What does the term “Karman vortex street” describe? a. The rate of change of fluid properties through control surfaces b. A pattern of vortices shed from a cylinder c. The continuous path of a fluid particle d. A line of fluid particles across a flow at an instant
The correct answer and explanation is:
The correct answer is:
b. A pattern of vortices shed from a cylinder
Explanation:
A Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow around bluff bodies, such as cylinders, placed in a fluid stream. This phenomenon was first described by Theodore von Kármán, a pioneering fluid dynamicist.
When a fluid flows past a cylindrical object (or any bluff body with a blunt shape), the flow cannot smoothly follow the shape of the body due to the boundary layer separation. Instead, alternating vortices form on either side of the object and are shed downstream in a staggered pattern. This alternating vortex shedding causes the fluid to form a regular, periodic pattern called a vortex street.
Why it happens:
- Flow separation: As fluid flows around the cylinder, it separates from the surface at points where the pressure gradient becomes adverse, creating regions of recirculating flow behind the object.
- Instability and shedding: The separated shear layers roll up into vortices. These vortices alternate sides because of an instability mechanism in the wake behind the cylinder.
- Strouhal number: The frequency of vortex shedding is characterized by the dimensionless Strouhal number, which relates vortex shedding frequency to flow velocity and cylinder diameter.
Significance and applications:
- Engineering impact: The Kármán vortex street can induce vibrations in structures such as bridges, chimneys, and marine cables due to alternating forces from vortex shedding. This phenomenon is important in designing structures to avoid resonance and fatigue.
- Flow visualization: The vortex street is often visualized in smoke or dye in wind tunnels and water channels, demonstrating fundamental fluid dynamic behavior.
- Nature and technology: This phenomenon also appears in nature, such as in cloud formations behind islands or the wake of fish and birds, and is studied in aerodynamics, meteorology, and oceanography.
Why other options are incorrect:
- a. The rate of change of fluid properties through control surfaces — This describes a general concept in fluid dynamics related to control volume analysis, not vortex streets.
- c. The continuous path of a fluid particle — This describes a streamline or pathline, not vortex shedding.
- d. A line of fluid particles across a flow at an instant — This describes a line of dye or smoke injected into the flow (a fluid filament or line of particles), not the vortex pattern.
In summary, the Kármán vortex street is a classic fluid dynamics phenomenon describing the periodic shedding of alternating vortices downstream of a cylinder or similar bluff body immersed in a flowing fluid.