Flow Configuration Research Articles

Reynolds stresses and turbulent heat fluxes in fan-shaped and cylindrical film cooling holes

Large eddy simulation (LES) is carried out to predict the thermal performance, unsteady flow patterns, and turbulence characteristics of the fan-shaped hole and the cylindrical hole cooling films. Velocity fields, Reynolds stresses, and turbulent heat flux distributions are the factors that ultimately determine the film cooling effectiveness by deciding the concentration of coolant and heat near the surface. Accurate prediction of these processes can provide a reliable data set for the design of film cooling systems. It can also help solve difficulties of improving RANS-based simulations in terms of their inaccuracies in matching measured flow rates for a given pressure drop across the film and the mixing process once the coolant is ejected from the film. Credible estimates and modest costs of the current LES originate from the multiblock hexahedra meshing, the turbulence inflow generator, and the numerical schemes implemented. Multiblock hexahedra meshes are adopted and generated by a quasi-automatic mesh generation algorithm utilizing template-based multi-block construction followed by elliptic smoothing. Computational resources are utilized in a way that mesh refinement exists only near cooling decorations of small size and wall boundaries, without the need for associated clustering in the far field which is commonly used in commercial packages. The high orthogonality and continuous smoothness throughout the computational domain are maintained to facilitate turbulence capturing. The synthetic inflow generator produces a random field matching a realistic set of two-point statistics based on eigen-reconstruction in a computationally inexpensive processing step. To prove that the current LES simulations of modest cost can reproduce with high accuracy, the discharge characteristics, velocity, and turbulence intensity profiles of cylindrical and fan-shaped cooling films are compared with published measured data on the same flow configurations. Results reveal that the set of modeling methodologies is valid for calculating film-cooling performance within an acceptable range of accuracy. The energy-carrying turbulence structures in and near the cooling holes are shown and their behaviors are linked to turbulent Reynolds stresses. Reynolds stresses and turbulent heat fluxes are compared for different blowing ratios and two types of cooling holes in the parametric study. Similarity patterns of velocity profiles, non-dimensional temperature profiles, Reynolds stress and turbulent heat flux profiles are discussed.

Stochastic model of organizational state transitions in a turbulent pipe flow

Flow configurations that display a significant degree of order, characterized by a well-defined number of low-speed streaks along the walls and quasi-streamwise vortical structures, hold valuable insights into the dynamics of turbulent pipes. Using stereoscopic particle image velocimetry, we investigate transitions between these organizational states (OS) in a large pipe rig facility. We find that the OS modes and their transitions exhibit a recurring statistical self-similar pattern, up to time horizons related to the flow outer scales. We also demonstrate that this distinctive statistical behavior can be accurately described by a lower-level Markovian model of the OS mode transitions.

High-resolution simulations of a turbulent boundary layer impacting two obstacles in tandem

We created a new publicly available data set of high-fidelity numerical simulations for two obstacles in tandem invested by a fully resolved turbulent boundary layer. In the three configurations of skimming flow, wake interference, and isolated roughness, we identify the regions of intense turbulent fluctuations, production and transport of turbulent kinetic energy, and examine the anisotropy-invariants map.

Numerical investigation of novel manifold microchannel heat sinks with countercurrent regions

