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Abstract Slipstream, which is the induced air movement generated by a high-speed train (HST) as it passes, is both a safety hazard to commuters and trackside workers, and can cause damage to infrastructure along track lines. Bogies have been shown to exert a strong influence on HST aerodynamics by altering the underbody flow, and their effects have been extensively studied previously from the perspectives of ballast flight and drag reduction. In contrast, the effect of bogies on slipstream characteristics, and especially on the structure of the wake, is less well understood. This study explicitly investigates the effect of bogies on HST slipstream characteristics based on two generic train configurations: a Simplified Train Model (M1) with the bogies covered by a flat surface, and a Full-featured Train Model (M2) with simplified bogie-sets. The bogie effects are revealed through a systematic comparison of flow structures, slipstream characteristics and aerodynamic forces between these configurations. Remarkably, this study discovers that the generation of the strong spanwise oscillation of the wake, observed especially in the presence of bogies, can be interpreted as due to seeding and amplification of a natural instability of the time-mean pair of counter-rotating vortices behind the tail, rather than through direct side-to-side vortex shedding from the bogie geometry. This paper also documents how the altered wake flow affects slipstream characteristics through statistical and gust analyses, and the effect of the bogies on aerodynamic loading by comparing the train surface pressure distributions between the configurations. [ABSTRACT FROM AUTHOR]

Understanding the induced movement of air as a high-speed train passes ( slipstream ) is important for commuter and track-side worker safety. Slipstream is affected by the movement of the train relative to the ground, but this is difficult to include in wind-tunnel tests. Using simulations based on the Improved Delayed Detached Eddy Simulation model, this study investigates the effect of relative ground motion on slipstream for three different ground/wheel configurations: a stationary ground with stationary wheels, a moving ground with stationary wheels, and a moving ground with rotating wheels. By examining the interaction between the train-induced flow structure and ground boundary layer, this study identifies two ways that the ground boundary layer changes slipstream: through directly altering the high slipstream velocity region due to the ground boundary-layer development, and through indirect widening of the wake by deformation of the trailing vortices. The altered aerodynamic loading on a train due to relative ground motion is visualised through the surface pressure distribution, allowing the resultant impact on drag and lift to be assessed. For wheel rotation, it is concluded that its effect is mainly restricted to be within the bogie regions, with limited influence on the wake behind the train. [ABSTRACT FROM AUTHOR]

How the positioning and length of a container placed on an arbitrary train wagon in an otherwise fully loaded train affects the local aerodynamics, and consequently the contribution to drag, is examined here. Results from scale-model wind-tunnel tests undertaken at a Reynolds number of 0.3 × 10 6 for a combination of 49 upstream and downstream gap spacings ( G f , G r ) are presented. Surface flow topology, pressure profiles and planar velocity fields are measured. G f dominated the drag variations, with G r only causing a secondary effect. The greatest drag reduction potential is found between gaps size of 1.77 W and 3.23 W , where W represents the wagon width. Over the range of G f and G r investigated, a number of distinct physical mechanisms were observed. These affect the separation size and the nature of boundary layer enveloping the wagon, which have a direct impact on the entrainment and shedding frequency of the wake. [ABSTRACT FROM AUTHOR]

The air movement induced by a high-speed train (HST) as it passes, the slipstream, is a safety hazard to commuters and trackside workers, and can cause damage to infrastructure along track lines. Because of its importance, many numerical studies have been undertaken to investigate this phenomenon. However, to the authors' knowledge, a systematic comparison of the accuracy of different turbulence models applied to the prediction of slipstream has not yet been conducted. This study investigates and evaluates the performance of three widely used turbulence models: URANS, SAS and DES, to predict the slipstream of a full-featured generic train model, and the results are compared with wind-tunnel experimental data to determine the fidelity of the models. Specifically, this research aims to determine the suitability of different turbulence modelling approaches, involving significantly different computational resources, for modelling different aspects of slipstream. [ABSTRACT FROM AUTHOR]

