| Summary: | 碩士 === 國立中央大學 === 機械工程研究所 === 83 === This thesis reports velocity measurements and their corre-
sponding flow statistics of a far-field round jet which
impinges from above on a sharp, vertically stable density
interface via laser-Doppler velocimeter in a water tank. The
initial flow Richardson and Reynolds numbers based on the local
jet scales at the density interface are remained constant and
ranged from 0 ≦ Ri ≦ 3 at Re = 4,600 and 0 ≦ Ri ≦ 2 at Re =
9,100. The rather narrow range of values of Ri is due to two
reasons : (1) potential refractive-index-fluctuation problems
environment across the density interface ant (2)large flow
Reynolds munbers. The principle measuring points are
distributed along the jet centerline from the initial density
interface (Z'= 0) to the jet maximum penetration depth (Z'max).
At each measuring points, both axial and radial directions of
velocities are simultaneously measrued. The goal is to
investigate the effects of Richardson and Reynolds numbers and
the penetration depth on the turbulent characteristic velocity,
W, which is commonly assumed to be the jet centerline axial
mean velocity at the density interface. It was found that :
(1)At Z'/Z'max = 0, the normalized turbulent characteristic
velocity (W/W ; W is the turbulent characte- ristic velocity
at the same position if there is no stratifi- cation dcreases
as Ri increases, probably independent of Re. The data can be
best fitted by an empirical relation of the form W/W = 0.8Ri
.(2) Traditionally, the entrainment rate was defined as E = we/
W, where we is the entrainment velocity and W was assumed to be
the same as W , not influenced by the stratification. The
current results suggest that such an assumption may need recon-
sideration. (3) W tends to decrease drastically as Z'/Z'max
increase. (4) When Z'/Z'max is close to unity, values of W/W
approach zero where the magnitude of turbulent shear stress at
Re = 9,100 is at least four times than that at Re = 4,600.
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