Successive Shock Research Articles

Acoustic Shock‐Induced Low Dielectric Loss in Glycine and Oxalic Acid‐Based Single Crystals

AbstractGlycinium oxalate (GO) and Bis(glycinium) oxalate (BGO) crystals are successfully grown using the slow evaporation solution growth technique. Following their growth, the crystals are subjected to a series of acoustic shock pulses. The effects of these shock pulses on the structural, optical, dielectric, and morphological properties of the crystals are comprehensively analyzed using various characterization techniques, including powder X‐ray diffraction (XRD), UV‐Visible spectroscopy, dielectric spectroscopy, and optical microscopy. Structural analysis through XRD reveals shifts in diffraction peak positions, indicating structural deformations. Fourier transform infrared spectroscopy analysis assesses the chemical stability of GO and BGO under shocked conditions. UV‐Visible spectroscopy shows alterations in optical transmission with successive shock pulses, attributed to structural and surface defects. Dielectric properties are investigated over a frequency range from 1 Hz to 1 MHz, revealing variations in dielectric constant and loss tangent, which provide insights into the electrical behavior of the materials under normal and shocked conditions. Optical and scanning electron microscopy examine surface morphology, visualizing defects induced by the shock pulses. This study highlights the significant impact of shock pulses on the structural properties, optical transmission, dielectric properties, and surface morphology of GO and BGO crystals, offering valuable information on their resilience under dynamic conditions and potential applications.

The impact of successive laser shock peening on surface integrity and residual stress distribution of laser powder-bed fused stainless steel 316L

Abstract The utilization of laser shock peening (LSP) in laser powder bed fused (LPBF) stainless steel (SS) 316L components enhances the mechanical characteristics and operational lifespan of the product quality through a significant reduction of residual stress and a noticeable increase in roughness parameters. The key objective of the study is to analyze the influence of consecutive laser shock peening (LSP) without ablative coating and low pulse energy on the surface properties, residual stress distribution, and microhardness of samples produced by LPBF with SS316L material. The surface quality of the sample subjected to consecutive laser shock peening shows a slight deterioration in its condition. This can be attributed to the combined impact of ablative surface and surface damage resulting from the production of high-energy plasma. However, the implementation of successive LSP results in a distinctive enhancement of compressive residual stresses (CRS) that are evenly distributed throughout the central axis and sharp edges. In contrast, the as-built condition exhibits non-uniform stress magnitudes. CRS observed in each LSP iteration exhibits a notable increase, reaching a maximum magnitude of −389 MPa compared to the initial stress level of 165 MPa in the as-built sample. This enhancement can be attributed to the repetitive impact of shock waves on the surface, leading to the formation of plastic deformation. The refinement of surface grains and the presence of favorable residual stresses were proven by the utilization of x-ray diffraction (XRD) studies and the Cos α plot. The XRD investigation also indicated the absence of any newly formed phases or secondary phases. A significant enhancement in microhardness was observed, with an increase of 58.3% achieved after the third consecutive peening process. The successive LSP samples displayed a gradual improvement in electrochemical behavior. Though the amplitude parameters increased after LSP, the increase in wear rate was observed.

Direct Numerical Simulation of Supersonic Gas Flow Through a Circular Cylindrical Channel

The results of the theoretical solution of the problem of braking a supersonic flow in a round pipe based on direct numerical simulation by integrating the Navier – Stokes equations without the use of additional models and empirical constants are shown. Shaded maps of density distribution depending on flow parameters are presented. The flow consists of successive rhombus-shaped shock waves distributed along the entire length of the channel. It is determined that the size of x-shaped structures depends on the flow parameters. At a lower Mach number, the rhombuses have a smaller size and, accordingly, their number increases along the length of the channel. The Reynolds number also affects the size of structures, however, it is less pronounced. With a lower Reynolds number, x-shaped structures have a smaller size. It is shown that over time the flow tends to a stationary state.

