103 research outputs found
Turbulence comes in bursts in stably stratified flows
There is a clear distinction between simple laminar and complex turbulent
fluids. But in some cases, as for the nocturnal planetary boundary layer, a
stable and well-ordered flow can develop intense and sporadic bursts of
turbulent activity which disappear slowly in time. This phenomenon is
ill-understood and poorly modeled; and yet, it is central to our understanding
of weather and climate dynamics. We present here a simple model which shows
that in stably stratified turbulence, the stronger bursts can occur when the
flow is expected to be more stable. The bursts are generated by a rapid
non-linear amplification of energy stored in waves, and are associated with
energetic interchanges between vertical velocity and temperature (or density)
fluctuations. Direct numerical simulations on grids of 2048^3 points confirm
this somewhat paradoxical result of measurably stronger events for more stable
flows, displayed not only in the temperature and vertical velocity derivatives,
but also in the amplitude of the fields themselves
Helicity dynamics in stratified turbulence in the absence of forcing
A numerical study of decaying stably-stratified flows is performed.
Relatively high stratification and moderate Reynolds numbers are considered,
and a particular emphasis is placed on the role of helicity (velocity-vorticity
correlations). The problem is tackled by integrating the Boussinesq equations
in a periodic cubical domain using different initial conditions: a non-helical
Taylor-Green (TG) flow, a fully helical Beltrami (ABC) flow, and random flows
with a tunable helicity. We show that for stratified ABC flows helicity
undergoes a substantially slower decay than for unstratified ABC flows. This
fact is likely associated to the combined effect of stratification and large
scale coherent structures. Indeed, when the latter are missing, as in random
flows, helicity is rapidly destroyed by the onset of gravitational waves. A
type of large-scale dissipative "cyclostrophic" balance can be invoked to
explain this behavior. When helicity survives in the system it strongly affects
the temporal energy decay and the energy distribution among Fourier modes. We
discover in fact that the decay rate of energy for stratified helical flows is
much slower than for stratified non-helical flows and can be considered with a
phenomenological model in a way similar to what is done for unstratified
rotating flows. We also show that helicity, when strong, has a measurable
effect on the Fourier spectra, in particular at scales larger than the buoyancy
scale for which it displays a rather flat scaling associated with vertical
shear
A New Measurement of the Temperature Density Relation of the IGM From Voigt Profile Fitting
We decompose the Lyman-{\alpha} (Ly{\alpha}) forest of an extensive sample of
74 high signal-to-noise ratio and high-resolution quasar spectra into a
collection of Voigt profiles. Absorbers located near caustics in the peculiar
velocity field have the smallest Doppler parameters, resulting in a low-
cutoff in the - set by the thermal state of intergalactic
medium (IGM). We fit this cutoff as a function of redshift over the range
, which allows us to measure the evolution of the IGM
temperature-density () relation parameters
and . We calibrate our measurements against Ly forest
simulations, using 21 different thermal models of the IGM at each redshift,
also allowing for different values of the IGM pressure smoothing scale. We
adopt a forward-modeling approach and self-consistently apply the same
algorithms to both data and simulations, propagating both statistical and
modeling uncertainties via Monte Carlo. The redshift evolution of shows a
suggestive peak at , while our evolution of is consistent with
and disfavors inverted temperature-density relations. Our
measured evolution of and are generally in good agreement with
previous determinations in the literature. Both the peak in the evolution of
at , as well as the high temperatures K
that we observe at , strongly suggest that a significant episode
of heating occurred after the end of HI reionization, which was most likely the
cosmic reionization of HeII.Comment: Accepted for publication in ApJ, 23 pages, 26 figures, machine
readable tables available onlin
A New Method to Directly Measure the Jeans Scale of the Intergalactic Medium Using Close Quasar Pairs
Although the baryons in the intergalactic medium (IGM) trace dark matter on
Mpc scales, small-scale (~100 kpc) fluctuations are suppressed by pressure
support, analogous to the classical Jeans argument. This Jeans filtering scale
has fundamental cosmological implications: it provides a thermal record of heat