Manifold microchannel (MMC) heat sink is a potential method to dissipate high heat fluxes of electronic devices. Traditional Z-type MMC is a simple configuration but usually encounters high temperature difference and pressure drop. In this work, we introduced countercurrent flows to alleviate these problems by revising the design of manifold configurations. Six cases with different manifold arrangements are numerically investigated using single-phase deionized water at mass flow rates ranging from 0.04 - 0.12 g/s. The results show that the maximum temperature, average temperature, and temperature difference of the revised arrangements are all lower than these of the traditional Z-type manifold arrangement. The cases with countercurrent regions can reduce the maximum temperature around 5 K compared with the original cases with parallel regions. The temperature difference of the ZU-type MMC with double-layer countercurrent flow configuration is lower than 4 K, which is about 25% of the original Z-type MMC. For the revised manifold configuration with countercurrent flows, the pressure drops also decrease due to the split of inlets and outlets of the manifold. The pressure drops of single-layer and double-layer countercurrent manifolds are respectively 27.13% and 33.36% lower than that of the case with traditional Z-type manifold. This leads to a higher comprehensive performance of revised cases with countercurrent flows. Besides the better temperature uniformity, the fluid flow becomes more uniform in the microchannels for the revised case. The improved ZU-type MMC with double-layer countercurrent flow can dissipate 1100 W/cm2 for single-phase flow with a temperature increase of 80 K and a pressure drop of 22 kPa at mass flow rate of 1.2 g/s.

The Potential of Multi-stage Pressure Retarded Osmosis for Blue Energy Harvesting

Objective: This study aims to investigate the potential benefits of using different multi-stage and multi-pass flow configurations for improving the efficiency of the pressure-retarded osmosis (PRO) process, which generates energy by utilizing the difference in osmotic pressure between two salinity solutions. Methods: To achieve the objective, the study used a numerical simulation based on the solution diffusion model. A flat sheet forward osmosis membrane was simulated to investigate the potential of the proposed multi-stage PRO configurations. Results: The results of the simulation showed that the multi-stage PRO arrangement generated 18% more power than the single-stage PRO system. Additionally, the multi-pass arrangement resulted in a 58% increase in power output. Conclusion: This study provides important insights into the development of optimized PRO membrane and module configurations to improve the efficiency of energy generation. The findings highlight the potential benefits of multi-stage and multi-pass flow configurations, which can help to advance the field of renewable energy generation using the PRO process.

Impact Analysis of Driver Takeover Under a Connected Environment: A Study of the External Effects on Vehicle Conversions

The moment we face cooperative adaptive cruise control (CACC), driver takeover, that is, drivers actively take over vehicles to avoid unknown risks, is a necessary concern in reality, especially with the distrust or low acceptance of automated driving technology in the early stage of CACC development. However, the impact of driver takeover on traffic flow has not been accurately presented, as researchers only focused on the taken-over vehicle and neglected the external effects on surrounding vehicles. Departing from the literature, this study provides a novel external perspective for the impact analysis of driver takeover under a connected environment, that is, it considers the resulting communication heterogeneity and specifies multi-class conversions in the entire traffic flow. To be specific, we subdivide the dynamic process of driver takeover into 10 vehicle conversions considering vehicle degradation and platoon reorganization, then analyze their contributions to traffic stability, emissions, and fuel consumption. What is also innovative is that two forms of driver takeover are first developed in this study, which is determined by whether drivers continue opening the communication device after the takeover. On this basis, this study indicates that driver takeover has the potential to cause greater heterogeneity of the traffic flow in contrast to that in the literature and proposes theoretical models for the changed traffic flow configuration and traffic stability after the takeover. Finally, numerical experiments verify the theoretical results with discussions on multiple factors, for example, vehicle proportion, platoon length, and driving behaviors. The findings extend the external effects of driver takeover on multi-class vehicle conversions in heterogeneity traffic flow and provide a more realistic basis for the evaluation of driver takeover.

Demonstration of direct ocean carbon capture using hollow fiber membrane contactors

