76
CHAPTER 3. PRINCIPLES OF SIMILITUDE
3.3.3 Hydraulic Model Distortion
Anytime one or more of the established similitude criteria are not satisfied in a model, the model is referred to as a distorted model. This strict
definition of model distortion means that in an undistorted model all the dimensionless pi-terms determined from the important independent variables
of the problem must be the same in the model as in the prototype.
For fluid flow problems of interest to coastal engineers, similitude of
all dimensionless pi-terms is impossible. Even when considering only the
Froude and Reynolds criteria, hydraulic models conducted at reduced scale
most likely will not satisfy both criteria simultaneously. Therefore, by
the strict definition, practically all coastal hydraulic models are distorted
models.
Hydraulic model engineers have chosen to relax the definition for model
distortion by limiting the term “distortion” to consideration of only geometric similitude, i.e.,
Models that maintain geometric similitude are referred to as
undistorted models.
Whereas...
Distorted models are physical models in which the horizontal
length scale and the vertical length scales are different. In other
words, a distorted model is not geometrically similar to the
prototype situation.
Throughout the remainder of this text, the terms “distorted model” and
“undistorted model” will conform to the above geometric definition.
In coastal engineering distorted models usually have a larger horizontal scale than vertical scale. This has the effect of lessening the required
horizontal space needed for the model while increasing slopes in the model.
Distorted models may be considered when departure from geometric
similitude serves some definite objective such as scaled depth or velocities;
however, the results from a distorted model are limited only to consideration
of this objective (Warnock 1950). For example, river flow models often
need to be constructed using distorted length scales because the physical
dimensions of a typical river cross-section would geometrically scale in such
a way that the model water depth may be less than a few centimeters
deep. In an undistorted model this would allow surface tension effects in
the model to play a significant role in the process, thus invalidating model
results. In this case, the objective of model distortion is to avoid surface
tension effects; however, interpretation of model results will become more
difficult because of scale effects introduced by the geometric distortion.
Distorted models offer several advantages:
CHAPTER 3. PRINCIPLES OF SIMILITUDE
3.3.3 Hydraulic Model Distortion
Anytime one or more of the established similitude criteria are not satisfied in a model, the model is referred to as a distorted model. This strict
definition of model distortion means that in an undistorted model all the dimensionless pi-terms determined from the important independent variables
of the problem must be the same in the model as in the prototype.
For fluid flow problems of interest to coastal engineers, similitude of
all dimensionless pi-terms is impossible. Even when considering only the
Froude and Reynolds criteria, hydraulic models conducted at reduced scale
most likely will not satisfy both criteria simultaneously. Therefore, by
the strict definition, practically all coastal hydraulic models are distorted
models.
Hydraulic model engineers have chosen to relax the definition for model
distortion by limiting the term “distortion” to consideration of only geometric similitude, i.e.,
Models that maintain geometric similitude are referred to as
undistorted models.
Whereas...
Distorted models are physical models in which the horizontal
length scale and the vertical length scales are different. In other
words, a distorted model is not geometrically similar to the
prototype situation.
Throughout the remainder of this text, the terms “distorted model” and
“undistorted model” will conform to the above geometric definition.
In coastal engineering distorted models usually have a larger horizontal scale than vertical scale. This has the effect of lessening the required
horizontal space needed for the model while increasing slopes in the model.
Distorted models may be considered when departure from geometric
similitude serves some definite objective such as scaled depth or velocities;
however, the results from a distorted model are limited only to consideration
of this objective (Warnock 1950). For example, river flow models often
need to be constructed using distorted length scales because the physical
dimensions of a typical river cross-section would geometrically scale in such
a way that the model water depth may be less than a few centimeters
deep. In an undistorted model this would allow surface tension effects in
the model to play a significant role in the process, thus invalidating model
results. In this case, the objective of model distortion is to avoid surface
tension effects; however, interpretation of model results will become more
difficult because of scale effects introduced by the geometric distortion.
Distorted models offer several advantages: