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Notes
- 1.
In the evening, the wind “goes to sleep” (but only near the ground!). – The reason: Turbulent motions lift cooler and therefore more dense air upwards, displacing the warmer air there downwards, with its lower density. Both require work, which is performed at the cost of the kinetic energy of the air, slowing the winds near the ground.
- 2.
For observations projected on the wall, the fixed setup of the basin sketched in Fig. 10.8 is sufficient. The eye of the observer follows the object, and thus sees the liquid flowing past it.
- 3.
With the exception of the special case treated in Eq. (10.20).
- 4.
To produce and maintain a vortex in water, it suffices to set the cylinder in rotation around its symmetry axis. The thickness of the boundary layer increases without limit as a function of time (Eq. (10.3)). The velocity distribution approaches more and more that of an irrotational vortex field with increasing distance from the surface of the cylinder (i.e. curl u = 0). – In air, with its small dynamic viscosity, vortices can be generated by the process described in the caption of Fig. 10.36 (see Fig. 10.37).
- 5.
Bell-shaped jellyfish use the recoil from water vortices which they produce as a means of propulsion.
- 6.
For rough calculations, one can keep in mind two useful approximations: Transverse force \(F_{\mathrm{a}}=\frac{1}{3}\varrho u^{2}A\), and resistance force \(F_{\mathrm{r}}\approx 1\) to 10 % of F a (\(A=\) area of the plate or airfoil).
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Lüders, K., Pohl, R.O. (2017). Motions in Liquids and Gases. In: Lüders, K., Pohl, R. (eds) Pohl's Introduction to Physics. Springer, Cham. https://doi.org/10.1007/978-3-319-40046-4_10
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