Classification of Flows, Laminar and Turbulent Flows

A flow can be Laminar, Turbulent or Transitional in nature. This becomes a very important classification of flows and is brought out vividly by the experiment conducted by Osborne Reynolds (1842 - 1912). Into a flow through a glass tube (Fig.7.2. ) he injected a dye to observe the nature of flow. When the speeds were small the flow seemed to follow a straight line path (with a slight blurring due to dye diffusion). As the flow speed was increased the dye fluctuates and one observes intermittent bursts. As the flow speed is further increased the dye is blurred and seems to fill the entire pipe. These are what we call Laminar, Transitional and Turbulent Flows.

Figure 7.2: Reynolds Experiment

A similar experiment may be conducted today using Hot Wire Anemometer, which measures instantaneous velocities at a point. The traces of velocity at the three regimes of flow are shown in Fig.7.3. It is clear that while the laminar flow has a predominant velocity in the main flow direction, turbulent flow has a significant component of velocity in the flow normal direction. While laminar flow is "orderly" turbulent flow is "Random" and "Chaotic".

 

Figure 7.3: Hot Wire Signals for Turbulent flow (top), Transitional flow (middle) and Laminar Flow (bottom)

It is also found that a flow in a pipe is laminar if the Reynolds Number (based on diameter of the pipe) is less than 2100 and is turbulent if it is greater than 4000. Transitional Flow prevails between these two limits. But it should be pointed out that people have preserved laminar flow at very high Reynolds number through carefully monitored conditions.

This is an annimation to perform the Reynolds Experiment.

(c) Aerospace, Mechanical & Mechatronic Engg. 2005
University of Sydney