The flow-induced deformation of a thin, elastic splitter plate attached to the rear of a circular cylinder and subjected to laminar pulsatile inflow is investigated. The cylinder and elastic splitter plate are contained within a narrow channel and the Reynolds number is mostly restricted to Re =100, primarily covering the two-dimensional flow regime. An in-house Fluid-Structure Interaction code is employed for simulations, which couples a sharp-interface immersed boundary method for the fluid dynamics with a finite-element method to treat the structural dynamics. The structural solver is implicitly (two-way) coupled with the flow solver using a partitioned approach. This implicit coupling ensures numerical stability at low structure-fluid density ratios. A power spectrum analysis of the time-varying plate displacement shows that the plate oscillates at more than a single frequency for pulsatile inflow, compared to a single frequency observed for steady inflow. The multiple frequencies obtained for the former case can be explained by beating between the applied and plate oscillatory signals. The plate attains a self-sustained time-periodic oscillation with a plateau amplitude in the case of steady flow, while the superimposition of pulsatile inflow with induced plate oscillation affects the plateau amplitude. Lock-in of the plate oscillation with the pulsatile inflow occurs at a forcing frequency that is twice of the plate natural frequency in a particular mode and this mode depends on the plate length. The plate displacement as well as pressure drag increases at the lock-in condition. The percentage change in the maximum plate displacement, and skin-friction and pressure drag coefficients on the plate, due to pulsatile inflow is quantified. The non-linear dynamics of the plate and its coupling with the pulsatile flow are briefly discussed. [ABSTRACT FROM AUTHOR]

This is a numerical investigation of the unsteady mixed convective flow over an isothermally heated circular cylinder. The flow is directed vertically downwards with buoyancy forces in the opposite direction. A parametric analysis was conducted at five Reynolds numbers Re = 10 , 20 , 30 , 40 , 100 , and for Richardson numbers between Ri = 0 and 0.5, to evaluate the influence of thermal buoyancy on the instantaneous flow behaviour, vortex-shedding characteristics, thermal fields, and mean heat-transfer rates. The simulations show that at the highest Reynolds number investigated, Re = 100 , the normal periodic von Kármán vortex street still characterises the flow as Richardson number is increased; however, the oscillation frequency decreases and the oscillation amplitude increases. For Re ≤ 40 , a new vortex street appears, named the " Buoyancy-Opposing Vortex Street ", due to thermal buoyancy modifying the von Kármán street. Interestingly, at very low Re = 20 and 10, the von Kármán street completely disappears in the heated wakes, whereas the "Buoyancy-Opposing Vortex Street" remains as the sole vortex street characterising the flow. • Unsteady mixed convective flow vertically downward directed to across a heated circular cylinder, is studied. • The analysis was conducted at Re = 10, 20, 30, 40, 100 and for Richardson number ranging between Ri = 0–0.5. • The aim is to evaluate the influence of the buoyancy effects on the vortex shedding attributes. • Unusually Kármán vortex street is generated for R e ⩽ 40 by heating the cylinder. • A new vortex street, namely the "Opposing-Buoyancy Vortex Street", is discovered by heating the Kármán vortex street. [ABSTRACT FROM AUTHOR]

A full-scale 48-foot shipping container was instrumented with surface pressure taps and loaded onto a number of single- and double-stacked container freight trains. Surface pressure data enabled the container pressure drag coefficient to be evaluated for a range of different train loading configurations, container positions along the train, and atmospheric wind conditions. Field-based measurements show that for low and high crosswind conditions surface pressure distributions measured on the front face of the instrumented container are in good agreement with those reported in past studies. However, the magnitude of the pressure drag coefficient was found to be typically 50% lower for all loading cases analysed compared with previous seemingly analogous wind-tunnel and numerical investigations. For high crosswind conditions, the drag coefficient was found to increase and correlated well with the level of asymmetry observed in the measured pressure distributions. This was true for all cases regardless of the position along the length of the train. In the absence of direct information of the incident free-stream wind conditions, the level of asymmetry in the pressure distributions was found to provide a viable indirect method for assessing the impact that crosswinds have on the aerodynamic drag of freight trains. • A full-scale shipping container was instrumented with surface pressure taps. • Pressure drag was evaluated for different loading configurations and container locations. • Drag coefficient was 50% lower compared with previous investigations. • An indirect method for assessing the effect of wind conditions on drag was established. • Estimates for aerodynamic efficiency and loading configuration considerations are made. [ABSTRACT FROM AUTHOR]

