An official website of the United States government
In this paper, nonlinearity associated with intense ultrasound is studied by using the one-dimensional motion of nonlinear shock wave in an ideal fluid. In nonlinear acoustics, the wave speed of different segments of a waveform is different, which causes distortion in the waveform and can result in the formation of a shock (discontinuity). Acoustic pressure of high-intensity waves causes particles in the ideal fluid to vibrate forward and backward, and this disturbance is of relatively large magnitude due to high-intensities, which leads to nonlinearity in the waveform. In this research, this vibration of fluid due to the intense ultrasonic wave is modeled as a fluid pushed by one complete cycle of piston. In a piston cycle, as it moves forward, it causes fluid particles to compress, which may lead to the formation of a shock (discontinuity). Then as the piston retracts, a forward-moving rarefaction, a smooth fan zone of continuously changing pressure, density, and velocity is generated. When the piston stops at the end of the cycle, another shock is sent forward into the medium. The variation in wave speed over the entire waveform is calculated by solving a Riemann problem. This study examined the interaction of shocks with a rarefaction. The flow field resulting from these interactions shows that the shock waves are attenuated to a Mach wave, and the pressure distribution within the flow field shows the initial wave is dissipated. The developed theory is applied to waves generated by 20 KHz, 500 KHz, and 2 MHz transducers with 50, 150, 500, and 1500 W power levels to explore the effect of frequency and power on the generation and decay of shock waves. This work enhances the understanding of the interactions of high-intensity ultrasonic waves with fluids.
more »
« less
Possibility of Fluid Flow Characterization via a Geophysical Signal: Experimental Study on the Krauklis Wave Under Fluid Flow
Abstract Krauklis waves are generated by pressure disturbances in fluid‐filled cavities and travel along the solid‐fluid interface. Their far‐field radiation, observed in seismic data from volcanoes or hydraulic fracturing, is known as long‐period events. Characterized by low velocity and resonance, Krauklis waves help estimate fracture size and discern fluids in saturated fractures. Despite numerous theoretical models analyzing Krauklis waves, the existing paradigms are founded on static flow conditions. However, in geological contexts, the assumption of static flow may not be valid. We developed an experimental apparatus using a tri‐layer model consisting of a pair of aluminum plates to examine the effect of fluid flow on Krauklis waves. We employed an infusion syringe pump to inject fluids into the fracture under different flow rates. We used water, oil, and an aqueous solution of Polyethylene glycol as fracture fluids. We calculated resonant frequency, phase velocity, and quality factor to characterize the Krauklis waves. Our findings reveal that an increase in flow rate leads to a higher phase velocity, higher quality factor, and a shift to higher resonant frequency when the flow is in the direction of initial wave propagation while decreasing amplitude. Additionally, when the flow is in the opposite direction of initial wave propagation, we note higher wave absorption and distortion of the Krauklis waves. Our observations unequivocally affirm that fluid flow leaves strong signatures on the Krauklis waves, providing a robust basis for characterizing fluid dynamics within geological settings through the analysis of Krauklis wave.
