Robert Herrendörfer

Evaporative cooling is a promising concept to improve proton exchange membrane fuel cells. While the particular concept based on gas diffusion layers (GDLs) modified with hydrophilic lines (HPILs) has recently been demonstrated, there is a lack in the understanding of the mass and heat transport processes. We have developed a 3-D, non-isothermal, macro-homogeneous numerical model focusing on one interface between a HPIL and an anode gas flow channel (AGFC). In the base case model, water evaporates within a thin film adjacent to the interfaces of the HPIL with the AGFC and with the hydrophobic anode GDL. The largest part of the generated water vapor leaves the cell via the AGFC. The transport to the cathode side is shown to be partly limited by the ab-/desorption into/from the membrane. The cooling due to the latent heat has a strong effect on the local evaporation rate. An increase of the mass transfer coefficient for evaporation leads to a transport limited regime inside the MEA while the transport via the AGFC is limited by evaporation kinetics.

Orientations of natural fault systems are subject to large variations. They often contradict classical Coulomb failure theory as they are misoriented relative to the regional Andersonian stress field. This is ascribed to local effects of structural or stress heterogeneities and reorientations of structures or stresses on the long term. To better understand the relation between fault orientation and regional stresses, we simulate spontaneous fault growth and its effect on the stress field. Our approach incorporates earthquake rupture dynamics, viscoelastoplastic brittle deformation and a rate‐ and state‐dependent friction formulation in a continuum mechanics framework. We investigate how strike‐slip faults orient according to local and far‐field stresses during their growth. We identify two modes of fault growth, seismic and aseismic, distinguished by different fault angles and slip velocities. Seismic fault growth causes a significant elevation of dynamic stresses and friction values ahead of the propagating fault tip. These elevated quantities result in a greater strike angle relative to the maximum principal regional stress than that of a fault segment formed aseismically. When compared to the near‐tip time‐dependent stress field the fault orientations produced by both growth modes follow the classical failure theory. We demonstrate how the two types of fault growth may be distinguished in natural faults by comparing their angles relative to the original regional maximum principal stress. A stress field analysis of the Landers‐Kickapoo fault suggests that an angle greater than ∼25° between two faults indicates seismic fault growth.

We present a 2‐D numerical modeling approach for simulating a wide slip spectrum in a viscoelastoplastic continuum. The key new model component is an invariant reformulation of the classical rate‐ and state‐dependent friction equations, which is designed for earthquake simulations along spontaneously evolving faults. Here we describe the methodology and demonstrate that it is accurate and stable in a setup consisting of a mature strike‐slip fault zone. We show that the nucleation and propagation of an earthquake are well resolved, as supported by a good agreement with various analytical approximations, including those of the nucleation and cohesive zone lengths. Results generally converge with respect to grid size, time step, and other numerical parameters. The convergence rate with respect to grid size depends on the internodal averaging scheme, is influenced by wave reflections, and deteriorates for inclined faults. The simulated slip spectrum, ranging from stable sliding at the loading rate to periodic aseismic slip to periodic seismic slip as a function of nucleation size, is in general agreement with the literature. In this simple setup, dynamic pressure does not play a significant role. By analyzing the role of viscous deformation, we identify and confirm by our simulations a theoretical viscosity threshold below which earthquakes cannot nucleate. This threshold is shown to depend on the reference strength of rate‐ and state‐dependent friction and the loading strain rate, which is in agreement with previous work on the brittle‐ductile transition.

Correlations between geodynamic parameters and interplate seismicity characteristics in subduction zones are generally weak due to the short instrumental record and multiparameter influences. To investigate the role of subduction velocity V_s and the width of the seismogenic zone W on maximum magnitude M_max, seismic rate τ, characteristic recurrence rate τ_c, and moment release rate MRR, we use synthetic data sets from simplified analog and numerical models to gain insight into natural subduction zones seismicity. Our models suggest that M_max increases with W and is unaffected by V_s, τ increases with V_s, τ_c increases with V_s/W, and MRR increases with V_s × W. In nature, only the positive correlation between V_s and τ is significant. Random sampling of our time series suggest that the positive correlation between V_s and τ can be observed with short observation time windows. Other correlations, including M_max versus W, become clear only for time window lengths longer than 1/τ_c.