Journal articles on the topic 'Pre-transitional boundary layer'

Transitional boundary layers exist on surfaces and bodies operating in viscous fluids at speeds such that the critical Reynolds number based on the distance from the leading edge is exceeded. The transition region is composed of a simultaneous mixture of both laminar and turbulent regimes occurring randomly in space and time. The turbulent regimes are known as turbulent spots, they grow rapidly with downstream distance, and they ultimately coalesce to form the beginning of fully-developed turbulent boundary-layer flow. It has been long suspected that such a region of unsteadiness may give rise to local pressure fluctuations and radiated sound that are different from those created by the fully-developed turbulent boundary layer at equivalent Reynolds number. This article reviews the available literature on this subject. The emphasis of this literature is on natural and artificially created transitional boundary layers under mostly incompressible conditions; hence, the word hydroacoustics in the title. The topics covered include the dynamics and local wall pressure fluctuations due to the passage of turbulent spots created in a deterministic way, the pressure fluctuations under transitioning boundary layers where the formation and location of spots are random, and the acoustic radiation from transition and its pre-cursor, the Tollmien-Schlichting waves. The majority of this review is for zero-pressure gradient flat plate flows, but the limited literature on axisymmetric body and plate flows with pressure gradient is included.

The response of the Blasius boundary layer to free-stream vortical disturbances of the convected gust type is studied. The vorticity signature of the boundary layer is computed through the boundary-region equations, which are the rigorous asymptotic limit of the Navier–Stokes equations for low-frequency disturbances. The method of matched asymptotic expansion is employed to obtain the initial and outer boundary conditions. For the case of forcing by a two-dimensional gust, the effect of a wall-normal wavelength comparable with the boundary-layer thickness is taken into account. The gust viscous dissipation and upward displacement due to the mean boundary layer produce significant changes on the fluctuations within the viscous region. The same analysis also proves useful for computing to second-order accuracy the boundary-layer response induced by a three-dimensional gust with spanwise wavelength comparable with the boundary-layer thickness. It also follows that the boundary-layer fluctuations of the streamwise velocity match the corresponding free-stream velocity component. The velocity profiles are compared with experimental data, and good agreement is attained.The generation of Tollmien–Schlichting waves by the nonlinear mixing between the two-dimensional unsteady vorticity fluctuations and the mean flow distortion induced by localized wall roughness and suction is also investigated. Gusts with small wall-normal wavelengths generate significantly different amplitudes of the instability waves for a selected range of forcing frequencies. This is primarily due to the disparity between the streamwise velocity fluctuations in the free stream and within the boundary layer.

AbstractDirect numerical simulations (DNS) are performed of a transient channel flow following a rapid increase of flow rate from an initially turbulent flow. It is shown that a low-Reynolds-number turbulent flow can undergo a process of transition that resembles the laminar–turbulent transition. In response to the rapid increase of flow rate, the flow does not progressively evolve from the initial turbulent structure to a new one, but undergoes a process involving three distinct phases (pre-transition, transition and fully turbulent) that are equivalent to the three regions of the boundary layer bypass transition, namely, the buffeted laminar flow, the intermittent flow and the fully turbulent flow regions. This transient channel flow represents an alternative bypass transition scenario to the free-stream-turbulence (FST) induced transition, whereby the initial flow serving as the disturbance is a low-Reynolds-number turbulent wall shear flow with pre-existing streaky structures. The flow nevertheless undergoes a ‘receptivity’ process during which the initial structures are modulated by a time-developing boundary layer, forming streaks of apparently specific favourable spacing (of about double the new boundary layer thickness) which are elongated streamwise during the pre-transitional period. The structures are stable and the flow is laminar-like initially; but later in the transitional phase, localized turbulent spots are generated which grow spatially, merge with each other and eventually occupy the entire wall surfaces when the flow becomes fully turbulent. It appears that the presence of the initial turbulent structures does not promote early transition when compared with boundary layer transition of similar FST intensity. New turbulent structures first appear at high wavenumbers extending into a lower-wavenumber spectrum later as turbulent spots grow and join together. In line with the transient energy growth theory, the maximum turbulent kinetic energy in the pre-transitional phase grows linearly but only in terms of ${u}^{\ensuremath{\prime} } $, whilst ${v}^{\ensuremath{\prime} } $ and ${w}^{\ensuremath{\prime} } $ remain essentially unchanged. The energy production and dissipation rates are very low at this stage despite the high level of ${u}^{\ensuremath{\prime} } $. The pressure–strain term remains unchanged at that time, but increases rapidly later during transition along with the generation of turbulent spots, hence providing an unambiguous measure for the onset of transition.

Abstract A recently developed transitional model for boundary-layer flows has been examined on a flat plate and the well-known S809 wind turbine blade. Proposed numerical model tries to simulate streamwise fluctuations, induced by freestream turbulence, in pre-transitional boundary-layer flows by introducing an additional transport equation for laminar kinetic energy term. This new approach can be used for modeling of transitional flows which are exposed to both the freestream turbulence intensity and streamwise pressure gradient, which are known as the most dominant factors in occurrence of transition. Computational method of this model is based on the solution of the Reynolds averaged Navier-Stokes (RANS) equations and the eddy-viscosity concept. The model includes three transport equations of laminar kinetic energy, turbulent kinetic energy and dissipation rate frequency. The present model is capable of predicting either natural or bypass transitional mechanisms, which may occur in attached boundary-layer flows. In addition, the model can simulate transition in the separated free shear layers and the subsequent turbulent re-attachment to form a laminar separation bubble. Flat plate was exposed to different freestream turbulence intensities and streamwise pressure gradients. Wind turbine blade was examined under two different Reynolds numbers, with one of them suitable for the occurrence of laminar separation bubbles on its surfaces. To evaluate the performance of this new model in resolving transitional boundary-layer flows, final results have been compared to those obtained through application of conventional turbulence models. Comparison of final results for the flat plate and the S809 aerofoil with available experimental data show very close agreements.

