Low Rotation Rates Research Articles

Influences of Background Rotation on Secondary Eyewall Formation of Tropical Cyclones in Idealized f‐Plane Simulations

AbstractThis study investigates the background rotational influences on the secondary eyewall formation (SEF) in tropical cyclones (TCs) in quiescent f‐plane environments. For given initial structures, simulated vortices tend to experience earlier SEF at lower latitudes. Yet the size of the secondary eyewall does not change monotonically with the latitudes. Specifically, ∼20°N provides the optimal amount of background rotation for the largest secondary eyewall size without considering other environmental forcings. Different background rotation rates affect SEF mainly by modulating the outer‐core convection as well as the wind structures. Specifically, the lower rotation rate causes more outer‐core surface fluxes, thus facilitating the outer rainbands (ORBs) at larger radii. Yet the secondary eyewall does not necessarily form at larger radii at lower latitudes since the transition from the ORBs to secondary eyewall is localized in a region of boundary layer (BL) convergence preceded by accelerated tangential winds. Budget analysis reveals that the differences in the acceleration of outer‐core tangential winds among vortices at different latitudes are dominated by the radial flux of absolute vorticity. Due to the non‐uniform influences of background rotation on the BL inflow and absolute vorticity, the most efficient spin‐up of outer‐core tangential winds is achieved at a medium latitude of 20°N, which leads up to SEF at the largest radii. By comparison, for TCs at lower (higher) latitudes, the lower outer‐core absolute vorticity (radial inflow) limits the acceleration of outer‐core tangential winds, thus placing SEF at smaller radii.

Stabilisation of the swing pattern of an anisotropic simple pendulum

The suppression of the effects of anisotropy on a pendulum by use of a rotating mount was initially envisaged by Léon Foucault, based on his observations of the vibrations of a rod clamped in a lathe. However, the method seems to never have been tried due to the practical difficulties involved. We report a computational study of the stabilisation of the swing pattern of a simple pendulum, showing anisotropic behaviour in a static configuration, by rotation of the system mount. When the mount is static, for most initial conditions the swing patterns quickly evolves into unstable, complex Lissajous-like patterns. When the pendulum mount is rotated faster than the pendulum frequency effects of anisotropy are suppressed, and the swing pattern stabilises to that of an isotropic 3D simple pendulum. Suppression of mount anisotropy influence occurs for relatively low rotation rates. We also study swing evolution in the presence of random variations in the orientation of the mount principal axes. The use of computational techniques confirms Foucault’s original observations and hypothesis and provides an interesting avenue for students to engage meaningfully with this historically important and inspiring experiment in a novel and challenging manner.

Vortex-induced vibration of a rotating cylinder with dual splitter plates

To explore suppression method on vortex-induced vibrations (VIVs) response of the rotating cylinders, the VIVs of two-degree-of-freedom rotating cylinders with dual splitter plates at a Reynolds number of 200 and a mass ratio of 2.6 are investigated via numerical simulations. The numerical results show that splitter plates are more effective at suppressing VIV in the cylinders with low rotation rates, and the suppression effect decreases with increasing rotation rate. Three flow patterns are defined [overshoot, merge shedding, and individual shedding], and the distributions of the flow patterns and wake patterns under different rotation rates and gap distances are discussed. The vibration–fluid force–wake interaction is analyzed, and the variation of flow patterns is accompanied by the sudden increase in amplitude and fluid force. In addition, the directional sensitivity of the lift and drag is discussed, the lift is more sensitive to the rotation rate, and the drag is more sensitive to the gap distance.

