@article{fazelpour_tang_daniels_2022, title={The effect of grain shape and material on the nonlocal rheology of dense granular flows}, volume={1}, ISSN={["1744-6848"]}, url={https://doi.org/10.1039/D1SM01237A}, DOI={10.1039/d1sm01237a}, abstractNote={Nonlocal rheologies allow for the modeling of granular flows from the creeping to intermediate flow regimes, using a small number of parameters. In this paper, we report on experiments testing how particle properties affect the model parameters used in the Kamrin & Koval cooperative nonlocal model, using particles of three different shapes (circles, ellipses, and pentagons) and three different materials, including one which allows for the measurement of stresses via photoelasticity. Our experiments are performed on a quasi-2D annular shear cell with a rotating inner wall and a fixed outer wall. Each type of particle is found to exhibit flows which are well-fit by nonlocal rheology, with each particle having a distinct triad of the local, nonlocal, and frictional parameters. While the local parameter b is always approximately unity, the nonlocal parameter A depends sensitively on both the particle shape and material. The critical stress ratio μs, above which Coulomb failure occurs, varies for particles with the same material but different shape, indicating that geometric friction can dominate over material friction.}, journal={Soft Matter}, author={Fazelpour, Farnaz and Tang, Zhu and Daniels, Karen E.}, year={2022}, month={Jan} }
 @article{tang_brzinski_shearer_daniels_2018, title={Nonlocal rheology of dense granular flow in annular shear experiments}, volume={14}, ISSN={1744-683X 1744-6848}, url={http://dx.doi.org/10.1039/c8sm00047f}, DOI={10.1039/c8sm00047f}, abstractNote={The flow of dense granular materials at low inertial numbers cannot be fully characterized by local rheological models; several nonlocal rheologies have recently been developed to address these shortcomings. To test the efficacy of these models across different packing fractions and shear rates, we perform experiments in a quasi-2D annular shear cell with a fixed outer wall and a rotating inner wall, using photoelastic particles. The apparatus is designed to measure both the stress ratio μ (the ratio of shear to normal stress) and the inertial number I through the use of a torque sensor, laser-cut leaf springs, and particle-tracking. We obtain μ(I) curves for several different packing fractions and rotation rates, and successfully find that a single set of model parameters is able to capture the full range of data collected once we account for frictional drag with the bottom plate. Our measurements confirm the prediction that there is a growing lengthscale at a finite value μs, associated with a frictional yield criterion. Finally, we newly identify the physical mechanism behind this transition at μs by observing that it corresponds to a drop in the susceptibility to force chain fluctuations.}, number={16}, journal={Soft Matter}, publisher={Royal Society of Chemistry (RSC)}, author={Tang, Zhu and Brzinski, Theodore A. and Shearer, Michael and Daniels, Karen E.}, year={2018}, pages={3040–3048} }
 @article{tang_reynolds_ressler_2016, title={X-RAY AND GAMMA-RAY EMISSION FROM MIDDLE-AGED SUPERNOVA REMNANTS IN CAVITIES. I. SPHERICAL SYMMETRY}, volume={227}, ISSN={["1538-4365"]}, url={https://doi.org/10.3847/1538-4365/227/2/28}, DOI={10.3847/1538-4365/227/2/28}, abstractNote={We present analytical and numerical studies of models of supernova-remnant (SNR) blast waves in one spatial dimension expanding into uniform media and interacting with a denser cavity wall. We predict the nonthermal emission from such blast waves: synchrotron emission at radio and X-ray energies, and bremsstrahlung, inverse-Compton emission (from cosmic-microwave-background seed photons; ICCMB), and emission from the decay of mesons produced in inelastic collisions between accelerated ions and thermal gas, at GeV and TeV energies. Accelerated-particle spectra are assumed to be power laws with exponential cutoffs at energies limited by the remnant age or (for electrons, if lower) by radiative losses. We compare the results with those from homogeneous (“one-zone”) models. Such models give fair representations of the 1D results for uniform media, but cavity-wall interactions produce effects for which one-zone models are inadequate. We study the time evolution of SNR morphology and emission with time. Strong morphological differences exist between ICCMB and -decay emission; at some stages, the TeV emission can be dominated by the former and the GeV by the latter, resulting in strong energy dependence of morphology. Integrated gamma-ray spectra show apparent power laws of slopes that vary with time, but do not indicate the energy distribution of a single population of particles. As observational capabilities at GeV and TeV energies improve, spatial inhomogeneity in SNRs will need to be accounted for.}, number={2}, journal={ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES}, publisher={American Astronomical Society}, author={Tang, Zhu and Reynolds, Stephen P. and Ressler, Sean M.}, year={2016}, month={Dec} }