Turbulence
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In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time. Flow that is not turbulent is called laminar flow. The (dimensionless) Reynolds number characterizes whether flow conditions lead to laminar or turbulent flow.
Consider the flow of water over a simple smooth object, such as a sphere. At very low speeds the flow is laminar; i.e., the flow is smooth (though it may involve vortices on a large scale). As the speed increases, at some point the transition is made to turbulent ("chaotic") flow. In turbulent flow, unsteady vortices appear on many scales and interact with each other. Drag due to boundary layer skin friction increases. The structure and location of boundary layer separation often changes, sometimes resulting in a reduction of overall drag. Because laminar-turbulent transition is governed by Reynolds number, the same transition occurs if the size of the object is gradually increased, or the viscosity of the fluid is decreased, or if the density of the fluid is increased.
Examples of turbulence
- Flow from a faucet or tap goes through several stages as the faucet is opened:
- Off. No flow.
- Dripping flow. Water emerges in discrete drops.
- Laminar flow. Water flows in a jet with a steady surface. In this regime, the surface of the jet is symmetric and steady. Its surface is so smooth that the shaft of water looks like a transparent lens.
- Turbulent flow. Above a certain critical flow rate, the water jet becomes unsteady and asymmetric. The stream ceases to be transparent, due to the extreme convolution of the surface of the fluid.
- Smoke rising from a cigarette -- for the first few centimetres it remains laminar, and then becomes unstable and turbulent.
- Flow over a golf ball. This can be best understood by considering the golf ball to be stationary, with air flowing over it. If the golf ball was smooth, the boundary layer flow over it would be laminar at typical conditions. However, the surface is dimpled to perturb the boundary layer and promote transition to turbulence. This results in higher skin friction, but moves the point of boundary layer separation rearward, resulting in lower form drag and lower overall drag.
According to an apocryphal story, Werner Heisenberg was asked what he would ask God, given the opportunity. His reply was: "When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first." A similar witticism has been attributed to Horace Lamb.
See also
External links
- Center for Turbulence Research (http://ctr.stanford.edu/), Stanford University
- Journal of Turbulence (http://www.iop.org/EJ/S/UNREG/journal/1468-5248/1) - Institute of Physics (http://www.iop.org/)
- http://turb.seas.ucla.edu/~jkim/sciam/turbulence.html - Scientific American articleda:Turbulens
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