An overview is provided of various direct numerical simulations (DNS) of transitional flows in turbine-related geometries. Two flow cases are considered: the first case concerns separating flow over a flat plate and the second case flows in turbine cascades. In the first case, in which Re=60,000, either an oscillating oncoming flow (1) or a uniform flow with and without oncoming turbulent free-stream fluctuations (2) is prescribed at the inlet. In both subcases (1) and (2), separation is induced by a contoured upper wall. In (1), the separated boundary layer is found to roll up due to a Kelvin-Helmholtz (KH) instability. This rolled-up shear layer is subject to spanwise instability and disintegrates rapidly into turbulent fluctuations. In (2), a massive separation bubble is obtained in the simulation without oncoming free-stream fluctuations. A KH instability is eventually triggered by numerical round-off error and is followed again by a rapid transition. With oncoming turbulent fluctuations, this KH instability is triggered much earlier and transition is enhanced, which leads to a drastic reduction in size of the separation bubble. The second case, concerning flow in turbine cascades, includes (1) flow in the T106 turbine cascade with periodically oncoming wakes at Re=51,800 and (2) flow and heat transfer in a MTU cascade with oncoming wakes and background turbulence at Re=72,000. In the simulation of flow in the T106 cascade with oncoming wakes, the boundary layer along the downstream half of the suction side is found to separate intermittently and subsequently rolls up due to a KH instability leading to separation-induced transition. At times when the wakes impinge separation is suppressed. In the simulations of flow around a MTU turbine blade, evidence of by-pass transition in the suction-side boundary-layer flow is observed while the pressure-side boundary layer remains laminar in spite of significant fluctuations present. In agreement with the experiments, the impinging wakes cause the heat transfer coefficient to increase significantly in the transitional suction-side region close to the trailing edge and by about 30% on the pressure side. The large increase in heat transfer in the pre-transitional suction-side region observed in the experiments could not be reproduced. The discrepancy is explained by differences in spectral contents of the turbulence in the oncoming wakes.

AbstractIt is very important to predict the bypass transition in the simulation of flows through turbomachinery. This paper presents a four-equation eddy-viscosity turbulence transition model for prediction of bypass transition. It is based on the SST turbulence model and the laminar kinetic energy concept. A transport equation for the non-turbulent viscosity is proposed to predict the development of the laminar kinetic energy in the pre-transitional boundary layer flow which has been observed in experiments. The turbulence breakdown process is then captured with an intermittency transport equation in the transitional region. The performance of this new transition model is validated through the experimental cases of T3AM, T3A and T3B. Results in this paper show that the new transition model can reach good agreement in predicting bypass transition, and is compatible with modern CFD software by using local variables.

This paper is concerned with the effect of free-stream turbulence on the pretransitional flat-plate boundary layer. It is assumed that either the turbulent Reynolds number or the downstream distance (or both) is small enough that the flow can be linearized. The dominant disturbances in the boundary layer, which are of the Klebanoff type, are governed by the linearized unsteady boundary-region equations, i.e. the linearized Navier–Stokes equations with the streamwise derivatives neglected in the viscous and pressure-gradient terms. The turbulence is represented as a superposition of vortical free-stream Fourier modes and the corresponding Fourier component solutions to the boundary-region equations are obtained numerically. The results are then superposed to compute the root mean square of the fluctuating streamwise velocity in the boundary layer produced by the actual free-stream turbulence. It is found that the disturbances computed with isotropic free-stream turbulence do not reach the levels measured in experiments. However, good quantitative agreement is obtained with the relatively low turbulent Reynolds number data of Kendall when the measured strong anisotropy of the low-frequency portion of his spectrum is accounted for. Data at higher turbulent Reynolds numbers are affected by nonlinearity, which manifests itself through the generation of small spanwise length scales. We attempt to model this within the context of the linear theory by choosing a free-stream spectrum whose energy is concentrated at larger transverse wavenumbers and achieve very good agreement with the data. The results suggest that even small deviations from pure isotropy can be an important factor in explaining the large amplitudes of the Klebanoff modes in the pre-transitional boundary layer, and also point to the importance of nonlinear effects. We discuss some additional effects that may need to be accounted for in order to obtain a complete description of the Klebanoff modes.

The objective of this investigation is to give experimental support to recent direct numerical simulation (DNS) results which demonstrated that in bypass transition the flow first breaks down to turbulence on the low-speed streaks (or so-called negative jets) that are lifted up towards the boundary-layer edge region. In order to do this, wall-normal profiles of the streamwise fluctuation velocity are presented in terms of maximum positive and negative values over a range of turbulence intensities (1.3–6%) and Reynolds numbers for zero pressure gradient flow upstream of, and including, transition onset. For all turbulence intensities considered, it was found that the peak negative fluctuation velocity increased in magnitude above the peak positive fluctuations and their positions relative to the wall shifted as transition onset approached; the peak negative value moved towards the boundary-layer edge and the peak positive value moved toward the wall. An experimental measure of the penetration depth (PD) of free-stream disturbances into the boundary layer has been gained through the use of the skewness function. The penetration depth (measured from the boundary-layer edge) scales as PD ∝ (ω Rexτw)−0.3), where ω is the frequency of the largest eddies in the free stream, Rex is the Reynolds number of the flow based on the streamwise distance from the leading edge and τw is the wall shear stress. The parameter dependence demonstrated by this scaling compares favourably with recent solutions to the Orr–Sommerfeld equation on the penetration depth of disturbances into the boundary layer. The findings illustrate the importance of negative fluctuation velocities in the transition process, giving experimental support to suggestions from recent DNS predictions that the breakdown to turbulence is initiated on the low-speed regions of the flow in the upper portion of the boundary layer. The representation of pre-transitional disturbances in time-averaged form is shown to be inadequate in elucidating which disturbances grow to cause the breakdown to turbulence.