Abstract: A detail numerical analysis of the effect of particle diameter of a packed bed of spherical particles on forced convection about an embedded circular cylinder is presented. This parametric study focusses on the two-phase energy (LTNE—local thermal non-equilibrium) model, which does not assume local thermal equilibrium (LTE) between the solid medium and the fluid. The investigation is performed for a cylinder-to-particle diameter ratio D cy /d p =10–100, at a wide ranges of Reynolds number Re D =1–250 and solid-to-fluid thermal conductivity ratio k r =0.01–1000. A comparison of predictions from the LTNE and LTE energy models is also made. This paper quantifies the influence of the key non-dimensional parameters on the heat transfer rate. It is also shown that although the presence of the porous materials around the heated cylinder enhances the overall heat transfer and increases the pressure drop in the bed compared to an empty channel, using a porous medium with large particle diameters increases considerably this enhancement in heat transfer and decreases significantly the unfavorable pressure drop. [Copyright &y& Elsevier]

An investigation into the control of the screech noise of two-dimensional underexpanded supersonic jets was carried out using the hydraulic analogy. Favorable agreement was found with previous experiments between the variations with pressure ratio of the measured screech-amplitude variation and the screech frequency. Remarkably, the intensity of the screech tone was able to be altered substantially by the positioning of a relatively small cylinder along the centerline of the jet flow. Two different cylinder diameters and two different Froude (Mach) numbers were tested. The normalized change in screech-tone intensity for all cases was found to correlate with the relative position in the shock cell of the intersection of the cylinder bow wave and the jet shear layer. [ABSTRACT FROM AUTHOR]

• Investigated flow control of a BFS at higher Reynolds numbers than previous studies. • Examined transient flow effects on the base pressure and detailed flow structure. • Proposed a model for the base pressure increase seen with high-frequency forcing. • Compared the response of the BFS to bluff bodies undergoing flow control. This work provides new insight into the transient flow effects of forcing a backward-facing step flow at high Reynolds number. The detailed flow structure and surface pressure is investigated over the range 118 , 000 ⩽ Re H ⩽ 472 , 000 using time-resolved particle-image velocimetry. Notably, this research considerably extends the Reynolds number ranges of previous studies. The effect of periodic shear-layer forcing on the global mean and fluctuating flow structure is examined over the forcing frequency range 0.036 ⩽ St H ⩽ 1.98. The effect of forcing on the base pressure, which has received relatively little attention, is also detailed. These studies allow us to characterise the relationship between the forcing frequencies, with a particular focus on frequencies near and significantly above the shear-layer instability. Importantly, despite the high Reynolds number, we are able to perturb the shear layer to control both the reattachment length and base pressure. Forcing at frequencies close to the shear-layer instability results in a significant reattachment length reduction, as has been well reported, with a corresponding base pressure reduction of up to 45%. At the highest forcing frequency, an increase in mean base pressure of 9.7% is found, with a corresponding increase in reattachment length of 3.9% over the unforced case. This high-frequency forcing reduced initial growth of the shear-layer instability and stabilised the latter half of the reattachment zone. This enabled more flow entrainment upstream to the step-base, resulting in a mean base-pressure increase. Although, this came at the cost of triple the base-pressure fluctuations, an important insight for practical flow-control applications. The spatial distribution of spectral power for key instabilities in the flow, and the influence of forcing on these distributions, is also examined. To our knowledge, the spatial distribution of these key instabilities, with or without control, has not been previously presented for high Reynolds numbers ( Re H > 10 4 ). [ABSTRACT FROM AUTHOR]