Enhancing optical and thermohydraulic performance of linear Fresnel collectors with receiver tube enhancers: A numerical study

Linear Fresnel collector system is a type of solar thermal utilization installation. It is mainly used to focus solar radiation on the receiver tube to generate high temperature thermal energy which can be applied in many aspects. This study investigates five different types of receiver tube models using water as the flowing medium, which leads to improving heat transfer performance by means of being equipped with enhancers. The optical results and thermohydraulic performance of the Linear Fresnel collector system are evaluated through Monte Carlo Ray Tracing and Computational Fluid Dynamics (CFD) simulation methods. The obtained optical results reveal that the distribution of energy flux density concentrating on the receiver tube surpasses that of the parabolic trough collector system. In order to maintain an energy reception rate of 90.0% or higher, the installation height range of the receiver tube should be between 1468–1532 mm. When the sun-tracking error increases from 0.0° to 1.0°, the overall optical efficiency of the Linear Fresnel collector system decreases from 98.9% to 79.7%. The thermohydraulic performance shows that the enhancers can generate a swirling flow as its main feature, which effectively promotes the homogenization of the temperature field and destroys the boundary layer near the receiver tube wall, thus greatly improving the convective heat transfer in the receiver tube. However, the introduction of the enhancers also means that the pressure drop loss of the receiver tube increases. Furthermore, the impact of the Kenics mixer's enhancer on heat transfer and flow characteristics is further amplified as the rotation rate decreases, owing to its alternating and complex structure. Through a comparative analysis of the influence of different receiver tube models on Nusselt number (Nu), Frictional resistance coefficient (f) and comprehensive evaluation criterion (PEC) under four rotation rate conditions (y = 6.3, 8.1, 11.34, 18.9), the conclusions present that the receiver tube with an enhancer at a low rotation rate has a larger Nu than that at a high rotation rate, while the Frictional resistance coefficient changes little at different rotation rates. Therefore, the PEC is higher when the receiver tube is equipped with a low-rotation-rate enhancer. Moreover, the receiver tube equipped with a Kenics mixer at a rotation rate of 6.3 was found to be the overall optimum case, with the PEC value varying from 1.47 to 1.73.

Investigation of vortex-induced vibrations of rotating cylinders with different surface roughnesses

The vortex-induced vibrations of a two-degree-of-freedom rough rotating cylinder at a low Reynolds number of 200 and a mass ratio of 2.6 are investigated via numerical simulations. The relevant calculation parameters are as follows: a rotation rate between zero and one, surface roughness height between 0% and 15%, and reduced velocity between 1 and 12. It is found that reasonable rough surface and rotational motion of the smooth cylinder are two effective factors for suppressing the vortex-induced vibration (VIV) response. Conversely, a rotating cylinder with a rough surface enhances the VIV response. Four wake patterns (2S, P + S, 2P, and multiple vortices patterns) are captured. At low rotation rates, with increasing roughness height, the wake pattern develops into a multiple vortex pattern after multiple evolutions. The variation in roughness at a high rotation rate does not correlate with a change in the wake pattern. The area of the cylindrical motion trajectory is positively correlated with the roughness height, and the time-averaged dimensionless displacements of the cross-flow and in-line flow directions increase with increasing roughness height.

Real model for micro swimmer and the study of the relationship between the swimming speed, pitch angle, and rotation rate for the flagellum

This study focuses on the fluid mechanics of a microswimmer and explores the relationship between speed, pitch angle, and rotation rate for the flagellar during bacterial swimming. Based on the simulation using MATLAB, it is concluded that when the pitch angle of the flagellar helix is in the range of 0 to 90 degrees, the value of swimming speed increases firstly and decreases. When the angle reaches 46.83 degrees, the speed reaches the maximum point. The radius of the body of the microswimmer is determined by the Buckingham Pi theory. After calculating by using the equations in the related paper and measuring by the real model, we derive that the relationship between swimming speed and the rotation rate for the flagellar filament should be proportional at the low rotation rate so that it can be obtained to optimize the artificial micro swimming device with higher swimming efficiency.