We study the nonlinear development and secondary instability of steady and unsteady Görtler vortices which are excited by free-stream vortical disturbances (FSVD) in a boundary layer over a concave wall. The focus is on low-frequency (long-wavelength) components of FSVD, to which the boundary layer is most receptive. For simplification, FSVD are modelled by a pair of oblique modes with opposite spanwise wavenumbers $\pm k_{3}$, and their intensity is strong enough (but still of low level) that the excitation and evolution of Görtler vortices are nonlinear. For the general case that the Görtler number $G_{\unicode[STIX]{x1D6EC}}$ (based on the spanwise wavelength $\unicode[STIX]{x1D6EC}$ of the disturbances) is $O(1)$, the formation and evolution of Görtler vortices are governed by the nonlinear unsteady boundary-region equations, supplemented by appropriate upstream and far-field boundary conditions, which characterize the impact of FSVD on the boundary layer. This initial-boundary-value problem is solved numerically. FSVD excite steady and unsteady Görtler vortices, which undergo non-modal growth, modal growth and nonlinear saturation for FSVD of moderate intensity. However, for sufficiently strong FSVD the modal stage is bypassed. Nonlinear interactions cause Görtler vortices to saturate, with the saturated amplitude being independent of FSVD intensity when $G_{\unicode[STIX]{x1D6EC}}\neq 0$. The predicted modified mean-flow profiles and structure of Görtler vortices are in excellent agreement with several steady experimental measurements. As the frequency increases, the nonlinearly generated harmonic component $(0,2)$ (which has zero frequency and wavenumber $2k_{3}$) becomes larger, and as a result the Görtler vortices appear almost steady. The secondary instability analysis indicates that Görtler vortices become inviscidly unstable in the presence of FSVD with a high enough intensity. Three types of inviscid unstable modes, referred to as sinuous (odd) modes I, II and varicose (even) modes I, are identified, and their relevance is delineated. The characteristics of dominant unstable modes, including their frequency ranges and eigenfunctions, are in good agreement with experiments. The secondary instability is intermittent when FSVD are unsteady and of low frequency. However, the intermittence diminishes as the frequency increases. The present theoretical framework, which allows for a detailed and integrated description of the key transition processes, from generation, through linear and nonlinear evolution, to the onset of secondary instability, represents a useful step towards predicting the pre-transitional flow and transition itself of the boundary layer over a blade in turbomachinery.

The ability to predict boundary layer transition locations accurately on turbomachinery airfoils is critical both to evaluate aerodynamic performance and to predict local heat-transfer coefficients with accuracy. Here we report on an effort to include empirical transition models developed in Part I of this report in a Reynolds averaged Navier-Stokes (RANS) solver. To validate the new models, two-dimensional design optimizations utilizing transitional RANS simulations were performed to obtain a pair of low-pressure turbine airfoils with the objective of increasing airfoil loading by 25%. Subsequent experimental testing of the two new airfoils confirmed pre-test predictions of both high and low Reynolds number loss levels. In addition, the accuracy of the new transition modeling capability was benchmarked with a number of legacy cascade and low-pressure turbine (LPT) rig data sets. Good agreement between measured and predicted profile losses was found in both cascade and rig environments. However, use of the transition modeling capability has elucidated deficiencies in typical RANS simulations that are conducted to predict component performance. Efficiency-versus-span comparisons between rig data and multi-stage steady and time-accurate LPT simulations indicate that loss levels in the end wall regions are significantly under predicted. Possible causes for the under-predicted end wall losses are discussed as well as suggestions for future improvements that would make RANS-based transitional simulations more accurate.

Here we report on an effort to include an empirically based transition modeling capability in a Reynolds Averaged Navier-Stokes solver. Well known empirical models for both attached- and separated-flow transition were tested against cascade data and found unsuitable for use in turbomachinery design. Consequently, a program was launched to develop models with sufficient accuracy for use in design. As a first step, accurate prediction of free stream turbulence development was identified as a prerequisite for accurate modeling. Additionally, a demonstrated capability to capture the effects of free stream turbulence on pre-transitional boundary layers became an impetus for the work. A computational fluid dynamics (CFD)-supplemented database of 104 experimental cascade cases was constructed to explore the development of new correlations. Dimensional analyses were performed to guide the work, and appropriate non-dimensional parameters were then extracted from CFD predictions of the laminar boundary layers existing on the airfoil surfaces prior to either transition onset or incipient separation. For attached-flow transition, onset was found to occur at a critical ratio of the boundary-layer diffusion time to a time scale associated with the energy-bearing turbulent eddies. In the case of separated-flow transition, it was found that the length of a separation bubble prior to turbulent reattachment was a simple function of the local momentum thickness at separation and the overall surface length traversed by a fluid element prior to separation. Both the attached- and separated-flow transition models were implemented into the design system as point-like trips.

Abstract. Aerosol particle number size distribution and chemical composition were measured at two low altitude sites, one urban and one relatively pristine valley, in Central Nepal during the 2009 pre-monsoon season (May–June). This is the first time that aerosol size distribution and chemical composition were measured simultaneously at lower elevation in the Middle Himalayan region in Nepal. The aerosol size distribution was measured using a Scanning Mobility Particle Sizer (SMPS, 14~340 nm), and the chemical composition of the filter samples collected during the field campaign was analyzed in the laboratory. Teflon membrane filters were used for ion chromatography (IC) and water-soluble organic carbon and nitrogen analysis. Quartz fiber filters were used for organic carbon and elemental carbon analysis. Multi-lognormal fits to the measured aerosol size distribution indicated a consistent larger mode around 100 nm which is usually the oldest, most processed background aerosol. The smaller mode was located around 20 nm, which is indicative of fresh but not necessarily local aerosol. The diurnal cycle of the aerosol number concentration showed the presence of two peaks (early morning and evening), during the transitional period of boundary layer growth and collapse. The increase in number concentration during the peak period was observed for the entire size distribution. Although the possible contribution of local emissions in size ranges similar to the larger mode cannot be completely ruled out, another plausible explanation is the mixing of aged elevated aerosol in the residual layer during the morning period as suggested by previous studies. Similarly, the evening time concentration peaks when the boundary layer becomes shallow concurrent with increase in local activity. A decrease in aerosol number concentration was observed during the nighttime with the development of cold (downslope) mountain winds that force the low level warmer air in the valley to rise. The mountain valley wind mechanisms induced by the topography along with the valley geometry appear to have a strong control in the diurnal cycle of the aerosol size distribution. During the sampling period, the chemical composition of PM2.5 was dominated by organic matter at both sites. Organic carbon (OC) comprised the major fraction (64~68%) of the aerosol concentration followed by ionic species (24~26% mainly SO42- and NH4+). Elemental Carbon (EC) compromised 7~10% of the total composition. A large fraction of OC was found to be water soluble (nearly 27% at both sites).

