Large-scale Laboratory Experiments Research Articles

Observed Dust Devil-Like Flow Structures In Large-Scale Turbulent Rayleigh-Bénard Convection

Turbulent Rayleigh-Bénard (RB) convection, a fluid layer heated from below and cooled from above, might be an appropriate tool to reproduce flow phenomena as occurring in a convective atmospheric boundary layer. One of these phenomena is the appearance of so-called dust devils, vortex like flow structures with typical dimensions up to a few tens of meters in diameter and up to a few hundreds of meters in height. Unlike their big brothers, the tornados, start to evolve close to the ground, but the physics of their evolution are still not fully understood. Since field measurements of dust devils are extremely hard to realize (due to their very rare and spontaneous appearance at arbitrarily varying locations), continuous flow measurements in large-scale laboratory experiments may help to overcome this problem. Such measurements in flows of several m³ in volume require optical measurement techniques like particle tracking velocimetry which gains access to the 3-dimensional Lagrangian velocity field. Special attention has to be drawn to the particle seeding of the flow and the imaging of particles. Particles must have neutral density compared to the fluid, and must be large enough to be detected by cameras over a range of several meters. They also should exhibit a prolonged lifetime. In this study, we present experiments conducted in the “Barrel of Ilmenau”, the world-wide largest Rayleigh-Bénard laboratory experiment with 7.15 m in diameter and up to 6.3 m in height using air as working fluid. Neutrally-buoyant helium-filled soap bubbles were injected into the flow and tracked using four cameras. Illumination was carried out with five high-power LEDs in pulsed mode. Reconstruction of the Lagrangian velocity field was performed with Shake-The-Box algorithm. Thus, the major focus of the presented work was to get access to the entire volumetric Lagrangian velocity field in the convection cell on a very long time. Our aim is to discover very rare flow events in turbulent convection, like e.g. dust devil-like vortices or other turbulent superstructures. Measurement data was analyzed to conclude about the applicability of particle tracking velocimetry as suitable measurement technique for large volumetric flows. Finally, an example of a discovered dust-devil like vortex is presented in detail.

Cubic-meter scale laboratory fault re-activation experiments to improve the understanding of induced seismicity risks

To understand fluid induced seismicity, we have designed a large-scale laboratory experiment consisting of a one-cubic-meter sandstone with an artificial fault cut and fluid-injection boreholes. The sandstone block is assembled in a true triaxial loading frame and equipped with 38 piezoelectric sensors to locate and characterise acoustic emission events. The differential stress on the artificial fault is increased in stages to bring it towards a critically stressed state. After each stage of differential stress increase, fluids are injected at low pressures through boreholes to test the potential of fault re-activation. In addition, a high-pressure injection was conducted that created a hydraulic fracture from the injection borehole towards the artificial fault. The newly generated fluid pathway resulted in an activation of the complete block through a stick–slip movement. We compare acoustic emission measurements from the laboratory experiment with seismicity observations from the field-scale CO2 injection at Decatur, Illinois, U.S., and conclude that the existence of fluid pathways plays a decisive role for the potential of induced seismicity.

Large Scale Laboratory Experiment: The Impact of the Hydraulic Characteristics of Flood Waves Caused by Gradual Levee Failure on Inundation Areas

As a levee failure and the consequent flooding cause significant financial losses and sometimes human casualties, they have led to considerable concern among city officials. Therefore, researchers have devoted considerable effort to investigating the hydraulic characteristics of sudden transient flow in the form of propagated waves to inundation areas during a levee and/or dam failure. A large number of studies, however, have mostly focused on simple one-dimensional cases investigated numerically and/or experimentally, and thus, important hydraulic characteristics, particularly near the failure zone, have not been adequately captured because of three-dimensional complexities. Taking these complexities into consideration, this study conducts a large-scale experiment to examine the characteristics of wave propagation in an open area caused by a gradual levee failure. From the experimental observations, this study provides the propagation speed of a wave front and suggests a formula for the maximum flood depth corresponding to the peak flood wave in the inundation area. We expect the findings to provide hydraulic engineers and scientists with fundamental insights into transient flow during a gradual levee failure. By contributing to our theoretical understanding, the measurements can also be used as validation tools for future numerical simulation and are likely to contribute to the establishment of emergency action plans that can help city officials cope with flood inundation.