The focus of membrane carbon capture to date has been primarily on point source capture, such as power plants and industrial capture. However, membrane technology can also play a role in negative emissions technology, such as direct air capture and direct ocean capture. Direct ocean capture has a few potential advantages over direct air capture, such as avoiding land use and coupling with offshore wind and offshore storage. The use and feasibility of hollow fiber membrane contactors (HFMCs) for direct ocean carbon capture with benign aqueous basic carbon dioxide solvents is assessed here through a multifaceted approach. A 1D HFMC model incorporates fluid dynamics and the chemical kinetics of both ocean water and aqueous sodium hydroxide solvent in order to simulate CO2 flux behavior in two flow configurations. Lab scale experiments of this system then guide a model refinement and validation process until experimental behaviors are predicted through computation. A preliminary technoeconomic assessment then uses computational and experimental results to estimate the carbon capture cost when the system is scaled to remove 0.98 Mtonnes CO2/year. Computational results suggest that higher seawater flow rates and temperatures relative to the sodium hydroxide solvent improve CO2 flux. The technoeconomic assessment suggests that HFMCs may only be cost-competitive if seawater pH is decreased at the membrane interface, thereby increasing the local concentration of dissolved carbon dioxide. These findings indicate that local pH swing on the seawater side will be necessary to feasibly remove carbon dioxide from seawater using HFMCs.

A design method for edge–cloud collaborative product service system: a dynamic event-state knowledge graph-based approach with real case study

ABSTRACT Product service system (PSS) is an effective approach to achieve win–win situation between service providers and consumers, and the design of PSS is the first step of its application. However, on the one hand, PSS is relatively an abstract concept, and what exactly should be considered when designing a PSS still require further exploration; on the other hand, the fast development of information and web technologies bring both opportunities and challenges for PSS design. For example, how to efficiently design the smart & connected products that support remote monitoring and control of service operation, how to build the dynamic service activity flow model that can be intuitively and visually read by human engineers and also can be conveniently deployed on computers, and how to realise edge–cloud collaboration between service providers and consumers. In this regard, a service requirement-oriented four-step generic PSS design method is established, including service mode selection and structured service order generation → smart & connected service product configuration → dynamic event-state knowledge graph-based service activity flow and service resource network configuration → industrial internet-based edge–cloud collaborative service delivering. Finally, a real design case of carbon block grinding and polishing PSS is used for verification. ABBREVIATIONS: AS: assembly service; C: controlling targets or objectives; I: input; M: enabling methods or mechanisms; MRO: maintenance, repair, and operation; MS: maintenance service; O: output; OEE: overall equipment effectiveness; PSS: product service system; PV: processing volume; R x E y -S z : the relation from Event y to the State z of Service activity x; R x S z -E y : the relation from the State z of Service activity x to Event y; StateAi Current: the current state of Service activity i; StateAi Futurek : the kth possible future state of Service activity i; StateAi Historyj : the jth historical state of service activity i; StateEy : the state of Event y; Tq : the qth time point in the time line.

Combining direct and indirect sparse data for learning generalizable turbulence models

Learning turbulence models from observation data is of significant interest in discovering a unified model for a broad range of practical flow applications. Either the direct observation of Reynolds stress or the indirect observation of velocity has been used to improve the predictive capacity of turbulence models. In this work, we propose combining the direct and indirect sparse data to train neural network-based turbulence models. The backpropagation technique and the observation augmentation approach are used to train turbulence models with different observation data in a unified ensemble-based framework. These two types of observation data can explore synergy to constrain the model training in different observation spaces, which enables learning generalizable models from very sparse data. The present method is tested in secondary flows in a square duct and separated flows over periodic hills. Both cases demonstrate that combining direct and indirect observations is able to improve the generalizability of the learned model in similar flow configurations, compared to using only indirect data. The ensemble-based method can serve as a practical tool for model learning from different types of observations due to its non-intrusive and derivative-free nature.