injected by UV photons during reionization events, determines the clumpiness of
the IGM, and sets the minimum mass scale for gravitational collapse, a key
quantity in galaxy formation. Unfortunately, it is extremely challenging to
measure via the standard analysis of purely longitudinal Lyman-alpha forest
spectra, because the thermal Doppler broadening of absorption lines is highly
degenerate with Jeans pressure smoothing. In this work we show that the Jeans
scale can be directly measured by characterizing the coherence of correlated
Lyman-alpha absorption in quasar pairs with separations small enough to resolve
it. We present a novel technique for this purpose, based on the probability
distribution function (PDF) of phase angle differences of homologous
longitudinal Fourier modes in close quasar pair spectra. A Bayesian formalism
is introduced based on the phase angle PDF, and MCMC techniques are used to
characterize the precision of a future Jeans scale measurement, and explore
degeneracies with other thermal parameters governing the IGM. A semi-analytical
model of the IGM is used to generate a grid of 500 thermal models from a dark
matter simulation. Our full parameter study indicates that a realistic sample
of only 20 close quasar pair spectra can pinpoint the Jeans scale to ~ 5%
precision, independent of the parameters governing the temperature-density
relation of the IGM. We show that this new method is insensitive to a battery
of systematics such as continuum fitting errors, imprecise knowledge of the
noise and spectral resolution, and metal-line absorption.Comment: Submitted to Apj. 28 pages, 15 figure
Active nematic flows confined in a two dimensional channel with hybrid alignment at the walls: A unified picture
Active nematic fluids confined in narrow channels are known to generate spontaneous flows when the activity is sufficiently intense. Recently, it was demonstrated [R. Green, J. Toner, and V. Vitelli, Phys. Rev. Fluids 2, 104201 (2017)] that if the molecular anchoring at the channel walls is conflicting, i.e., perpendicular on one plate and parallel on the other, flows are initiated even in the zero activity limit. An analytical laminar velocity profile for this specific configuration was derived within a simplified nematohydrodynamic model in which the nematic order parameter is a fixed-magnitude unit vector n. The solution holds in a regime where the flow does not perturb the nematic order imposed by the walls. In this study, we explore systematically active flows in this confined geometry with a more general theoretical model that uses a second-rank tensor order parameter Q to express both the magnitude and orientation of the nematic phase. The Q-model allows for the presence of defects and biaxial, in addition to uniaxial, molecular arrangements. Our aim is to provide a unified picture, beyond the limiting regime explored previously, to serve as a guide for potential microfluidic applications that exploit the coupling between the orientational order of the molecules and the velocity field to finely control the flow and overcome the intrinsic difficulties of directing and pumping fluids at the microscale. We reveal how the nematic-flow coupling is not only dependent on geometrical constraints, but is also highly sensitive to material and flow parameters. We specifically stress the key role played by the activity and the flow aligning parameter, and we show that solutions mostly depend on two dimensionless parameters. We find that for large values of the activity parameter, the flow is suppressed for contractile particles while it is either sustained or suppressed for extensile particles depending on whether they tend to align or tumble when subject to shear. We explain these distinct behaviors by an argument based on the results of the stability analysis applied to two simpler configurations: active flows confined between parallel plates with either orthogonal or perpendicular alignment at both walls. We show that the analytical laminar solution derived for the n model in the low activity limit is found also in the Q model, both analytically and numerically. This result is valid for both contractile and extensile particles and for a flow-tumbling as well as aligning nematics. We remark that this velocity profile can be derived for generic boundary conditions. To stress the more general nature of the Q model, we conclude by providing a numerical example of a biaxial three-dimensional thresholdless active flow for which we show that biaxiality is especially relevant for a weakly first-order isotropic-nematic phase transition
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