Before discussing about VIV, we may take a look at flow around circular cylinder. Flow around cylinder usually characterized by Reynold number (based on the diameter of the circular member).
Reynold number is defined as:
where v = mean velocity of the object relative to the fluid; L = characteristic linear dimension; μ = dynamic viscosity of the fluid; ν = kinematic viscosity; ρ = density of the fluid.
At very low Reynold numbers, the streamlines of the resulting flow is perfectly symmetric as explained in potential theory. However, with the increasing Reynold number, the streamline becomes assymetric.
When a body is immersed in a fluid and is in relative motion, the drag is defined as the component of the resultant force working on the body, in the direction of the relative motion. Drag = pressure drag + skin friction drag.
Flow past a circular cylinder:
Reynold number < 0.5; therefore inertia effect can be ignored and pressure recovery is nearly complete.
Reynold number ranged between 5 – 40; in this case, separation of boundary layers occurs.
With the increasing Reynold number, there is tendency of eddies elongation which then begin to oscillate until Reynold number of 90, depending on free stream turbulence level. The streams break away from the cylinder.
With further additional of Reynold number, the laminar streamline transforms into turbulence.
Then, as Reynold number getting bigger, separation of boundary layer begin to existence.
Then, vortex begins to appear and 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.
VORTEX-INDUCED VIBRATION (VIV)
Vortex-Induced Vibration, abbreviated as VIV, are motions induced on bodies facing an external flow by periodical irregularities on the flow. A simple example of VIV is an underwater cylinder, offshore pipelines.
VIV happens 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, and leading to motion transverse to the flow. This motion changes the nature of the vortex formation leading to a limited motion amplitude.
There are two types of VIV, self-excited oscillations and forced oscillation.
1. Self-excited oscillations
This type of VIV occurs naturally. For instance, when the vortex-shedding frequency and the natural frequency are approximately the same. This is the real VIV, vortex-induced vibration.
2. Forced oscillations
This VIV occurs at velocities and amplitudes which are preset and can be controled indepedently of fluid velocity. This is not the “real” VIV, this is vibration-induced vortices.
In order to prevent VIV phenomenon, some offshore structures are design with strakes to suppress VIV. Strakes can be seen in the following figure:
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