High-Fidelity Simulation of Mixing Phenomena in Large Enclosures

In the present work, two large eddy simulations (LESs) of single isothermal jets discharging into large enclosure facilities are proposed. The geometries and tested flow conditions correspond to scaled experiments of the upper plenum of high-temperature gas-cooled reactors. More specifically, two reference experiments were conducted at Texas A&M University and Michigan University. The objective of the present work is to validate these simulations with their corresponding reference experiments. The proposed LES models are performed with NekRS, a spectral element code with graphics processing unit capabilities developed at Argonne National Laboratory. These simulations were performed on the Summit supercomputer at Oak Ridge National Laboratory. For validation purposes, first- and second-order statistics from the computational fluid dynamics (CFD) calculation are compared with measurements obtained from the experiments. The models proved to be accurate, as these results are in good agreement. Additionally, flow visualization is provided showing that these models are able to retrieve similar effects to what are described in the literature for this type of flow configuration. Finally, the proposed models are part of a broader effort under the current Integrated Research Project of Nuclear Energy Advanced Modeling and Simulation 1.1, whose main objective is to deliver fast-running models to accurately predict complex physical phenomena, including for instance, turbulent mixing and thermal stratification. In this regard, the CFD models proposed here will be used to generate a high-fidelity data set to be applied in conjunction with data-driven methods to improve turbulence modeling closures.

Grain size reduction by plug flow in the wet oceanic upper mantle explains the asthenosphere's low seismic Q zone

The prominent seismic low-velocity zone (LVZ) in the oceanic low-viscosity asthenosphere is approximately coincident with the low seismic Q zone (LQZ; a zone of high seismic attenuation). Small olivine grain sizes could link these seismic and rheological properties because they reduce viscosity, seismic velocity and seismic Q. Because rock deformation reduces grain sizes, the asthenosphere's seismic properties can place constraints on asthenospheric flow. To determine dominant flow patterns, we develop a self-consistent analytical 1-D channel flow model that accounts for upper mantle rheology and its dependence on flow-modified grain-sizes, water content, and melt fraction, for flow driven by both surface plate motions (Couette flow) and/or horizontal pressure gradients (Poiseuille flow). We find that Couette flow dominates if the upper mantle is dry, and plug flow (a Poiseuille flow for power law rheology) dominates if it is weakened by wet conditions. A plug flow configuration spanning the upper 670 km of the mantle best explains the LQZ in the asthenosphere, and can be attributed to significant grain-size reduction due to extensive shearing across the asthenosphere. This flow configuration also explains high seismic Q values below the asthenosphere associated with minimal shear deformation and large grain sizes above the mantle transition zone. We suggest that the asthenospheric LQZ and LVZ seismic anomalies can be largely explained by grain-size variations associated with plug flow in the wet upper mantle.

Magneto-Convective Transport of Immiscible Binary Fluids in Inclined Channel

Abstract This work theoretically investigates the interfacial transport of immiscible fluid layers in an inclined fluidic channel in the presence of magnetofluidic actuation. Immiscible binary system consists of both non-Newtonian fluid (top layer) and Newtonian fluid (bottom layer), while the Carreau fluid model is used to describe the rheology of non-Newtonian layer. We develop a theoretical framework consistent with the homotopy analysis method (HAM) to obtain the approximate analytical solutions for the underlying thermofluidic transport features. By depicting the auxiliary parameter curve (ℏ-curve) of flow velocity and temperature distribution, we ascertain the effective reliability of the theoretical method developed here. We demonstrate both velocity and temperature variations in the channel for a set of involving parameters pertinent to this analysis. Albeit the flow configuration considered in this analysis is not complex, yet, the method developed here seems to be efficient in capturing underling transport features, retaining the simultaneously acted implications of fluid rheology and magnetohydrodynamics. Form the consistency observed in predicting the flow velocity for any values of shear-thinning parameter, including nonintegers, our semi-analytical method is deemed pertinent to predict the thermohydrodynamics of immiscible multilayer system even by accurately capturing the intervening effects of fluid rheology and applied fields.