Local thermal equilibrium ( L T E ) is a frequently-employed hypothesis when analysing convection heat transfer in porous media. However, investigation of the non-equilibrium phenomenon exhibits that such hypothesis is typically not true for many circumstances such as rapid cooling or heating, and in industrial applications involving immediate transient thermal response, leading to a lack of local thermal equilibrium ( L T E ). Therefore, for the sake of appropriately conduct the technological process, it has become necessary to examine the validity of the L T E assumption before deciding which energy model should be used. Indeed, the legitimacy of the L T E hypothesis has been widely investigated in different applications and different modes of heat transfer, and many criteria have been developed. This paper summarises the studies that investigated this hypothesis in forced, free, and mixed convection, and presents the appropriate circumstances that can make the L T E hypothesis to be valid. For example, in forced convection, the literature shows that this hypothesis is valid for lower Darcy number, lower Reynolds number, lower Prandtl number, and/or lower solid phase thermal conductivity; however, it becomes invalid for higher effective fluid thermal conductivity and/or lower interstitial heat transfer coefficient. [ABSTRACT FROM AUTHOR]

To date, it has been shown that the vibration response of an elastically mounted sphere undergoing vortex-induced vibration (VIV) can be controlled by imposing rotary oscillations at frequencies close to the vibration frequency. Here, we demonstrate that rotary oscillations imposed at significantly higher frequencies can be used to directly influence shear-layer vortex shedding and consequently reduce vibration. This approach contrasts with aiming to directly target the large-scale wake structures, using lower frequency perturbations. The oscillation frequencies imposed were between 5 and 35 times the natural frequency of the system and the amplitude of the rotational velocities were only 10% of the free-stream velocity. The effects of the rotary oscillations were found to vary significantly across sphere vibration modes. In the mode III transition regime significant attenuation of the vibration response was observed for a narrow band of rotary oscillation frequencies. Time-resolved particle image velocimetry revealed that the shear-layer vortex structures locked to the forcing frequency, where suppression of the vibration response occurred. Optimal tuning of the oscillation frequency reduced the vibration amplitude in the mode III transition regime by 84%, with a rotational velocity amplitude of only 10% of freestream. These results show low-amplitude shear-layer forcing is a promising method of more efficiently suppressing VIV of three-dimensional geometries. [ABSTRACT FROM AUTHOR]

A novel full-scale field test was undertaken to assess the aerodynamic performance of shipping containers loaded on inter-modal freight trains. The aerodynamic performance of an instrumented 48 ​ft container, located 185 ​m downstream of the locomotive, is assessed in the context of surface pressure, weather station, and GPS data sets. Previous studies on the aerodynamics of trains have been largely limited to low-resolution, reduced-order and scaled; field, numerical and wind-tunnel studies; respectively. The objective here was to determine the capacity of this novel field-based method to assess the aerodynamic performance of full-scale train containers for a large range of operating conditions. For low wind conditions, where the yaw angle is predicted to be low, measured surface pressure distributions on the front and base of the container are similar to that of past work however the magnitude of the drag coefficient was much lower, by up to 65%. This suggests that previous studies are yet to fully detail the drag profile of containers located at large downstream distance. Observed asymmetry of the pressure distribution on the front of the container are generally consistent with wind conditions measured at nearby weather stations and can be used as a proxy to determine the wind yaw angle at the train. • A novel full-scale field test was undertaken on a freight train by instrumenting a shipping container. • This was done in the context of weather station, GPS, measurement uncertainty and pressure data. • The pressure distribution on the front and base was similar to past work however the magnitude of the drag coefficient was lower. • Pressure distribution asymmetry on the front of the container was consistent with wind conditions seen at weather stations. • The asymmetry can be used as a proxy to determine the wind yaw angle. [ABSTRACT FROM AUTHOR]