Hubble Space Telescope survey of Magellanic Cloud star clusters: UV-dim stars in young clusters

ABSTRACT Young and intermediate-age star clusters of both Magellanic Clouds exhibit complex colour–magnitude diagrams. In addition to the extended main-sequence turn-offs (eMSTOs), commonly observed in star clusters younger than ∼2 Gyr, the clusters younger than ∼800 Myr exhibit split main sequences (MSs). These comprise a blue MS, composed of stars with low rotation rates, and a red MS, which hosts fast-rotating stars. While it is widely accepted that stellar populations with different rotation rates are responsible for the eMSTOs and split MSs, their formation and evolution are still debated. A recent investigation of the ∼1.7-Gyr-old cluster NGC 1783 detected a group of eMSTO stars extremely dim in ultraviolet (UV) bands. Here, we use multiband Hubble Space Telescope photometry to investigate five star clusters younger than ∼200 Myr, including NGC 1805, NGC 1818, NGC 1850, and NGC 2164 in the Large Magellanic Cloud, and the Small Magellanic Cloud cluster NGC 330. We discover a group of bright MS stars in each cluster that are significantly dim in the F225W and F275W bands, similar to what is observed in NGC 1783. Our result suggests that UV-dim stars are common in young clusters. The evidence that most of them populate the blue MS indicates that they are slow rotators. As a by-product, we show that the star clusters NGC 1850 and BHRT 5b exhibit different proper motions, thus corroborating the evidence that they are not gravitationally bound.

Stability characteristics of the spiral Poiseuille flow induced by inner or outer wall rotation

In this paper we reveal, that the stability behavior of the spiral Poiseuille flow in an annular gap with rotating inner or outer cylinder exhibits a striking similarity at low and intermediate rotation rates, while both cases differ significantly at higher rotation rates. Based on the work of Vasanta Ram (2019) we formulate the flow problem by means of the inner swirl parameter (Si), the outer swirl parameter (So) and the curvature parameter (ϵ). Six different values of the curvature parameter {0.005; 0.025; 0.111; 0.25; 0.333; 0.785} and a swirl range of 10−5<Si;So<105 are investigated. We reveal that for the rotating outer cylinder case three characteristic jumps of the critical axial and azimuthal wavenumbers can be identified. Similar features can be identified for the inner rotation case. Based on this, we define three regimes delimited for both the rotating inner and rotating outer cylinder which enables us to perform a systematic comparison. The first regime transition occurs at low values of the swirl parameter (Si; So>10−4) and is accompanied by a rapid decrease of the critical Reynolds number. Our computations reveal that this regime transition does not occur for a small curvature parameter of ϵ= {0.005; 0.025} in the outer rotation case. The second wavenumber jump occurs at higher swirl parameters of around Si>2.5 and So>1.6. While the critical Reynoldsnumber decreases monotonously in the inner cylinder rotating case, we show that in the case of rotating outer cylinder a rapid increase of Rec is observed as So increases further. To gain an insight into the mechanisms involved that induce such wavenumber jumps, a detailed comparative study on the associated Reynolds shear stress distributions is performed. For larger ϵ it is shown that the distributions of the Reynolds shear stresses change significantly over the first jump exhibiting similar features for both flow cases, leading to the conclusion that in both cases the same centrifugal instability is triggered here. Instead, the distributions of the Reynolds shear stresses before and after the second jump are fundamentally different for inner and outer rotation. While the Reynolds shear stresses spread over the whole gap height for the inner rotation case, they are nonzero only close to a small regime near the inner cylinder for the outer rotation case at high rotation rates. Overall, analyzing the shear stresses, we reveal that a similar instability arises in the Spiral Poiseuille flow with inner and outer cylinder rotation at intermediate swirls, but at higher swirl outer rotation seems to give rise to an instability associated with a critical layer that is inherently different to the inner rotation case.

Suspension of large inertial particles in a turbulent swirling flow

We present experimental observations of the spatial distribution of large inertial particles suspended in a turbulent swirling flow at high Reynolds number. The plastic particles are heavier than the working fluid so that their dynamics results from a competition between gravitation and turbulent agitation. Two suspension regimes are observed. At low rotation rate particles are confined near the bottom for any size or density. At high rotation rate, particles are loosely confined: small particles are nearly homogeneously distributed and large ones found near the top as if gravity was reversed. We discuss these observations using a minimal random walk model accounting for particle inertia.