Temporal evolution of scour at bridge abutments in compound channels

Bridge abutment-induced sediment scour in compound river channels is time-sensitive during floods, and, thus, estimating the scour rate has significance in design. In the current study, large-scale laboratory experiments were done to examine the influence of multiple factors on scour evolution, including channel geometry, bed mobility (clear-water or live-bed), and lateral (embankment intrusion) and vertical (deck submergence) flow contractions. A new method is applied for scour measurement and determination of characteristic scour trends. Results show that the non-dimensional time scale is related to flow intensity and may peak before reaching the clear-water scour threshold; the corresponding equations also are derived. It is found that the use of scour countermeasures has a significant impact on the scour rate, and the existing equations for unprotected abutments have limitations. Under clear-water conditions, the scour rate is closely related to the overall flow contraction, which can be better reflected by the ratio of the approaching unit discharge to that at the bridge section. Specifically, the effect of vertical flow contraction on scour rate is more dominant than other factors. Two scour rate curves are derived to suit different structural and flow contraction types for clear-water scour. In contrast, the live-bed scour rate data are generally consistent and fall in a narrow band, showing less dependence on other factors. Finally, a new design approach is established to estimate the time scale and scour rate using the new equations proposed in this paper.

Interactive effect of temperature and plant species on nitrogen cycling and treatment in stormwater biofiltration systems

Stormwater biofiltration systems (also known as biofilters, bioretention, rain gardens) are engineered nature-based solutions, which help mitigate aquatic nitrogen pollution arising from storm runoff. These systems are being increasingly used in a range of climates across the world. A decline in treatment performance is frequently observed in cold weather conditions. While plant species comprise an important design factor influencing system performance, the effect of temperature on the fate of dissolved nitrogen forms, namely ammonium (NH4+) and nitrate (NO3−), in the presence of different plant species in these systems remains unclear. A large scale laboratory experiment was undertaken that measured potential rates of nitrification, denitrification and dissimilatory nitrate reduction to ammonium (DNRA) as well as the microbial community structure to investigate nitrogen fate and hence removal under two different temperature conditions (2 °C and 15 °C) in the presence of four distinct plant species. The results indicate that lower nitrification rates (reduced by a factor of 4) coupled with potential media NH4+ desorption could be contributing to reduced NH4+ removal during cold conditions. Planting with species exhibiting good nutrient uptake capacity can reduce the extent of this performance decline. While NO3− reduction generally remains problematic during cold weather (<0 to 55% reduction), which may not be significantly different from warmer periods, the study demonstrated that the denitrification potential and gene abundance (nap, nar, NirS, norB, nosZ) to be higher than those of nitrification (amoA). Denitrification may not proceeding at optimal rates due to lack of conducive environmental conditions. Nitrogen transformation via DNRA was found to be relatively insignificant. Future studies should investigate the potential of employing cold-resilient plant species to maintain both NH4+ and NO3− removal in cold weather conditions.

Experimental and Numerical Study on the Wave, Surge, and Structure Interactions on a Coastal Residential Building

Lee, D.; Park, H.; Shin, S., and Cox, D.T., 2021. Experimental and numerical study on the wave, surge, and structure interactions on a coastal residential building. In: Lee, J.L.; Suh, K.-S.; Lee, B.; Shin, S., and Lee, J. (eds.), Crisis and Integrated Management for Coastal and Marine Safety. Journal of Coastal Research, Special Issue No. 114, pp. 156–160. Coconut Creek (Florida), ISSN 0749-0208. Residential structures in the low-lying coastal region are often suffered from devastate coastal disasters, like storms and tsunamis. To minimize the loss and improve the resilience of a coastal community, it is crucial to understand structural damage and failure mechanism under extreme from extreme coastal events. Large-scale laboratory experiments of wood-framed residential houses were conducted to investigate the wave and surge effect on the structure and the structural failure mechanism. The tests were conducted by changing a surge level, wave period, and wave height condition, and measure hydro-kinematics and structural damage process. LIDAR scanning was conducted to collect the data of damage proportion on the structure during series of wave and surge conditions. A three-dimensional RANS model, olaFlow, was used to verify our numerical model, and applied to understand hydrodynamics near the structure. Three different wave breaking conditions, including non-breaking, breaking, and broken wave are tested and consequent wave induced forces on structure are compared for both undamaged structure and damaged structure conditions. The numerical model results showed a good agreement with the experimental results in water surface and velocity. The numerical results of wave induced force in damaged conditions showed 28∼43% smaller than those in undamaged conditions. This finding highlight that the damage level of a structure could affect predicting wave load on the structure.