more »
« less
- Award ID(s):
- 2021768
- PAR ID:
- 10558796
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 129
- Issue:
- 12
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
ABSTRACT This work proposes a novel approach for coupling non‐isothermal fluid dynamics with fracture mechanics to capture thermal effects within fluid‐filled fractures accurately. This method addresses critical aspects of calculating fracture width in enhanced geothermal systems, where the temperature effects of fractures are crucial. The proposed algorithm features an iterative coupling between an interface‐capturing phase‐field fracture method and interface‐tracking thermo‐fluid‐structure interaction using arbitrary Lagrangian–Eulerian coordinates. We use a phase‐field approach to represent fractures and reconstruct the geometry to frame a thermo‐fluid‐structure interaction problem, resulting in pressure and temperature fields that drive fracture propagation. We developed a novel phase‐field interface model accounting for thermal effects, enabling the coupling of quantities specific to the fluid‐filled fracture with the phase‐field model through the interface between the fracture and the intact solid domain. We provide several numerical examples to demonstrate the capabilities of the proposed algorithm. In particular, we analyze mesh convergence of our phase‐field interface model, investigate the effects of temperature on crack width and volume in a static regime, and highlight the method's potential for modeling slowly propagating fractures.more » « less
-
null (Ed.)Abstract Seismic waves carrying tiny perturbing stresses can trigger earthquakes in geothermal and volcanic regions. The underlying cause of this dynamic triggering is still not well understood. One leading hypothesis is that a sudden increase in the fluid-pore pressure in the fault zone is involved, but the exact physical mechanism is unclear. Here, we report experimental evidence in which a fluid-filled fracture was shown to be able to amplify the pressure of an incoming seismic wave. We built miniature pressure sensors and directly placed them inside a thin fluid-filled fracture to measure the fluid pressure during wave propagation. By varying the fracture aperture from 0.2 to 9.2 mm and sweeping the frequency from 12 to 70 Hz, we observed in the lab that the fluid pressure in the fracture could be amplified up to 25.2 times compared with the incident-wave amplitude. Because an increase of the fluid pressure in a fault can reduce the effective normal stress to allow the fault to slide, our observed transient pressure surge phenomenon may provide the mechanism for earthquake dynamic triggering.more » « less
-
SUMMARY A better understanding of damage accumulation before dynamic failure events in geological material is essential to improve seismic hazard assessment. Previous research has demonstrated the sensitivity of seismic velocities to variations in crack geometry, with established evidence indicating that initial crack closure induces rapid changes in velocity. Our study extends these findings by investigating velocity changes by applying coda wave interferometry (CWI). We use an array of 16 piezoceramic transducers to send and record ultrasonic pulses and to determine changes in seismic velocity on intact and faulted Westerly granite samples. Velocity changes are determined from CWI and direct phase arrivals. This study consists of three sets of experiments designed to characterize variations in seismic velocity under various initial and boundary conditions. The first set of experiments tracks velocity changes during hydrostatic compression from 2 and 191 MPa in intact Westerly granite samples. The second set of experiments focuses on saw-cut samples with different roughness and examines the effects of confining pressure increase from 2 to 120 MPa. The dynamic formation of a fracture and the preceding damage accumulation is the focus of the third type of experiment, during which we fractured an initially intact rock sample by increasing the differential stress up to 780 MPa while keeping the sample confined at 75 MPa. The tests show that: (i) The velocity change for rough saw cut samples suggests that the changes in bulk material properties have a more pronounced influence than fault surface apertures or roughness. (ii) Seismic velocities demonstrate higher sensitivity to damage accumulation under increasing differential stress than macroscopic measurements. Axial stress measured by an external load cell deviates from linearity around two-third through the experiment at a stress level of 290 MPa higher than during the initial drop in seismic velocities. (iii) Direct waves exhibit strong anisotropy with increasing differential stress and accumulating damage before rock fracture. Coda waves, on the other hand, effectively average over elastic wave propagation for both fast and slow directions, and the resulting velocity estimates show little evidence for anisotropy. The results demonstrate the sensitivity of seismic velocity to damage evolution at various boundary conditions and progressive microcrack generation with long lead times before dynamic fracture.more » « less
-
Abstract Englacial water transport is an integral part of the glacial hydrologic system, yet the geometry of englacial structures remains largely unknown. In this study, we explore the excitation of fluid resonance by small amplitude waves as a probe of englacial geometry. We model a hydraulic network consisting of one or more tabular cracks that intersect a cylindrical conduit, subject to oscillatory wave motion initiated at the water surface. Resulting resonant frequencies and quality factors are diagnostic of fluid properties and geometry of the englacial system. For a single crack–conduit system, the fundamental mode involves gravity-driven fluid sloshing between the conduit and the crack, at frequencies between 0.02 and 10 Hz for typical glacial parameters. Higher frequency modes include dispersive Krauklis waves generated within the crack and tube waves in the conduit. But we find that crack lengths are often well constrained by fundamental mode frequency and damping rate alone for settings that include alpine glaciers and ice sheets. Branching crack geometry and dip, ice thickness and source excitation function help define limits of crack detectability for this mode. In general, we suggest that identification of eigenmodes associated with wave motion in time series data may provide a pathway toward inferring englacial hydrologic structures.more » « less