Same-sided successive-shock HED instability experiments

Inertial confinement fusion (ICF) and high-energy density (HED) physics experiments experience complicated forcing for instability growth and mix due to the ubiquitous presence of multiple shocks interacting with perturbations on multiple material interfaces. One common driver of instability growth is successive shocks from the same direction. However, there is a severe lack of analytic work and modeling validation for same-sided successive shocks since they are extremely difficult to achieve with conventional (non-HED) drivers. Successive shocks access a large instability parameter space; idealized fluid theory [K. O. Mikaelian, Phys. Rev. A 31, 410 (1985)] predicts 15 different interface evolution scenarios for a sinusoidal perturbation. Growth becomes more complex for multi-mode, compressible HED systems. The Mshock campaign is the first experiment in any fluid regime to probe a wide portion of successive shock parameter space. This is enabled by our development of a hybrid direct/indirect drive platform capable of creating independently controllable successive shocks on the National Ignition Facility. These experiments have delivered the first data capable of rigorously challenging our models and their ability to accurately capture Richtmyer–Meshkov growth under successive shocks. Single-mode and two-mode experiments have successfully demonstrated the ability to access and control the various growth scenarios of the shocked interface, including re-inversion, freeze out, and continued growth. Simulations and theoretical modeling are shown to accurately capture the experimental observations in the linear growth phase, giving us confidence in our ICF/HED design codes.

Characteristics of “Dust” Fluxes from the Surface of Copper and Lead Liners Exposed to One or Two Successive Shock Waves

Comparative experimental studies of the shock-induced particle ejection (“dusting”) from the free rough (Rz20) surface of copper and lead liners exposed to one or two successive shock waves separated in time by 0.2 μs have been carried out for the first time. This situation usually occurs in cumulative systems for the compression of the plasma by cylindrical or spherical liners shock or quasi-isentropically accelerated by explosion products. Using pulsed X-ray diffraction, laser optical recording, piezoelectric pressure sensors, and heterodyne interferometry, a qualitative picture has been studied and the quantitative characteristics of particle ejection from the free surface such as the velocities of the free surface and the particle flux front and the density (mass) distribution of particle flux in the direction of its motion, which are necessary for more accurate determination of features and the development of more appropriate models of the effect, have been evaluated.

Experimental study on a light–heavy interface evolution induced by two successive shock waves

Richtmyer–Meshkov instability induced by two successive shock waves is experimentally studied in a specific shock tube. To create two successive shock waves synchronously, a driver section is added between the driver and driven sections of the standard shock tube, and an electronically controlled membrane rupture equipment is adopted. The shock-tube flow after the membranes rupture is well described by combining the shock relations, isentropic wave relations with compatibility relations across the contact surface (region). The new shock tube is capable of generating two successive shock waves with controllable strengths and time interval, and provides a relatively ‘clean’ wave system. Then the developments of single-mode light–heavy interfaces with different initial conditions induced by two successive shock waves are investigated. The initial amplitudes are all small enough such that the first-shocked interface is within the linear growth regime at the arrival of the second shock. The results show that if the pre-second-shock perturbation amplitude is small, the linear, nonlinear and modal evolutions of the double-shocked interface can be reasonably predicted by the existing models proposed for predicting the perturbation growth induced by a single shock. For the double-shocked interface, the second shock provides an additional perturbation velocity field to the original one introduced by the first shock impact. The validity of the linear superposition model indicates that the linear superposition of these two perturbation velocity fields is satisfied. Therefore, a double-shocked interface evolves similarly to a single-shocked interface provided that their postshock amplitudes and linear growth rates are the same.

Finite-time singularity associated with the deflagration-to-detonation transition on the tip of an elongated flame-front in a tube

The critical condition for the deflagration to detonation transition (DDT) on the tip of elongated flames in tubes is revisited. Outstanding experiments and numerics, performed in 2010, have shown that a train of successive shock waves is produced by the self-accelerating flame-front. A pre-conditioned state is thus generated just prior to the DDT in a slice of unreacted gas adjacent to the flame front with a temperature too low for self-ignition, leaving unexplained the abrupt transition. Using a simplified one-dimensional model, the objective of the theoretical analysis of this short note is to identify the pre-conditioned state just prior to the spontaneous formation of a flow singularity on the flame front. This finite-time singularity responsible for the DDT is produced by the least further increase in propagation velocity beyond a critical value. The attention is focused on the unsteady compression waves emitted in the unreacted gas by the self-accelerating flame front. The unsteady effects were overlooked by the previous theoretical analyses.