Abstract. Aerosol particle number size distribution and chemical composition were measured at two low altitude sites, one urban and one relatively pristine valley, in Central Nepal during the 2009 pre-monsoon season (May–June). This is the first time that aerosol size distribution and chemical composition were measured simultaneously at lower elevations in the middle Himalayan region in Nepal. The aerosol size distribution was measured using a Scanning Mobility Particle Sizer (SMPS, 14–340 nm), and the chemical composition of the filter samples collected during the field campaign was analyzed in the laboratory. Teflon membrane filters were used for ion chromatography (IC) and water-soluble organic carbon and nitrogen analysis. Quartz fiber filters were used for organic carbon and elemental carbon analysis. Multi-lognormal fits to the measured aerosol size distribution indicated a consistent larger mode around 100 nm which is usually the oldest, most processed background aerosol. The smaller mode was located around 20 nm, which is indicative of fresh but not necessarily local aerosol. The diurnal cycle of the aerosol number concentration showed the presence of two peaks (early morning and evening), during the transitional periods of boundary layer growth and collapse. The increase in number concentration during the peak periods was observed for the entire size distribution. Although the possible contribution of local emissions in size ranges similar to the larger mode cannot be completely ruled out, another plausible explanation is the mixing of aged elevated aerosol in the residual layer during the morning period as suggested by previous studies. Similarly, the evening time concentration peaks when the boundary layer becomes shallow concurrent with increase in local activity. A decrease in aerosol number concentration was observed during the nighttime with the development of cold (downslope) mountain winds that force the low level warmer air in the valley to rise. The mountain valley wind mechanisms induced by the topography along with the valley geometry appear to have a strong control in the diurnal cycle of the aerosol size distribution. During the sampling period, the chemical composition of PM2.5 was dominated by organic matter at both sites. Organic carbon (OC) comprised the major fraction (64–68%) of the aerosol concentration followed by ionic species (24–26%, mainly SO42− and NH4+). Elemental Carbon (EC) compromised 7–10% of the total composition and 27% of OC was found to be water soluble at both sites. The day-to-day variability observed in the time series of aerosol composition could be explained by the synoptic scale haze that extended to the sampling region from the Indian Gangetic Plain (IGP), and rainfall occurrence. In the presence of regional scale haze during dry periods, the mean volume aerosol concentration was found to increase and so did the aerosol mass concentrations.

This paper presents analyses of particle image velocimetry measurements from a boundary layer on a flat plate subject to grid-generated free-stream turbulence. The pre-transition region and early stages of breakdown to turbulent spots are explored by means of quadrant analysis and quadrant hole analysis. By isolating the contributors to the Reynolds shear stresses, it is possible to identify coherent structures within the flow that are responsible for the production of TKE. It is found that so called ‘ejection’ events are the most significant form of disturbance, exhibiting the largest amplitude behaviour with increased negative spanwise vorticity. Sweep events become increasingly large close to the wall with increased Reynolds number and intermittency.

Streaks in pre-transitional boundary layers are analysed and their properties are extracted from direct numerical simulation data. Streaks that induce breakdown to turbulence via secondary instability are shown to differ from the remainder of the population in various attributes. Conditionally averaged flow fields establish that they are situated farther away from the wall, and have a larger cross-sectional area and higher peak amplitude. The analysis also shows that the momentum thickness acts as a similarity parameter for the properties of the streaks. Probability density functions of the streak amplitude, area, and shear along the streaks, collapse among the various pressure gradients when plotted as a function of the momentum thickness. A prediction scheme for laminar–turbulent transition based on artificial neural networks is presented, which can identify the streaks that will eventually induce the formation of turbulent spots. In comparison to linear stability theory, the approach achieves a higher prediction accuracy at considerably lower computational cost.

AbstractThe influence of harmonic spanwise wall motion on bypass transition in boundary layers is investigated using direct numerical simulations. It is shown that the appropriate choice of the forcing parameters can achieve a substantial stabilization of the laminar flow regime. However, an increase of the forcing amplitude or period beyond their optimal values diminishes the stabilizing effect, and leads to breakdown upstream of the unforced case. For the optimal wall-oscillation parameters, the reduction in propulsion power substantially outweighs the power requirement of the forcing. The mechanism of transition delay is examined in detail. Analysis of the pre-transitional streaks shows that the wall oscillation substantially reduces their average amplitude, and eliminates the most energetic streaks. As a result, the secondary instabilities that precede breakdown to turbulence are substantially weakened – an effect demonstrated by linear stability analyses of flow fields from direct numerical simulations. The outcome is transition delay owing to a significant reduction in the frequency of occurrence of turbulent spots and a downstream shift in their average inception location. Finally, it is shown that the efficiency of the forcing can be further improved by replacing the sinusoidal time dependence of the wall oscillation with a square wave.

Opposition control of artificially initiated turbulent spots in a laminar boundary layer was carried out in a low-turbulence wind tunnel with the aim to delay transition to turbulence by modifying the turbulent structure within the turbulent spots. The timing and duration of control, which was carried out using wall-normal jets from a spanwise slot, were pre-determined based on the baseline measurements of the transitional boundary layer. The results indicated that the high-speed region of the turbulent spots was cancelled by opposition control, which was replaced by a carpet of low-speed fluid. The application of the variable-interval time-averaging technique on the velocity fluctuation signals demonstrated a reduction in both the burst duration and intensity within the turbulent spots, but the burst frequency was increased.

The objective of the present research is to develop new fundamental knowledge of the entropy generation process in laminar flow with significant fluctuations (called pre-transition) and during transition prematurely induced by strong freestream turbulence (bypass transition). Results of direct numerical simulations are employed. In the pre-transitional boundary layer, the perturbations by the streaky structures modify the mean velocity profile and induce a “quasi-turbulent” contribution to indirect dissipation. Application of classical laminar theory leads to underprediction of the entropy generated. In the transition region the pointwise entropy generation rate (S′′′)+ initially increases near the wall and then decreases to correspond to the distribution predicted for a fully-turbulent boundary layer as the flow progresses downstream. In contrast to a developed turbulent flow, the term for turbulent convection in the turbulence kinetic energy balance is significant and can play an important role in some regions of the transitioning boundary layer. More turbulent energy is produced than dissipated and the excess is convected downstream as the boundary layer grows. Since it is difficult to measure and predict true turbulent dissipation rates (and hence, entropy generation rates) exactly other than by expensive direct numerical simulations, a motivation for this research is to evaluate approximate methods for possible use in experiments and design. These new results demonstrate that an approximate technique, used by many investigators, overestimates the dissipation coefficient Cd by up to seventeen per cent. For better predictions and measurements, an integral approach accounting for the important turbulent energy flux is proposed and validated for the case studied.

