Turbulent flow around an obstacle; the flow further away is laminar
Laminar and turbulent water flow over the hull of a submarine
Turbulence creating a vortex on an airplane wing
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by low-momentum diffusion, high momentum convection, and pressure and velocity variation with time. Flow that is not turbulent is called laminar flow. The (dimensionless) Reynolds number characterizes whether flow conditions lead to laminar or turbulent flow.
If you 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 and the drag is relatively low. As the speed increases, at some point the transition is made to turbulent ("chaotic") flow, where there is a large increase in drag, and often vortices behind the object. The same transition occurs if you gradually increase the size of the object or decrease the viscosity of the fluid.
Drip. Which would be modeled as a free boundary problem
Laminar flow. A smooth and regular flow which looks somewhat solid because the flow maintains a fixed shape. In this regime, the water acts as a transparent lens.
Turbulent flow. At a certain point the water will start to break up. The water no longer stays contained in a stable region in space. The stream ceases to be transparent, due to the extreme convolution of the surface of the fluid.
Smoke from a cigarette -- for the first few centimetres it remains laminar, and then becomes unstable and turbulent.
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."
Modeling blood flow in the human aorta according to this criterion leads to the expectation of some turbulence in the center of the flow.
Blood flow in the human body is remarkably free of turbulence, but sounds attributed to turbulence are sometimes detected by stethescope associated with the aorta.
But when the velocity profile for tube flow is taken into account, it is found that the maximum velocity of flow is twice the effective value, so a velocity of 66 cm/s would be expected to produce turbulence in the center of the aorta.
This difference in the streaklines is due to the orderly nature of laminar flow in the first case, and the fluctuating character of turbulentflow in the latter.
For pipe flow, the transition from laminar to turbulentflow occurs at Re = 2100.
In wall bounded turbulentflows, the viscous sub-layer is thought of as a relatively quiescent region punctuated with violent quasi-periodic activity such as bursting.
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