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مکانیک::
همراستا با جریان، در جهت جریان، در جهت جریان
(Here, u, v, w refer to the streamwise, span- wise and wall-normal components of fluctuating velocity respectively, and Ut is the friction velocity.)
It is emphasized that the physical understanding of these relationships rests essentially on the fact that the normal to the wall component and the shear-stress component at a given height are mainly due to the influence of eddies with 'centres' at or close to that height, whereas the streamwise and spanwise fluctuations at a given distance from the wall are due to all eddies with heights greater than the wall-normal height.
Related to this is an anecdote told by Tony Perry about the attached eddy hypothesis and k-1 law (where k is streamwise wavenumber).
Perry & Abell (1975) presented the first serious assault, using dynamically calibrated hot- wires, to uncover the scaling laws of the streamwise fluctuating velocity in pipe flows, a topic where a compilation of the previous experimental data highlighted significant disparities and thus considerable uncertainty.
A key new feature of these studies has been the documentation of characteristic eddy shapes that take the form of hairpin-shaped vortices, which appear with a range of scales and a preferred streamwise organization where the individual vortices align to form hairpin packets.،Moreover, these structures can experience VIV in direction transverse to the fluid flow [1], in the streamwise
The next three sections are devoted to transverse-only VIV, combined transverse and streamwise VIV and VIV of flexible cylinders.
Figures 2.4 (a) and (b) show the circular cylinders which are subjected to a fluid flow, elastically mounted and free to vibrate in the streamwise and cross-flow (CF) directions, respectively.
Figure 2.4: (a) and (b), schematic representation of an elastically mounted circular cylinder subjected to a uniform flow of velocity V which is free to vibrate in streamwise and transverse directions, respectively
(c), typical amplitude of streamwise (blue dash line) and transverse (green line) oscillations vs. reduced velocity.،
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