Engineering Background

            In fluid mechanics, a tool used to measure the equivalence of systems and to assess the conditions of pipe flow is the Reynolds number. The Reynolds number can be calculated using the following equation:

where

Reynolds numbers higher than 40,000 generally correspond to turbulent flow conditions. Turbulent flow is desirable for wind tunnel testing because it accurately reproduces normal flow conditions around a truck at high speeds. In turbulent flow, air velocity is relatively uniform over the diameter of the tube, as shown in Fig 1.

The flow at the center of the tube is especially uniform and is consequently the best location for a model to be placed if one's aim is to place it under conditions accurately replicating a normal environment.

Fig 1: Turbulent flow in wind tunnel

Reynolds numbers below 40,000 are indicative of laminar flow. Laminar flow in a pipe results in a great variation in flow speed between the edges of the pipe and the center (Fig 2). Consequently, laminar flow requires very careful positioning of the model in the wind tunnel and the flow does not mimic normal conditions accurately.

Fig 2: Laminar flow in wind tunnel

 

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