Turbulence always need a surface in order to be created, an aeroplane wing or the walls of a pipe per example.
How free jets become turbulent when they are just surrounded by air?
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$\begingroup$ Can you explain what you mean by a free jet? $\endgroup$– YashasCommented Feb 10, 2017 at 9:11
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$\begingroup$ Why do you think there needs to be a surface to generate turbulence? $\endgroup$– tpg2114Commented Feb 11, 2017 at 19:28
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$\begingroup$ @tpg2114 Because turbulence is generated by the fact that liquids and gases have viscosity $\endgroup$– user143115Commented Feb 11, 2017 at 20:25
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$\begingroup$ @sofky and viscosity only exists in a liquid or gas at physical walls and not anywhere else? $\endgroup$– tpg2114Commented Feb 11, 2017 at 20:34
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1$\begingroup$ As TRF has nicely explained below, you don't need walls to create turbulence. All you need is velocity gradients. Further, a statement like "I cannot imagine viscosity without the no-slip condition" makes no sense. Viscosity is a fluid property and has nothing to do with any specific flow properties, or the structure of the flow field. Finally, viscosity creates shear stresses whenever the velocity field is rotational, $\nabla\times\mathbf u\ne0$. $\endgroup$– PirxCommented Feb 11, 2017 at 22:13
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1 Answer
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I believe you are referring to a free shear layer as consequence of a jet. Due to the imbalance between the fluid velocity in the jet and the quiescent ambient fluid, a shear layer will exist at the edge of the jet. Shear layers are unstable, meaning some disturbances will amplify into an instability wave or vortical structure. A very common instability regarding shear layers is the Kelvin-Helmholtz instability. As the vortical structures grow on the shear layer, a phenomena known as convective transport takes place between the ambient and jet streams of fluid. Additionally, inside of these vortical structures exist smaller structures at a variety of scales, which result in mixing. Hence, eventually the two fluid streams will begin mixing together. As these instabilities continue to grow, they eventually result in total turbulence downstream in the jet. Below is a computational image of such a flow (although this case is studying compressible jet instabilities).
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1$\begingroup$ +1 To add to the answer: In the absence of other effects such as density gradients etc., boundaries are required for creation of vorticity (viscous effects diffuse them), and all turbulent flows are necessarily vortical in 3-dimensions. A jet of water issuing into air already contains vorticity (created at pipe walls), and these tangle up to create turbulence, perhaps even before water exits the pipe. K-H instability is based on another way of generating vorticity (explained in answer), but for water jet in air this mechanism may not be the most effective. $\endgroup$– DeepCommented Feb 13, 2017 at 8:19