Shock loads change the resistance, resiliency, and productivity of anaerobic co-digestion of municipal sludge and fats, oils, and greases

Accidental organic overloading (shock loading) is a common cause of failure in the anaerobic co-digestion of fats, oils, and greases (FOG). Nonetheless, questions still remain about how shock loads alter an anaerobic digester's biogas productivity and response to future organic overloads. To address this knowledge gap, this study utilized short increases in OLR, or shock loads, to adapt one reactor to FOG co-digestion and compare its response to that of a non-adapted digester tested at similar organic loads. A long-term study was also conducted where sequential shock events and disturbance-to-failure events (with organic loading rates ranging from 2.5 to 9.0 g VS/L/d) were employed to study their effects on reactor stability and performance. When exposed to moderate and large shock loads (3.0 and 9.0 g VS/L/d, respectively), the anaerobic digester previously exposed to FOG shock loads had greater levels of resistance and resiliency. For a moderate shock, it took the non-adapted reactor 29.8, 43, 44, and 19 days longer than the adapted reactor to recover to the baseline levels for methane content, organic acids (OA), pH, and mcrA concentrations, respectively. For a large shock, the adapted and non-adapted reactors saw more similar recovery times with the non-adapted reactor requiring an additional 11, 18, 19, and 0 days longer than the adapted reactor to recover to baseline levels for methane content, OA, pH, and mcrA concentrations, respectively. However, exposure to successive large shock loads decreased the anaerobic digester's resistance and resilience. The length of time it took for the organic acid accumulation and methane content values to return to their pre-shock values generally increased with each successive large shock load. For OA, the reactor required 35, 39, and 43 days to recover from the first, second, and third large shocks, respectively, while the methane content recovery times were 27.5, 26.5, and 34.5 days for the same shock periods. Thus, this work demonstrates that there is a tipping point in which FOG shock loads, whether intentional or accidental, go from improving the overall robustness of an anaerobic digester to significantly deteriorating its overall performance.

Comparative study on single-incident and dual-incident shock wave/turbulent boundary layer interactions with identical total deflection angle

Interactions between the boundary layer and two successive incident shock waves (ISWs) often occur in the supersonic mixed-compression inlets. However, the flow mechanism involved in such interactions has been studied rarely. In this study, we investigate experimentally and analytically the turbulent boundary layer separation flow induced by the single ISW and dual ISWs at the identical total deflection angles in a Mach 2.73 flow. Schlieren photography, wall pressure measurement and surface oil-flow visualisation are employed to diagnose the flow field. Experiments with the impingement points of the two ISWs intersecting on the bottom wall exhibit a separated flow with a triangle-like separation bubble, namely the first kind of dual-ISW/turbulent boundary layer interaction (ISWTBLI). Comparative studies show that various flow features of this kind of dual-ISWTBLI, including the extent of the separation region, pressure distribution and surface-flow topological structures, are nearly identical to those of the single-ISWTBLI with an identical total deflection angle. As the distance between the two ISWs increases, the shape of the separation region in the dual-ISWTBLI changes from triangle-like to quadrilateral-like, and the height of the separation region decreases accordingly, forming the second kind of dual-ISWTBLI. Furthermore, an inviscid model is developed for the dual-ISWTBLI to describe the complex shock wave system and elucidate the cause of a quadrilateral-like separation bubble in the second kind of dual-ISWTBLI. Moreover, based on a previous work by Souverein et al. (J. Fluid Mech., vol. 714, 2013, pp. 505–535) on the single-ISWTBLI, a modified scaling method is established for the first kind of dual-ISWTBLI.