Effect of stellar rotation on the development of post-shock instabilities during core-collapse supernovae

Context. The growth of hydrodynamical instabilities is key to triggering a core-collapse supernova explosion during the phase of stalled accretion shock, immediately after the birth of a proto-neutron star (PNS). Stellar rotation is known to affect the standing accretion shock instability (SASI) even for small rotation rates, but its effect on the onset of neutrino-driven convection is still poorly known. Aims. We assess the effect of stellar rotation on SASI when neutrino heating is taken into account as well as the effect of rotation on neutrino-driven convection. The interplay of rotation with these two instabilities affects the frequency of the mode m = 2, which can be detected with gravitational waves at the onset of a supernova explosion. Methods. We used a linear stability analysis to study the dynamics of the accreting gas in the equatorial plane between the surface of the PNS and the stationary shock. We explored rotation effects on the relative strength of SASI and convection by considering a large range of specific angular momenta and neutrino luminosities. Results. The nature of the dominant non-axisymmetric instability developing in the equatorial post-shock region depends on both the convection parameter, χ, and the rotation rate. Equatorial convective modes with χ ≳ 5 are hampered by differential rotation. At smaller χ, however, mixed SASI-convective modes with a large angular scale, m = 1, 2, 3, can take advantage of rotation and become dominant for relatively low rotation rates, at which centrifugal effects are small. For rotation rates exceeding ∼30% of the Keplerian rotation at the PNS surface, a new instability regime is characterised by a frequency that, when measured in units of the post-shock velocity and radius, vsh/rsh, is nearly independent of the convection parameter, χ. A strong prograde m = 2 spiral dominates over a large parameter range and is favorable to the production of gravitational waves. In this regime, a simple linear relation exists between the oscillation frequency of the dominant mode and the specific angular momentum of the accreted gas. Conclusions. Three different regimes of post-shock instabilities can be distinguished depending on the rotation rate. For low rotation rates (less than 10% of the Keplerian rotation at the PNS surface), differential rotation has a linear destabilising effect on SASI and a quadratic stabilising or destabilising effect on the purely convective equatorial modes depending on their azimuthal wavenumber. Intermediate rotation rates (10% to 30% of the Keplerian rotation) lead to the emergence of mixed SASI-convection-rotation modes that involve large angular scales. Finally, strong rotation erases the influence of the buoyancy and heating rate on the instability. This independence allows for a reduction in the parameter space, which can be helpful for gravitational wave analysis.

Modeling the perceptions of Rumbling, humming and booming in the context of vehicle interior sounds

Sensations associated with an auditory perception of low-frequency tonal components are humming, rumbling and booming. While humming is commonly associated with an audible low-frequency tone, rumbling requires a temporal level fluctuation of such a tonal component and booming is associated with sounds that, in addition to the low-frequency tone, contain higher tonal components, which are related to the low-frequency tone. This study presents a model that predicts the magnitude of these sensations for interior sounds of vehicles with combustion engines running at low rotation rates, where low-frequency components are often audible. For the model development, own experimental data of recorded interior sounds and altered versions of these sounds were used. The model combines an auditory processing model with a Gaussian Process Regression approach. Key features of the auditory processing model are that it simulates the frequency-place transformation at the auditory periphery with a bank of auditory band pass filters and the frequency selectivity to temporal amplitude modulations with a bank of modulation band pass filters. It is shown that auditory filters up to a center frequency of 1000 Hz and modulation filters of up to 28 Hz are required for a good prediction of the data.