Theoretical analysis and experimental investigation of air-bubble-stream-induced water circulation

Air-bubble curtain is an amount of air injected vertically into a water body. The generation of such a flow and the lack of a continuous interface cannot be described by a smooth mathematical function. Therefore, a two-phase flow model is introduced. A numerical model for the concurrent flow of buoyant bubbles continuously flowing into a 2D water field, and the water flow (generated by the bubbles), is formulated and solved. The two-phase flow model consists of the 2D Navier equations for the water phase (continuous phase) and of the active Lagrangian particles for the simulation of the air bubbles (discrete phase). The coupling of the two phases is done through the continuity and the momentum equilibriums. The numerical solution by explicit second-order Finite Differences (FD) scheme leads from a cold start to steady flow conditions, resolving for the water velocities vector field and the air bubbles’ concentration distribution. The flow configuration is repeated in laboratory conditions, and the velocity field is measured by the particle image velocimetry (PIV) technique. In this work, the numerical two-phase flow model and the hardware aspects of our measurement device are analyzed, followed by the comparison of the numerical and experimental results. This empowers the validity and credibility of the algorithm introduced. Finally, interesting conclusions are drawn regarding the operational use of the model.

Effective control of two-dimensional Rayleigh–Bénard convection: Invariant multi-agent reinforcement learning is all you need

Rayleigh–Bénard convection (RBC) is a recurrent phenomenon in a number of industrial and geoscience flows and a well-studied system from a fundamental fluid-mechanics viewpoint. In the present work, we conduct numerical simulations to apply deep reinforcement learning (DRL) for controlling two-dimensional RBC using sensor-based feedback control. We show that effective RBC control can be obtained by leveraging invariant multi-agent reinforcement learning (MARL), which takes advantage of the locality and translational invariance inherent to RBC flows inside wide channels. MARL applied to RBC allows for an increase in the number of control segments without encountering the curse of dimensionality that would result from a naive increase in the DRL action-size dimension. This is made possible by the MARL ability for re-using the knowledge generated in different parts of the RBC domain. MARL is able to discover an advanced control strategy that destabilizes the spontaneous RBC double-cell pattern, changes the topology of RBC by coalescing adjacent convection cells, and actively controls the resulting coalesced cell to bring it to a new stable configuration. This modified flow configuration results in reduced convective heat transfer, which is beneficial in a number of industrial processes. We additionally draw comparisons with a conventional single-agent reinforcement learning (SARL) setup and report that in the same number of episodes, SARL is not able to learn an effective policy to control the cells. Thus, our work both shows the potential of MARL for controlling large RBC systems and demonstrates the possibility for DRL to discover strategies that move the RBC configuration between different topological configurations, yielding desirable heat-transfer characteristics.

Microramp wake impinging on canonical shock/boundary-layer interaction

We analyze the influence of microramp vortex generators (mVGs) on a canonical oblique shock wave/turbulent boundary-layer interaction (SBLI) in terms of mean flow field and unsteady dynamics. The flow configuration of our wall-resolved large-eddy simulations (LES) reproduces the experiment of Bo et al. [“Experimental investigation of the micro-ramp based shock wave and turbulent boundary-layer interaction control,” Phys. Fluids 24, 055110 (2012)]: a rake of microramps is inserted upstream of the SBLI, protruding by 0.476δ in a turbulent boundary layer (TBL) at free-stream Mach number M = 2.7 and corresponding to a Reynolds number based on the displacement thickness of Reθ=3600. The long integration time of 1672 Lsep/U∞ allows an accurate characterization of the low-frequency dynamics of the SBLI under the influence of the microramps. With respect to the reference SBLI without control devices, the mean flow field shows a new spatial organization of the recirculation bubble due to the mVGs' wake. The alternating high and low-speed zones in the near-wall region of the incoming TBL, induced by the counter-rotating streamwise vortices generated by the mVGs, trigger spanwise corrugations of the separation and reattachment lines and locally alter the reverse flow region. Tornado-like vortices are found in the vicinity of these zones, yielding a new fluid collection mechanism of the reverse flow region. These vortices redirect the fluid coming from regions outside of the wake in the incoming TBL to three key spanwise exit locations located in between the mVGs and at their centerline. Interestingly, power spectral densities of wall-pressure probes show a damping of the low-frequency dynamics of the reflected shock foot for spanwise stations aligned with the mVGs' wake, whereas this activity appears to be reinforced in the planes located in between the mVGs. However, we found no evidence of unsteady forcing linked to the high-frequency shedding of the coherent structures developing in the wake of the microramps. Dynamic mode decomposition highlights a significant change in the low-frequency dynamics, mostly affecting the mass budget of the recirculation bubble. The breathing of the recirculation zone that occurs at StL=0.1 for the SBLI without control devices (with StL=fLsep/U∞) appears to shift toward a lower frequency of StL=0.05. Remembering that the reflected shock foot motion is related to frequencies in the range StL=[0.03−0.05], the SBLI with upstream mVGs seems to highlight a synchronization of this motion with the breathing of the separation bubble.

