28,714 research outputs found
Meso-scale turbulence in living fluids
Turbulence is ubiquitous, from oceanic currents to small-scale biological and
quantum systems. Self-sustained turbulent motion in microbial suspensions
presents an intriguing example of collective dynamical behavior amongst the
simplest forms of life, and is important for fluid mixing and molecular
transport on the microscale. The mathematical characterization of turbulence
phenomena in active non-equilibrium fluids proves even more difficult than for
conventional liquids or gases. It is not known which features of turbulent
phases in living matter are universal or system-specific, or which
generalizations of the Navier-Stokes equations are able to describe them
adequately. Here, we combine experiments, particle simulations, and continuum
theory to identify the statistical properties of self-sustained meso-scale
turbulence in active systems. To study how dimensionality and boundary
conditions affect collective bacterial dynamics, we measured energy spectra and
structure functions in dense Bacillus subtilis suspensions in quasi-2D and 3D
geometries. Our experimental results for the bacterial flow statistics agree
well with predictions from a minimal model for self-propelled rods, suggesting
that at high concentrations the collective motion of the bacteria is dominated
by short-range interactions. To provide a basis for future theoretical studies,
we propose a minimal continuum model for incompressible bacterial flow. A
detailed numerical analysis of the 2D case shows that this theory can reproduce
many of the experimentally observed features of self-sustained active
turbulence.Comment: accepted PNAS version, 6 pages, click doi for Supplementary
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Modelling of Diesel fuel properties through its surrogates using Perturbed-Chain, Statistical Associating Fluid Theory
The Perturbed-Chain, Statistical Associating Fluid Theory equation of state is utilised to model the effect of pressure and temperature on the density, volatility and viscosity of four Diesel surrogates; these calculated properties are then compared to the properties of several Diesel fuels. Perturbed-Chain, Statistical Associating Fluid Theory calculations are performed using different sources for the pure component parameters. One source utilises literature values obtained from fitting vapour pressure and saturated liquid density data or from correlations based on these parameters. The second source utilises a group contribution method based on the chemical structure of each compound. Both modelling methods deliver similar estimations for surrogate density and volatility that are in close agreement with experimental results obtained at ambient pressure. Surrogate viscosity is calculated using the entropy scaling model with a new mixing rule for calculating mixture model parameters. The closest match of the surrogates to Diesel fuel properties provides mean deviations of 1.7% in density, 2.9% in volatility and 8.3% in viscosity. The Perturbed-Chain, Statistical Associating Fluid Theory results are compared to calculations using the Peng–Robinson equation of state; the greater performance of the Perturbed-Chain, Statistical Associating Fluid Theory approach for calculating fluid properties is demonstrated. Finally, an eight-component surrogate, with properties at high pressure and temperature predicted with the group contribution Perturbed-Chain, Statistical Associating Fluid Theory method, yields the best match for Diesel properties with a combined mean absolute deviation of 7.1% from experimental data found in the literature for conditions up to 373°K and 500 MPa. These results demonstrate the predictive capability of a state-of-the-art equation of state for Diesel fuels at extreme engine operating conditions
Heavy quark energy loss far from equilibrium in a strongly coupled collision
We compute and study the drag force acting on a heavy quark propagating
through the matter produced in the collision of two sheets of energy in a
strongly coupled gauge theory that can be analyzed holographically. Although
this matter is initially far from equilibrium, we find that the equilibrium
expression for heavy quark energy loss in a homogeneous strongly coupled plasma
with the same instantaneous energy density or pressure as that at the location
of the quark describes many qualitative features of our results. One
interesting exception is that there is a time delay after the initial collision
before the heavy quark energy loss becomes significant. At later times, once a
liquid plasma described by viscous hydrodynamics has formed, expressions based
upon assuming instantaneous homogeneity and equilibrium provide a
semi-quantitative description of our results - as long as the rapidity of the
heavy quark is not too large. For a heavy quark with large rapidity, the
gradients in the velocity of the hydrodynamic fluid result in qualitative
consequences for the 'drag' force acting on the quark. In certain
circumstances, the force required to drag the quark through the plasma can
point opposite to the velocity of the quark, meaning that the force that the
plasma exerts on a quark moving through it acts in the same direction as its
velocity. And, generically, the force includes a component perpendicular to the
direction of motion of the quark. Our results support a straightforward
approach to modeling the drag on, and energy loss of, heavy quarks with modest
rapidity in heavy ion collisions, both before and after the quark-gluon plasma
hydrodynamizes, and provide cautionary lessons at higher rapidity.Comment: 22 pages, 20 figure
Static and dynamic aeroelastic characterization of an aerodynamically heated generic hypersonic aircraft configuration
This work-in-progress presentation describes an ongoing research activity at the NASA Langley Research Center to develop analytical methods for the prediction of aerothermoelastic stability of hypersonic aircraft including active control systems. The objectives of this research include application of aerothermal loads to the structural finite element model, determination of the thermal effects on flutter, and assessment of active controls technology applied to overcome any potential adverse aeroelastic stability or response problems due to aerodynamic heating- namely flutter suppression and ride quality improvement. For this study, a generic hypersonic aircraft configuration was selected which incorporates wing flaps, ailerons and all-moveable fins to be used for active control purposes. The active control systems would use onboard sensors in a feedback loop through the aircraft flight control computers to move the surfaces for improved structural dynamic response as the aircraft encounters atmospheric turbulence
Rheological constitutive equation for model of soft glassy materials
We solve exactly and describe in detail a simplified scalar model for the low
frequency shear rheology of foams, emulsions, slurries, etc. [P. Sollich, F.
Lequeux, P. Hebraud, M.E. Cates, Phys. Rev. Lett. 78, 2020 (1997)]. The model
attributes similarities in the rheology of such ``soft glassy materials'' to
the shared features of structural disorder and metastability. By focusing on
the dynamics of mesoscopic elements, it retains a generic character.
Interactions are represented by a mean-field noise temperature x, with a glass
transition occurring at x=1 (in appropriate units). The exact solution of the
model takes the form of a constitutive equation relating stress to strain
history, from which all rheological properties can be derived. For the linear
response, we find that both the storage modulus G' and the loss modulus G''
vary with frequency as \omega^{x-1} for 1<x<2, becoming flat near the glass
transition. In the glass phase, aging of the moduli is predicted. The steady
shear flow curves show power law fluid behavior for x<2, with a nonzero yield
stress in the glass phase; the Cox-Merz rule does not hold in this
non-Newtonian regime. Single and double step strains further probe the
nonlinear behavior of the model, which is not well represented by the BKZ
relation. Finally, we consider measurements of G' and G'' at finite strain
amplitude \gamma. Near the glass transition, G'' exhibits a maximum as \gamma
is increased in a strain sweep. Its value can be strongly overestimated due to
nonlinear effects, which can be present even when the stress response is very
nearly harmonic. The largest strain \gamma_c at which measurements still probe
the linear response is predicted to be roughly frequency-independent.Comment: 24 pages, REVTeX, uses multicol, epsf and amssymp; 20 postscript
figures (included). Minor changes to text (relation to mode coupling theory,
update on recent foam simulations etc.) and figures (emphasis on low
frequency regime); typos corrected and reference added. Version to appear in
Physical Review
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