Evolution of hole surface integrity in successive processing using sequential milling and laser shock peening

Part's surfaces are usually produced by successive/integration machining process and the performance of the part is directly tied to the generated surface integrity. Holes are commonly used in many critical engineering structures to connect parts, whose surface primarily characterizes a high risk of fretting or contact fatigue failure. In this paper, the evolution of metallurgical and mechanical surface integrity of Ti-6Al-4V alloy hole surface which machined using multi-step milling and successive laser shock peening (LSP) treatment was investigated. Firstly, a special workpiece with various hole surface pieces was designed and four different sequential milling processes were set to conduct the milling experiments. A comprehensive analysis of the evolution of surface roughness, microstructure, microhardness, and residual stress along the sequential processing steps was carried out. The results showed that the forms of surface integrity evolution could be classified into mono-evolution and coupling evolution, which were correlated to the combined effect of the milling parameters, cutter-workpiece engagement nature, milling sequence, milling mode, and the coolant/lubricant applied strategy. Mono-evolution was predominated in material removal sequences (milling sequences), while the coupling evolution was dependent on the radial depth of cut (DoC, ~50 μm). By comparison, coupling evolution was prominent in non-material removal related sequences (LSP treatment). Due to the conformal contact nature of tool-workpiece engagement and the flood cooling strategy, the microhardness in the subsurface layer was not significantly altered in milling sequences, except that the up-milling normally produced a surface with lower hardness and the down-milling produced almost unchanged surface hardness compared with bulk material. The residual stress was a result of the coupling effect of the residual stress field introduced in the previous and current machining sequences, whose in-depth distribution could be represented as an exponential decay function. The degree of influence of the machining sequences and their interactions on the final residual stresses could be evaluated by utilizing a set of weighting coefficients. This research indicated that understanding the effect of successive processing on the surface integrity evolution could help in planning of appropriate processes to achieve desirable surface integrity.

Diffusive Shock Acceleration by Multiple Weak Shocks

The intracluster medium (ICM) is expected to experience on average about three passages of weak shocks with low sonic Mach numbers, M ≲ 3, during the formation of galaxy clusters. Both protons and electrons could be accelerated to become high energy cosmic rays (CRs) at such ICM shocks via diffusive shock acceleration (DSA). We examine the effects of DSA by multiple shocks on the spectrum of accelerated CRs by including in situ injection/acceleration at each shock, followed by repeated re-acceleration at successive shocks in the test-particle regime. For simplicity, the accelerated particles are assumed to undergo adiabatic decompression without energy loss and escape from the system, before they encounter subsequent shocks. We show that in general the CR spectrum is flattened by multiple shock passages, compared to a single episode of DSA, and that the acceleration efficiency increases with successive shock passages. However, the decompression due to the expansion of shocks into the cluster outskirts may reduce the amplification and flattening of the CR spectrum by multiple shock passages. The final CR spectrum behind the last shock is determined by the accumulated effects of repeated re-acceleration by all previous shocks, but it is relatively insensitive to the ordering of the shock Mach numbers. Thus multiple passages of shocks may cause the slope of the CR spectrum to deviate from the canonical DSA power-law slope of the current shock.

Finite Element and Experimental Investigation on the Effect of Repetitive Shock in Corrugated Cardboard Packaging

The primary concern of the current study is estimating the repetitive shock induced damages leading to cumulative fatigue on corrugated cardboard boxes experimentally and numerically. Repetitive shock tests were performed on boxes using a vibration table to construct a Damage Boundary Curve (DBC). To computationally determine this curve, a finite element approach is proposed using an elastoplastic homogenization model for corrugated cardboard. The proposed model was implemented in the finite element software ABAQUS. Thanks to adopted model simplifications, a box can be easily and reliably modelled as a homogenized structure. A calibration method is used to compute a set of effective parameters in homogenized model in order to keep its behavior qualitatively and quantitatively close to the response of a full structural model. For verification, the identified model is used to simulate the box compression test. To replicate the experimental tests, simulations of successive repetitive shock pulses are carried with the proposed model for oligocyclique and limited endurance fatigue. To reduce computational costs, we propose a simple method for unlimited endurance fatigue by extrapolating a trend line after some training cycles. The proposed method shows good agreement with experimental results.