Global instability phenomenon as a physical mechanism controlling dynamics of a nitrogen-diluted hydrogen flame

We investigate the dynamics of counter-current reacting flow under varying strengths of the counterflow and report the occurrence of global instability phenomena. This clarifies the role of global instability in the transition between attached and lifted flames. The research is performed with the help of the Large Eddy Simulation method using a high-order numerical method. The combustion process is modelled by applying detailed chemical kinetics for hydrogen combustion with chemical reaction terms computed from the resolved scale. The flow configuration consists of a central jet of nitrogen-diluted hydrogen fuel surrounded by an annular nozzle with a suction slot enforcing counterflow in the vicinity of the fuel stream. Suction strength is controlled by the ratio between the velocity in the suction slot (Usuc) and the velocity of the fuel jet (Uj) (I=Usuc/Uj). The Reynolds number based on the fuel parameters and the central nozzle diameter is Re=1600. Two values of the suction strength are considered (I=0.1,0.2) as well as the case without suction (I=0.0). A hot oxidizer (air) provided in the region outside the co-axial nozzle causes fuel auto-ignition far from the injection system after which the flame propagates downstream. It is shown that increasing the suction strength postpones the ignition and shifts its axial location downstream. Depending on the I parameter the flame stabilizes as attached (I=0.0,0.1) or lifted (I=0.2). It is shown that the flames in the cases with I=0.0,0.1 are very similar to each other appearing as laminar non-premixed flames both in the nozzle vicinity and far downstream. The flame in case I=0.2 is turbulent and much more dynamic. Its lifting is the result of global instability appearing at increased suction in the region of counter-current flow. It is shown that the global instability induces strong toroidal vortices that create a premixed reaction zone at the flame base. This flow structuring prevents upstream flame propagation and intensifies the mixing process ensuring almost complete fuel burning in a short distance from the nozzle.

Physical/numerical duality of explicit/implicit subgrid-scale modelling

The development of implicit approaches has prompted debate on the actual usefulness of any explicit subgrid-scale modelling in large-eddy simulation. This question is addressed here by considering two generic turbulent flows: (i) the Taylor-Green vortex problem; (ii) the pipe flow. For both flow configurations, implicit modelling is found to overtake the very popular Smagorinsky model. To understand this robust observation, an analysis in the Fourier space is presented for the Taylor-Green vortex problem. The concept of spectral eddy viscosity, widely used in the pioneer work of Marcel Lesieur in two-point closure and subgrid-scale modelling, is revisited in a general framework based on explicit/implicit subgrid-scale modelling. In particular, the essentially anisotropic nature of implicit modelling is exhibited, as a favourable feature in terms of consistency with the computational mesh. Smagorinsky's model, considered as a generic explicit subgrid-scale model in the framework of Boussinesq's hypothesis, is found to be highly sensitive to numerical errors. Removing the latter is easy but makes computationally inefficient this type of explicit modelling. Comparisons between a priori and a posteriori spectral eddy viscosities show that neither Smagorinsky's model nor implicit modelling can mimic the expected spectral behaviour. Smagorinsky's model is observed to be weakly scale-selective with a poor ability to actually filter the solution. The feature of scale-selectivity is well replicated by implicit modelling which exhibits excellent capabilities for filtering. However, its lack of influence at the largest scales is against the expected behaviour for the spectral eddy viscosity at low wavenumber through the establishment of a non-zero plateau value. This lack of consistency of implicit LES could be overcome thanks to an extra explicit modelling but the attempt to mix Smagorinsky's model and implicit LES is not successful in this study. The potential of implicit large-eddy simulation is also exhibited for the accurate computation of near-wall turbulence inside a pipe flow despite the use of a regular Cartesian mesh with an immersed boundary method. Interestingly, the resulting coarse wall-normal resolution in the near-wall direction does not prevent the reliable prediction of statistical profiles up to the capture of subgrid-scale details. It is suggested that the regularisation associated with implicit modelling is a necessary condition to reach numerical accuracy. However, to faithfully represent the large-scale dynamics, present results confirm that non-local triad interactions must be taken into account as widely discussed in the inspiring textbook Lesieur [Turbulence in fluids. 4th ed. Springer; 2008] of Marcel Lesieur.