The effect of structural nonlinearity on vortex-induced vibration of a rigid circular cylinder has been studied computationally for a fixed mass ratio of m ∗ = 2. 546 at R e = 150. The arrangement of the springs and damper is similar to setup for the Standard Linear Solid (SLS) model used to model a viscoelastic material. One linear spring is in series with the damper and another nonlinear spring is parallel with the damper. The nonlinear structural system is governed by the following three parameters: (a) the ratio of the linear spring constants (R), (b) damping ratio (ζ), and (c) nonlinearity strength (λ). The focus of the present study is to examine the response of the cylinder to VIV by changing ζ and λ. The peak amplitude decreases in comparison with a linear spring, as spring softening (λ < 0) is increased; in contrast, the peak amplitude increases for a hardening spring (λ > 0). The equivalent reduced velocity (U r e q), a measure of nonlinearity, is affected by damping, showing the non-monotonic variation with ζ. There exists a critical value ζ ≈ 1 below which the equivalent reduced velocity decreases and beyond which U r e q increases. We also observed at high values of λ (e.g. λ = 4), the peak lift force coefficient is almost constant over a wide range of reduced velocity, with the absence of the lower branch for a very low (ζ = 0. 001) and high (ζ = 10) values of damping. This near constant amplitude range suggests a hardening spring may be useful for extending the operational range for energy extraction applications. Finally, increased system nonlinearity leads to considerably richer spectral content in the displacement and force signals, reflected in the wake development. [ABSTRACT FROM AUTHOR]

This paper reports on an extensive numerical investigation of the effects of pivot location and mass ratio ( m ∗ = solid/fluid mass) on flow-induced vibration (FIV) of a foil undergoing fully passive two-degree-of-freedom (2-DOF) plunging and pitching motion in a two-dimensional free-stream flow. Here, the normalised pivot location is defined by x = x p ∕ c , with c the foil length and x p the distance to the foil leading edge. A comprehensive set of numerical simulations were conducted employing an Immersed Boundary Method at a Reynolds number of 400. By analysing the FIV dynamics for three selected mass ratios, m ∗ = 5 , 20 and 200, at two pivot locations, x = 0. 35 and 0.50, it is found that there are two types (type-I and type-II) of FIV responses, one is primarily a driven static instability while the other is strongly associated with vortex shedding. Interestingly, for x = 0. 50 , which is close to the mass centre, increasing the mass ratio can favour suppression of the chaotic response. Importantly, it is shown that there exists a critical mass ratio, above which the foil oscillations are suddenly suppressed. The findings indicate that the combined effects of eccentricity and mass ratio on the foil dynamics can be profound. [ABSTRACT FROM AUTHOR]

In this paper, a numerical investigation of natural convection in a porous medium confined by two horizontal eccentric cylinders is presented. The cylinders are impermeable to fluid motion and retained at uniform different temperatures. While, the annular porous layer is packed with glass spheres and fully-saturated with air, and the cylindrical packed bed is under the condition of local thermal non-equilibrium. The mathematical model describing the thermal and hydrodynamic phenomena consists of the two-phase energy model coupled by the Brinkman-Forchheimer-extended Darcy model under the Boussinesq approximation. The non-dimensional derived system of formulations is numerically discretised and solved using the spectral-element method. The investigation is conducted for a constant cylinder/particle diameter ratio ( D i / d) = 30, porosity (ε) = 0.5, and solid/fluid thermal conductivity ratio ( k r ) = 38.6. The effects of the vertical, horizontal and diagonal heat source eccentricity (−0.8 ⩽ e ⩽ 0.8) and the annulus radius ratio (1.5 ⩽ RR ⩽ 5.0) on the temperature and velocity distributions as well as the overall heat dissipation within both the fluid and solid phases, for a broad range of Rayleigh number (104 ⩽ Ra ⩽ 8 × 10 7 ). The results show that uni-cellular, bi-cellular and tri-cellular flow regimes appear in the vertical eccentric annulus at the higher positive eccentricity e = 0.8 as Rayleigh number increases. However, in the diagonal eccentric annulus, the multi-cellular flow regimes are shown to be deformed and the isotherms are particularly distorted when Rayleigh number increases. In contrast, in the horizontal eccentric annulus, it is found that whatever the Rayleigh number is only an uni-cellular flow regime is seen. In addition, it is shown that the fluid flow is always unstable in the diagonal eccentric geometry at e = 0.8 for moderate and higher Rayleigh numbers. However, it loses its stability in the vertical eccentric geometry only at two particular cases, while it is always stable in the horizontal eccentric geometry, for all eccentricities and Rayleigh numbers. [ABSTRACT FROM AUTHOR]

