Evolution Of Velocity Research Articles

Turbulent disruption of density staircases in stratified shear flows

The formation of step-like ‘density staircase’ distributions induced by stratification and turbulence has been widely studied, and can be explained by the ‘instability’ of a sufficiently strongly stably stratified turbulent flow due to the decrease of the turbulent density flux with increasing stratification via the ‘Phillips mechanism’ (Phillips, Deep Sea Research and Oceanographic Abstracts, vol. 19, 1972, pp. 79–81. Elsevier). However, such density staircases are not often observed in ocean interiors, except in regions where double-diffusion processes are important, leading to thermohaline staircases. Using reduced-order models for the evolution of velocity and density gradients, we analyse staircase formation in stratified and sheared turbulent flows. Under the assumption of inertial scaling $\epsilon \sim U^3/L$ for the kinetic energy dissipation rate $\epsilon$ , where $U$ and $L$ are characteristic velocity and length scales, we determine ranges of bulk Richardson numbers ${Ri}_{b}$ and turbulent Prandtl numbers ${Pr}_{T}$ for which staircases can potentially form and show that the Phillips mechanism only survives in the limit of sufficiently small turbulent Prandtl numbers. For relevant oceanic parameters, a range of turbulent Prandtl numbers above which the system is not prone to staircases is found to be ${Pr}_T \simeq 0.5\unicode{x2013}0.8$ . Since several studies indicate that the turbulent Prandtl number in stably stratified turbulence and in ocean interiors is usually above this threshold, this result supports the empirical observation that staircases are not favoured in ocean interiors in the presence of relatively homogeneous and sustained turbulence. We also show that our analysis is robust to other scalings for $\epsilon$ (such as the more strongly stratified scaling $\epsilon \sim U^{2}N_{c}$ , where $N_{c}$ is a characteristic value of the buoyancy frequency), supporting our results in both shear-dominated and buoyancy-dominated turbulent regimes as well as in weakly and strongly stratified regimes.

Synthetic red supergiant explosion model grid for systematic characterization of Type II supernovae

Abstract A new model grid containing 228016 synthetic red supergiant explosions (Type II supernovae) is introduced. Time evolution of spectral energy distributions from 1 to 50000 Å (100 frequency bins in a log scale) is computed at each time step up to 500 d after explosion in each model. We provide light curves for the filters of Vera C, Rubin Observatory’s Legacy Survey of Space and Time (LSST), the Zwicky Transient Facility, the Sloan Digital Sky Survey, and the Neil Gehrels Swift Observatory, but light curves for any photometric filters can be constructed by convolving any filter response functions to the synthetic spectral energy distributions. We also provide bolometric light curves and photosphere information such as photospheric velocity evolution. The parameter space covered by the model grid is five progenitor masses (10, 12, 14, 16, and 18 M$_{\odot}$ at the zero-age main sequence, solar metallicity), ten explosion energies (0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 × 1051 erg), nine 56Ni masses (0.001, 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, and 0.3 M$_{\odot}$), nine mass-loss rates (10−5.0, 10−4.5, 10−4.0, 10−3.5, 10−3.0, 10−2.5, 10−2.0, 10−1.5, and 10−1.0 M$_{\odot}$ yr−1 with a wind velocity of 10 km s−1), six circumstellar matter radii (1, 2, 4, 6, 8, and 10 × 1014 cm), and ten circumstellar structures (β = 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0). 56Ni is assumed to be uniformly mixed up to the half-mass of a hydrogen-rich envelope. This model grid can be a base for rapid characterizations of Type II supernovae with sparse photometric sampling expected in LSST through a Bayesian approach, for example. The model grid is available at https://doi.org/10.5061/dryad.pnvx0k6sj.