AbstractThe nature of the formation of sub-grain boundaries within sequential lateral solidification (SLS) processed thin silicon films has been examined using single crystal Si films as a pre- cursor material. Experimental details include the use of an excimer laser projection system and straight-slit beamlets to produce directionally solidified microstructures. Within the SLS processed silicon-on-insulator (SOI) films, three microstructurally distinct regions are identi-fied: (1) an initial planar defect-free area (the extent of which can depend on laser fluence and orientation); (2) a transitional area within which the sub-grain boundaries appear and propagate in a well-defined direction relative to the crystallographic orientation; and (3) a final area characterized by sub-grain boundaries aligning approximately to the scan direction, and the in-plane texture becoming more random. We discuss the results within the context of a plastic deformation model of sub-grain boundary formation.

AbstractPharmacologically disrupting fear memory reconsolidation dramatically reduces fear behaviour. For example, 2–3 min of tarantula exposure followed by 40 mg of propranolol HCl (i.e., a reconsolidation intervention) abruptly decreased spider avoidance, an effect that persisted one year later. However, the success of reconsolidation interventions is not guaranteed: Pavlovian fear-conditioning research shows that the window to target memory reconsolidation is small and easy to miss. If exposure is too long to trigger reconsolidation, but too short for extinction learning, an inactive transitional limbo state occurs, rendering the fear memory unchanged and insensitive to amnesic agents. In this pre-registered study, we aimed to find this behaviourally-controlled boundary condition. Spider-fearful participants underwent a ~3 min (n = 23) or ~14 min (n = 20) exposure to a tarantula, intended to trigger reconsolidation or the limbo state respectively, followed by 40 mg of propranolol. We expected greater spider fear reduction after 3 than 14 min of exposure. Unexpectedly, there were no group differences on any outcome measures. In both groups, Bayesian analysis revealed a marked reduction in fear behaviour towards a generalisation stimulus (a house spider) accompanied by lower self-reported distress, with a sharp decline in spider fear scores two days after treatment that persisted one year later. Possible explanations include that the boundary conditions of reconsolidation are wider in older and stronger memories than experimentally-induced fears, or that alternative processes caused the treatment effects. Although the mechanism is unclear, these results carry a tentative promising message for the potential of brief reconsolidation-targeting interventions to mitigate irrational fears.

The Velké Vrbno Dome crops-out at the boundary between the Brunovistulian Terrane and the internal parts of the Bohemian Massif. Here, eclogite boudins occur within an Ediacaran volcano-sedimentary sequence. Strong Nb depletion (Nb/Nb* = 0.19 – 0.82) combined with moderately positive Nd isotopic compositions (εNd(i) = +3.89 – +5.77) are used to argue for emplacement of the eclogite protoliths in a transitional supra-subduction to continental-rift setting. Conversely, heterogeneously enriched large ion lithophile elements and highly radiogenic Sr isotopic ratios (87Sr/86Sr = 0.705–0.720) are interpreted to have been modified following fluid infiltration subsequent to eclogite-facies metamorphism.U-Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircon from the rift-type eclogite indicates Early Cambrian emplacement (c.535 Ma) following episodic Ediacaran volcanic arc activity. Moreover, a continental setting is emphasised by zircon dating of a mylonitic orthogneiss, revealing a fragment of Palaeoproterozoic (c.2000 Ma) basement, the first such finding within the Brunovistulian Terrane sensu stricto.The new data from eclogite confirm that rifting in this segment of Gondwana pre-dated the Ordovician opening of the Rheic Ocean and therefore that the suture between Brunovistulia and the rest of the Bohemian Massif likely represents the vestige of an older hyperextended basin or oceanic tract.Supplementary material: Previously unpublished single zircon evaporation ages from Ediacaran orthogneiss from the Velké Vrbno Dome (supplement A); detailed analytical methodology (supplement B); whole rock geochemical data (supplement C); and U-Pb LA-ICP-MS zircon data (supplement d). https://doi.org/10.6084/m9.figshare.c.5233079

