1,657 research outputs found
Coherent pressure structures in turbulent channel flow
Most of the studies on pressure fluctuations in wall-bounded turbulent flows
aim at obtaining statistics as power spectra and scaling laws, especially at
the walls. In the present study we study energetic coherent pressure structures
of turbulent channel flows, aiming at a characterization of dominant coherent
structures throughout the channel. Coherent structures are detected using
spectral proper orthogonal decomposition (SPOD) and modeled using resolvent
analysis, similar to related works dealing with velocity fluctuations, but
using pressure fluctuations as the output of interest. The resolvent operator
was considered with and without the Cess eddy viscosity model. Direct numerical
simulations (DNSs) of incompressible turbulent channel flows at friction
Reynolds numbers of approximately 180 and 550 were employed as databases. Three
representative dominant structures emerged from a preliminary spectral
analysis: near-wall, large-scale and spanwise-coherent structures. For
frequency-wavenumber combinations corresponding to these three representative
structures, SPOD results show a strong dominance of the leading mode,
highlighting low-rank behavior of pressure fluctuations. The leading resolvent
mode closely agrees with the first SPOD mode, providing support to studies that
showed better performance of resolvent-based estimators when predicting
pressure fluctuations compared to velocity fluctuations. The dominant
mechanisms of the analyzed modes are seen to be the generation of
quasi-streamwise vortices with pressure fluctuations appearing close to vortex
centers. A study on the individual contributions of the nonlinear terms
(treated as forcing in resolvent analysis) to the pressure output reveals that
each forcing component plays a constructive role to the input-output
formulation, which also helps understanding the weaker role of forcing color in
driving pressure fluctuations.Comment: 24 pages, 23 figure
Reduced-order Galerkin models of plane Couette flow
Reduced-order models were derived for plane Couette flow using Galerkin
projection, with orthonormal basis functions taken as the leading
controllability modes of the linearised Navier-Stokes system for a few low
wavenumbers. Resulting Galerkin systems comprise ordinary differential
equations, with a number of degrees of freedom ranging from 144 to 600, which
may be integrated to large times without sign of numerical instability. The
reduced-order models so obtained are also found to match statistics of direct
numerical simulations at Reynolds number 500 and 1200 with reasonable accuracy,
despite a truncation of orders of magnitude in the degrees of freedom of the
system. The present models offer thus an interesting compromise between
simplicity and accuracy in a canonical wall-bounded flow, with relatively few
modes representing coherent structures in the flow and their dominant dynamics.Comment: 10 pages, 4 figure
On the Low-Frequency Dynamics of Turbulent Separation Bubbles
The low-frequency modal and non-modal stability characteristics of an
incompressible, pressure-gradient-induced turbulent separation bubble (TSB) are
investigated with the objective of studying the mechanism responsible for the
low-frequency contraction and expansion (breathing) commonly observed in
experimental studies. The configuration of interest is a TSB generated on a
flat test surface by a succession of adverse and favourable pressure gradients.
The base flow selected for the analysis is the average TSB from the direct
numerical simulation of Coleman et al. (J. Fluid Mech., vol. 847, 2018). Global
linear stability analysis reveals that the flow is globally stable for
wavenumbers. The mode closest to the stability threshold appears to occur at
zero frequency and low, non-zero spanwise wavenumber. Resolvent analysis is
then employed to examine the forced dynamics of the flow. At low frequency, a
region of low, non-zero spanwise wavenumber is also discernible, where the
receptivity appears to be driven by the identified weakly damped global mode.
The results from resolvent analysis are compared to the unsteady experimental
database of Le Floc'h et al. (J. Fluid Mech., vol. 902, 2020) in a similar TSB
flow. The alignment between the optimal response and the first spectral proper
orthogonal decomposition mode computed from the experiments is shown to exceed
95 %, while the spanwise wavenumber of the optimal response is consistent with
that of the low-frequency breathing motion captured experimentally. This
indicates that the fluctuations observed experimentally at low frequency
closely match the response computed from resolvent analysis. Based on these
results, we propose that the forced dynamics of the flow, driven by the weakly
damped global mode, serve as a plausible mechanism for the origin of the
low-frequency breathing motion commonly observed in experimental studies of
TSBs
Lift-up, Kelvin-Helmholtz and Orr mechanisms in turbulent jets
Three amplification mechanisms present in turbulent jets, namely lift-up, Kelvin–Helmholtz and Orr, are characterized via global resolvent analysis and spectral proper orthogonal decomposition (SPOD) over a range of Mach numbers. The lift-up mechanism was recently identified in turbulent jets via local analysis by Nogueira et al. (J. Fluid Mech., vol. 873, 2019, pp. 211–237) at low Strouhal number ( St ) and non-zero azimuthal wavenumbers ( m ). In these limits, a global SPOD analysis of data from high-fidelity simulations reveals streamwise vortices and streaks similar to those found in turbulent wall-bounded flows. These structures are in qualitative agreement with the global resolvent analysis, which shows that they are a response to upstream forcing of streamwise vorticity near the nozzle exit. Analysis of mode shapes, component-wise amplitudes and sensitivity analysis distinguishes the three mechanisms and the regions of frequency–wavenumber space where each dominates, finding lift-up to be dominant as St/m→0 . Finally, SPOD and resolvent analyses of localized regions show that the lift-up mechanism is present throughout the jet, with a dominant azimuthal wavenumber inversely proportional to streamwise distance from the nozzle, with streaks of azimuthal wavenumber exceeding five near the nozzle, and wavenumbers one and two most energetic far downstream of the potential core
Transition to chaos in a reduced-order model of a shear layer
The present work studies the non-linear dynamics of a shear layer, driven by
a body force and confined between parallel walls, a simplified setting to study
transitional and turbulent shear layers. It was introduced by Nogueira \&
Cavalieri (J. Fluid Mech. 907, A32, 2021), and is here studied using a
reduced-order model based on a Galerkin projection of the Navier-Stokes system.