Multi-scalar mixing in turbulent coaxial jets

Although natural and industrial flows often transport and mix multiple scalars, relatively few studies of turbulent multi-scalar mixing have been undertaken. In the present work, a novel three-wire thermal-anemometry-based probe – capable of simultaneously measuring velocity, helium concentration and temperature – is used to investigate the evolution of multiple scalars ($\phi _1$,$\phi _2$,$\phi _3$) and velocity in turbulent coaxial jets. The jets consist of (i) a centre jet containing a mixture of helium and air ($\phi _1$), (ii) an annular jet containing pure (unheated) air ($\phi _2$) and (iii) a coflow of (pure) heated air ($\phi _3$). Axial measurements are made at three different momentum flux ratios ($M=0.77, 2.1, 4.2$). Increasing$M$was observed to result in complex, competing effects. Larger momentum flux ratios cause the potential core of the centre jet to decrease in size, greater scalar fluctuations and more rapid correlation of$\phi _1$and$\phi _2$. However, at the same time, certain statistics, including those describing the velocity field, evolve more slowly. Moreover, the flow near the beginning of the fully merged region appears to be less mixed at higher values of$M$. The present work finally demonstrates that differences can be observed in the evolution and mixing of coaxial jets between those in which$M<1$and those in which$M>1$, thereby presenting an opportunity by which the mixing process in coaxial jets may be controlled.

Effects of sub‐mm cylindrical voids on detonation performance in PBX 9501

AbstractInternal features of varying scale and geometry are always present in explosives systems. Below a critical length, dependent on the explosive, these features can operate as a driving force for energy concentration and reaction, known as hotspots. At larger length scales, internal features result in jetting and detonation wave shaping, allowing for bulk work to be done by the explosive as is seen in shape charges. To date, a large volume of work has been performed to simulate hotspot ignition and large‐scale wave shaping. However, little experimental data exists on the effects of features in intermediate length scales, 0.1 to 1 millimeter. It has been observed in many tests that these small‐scale features can influence the high explosive (HE) performance and in some cases cause substantial damage to adjacent systems. This work provides quantitative data on the effects of machined voids moderately above more typical hotspot lengths, 0.3–0.8 mm in diameter, in PBX 9501 pressed to 1.785 g cm−3±2.5 mg cm−3. Streak imaging was used to visualize void collapse, jet velocity, re‐initiation time, and wave shape evolution. Delay in detonation front propagation time was found to be linearly dependent on the void diameter and jet velocity was found to be independent within the tested range and resolution. Cut‐back experiments were used to investigate wave shape distortion and evolution downstream of the void, showing consistent growth and decay shapes across all void sizes. Simulations using CTH, a hydrocode by Sandia National Laboratory, were used to investigate void collapse showing agreement with the trends of experimental results but yielded inaccurate wave shape development delay values. Both simulation and experimental results identified several re‐initiation mechanisms with jetting and subsequent double shocking of localized HE being the dominant mechanisms for the void sizes that were studied.

A hybrid projection/data-driven reduced order model for the Navier-Stokes equations with nonlinear filtering stabilization

We develop a Reduced Order Model (ROM) for the Navier-Stokes equations with nonlinear filtering stabilization. Our approach, that can be interpreted as a Large Eddy Simulation model, combines a three-step algorithm called Evolve-Filter-Relax (EFR) with a computationally efficient finite volume method. The main novelty of our ROM lies in the use within the EFR algorithm of a nonlinear, deconvolution-based indicator function that identifies the regions of the domain where the flow needs regularization. The ROM we propose is a hybrid projection/data-driven strategy: a classical Proper Orthogonal Decomposition Galerkin projection approach for the reconstruction of the velocity and the pressure fields and a data-driven reduction method to approximate the indicator function used by the nonlinear differential filter. This data-driven technique is based on interpolation with Radial Basis Functions. We test the performance of our ROM approach on two benchmark problems: 2D and 3D unsteady flow past a cylinder at Reynolds number 0≤Re≤100. The accuracy of the ROM is assessed against results obtained with the full order model for velocity, pressure, indicator function and time evolution of the aerodynamics coefficients.

