The relation between Turbulence, Reynolds, and Wind Tunnel Testing

In this video, we will be discussing Reynolds number and Turbulence. Two closely related items. The Reynolds Number The Reynolds number provides the ratio between the inertia forces and the viscous forces of a flow. It is calculated by dividing the product of the flow velocity, characteristic length and density by the viscosity. You can see it in this formula (00:00:18). Laminar flow At low Reynolds numbers, the damping effect of the viscosity is larger than the inertia forces that want to disturb the flow: fluid particles move on steady parallel trajectories. It looks very clean, very orderly. And that's why we call this laminar flow. Turbulent flow At high Reynolds numbers, the inertia forces are big enough to overcome this damping effect and you will start to see nervous movements of particles superimposed onto the main flow. The trajectories are no longer parallel but they feature many local direction variations and swirls within the flow. We call this turbulent flow. Transition point As the Reynolds number increases, somewhere between laminar and turbulent flow there is a transition point or transition zone. As density, viscosity and object dimensions typically stay the same within a certain case, this often happens as the velocity increases beyond a certain point, a critical point. Example A well-known example of this is the flow out of a faucet. If you turn it open only slightly, you will see a stable, clear stream of water. Turn it open completely and you will see a nervous flow full of bubbles. Practical use So how can you use this Reynolds number in your own application? Unless you are running a case that is exactly identical to one that has been well tested, you cannot compare directly. Because the Reynolds number is a subjective thing, there is no clear definition of the characteristic length. It's more of a subjective method to characterize a flow rather than a real physical property. But by calculating the Reynolds number for your application, you can roughly compare to other applications, to get the first idea on whether the flow will be laminar or turbulent in your case. Using the Reynolds number you can analyze the flow of a scaled model, that you have tested in the wind tunnel for example, and extrapolate to a full size one. That's pretty useful for wind tunnel testing. ----------------------------------------------------------------------------------------------------------- Wouter Remmerie Wouter is the Founder of AirShaper, an online, virtual wind tunnel. With this tool and these videos, we want to make aerodynamics accessible to everyone! Interested in more content like this on the field of aerodynamics? Make sure to click that subscribe button, we post new videos every week! Looking for a way to test your Aerodynamics projects without all the hassle and the huge costs coming with it? Check out and see how easy it can be!

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