Dissertations and Theses

Date of Award

2021

Document Type

Dissertation

Department

Mechanical Engineering

First Advisor

Yiannis Andreopoulos

Abstract

The instability and transition to turbulence and its evolution in pulsatile pipe flows involving both, reverse and no-reverse flows, is the primary focus of this experimental work. A piston driven by a programable DC servo motor was used to set up a water flow system and provide the pulsation characteristics. Time-resolved particle image velocimetry data were acquired in a refractive index matching setup by using a continuous wave laser and a high-frame-rate digital camera. The position of the piston was continuously recorded by a laser proximity sensor. Different experiments were carried out with Reynolds numbers in the range of 535 4825 and Womersley numbers from 6.3 to 23.82. The non-stationarity of the data was addressed by incorporating trend removal methods involving low- and high-pass filtering of the data, and by using the empirical mode decomposition together with the relevant Hilbert-Huang transform to determine the intrinsic mode functions. This later method is more appropriate for nonlinear and non-stationary cases, for which traditional analysis involving classical Fourier decomposition is not directly applicable. It was found that transition to turbulence is a spontaneous event covering the whole near wall region. The instantaneous vorticity profiles suggest the development of a large-scale ring-like attached wall vortical layer with smaller vortices of higher frequencies than the pulsation one superimposed which point to Kelvin-Helmholtz type of instability. At low frequency of pulsation, transition is delayed to higher Reynolds numbers, where the appearance of turbulence is not modulated anymore by the frequency of pulsation. Inflectional instability of the velocity profile leads to flow separation and the formation of a major roll-up structure with the K-H vortices superimposed. This structure breaks down in the longitudinal direction into smaller turbulent patches with vortical content which appears to be the prevailing structural content of the flow in each of the investigated Re numbers. At higher Re numbers, the strength and extent of the vortices are larger and substantial disturbances appear in the free stream region of the flow which are typical of pipe flows at transitional Re numbers. Turbulence appears to be produced at the locations of maximum or minimum vorticity within the attached wall vortical layer (WVL), in the ridges between the K-H vortices around the separated WVL and the upstream side of the secondary vortex where the flow impinges the wall.

Available for download on Thursday, August 18, 2022

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