Non-equilibrium strain induces hysteresis and anisotropy in the quasi-static and dynamic elastic behavior of sandstones: Theory and experiments

Materials with grain contacts or partially closed cracks exhibit anomalous elastic behavior: hysteresis in quasi-static experiments and slow dynamics in fast dynamic ones. Albeit the behavior in the two cases (which correspond to very different strain ranges) appears different, it should stem from the same physics and, thus, could be modeled by a universal equation of state. We propose a modification of the standard acoustoelastic theory, introducing the concept of conditioning induced non-equilibrium strain, which allows us to predict the evolution of elastic wave velocity in both quasi-static and dynamic ranges, including the velocity anisotropy induced by external uniaxial loading.

The evolution of fast turbulent deflagrations to detonations

We use advanced experimental techniques to explore turbulence-induced deflagration-to-detonation transition (tDDT) in hydrogen–air mixtures. We analyze the full sequence of turbulent flame evolution from fast deflagration-to-detonation using simultaneous direct measurements of pressure, turbulence, and flame, shock, and flow velocities. We show that fast turbulent flames that accelerate and develop shocks are characterized by turbulent flame speeds that exceed the Chapman–Jouguet deflagration speed in agreement with the tDDT theory and direct numerical simulation (DNS) results. Velocity and pressure evolutions are provided to detail the governing mechanisms that drive turbulent flame acceleration. Turbulent flame speeds and fluctuations are examined to reveal flow field characteristics of the tDDT process. This work contributes to the understanding of fundamental mechanisms responsible for spontaneous initiation of detonations by fast turbulent flames.

Energy partitioning in laser-induced millimeter-sized spherical cavitation up to the fourth oscillation

To investigate the energy partitioning up to the fourth oscillation of a millimeter-scale spherical cavitation bubble induced by laser, we used nanosecond laser pulses to generate highly spherical cavitation bubbles and shadowgraphs to measure the radius-time curve. Using the extended Gilmore model and considering the continuous condensation of the vapor in the bubble, the time evolution of the bubble radius, bubble wall velocity, and pressure in the bubble is calculated till the 4th oscillation. Using Kirkwood-Bethe hypothesis, the evolution of velocity and pressure of shock wave at the optical breakdown, the first and second collapses are calculated. The shock wave energy at the breakdown and bubble collapse is directly calculated by numerical method. We found the simulated radius-time curve fits well with experimental data for the first four oscillations. The energy partition at the breakdown is the same as that in previous studies, the ratio of shock wave energy to bubble energy is about 2:1. In the first collapse and the second collapse, the ratio of shock wave energy to bubble energy is 14.54:1 and 2.81:1 respectively. In the third and fourth collapses, the ratio is less, namely than 1.5:1 and 0.42:1 respectively. The formation mechanism of the shock wave at the collapse is analyzed. The breakdown shock wave is mainly driven by the expansion of the supercritical liquid resulting from the thermalization of the energy of the free electrons in the plasma, and the collapse shock wave is mainly driven by the compressed liquid around the bubble.

Type Ia supernovae SN 2013bz, PSN J0910 + 5003, and ASASSN-16ex: similar to 09dc-like?

ABSTRACT We present optical photometric and spectroscopic studies of three supernovae (SNe): SN 2013bz, PSN J0910 + 5003, and ASASSN-16ex (SN 2016ccj). UV–optical photometric data of ASASSN-16ex obtained with the Swift Ultraviolet/Optical Telescope (UVOT) are also analysed. These objects were initially classified as 09dc-like type Ia SNe. The decline-rate parameters (Δm15(B)true) are derived as 0.92 ± 0.04 (SN 2013bz), 0.70 ± 0.05 (PSN J0910 + 5003), and 0.73 ± 0.03 (ASASSN-16ex). The estimated B-band absolute magnitudes at maximum, −19.61 ± 0.20 mag for SN 2013bz, −19.44 ± 0.20 mag for PSN J0910 + 5003, and −19.78 ± 0.20 mag for ASASSN-16ex, indicate that all three objects are relatively bright. The peak bolometric luminosities for these objects are derived as $\log L_\text{bol}^\text{max} =$ 43.38 ± 0.07, 43.26 ± 0.07, and 43.40 ± 0.06 erg s−1, respectively. The spectral and velocity evolution of SN 2013bz is similar to that of a normal SN Ia, hence it appears to be a luminous, normal type Ia supernova. On the other hand, the light curves of PSN J0910 + 5003 and ASASSN-16ex are broad and exhibit properties similar to 09dc-like SNe Ia. Their spectroscopic evolution shows similarity with 09dc-like SNe: strong C ii lines are seen in the pre-maximum spectra of these two events. Their photospheric velocity evolution is similar to SN 2006gz. Further, in the UV bands, ASASSN-16ex is very blue, like other 09dc-like SNe Ia.