Analysis of a stable bathtub vortex in a rotating container

Rotating flows with free-surface vortices can be found in many engineering applications, such as pump and turbine intakes, vessels, and nuclear reactors. The need to address rather different flow regions existing in such flows, such as Ekman and Stewartson layers and the line vortex zone, in a coupled manner, makes modeling of free-surface rotating flows very challenging. In this work, the flow field of a free-surface vortex, created in a rotating cylinder with a drain hole in its bottom, is investigated numerically and analytically. Above the drain hole of the cylinder, a free-surface vortex, accompanied by axial velocity, is created. This axial velocity profile is governed by the Ekman boundary layer far from the axis and by the drainage in its proximity. The experiments of Andersen et al. [“Anatomy of a bathtub vortex,” Phys. Rev. Lett. 91(10), 104502 (2003a); “The bathtub vortex in a rotating container,” J. Fluid Mech. 556, 121–146 (2006)] on the so-called bathtub vortex are numerically modeled with the finite volume method. The simulations are validated with the available measurements from the experiments. Using the simulation results, self-similar and non-self-similar models, describing the velocity fields in the Ekman boundary layer, are compared and tested. It is shown that the self-similar model is more accurate than the non-self-similar model. It is also demonstrated that the analytical model of Andersen et al. [“Anatomy of a bathtub vortex,” Phys. Rev. Lett. 91(10), 104502 (2003a); “The bathtub vortex in a rotating container,” J. Fluid Mech. 556, 121–146 (2006)], when modified as suggested in the present study, is capable of predicting the free-surface profile for low rotation rates. However, for high rotation rates, only the numerical simulation can predict the relation between the flow field within the liquid and the free-surface profile.

Numerical Studies of Bubbles in Swirling Channel Flows

Abstract The motion of bubbles in upflow in a vertical rotating closed channel is examined numerically, using a front tracking/finite volume method. The flow is driven upward by a constant pressure gradient. The Reynolds number is low enough so that the flow remains laminar and the Eötvös number is sufficiently low so the bubbles remain nearly spherical. A bubble is placed between the centerline and the walls and for low rotation rate the bubble moves to a wall, due to the lift force and the fluid shear near the walls, but for higher rotation rate the bubble moves to the center of the channel, due to the radial pressure gradient established by the rotation. For intermediate rotation rates, we find bubbles where the lift force and the pressure gradient balance and the bubbles remain between the centerline and the walls. We also examine the collective motion of a few bubbles and show that their dynamics are similar to what is observed for a single bubble.

Turntable IMU Calibration Algorithm Based on the Fourier Transform Technique

The paper suggests a new approach to calibration of a micromechanical inertial measurement unit. The data are collected on a simple rotating turntable with horizontal (or close to) rotation axis. For such a turntable, an electric screwdriver with fairly low rotation rate can be used. The algorithm is based on the Fourier transform applied to the rotation experimental data, implemented as FFT. The frequencies and amplitudes of the spectral peaks are calculated and collected in a small set of data, and calibration is done explicitly with these data. Calibration of an accelerometer triad and choosing the IMU coordinate frame are reduced to approximating the collected data with an ellipsoid in three dimensions. With rotation frequency calculated as the peak frequency of accelerometer readings, calibration of the gyros is a straightforward linear least square problem. The algorithm is purely algebraic, requires no iterations and no initial guess on the parameters, and thus encounters no convergence problems. The algorithm was tested both with simulated and experimental data, with some promising results.

Combining impedance and hydrodynamic methods in electrocatalysis. Characterization of Pt(pc), Pt5Gd, and nanostructured Pd for the hydrogen evolution reaction

Electrochemical hydrodynamic techniques typically involve electrodes that move relative to the solution. Historically, approaches involving rotating disc electrode (RDE) configurations have become very popular, as one can easily control the electroactive species’ mass transport in those cases. The combination of cyclic voltammetry and RDE is nowadays one of the standard characterization protocols in electrocatalysis. On the other hand, impedance spectroscopy is one of the most informative electrochemistry techniques, enabling the acquisition of information on the processes taking place simultaneously at the electrode/electrolyte interface. In this work, we investigated the hydrogen evolution reaction (HER) catalyzed by polycrystalline Pt (Pt(pc)) and Pt5Gd disc electrodes and characterized them using RDE and electrochemical impedance spectroscopy techniques simultaneously. Pt5Gd shows higher HER activities than Pt in acidic and alkaline media due to strain and ligand effects. The mechanistic study of the reaction showed that the rotation rates in acidic media do not affect the contribution of the Volmer–Heyrovsky and Volmer–Tafel pathways. However, the Volmer–Heyrovsky pathway dominates at lower rotation rates in alkaline media. Besides, the HER in acidic solutions depends more strongly on mass diffusion than in alkaline media. In addition to simple and clearly defined systems, the combined method of both techniques is applicable for systems with greater complexity, such as Pd/C nanostructured catalysts. Applying the above-presented approach, we found that the Volmer–Tafel pathway is the dominating mechanism of the HER for this catalytic system.