Fuel convergence sensitivity in indirect drive implosions

In inertial confinement fusion experiments at the National Ignition Facility, a spherical shell of deuterium–tritium fuel is imploded in an attempt to reach the conditions needed for fusion, self-heating, and eventual ignition. Since theory and simulations indicate that ignition efficacy in 1D improves with increasing imploded fuel convergence ratio, it is useful to understand the sensitivity of the scale-invariant fuel convergence on all measurable or inferable 1D parameters. In this paper, we develop a simple isobaric and isentropic compression scaling model incorporating sensitivity to the in-flight adiabat inferred from shock strengths, to measured implosion velocity, and to known initial ablator and fuel aspect ratio and mass ratio. The model is first benchmarked to 1D implosion simulations spanning a variety of relevant implosion designs. We then use the model to compare compressibility trends across all existing indirect-drive layered implosion data from the facility spanning three ablators [CH, carbon (C), and Be], for which in-flight fuel adiabats varied from 1.6 to 5 by varying the number of drive shocks from 2 to 4, peak implosion velocities varied by 1.4×, capsule radii by 50%, and initial fuel aspect ratios by 1.4×. We find that the strength of the first shock is the dominant contributor setting the maximum fuel convergence. We also observe additional sensitivities to successive shock strengths and fuel aspect ratios that improve the agreement between the expected and measured compression for carbon and Be designs with adiabats above 3. A principal finding is that the adiabat 2.5 C-shell designs exhibit less convergence than CH-shell designs of similar inferred in-flight adiabat.

Structural, optical and magnetic properties of silver oxide (AgO) nanoparticles at shocked conditions

Silver oxide nanoparticles (AgO NPs) were synthesized by implementing chemical reduction process and two different samples of AgO NPs were loaded with 100 as well as 200 pulses of successive shock waves of Mach number 2.2 so as to investigate the aftermath effect. Various characteristic analytical techniques were carried out for the samples utilizing Powder X-ray diffractometer (PXRD), Ultra-Violet diffuse reflectance spectroscope (UV-DRS), Field emission scanning electron microscope (FE-SEM) and Vibrating sample magnetometer (VSM) to find stabilities of crystallographic, spectroscopic, morphological and magnetic behaviors and also to analyze the changes occurring on the properties of the test material due to the impulsion of shock waves. From the obtained XRD patterns, the mean value of grain sizes of unshocked, 100 and 200 shocked AgO NPs is found to be 20, 20.6 and 26 nm, respectively. FE-SEM images reveal that the mean values of particle sizes increase as the counting of shock pulses are increased. The particles sizes are computed to be 72 nm, 112 nm, and 162 nm for the unshocked, 100 and 200 shocked AgO NPs, respectively. Whereas, the values of magnetization of AgO NPs have reduced with regard to the number of shock waves in such a way that the value of 2.183 emu/g for the unshocked AgO NPs is found to have reduced to 1.801 and 1.416 emu/g for 100 and 200 shocks, receptively. The band gap energy of the unshocked and shocked AgO NPs is observed to have the same value of 2.1 eV. From the experiential results, it is very well established that the title material’s properties such as crystalline and molecular structure, band gap energy are stable; whereas, the magnetic properties and surface morphology undergo subtle changes at shocked conditions.

Pressure-less joining of SiCf/SiC composites by Y2O3–Al2O3–SiO2 glass: Microstructure and properties

In this study, Y2O3–Al2O3–SiO2 (YAS) glass was prepared from Y2O3, Al2O3, and SiO2 micron powders. Thermal expansion coefficient of as-obtained YAS glass was about 3.9 × 10−6, matching-well with that of SiCf/SiC composites. SiCf/SiC composites were then brazed under pressure-less state by YAS glass and effects of brazing temperature on microstructures and properties of resulting joints were investigated. The results showed that glass powder in brazed seam sintered and precipitated yttrium disilicate, cristobalite, and mullite crystals after heat treatment. With the increase in temperature, joint layer gradually densified and got tightly bonded to SiCf/SiC composite. The optimal brazing parameter was recorded as 1400 °C/30 min and shear strength of the joint was 51.7 MPa. Formation mechanism of glass-ceramic joints was proposed based on combined analysis of microstructure and fracture morphology of joints brazed at different temperatures. Thermal shock resistance testing of joints was also carried out, which depicted decline in shear strength with the increase of thermal shock times. The strength of the joint after three successive thermal shock cycles at 1200 °C was 35.6 MPa, equivalent to 69% of that without thermal shock.