Utilization of alkali-activated construction demolition waste for sandy soil improvement with large-scale laboratory experiments

Using alkali-activated materials to treat waste materials is becoming more prevalent in geotechnical engineering. Adding these materials into soil particles as a cementitious material can improve the geotechnical properties of problematic soils, and minimize environmental pollution. The present study focused on the feasibility of using alkali-activated construction demolition waste (CDW) as a stabilizer for large-scale soil improvement applications. The CDW was used as a source of aluminosilicate; sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium oxide (CaO) (at concentrations of 2 M, 6 M, 10 M, and 14 M) were used as the alkaline media. CDW (16%) was selected as the source of alkali-activated material. The optimal concentrations of NaOH, KOH, and CaO were found to be 10 M, 10 M, and 6 M, respectively. Moreover, 21 days and 39 °C yielded the best results at the bearing capacity values. Thus, the use of CDW as an alkali activator is a better remedy for soil engineering properties, and contributes to soil improvement.

Three-dimensional numerical simulation of wave-induced scour around a pile on a sloping beach

A fully-coupled (hydrodynamic and morphologic) numerical model based on the open-source CFD package OpenFOAM is presented and utilized to simulate breaking wave induced scour around a pile on a sloping beach. The hydrodynamic model is based on RANS equations together with k-ω turbulence closure and volume of fluid method for capturing the free surface. These are then coupled with both bed load and suspended load transport descriptions, which drive resultant morphology of the bed. The model is validated against large-scale laboratory experiments involving interaction of breaking solitary waves with a pile and induced flow and scour. The flow features and hydrodynamic characteristics around the pile are described in detail, and the scouring mechanism at different locations of the pile is thoroughly analyzed. The simulated flow field confirms the formation of horseshoe vortex in front of the pile induced by plunging breaking wave. The temporal variation of scour depth at the side of the pile is satisfactorily simulated while those at the offshore and onshore are under-predicted. The scour depth at the side of the pile increases twice during run-up and draw-down. The scour at the onshore location of the pile starts to increase rapidly at the end of wave drawdown.

Predicting social tipping and norm change in controlled experiments

The ability to predict when societies will replace one social norm for another can have significant implications for welfare, especially when norms are detrimental. A popular theory poses that the pressure to conform to social norms creates tipping thresholds which, once passed, propel societies toward an alternative state. Predicting when societies will reach a tipping threshold, however, has been a major challenge because of the lack of experimental data for evaluating competing models. We present evidence from a large-scale laboratory experiment designed to test the theoretical predictions of a threshold model for social tipping and norm change. In our setting, societal preferences change gradually, forcing individuals to weigh the benefit from deviating from the norm against the cost from not conforming to the behavior of others. We show that the model correctly predicts in 96% of instances when a society will succeed or fail to abandon a detrimental norm. Strikingly, we observe widespread persistence of detrimental norms even when individuals determine the cost for nonconformity themselves as they set the latter too high. Interventions that facilitate a common understanding of the benefits from change help most societies abandon detrimental norms. We also show that instigators of change tend to be more risk tolerant and to dislike conformity more. Our findings demonstrate the value of threshold models for understanding social tipping in a broad range of social settings and for designing policies to promote welfare.

Compressibility characteristics of TDA from OTR (off-the-road) tires: A numerical approach

This paper aims to use the discrete element method (DEM) to study the performance of discarded off-the-road (OTR) tire chips under compression. The results of small-scale laboratory tests were used to calibrate a model built in PFC3D. This model was validated using the results of a large-scale laboratory experiment. The results show that the discrepancy between the calculated strain and the strain measured in the laboratory was 1.94%. Furthermore, the calculated elastic modulus was 0.11 MPa lower than the elastic modulus determined in laboratory measurements. A sensitivity analysis done on the parameters included in the simulation indicated that Poisson’s ratio and the local damping ratio had a significant impact on the simulation results, whereas the loading rate, particle size and scale effect of the loading cylinder had a limited effect on the simulation results. The compressibility characteristics of OTR tire chips in the field embankment were simulated using a cuboid filled with the same size clumps as the TDA chips used in laboratory tests, and the simulation results were compared with available information. The results indicated that the parameters calibrated based on the small-scale laboratory tests can be used to predict the deformation of OTR tire chips in the field. The discrepancy between the calculation and field measurements for strain was 1%. Due to the extremely low winter temperatures in Edmonton (as low as −40 °C), the impact of temperature on the mechanical properties of OTR tire chips was also considered by varying the values of elastic modulus used in the simulation. The results show that the tire chip embankment had a larger strain in summer than in winter as a result of the high elastic modulus in warmer temperatures; however, the difference between the summer and winter strain values is only 0.4%. This paper presents valuable results on the deformation characteristics of OTR tire chips, using a combination of field measurements and laboratory measurements, and exploring the impact of temperature on the deformation of OTR chips using a numerical approach.

