Realistic Fluid Properties Research Articles

Steady‐State Effective Normal Stress in Subduction Zones Based on Hydraulic Models and Implications for Shallow Slow Earthquakes

AbstractThe spatial distribution of effective normal stress, , is essential for understanding the fault motion. Although Rice (1992, https://doi.org/10.1016/s0074-6142(08)62835-1) proposed a steady‐state solution for a vertical strike‐slip fault zone with constant fluid properties, models that are based on the concept by Rice (1992, https://doi.org/10.1016/s0074-6142(08)62835-1) and are applicable for other tectonic settings have not yet been developed. Such a model is particularly important in subduction zones because the relationship between low and slow earthquakes is often discussed. To quantitatively examine the causes of a local decrease in on a shallow region of the subduction zone, we performed model calculations that incorporated mechanisms characteristic to subduction zones. Our basic model, which considers the effect of smectite dehydration and the mechanical effect of subduction, yields results that are consistent with those reported by Rice (1992, https://doi.org/10.1016/s0074-6142(08)62835-1): the gradient of remarkably decreases with the increase in depth, whereas the realistic fluid properties rule out nearly constant at depth. We obtained a monotonic increase in with the increase in depth for the physically sound solutions and failed to generate a local decrease in . The presence of a splay fault and fluid leakage though it cannot decrease locally. We found that a local decrease in permeability decreased locally around an impermeable zone and, thus, possibly led to the occurrence of shallow slow earthquakes. The water release caused by the dehydration reaction may not be the dominant factor, although smectite dehydration releases silica and promotes its precipitation.

Decompaction Weakening as a Mechanism of Fluid Focusing in Hydrothermal Systems

AbstractWe present a mathematical model of nonreactive fluid flow in a compacting porous medium. The model differs from previous formulations by considering fluid transport in the frame of reference moving with the solid phase. Such an approach guarantees material balance for the fluid and solid phases not only in the small‐porosity limit but also for large values of porosity. Using the numerical implementation of the proposed model, we simulate magmatic fluid transport in the Earth's upper crust. We account for the thermal softening of rocks, the plastic deformation of the solid matrix through decompaction weakening, and realistic fluid properties in a wide range of depths, including those above and below the brittle‐ductile transition (BDT). We show that our simulation approach can resolve the localized flow in the ductile zone and numerous hydrothermal convection cells in the brittle zone. We investigate the influence of decompaction weakening on high‐porosity channels forming in the ductile zone and their interaction with the convection in the brittle zone. We show that compaction causes magmatic fluid focusing and accumulation in high‐porosity lenses beneath the low‐porosity BDT zone. We show that magmatic fluid transfer through the BDT occurs mainly through the roofs of the lenses, which results in a plume of hydrothermal convection always sitting atop every lens. Other plumes between the lenses are associated with the convection of meteoric water that transfers only heat from the BDT to the surface. The simulations indicate that the lenses can be tracked by measuring certain parameters at the surface.

Physical effects of water droplets interacting with turbulent premixed flames: A Direct Numerical Simulation analysis

Three-dimensional carrier-phase Direct Numerical Simulations (DNS), combined with a Lagrangian representation of individual droplets, have been employed in this parametric study to examine the physical effects of liquid water mist interacting with laminar and turbulent premixed stoichiometric n-heptane/air flames. Significant reductions of flame temperature and burning velocity have been observed in the presence of water droplets. In agreement with the laws governing evaporation, a strongly non-linear influence of the droplet size on the overall burning rate has been noted, whereas the influence of water loading is fairly linear. Different regimes of droplet-flame interaction are known to exist and this has been investigated by numerical experiments focusing on the influence of the latent heat of vaporization. When using realistic fluid properties of water, the cooling effect associated to evaporating droplets outweighs the dilution effect due to the local decrease of fuel and oxidizer concentrations. Under turbulent conditions, the effectiveness of the droplets in reducing the overall burning rate changes owing to the transient nature of the droplet-flame interaction. Furthermore, it was found that the evaporating droplets significantly diminish the flame-generated turbulence and this leads to weaker turbulent wrinkling of the flame surface as compared to gaseous premixed reference simulations without droplets. Based on a comparison of the time scales representing droplet evaporation and the droplet residence time within the flame, a reduced-order model is proposed to account for both the cooling and dilution effects with respect to flame temperature and laminar burning velocity.

