expected frompotential theory. However as the Reynoldsnumber is increased the flow becomes

    assymetric.

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    Introduction to Fluid Flows:

    Regimes of external flow.

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    Profile Drag

    When a body is immersed in a fluid and is

    in relative motion with respect to it , thedrag is defined as that component of theresultant force acting on the body which isin the direction of the relative motion.

    Profile Drag = Pressure drag + Skin frictiondrag.

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    Flow past a circular cylinder :

    Re < 0.5

    Inertia effects are negligible pressure recovery is nearly complete.

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    Flow past a circular cylinder :

    2

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    Flow past a circular cylinder :

    Further increase of Re

    Tends to elongate the eddies which thenbegin to oscillate until at about Re=90

    depending on free stream turbulence level.They break away from the cylinder.

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    Flow past a circular cylinder :

    This process is further intensifies by

    further increase of Re leads to VortexStreet

    ( Von Karman Vortex Street.)

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    Flow past a circular cylinder :

    Up to Re values of 2 x 105 the boundary

    layer is laminar but at that valueapproximately depending on the intensityof the free stream turbulence, it changesto turbulence before separation .

    The effect of this is that separation points movefurther back and hence there is a marked drop inthe value of CD

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    Further

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    Flow past a circular cylinder :

    At Re > 107 the value of CD appears to be

    independent of Re but there areinsufficient experimental data available forthis end of the range.

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    Flow past a circular cylinder :

    For higher Re Numbers Vortices disappear

    because of high rate of shear and are thenreplaced by a highly turbulent wake.

    This produces an increase in the value of CD

    and here pressure drag is nearly responsible forall drag.

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    Flow past a circular cylinder :

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    Separation of Boundary Layer

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    Cp

    vs Angular position

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    Separation angle with Reynolds Number

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    Flow past a circular cylinder :

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    Fluid Structure interaction

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    Vortex Shedding

    Vortex shedding is an unsteady-flow that

    takes place in special flow velocities(according to the size and shape of thecylindrical body).

    In this flow, vortices are created at theback of the body and detach periodically

    from either side of the body.

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    Vortex shedding is caused when air flows

    past a blunt object. The airflow past the object creates

    alternating low-pressure vortices on thedownwind side of the object.

    The object will tend to move toward the

    low-pressure zone.

    Vortex Shedding

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    Vortex Shedding

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    Karman Vortex Street.

    Von Krmn

    vortex street isa repeatingpattern ofswirling vorticescaused by theunsteadyseparation offlow over bluffbodies

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    Some points to note : The frequency of vortex shedding is definite andis related to the Reynolds number (flow velocity,

    viscosity of fluid, and the diameter of the cylinder). The frequency of vortex shedding is the sameas the vibrating frequency of the cylinder induced

    by the flow. If the density and viscosity of the fluid areknown and the diameter of the cylinder is given,the frequency measured at the cylinder can beused to represent the flow velocity.

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    Karman Vortex Street

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    What is VIV ?

    Vortex-induced vibrations (VIV) are

    motions induced on bodies facing anexternal flow by periodical irregularities onthis flow.

    The classical example is the VIV of anunderwater cylinder.

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    How and why VIV ?

    When the vortices are not formed symmetrically aroundthe body (with respect to its mid plane), different lift

    forces develop on each side of the body, thus leading tomotion transverse to the flow.

    This motion changes the nature of the vortex formation

    in such a way as to lead to a limited motion amplitude.(differently from what would be expected in a case ofresonance).

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    Strouhal instability

    The Strouhal number relates the frequency of

    shedding to the velocity of the flow and acharacteristic dimension of the body (diameter inthe case of a cylinder).

    fv -vortex shedding frequency of a body at rest

    (Strouhal frequency)

    Dc is the diameter of the circular cylinder

    V is the velocity of the ambient flow.

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    The phenomenon of lock-in happens when

    the vortex shedding frequency becomesclose to a natural frequency of vibration ofthe structure. When this happens large

    and damaging vibrations can result.

    Strouhal instability

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    Strouhal Number Vs Reynolds

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    Strouhal Number Vs Reynolds

    number for cylinder..

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    Types of VIV:

    Self-excited oscillations - this type of VIV is whatoccurs naturally, i.e., when the vortex-shedding

    frequency and the natural frequency areapproximately the same. (This is the real VIV this isvortex-induced vibration)

    Forced oscillations occurs at velocities andamplitudes which are preset and can be controlled

    independently of fluid velocity. (This is not the realVIV this is vibration-induced vortices).

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    Need to understand VIV ?

    VIV manifests itself on many different

    branches of engineering, from cables toheat exchanger tube arrays.

    Vortex-induced vibration (VIV) is animportant source of fatigue damage ofoffshore oil exploration and production

    risers.

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    E l

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    Example.

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    Map of VortexSynchronization

    patterns

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