A partitioned model for predicting the spatial development of flow in ecological vegetated rivers with submerged vegetation

The evolution of the velocity caused by submerged vegetation in river courses with regular cross sections has been poorly investigated using analytical methods. Fully understanding the longitudinal profiles of the velocity is significant and essential to study the hydrodynamic processes taking place in wetland ecosystems, which is the most important indicator of energy transfer and material exchange in fluvial ecosystems. Based on the experimental data in the literature, the streamwise velocity gradually is found to decrease in the submerged vegetation and increase in the free surface layer. A four-panel analytical model derived from exponential decay theory was developed to predict the streamwise velocity occurring both upstream and within vegetation along the main flow direction in a rectangular open channel with a submerged vegetation patch. We aim at the development of velocity, which is averaged across the vegetation and free surface layers separately, and the length scales shown in the velocity evolution. The explicit calculation expression for each parameter in the analytical model was provided, and sensitivity analysis was conducted by changing the parameters to check how the predicted velocity changed. The change in the drag coefficient of vegetation (CD) and the velocity at the leading edge of the vegetation (U0) influenced the development of velocity. The former mainly enlarges the velocity difference between the vegetation and free surface layers after the leading edge as it increases, and the latter mainly diminishes the velocity difference before the leading edge as it increases. The error statistics is conducted and shows that the predicted and measured velocities are consistent within a relative error of 11%, indicating that the partitioned model is applicable in predicting longitudinal velocity in flows with submerged vegetation of various flexibility. The theoretical analysis of velocity development paves the way for the future studies on the flow turbulent structure and sediment transport in partially vegetated rivers.

Mass-loss effects on the non-Darcy seepage characteristics of broken rock mass with different clay contents

A seepage testing system was designed and a series of seepage experiments on broken rock was conducted using different original porosity conditions and clay contents. The mass-loss process of the broken rock and the change in water flow velocity were investigated. After the mass-loss test, the non-Darcy seepage characteristics of the broken rock were tested through a step-by-step pressure-reduced seepage test. The experimental results show that the mass-loss and water velocity evolution during the water inrush could be divided into four stages: acceleration, stable with slight fluctuations, reacceleration, and stable. The lost-mass and change in water velocity were positively correlated with the clay contents and negatively correlated with the original porosity. By introducing the evolution equation of the Kozeny-Carman equation and the liquid limit index which characterises the effective particle size, the prediction model of the permeability coefficient was built. Six prediction models of the non-Darcy coefficient were verified against the testing results. The prediction model of the critical flow velocity from a Darcy flow to a non-Darcy flow using the Forchheimer number was also established. The results could provide an important reference for understanding water inrush mechanisms, adopting effective control measures for water inrush events, and calculating the water influx of tunnels.

