94
CHAPTER 4. HYDRODYNAMIC MODELS
pointed out that the convective variation in the flow velocity is not modeled
in this tunnel, and mass transport must be introduced as a separate item.
King, et al. (1984) described a versatile flow tunnel capable of irregular
oscillation along with superimposed steady currents. The flow tunnel can
be tilted to simulate beach slopes. King, et al. also presented a table summarizing 8 other flow tunnels that had been reported in the literature prior
to 1984.
Unidirectional Currents in Short-Wave Models
Many coastal problems involve unidirectional or multidirectional currents
along with the action of short waves. These flows may stem from tidal
currents through inlets, river or estuarine discharge, wind-driven currents,
or large-scale coastal currents; and when considered in combination with
short-wave-induced currents, the resulting hydrodynamic picture can become quite cloudy.
Currents can be mechanically created in small-scale short-wave models
through the use of pumps and manifolds. In short-wave models the nonlinear interaction between the waves and currents will be faithfully reproduced
provided the currents are properly scaled. Current flows are governed by
the same set of equations (Eqns. 4.2-4.5) as the short-wave motion, hence
the currents must conform to the same criteria found for short waves. The
scale for current velocity is found from Condition 1 (Eqn. 4.19) as
Nv = \/N^
(4.25)
Additionally, correct reproduction of time variations in nonsteady flows
requires similitude of the Strouhal number (Condition 2, Eqn. 4.20), which
when combined with Condition 1 leads to the time scale for current variations,
which is the same as the time scale for wave period (Eqn. 4.21).
Recognize that our usual inability to satisfy the Reynolds criterion
(given by Condition 3) means that similitude of combined currents and
short waves is restricted to the region of the flow field that is considered
to be inviscid. Current-induced boundary layers may not be in similitude
if viscous forces are significant or if boundary roughness is not correctly
scaled. Similitude requirements for boundary layers and shear stresses are
discussed shortly.
CHAPTER 4. HYDRODYNAMIC MODELS
pointed out that the convective variation in the flow velocity is not modeled
in this tunnel, and mass transport must be introduced as a separate item.
King, et al. (1984) described a versatile flow tunnel capable of irregular
oscillation along with superimposed steady currents. The flow tunnel can
be tilted to simulate beach slopes. King, et al. also presented a table summarizing 8 other flow tunnels that had been reported in the literature prior
to 1984.
Unidirectional Currents in Short-Wave Models
Many coastal problems involve unidirectional or multidirectional currents
along with the action of short waves. These flows may stem from tidal
currents through inlets, river or estuarine discharge, wind-driven currents,
or large-scale coastal currents; and when considered in combination with
short-wave-induced currents, the resulting hydrodynamic picture can become quite cloudy.
Currents can be mechanically created in small-scale short-wave models
through the use of pumps and manifolds. In short-wave models the nonlinear interaction between the waves and currents will be faithfully reproduced
provided the currents are properly scaled. Current flows are governed by
the same set of equations (Eqns. 4.2-4.5) as the short-wave motion, hence
the currents must conform to the same criteria found for short waves. The
scale for current velocity is found from Condition 1 (Eqn. 4.19) as
Nv = \/N^
(4.25)
Additionally, correct reproduction of time variations in nonsteady flows
requires similitude of the Strouhal number (Condition 2, Eqn. 4.20), which
when combined with Condition 1 leads to the time scale for current variations,
which is the same as the time scale for wave period (Eqn. 4.21).
Recognize that our usual inability to satisfy the Reynolds criterion
(given by Condition 3) means that similitude of combined currents and
short waves is restricted to the region of the flow field that is considered
to be inviscid. Current-induced boundary layers may not be in similitude
if viscous forces are significant or if boundary roughness is not correctly
scaled. Similitude requirements for boundary layers and shear stresses are
discussed shortly.