Setting the Mood Weirdly, everything feels the same. There’s absolutely no distinction for me between news, work, walking, gaming, Netflix, rock collecting, scrolling, messaging. I don’t know how this happened, but everything has simply blurred together. There’s a dreadful and yet soothing sameness to it, scrolling through images on Instagram, scrolling Netflix, walking the dog, scrolling the news, time scrolling by as I watch face after face appear or disappear on my screen, all saying something, yet saying nothing. Is this the rhythm of crisis in a slow apocalypse? Really, would it be possible for humans to just bore themselves into oblivion? Because in the middle of a pandemic, boredom feels in my body the same as doom ... just another swell that passes, like my chest as it rises and falls with my breath. This opening anecdote comes from combining narratives in two studies we conducted online during the COVID-19 pandemic in 2020: a global study, Massive and Microscopic Sensemaking: Autoethnographic Accounts of Lived Experience in Times of Global Trauma; and an Australian project, The Shut-In Worker: Working from Home and Digitally-Enabled Labour Practices. The Shut-In Worker project aimed to investigate the thoughts, beliefs, and experiences of Australian knowledge workers working from home during lockdown. From June to October 2020, we recruited twelve households across two Australian states. While the sample included households with diverse incomes and living arrangements—from metropolitan single person apartment dwellers to regional families in free standing households—the majority were relatively privileged. The households included in this study were predominantly Anglo-Australian and highly educated. Critically, unlike many during COVID-19, these householders had maintained their salaried work. Participating households took part in an initial interview via Zoom or Microsoft Teams during which they took us on workplace tours, showing us where and how the domestic had been requisitioned for salaried labour. Householders subsequently kept digital diaries of their working days ahead of follow up interviews in which we got them to reflect on their past few weeks working from home with reference to the textual and photographic diaries they had shared with us. In contrast to the tight geographic focus of The Shut-In Worker project and its fairly conventional methodology, the Massive and Microscopic Sensemaking project was envisaged as a global project and driven by an experimental participant-led approach. Involving more than 150 people from 26 countries during 2020, the project was grounded in autoethnography practice and critical pedagogy. Over 21 days, we offered self-guided prompts for ourselves and the other participants—a wide range of creative practitioners, scholar activists, and researchers—to explore their own lived experience. Participants with varying degrees of experience with qualitative methods and/or autoethnography started working with the research questions we had posed in our call; some independently, some in collaboration. The autoethnographic lens used in our study encouraged contributors to document their experience from and through their bodies, their situated daily routines, and their relations with embedded, embodied, and ubiquitous digital technologies. The lens enabled deep exploration and evocation of many of the complexities, profound paradoxes, fears, and hopes that characterise the human and machinic entanglements that bring us together and separate the planetary “us” in this moment (Markham et al. 2020). In this essay we draw on anecdotes and narratives from both studies that speak to the “Zoom experience” during COVID-19. That is, we use Zoom as a socio-technical pivot point to think about how the experience of liminality—of being on/off screen and ambiently in between—is operating to shift both our micro practices and macro structures as we experience and struggle within the rupture, “event”, and conjuncture that marks the global pandemic. What we will see is that many of those narratives depict disjointed, blurry, or confusing experiences, atmospheres, and affects. These liminal experiences are entangled in complex ways with the distinctive forms of commercial infrastructure and software that scaffold video conferencing platforms such as Zoom. Part of what is both enabling and troubling about the key proprietary platforms that increasingly host “public” participation and conversation online (and that came to play a dominant role during COVID19) in the context of what Tarleton Gillespie calls “the internet of platforms” is a sense of the hidden logics behind such platforms. The constant sense of potential dis/connection—with home computers becoming ambient portals to external others—also saw a wider experience of boundarylessness evoked by participants. Across our studies there was a sense of a complete breakdown between many pre-existing boundaries (or at least dotted lines) around work, school, play, leisure and fitness, public and media engagement, and home life. At the same time, the vocabulary of confinement and lockdown emerged from the imposition of physical boundaries or distancing between the self and others, between home and the outside world. During the “connected confinement” of COVID-19, study participants commonly expressed an affective sensation of dysphoria, with this new state of in betweenness or disorientation on and off screen, in and out of Zoom meetings, that characterises the COVID-19 experience seen by many as a temporary, unpleasant disruption to sociality as usual. Our contention is that, as disturbing as many of our experiences are and have been during lockdown, there is an important, ethically and politically generative dimension to our global experiences of liminality, and we should hold on to this state of de-normalisation. Much ink has been spilled on the generalised, global experience of videoconferencing during the COVID-19 pandemic. A line of argument within this commentary speaks to the mental challenge and exhaustion—or zoom fatigue as it is now popularly termed—that many have been experiencing in attempting to work, learn, and live collectively via interactive screen technologies. We suggest zoom fatigue stands in for a much larger set of global social challenges—a complex conjuncture of microscopic ruptures, decisions within many critical junctures or turning points, and slow shifts in how we see and make sense of the world around us. If culture is habit writ large, what should we make of the new habits we are building, or the revelations that our prior ways of being in the world might not suit our present planetary needs, and maybe never did? Thus, we counter the current dominant narrative that people, regions, and countries should move on, pivot, or do whatever else it takes to transition to a “new normal”. Instead, drawing on the work of Haraway and others interested in more than human, post-anthropocenic thinking about the future, this essay contends that—on a dying planet facing major global challenges—we need to be embracing liminality and “staying with the trouble” if we are to hope to work together to imagine and create better worlds. This is not necessarily an easy step but we explore liminality and the affective components of Zoom fatigue here to challenge the assumption that stability and certainty is what we now need as a global community. If the comfort experienced by a chosen few in pre-COVID-19 times was bought at the cost of many “others” (human and more than human), how can we use the discomfort of liminality to imagine global futures that have radically transformative possibilities? On Liminality Because liminality is deeply affective and experienced both individually and collectively, it is a difficult feeling or state to put into words, much less generalised terms. It marks the uncanny or unstable experience of existing between. Being in a liminal state is marked by a profound disruption of one’s sense of self, one’s phenomenological being in the world, and in relation to others. Zoom, in and of itself, provokes a liminal experience. As this participant says: Zoom is so disorienting. I mean this literally; in that I cannot find a solid orientation toward other people. What’s worse is that I realize everyone has a different view, so we can’t even be sure of what other people might be seeing on their screen. In a real room this would not be an issue at all. The concept of liminality originally came out of attempts to capture the sense of flux and transition, rather than stasis, that shapes culture and community, exemplified during rites of passage. First developed in the early twentieth century by ethnographer and folklorist Arnold van Gennep, it was later taken up and expanded upon by British anthropologist Victor Turner. Turner, best known for his work on cultural rituals and rites of passage, describes liminality as the sense of “in betweenness” experienced