Experimental research and energy analysis of a new type of dry ice powder pneumatic rock breaking technology

When the traditional drill and blast method is applied to rock crushing projects, it has strong vibration, loud noise and dust pollution, so it cannot be used in densely populated areas such as urban public works. We developed a supercritical CO2 true triaxial pneumatic rock-breaking experimental system, and conducted laboratory and field tests of dry ice powder pneumatic rock-breaking. The characteristics of the blast-induced vibration velocity waveform and the evolution of the vibration velocity and frequency with the focal distance were analyzed and discussed. The fracturing mechanism of dry ice powder pneumatic rock breaking is studied. The research results show that: (1) The vibration velocity induced by dry ice powder pneumatic rock breaking decays as a power function with the increase of the focal distance; (2) The vibration frequency caused by dry ice powder pneumatic rock breaking is mainly distributed in 1–120 Hz. Due to the dispersion effect, the dominant frequency of 10–30 Hz appears abnormally attenuated; (3) The traditional CO2 phase change fracturing energy calculation formula is also applicable to dry ice pneumatic rock breaking technology, and the trinitrotoluene (TNT) equivalent of fracturing energy is applicable to the Sadovsky formula; (4) Dry ice powder pneumatic rock breaking is shock wave and high-energy gas acting together to fracture rock, which can be divided into three stages, among which the gas wedge action of high-energy gas plays a dominant role in rock mass damage.

Quantifying fabric anisotropy of granular materials using wave velocity anisotropy: a numerical investigation

Fabric anisotropy is a sought-after micro index to correlate macro mechanical responses of granular materials. In this work, the discrete-element method is utilised to simulate multi-directional bender element tests in granular soils to obtain the evolution of wave velocities during drained conventional triaxial (CT) and true triaxial (TT) tests; the contact normal-based fabric is simultaneously monitored for bridging the fabric anisotropy and wave velocity anisotropy. The results show that stress-normalised wave velocities and microscopic fabric, including contact normal distribution and coordination number, remain nearly constant until a stress ratio threshold is reached. After the threshold value is reached, stress-normalised wave velocities start to decrease, especially in the minor principal stress direction, accompanied by significant adjustment of coordination number and fabric anisotropy. The results also reveal that the normalised wave velocity depends on the contact normal densities in the wave propagation and particle oscillation directions. With the contact normal distribution represented by a density function, a good linear relationship between the microscopic fabric anisotropy and macroscopic wave velocity anisotropy is obtained for both CT and TT tests.

Migration of the Shear Margins at Thwaites Glacier: Dependence on Basal Conditions and Testability Against Field Data

AbstractProjections of global sea level depend sensitively on whether Thwaites Glacier, Antarctica, will continue to lose ice rapidly. Prior studies have focused primarily on understanding the evolution of ice velocity and whether the reverse‐sloping bed at Thwaites Glacier could drive irreversible retreat. However, the overall ice flux to the ocean and the possibility of irreversible retreat depend not only on the ice speed but also on the width of the main ice trunk. Here, we complement prior work by focusing specifically on understanding whether the lateral boundaries of the main ice trunk, termed shear margins, might migrate over time. We hypothesize that the shear margins at Thwaites Glacier will migrate on a decadal timescale in response to continued ice thinning and surface steepening. We test this hypothesis by developing a depth‐averaged, thermomechanical free‐boundary model that captures the complex topography underneath the glacier and solves for both the ice velocity and for the position of the shear margins. We find that both shear margins are prone to migration in response to ice thinning with basal strength and surface slope steepening determining their relative motion. We construct four end‐member cases of basal strength that represent different physical properties governing friction at the glacier bed and present two cases of ice thinning to contrast the effects of surface steepening and ice thinning. We test our model by hindcasting historic data and discuss how data from ongoing field campaigns could further be used to test our model.

Hydrodynamic effect and Fluid-Structure coupled vibration of underwater flexible caudal fin actuated by Macro fiber composites