By considering a confined shear layer with free-slip boundary conditions on the
walls, periodic boundary conditions in streamwise and spanwise directions may
be used, simplifying the system and enabling the use of methods of dynamical
systems theory. A basis of eight modes is used in the Galerkin projection,
representing the mean flow, Kelvin-Helmholtz vortices, rolls, streaks and
oblique waves, structures observed in the cited work, and also present in shear
layers and jets. A dynamical system is obtained, and its transition to chaos is
studied. Increasing Reynolds number leads to pitchfork and Hopf
bifurcations, and the latter leads to a limit cycle with amplitude modulation
of vortices, as in the DNS by Nogueira \& Cavalieri. Further increase of
leads to the appearance of a chaotic saddle, followed by the emergence of
quasi-periodic and chaotic attractors. The chaotic attractors suffer a merging
crisis for higher , leading to chaotic dynamics with amplitude modulation
and phase jumps of vortices. This is reminiscent of observations of coherent
structures in turbulent jets, suggesting that the model represents dynamics
consistent with features of shear layers and jets.Comment: 28 pages, 18 figure
Evaluation of PSE as a Model for Supersonic Jet Using Transfer Functions
Parabolized Stability Equations (PSE) have been shown to model wavepackets and, consequently, the near field of turbulent jets with reasonable accuracy. Because of these capabilities, PSE is a promising reduced-order model to derive control laws that could be employed to reduce the sound generation of a jet. The purpose of this work is to apply PSE to obtain time-domain transfer functions that could estimate both the fluid-dynamic and the acoustic fields of a supersonic jet. The results of this model were compared to results obtained from a database of a well-validated large-eddy simulation of a supersonic jet.
Based on the unsteady pressure data at a input position, the time-domain pressure field was estimated using transfer functions obtained using PSE and an empirical method based on the LES data. The prediction scheme employed is a single-input-single-output (SISO), linear model. The unsteady pressure predicted by PSE showed good agreement with the LES results, especially if the input position is outside the mixing layer. For this region, the prediction capabilities of PSE are comparable to those of empirical transfer functions. The agreement is good even for output points taken in the acoustic field, showing that it is possible to estimate the time-domain behaviour of Mach-wave radiation using transfer functions.
This indicates that PSE could not only be used to predict the sound generation, but also to open up new potentialities to attenuate noise by means of closed-loop control of the flow. The exploration of the regions where the method displayed good agreement, presented in this work, can guide the positioning of sensors and actuators for experimental implementation of closed-loop control in a jet
An update on the human and animal enteric pathogen Clostridium perfringens
Clostridium perfringens, a rapid-growing pathogen known to secrete an arsenal of >20 virulent toxins, has been associated with intestinal diseases in both animals and humans throughout the past century. Recent advances in genomic analysis and experimental systems make it timely to re-visit this clinically and veterinary important pathogen. This Review will summarise our understanding of the genomics and virulence-linked factors, including antimicrobial potentials and secreted toxins of this gut pathogen, and then its up-to-date clinical epidemiology and biological role in the pathogenesis of several important human and animal-associated intestinal diseases, including pre-term necrotising enterocolitis. Finally, we highlight some of the important unresolved questions in relation to C. perfringens-mediated infections, and implications for future research directions
Dessins, their delta-matroids and partial duals
Given a map on a connected and closed orientable surface, the
delta-matroid of is a combinatorial object associated to which captures some topological information of the embedding. We explore how
delta-matroids associated to dessins d'enfants behave under the action of the
absolute Galois group. Twists of delta-matroids are considered as well; they
correspond to the recently introduced operation of partial duality of maps.
Furthermore, we prove that every map has a partial dual defined over its field
of moduli. A relationship between dessins, partial duals and tropical curves
arising from the cartography groups of dessins is observed as well.Comment: 34 pages, 20 figures. Accepted for publication in the SIGMAP14
Conference Proceeding
- …