Flow separation at a vertical step where the turbulence is manifested

primarily in the vertical plane will not have any significant scale effect

in geometrically distorted models.

fluid motion. Consequently, a scale effect will occur between a

prototype where flow turbulence is generated at a sloping boundary.

However, the scale effect is strongest near the bottom and appears to

lessen closer to the free surface. Also, the scale effect seemed to be

restricted to the immediate vicinity of the jet boundary; however,

potential impacts farther downstream were not evaluated due to the

limited measurement region of the experiments.

Importance of turbulent scale effects in a geometrically distorted physical

model relates directly to the processes being studied, and whether or not flow

turbulence is a dominant forcing of that problem. Specifically, the Alaska

District is considering construction of a large physical model of Cook Inlet with

a geometric distortion of 4 (horizontal/vertical length scale), and the Alaska

District engineers are concerned about potential turbulence scale effects because

flow separation and gyre formation are known to be important influences at Cook

Inlet.

Based on (a) performance of the 3-D flow table model with horizontal-to-

vertical distortion of 15, (b) theoretical analyses of potential scale effects, and

(c) flow table experiments, it is the opinion of the report authors that turbulent

scale effects in the proposed distorted physical model of Cook Inlet would not

significantly influence model results. In other words, hydrodynamic flow

patterns, regions of flow separation, generation of large-scale gyres, and results

of sediment tracer and dye injection experiments from the distorted model would

closely resemble those of an undistorted model. Differences will occur in the

immediate vicinity of flow separation boundaries closer to the bottom, but these

differences are expected to be localized and should not influence overall flow

patterns. Three-dimensional flow structures will also have a scale effect with

vertical velocity components being stronger than they should be. However, this

is partially offset by steepening of slopes in the distorted model that will decrease

the vertical turbulent fluctuations. An important consideration in large distorted

physical models is providing sufficient bottom and boundary roughness to assure

a fully turbulent boundary layer.

Geometrically distorted physical models retain all of the nonlinearities

inherent in complex flow situations, including turbulence generation. Even

though known scale effects will alter the value of some turbulence terms, the

overall turbulence flow field will still exhibit most of the behavior expected from

an undistorted physical model. For cases where turbulence and 3-D flow patterns

dominate, the distorted physical model provides more reality than depth-averaged

numerical models or even 3-D numerical flow models which discard or linearize

the convective accelerations and/or simplify terms related to turbulence.

Consequently, where space limitations prohibit construction of an undistorted

model, geometrically distorted physical model may be the best tool for

examining engineering problems related to flow turbulence and 3-D flow fields.

92

Chapter 8 Summary and Conclusions

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