Underwater bionic robotics or vehicles using oscillating flexible structures as power sources are attracting considerable attention. In this study, the hydrodynamic effect and fluid–structure coupled vibration of a flexible caudal fin actuated by macro fiber composites(MFC) are investigated. A fluid–structure coupled dynamic equation of the MFC-actuated caudal fin with a variable cross-section is established, and both the internal moment generated by the MFC and the complex-valued hydrodynamic forces are considered. Distributions and evolutions of the flow velocity and pressure in the vicinity of the oscillating caudal fin with different parameters are visualized through comprehensive parametric CFD simulations. Simulation results show that the presence of the gap introduces significant flow reshaping effects to the flow distribution and hydrodynamics. Distributions of the flow velocities and concentrated pressure regions depend highly on the gap width of the forked caudal fin. Then, a revised hydrodynamic function depending on three nondimensional governing parameters is developed, this is, the characteristic oscillating frequency, the width ratio, and the gap-to-width ratio. Formulas for the real and -imaginary parts of the complex-valued hydrodynamic function are developed respectively through the comprehensive CFD framework. Theoretical and experimental results show that the revised hydrodynamic function and developed model are capable of estimating the hydrodynamic effect and underwater dynamic response of the MFC-actuated caudal fin. Compared with the classical hydrodynamic function for the rectangular beam, the expression for the beam overestimates the added mass effect exerted on the caudal fin, and underestimates the hydrodynamic damping effect, thus leading to a smaller estimated resonant frequency and bandwidth, but a higher predicted response peak. These findings are useful for the robotic fish or autonomous underwater vehicles which are propelled by oscillating foils actuated by smart actuators.

Selection of solidification pathway in rapid solidification processes

Rapid solidification processing of alloys enables the formation of exotic nonequilibrium microstructures. However, the interrelationship between the processing parameters and the resulting microstructure is yet to be fully understood. In melt spinning (MS) and additive manufacturing (AM) of rapidly solidified alloys, opposite microstructure development sequences were observed. A fine-to-coarse microstructural transition is typically observed in melt-spun ribbons, whereas melt pools in AM exhibit a coarse-to-fine transition. In this paper, the microstructural evolutions during these two processes are investigated using phase-field modeling. The variation of all key variables of the solid-liquid interface (temperature, composition, and velocity) throughout the entire rapid solidification of AM and MS processes was acquired with high accuracy. It is found that the onset of nucleation determines the selection of the solidification pathway and, consequently, the evolution of temperature and velocity of the interface during the rapid solidification. The switching of control mechanisms of the solid-liquid interface, which happens in both processes but in opposite directions, is found to cause the velocity jump and disrupt the microstructure development.

Shock waves, overpressure, and spontaneous ignition of pressurized hydrogen in T-shaped tubes

The accidental release of high-pressure hydrogen into a tube can induce self-ignition, and special tube configurations can complicate their characteristics. Hence, this paper studies the shock waves and self-ignition in tubes with T-shaped structure. Results show that the shock wave intensity decreases after passing through the T-shaped region, and its intensity is lower in the branch pipeline. The flow field is extremely complex after shock waves enter the T-shaped region, accompanied by different shockwave structures (e.g. barrel shock and Mach disk), multiple reflections, and shock-shock interaction. In addition, the location of the bifurcation point affects not only the downstream velocity evolution but also the time of upstream pressure to reach the peak pressure. When the bifurcation point is the closest to the rupture disc, spontaneous ignition is least likely to take place. The flame intensity decreases after passing via a T-shaped zone. Besides, if the T-shaped zone is far from rupture disc, even extinguishment can occur.

Development of a calcium sulfoaluminate-Portland cement binary system for twin-pipe 3D concrete printing

