chapter  4
49 Pages

Principles of air jets and plumes

Potential core region This is the region immediately downstream of the supply opening where mixing of the jet fluid with the surrounding fluid is not complete. The length of the core depends on the type of opening and the turbulence of the air supply but usually it extends to 5-10 equivalent opening diameters. In this region the centreline velocity, Um, is constant and equal to the supply velocity, Uo. Characteristic decay region After the consumption of the potential core by the free

shear layer the centreline velocity begins to gradually decrease so that:

Um/Uo ∝ 1/xn (4.1) where x is the distance from the supply and n is an index which has a value between 0.33 and 1.0. The extent of this region and the value of n depend on the shape of the supply opening and it is usually associated with large aspect ratio (ratio of length to height) openings, i.e. it is negligible for circular or square openings. Axisymmetric decay region This is a region dominated by a highly turbulent flow

generated by viscous shear at the edge of the shear layer. For three-dimensional jets

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this is usually referred to as the ‘fully developed flow region’ where the spread angle of the jet is a constant whose value depends on the geometry of the opening. It is the predominant region for a jet discharging from a low aspect ratio opening where it extends to about 100 equivalent diameters. It is not so significant for high aspect ratio openings. The centreline velocity here decreases inversely with the distance from the opening, i.e.:

Um/Uo ∝ 1/x (4.2)

Terminal region This is a region of rapid diffusion and the jet becomes indistinguishable from the surrounding air. The centreline velocity decays with the square of the distance, i.e.:

Um/Uo ∝ 1/x2 (4.3)

A free jet may be produced by a circular, cylindrical, rectangular or infinitely long opening discharging air into a large enclosure containing a stagnant mass of air. The extent of each of these four regions will primarily depend on the geometry of the opening and, to a lesser extent, on the turbulence characteristics at the opening.