1,721,001 research outputs found
The planar X-junction flow: stability analysis and control
Full text linkThe bifurcations and control of the flow in a planar X-junction are studied via linear stability analysis and direct numerical simulations. This study reveals the instability mechanisms in a symmetric channel junction and shows how these can be stabilized or destabilized by boundary modification. We observe two bifurcations as the Reynolds number increases. They both scale with the inlet speed of the two side channels and are almost independent of the inlet speed of the main channel. Equivalently, both bifurcations appear when the recirculation zones reach a critical length. A two-dimensional stationary global mode becomes unstable first, changing the flow from a steady symmetric state to a steady asymmetric state via a pitchfork bifurcation. The core of this instability, whether defined by the structural sensitivity or by the disturbance energy production, is at the edges of the recirculation bubbles, which are located symmetrically along the walls of the downstream channel. The energy analysis shows that the first bifurcation is due to a lift-up mechanism. We develop an adjustable control strategy for the first bifurcation with distributed suction or blowing at the walls. The linearly optimal wall-normal velocity distribution is computed through a sensitivity analysis and is shown to delay the first bifurcation from Re = 82.5 to Re = 150. This stabilizing effect arises because blowing at the walls weakens the wall-normal gradient of the streamwise velocity around the recirculation zone and hinders the lift-up. At the second bifurcation, a three-dimensional stationary global mode with a spanwise wavenumber of order unity becomes unstable around the asymmetric steady state. Nonlinear three-dimensional simulations at the second bifurcation display transition to a nonlinear cycle involving growth of a three-dimensional steady structure, time-periodic secondary instability and nonlinear breakdown restoring a two-dimensional flow. Finally, we show that the sensitivity to wall suction at the second bifurcation is as large as it is at the first bifurcation, providing a possible mechanism for destabilization
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Forcing of globally unstable jets and flames
Full text linkIn the analysis of thermoacoustic systems, a flame is usually characterised
by the way its heat release responds to acoustic forcing. This
response depends on the hydrodynamic stability of the flame. Some
flames, such as a premixed bunsen flame, are hydrodynamically globally
stable. They respond only at the forcing frequency. Other flames,
such as a jet diffusion flame, are hydrodynamically globally unstable.
They oscillate at their own natural frequencies and are often assumed
to be insensitive to low-amplitude forcing at other frequencies.
If a hydrodynamically globally unstable flame really is insensitive to
forcing at other frequencies, then it should be possible to weaken
thermoacoustic oscillations by detuning the frequency of the natural
hydrodynamic mode from that of the natural acoustic modes. This
would be very beneficial for industrial combustors.
In this thesis, that assumption of insensitivity to forcing is tested
experimentally. This is done by acoustically forcing two different selfexcited
flows: a non-reacting jet and a reacting jet. Both jets have
regions of absolute instability at their base and this causes them to
exhibit varicose oscillations at discrete natural frequencies. The forcing
is applied around these frequencies, at varying amplitudes, and
the response examined over a range of frequencies (not just at the
forcing frequency). The overall system is then modelled as a forced
van der Pol oscillator.
The results show that, contrary to some expectations, a hydrodynamically
self-excited jet oscillating at one frequency is sensitive to
forcing at other frequencies. When forced at low amplitudes, the jet
responds at both frequencies as well as at several nearby frequencies,
and there is beating, indicating quasiperiodicity. When forced at
high amplitudes, however, it locks into the forcing. The critical forcing
amplitude required for lock-in increases with the deviation of the
forcing frequency from the natural frequency. This increase is linear,
indicating a Hopf bifurcation to a global mode.
The lock-in curve has a characteristic ∨ shape, but with two subtle
asymmetries about the natural frequency. The first asymmetry concerns
the forcing amplitude required for lock-in. In the non-reacting
jet, higher amplitudes are required when the forcing frequency is above
the natural frequency. In the reacting jet, lower amplitudes are required
when the forcing frequency is above the natural frequency. The
second asymmetry concerns the broadband response at lock-in. In the
non-reacting jet, this response is always weaker than the unforced response,
regardless of whether the forcing frequency is above or below
the natural frequency. In the reacting jet, that response is weaker
than the unforced response when the forcing frequency is above the
natural frequency, but is stronger than it when the forcing frequency
is below the natural frequency.
In the reacting jet, weakening the global instability – by adding coflow
or by diluting the fuel mixture – causes the flame to lock in at lower
forcing amplitudes. This finding, however, cannot be detected in the
flame describing function. That is because the flame describing function
captures the response at only the forcing frequency and ignores all
other frequencies, most notably those arising from the natural mode
and from its interactions with the forcing. Nevertheless, the flame describing
function does show a rise in gain below the natural frequency
and a drop above it, consistent with the broadband response.