Influence of tensile strength of geogrid and subgrade modulus on layer coefficients of granular bases

This paper presents a series of large-scale laboratory experiments conducted on pavement sections to evaluate the performance of geogrid-reinforced base layer overlying various subgrade conditions (resilient modulus ranging between 10 MPa and 72 MPa). Four types of geogrids with different tensile strengths, aperture sizes, and polymer types were used to investigate the influence of geogrids on the structural coefficients in terms of modulus improvement factor (MIF) and base layer coefficient ratios (LCRs). Further, a thorough analysis was performed on a three-layer flexible pavement system to propose firstly new models for base layer coefficients (a2u) for control (unreinforced) sections; and secondly, LCR equations and layer coefficient models for geogrid-reinforced base layers (a2r). The experimental results indicate that the stiffness of the subgrade significantly influences the performance of the pavement system. The optimum reinforcement depth was found to be one-third of the base layer thickness measured from the top surface. For an optimum geogrid configuration on a weak subgrade, a maximum LCR value obtained was about 1.8. As-built pavement section of Interstate Highway-8, Arizona, was evaluated to validate the proposed models. Based on the study, limiting values for MIF and LCR for geogrid-reinforced bases were proposed. Finally, a set of design charts were provided to determine the granular layer (base and subbase) thickness for a given subgrade and traffic conditions and a practical range of base layer resilient modulus values (200–400 MPa).

Efficacy of locally-available cleaning methods and household chlorination at inhibiting biofilm development in jerricans used to store household drinking water

Jerricans, commonly used to store household drinking water, are challenging to clean and biofilms may develop inside containers storing contaminated water. A large-scale laboratory experiment was conducted to test methods to inhibit biofilm growth.

Large-Scale Laboratory Experiments on Mussel Dropper Lines in Ocean Surface Waves

The rapid growth of marine aquaculture around the world accentuates issues of sustainability and environmental impacts of large-scale farming systems. One potential mitigation strategy is to relocate to more energetic offshore locations. However, research regarding the forces which waves and currents impose on aquaculture structures in such conditions is still scarce. The present study aimed at extending the knowledge related to live blue mussels (Mytilus edulis), cultivated on dropper lines, by unique, large-scale laboratory experiments in the Large Wave Flume of the Coastal Research Center in Hannover, Germany. Nine-months-old live dropper lines and a surrogate of 2.0 m length each are exposed to regular waves with wave heights between 0.2 and 1.0 m and periods between 1.5 and 8.0 s. Force time histories are recorded to investigate the inertia and drag characteristics of live mussel and surrogate dropper lines. The surrogate dropper line was developed from 3D scans of blue mussel dropper lines, using the surface descriptor Abbott–Firestone Curve as quality parameter. Pull-off tests of individual mussels are conducted that reveal maximum attachment strength ranges of 0.48 to 10.55 N for mussels that had medium 3.04 cm length, 1.60 cm height and 1.25 cm width. Mean drag coefficients of CD = 3.9 were found for live blue mussel lines and CD = 3.4 for the surrogate model, for conditions of Keulegan–Carpenter number (KC) 10 to 380, using regular wave tests.

Numerical simulation of the mechanical behaviour of steel pipe bends under strong cyclic loading

The present study uses advanced numerical tools to examine the mechanical behavior of steel pipe bends (elbows), subjected to strong cyclic loading, associated with repeated plastic deformations of alternate sign. The elbows are modeled with finite elements, accounting for the measured elbow geometry and the actual properties of steel elbow material. The main feature of the present work is the simulation of material response using an advanced cyclic-plasticity model, based on the bounding-surface concept, implemented through an in-house material subroutine. Upon appropriate calibration with material tests in strip specimens, extracted from the bends under consideration, the constitutive model is employed for predicting the mechanical response of large-scale physical laboratory experiments. Very good predictions are obtained in terms of global load-displacement response, as well as in terms of local strains and their accumulation over the loading cycles (ratcheting) at specific elbow locations. This indicates that the bounding-surface model can be an efficient tool for predicting accurately the mechanical response of piping components under severe cyclic loading conditions. Finally, using the validated numerical models, extensive results are obtained, focusing on the effects of internal pressure on strain accumulation at specific locations of the elbow, for three values of pipe thickness.