In this study, we perform a numerical investigation to better comprehend the domination and the effect of strong thermal buoyancy on hydrodynamic and thermal characteristics, vortex shedding in particular, of a horizontal circular heated cylinder subject to a vertically upward laminar flow stream. The Reynolds number is varied in the range 20 ≤ R e ≤ 150 while maintaining the Prandtl number constant at P r = 7.1. The effect of thermal buoyancy on the system is determined by varying the Richardson number up to R i = 5. The behaviour is determined by solving the unsteady laminar two-dimensional Navier–Stokes and standard energy equations numerically, utilising the spectral-element method with buoyancy considered using the Boussinesq approximation. Representative vorticity, streamline, and thermal patterns are presented, and the average Nusselt numbers are plotted as a function of the Richardson number for a set of Reynolds numbers to explain, in detail, the role of superimposed thermal buoyancy on the wake flow and rates of heat transfer. The predictions demonstrate that the wake flow exhibits unsteady periodic characteristics for the selected moderate Reynolds numbers, and as the buoyancy parameter increases, the fluid flow and temperature fields become less stable. Subsequently, it was observed that heat transfer augmented considerably, the vortex shedding stopped abruptly, the wake converted to steady twin vortices beyond certain critical Richardson numbers, and the Nusselt numbers suddenly reduced to minimum values. These critical values are determined to increase with the Reynolds number. The results also confirm that for increased heating, the recirculating flow in the cylinder near the wake disappears and flow separation occurs only at the backward stagnation point. [ABSTRACT FROM AUTHOR]

The effect of a viscoelastic-type structural support on vortex-induced vibration (VIV) of a circular cylinder has been studied computationally for a fixed mass ratio ( m ∗ = 2. 546) and Reynolds number (R e = 150). Unlike the classical case of VIV where the structural support consists of a spring and damper in parallel, this study considers two springs and one damper, where the two springs are in parallel and the damper is in series with one of the springs. This spring/damper arrangement is similar to the Standard Linear Solid (SLS) model used for modelling viscoelastic behaviour. The viscoelastic support (SLS type) is governed by the following two parameters: (a) the ratio of spring constants (R), and (b) the damping ratio (ζ). The focus of the present study is to examine and understand the varied response of the cylinder to VIV as these parameters are varied. For small ζ and R , the cylinder response shows characteristics similar to the classical case, where the amplitude response is composed of an upper- and the lower-type branch. The presence of upper-type branch at low R e is evident through the peak lift force, frequency and phase response of the cylinder. As the damping ratio is increased, the vibration amplitude decreases and hence the upper-type branch disappears. There exists a critical value of ζ = 1 beyond which the amplitude again increases asymptotically. The non-monotonic variation of amplitude response with ζ is presented in the form of the "Griffin plot". The amplitude, force, frequency and phase-difference response of cylinder were found to be mirror symmetric in log (ζ) about ζ = 1. In addition, the effect of R at the critical value of damping, ζ = 1 , was studied. This show that the amplitude decreases with an increase of R , with suppression of the response branches for high R values. The results suggest that a careful tuning of the damping may be effectively employed both to enhance power output for energy extraction applications or to suppress flow-induced vibration. [ABSTRACT FROM AUTHOR]