Rod-climbing rheometry revisited.

The rod-climbing or "Weissenberg" effect in which the free surface of a complex fluid climbs a thin rotating rod is a popular and convincing experiment demonstrating the existence of elasticity in polymeric fluids. The interface shape and steady-state climbing height depend on the rotation rate, fluid elasticity (through the presence of normal stresses), surface tension, and inertia. By solving the equations of motion in the low rotation rate limit for a second-order fluid, a mathematical relationship between the interface deflection and the fluid material functions, specifically the first and second normal stress differences, emerges. This relationship has been used in the past to measure the climbing constant, a combination of the first (Ψ1,0) and second (Ψ2,0) normal stress difference coefficients from experimental observations of rod-climbing in the low shear rate limit. However, a quantitative reconciliation of such observations with the capabilities of modern-day torsional rheometers is lacking. To this end, we combine rod-climbing experiments with both small amplitude oscillatory shear (SAOS) flow measurements and steady shear measurements of the first normal stress difference from commercial rheometers to quantify the values of both Ψ1,0 and Ψ2,0 for a series of polymer solutions. Furthermore, by retaining the oft-neglected inertial terms, we show that the "climbing constant"  = 0.5Ψ1,0 + 2Ψ2,0 can be measured even when the fluids, in fact, experience rod descending. A climbing condition derived by considering the competition between elasticity and inertial effects accurately predicts whether a fluid will undergo rod-climbing or rod-descending. Our results suggest a more general description, "rotating rod rheometry" instead of "rod-climbing rheometry", to be more apt and less restrictive. The analysis and observations presented in this study establish rotating rod rheometry combined with SAOS measurements as a prime candidate for measuring normal stress differences in complex fluids at low shear rates that are often below commercial rheometers' sensitivity limits.

Rotational remanent magnetization as a magnetic mineral diagnostic tool at low rotation rates

SUMMARY Prior work on rotational remanent magnetization (RRM) and rotational anhysteretic remanent magnetization (ARMROT) has demonstrated promise for magnetic mineral identification in earth materials. One challenge has been to calibrate the measurements to magnetic mineral types and microstructural controls, since previous studies have used differing spin rates, alternating field (AF) intensities and decay times, which hinders a comparison of data sets. Using a RAPID magnetometer we show that the range of usable practical rotation rates is 0.25–3 Hz [rps] which allows a wide range of RRM and ARMROT characteristics to be utilized (at 100 mT AF field, 100 μT bias field). Sets of magnetic mineral extracts from sediments, and well characterized rock samples that contain the key magnetic minerals magnetite, pyrrhotite and greigite are used for a calibration of the RRM-ARMROT behaviour. Detrital pyrrhotite and pyrrhotite-bearing phyllites have largely small positive effective field (Bg) values (up to 6 μT), with differences in Bg and ARMROT ratios at 0.5 and 2.5 Hz [rps] allowing grain size discrimination. The positive Bg values, and changes in RRM and ARMROT with rotation rates allow distinction of pyrrhotite from magnetite and diagenetic greigite. Diagenetic greigite has Bg values of –83 to –109 μT (at 0.5 Hz [rps]) and unusual RRM variation at low rotation rates caused by anisotropy affects. In contrast to previous work, based on crushed and sized natural magnetite at high spin rates, Bg for single domain magnetite from intact bacterial magnetofossils from Upper Cretaceous Chalk has some of the lowest Bg (0–1 μT) and displays a steep decline in ARMROT with increasing rotation rates. A simple tool for particle size characterization of magnetite may be the ratio of ARMROT at spin rates 2.5 and 0.5 Hz [rps]. Stability of RRM is better studied using RRM acquisition with increasing AF field intensity, since static demagnetization imparts a nuisance gyroremanence along the field axis. Mineral microstructure, dislocations and particle interactions are likely additional effects on RRM behaviour that need more investigation.