A lattice Boltzmann model for multi-component two-phase gas-liquid flow with realistic fluid properties

Multi-component multi-phase flows are of significant interests in nature and engineering problems of different fields. Modeling the phenomena involved in multi-phase flows is challenging, attributed to the complexity in simulating phase interface dynamics and diffusion processes. Owing to its kinetic nature, lattice Boltzmann (LB) method emerges as an attractive computational approach, in dealing with complicated fluid flow problems and microstructure geometries with effectiveness of parallelized processing. However, some critical drawbacks of basic multi-phase LB models, such as the low density and kinematic viscosity ratios, thermodynamic inconsistency, and dependence of surface tension and density distribution on relaxation times, limit its application in realistic multi-component multi-phase systems. Based on the original pseudopotential model and progresses in single-component multi-phase model, a multi-component LB model was proposed to study the two-phase gas-liquid flow with realistic fluid properties. The importance of improved model is in simultaneously realizing the realistic fluid flow characteristics for multi-component two-phase system, including high density and viscosity ratios, good thermodynamic consistency, independently tunable surface tension and appropriate two-phase boundaries. The proposed LB model is validated with Laplace law and visualization experiment results, and the effects of surface tension, gas flow velocity and wall wettability on dynamic behaviors of fluid flow are investigated.

Direct numerical simulation of a turbulent bubbly flow in a vertical channel: Towards an improved second-order reynolds stress model

Two-phase turbulence has been studied using a Direct Numerical Simulation (DNS) of an upward turbulent bubbly flow in a so-called plane channel. Fully deformable monodispersed bubbles are tracked by a Front-Tracking algorithm implemented in TrioCFD code on the TRUST platform. Realistic fluid properties are used to represent saturated steam and water in pressurised water reactor (PWR) conditions. The large number of bubbles creates a void fraction of 10%. The Reynolds friction number is 180. Time- and space-averaging is used to compute the main variables of the averaged scale description (e.g. void fraction, liquid and vapour velocities…) along with the Reynolds stresses and the turbulent dissipation rate tensor. Altogether, they provide reference profiles to assess and further improve Reynolds Stress models. A low-Reynolds version of the SSG model (Speziale et al., 1991) called EBRSM (Manceau and Hanjalić, 2002; Manceau, 2005) is applied in the context of two-phase flows with additional interfacial production terms. The model has been implemented and tested in the two-fluid Euler-Euler model of NEPTUNE_CFD code. The comparison with DNS demonstrates that the interfacial momentum closure plays a dominant role over the turbulent closure hypothesis in the present physical conditions.

Tests of diffusion-free scaling behaviors in numerical dynamo datasets

Many dynamo studies extrapolate numerical model results to planetary conditions by empirically constructing scaling laws. The seminal work of Christensen and Aubert (2006) proposed a set of scaling laws that have been used throughout the geoscience community. These scalings make use of specially-constructed parameters that are independent of fluid diffusivities, anticipating that large-scale turbulent processes will dominate the physics in planetary dynamo settings. With these ‘diffusion-free’ parameterizations, the results of current numerical dynamo models extrapolate directly to fully-turbulent planetary core systems; the effects of realistic fluid properties merit no further investigation. In this study, we test the validity of diffusion-free heat transfer scaling arguments and their applicability to planetary conditions. We do so by constructing synthetic heat transfer datasets and examining their scaling properties alongside those proposed by Christensen and Aubert (2006). We find that the diffusion-free parameters compress and stretch the heat transfer data, eliminating information and creating an artificial alignment of the data. Most significantly, diffusion-free heat transfer scalings are found to be unrelated to bulk turbulence and are instead controlled by the onset of non-magnetic rotating convection, itself determined by the viscous diffusivity of the working fluid. Ultimately, our results, in conjunction with those of Stelzer and Jackson (2013) and King and Buffett (2013), show that diffusion-free scalings are not validated by current-day numerical dynamo datasets and cannot yet be extrapolated to planetary conditions.

Are standard values the best choice? A critical statement on rheological soil fluid properties viscosity and surface tension

The rheological properties of a fluid in terms of viscosity η and surface tension γ are often neglected in studies on hydrostatic and hydrodynamic phenomena. However, their measurement seems to provide us with more benefit than it takes time and effort. Especially regarding transport of fluids in soil, the exact definition of the fluid properties is necessary for predicting water transport correctly.In this paper we review and summarize the available scientific knowledge on rheological soil fluid properties and supplement it with our own investigations on viscosity and surface tension of salt and soil solutions. Our results showed neither a clear linear relationship between η and salt concentration in aqueous solutions nor to the kind of cation or anion of the dissolved salt whereas the surface tension of salt solutions generally increased linearly with molar concentration though at different rates. As an example, a 1M MgSO4 solution doubled η but increased γ only by 3%, whereas 1M MgCl2 caused 4% increase in γ, but still 50% increase in η. Viscosity of soil solutions depended on soil–water ratio as well as fertilization of the soil. The largest deviation to standard η of water was about 6% at 40% gravimetric water content.Concluding from the literature review and our own findings, we recommend to additionally measure rheology (namely viscosity and surface tension) of the soil solution in order to improve modeling of hydrostatic and hydrodynamic phenomena in soils by utilizing more realistic fluid properties which might deviate significantly from the usually employed standard values. The mathematical error cumulates with increasing salt concentration as a deviation of 10% to the standard value also causes an error of 10% in linear relationships derived from that parameter, e.g. the equation of Hagen–Poiseuille.Future investigations should focus on the manifold single effects as well as on the interactions of different dissolved components in the soil solution on viscosity and surface tension and how these are influenced by temperature, pressure (i.e. shear rate) and concentration.

