Date of Award:

2014

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Advisor/Chair:

Blake P. Tullis

Abstract

Weirs are commonly used as spillways to release flows from a reservoir. The free-falling jet on the downstream side of the weir is called the nappe. Under certain hydraulic conditions, determined mainly by the size, design, and construction of the weir, nappe oscillation, otherwise known as nappe vibration, can occur. Characteristics of this dynamic behavior include excessive acoustic energy manifested as sound pressure waves and lowfrequency noise accompanied by horizontal waves or banding on the nappe. Mitigation of this process may be required, especially if the weir operates in close proximity to occupied structures. Instability of water jets moving through air has been a topic of study for over a century, although studies specific to curvilinear weir nappe flow are less common. The objective of this research is to further the understanding related to the mechanisms that cause nappe vibration, document the occurrence conditions, and investigate mitigation techniques.

Research was conducted at the Utah Water Research Laboratory (UWRL) using three physical models:

  • A 6 ft wide x 3.5 ft tall weir with a quarter round crest (model #1)
  • A 15.4 ft wide x 11 ft tall weir with a broad crest (model #2)

  • A 16 ft wide x 12 ft tall weir with a quarter round crest (model #3)

Testing included confined and unconfined nappe conditions (open air cavity behind the nappe vs. closed air cavity) for model #1 and model #3. Vibration frequencies were recorded and analyzed using an accelerometer and microphone. Comparisons were made between the three models of different scale. Testing included modifications to the weir crest and the weir apron to study the effect on the behavior of the nappe.

The results of this study are presented, including a review of previous literature and theories. The mechanisms that sustain and amplify the nappe vibration phenomenon varied at the different size-scales, while evidence exists that the root cause of initial instability leading to self-induced vibration can be traced to the same source. The results of this study should be of practical use to engineers, researchers, and those concerned with dam safety.

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