Experimental study of acoustic superradiance from a rotating absorber

The concept that classic waves reflected from a rotating absorbing cylinder will be amplified is analogously linked to the Penrose superradiance that may extract energy from a rotating black hole. The superradiance of acoustic waves carrying orbital angular momentum (OAM) from a sound absorber has been demonstrated in a recently published experiment using two rotating microphones. The experiment showed that the waves were amplified by up to 30% when the rotation rate of the absorber satisfied the Zel’dovich condition. Here, we proposed an experimental method to detect acoustic superradiance by static microphones. We demonstrated that the acoustic waves transmitted through a perforated absorber were amplified (by up to 1000%) even when the Zel’dovich condition was not satisfied. The experiment also showed that when the Doppler-shifted frequency of an observer was 0 Hz, the acoustic amplitude was very weak regardless of the rotation rate of the absorber. Our work was worthwhile not only in the proposed experimental method that was well-suited for observing the acoustic OAM wave but also in the high amplification achieved at a low rotation rate of the perforated absorber, showing a great prospect in practical applications such as amplifying the information-carrying OAM waves for high-speed acoustic communication.

Computational modelling of turbulent Taylor–Couette flow for bearing chamber applications: A comparison of unsteady Reynolds-averaged Navier–Stokes models

The capability to accurately model fluid flow within rotating Taylor–Couette systems has a primary role in informing computational investigations of rotating machinery. There is considerable uncertainty regarding selection of modelling approach, including a suitable turbulence model, that can accurately resolve turbulence within such complex flows while remaining computationally feasible for industrially relevant applications. This paper presents a numerical comparison of axisymmetric and three-dimensional unsteady Reynolds-averaged Navier–Stokes (URANS) turbulence models within ANSYS Fluent. The CFD geometries are representative of ones for which there are published experimental measurements. For the Taylor–Couette study, investigation into inner cylinder start-up procedure, based on previous published findings, confirmed that the final state of the flow is highly dependent on the initial conditions and acceleration rate. Once Taylor vortices form and stabilise, they are not disrupted by small steps in inner cylinder speed, allowing computationally efficient accelerations. Investigations into applying rotational periodicity were unsuccessful, resulting in a significantly reduced core velocity. Axisymmetric predictions provided reasonable agreement with experimental data only at low rotation rates. A good prediction of the velocity flow field was obtained for three-dimensional simulations of the full 360° domain with differences of less than 5% for radial velocities. Among the URANS models, the standard k-ω model and baseline Reynolds stress model (BSL-RSM) provided the closest agreement to published experimental data. In the paper, the developed Taylor–Couette turbulence modelling methodology is extended to a bearing chamber geometry. Analysis of the secondary vortex flow field is compared both qualitatively and quantitatively to published bearing chamber experimental measurements. Overall, whilst a good agreement is still found using the standard k-ω turbulence model, discrepancies arise with the BSL-RSM. However, for this more complex bearing chamber environment compared to a Taylor–Couette flow, the shear stress transport k-ω turbulence model provided the closest agreement and is recommended for future bearing chamber modelling.

A 16 au Binary in the Class 0 Protostar L1157 MMS

Abstract We present Very Large Array observations toward the Class 0 protostar L1157 MMS at 6.8 and 9 mm with a resolution of ∼0.″04 (14 au). We detect two sources within L1157 MMS and interpret these sources as a binary protostar with a separation of ∼16 au. The material directly surrounding the binary system within the inner 50 au radius of the system has an estimated mass of 0.11 M ☉, calculated from the observed dust emission. We interpret the observed binary system in the context of previous observations of its flattened envelope structure, low rates of envelope rotation from 5000 to 200 au scales, and an ordered, poloidal magnetic field aligned with the outflow. Thus, L1157 MMS is a prototype system for magnetically regulated collapse, and the presence of a compact binary within L1157 MMS demonstrates that multiple star formation can still occur within envelopes that likely have dynamically important magnetic fields.