Phase separation, brine formation, and salinity variation at Black Smoker hydrothermal systems

We present the first fully transient 2‐D numerical simulations of black smoker hydrothermal systems using realistic fluid properties and allowing for all phase transitions possible in the system H2O‐NaCl, including phase separation of convecting seawater into a low‐salinity vapor and high‐salinity brine. We investigate convection, multiphase flow, and phase segregation at pressures below, near, and above the critical point of seawater. Our simulations accurately predict the range in vent salinities, from 0.05 to 2.5 times seawater salinity measured at natural systems. In low‐pressure systems at ∼1500 m water depth, phase separation occurs in boiling zones stretching from the bottom of the hydrothermal cell to the seafloor. Low‐salinity vapors and high‐salinity brines can vent simultaneously, and transient variations in vent fluid salinities can be rapid. In high‐pressure systems at roughly ∼3500 m water depth, phase separation is limited to the region close to the underlying magma chamber, and vent fluids consist of a low‐salinity vapor mixed with a seawater‐like fluid. Therefore, vent salinities from these systems are much more uniform in time and always below seawater salinity as long as phase separation occurs in the subseafloor. Only by shutting down the heat source can, in the high‐pressure case, the brine be mined, resulting in larger than seawater salinities. These numerical results are in good agreement with long‐term observations from several natural black smoker systems.

Two‐dimensional numerical models of open‐top hydrothermal convection at high Rayleigh and Nusselt numbers: Implications for mid‐ocean ridge hydrothermal circulation

Mid‐ocean ridges host vigorous hydrothermal systems that remove large quantities of heat from the oceanic crust. Inferred Nusselt numbers (Nu), which are the ratios of the total heat flux to the heat flux that would be transported by conduction alone, range from 8 to several hundred. Such vigorous convection is not fully described by most numerical models of hydrothermal circulation. A major difficulty arises at high Nu from the numerical solution of the temperature equation. To avoid classical numerical artifacts such as nonphysical oscillatory behavior and artificial diffusion, we implement the Multidimensional Positive Definite Advection Transport Algorithm (MPDATA) technique, which solves the temperature equation using an iterated upwind corrected scheme. We first validate the method by comparing results for models with uniform fluid properties in closed‐ and open‐top systems to existing solutions with Nu ≤ ∼20. We then incorporate realistic fluid properties and run models for Nu up to 50–60. Solutions are characterized by an unstable bottom thermal boundary layer where thermal instabilities arise locally. The pattern of heat extraction is periodic to chaotic. At any Nu > ∼13 the venting temperatures in a given plume are chaotic and oscillate from ∼350° to 450°C. Individual plumes can temporarily stop short of the surface for intervals ranging from tens to hundreds of years at times when other plumes vent with an increased flow rate. The solutions also display significant recirculation, and as a result large areas of downflow are relatively warm with temperatures commonly exceeding 150°C at middepths. Our results have important implications for mid‐ocean ridge hydrothermal systems and suggest the following: (1) The reaction zones of mid‐ocean ridge hydrothermal systems are enlarged by thermal instabilities that migrate laterally toward upflow zones. This will substantially increase the volume of rock involved in chemical reactions compared to steady state configurations. (2) Hydrothermal discharge can stop temporarily as zones of venting are dynamically replaced by zones of seawater recharge. (3) Anhydrite precipitation occurring at temperatures exceeding ∼150°C will likely occur throughout a large portion of recharge zone and will not necessarily clog downflow pathways as efficiently as has been recently inferred.

Seismic properties of pore fluids

Pore fluids strongly influence the seismic properties of rocks. The densities, bulk moduli, velocities, and viscosities of common pore fluids are usually oversimplified in geophysics. We use a combination of thermodynamic relationships, empirical trends, and new and published data to examine the effects of pressure, temperature, and composition on these important seismic properties of hydrocarbon gases and oils and of brines. Estimates of in‐situ conditions and pore fluid composition yield more accurate values of these fluid properties than are typically assumed. Simplified expressions are developed to facilitate the use of realistic fluid properties in rock models. Pore fluids have properties that vary substantially, but systematically, with composition, pressure, and temperature. Gas and oil density and modulus, as well as oil viscosity, increase with molecular weight and pressure, and decrease with temperature. Gas viscosity has a similar behavior, except at higher temperatures and lower pressures, where the viscosity will increase slightly with increasing temperature. Large amounts of gas can go into solution in lighter oils and substantially lower the modulus and viscosity. Brine modulus, density, and viscosities increase with increasing salt content and pressure. Brine is peculiar because the modulus reaches a maximum at a temperature from 40 to 80°C. Far less gas can be absorbed by brines than by light oils. As a result, gas in solution in oils can drive their modulus so far below that of brines that seismic reflection bright spots may develop from the interface between oil saturated and brine saturated rocks.