4.2. SHORT-WAVE HYDRODYNAMIC MODELS
127
Mass Transport by Waves and Currents. Mass transport of the discharged effluent by short waves and currents will be in similitude by virtue
of the Froude scaling criterion and by maintaining a model Reynolds number in the rough turbulent range. Generally, waves and currents in coastal
hydraulic models are sufficiently large to assure the Reynolds number is
high enough. For cases were mass transport velocities are low, Lavender and Cowley (1975) recommended a minimum Reynolds number of 600
where the velocity is the minimum velocity expected and the length is taken
as the minimum depth.
Diffusion by Ambient Turbulence. The physical process of diffusion
by ambient turbulence is usually considered unimportant relative to the
effects of the other physical mechanisms discussed above. However, a geometrically undistorted Froude-scaled model will provide similitude of the
horizontal and vertical diffusion according to Lavender and Cowley (1975).
Evaporative Cooling. The most difficult aspect of thermal dispersion
to reproduce in the laboratory is the cooling of water by heat exchange
between the fluid and the ambient atmosphere. If attempts are made to
simulate prototype climate in the laboratory, the model must be distorted,
which invalidates the short-wave properties of the model and many of the
thermal similitude aspects.
Fliigge and Schwarze (1974) gave details of scale relationships for heat
input and heat exchange between fluid and atmosphere. The heat input
scale ratio for an undistorted short-wave model was given as
Nh.. = (AT£)5/2
(4.89)
and assuming the heat exchange coefficient is the same between prototype
and model, the scale for heat exchange was determined to be
Nh„ = (NLf
(4.90)
which is the same as the area scale.
Because the heat input and heat exchange scales are different, the result is evaporative loss of heat in the model that is more than it should be.
Fliigge and Schwarze (1974) recommended a reduction factor to determine
the actual heat exchange coefficient in the laboratory. Lavender and Cowley (1975) suggested that a more practical approach would be to “control
the model room climate to inhibit heat exchange between the fluid and atmosphere, thus providing conservative results with respect to environmental
effects of thermal discharges”.
127
Mass Transport by Waves and Currents. Mass transport of the discharged effluent by short waves and currents will be in similitude by virtue
of the Froude scaling criterion and by maintaining a model Reynolds number in the rough turbulent range. Generally, waves and currents in coastal
hydraulic models are sufficiently large to assure the Reynolds number is
high enough. For cases were mass transport velocities are low, Lavender and Cowley (1975) recommended a minimum Reynolds number of 600
where the velocity is the minimum velocity expected and the length is taken
as the minimum depth.
Diffusion by Ambient Turbulence. The physical process of diffusion
by ambient turbulence is usually considered unimportant relative to the
effects of the other physical mechanisms discussed above. However, a geometrically undistorted Froude-scaled model will provide similitude of the
horizontal and vertical diffusion according to Lavender and Cowley (1975).
Evaporative Cooling. The most difficult aspect of thermal dispersion
to reproduce in the laboratory is the cooling of water by heat exchange
between the fluid and the ambient atmosphere. If attempts are made to
simulate prototype climate in the laboratory, the model must be distorted,
which invalidates the short-wave properties of the model and many of the
thermal similitude aspects.
Fliigge and Schwarze (1974) gave details of scale relationships for heat
input and heat exchange between fluid and atmosphere. The heat input
scale ratio for an undistorted short-wave model was given as
Nh.. = (AT£)5/2
(4.89)
and assuming the heat exchange coefficient is the same between prototype
and model, the scale for heat exchange was determined to be
Nh„ = (NLf
(4.90)
which is the same as the area scale.
Because the heat input and heat exchange scales are different, the result is evaporative loss of heat in the model that is more than it should be.
Fliigge and Schwarze (1974) recommended a reduction factor to determine
the actual heat exchange coefficient in the laboratory. Lavender and Cowley (1975) suggested that a more practical approach would be to “control
the model room climate to inhibit heat exchange between the fluid and atmosphere, thus providing conservative results with respect to environmental
effects of thermal discharges”.