Multiple Large‐Scale Dynamical Pathways for Pan–Atlantic Compound Cold and Windy Extremes

AbstractWinter cold spells over North America have been correlated with European wind extremes, but the physical mechanisms behind such “pan‐Atlantic” compound extremes have not been clarified yet. In this study, we propose that pan–Atlantic cold and windy extremes occur following two possible dynamical pathways. The first one involves the propagation of a Rossby wave train from the Pacific Ocean, associated with windstorms over north‐western Europe in the 5–10 days after the cold spell peak. The second is associated with a high‐latitude anticyclone over the North Atlantic and an equatorward‐shifted jet, leading to windstorms over south‐western Europe already in the days preceding the cold spell peak. European windstorms are thus systematically tied to North American cold spells according to the different flow configuration. The analysis underscores that apparently similar surface extremes might be driven by different processes, and conflating them together could lead to misleading conclusions.

Role of slip in the stability of viscoelastic liquid flow through a channel

For a horizontal channel flow of a weakly viscoelastic liquid that follows the constitutive model of Walters’ liquid B′′, a linear stability analysis is performed when the channel walls are slippery. In this case, the viscoelastic channel flow is driven by the streamwise pressure gradient. We explore the primary instability for a symmetric slip flow with the same slip length of the channel walls, an asymmetric slip flow with different slip lengths of the channel walls, and an analogy to the Poiseuille–Couette flow of a viscoelastic liquid with a slip effect on the lower wall but no slip effect on the upper wall. We observe that the viscoelastic parameter shows a destabilizing impact, but the slip length shows a stabilizing impact on the most unstable shear mode in all three flow configurations. Moreover, the Poiseuille and Poiseuille–Couette flows for the viscoelastic liquid are linearly more unstable to infinitesimal disturbances than those of the Newtonian liquid. As a result, wall velocity needs to be higher than in the Newtonian liquid to stabilize the viscoelastic Poiseuille–Couette flow. Using the asymptotic analysis, we have determined the critical value of the slip length, or equivalently, the critical value of the lower wall velocity, above which the asymmetric slip flow and the Poiseuille–Couette flow of the viscoelastic liquid are permanently stable to infinitesimal disturbances.

The Yih Instability in Layered Lava Flow May Initiate the Pāhoehoe to ‘a‘ā Lava Transition

AbstractMost basaltic lavas begin flowing as pāhoehoe but sometimes transition into ‘a‘ā. Field observations and previous models have demonstrated that the rheology of smooth, liquid‐like pāhoehoe is distinct from rough, pasty ‘a‘ā, but the cause of this dramatic and rapidly occurring change in rheology has remained unclear. Here, we propose that the pāhoehoe to ‘a‘ā transition could be initiated by the Yih instability, an internal shear instability, in layered pāhoehoe flow. We show that the conditions under which instability arises depend on both extrinsic and intrinsic factors and derive a non‐dimensional regime diagram. We test the model's prediction of stable flow configurations against field observations of solidified lava flows.