Vortex Induced Vibrations

Size: px

Start display at page:

Download "Vortex Induced Vibrations"

Kenneth Carpenter
6 years ago
Views:

1 Vortex Induced Vibrations By: Abhiroop Jayanthi Indian Institute of Technology, Delhi

2 Some Questions! What is VIV? What are the details of a steady approach flow 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?

3 Contents 1 2 Introduction to Fluid flows (Instabilities and Bifurcations) Bifurcation of flow around a cylinder (Karman Vortex Street) 3 Vortex Induced Vibrations. 4 Galloping Vs VIV

4 Flow around a circular cylinder? At very low Reynolds numbers (based on the diameter of the circular member) the streamlines of the resulting flow is perfectly symmetric as expected from potential theory. However as the Reynolds number is increased the flow becomes assymetric.

5 Introduction to Fluid Flows: Regimes of external flow.

6 Profile Drag When a body is immersed in a fluid and is in relative motion with respect to it, the drag is defined as that component of the resultant force acting on the body which is in the direction of the relative motion. Profile Drag = Pressure drag + Skin friction drag.

7 Flow past a circular cylinder : Re < 0.5 Inertia effects are negligible pressure recovery is nearly complete.

8 Flow past a circular cylinder : 2 <Re<30 Separation of boundary layers occurs

9 Flow past a circular cylinder : Further increase of Re Tends to elongate the eddies which then begin to oscillate until at about Re=90 depending on free stream turbulence level. They break away from the cylinder.

10 Flow past a circular cylinder : This process is further intensifies by further increase of Re leads to Vortex Street ( Von Karman Vortex Street.)

11 Flow past a circular cylinder : Up to Re values of 2 x 10 5 the boundary layer is laminar but at that value approximately depending on the intensity of the free stream turbulence, it changes to turbulence before separation. The effect of this is that separation points move further back and hence there is a marked drop in the value of C D

12 Further

13 Flow past a circular cylinder : At Re > 10 7 the value of C D appears to be independent of Re but there are insufficient experimental data available for this end of the range.

14 Flow past a circular cylinder : For higher Re Numbers Vortices disappear because of high rate of shear and are then replaced by a highly turbulent wake. This produces an increase in the value of C D and here pressure drag is nearly responsible for all drag.

15 Flow past a circular cylinder :

16 Separation of Boundary Layer

17 C p vs Angular position

18 Separation angle with Reynold s Number

19 Flow past a circular cylinder :

20 Fluid Structure interaction

21 Vortex Shedding Vortex shedding is an unsteady-flow that takes place in special flow velocities (according to the size and shape of the cylindrical body). In this flow, vortices are created at the back of the body and detach periodically from either side of the body.

22 Vortex Shedding Vortex shedding is caused when air flows past a blunt object. The airflow past the object creates alternating low-pressure vortices on the downwind side of the object. The object will tend to move toward the low-pressure zone.

23 Vortex Shedding

24 Karman Vortex Street. Von Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow over bluff bodies

25 Some points to note : The frequency of vortex shedding is definite and is related to the Reynolds number (flow velocity, viscosity of fluid, and the diameter of the cylinder). The frequency of vortex shedding is the same as the vibrating frequency of the cylinder induced by the flow. If the density and viscosity of the fluid are known and the diameter of the cylinder is given, the frequency measured at the cylinder can be used to represent the flow velocity.

26 Karman Vortex Street

27 What is VIV? Vortex-induced vibrations (VIV) are motions induced on bodies facing an external flow by periodical irregularities on this flow. The classical example is the VIV of an underwater cylinder.

28 How and why VIV? When the vortices are not formed symmetrically around the body (with respect to its mid plane), different lift forces develop on each side of the body, thus leading to motion 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 of resonance).

29 Strouhal instability The Strouhal number relates the frequency of shedding to the velocity of the flow and a characteristic dimension of the body (diameter in the case of a cylinder). f v -vortex shedding frequency of a body at rest (Strouhal frequency) D c is the diameter of the circular cylinder V is the velocity of the ambient flow.

30 Strouhal instability The phenomenon of lock-in happens when the vortex shedding frequency becomes close to a natural frequency of vibration of the structure. When this happens large and damaging vibrations can result.

31 Strouhal Number Vs Reynolds number for cylinder..

32 Types of VIV: Self-excited oscillations - this type of VIV is what occurs naturally, i.e., when the vortex-shedding frequency and the natural frequency are approximately the same. (This is the real VIV this is vortex-induced vibration) Forced oscillations occurs at velocities and amplitudes which are preset and can be controlled independently of fluid velocity. (This is not the real VIV this is vibration-induced vortices).

33 Need to understand VIV? VIV manifests itself on many different branches of engineering, from cables to heat exchanger tube arrays. Vortex-induced vibration (VIV) is an important source of fatigue damage of offshore oil exploration and production risers.

34 Example.

35 Map of Vortex Synchronization patterns

36 Map of Vortex Synchronization patterns

37 Elastically mounted cylinder. (High Mass Ratio)

38 Classical definition Vs New findings. Classical definition of lock-in or synchronization is often perceived as the regime where the frequency of oscillation (f), as well as the vortex formation frequency (f V ), are close to the natural frequency (f N ) of the structure throughout the regime of large-amplitude vibration However, recent studies show a dramatic departure from this classical result; bodies can conceivably vibrate with large amplitude, at hundreds of times the natural frequency!

39 Findings The phenomenon of lock-in, or synchronization traditionally means that the ratio f*=f/f N remains close to unity for high mass ratio. However, for light bodies in water, for m*= 2.4 the body oscillates at a higher frequency (f*= 1.4). Therefore, one might define synchronization as the matching of the frequency of the periodic wake vortex mode with the body oscillation frequency.

40 How do we reduce VIV? This is the important question! It may be best to design around VIV. In other words, let s learn how to predict VIV and then avoid the situations that will produce VIV. The circular cylinder will always be the preferred shape and the fluctuating lift will always be there, VIV or no VIV. Since we can t avoid the shedding of vortices, let s try to learn to avoid the situations that produce VIV

41 Some ways of achieving this.

42 Use of Control Cylinders.

43 Galloping Vs VIV Two well-known phenomena in the problems of fluid/structure interaction are vortex-induced vibration (VIV) and galloping. VIV is associated with synchronization, or lock-in of the structural oscillation frequency with the vortex-shedding frequency, Lock-in occurs at reduced velocities where the vortex shedding frequency is comparable to the natural frequency of the structure. Galloping is driven by a time-averaged fluid force which develops in phase with the structural velocity and has a frequency many times lower than that of vortex shedding. galloping is prevalent at higher reduced velocities where the frequency of oscillation is lower than the vortex-shedding frequency.

44 Thank you.

Numerical Investigation of the Fluid Flow around and Past a Circular Cylinder by Ansys Simulation

, pp.49-58 http://dx.doi.org/10.1457/ijast.016.9.06 Numerical Investigation of the Fluid Flow around and Past a Circular Cylinder by Ansys Simulation Mojtaba Daneshi Department of Mechanical Engineering,