Fracture of warm S2 columnar freshwater ice: size and rate effects

Large scale laboratory experiments on size and rate effects on the fracture of warm columnar freshwater ice have been conducted with floating edge-cracked rectangular plates loaded at the crack mouth. The largest test plate size had dimensions of 19.5m x 36m. The overall crack-parallel dimension covered a size range of 1:39, possibly the largest for ice tested under laboratory conditions. The loading rates applied led to test durations from fewer than 2 seconds to more than 1000 seconds, leading to an elastic response at the highest rates to a viscoelastic response at the lower rates. Methods for both the linear elastic fracture mechanics (LEFM) and a non-linear viscoelastic fictitious crack model (VFCM) were derived to analyze the data and calculate values for the apparent fracture toughness, crack opening displacement, stress-separation curve, fracture energy, and size of the process zone near a crack tip. Issues of notch sensitivity and minimum size requirements for polycrystalline homogeneity were addressed. Both size and rate effects were observed, as well as how these two factors are interrelated in the fracture of columnar freshwater ice. There was a size effect at low rates but no size effect at high rates. There was a rate effect for the larger test sizes but a weaker or no rate effect for the smallest test size.

Observations of wave attenuation, scour, and subsurface pore pressures acrossbthree marsh restoration sill structures on a sandy bed

With rising sea levels and more frequent exposure to extreme storms, coastlines worldwide are vulnerable to increased erosion and loss of natural marsh lands. In an effort to lessen these impacts, there is a growing practice of adapting hard or “gray” coastline protection techniques to more nature-based features that promote habitat and ecosystem health. Living shoreline marsh restorations utilize natural and naturebased materials to protect marsh shores from erosion while also allowing intertidal flushing to promote the health and diversity of the marsh. Our study investigates three types of living shoreline sill designs exposed to average and storm-energy wave conditions at varying water levels. The sills were designed to mimic constructed sills in practice (rock, oyster shell, tree root wads), but more generally vary in structure porosity and material dissipation potential. Large-scale laboratory experiments were conducted in the large wave flume at the O.H. Hinsdale Wave Research Laboratory. Wave transmission and reflection are used to demonstrate wave attenuation capability of each sill structure. Scour of the sill, bedload sediment transport rates on the seaward and shoreward sides of the sill, and sediment pore-water vertical hydraulic gradients were used to demonstrate the potential for sediment transport and liquefaction. Results will contribute to understanding the effect of sill material porosity and mass on structure stability, and the effectiveness of using green living shoreline sill structures in the continued effort to establish design criteria for living shoreline implementation.

Establishment of Intrinsic Permeability of Coarse Open-Graded Materials: Review and Analysis of Existing Data from Natural Air Convection Tests

This paper presents a review and analysis of large-scale air convection tests and the establishment of intrinsic permeability in coarse open-graded materials. Natural air convection can make a significant contribution to heat transfer during cooling periods. In seasonally freezing environments this can result in excessive frost penetration and subsequent frost-related problems. Intrinsic permeability largely defines the onset of convective heat transfer in granular materials. Conventional methods for measuring intrinsic permeability cannot be applied to very coarse materials. Large-scale laboratory experiments on natural air convection can serve as an alternative method for determining this crucial parameter. This paper gives an overview of four different experimental test setups for measuring natural air convection, all differing in physical shape, boundary conditions and heat flux/temperature measurement devices. Comparison between these is difficult because the air convection pattern can differ and in some cases the shape and number of convection cells cannot be validated. Most of the studies available in the literature use theoretical equations to approximate intrinsic permeability. A method based on the analytical Nu-Ra number relationship is employed to establish the values of intrinsic permeability. Tests that provide enough data to enable the use of the Nu-Ra relationship are very limited. The overall results show a reasonable correlation between experiment-based intrinsic permeability and theoretical approximation. However, several issues must be addressed: first, differences may exist between the intrinsic permeability of natural and of crushed materials due to the shape effect. Second, the method used is in theory valid only for two-dimensional air convection within a square enclosure heated from below. Yet the results show that this method could be extended to other conditions with a certain degree of confidence. Third, a good estimate of intrinsic permeability is possible only with accurate experimental measurement.

