Self organized spatial‐temporal structure within the fractured Vadose Zone: Influence of fracture intersections

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[1] Recent experimental evidence suggests that the capillary heterogeneity associated with fracture intersections can act to impose temporal and spatial structure on network-scale flows. A simple intersection between orthogonal fractures, one horizontal and the other vertical, has been shown to integrate unsaturated flows. At low flows the intersection forms a capillary barrier that accumulates water in a growing pool. Eventually, the retaining meniscus snaps, discharging a pulse of water. Here we develop a mechanistic explanation for this observed behavior and experimentally consider three perturbations to the geometry of the simple orthogonal intersection. Two of the perturbations also act as capillary barriers, while the third formed a capillary bridge across the intersection. At low flow, all of our experimental intersections imposed a temporal signal, with the nature of that signal dependent on intersection geometry and participation by the horizontal fractures in dynamic storage. At high flow a continuous fluid tendril spanned the system from inlet to outlet with water pooled above the intersection caused by a narrow fluid connection that restricted flow across the intersection. Results from all experiments suggest that pulsation is critically sensitive to small variations in the geometry of fracture intersections and storage in the horizontal fractures. When combined with dependency on supply rate, this sensitivity can generate pulsation of flow across a wide range of time periods and discharge volumes.

Section: Discussionmentioning

confidence: 99%

Influence of fracture intersections under unsaturated, low‐flow conditions

Wood

Nicholl

Glass

2005

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“…The cycles of accumulation and discharge not only affect the size and frequency of the infiltrating liquid locally (Wood et al, 2005), but also cause flow pulsation in a fracture network (Glass et al, 2002). During liquid discharging and passing a bifurcating intersection, flow path selection and volume partitioning occur, which lead to flow pathway shifts and structural evolution in the networks (Glass & LaViolette, 2004). To better understand the liquid partitioning process, visualization experiments (Kordilla et al, 2017; Yang et al, 2019) have been performed to examine the fluid splitting behavior under different flow modes, from discrete droplet flows to continuous rivulets.…”

Section: Introductionmentioning

confidence: 99%

Splitting Dynamics of Liquid Slugs at a T‐Junction

Xue

Yang

et al. 2020

Understanding the mechanisms of liquid movement through fracture intersections is important for prediction of fluid flow and solute transport in unsaturated fractured media. Here we present a quasi‐static model to predict the dynamic splitting behavior of liquid slugs at a T‐junction, as a simplified representation of a fracture intersection and consisting of a main channel and a branch channel. The proposed model is validated against carefully controlled visualization experiments. We find that there exists a critical initial slug length at which the splitting behavior shifts from flow dominated by the main channel to that dominated by the branch. The influence of key parameters, including the inclination angle of the junction, the channel widths, and the dynamic contact angles, on the splitting dynamics is systematically investigated. We show that the splitting ratio depends nonmonotonically on the relative width of the branch channel to the main channel. Furthermore, it is demonstrated that the dynamic contact angles have a profound impact on the splitting ratios and meniscus velocities. It is shown that taking velocity‐dependent contact angle into account is essential to predict the meniscus velocities and the dynamic flow process.

“…The accumulation and release of water results in pulsating flows with magnitudes of temporal outflow fluctuations much higher than dripping dynamics in individual fractures alone can explain (Su et al, ), highlighting the importance of fracture intersections (specifically the subvertical fractures) as potential storage zones. Furthermore, the partitioning process at fracture intersections leads to flow focusing or splitting (Wood et al, ; Wood & Huang, ), which essentially controls the lateral and vertical dispersion characteristics of preferential flow paths (zones) (Glass & LaViolette, ). Hence, the analysis of flow partitioning at fracture intersections can ultimately lead to the interpretation and understanding of large‐scale vertical flow structures.…”

Section: Introductionmentioning

confidence: 99%

Effect of Unsaturated Flow Modes on Partitioning Dynamics of Gravity‐Driven Flow at a Simple Fracture Intersection: Laboratory Study and Three‐Dimensional Smoothed Particle Hydrodynamics Simulations

Kordilla

Noffz

Dentz

et al. 2017

In this work, we study gravity‐driven flow of water in the presence of air on a synthetic surface intersected by a horizontal fracture and investigate the importance of droplet and rivulet flow modes on the partitioning behavior at the fracture intersection. We present laboratory experiments, three‐dimensional smoothed particle hydrodynamics (SPH) simulations using a heavily parallelized code, and a theoretical analysis. The flow‐rate‐dependent mode switching from droplets to rivulets is observed in experiments and reproduced by the SPH model, and the transition ranges agree in SPH simulations and laboratory experiments. We show that flow modes heavily influence the “bypass” behavior of water flowing along a fracture junction. Flows favoring the formation of droplets exhibit a much stronger bypass capacity compared to rivulet flows, where nearly the whole fluid mass is initially stored within the horizontal fracture. The effect of fluid buffering within the horizontal fracture is presented in terms of dimensionless fracture inflow so that characteristic scaling regimes can be recovered. For both cases (rivulets and droplets), the flow within the horizontal fracture transitions into a Washburn regime until a critical threshold is reached and the bypass efficiency increases. For rivulet flows, the initial filling of the horizontal fracture is described by classical plug flow. Meanwhile, for droplet flows, a size‐dependent partitioning behavior is observed, and the filling of the fracture takes longer. For the case of rivulet flow, we provide an analytical solution that demonstrates the existence of classical Washburn flow within the horizontal fracture.