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Subcritical Transition to Turbulence in Taylor-Couette FlowBorrero, Daniel 12 1900 (has links)
Turbulence is ubiquitous in naturally-occurring and man-made flows. Despite its importance in scientific and engineering applications, the transition from smooth laminar flow to disorganized turbulent flow is poorly understood. In some cases, the transition can be understood in the context of linear stability theory, which predicts when the underlying laminar solution will become unstable as a parameter is varied. For a large class of flows, however, this approach fails spectacularly, with theory predicting that the laminar flow is stable but experiments and simulations showing the emergence of spatiotemporal complexity. In this dissertation, the direct or subcritical transition to turbulence in Taylor-Couette flow (i.e., the flow between independently rotating co-axial cylinders) is studied experimentally. Chapter 1 discusses different scenarios for the transition to turbulence and recent advances in understanding the subcritical transition within the framework of dynamical systems theory. Chapter 2 presents a comprehensive review of earlier investigations of linearly stable Taylor-Couette flow. Chapter 3 presents the first systematic study of long-lived super-transients in Taylor-Couette flow with the aim of determining the correct dynamical model for turbulent dynamics in the transitional regime. Chapter 4 presents the results of experiments regarding the stability of Taylor-Couette flow to finite-amplitude perturbations in the form of injection/suction of fluid from the test section. Chapter 5 presents numerical investigations of axisymmetric laminar states with realistic boundary conditions. Chapter 6 discusses in detail the implementation of time-resolved tomographic particle image
velocimetry (PIV) in the Taylor-Couette geometry and presents preliminary tomographic PIV measurements of the growth of turbulent spots from finite-amplitude perturbations. The main results are summarized in Chapter 7.
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Measurement of three-dimensional coherent fluid structure in high Reynolds number turbulent boundary layersClark, Thomas Henry January 2012 (has links)
The turbulent boundary layer is an aspect of fluid flow which dominates the performance of many engineering systems - yet the analytic solution of such flows is intractable for most applications. Our understanding of boundary layers is therefore limited by our ability to simulate and measure them. Tomographic Particle Image Velocimetry (TPIV) is a recently developed technique for direct measurement of fluid velocity within a 3D region. This allows new insight into the topological structure of turbulent boundary layers. Increasing Reynolds Number increases the range of scales at which turbulence exists; a measurement technique must have a larger 'dynamic range' to fully resolve the flow. Tomographic PIV is currently limited in spatial dynamic range (which is also linked to the spatial and temporal resolution) due to a high degree of noise. Results also contain significant bias error. This work proposes a modification of the technique to use more than two exposures in the PIV process, which (for four exposures) is shown to improve random error by a factor of 2 to 7 depending on experimental setup parameters. The dynamic range increases correspondingly and can be doubled again in highly turbulent flows. Bias error is reduced by up to 40%. An alternative reconstruction approach is also presented, based on application of a reduction strategy (elimination of coefficients based on a first guess) to the tomographic weightings matrix Wij. This facilitates a potentially significant increase in computational efficiency. Despite the achieved reduction in error, measurements contain non-zero divergence due to noise and sampling errors. The same problem affects visualisation of topology and coherent fluid structures. Using Projection Onto Convex Sets, a framework for post-processing operators is implemented which includes a divergence minimisation procedure and a scale-limited denoising strategy which is resilient to 'false' vectors contained in the data. Finally, developed techniques are showcased by visualisation of topological information in the inner region of a high Reynolds Number boundary layer (δ+ = 1890, Reθ = 3650). Comments are made on the visible flow structures and tentative conclusions are drawn.
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Spark induced flow in quiescent airBhavini Singh (10586768) 07 May 2021 (has links)
<p>Nanosecond spark plasma actuators provide an opportunity to
reduce pollutants by promoting efficient combustion in engines or provide
targeted, tunable, flow control over vehicles, due to their ability to
influence flow and combustion through multiple mechanisms. The plasma actuators
can be physically unobtrusive, can be turned on and off and their low duty
cycle, large bandwidth, and light weight make them more appealing than other
control approaches. One method by which these plasma actuators interact with
the environment is by inducing a complex local flow field and in order, to
design scalable, high frequency actuators effectively, it is necessary to first
understand the flow induced by a single spark discharge. Most experimental
analysis on the flow induced by spark discharges has been restricted to qualitative
descriptions of the flow field, primarily due to the difficulties associated
with measuring such a transient and highly complex flow with sufficient
spatiotemporal resolution. Quantitative, experimental characterization of the
flow induced by a spark discharge remains lacking. </p><p> </p><p>A spark discharge produces a shock wave and a hot gas kernel
with a complex flow field following the shock. In this work, combined experimental
and theoretical characterization of the spark induced flow is performed through
a series of high spatiotemporal resolution measurements of the density and velocity
fields and reduced-order modeling. The work investigates the mechanisms driving
the cooling and vorticity generation in spark induced flow and the 3D nature of
the flow field. Planar (2D-3C) and volumetric (3D-3C) velocity measurements are
taken using stereoscopic particle image velocimetry (SPIV) and tomographic PIV,
respectively. Density measurements are taken using background oriented
schlieren (BOS) and high speed schlieren imaging is used to capture the shock
wave induced by the spark.</p><p> </p><p>The work shows that spark plasma discharges induce vortex
rings whose vorticity is likely generated due to baroclinic torque arising from
the non-uniform strength of the induced shock wave. The hot gas kernel cools in
two stages: an initially fast cooling regime, followed by a slower cooling
process. Reduced order analytical models are developed to describe the cooling
observed in the fast regime and the role of the vortex rings in the entrainment
of cold ambient gas and the cooling of the hot gas kernel. The results show
that the vortex rings entrain ambient gas and drive cooling in the fast, convective
regime, cooling approximately 50% of the hot gas within the first millisecond
of the induced flow. An increase in the electrical energy deposited in the
spark gap increases the shock strength and curvature and increases the vortex
ring strength, thereby increasing the cooling rate and expansion of the hot gas
kernel. The volumetric velocity measurements capture one of the two induced vortex
rings and provide a framework for the improvements needed in future tomographic
PIV experiments of the spark induced flow field, necessary in assessing the 3D
nature of the induced vortex rings.</p><p> </p><p>
The results of this work provide the first set of
quantitative, experimental data on flow induced by nanosecond spark discharges
that can be used for validation of computational fluid dynamics (CFD) simulations.
The results demonstrate that spark plasmas induce vortex ring-driven mixing
flows and the results on mixing and cooling of the hot gas kernel can be
extended to any passive scalars present in the flow field as well as inform
pulsation frequencies and actuator designs for flow and combustion control. The
results from the reduced order modeling can inform future studies and
applications of nanosecond spark discharges and can be extended to a variety of
other types of plasma discharges like laser sparks, long duration sparks and
surface discharges with similar induced flow fields.<br></p>
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