Continuous Release

The continuous release profile extends from the source downwind (Fig. E.1). An elevated, heavy vapour/aerosol release starts out with a circular cross section. Upon touching down, the cross section becomes a truncated ellipse, and the cloud levels off as the vertical component of momentum is converted into downwind and cross-wind momentum. Aerosol droplets may rain out shortly after touchdown. Rain-out produces a pool which spreads and vaporises. If spilled onto water, part of the material may also dissolve. The vapour from the pool is added back to the plume, as a function of time. The plume can become buoyant after evaporating all aerosol droplets and picking up heat by ground conduction, or by condensing water picked up over a wet surface. A buoyant plume lifts off and rises until constrained by the mixing layer.

The Cartesian co-ordinates x, y, z correspond to the downwind, cross-wind (lateral horizontal) and vertical directions, respectively; x=0 corresponds to the point of release, y = 0 to the plume centre-line and z = 0 to ground-level. In addition to these cartesian co-ordinates use is made of the 'cloud' coordinates s and Z Here s is the arclength measured along the plume centre, with s=0 corresponding to the point of release. The concentration profile for continuous release (Plume) is given by [203; 9]:



Fh( y) = exp-

(y )


p us ys z

V2S y



V2s z

and and oy, = standard deviation of horizontal profile of cloud concentration (m)

oz, = standard deviation of vertical profile of cloud concentration (m)

y = the crosswind distance from the cloud axis (m)

Z = distance from Plume center-line (m)

circular cross-section truncated cross-section circular cross-section truncated cross-section

Fig. E.1 UDM cloud geometry for continuous release [203]

The co-ordinate Z indicates the direction perpendicular to the plume centre-line and perpendicular to the y-direction. The angle between the plume centre-line and the horizontal is denoted by 0 = 0(s), and the vertical plume height above the ground by z0 = zcid(s). Thus z and Z are related to each other by z = zcld + Z cos(0).

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