Slipstream is the induced movement of air as a high-speed train (HST) passes. Previous studies have shown that the development of slipstream is highly geometry dependent, including both the geometries of the trains and nearby objects. Much effort has been channelled into reducing slipstream through optimisation of the train geometry; however, the impact of the rails on train aerodynamics remains largely unexplored. This study analyses the effect of rails on HST slipstream characteristics by systematically comparing the wakes for two geometric configurations incorporating: No Rails (NR) and With Rails (WR). The train model remains identical in both configurations, with the only difference being whether rails are included. This study highlights the potential effects of rails on HST slipstream characteristics, and reveals the underlying mechanism of how the rails shape the wake flow structures. By examining both mean and time-dependent flow features, the simulations show that the rails can significantly alter slipstream characteristics, especially the downstream evolution of the wake, effectively by reducing the lateral movement of the mean streamwise vortex structures, despite the relatively small length scale of the rail cross-section. Perhaps surprisingly, the slipstream measured at the standard distance from the train centerplane, is found be significantly reduced. • The study examines the differences between two models, with and without rails, as both models were adopted in the literature. • This study highlights the potential effects of rails on high-speed train slipstream. • This study further reveals the underlying mechanism of how the rails shape the wake structures. • This study provides insight in formulating the best practice for numerically predicting high-speed train slipstream. [ABSTRACT FROM AUTHOR]

In this paper, we study the vortex-shedding mechanism of a square cylinder subjected to a mean flow with a superimposed pulsatile flow, under symmetric vortex shedding conditions. The near-body vortical events are interpreted in accordance with the two-dimensional vorticity transport equation, and the interactions between different vortical regions are quantified by circulation balance for a control volume in the near wake. Strong wake counter flow that splits into two branches, with one branch considerably disrupting the boundary layer while other branch cuts off supply of vorticity to the shear layer, is found to be an essential feature of symmetric vortex shedding. To further clarify the roles of correlated terms contributing to the vorticity generation and transport in the wake, the Reynolds number was varied while keeping other excitation parameters constant. This results in the intensity of symmetric vortex shedding increasing with Reynolds number. The increase in wake counter-flow strength with increased Reynolds number cannot be explained either by a relative velocity between the mean and pulsatile flow components or relative acceleration between bluff body and fluid. Based on the results it is inferred that the brief rolling up of the shear layer to form a wake vortex during a part of the pulsation cycle, together with the associated unsteady tangential pressure gradient around the body, contribute to the stronger wake counter flow at higher Reynolds numbers. At lower pulsation amplitudes, the tendency of the fluid in the wake to form strong counter flow can interact significantly with the transverse entrainment flow to alter the natural vortex-shedding mechanism, to give different vortex-shedding modes. • Strong wake counter flow is an essential feature of symmetric vortex shedding. • Wake counter flow depends on wall–vorticity interactions in the near wake. • The nonlinear vorticity production due to wake counter flow can be significant. • Vorticity generated at base wall can also be shed in to wake. • The differing mechanisms of vortex formation can interact in the near wake. [ABSTRACT FROM AUTHOR]

This study experimentally investigates the transverse flow-induced vibration (FIV) of an elastically mounted cube at three different incidence angles of α = 0 ∘ , 2 0 ∘ and 4 5 ∘ , corresponding to two centreplane mirror-symmetric cases and one asymmetric case. The FIV response is characterised by analysing the vibration amplitude and frequency responses, together with the fluid force coefficients and phases, over the reduced velocity range of 1. 2 ⩽ U ∗ ⩽ 16. Here, U ∗ = U ∕ (f nw H) , with U the free-stream velocity, f nw the natural frequency of the system in quiescent fluid (water) and H the frontal projected width of the body. It was found in the α = 0 ∘ case that two synchronisation regions could be identified, where the periodic body vibration was synchronised with oscillatory shear layers. The vibration amplitude was found to increase with U ∗ in the second synchronisation region, with the largest value of A max ∗ ≃ 0. 74 observed at the highest U ∗ value tested. In the asymmetric orientation case of α = 2 0 ∘ , a synchronisation region occurred over 6. 0 < U ∗ < 8. 6 , where the amplitude tended to increase to its local peak of A max ∗ ≃ 0. 25 at U ∗ = 8. 6. For higher U ∗ values, synchronisation was lost, but the cube still exhibited high amplitude oscillations. However, in the α = 4 5 ∘ case, while the vibration amplitude tended to increase with U ∗ (i.e. A max ∗ ≈ 0. 3 at U ∗ = 16), the FIV response was found to be desynchronised over the entire U ∗ range. The findings indicate that body vibration is strongly coupled with the oscillatory separating shear layers for all α cases, which can result in significant vibration. [ABSTRACT FROM AUTHOR]