Many of these features can be predicted by the forced van der Pol
oscillator. They include (i) the coexistence of the natural and forcing
frequencies before lock-in; (ii) the presence of multiple spectral peaks
around these competing frequencies, indicating quasiperiodicity; (iii)
the occurrence of lock-in above a critical forcing amplitude; (iv) the
∨-shaped lock-in curve; and (v) the reduced broadband response at
lock-in. There are, however, some features that cannot be predicted.
They include (i) the asymmetry of the forcing amplitude required
for lock-in, found in both jets; (ii) the asymmetry of the response at
lock-in, found in the reacting jet; and (iii) the interactions between
the fundamental and harmonics of both the natural and forcing frequencies,
found in both jets
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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Global stability and control of swirling jets and flames
Full text linkLarge-scale unsteady flow structures play an influential role in the dynamics of many practical flows, such as those found in gas turbine combustion chambers. This thesis is concerned primarily with large-scale unsteady structures that arise due to self-sustained hydrodynamic oscillations, also known as global hydrodynamic instability. Direct numerical simulation (DNS) of the Navier--Stokes equations in the low Mach number limit is used to obtain a steady base flow, and the most unstable direct and adjoint global modes. These are combined, using a structural sensitivity framework, to identify the region of the flow and the feedback mechanisms that are responsible for causing the global instability. Using a Lagrangian framework, the direct and adjoint global modes are also used to identify the regions of the flow where steady and unsteady control, such as a drag force or heat input, can suppress or promote the global instability. These tools are used to study a variety of reacting and non-reacting flows to build an understanding of the physical mechanisms that are responsible for global hydrodynamic instability in swirling diffusion flames. In a non-swirling lifted jet diffusion flame, two modes of global instability are found. The first mode is a high-frequency mode caused by the instability of the low-density jet shear layer in the premixing zone. The second mode is a low-frequency mode caused by an instability of the outer shear layer of the flame. Two types of swirling diffusion flames with vortex breakdown bubbles are considered. They show qualitatively similar behaviour to the lifted jet diffusion flames. The first type of flame is unstable to a low-frequency mode, with wavemaker located at the flame base. The second type of flame is unstable to a high-frequency mode, with wavemaker located at the upstream edge of the vortex breakdown bubble. Feedback from density perturbations is found to have a strong influence on the unstable modes in the reacting flows. The wavemaker of the high-frequency mode in the reacting flows is very similar to its non-reacting counterpart. The low-frequency mode, however, is only observed in the reacting flows. The presence of reaction increases the influence of changes in the base flow mixture fraction profiles on the eigenmode. This increased influence acts through the heat release term. These results emphasize the possibility that non-reacting simulations and experiments may not always capture the important instability mechanisms of reacting flows, and highlight the importance of including heat release terms in stability analyses of reacting flows
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Proceedings of the NFFDy Summer Programme on ‘Data in Fluids’
Full text linkThis report collates the results of eight scoping studies carried out at the first NFFDy Summer Programme, with the theme ‘Data in Fluids’, held at the Department of Engineering in Cambridge from 10 July to 18 August 2023. The 11 NFFDy fellows and 15 other early career researchers who took part in the programme will develop their findings further with a view to publishing journal papers from them
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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Sensitivity analysis of low-density jets and flames
Full text linkThis work represents the initial steps in a wider project that aims to map out the
sensitive areas in fuel injectors and combustion chambers. Direct numerical simulation
(DNS) using a Low-Mach-number formulation of the Navier–Stokes equations is used to
calculate direct-linear and adjoint global modes for axisymmetric low-density jets and
lifted jet diffusion flames. The adjoint global modes provide a map of the most sensitive
locations to open-loop external forcing and heating. For the jet flows considered here,
the most sensitive region is at the inlet of the domain.
The sensitivity of the global-mode eigenvalues to force feedback and to heat and
drag from a hot-wire is found using a general structural sensitivity framework. Force
feedback can occur from a sensor-actuator in the flow or as a mechanism that drives
global instability. For the lifted flames, the most sensitive areas lie between the inlet
and flame base. In this region the jet is absolutely unstable, but the close proximity
of the flame suppresses the global instability seen in the non-reacting case. The lifted
flame is therefore particularly sensitive to outside disturbances in the non-reacting zone.
The DNS results are compared to a local analysis. The most absolutely unstable region
for all the flows considered is at the inlet, with the wavemaker slightly downstream
of the inlet. For lifted flames, the region of largest sensitivity to force feedback is near
to the location of the wavemaker, but for the non-reacting jet this region is downstream
of the wavemaker and outside of the pocket of absolute instability near the inlet.
Analysing the sensitivity of reacting and non-reacting variable-density shear flows
using the low-Mach-number approximation has up until now not been done. By including
reaction, a large forward step has been taken in applying these techniques to real
fuel injectors
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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