More Like this

1. An open-source solver for partially-saturated flow and multicomponent reactive transport in the hyporheic zone

   https://doi.org/10.1016/j.envsoft.2025.106583  

   Li, Pei; Wallace, Corey; Soltanian, Mohamad Reza (July 2025, Environmental Modelling and Software)

   Numerical models have been extensively used to understand and predict flow and reactive transport processes in the hyporheic zone. However, most models focus on fully saturated riverbeds without accounting for surface water stage fluctuations related to precipitation and flooding. To capture the complete picture of hyporheic processes in riverbeds and riverbanks, we developed a fully-coupled multiphase reactive transport solver using the Open Source Field Operation And Manipulation (OpenFOAM) platform. This solver captures surface water stage fluctuations and partially-saturated flow in fluvial sediment using VoF two-phase flow and extendedDarcy’s Law two-phase flow models for surface and subsurface domains, respectively. The transport models designed for partially saturated conditions in both domains are implemented. A geochemical reaction module, PhreeqcRM, is integrated into the solver to facilitate complex geochemical reaction networks. A two-way conservative flux boundary condition is implemented at the surface-subsurface interface to realistically map fluxes. The solver’s capability is illustrated through a variety of hyporheic-related problems across spatial scales. These include laboratory experiments and reactive transport in two and three dimensions, from the bedform scale to multiscale riverbeds and riverbanks with fluctuating surface water flow. This novel solver allows for quantifying dynamics in the hyporheic zone with fewer simplifications. Based on the code structure and parallel design of OpenFOAM, the solver can simulate large, three-dimensional (3D) multiscale cases. The code, examples, and pre- and post-processing scripts are all open source, providing community access to use and modify them as desired. 

   more » « less
2. The shear viscosity of parton matter under anisotropic scatterings

   https://doi.org/10.1140/epjc/s10052-022-10892-y  

   MacKay, Noah M.; Lin, Zi-Wei (October 2022, The European Physical Journal C)

   Abstract The shear viscosity$$\eta $$ $\eta$of a quark–gluon plasma in equilibrium can be calculated analytically using multiple methods or numerically using the Green–Kubo relation. It has been realized, which we confirm here, that the Chapman–Enskog method agrees well with the Green–Kubo result for both isotropic and anisotropic two-body scatterings. We then apply the Chapman–Enskog method to study the shear viscosity of the parton matter from a multi-phase transport model. In particular, we study the parton matter in the center cell of central and midcentral Au + Au collisions at 200AGeV and Pb + Pb collisions at 2760AGeV, which is assumed to be a plasma in thermal equilibrium but partial chemical equilibrium. As a result of using a constant Debye mass or cross section$$\sigma $$ $\sigma$for parton scatterings, the$$\eta /s$$ $\eta /s$ratio increases with time (as the effective temperature decreases), contrary to the trend preferred by Bayesian analysis of the experimental data or pQCD results that use temperature-dependent Debye masses. At$$\sigma =3$$ $\sigma =3$mb that enables the transport model to approximately reproduce the elliptic flow data of the bulk matter, the average$$\eta /s$$ $\eta /s$of the parton matter in partial equilibrium is found to be very small, between one to two times$$1/(4\pi )$$ $1/\left(4\pi \right)$. 

   more » « less
3. Time‐And‐Space Averaging Applied to Intermittent Multiphase Flow Experiments

   https://doi.org/10.1029/2023WR036577  

   Wang, Tingting; McClure, James_E; Da_Wang, Ying; Berg, Steffen; Chen, Cheng; Mostaghimi, Peyman; Armstrong, Ryan_T (June 2024, Water Resources Research)

   Abstract Various researchers have studied fluctuations in pore‐scale phase occupancy during multiphase flow in porous media using synchrotron‐based X‐ray microcomputed tomography (micro‐CT). However, the impact of these fluctuations on the concept of a representative volume is not yet fully understood. In this study, we performed spatial and temporal averaging of multiphase flow experiments visualized with synchrotron‐based micro‐CT, focusing on oil saturation as the key parameter to determine a representative time‐and‐space average. Our findings revealed that a saturation value representative of both time and space was achieved during fractional flow experiments in drainage mode with fractional flows of 0.8, 0.5, and 0.3. Furthermore, we computed a range of relative permeabilities on the basis of whether momentaneous saturation or time‐and‐space averaged saturation was utilized for direct simulation. Our results highlighted the importance of time‐and‐space averaging in determining a representative relative permeability and indicated that the temporal and spatial scales covered in a typical micro‐CT flow experiment were sufficient to obtain a representative saturation value for sandstone rock under intermittent flow conditions. 

   more » « less
4. General validity of the second fluctuation-dissipation theorem in the nonequilibrium steady state: Theory and applications

   https://doi.org/10.1088/1402-4896/acfce5  

   Zhu, Yuanran; Lei, Huan; Kim, Changho (October 2023, Physica Scripta)

   In this paper, we derive a generalized second fluctuation-dissipation theorem (FDT) for stochastic dynamical systems in the steady state and further show that if the system is highly degenerate, then the classical second FDT is valid even when the exact form of the steady state distribution is unknown. The established theory is built upon the Mori-type generalized Langevin equation for stochastic dynamical systems and hence generally applies to nonequilibrium systems driven by stochastic forces. These theoretical results enable us to construct a data-driven nanoscale fluctuating heat conduction model based on the second FDT. We numerically verify that our heat transfer model yields better predictions than the Green-Kubo formula for systems far from the equilibrium. 

   more » « less
5. Turbulence-level dependence of cosmic ray parallel diffusion

   https://doi.org/10.1093/mnras/staa2533  

   Reichherzer, P; Becker Tjus, J; Zweibel, E G; Merten, L; Pueschel, M J (September 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Understanding the transport of energetic cosmic rays belongs to the most challenging topics in astrophysics. Diffusion due to scattering by electromagnetic fluctuations is a key process in cosmic ray transport. The transition from a ballistic to a diffusive-propagation regime is presented in direct numerical calculations of diffusion coefficients for homogeneous magnetic field lines subject to turbulent perturbations. Simulation results are compared with theoretical derivations of the parallel diffusion coefficient’s dependences on the energy and the fluctuation amplitudes in the limit of weak turbulence. The present study shows that the widely used extrapolation of the energy scaling for the parallel diffusion coefficient to high turbulence levels predicted by quasi-linear theory does not provide a universally accurate description in the resonant-scattering regime. It is highlighted here that the numerically calculated diffusion coefficients can be polluted for low energies due to missing resonant interaction possibilities of the particles with the turbulence. Five reduced-rigidity regimes are established, which are separated by analytical boundaries derived in this work. Consequently, a proper description of cosmic ray propagation can only be achieved by using a turbulence-level-dependent diffusion coefficient and can contribute to solving the Galactic cosmic ray gradient problem. 

   more » « less