A liquid marble micro-bioreactor is prepared by placing a drop of murine embryonic stem cell (ESC) (Oct4B2-ESC) suspension onto a polytetrafluoroethylene (PTFE) particle bed. The Oct4B2-ESC aggregates to form embryoid bodies (EBs) with relatively uniform size and shape in a liquid marble within 3 d. For the first time, the feasibility of differentiating ESC into cardiac lineages within liquid marbles is being investigated. Without the addition of growth factors, suspended EBs from liquid marbles express various precardiac mesoderm markers including Flk-1, Gata4, and Nkx2.5. Some of the suspended EBs exhibit spontaneous contraction. These results indicate that the liquid marble provides a suitable microenvironment to induce EB formation and spontaneous cardiac mesoderm differentiation. Some of the EBs are subsequently plated onto gelatin-coated tissue culture dishes. Plated EBs express mature cardiac markers atrial myosin light chain 2a (MLC2a) and ventricular myosin light chain (MLC2v), and the cardiac structural marker α-actinin. More than 60% of the plated EBs exhibit spontaneous contraction and express mature cardiomyocyte marker cardiac troponin T (cTnT), indicating that these EBs have differentiated into functional cardiomyocytes. Together, these results demonstrate that the liquid-marble technique is an easily employed, cost effective, and efficient approach to generate EBs and facilitating their cardiogenesis., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The architecture of the branched root system of plants is a major determinant of vigor. Water availability is known to impact root physiology and growth; however, the spatial scale at which this stimulus influences root architecture is poorly understood. Here we reveal that differences in the availability of water across the circumferential axis of the root create spatial cues that determine the position of lateral root branches. We show that roots of several plant species can distinguish between a wet surface and air environments and that this also impacts the patterning of root hairs, anthocyanins, and aerenchyma in a phenomenon we describe as hydropatterning. This environmental response is distinct from a touch response and requires available water to induce lateral roots along a contacted surface. X-ray microscale computed tomography and 3D reconstruction of soil-grown root systems demonstrate that such responses also occur under physiologically relevant conditions. Using early-stage lateral root markers, we show that hydropatterning acts before the initiation stage and likely determines the circumferential position at which lateral root founder cells are specified. Hydropatterning is independent of endogenous abscisic acid signaling, distinguishing it from a classic water-stress response. Higher water availability induces the biosynthesis and transport of the lateral root-inductive signal auxin through local regulation of tryptophan aminotransferase of Arabidopsis 1 and PIN-formed 3, both of which are necessary for normal hydropatterning. Our work suggests that water availability is sensed and interpreted at the suborgan level and locally patterns a wide variety of developmental processes in the root.

The feasibility of a novel stirred bioreactor, the rotating aerial disk (RAD) design, was tested in this study. The novelty lies in its method of medium recirculation by convective airflow using a non-contact planar disc, a variation on a physically defined theoretical model. Computational predictions of improved oxygenation were confirmed by increases in measured dissolved oxygen, even at Reynolds numbers (100-200) where flow is mostly laminar. EL-4 mouse lymphoma cells grown for the first time as suspension cultures in the RAD bioreactor, were mechanically re-organization into dense, circular three-dimensional colonies (diameter 3-5 mm, thickness 5-800 microm), more rapidly than we have observed previously. Cell proliferation in the RAD vessels was similar to static cultures, although lactate production from glucose was significantly lower, suggesting a shift toward aerobic glycolysis. This possible reversal of the 'Warburg effect' was accompanied by a decrease in mitochondrial activity, perhaps reflecting a more quiescent cytoplasmic state. The RAD device may be useful as scalable, three-dimensional solid tumor model under more physiological conditions then static culture.