Terminology. Other examples of penetrable roughnesses

The term canopy was used to characterize the above structures subjected to a flow. The term originated, perhaps, from geographical science but was naturally adopted by the meteorology of the Earth s vegetative cover. This term however has not been so natural for other research fields. In addition, the term does not have any inheritance to the general area of fluid mechanics that investigates these structures now. 

Even meteorologists used another terms, a very rough or random surface at the early history of their recent theories, [522], The terms penetrable obstruction or a grid [554] are evidently incorrect for such extensive and lengthy structures. The term the porous medium also used by some researchers resembles some features of the object under investigation but has already been employed in filtration theory and thus is associated with theoretical approaches of the latter theory. The term a layer with distributed force suggested by Hunt et al. [50, 319] clearly expresses the mathematical idea used but does not reflect the physical phenomenon under focus. 

The collection of canopies or EPR flows discussed above can be extended by some other examples of such flows to demonstrate the even wider importance of the theme. 

Heat exchangers in engineering often contain elements deeply protruding into the flow like those presented in Fig. 1.16 to provide the effective heat removal. Some interesting examples named porous matrix were reported by Pedras and Lemos [485] and some other authors [2, 8, 600], Penetrable roughnesses are also applied in engineering. 

There are two uncommon examples to conclude. In all the situations considered, a gas or a liquid was moved and influenced by a penetrable structure, a PR that could be motionless or set in a motion itself by the medium. The droplet jet like that from a shower provides a situation when porous medium moves and induces a motion of the surrounding air, Fig. 1.17,A. A theoretical approach was suggested by Hunt [230] to consider droplets as a porous medium. A similar situation occurs when a bubble column arises from a source deeply in the sea or ocean, Fig. 1.17,B. Bubbles form a porous medium that induces the motion of the surrounding water. Of course, particular equations should be employed in each case, either for droplets or bubbles. The broadening of the jets can be explained in such models. Another approach to bubbles as to obstructions is suggested in the Chapter 7. 

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