![]() A turbulent flow increases the amount of air resistance and noise however, a turbulent flow also accelerates heat conduction and thermal mixing. We find that the majority of flows observed in our daily lives is turbulent.Ī turbulent flow can be either an advantage or disadvantage. The calculated value for the Reynolds number is 400,000, far greater than the aforementioned approximate value of 2,000 - 4,000 denoting the conditions when the flow transitions from laminar to turbulent. The Reynolds number is calculated using the following equation for the flow of air. Figure 3.18 shows the air flow around a bicyclist traveling at 14.4 km/h. In this final example, we can determine whether the air flow around a bicyclist is laminar or turbulent. These values are just approximated and the actual values will vary depending on the state or conditions of the flow. The range of Reynolds number for the transition from laminar to turbulent flow in a pipe is generally 2,000 to 4,000. Thus, the Reynolds number of the fluid will be large and the fluid flow will be turbulent. On the other hand, when the fluid viscosity is small or the velocity is high, the inertial force becomes dominant. When this occurs, the Reynolds number will be small and the fluid flow will be laminar. The equation for the Reynolds number says the air velocity should be 100 km/h since the characteristic length has been cut in half to maintain a constant Reynolds number.Ī closer look at the Reynolds number reveals the following: When the fluid viscosity is large or the fluid velocity is low, the viscous force becomes dominant. Suppose you want to simulate the air flow around an automobile moving at a speed of 50 km/h by using a half-size model in a wind tunnel as shown in Figure 3.17. This law is called the Reynolds' law of similarity.Ĭonsider another example. Thus, the behavior of the two flows will be essentially the same. When two flows are geometrically similar and have the same Reynolds number, this means their ratio between their inertial forces and their viscous forces will be the same. Hence, the Reynolds number is the ratio of the inertial force to the viscous force. The numerator of the equation expresses the inertial force. The denominator of the equation expresses the viscous force of the fluid. (Here, it is the average velocity of the flow passing through a cross-section of the pipe) (In this example, L is the inside diameter of the pipe) The Reynolds number Re is defined by the following equation: This number is called the Reynolds number. In the experiment, Reynolds discovered a dimensionless number could be used to classify flows as either laminar or turbulent. However, when the water velocity was high, the ink started in a straight line but began oscillating and quickly dispersed throughout the pipe. The result showed that, when the water velocity was low, the ink moved downstream in a continuous straight line as shown in (a) of Figure 3.16. In the experiment, flows were visualized by pouring a stream of ink into a pipe in which water flows. In 1883, a British scientist named Osborne Reynolds (1842-1912) classified flows as either laminar or turbulent from a series of experiments known as the Reynolds’ experiment. The surface of the water creates waves as shown in (b). The more the flow increases the more turbulent the water flow becomes. When the tap is turned on just a small amount, water flows straight down in a very well-behaved and predictable stream as shown in (a) of Figure 3.15. On the other hand, a flow with irregular, unpredictable motion is called turbulent flow.Ĭonsider water running from a tap to illustrate the two states. ![]() A fluid flow with regular, predictable motion is called laminar flow. Chapter 3 Basics of Flow V 3.2.4 Laminar flow and turbulent flowĪ flow has two states: laminar and turbulent.
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