Document Type

Dissertation

Date of Degree

Summer 2015

Degree Name

PhD (Doctor of Philosophy)

Degree In

Civil and Environmental Engineering

First Advisor

George Constantinescu

Second Advisor

Marian Muste

Abstract

Investigating flow and turbulence structure around a barrier mounted on the ground or placed in its vicinity is a fundamental problem in wind engineering because of many practical applications related to protection against adverse effects induced by major wind storms (e.g., hurricanes) and snow events (e.g., snow fences used to reduce adverse effects of snow drifting on the roads). In this work the focus is on the case when the obstacle/barrier is porous and the shape of the obstacle is close to a high-aspect-ratio rectangular cylinder situated in the vicinity of the ground. The study employs a range of numerical and experimental techniques to achieve this goal that include 3D LES and 2D RANS numerical simulations, and RTK survey and 3D photogrammetry techniques to measure ground elevations and snow deposits in the field.

In the first part of the study, high-resolution large eddy simulations are used to understand the fundamental flow physics of flow past 2D solid and porous vertical plates with a special focus on describing the unsteady wind loads on the obstacle, vortical structure of the turbulent wake, spectral content of the wake, the separated shear layers and of the characteristics of the large-scale vortex shedding behind the plate, if present. Results show that LES can accurately predict mean flow and turbulence statistics around solid/porous cylinders. Then, a detailed parametric study of flow past vertical solid and porous plates situated in the vicinity of a horizontal bed is performed for the purpose of understanding changes in the mean flow structure, turbulence statistics and dynamics of large scale coherent structures as a function of the main nondimensional geometrical parameters (bottom gap for solid and porous plates, and porosity and average hole size of porous plates) and flow variables (e.g., bed roughness) that affect the wake flow. In particular, the LES flow fields allowed clarifying how the interactions between the bottom and the top separated shear layers change with increasing bottom gap and what is the effect of the bleeding flow on the interactions between the separated shear layers that determine the coherence of the large-scale eddies at large distances from the wake.

In the second part of the thesis, a novel methodology based on field monitoring of the snow deposits and RANS numerical simulations is proposed to improve the design of snow fences and in particular the design of lightweight plastic snow fences that are commonly used to protect roads in the US Midwest against the snow drifting. The goal of the design optimization procedure is to propose a snow fence design that can retain a considerable amount of snow within a shorter downwind distance compared to fences of standard design. A major contribution of the present thesis was the development of a novel non-intrusive image-based technique that can be used to quantitatively estimate the temporal evolution of the volume of snow trapped by a fence over long periods of time. This technique is based on 3-D close range photogrammetry. Results showed that this technique can produce estimations of the snow deposits of comparable accuracy to that given by commonly used methods. This is the first application of this type of techniques to measurements of the snow deposits.

Public Abstract

Investigating flow around an obstacle/barrier is a fundamental problem in wind engineering because of many practical applications related to protection against adverse effects induced by major wind storms (e.g., hurricanes) and snow events (e.g., snow fences used to reduce adverse effects of snow drifting on the roads). In this work the focus is on the case when a rectangular porous vertical plate is situated in the vicinity of the ground. The study employs a range of numerical and experimental techniques to achieve this goal.

In the first part of the study, high-resolution three dimensional large eddy simulations (LES) are used to understand the fundamental flow physics of flow past solid and porous vertical plates with a special focus on describing the unsteady wind loads on the plate, turbulence structures behind the plate and bed friction velocity. Results show that LES can accurately predict mean flow and turbulence statistics around solid/porous cylinders. Then, a detailed parametric study of flow past vertical solid and porous plates situated in the vicinity of a horizontal bed is performed for the purpose of understanding changes in the mean flow structure, turbulence statistics and dynamics of large scale coherent structures as a function of the main geometrical parameters (relative bottom gap for solid and porous plates, and porosity and average hole size of porous plates) and flow variable (bed roughness) that affect the wake flow. In particular, the LES flow fields allowed clarifying how the potential region of particles deposition and sediment entrainment vary with the main geometrical parameters and flow variable.

In the second part of the thesis, a novel joint methodology based on both field monitoring of the snow deposits and two dimensional RANS numerical simulations is proposed to improve the design of snow fences and in particular the design of lightweight plastic snow fences that are commonly used to protect roads in the US Midwest against the snow drifting. The goal of the design optimization procedure is to propose a snow fence design that can retain a considerable amount of snow within a shorter downwind distance compared to fences of standard design. Results showed that snow fences with new design proposed in this study are a better option to protect roadways against snow drifting in regions with narrow right of ways. Another major contribution of this study is to propose, develop protocols and test a novel image-based technique that can be used to quantify the snow deposits trapped by a snow fence over time. This technique is based on three dimensional close range photogrammetry (CRP) which is a non-intrusive method that eliminates the need to perform man-made measurements during the storms, which are difficult and sometimes dangerous to perform. The protocols developed in this study allowed continuous measurements of snow deposits over time using images remotely that eliminates the need to visit the site for field measurements during the harsh weather. This is the first application of this type of techniques to measurements of the snow deposits trapped by a fence.

Keywords

publicabstract, Flow past porous barriers, Fundamental flow physics of flow past solid and porous vertical plates, Large Eddy Simulations, Numerical simulations, Snow drifting, Snow fence

Pages

xxv, 348

Bibliography

337-348

Comments

This thesis has been optimized for improved web viewing. If you require the original version, contact the University Archives at the University of Iowa: http://www.lib.uiowa.edu/sc/contact/.

Copyright

Copyright 2015 Keshav Basnet

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