2,823 research outputs found
Spectra of turbulence in dilute polymer solutions
We investigate turbulence in dilute polymer solutions when polymers are
strongly stretched by the flow. We establish power-law spectrum of velocity,
which is not associated with a flux of a conserved quantity, in two cases. The
first case is the elastic waves range of high Reynolds number turbulence of
polymer solutions above the coil-stretch transition. The second case is the
elastic turbulence, where chaotic flow is excited due to elastic instabilities
at small Reynolds numbers.Comment: 14 pages, RevTe
Model-robust regression and a Bayesian ``sandwich'' estimator
We present a new Bayesian approach to model-robust linear regression that
leads to uncertainty estimates with the same robustness properties as the
Huber--White sandwich estimator. The sandwich estimator is known to provide
asymptotically correct frequentist inference, even when standard modeling
assumptions such as linearity and homoscedasticity in the data-generating
mechanism are violated. Our derivation provides a compelling Bayesian
justification for using this simple and popular tool, and it also clarifies
what is being estimated when the data-generating mechanism is not linear. We
demonstrate the applicability of our approach using a simulation study and
health care cost data from an evaluation of the Washington State Basic Health
Plan.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS362 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Beyond cognitive-behavioral therapy for fibromyalgia: addressing stress by emotional exposure, processing, and resolution
I comment on the well-designed trial by Alda and colleagues reported in a recent issue of Arthritis Research and Therapy which demonstrated some benefits of cognitive-behavioral therapy (CBT) for fibromyalgia (FM). CBT in this and other studies provides statistically significant but rather modest benefits for FM. This may be because CBT does not directly address the high rates of victimization, post-traumatic stress disorder, and emotional avoidance experienced by a substantial number of patients with FM. Interventions that encourage emotional exposure, processing, and resolution of stressful or traumatic experiences and relationships hold potential for larger effects for many patients and need to be tested
Single polymer dynamics: coil-stretch transition in a random flow
By quantitative studies of statistics of polymer stretching in a random flow
and of a flow field we demonstrate that the stretching of polymer molecules in
a 3D random flow occurs rather sharply via the coil-stretch transition at the
value of the criterion close to theoretically predicted.Comment: 4 pages, 5 figure
Hydrogen solubility in zirconium intermetallic second phase particles
The enthalpies of solution of H in Zr binary intermetallic compounds formed
with Cu, Cr, Fe, Mo, Ni, Nb, Sn and V were calculated by means of density
functional theory simulations and compared to that of H in {\alpha}-Zr. It is
predicted that all Zr-rich phases (formed with Cu, Fe, Ni and Sn), and those
phases formed with Nb and V, offer lower energy, more stable sites for H than
{\alpha}-Zr. Conversely, Mo and Cr containing phases do not provide
preferential solution sites for H. In all cases the most stable site for H are
those that offer the highest coordination fraction of Zr atoms. Often these are
four Zr tetrahedra but not always. Implications with respect to H-trapping
properties of commonly observed ternary phases such as Zr(Cr,Fe)2, Zr2(Fe,Ni)
and Zr(Nb,Fe)2 are also discussed.Comment: manuscript accepted for publication in Journal of Nuclear Materials
(2013
Small scale statistics of viscoelastic turbulence
The small scale statistics of homogeneous isotropic turbulence of dilute
polymer solutions is investigated by means of direct numerical simulations of a
simplified viscoelastic fluid model. It is found that polymers only partially
suppress the turbulent cascade below the Lumley scale, leaving a remnant energy
flux even for large elasticity. As a consequence, fluid acceleration in
viscoelastic flows is reduced with respect to Newtonian turbulence, whereas its
rescaled probability density is left unchanged. At large scales the velocity
field is found to be unaffected by the presence of polymers.Comment: 7 pages, 4 figure
Efficient Measurement Error Correction with Spatially Misaligned Data
Association studies in environmental statistics often involve exposure and outcome data that are misaligned in space. A common strategy is to employ a spatial model such as universal kriging to predict exposures at locations with outcome data and then estimate a regression parameter of interest using the predicted exposures. This results in measurement error because the predicted exposures do not correspond exactly to the true values. We characterize the measurement error by decomposing it into Berkson-like and classical-like components. One correction approach is the parametric bootstrap, which is effective but computationally intensive since it requires solving a nonlinear optimization problem for the exposure model parameters in each bootstrap sample. We propose a less computationally intensive alternative termed the ``parameter bootstrap\u27\u27 that only requires solving one nonlinear optimization problem, and we also compare bootstrap methods to other recently proposed methods. We illustrate our methodology in simulations and with publicly available data from the Environmental Protection Agency
- …