Recently, the twin-pipe pumping (TPP) system has been developed for 3D concrete printing which involves pumping two mixtures, typically a Portland cement-based mixture and a pure limestone powder-based mixture that contains an accelerator. Following this, the two mixtures are allowed to be intermixed in a static mixer placed just prior to the nozzle which causes immediate stiffening and a very high buildability for the printed concrete post-extrusion. However, the flow division pattern produced by the mixing baffles of the static mixer results in striations consisting of unmixed limestone powder resulting in weak zones that lower the mechanical strength of the printed element. In this study, we present a TPP system consisting of Portland cement-based mixtures and a borated calcium sulfoaluminate (CSA) cement-based mixture. The mixtures are capable of reacting independently with water as compared to the pure limestone powder-based mixture. After getting blended in the static mixer right before extrusion, the retardation in the borated CSA cement is destroyed by the alkaline component contained in the other stream, leading to the rapid hardening and a high construction rate post extrusion from the nozzle. The hydration characteristics were studied using a Vicat penetrometer, isothermal calorimetry, and ultrasonic pulse velocimetry, while the early age mechanical behavior was characterized by using uniaxial unconfined compression and slow penetration tests. Results indicate that Portland cement alone cannot produce enough alkalinity to destroy the ulexite phase and reinitiate the hydration of CSA cement, while adding a nominal dosage of calcium hydroxide can provide enough increase in pH to design a system suitable for the stiffening control. However, abundant calcium hydroxide would reduce the hydration rate at later ages due to the dilution effect, as indicated by the P-wave velocity evolution and yield stress evolution. In addition, the hardened behavior of printed elements was studied by performing compression and flexural tests. Results show that the CSA-Portland cement binary binder system avoids the formation of weak zones and hence improves the mechanical behavior.

Characterization of Tensile and Fatigue Damages in Composite Structures Using Lamb Wave for Improved Structural Health Monitoring

The majority of ultrasonic characterizations are done on thermoplastics, with just a few articles available on the characterization of thermoset resin characteristics. A non-destructive methodology for monitoring fatigue and static deformation induced by mechanical loading on a fiber-reinforced plastic is presented. However, these materials’ dynamics of elastic waves are considerably more complicated. A large part was devoted to the calculation of dispersion curves of guided waves in composites. Therefore, this study presents a thorough description of the Glass/Epoxy system by comparing ultrasonic and mechanical data. Ultrasonic wave propagation at high frequencies, functioning as a dynamic mechanical deformation, may be utilized to calculate longitudinal and shear moduli during static and dynamic loading. The evolution of attenuation and velocity during loading is linked to the significant changes that occur during the aging process. The experimental transfer function is determined by the Fourier transform of all the obtained ultrasonic echoes.

Resolving minute temporal seismic velocity changes induced by earthquake damage: the more stations, the merrier?

SUMMARYGround shaking induced by earthquakes often introduces transient changes in seismic velocity monitored with ambient noise. These changes are usually attributed to relaxation behaviour following the coseismic damage in the subsurface and are of relevance for post-seismic hazard mitigation. However, the velocity evolution associated with this phenomenon can occur at very small timescales and amplitudes that are not resolved with seismic interferometry and are therefore challenging to link to laboratory experiments. A way to improve the temporal resolution of the velocity time-series is to test whether the estimation of the relative seismic velocity changes dv/v obeys the ergodic hypothesis in which the joint use of colocated stations would lead to better resolved measurements. In this study, we present results from a dense seismic array that was deployed for 2 weeks at the remarkable Patache site in Chile. Thanks to high temporal averaging capabilities, we are able to resolve seismic velocity changes in the 3–6 Hz frequency band at a 10-min resolution around the occurrence of a moderate earthquake (PGV ∼1 cm s–1). We report a velocity drop of ∼0.4 per cent in the first 10 min after ground shaking. Half of this initial drop was recovered within the 2 following days. The shape of the recovery follows a log-linear shape over the whole observed recovery phase, analogous to slow dynamics experiments. When normalized by the total amount of processed data, we show that the ergodic hypothesis almost perfectly holds in our network: the dv/v signal-to-noise ratio (SNR) obtained when averaging a few observation with large stacking durations for the correlation functions is almost equal to the SNR when using a large number of observations with small stacking durations. To understand if the ergodicity is linked to a particular site property, we use the array capabilities to identify the surf at the shoreline as the source of the noise and to derive a 1-D shear velocity profile with the focal spot imaging technique and a transdimensional Bayesian inversion framework. The inversion shows that hard rocks lie close to the surface indicating that this material hosts the observed shallow velocity changes. We discuss our high-resolution measurements and attribute them to a stable noise source excited by the shore, the ergodicity property and an ideal subsurface structure. Finally, we discuss the effect of moderate earthquakes on subsurface damage and the potential relaxation processes in hard rocks.