ВИЗНАЧЕННЯ ВПЛИВУ ТЕХНОЛОГІЧНИХ ЧИННИКІВ НА ПРОЦЕС ІНТЕНСИФІКАЦІЇ ПНЕВМОУДАРНОГО ШТАМПУВАННЯ ВИТЯГУВАННЯМ І ТОЧНІСТЬ ШТАМПОВАНИХ ДЕТАЛЕЙ

This work is devoted to experimental studies to determine the influence of technological factors on the process of intensification of pneumatic shock punching by drawing and accuracy of stamped products. The process of pneumatic shock extraction, as well as other impulse methods of extraction (explosive, electrohydraulic and magneto-pulse) is complicated by the high speed of deformation and, accordingly, by the occurrence of considerable inertial forces that counteract the movement of the flange of the extracted part. In the case of pneumatic shock reduction of the deformation rate can be achieved by adjusting the relative weight of the striker or by applying a two-stroke stretching. It is experimentally confirmed that the greatest influence on the limit values of the stamping ability in pneumatic impact stamping have the properties of the workpiece material, its surface properties, the scheme of the deformation process, the rate of deformation, the geometry of the matrix or punch, the value of contact friction and intensification techniques. Experiments have shown that the following factors have the greatest influence on the ultimate degree of drawing and the quality of the products: the condition of the surface and the physical and mechanical properties of the workpiece material; the radius of curvature of the working edge of the matrix; the relative thickness of the workpiece; workpiece loading rate (parameter α); the frictional properties of the matrix material and the workpiece; quality of greasing; technological scheme of operation. For two-stroke stretching, the stretching limit is increased by 8-10%. Also, when using two-stroke load, as shown by experiments, the amount of thinning in dangerous sections of the finished product is reduced. The reduction of radial deformation in the centre of the samples reaches 20-25%. The accuracy of pneumatic shock punching by a liquid is very high, since the deviation of the measured diameters does not exceed 0.1 mm. The surface quality of the pneumatic impacted products is determined only by the surface quality of the starting material. The pneumatic shock method of punching using a two-stage folded dash allows to increase the degree of drawing, accuracy and quality by means of a two-stroke loading, that is, to intensify the process of deep drawing of sheet products. The intensification is due to the successive double shock load for one cycle of the PUSH installation – the first blow is a formative, and the subsequent blow of the second stage is a calibration.

Correlation of hardness and surface microcracking in ITER specification tungsten exposed at QSPA Kh-50

In this work, we have investigated the evolution of the hardness of tungsten under successive thermal shock pulses induced under plasma exposure at quasi-stationary plasma accelerator QSPA-Kh50. The applied conditions represent localized modes of plasma instabilities expected under operation in ITER. The base temperature of 300 °C and deposited heat load of 0.45 MJ/m2 is known to be close to the cracking threshold, which is chosen in this study on purpose. Nanoindentation and microstructural characterization (identification of microcracks) is applied to the samples exposed to 10, 50, 70 and 100 pulses to reveal ability of nanoindentation technique to capture the threshold for the microcrack formation. Knowing that under the selected exposure conditions, the subsurface region of the material is a subject to the recrystallization, which is induced by the overheating during the plasma discharge, nanoindentation measurements are performed on the same tungsten grade annealed at 1300 °C, 1500 °C and 1800 °C to achieve different degree of recrystallization. It is shown that multiple microcracks appear after the 50th cycle which correlates with the reduction of the hardness corresponding to the massive grain growth. FEM analysis is applied to identify stress/temperature distribution across the sample depth to clarify the nucleation location, expected to occur in the region with the highest stress concentration close to the ductile to brittle transition temperature.

