5.2. RUBBLE-MOUND STRUCTURES
177
Rubble-mound structure models must be geometrically
undistorted in length scale.
It is important to note that the geometric scaling of the model extends to
providing a reasonable approximation of the shape and size distributions of
the primary and underlayer armor units.
The condition given by Eqn. 5.10 provides for similarity in the armor
layer Froude number, and it is the same condition specified by the Froude
scaling criterion for hydrodynamic models. Therefore, our second requirement is
Flow hydrodynamics in a rubble-mound structure model
must conform to the Froude criterion.
The Froude criterion states that inertial forces relative to gravity forces
scale correctly; and according to Hudson, et al. (1979), form drag relative
to gravity forces scale nearly correctly (depends on the form of armor unit,
its weight, and the size of wave).
The condition given by Eqn. 5.11 is recognized as similarity of the armor layer Reynolds Number based on the characteristic linear dimension of the armor units (usually the mean diameter for quarrystone). It is
impossible to satisfy completely this criterion at reduced scale. However,
if the model is conducted at a large enough scale to assure that the flow
through the primary armor layer remains turbulent, then this criterion is
reasonably well satisfied. Thus, our third criterion is
Rubble-mound structure models must have turbulent
flow conditions throughout the primary armor layer.
However, if the flow velocities and the size of the units are small, viscous
forces may be greater in the model resulting in a scale effect; therefore, it
is best to operate at larger scales when possible.
The condition represented by Eqn. 5.12 states that the effects of surface roughness of the armor units in the prototype must be the same in
the model. The resistance to movement offered by surface roughness in
prototype-scale quarrystone or armor units is considered negligible. In the
model, attempts are made to decrease the relative roughness of the structural units by making their surfaces as smooth as possible1. If there is
appreciable friction between armor units in the model, then the model will
show higher stability than its prototype equivalent. This could lead to
potentially unsafe design.
1 Painting model armor layer stones is one technique used to reduce the contact friction
between units.
177
Rubble-mound structure models must be geometrically
undistorted in length scale.
It is important to note that the geometric scaling of the model extends to
providing a reasonable approximation of the shape and size distributions of
the primary and underlayer armor units.
The condition given by Eqn. 5.10 provides for similarity in the armor
layer Froude number, and it is the same condition specified by the Froude
scaling criterion for hydrodynamic models. Therefore, our second requirement is
Flow hydrodynamics in a rubble-mound structure model
must conform to the Froude criterion.
The Froude criterion states that inertial forces relative to gravity forces
scale correctly; and according to Hudson, et al. (1979), form drag relative
to gravity forces scale nearly correctly (depends on the form of armor unit,
its weight, and the size of wave).
The condition given by Eqn. 5.11 is recognized as similarity of the armor layer Reynolds Number based on the characteristic linear dimension of the armor units (usually the mean diameter for quarrystone). It is
impossible to satisfy completely this criterion at reduced scale. However,
if the model is conducted at a large enough scale to assure that the flow
through the primary armor layer remains turbulent, then this criterion is
reasonably well satisfied. Thus, our third criterion is
Rubble-mound structure models must have turbulent
flow conditions throughout the primary armor layer.
However, if the flow velocities and the size of the units are small, viscous
forces may be greater in the model resulting in a scale effect; therefore, it
is best to operate at larger scales when possible.
The condition represented by Eqn. 5.12 states that the effects of surface roughness of the armor units in the prototype must be the same in
the model. The resistance to movement offered by surface roughness in
prototype-scale quarrystone or armor units is considered negligible. In the
model, attempts are made to decrease the relative roughness of the structural units by making their surfaces as smooth as possible1. If there is
appreciable friction between armor units in the model, then the model will
show higher stability than its prototype equivalent. This could lead to
potentially unsafe design.
1 Painting model armor layer stones is one technique used to reduce the contact friction
between units.