Consider a situation when a water tap is turned on slowly, the water flow is smooth initially but loses its smoothness when the speed of the output is increased. Similarly, Have you ever seen a stream passing underneath the bridge? What do you first observe when you see a streamline? The answer is the most common factors of the stream such as speed, width, amount of water flowing etc. One of the primary characteristics of any stream is its flow. We refer to it as streamflow.
Streamline flow in the case of fluids is referred to as the type of flow where the fluids flow in separate layers without mixing or disruption occurring in between the layers at a certain point. The velocity of each fluid particle flowing will remain constant with time in a streamlined flow. In the case of low fluid velocities, the fluid will flow without any sort of lateral mixing (mixing fluids at right angles to the flow direction) because of the lack of turbulent velocity fluctuations. The fluid particles tend to follow a particular order where the movement or motion of fluid particles will be based on particles flowing in a straight line parallel to the pipe wall. The movement happens in a way that the adjacent layers of the fluid will smoothly slide past each other.
Notice water flowing from a tap at a different flow rate. You will observe that when the flow rate is low, the water flowing from the tap will run smoothly and the water will have a smooth texture. However, if you gradually keep on increasing the flow rate you will begin to see the disturbances and voids after a particular point of increasing the flow rate, thus giving turbulent flow. When you introduce a stream of ink in the first case (where water is flowing smoothly), the ink will not mix with other layers. This type of condition is called streamlined flow. However, if we introduce the ink in a turbulent flow, the ink will mix with other layers of water. Now, in both cases, the introduction of ink will give different results.
Steady flow is achieved at low flow speeds. Beyond a limiting value, called critical speed, this flow loses steadiness and becomes turbulent. One sees this when a fast-flowing stream encounters rocks, small foamy whirlpool-like regions called white water rapids are formed. The turbulence in the airplane that the passengers experience during their journey could also be related to this similar flow of air.
The distinction between laminar and turbulent flow was first studied and theorized by Osborne Reynolds in the second half of the 19th century. His first publication on this topic is considered a milestone in the study of fluid dynamics. This work was based on the experiment used by Reynolds to show the transition from the laminar to the turbulent flow.
The experiment consisted of examining the behavior of water flow in a large glass pipe. To visualize the flow, Reynolds injected a small vein of dyed water into the flow and observed its behavior at different flow rates. When the velocity was low, the dyed layer remained distinct through the entire length of the pipe. When the velocity was increased, the vein broke up and diffused throughout the tube’s cross-section. Thus, Reynolds exhibited the existence of two different flow regimes, called laminar flow and turbulent flow, separated by a transition phase. He also identified several factors that affect the occurrence of this transition.
Laminar flows have both academic and industrial applications. From the industrial point of view, the laminar regime is usually developed inflows with low velocity, low density or high viscosity. This is usually the case of natural convection or ventilation systems working at low velocity. Many flows in the laminar regime are used as benchmarks for the development of advanced simulation techniques. This is the case of the “lid-driven cavity”.
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Sommerfeld – Ia800309.Us.archive.org. https://ia800309.us.archive.org/32/items/1461474604_Arnold_Sommerfeld/1461474604_Arnold.pdf.
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