The accretion disc-jet connection in blazars

The power spectral density (PSD) of the X-ray emission variability from the accretion disc-corona region of black hole X-ray binaries and active galactic nuclei has a broken power law shape with a characteristic break time-scale. If the disc and the jet are connected, the jet variability may also contain a characteristic time-scale related to that of the disc-corona. Recent observations of the blazar Mrk 421 have confirmed the broken power law shape of the PSD of its jet X-ray variability. We model the time variability of a blazar, in which emitting particles are assumed to be accelerated by successive shock waves flowing down the jet with a varying inter-shock time-scale. We investigate the possible relation between the characteristic time-scales in the disc and jet variability based on the above model, along with mathematically and physically simulated disc variability. We find that both the PSD of the jet and disc variability may have a broken power law shape but the break time-scales are not related in general except only in systems with a small range of BH mass. The break in the jet and the disc PSD are connected to the interval between large amplitude outbursts in the jet (inter-shock time-scale) and to the viscous time-scale in the disc, respectively. In frequency bands where multiple emission processes are involved or emission is from lower energy particles, the break in the PSD may not be prominent enough for detection.

Smallholders' uneven capacities to adapt to climate change amid Africa's 'green revolution': Case study of Rwanda's crop intensification program.

Development programs and policies can influence smallholder producers' abilities to adapt to climate change. However, gaps remain in understanding how households' adaptive capacities can become uneven. This paper investigates how development transitions-such as the recent adoption of 'green revolution' agricultural policies throughout sub-Saharan Africa-intersect with cross-scale social-environmental processes to unevenly shape smallholders' adaptive capacities and adaptation pathways. Drawing on quantitative and qualitative material from a multi-season study in Rwanda, we investigate smallholder adaptation processes amid a suite of rural development interventions. Our study finds that adaptive capacities arise differentially across livelihood groups in the context of evolving environmental, social, and political economic processes. We show how social institutions play key roles in shaping differential adaptation pathways by enabling and/or constraining opportunities for smallholders to adapt livelihood and land use strategies. Specifically, Rwanda's Crop Intensification Program enables some wealthier households to adapt livelihoods by generating income through commercial agriculture. At the same time, deactivation of local risk management institutions has diminished climate risk management options for most households. To build and employ alternate livelihood practices such as commercial agriculture and planting woodlots for charcoal production, smallholders must negotiate new institutions, a prerequisite for which is access to capitals (land, labor, and nonfarm income). Those without entitlements to these are pulled deeper into poverty with each successive climatic shock. This illustrates that adaptive capacity is not a static, quantifiable entity that exists in households. We argue that reconceptualizing adaptive capacity as a dynamic, social-environmental process that emerges in places can help clarify complex linkages among development policies, livelihoods, and adaptation pathways. To ensure more equitable and climate-resilient agricultural development, we stress the need to reformulate policies with careful attention to how power structures and entrenched social inequalities can lead to smallholders' uneven capacities to adapt to climate change.

Disentangling the Circumnuclear Environs of Centaurus A. III. An Inner Molecular Ring, Nuclear Shocks, and the CO to Warm H2 Interface

Abstract We present the distribution and kinematics of the molecular gas in the circumnuclear disk (CND; 400 pc × 200 pc) of Centaurus A with resolutions of ∼5 pc (0.″3) and shed light onto the mechanism feeding the active galactic nucleus (AGN) using CO(3–2), HCO+(4–3), HCN(4–3), and CO(6–5) observations obtained with ALMA. Multiple filaments or streamers of tens to a hundred parsec scale exist within the CND, which form a ring-like structure with an unprojected diameter of 9″ × 6″ (162 pc × 108 pc) and a position angle P.A. ≃ 155°. Inside the nuclear ring, there are two leading and straight filamentary structures with lengths of about 30–60 pc at P.A. ≃ 120° on opposite sides of the AGN, with a rotational symmetry of 180° and steeper position–velocity diagrams, which are interpreted as nuclear shocks due to non-circular motions. Along the filaments, and unlike other nearby AGNs, several dense molecular clumps present low HCN/HCO+(4–3) ratios (≲0.5). The filaments abruptly end in the probed transitions at r ≃ 20 pc from the AGN, but previous near-IR H2(J = 1–0)S(1) maps show that they continue in an even warmer gas phase (T ∼ 1000 K), winding up in the form of nuclear spirals, and forming an inner ring structure with another set of symmetric filaments along the N–S direction and within r ≃ 10 pc. The molecular gas is governed primarily by non-circular motions, being the successive shock fronts at different scales where loss of angular momentum occurs, a mechanism that may feed efficiently powerful radio galaxies down to parsec scales.