Numerical investigation of monochromatic wave interaction with a vertical seawall located on a reef flat

Existing design codes for coastal defense structures built on coral reefs are scarce compared to those for traditional sandy or muddy beaches. A three-dimensional numerical wave flume is developed to investigate monochromatic wave interaction with a vertical seawall located on the reef flat. The Navier-Stokes equations for two-phase incompressible flow, combining with methods of Large Eddy Simulation (LES) for turbulence closure and Volume of Fluid (VOF) for tracking the free surface, are solved. A set of large-scale laboratory experiments are performed to validate the adopted model in terms of free surface, velocity, dynamic pressure as well as overtopping discharge around the seawall. The model is first applied to examine the cross-shore evolution of waves and flows associated with breaking wave interaction with the seawall. Subsequently, a series of numerical simulations are implemented to study the impacts of different hydrodynamic (incident wave height, incident wave period, and reef-flat wave level) as well as reef and seawall morphological factors (fore-reef slope, seawall position and seawall crest width) on the overtopping discharge over the seawall as well as the cross-shore wave impact force acting on the seawall. Finally, new empirical formulas are proposed, respectively, to predict the overtopping discharge and the wave force.

Does geographical origin affect lateralization and male mating success in Rhyzopertha dominica beetles?

The lesser grain borer, Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae), is an important pest of stored grains globally. In this study, the impact of geographical origin on male mating success and lateralization in three strains of R. dominica (i.e., a Greek strain, a Romanian strain and a Turkish strain) was investigated. Concerning the Greek strain, even though most males showed a left-biased approach, males performing right-biased copulation attempts achieved the highest proportion of successful copulations. Males of the Romanian and Turkish strains exhibited right-biased approaches and copulation attempts, resulting in higher mating success over the males that approached and performed copulation attempts from the left or backside of females. Copula was significantly longer than mate recognition and precopula phases. Furthermore, copula duration was significantly longer in the Turkish strain over the Romanian strain, while copula duration differences between Greek and Romanian individuals, as well as between Greek and Turkish individuals, were not significant. Unsuccessful sexual interactions were characterized by a longer precopula in Greek and Romanian strains, but not in the Turkish strain. Our results add basic knowledge to the mating behaviour of R. dominica, with emphasis on impact of the geographical origin and laterality on male mating success, which may be helpful for optimizing mass-rearing techniques of stored-product pests needed for large-scale laboratory or semi-field experiments, as well as to contribute to the development of behaviour-based control tools.

Nucleation of Stick-Slip Instability Within a Large-Scale Experimental Fault: Effects of Stress Heterogeneities Due to Loading and Gouge Layer Compaction.

Geodetic observations and large‐scale laboratory experiments show that seismic instability is preceded by slow slip within a finite nucleation zone. In laboratory experiments rupture nucleation is studied mostly using bare (rock) interfaces, whereas upper crustal faults are typically filled with gouge. To investigate effects of gouge on rupture nucleation, we performed a biaxial shearing experiment on a 350 mm long saw‐cut fault filled with gypsum gouge, at room temperature and a minimum horizontal stress σ 2 = 0.3–5 MPa. The gouge layer was sandwiched between polymethylmethacrylate (PMMA) plates For reference also a fault without gouge was deformed. Strain gauges and Digital Image Correlation were used to monitor the deformation field along the fault zone margins. Stick‐slip behavior occurred on both the gouge‐filled fault and the PMMA fault. Nucleation of instability on the PMMA fault persistently occurred from one location 2/3 to 3/4 along the fault adjacent to a slow slip zone at the fault end, but nucleation on the gouge‐filled fault was more variable, nucleating at the ends and/or at approximately 2/3 along the fault, with precursory slip occurring over a large fraction of the fault. Nucleation correlated to regions of high average fault stress ratio τ/σ n, which was more variable for the gouge‐filled fault due to small length scale variations in normal stress caused by heterogeneous gouge compaction. Rupture velocities and slip rates were lower for the gouge‐filled fault than for the bare PMMA fault. Stick‐slip persisted when σ 2 was lowered and the nucleation zone length increased, expanding from the center to the sample ends before transitioning into instability.