11 research outputs found
Mixing across stable density interfaces in forced stratified turbulence
Understanding how turbulence enhances irreversible scalar mixing in
density-stratified fluids is a central problem in geophysical fluid dynamics.
While isotropic overturning regions are commonly the focus of mixing analyses,
we here investigate whether significant mixing may arise in anisotropic
statically-stable regions of the flow. Focusing on a single forced direct
numerical simulation of stratified turbulence, we analyze spatial correlations
between the vertical density gradient and the
dissipation rates of kinetic energy and scalar variance , the
latter quantifying scalar mixing. The domain is characterized by relatively
well-mixed density layers separated by sharp stable interfaces that are
correlated with high vertical shear. While static instability is most prevalent
within the mixed layers, much of the scalar mixing is localized to the
intervening interfaces, a phenomenon not apparent if considering local static
instability or alone. While the majority of the domain is
characterized by the canonical flux coefficient
, often assumed in ocean mixing
parameterizations, extreme values of within the statically-stable
interfaces, associated with elevated , strongly skew the bulk
statistics. Our findings suggest that current parameterizations of turbulent
mixing may be biased by undersampling, such that the most common, but not
necessarily the most significant, mixing events are overweighted. Having
focused here on a single simulation of stratified turbulence, it is hoped that
our results motivate a broader investigation into the role played by stable
density interfaces in mixing, across a wider range of parameters and forcing
schemes representative of ocean turbulence.Comment: 17 pages, 7 figures. Version accepted for publication in the Journal
of Fluid Mechanics. DOI link to final typeset version provide
Inertial enhancement of the polymer diffusive instability
Beneitez et al. (2023b) have recently discovered a new linear "polymer
diffusive instability" (PDI) in inertialess viscoelastic rectilinear shear flow
of a FENE-P fluid with polymer stress diffusion. Here, we examine the impact of
inertia on the PDI, which we delineate for both plane Couette and channel
configurations under varying Weissenberg number , polymer stress diffusivity
, solvent-to-total viscosity and Reynolds number ,
considering Oldroyd-B and FENE-P constitutive relations. Both the prevalence of
the instability in parameter space and the associated growth rates are found to
significantly increase with . For instance, as increases with
fixed, the instability emerges at progressively lower values of and
than in the inertialess limit, and the associated growth rates
increase linearly with when all other parameters are fixed. This
strengthening of PDI with inertia and the fact that stress diffusion is always
present in time-stepping algorithms, either implicitly as part of the scheme or
explicitly as a stabiliser, implies that the instability is likely operative in
computational work using the popular Oldroyd-B and FENE-P constitutive models.
The fundamental question now is whether PDI is physical and observable in
experiments, or is instead an artifact of the constitutive models that must be
suppressed.Comment: 10 pages, 3 figure
Prandtl number effects on extreme mixing events in forced stratified turbulence
Relatively strongly stratified turbulent flows tend to self-organise into a
'layered anisotropic stratified turbulence' (LAST) regime, characterised by
relatively deep and well-mixed density 'layers' separated by relatively thin
'interfaces' of enhanced density gradient. Understanding the associated mixing
dynamics is a central problem in geophysical fluid dynamics. It is challenging
to study 'LAST' mixing, as it is associated with Reynolds numbers and Froude numbers , ( and being
characteristic velocity and length scales, being the kinematic viscosity
and the buoyancy frequency). Since a sufficiently large dynamic range
(largely) unaffected by stratification and viscosity is required, it is also
necessary for the buoyancy Reynolds number where is the (appropriately volume-averaged) turbulent kinetic
energy dissipation rate. This requirement is exacerbated for oceanically
relevant flows, as the Prandtl number in
thermally-stratified water (where is the thermal diffusivity), thus
leading (potentially) to even finer density field structures. We report here on
four forced fully resolved direct numerical simulations of stratified
turbulence at various Froude () and Prandtl numbers ()
forced so that , with resolutions up to . We find that, as increases, emergent 'interfaces' become finer and
their contribution to bulk mixing characteristics decreases at the expense of
the small-scale density structures populating the well-mixed 'layers'. However,
extreme mixing events (as quantified by significantly elevated local
destruction rates of buoyancy variance ) are always preferentially
found in the (statically stable) interfaces, irrespective of the value of .Comment: 10 pages, 4 figure
Innovation and HRM : absences and politics
This article analyses the role of HRM practices in the implementation of an innovative cross-functional approach to new product development (concurrent engineering, CE) in Eurotech Industries. Contrary to CE methodology stipulations, and despite supportive conditions, HRM received scant attention in the implementation process. Organizational power and politics were clearly involved in this situation, and this article explores how their play created such HRM ‘absences’. The article builds on a four-dimensional view of power in order to provide a deeper understanding of the embedded, interdependent and political nature of HRM practice and innovation.<br /
Collective vibrations of confined levitating droplets
We report a new type of fluid-based driven dissipative oscillator system
consisting of a lattice of millimetric fluid droplets bouncing on a vertically
vibrating liquid bath and bound within an annular ring. We characterize the
system behavior as it is energized through a progressive increase in the bath's
vibrational acceleration. Depending on the number of drops, the onset of motion
of the lattice may take the form of either out-of-phase oscillations or a
striking solitary wave-like instability. Theoretical modeling demonstrates that
these behaviors may be attributed to different bifurcations at the onset of
instability. The results presented here demonstrate the potential and utility
of the walking droplet system as a platform for investigating wave-mediated,
inertial, non-equilibrium particle dynamics at the macroscale.Comment: 15 pages (incl. references) and 4 figure
The Stability of a Hydrodynamic Bravais Lattice
We present the results of a theoretical investigation of the stability and collective vibrations of a two-dimensional hydrodynamic lattice comprised of millimetric droplets bouncing on the surface of a vibrating liquid bath. We derive the linearized equations of motion describing the dynamics of a generic Bravais lattice, as encompasses all possible tilings of parallelograms in an infinite plane-filling array. Focusing on square and triangular lattice geometries, we demonstrate that for relatively low driving accelerations of the bath, only a subset of inter-drop spacings exist for which stable lattices may be achieved. The range of stable spacings is prescribed by the structure of the underlying wavefield. As the driving acceleration is increased progressively, the initially stationary lattices destabilize into coherent oscillatory motion. Our analysis yields both the instability threshold and the wavevector and polarization of the most unstable vibrational mode. The non-Markovian nature of the droplet dynamics renders the stability analysis of the hydrodynamic lattice more rich and subtle than that of its solid state counterpart
Multidisciplinary community mental health team staff's experience of a ‘skills level’ training course in cognitive analytic therapy
This study sought to explore community mental health teams' (CMHTs) experiences of receiving an innovative introductory level training in cognitive analytic therapy (CAT). CMHTs are important providers of care for people with mental health problems. Although CMHTs have many strengths, they have been widely criticized for failing to have a shared model underlying practice. Inter-professional training which develops shared therapeutic models from which to plan care delivery is, therefore, essential. We have been developing such a training based on the psychotherapeutic principles of CAT. Twelve community mental health staff (six mental health social workers and six community psychiatric nurses) were interviewed by an independent interviewer following the completion of the training programme. The interviews were analysed using a qualitative thematic analysis. The analysis revealed that the programme increased the participants' self-assessed therapeutic confidence and skill and fostered the development of a shared model within the team, although the training was also perceived as adding to workload. The results of this study suggest that whole-team CAT training may facilitate cohesion and also suggest that having some shared common language is important in enabling and supporting work with ‘difficult’ and ‘complex’ clients, for example, those with personality disorders. Further development of such training accompanied by rigorous evaluation should be undertaken