as one moves from one status (say that of a child) to another (formal recognition of adulthood). For Turner, community life and the formation of societies more broadly involves periods of transition, threshold moments in which both structures and anti-structures become apparent. Bringing liminality into the contemporary digital moment, Zizi Papacharissi discusses the concept in collective terms as pertaining to the affective states of networked publics, particularly visible in the development of new social and political formations through wide scale social media responses to the Arab Spring. Liminality in this context describes the “not yet”, a state of “pre-emergence” or “emergence” of unformed potentiality. In this usage, Papacharissi builds on Turner’s description of liminality as “a realm of pure possibility whence novel configurations of ideas and relations may arise” (97). The pandemic has sparked another moment of liminality. Here, we conceptualise liminality as a continuous dialectical process of being pushed and pulled in various directions, which does not necessarily resolve into a stable state or position. Shifting one’s entire lifeworld into and onto computer screens and the micro screens of Zoom, as experienced by many around the world, collapses the usual functioning norms that maintain some degree of distinction between the social, intimate, political, and work spheres of everyday life. But this shift also creates new boundaries and new rules of engagement. As a result, people in our studies often talked about experiencing competing realities about “where” they are, and/or a feeling of being tugged by contradictory or competing forces that, because they cannot be easily resolved, keep us in an unsettled, uncomfortable state of being in the world. Here the dysphoric experiences associated not just with digital liminality but with the broader COVID-19 epidemiological-socio-political conjuncture are illustrated by Sianne Ngai’s work on the politics of affect and “ugly feelings” in the context of capitalism’s relentlessly affirmative culture. Rather than dismissing the vague feelings of unease that, for many of us, go hand in hand with late modern life, Ngai suggests that such generalised and dispersed affective states are important markers of and guides to the big social and cultural problems of our time—the injustices, inequalities, and alienating effects of late capitalism. While critical attention tends to be paid to more powerful emotions such as anger and fear, Ngai argues that softer and more nebulous forms of negative affect—from envy and anxiety to paranoia—can tell us much about the structures, institutions, and practices that frame social action. These enabling and constraining processes occur at different and intersecting levels. At the micro level of the screen interface, jarring experiences can set us to wondering about where we are (on or off screen, in place and space), how we appear to others, and whether or not we should showcase and highlight our “presence”. We have been struck by how people in our studies expressed the sense of being handled or managed by the interfaces of Zoom or Microsoft Teams, which frame people in grid layouts, yet can shift and alter these frames in unanticipated ways. I hate Zoom. Everything about it. Sometimes I see a giant person, shoved to the front of the meeting in “speaker view” to appear larger than anyone else on the screen. People constantly appear and disappear, popping in and out. Sometimes, Zoom just rearranges people seemingly randomly. People commonly experience themselves or others being resized, frozen, or “glitched”, muted, accidentally unmuted, suddenly disconnected, or relegated to the second or third “page” of attendees. Those of us who attend many meetings as a part of work or education may enjoy the anonymity of appearing at a meeting without our faces or bodies, only appearing to others as a nearly blank square or circle, perhaps with a notation of our name and whether or not we are muted. Being on the third page of participants means we are out of sight, for better or worse. For some, being less visible is a choice, even a tactic. For others, it is not a choice, but based on lack of access to a fast or stable Internet connection. The experience and impact of these micro elements of presence within the digital moment differs, depending on where you appear to others in the interface, how much power you have over the shape or flow of the interaction or interface settings, or what your role is. Moving beyond the experience of the interface and turning to the middle range between micro and macro worlds, participants speak of attempting to manage blurred or completely collapsed boundaries between “here” and “there”. Being neither completely at work or school nor completely at home means finding new ways of negotiating the intimate and the formal, the domestic and the public. This delineation is for many not a matter of carving out specific times or spaces for each, but rather a process of shifting back and forth between makeshift boundaries that may be temporal or spatial, depending on various aspects of one’s situation. Many of us most likely could see the traces of this continuous shifting back and forth via what Susan Leigh Star called “boundary objects”. While she may not have intended this concept in such concrete terms, we could see these literally, in the often humorous but significantly disruptive introduction of various domestic actants during school or work, such as pets, children, partners, laundry baskets, beds, distinctive home decor, ambient noise, etc. Other trends highlight the difficulty of maintaining zones of work and school when these overlap with the rest of the physical household. One might place Post-it Notes on the kitchen wall saying “I’m in a Zoom meeting so don’t come into the living room” or blur one’s screen background to obscure one’s domestic location. These are all strategies of maintaining ontological security in an otherwise chaotic process of being both here and there, and neither here nor there. Yet even with these strategies, there is a constant dialectical liminality at play. In none of these examples do participants feel like they are either at home or at work; instead, they are constantly shifting in between, trying to balance, or straddling physical and virtual, public and private, in terms of social “roles” and “locations”. These negotiations highlight the “ongoingness” of and the labour involved in maintaining some semblance of balance within what is inherently an unbalanced dialectical process. Participants talked about and showed in their diaries and pictures developed for the research projects the ways they act through, work with, or sometimes just try to ignore these opposing states. The rise of home-based videoconferencing and associated boundary management practices have also highlighted what has been marginalised or forgotten and conversely, prioritised or valorised in prior sociotechnical assemblages that were simply taken for granted. Take for example the everyday practices of being in a work versus domestic lifeworld; deciding how to handle the labor of cleaning cups and dishes used by the “employees” and “students” in the family throughout the day, the tasks of enforcing school attendance by children attending classes in the family home etc. This increased consciousness—at both a household and more public level—of a previously often invisible and feminised care economy speaks to larger questions raised by the lockdown experience. At the same time as people in our studies were negotiating the glitches of screen presence and the weird boundarylessness of home-leisure-domestic-school-work life, many expressed an awareness of a troubling bigger picture. First, we had just the COVID lockdowns, you know, that time where many of us were seemingly “all together” in this, at home watching Tiger King, putting neighborly messages in our windows, or sharing sourdough recipes on social media. Then Black Lives Matters movements happened. Suddenly attention is shifted to the fact that we’re not all in this together. In Melbourne, people in social housing towers got abruptly locked down without even the chance to go to the store for food first, and yet somehow the wealthy or celebrity types are not under this heavy surveillance; they can just skip the mandatory quarantine. ... We can’t just go on with things as usual ... there are so many considerations now. Narratives like these suggest that while 2020 might have begun with the pandemic, the year raised multiple other issues. As many things have been destabilised, the nature or practice of everyday life is shifting under our feet. Around the world, people are learning how to remain more distanced from each other, and the rhythms of temporal and geographic movement are adapting to an era of the pandemic. Simultaneously, many people talk about an endlessly arriving (but never quite here) moment when things will be back to normal, implying not only that this feeling of uncertainty will fade, but also that the zone of comfort is in what was known and experienced previously, rather than in a state of something radically different. This sentiment is strong despite the general agreement that “we will never [be able to] go back to how it was, but [must] proceed to some ‘new normal’”. Still, as the participant above suggests, the pandemic has also offered a much broader challenge to wider, taken-for-granted social, political, and economic structures that underpin late capitalist nations in particular. The question then becomes: How do we imagine “moving on” from the pandemic, while learning from the disruptive yet critical moment it has offered us as a global community? Learning from Liminality I don’t want us to go back to “normal”, if that means we are just all commuting in our carbon spitting cars to work and back or traveling endlessly and without a care for the planet. COVID has made my life better. Not having to drive an hour each way to work every day—that’s a massive benefit. While it’s been a struggle, the tradeoff is spending more time with loved ones—it’s a better quality of life, we have to rethink the place of work. I can’t believe how much more I’ve been involved in huge discussions about politics and society and the planet. None of this would have been on my radar pre-COVID. What would it mean then to live with as well as learn from the reflexive sense of being and experience associated with the dis-comforts of living on and off screen, a Zoom liminality, if you will? These statements from participants speak precisely to the budding consciousness of new potential ways of being in a post-COVID-19 world. They come from a place of discomfort and represent dialectic tensions that perhaps should not be shrugged off or too easily resolved. Indeed, how might we consider this as the preferred state, rather than being simply a “rite of passage” that implies some pathway toward more stable identities and structured ways of being? The varied concepts of “becoming”, “not quite yet”, “boundary work”, or “staying with the trouble”, elaborated by Karen Barad, Andrew Pickering, Susan Leigh Star, and Donna Haraway respectively, all point to ways of being, acting, and thinking through and with liminality. All these thinkers are linked by their championing of murky and mangled conceptions of experience and more than human relations. Challenging notions of the bounded individual of rational humanism, these post-human scholars offer an often-uncomfortable picture of being in and through multiplicity, of modes of agency born out of a slippage between the one and the many. While, as we noted above, this experience of in betweenness and entanglement is often linked to emotions we perceive as negative, “ugly feelings”, for Barad et al., such liminal moments offer fundamentally productive and experimental modalities that enable possibilities for new configurations of being and doing the social in the anthropocene. Further, liminality as a concept potentially becomes radically progressive when it is seen as both critically appraising the constructed and conventional nature of prior patterns of living and offering a range of reflexive alternatives. People in our studies spoke of the pandemic moment as offering tantalizing glimpses of what kinder, more caring, and egalitarian futures might look like. At the same time, many were also surprised by (and skeptical of) the banality and randomness of the rise of commercial platforms like Zoom as a “choice” for being with others in this current lifeworld, emerging as it did as an ad hoc, quick solution that met the demands of the moment. Zoom fatigue then also suggests a discomfort about somehow being expected to fully incorporate proprietary platforms like Zoom and their algorithmic logics as a core way of living and being in the post-COVID-19 world. In this sense the fact that a specific platform has become a branded eponym for the experience of online public communicative fatigue is telling indeed. The unease around the centrality of video conferencing to everyday life during COVID-19 can in part be seen as a marker of anxieties about the growing role of decentralized, private platforms in “replacing or merging with public infrastructure, [thereby] creating new social effects” (Lee). Further, jokes and off-hand comments by study participants about their messy domestic interiors being publicized via social media or their boss monitoring when they are on and offline speak to larger concerns around surveillance and privacy in online spaces, particularly communicative environments where unregulated private platforms rather than public infrastructures are becoming the default norm. But just as people are both accepting of and troubled by a growing sense of inevitability about Zoom, we also saw them experimenting with a range of other ways of being with others, from online cocktail parties to experimenting with more playful and creative apps and platforms. What these participants have shown us is the need to “stay with the trouble” or remain in this liminal space as long as possible. While we do not have the space to discuss this possibility in this short provocation, Haraway sees this experimental mode of being as involving multiple actants, human and nonhuman, and as constituting important work in terms of speculating and figuring with various “what if” scenarios to generate new possible futures. As Haraway puts it, this process of speculative figuring is one of giving and receiving patterns, dropping threads, and so mostly failing but sometimes finding something that works, something consequential and maybe even beautiful, that wasn’t there before, of relaying connections that matter, of telling stories in hand upon hand, digit upon digit, attachment site upon attachment site, to craft conditions for flourishing in terran worlding. This struggle of course takes us far beyond decisions about Zoom, specifically. This deliberately troubling liminality is a process of recognizing old habits, building new ones, doing the hard work of reconsidering broader social formations in a future that promises more trouble. Governments, institutions, corporate entities, and even social movements like Transition Towns or #BuildBackBetter all seem to be calling for getting out of this liminal zone, whether this is to “bounce back” by returning to hyper-consumerist, wasteful, profit-driven modes of life or the opposite, to “bounce forward” to radically rethink globalization and build intensely localized personal and social formations. Perhaps a third alternative is to embrace this very transitional experience itself and consider whether life on a troubled, perhaps dying planet might require our discomfort, unease, and in-betweenness, including acknowledging and sometimes embracing “glitches” and failures (Nunes). Transitionality, or more broadly liminality, has the potential to enhance our understanding of who and what “we” are, or perhaps more crucially who “we” might become, by encompassing a kind of dialectic in relation to the experiences of others, both intimate and distant. As many critical commentators before us have suggested, this necessarily involves working in conjunction with a rich ecology of planetary agents from First People’s actors and knowledge systems--a range of social agents who already know what it is to be liminal to landscapes and other species--through and with the enabling affordances of digital technologies. This is an important, and exhausting, process of change. And perhaps this trouble is something to hang on to as long as possible, as it preoccupies us with wondering about what is happening in the lines between our faces, the lines of the technologies underpinning our interactions, the taken for granted structures on and off screen that have been visibilized. We are fatigued, not by the time we spend online, although there is that, too, but by the recognition that the world is changing. References Barad, Karen. Meeting the Universe Halfway: Quantum Physics and the Entanglement of Matter and Meaning. Duke UP, 2006. Gillespie, Tarleton. Custodians of the Internet: Platforms, Content Moderation, and the Hidden Decisions That Shape Social Media. Yale UP 2018. Haraway, Donna J. “SF: Science Fiction, Speculative Fabulation, String Figures, So Far.” Ada New Media 3 (2013). <http://adanewmedia.org/2013/11/issue3-haraway>. Lee, Ashlin. “In the Shadow of Platforms: Challenges and Opportunities for the Shadow of Hierarchy in the Age of Platforms and Datafication.” M/C Journal 24.2 (2021). <http://doi.org/10.5204/mcj.2750>. Markham, Annette N., et al. “Massive and Microscopic Sensemaking during COVID-19 Times.” Qualitative Inquiry Oct. 2020. <https://doi.org/10.1177/1077800420962477>. Ngai, Sianne. Ugly Feelings. Harvard UP, 2005. Nunes, Mark. 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