Streamwise Position Research Articles

Shear-Rate-Independent Diffusion in Granular Flows.

We computationally study the behavior of the diffusion coefficient D in granular flows of monodisperse and bidisperse particles spanning regions of relatively high and low shear rate in open and closed laterally confined heaps. Measurements of D at various flow rates, streamwise positions, and depths collapse onto a single curve when plotted as a function of γd2, where d is the local mean particle diameter and γ is the local shear rate. When γ is large, D is proportional to γd2, as in previous studies. However, for γd2 below a critical value, D is independent of γd2. The acceleration due to gravity g and particle stiffness (or, equivalently, the binary collision time t(c)) together determine the transition in D between regimes. This suggests that while shear rate and particle size determine diffusion at relatively high shear rates in surface-driven flows, diffusion at low shear rates is an elastic phenomenon with time and length scales dependent on gravity (sqrt d/g) and particle stiffness (t(c)sqrt(dg), respectively.

Velocity derivative skewness in fractal-generated, non-equilibrium grid turbulence

The evolution of the velocity derivative skewness, S(∂u/∂x), is investigated along two streamwise axes and four transverse positions in the wake of a square-fractal-element grid. In the near-field, the produced turbulence exhibits non-equilibrium characteristics including Cϵ∼ReMα/ReLβ. In the far-field, the turbulence agrees with canonical grid turbulence results and Cϵ is approximately constant. It is found that in the non-equilibrium region, the value of −S(∂u/∂x) is dependent on both streamwise and transverse positions, but after a sufficient decay period, it takes on a near constant value in the far-field. It is demonstrated that the evolution Cϵ approximately corresponds to that of −S(∂u/∂x), which is suggestive that some of the non-equilibrium properties are likely a result of residual strain from the turbulence generating conditions.

An optical approach for quantitative characterization of slug bubble interface profiles in a two-phase microchannel flow

A new measurement technique is developed for quantitatively mapping the liquid–gas interface profiles of air bubbles in an adiabatic microchannel slug flow environment. Water seeded with 0.5μm-diameter fluorescent polystyrene particles is pumped through a single acrylic microchannel of 500μm×500μm square cross section. A periodic slug flow is achieved by the controlled injection of air into the channel. Particles are constrained to the liquid phase, and their distribution in the flow is visualized through an optical microscope in an epifluorescent configuration with pulsed laser illumination to resolve the instantaneous liquid–gas interface profile to within ±2.8μm in the focal plane.This approach is able to identify the interface profile within individual focal planes at various depths within the channel, unlike conventional backlit optical profile detection approaches that can only resolve the interface at the midplane. A similar particle-tracking technique was previously demonstrated for interface reconstruction in annular flows; however, the additional noise within images due to the reflection and refraction of background light at the compound-curvature interfaces characteristic of slug bubbles requires texture-based image analysis to obtain interface profiles. The varying interface profile of the slug bubbles in the streamwise direction also greatly complicates the tracking procedures for achieving a three-dimensional reconstruction of slug bubbles based on the measured two-dimensional interface profiles, which requires spatial alignment of the streamwise position of liquid–vapor interfaces realized at varying depths within the channel. This is addressed during reconstruction by using the measured mid-plane slug bubble cap curvature to assign the relative streamwise positions of interface profiles obtained at other measured depths. The characterization of two different selected bubble morphologies presented here demonstrates a critical improvement in metrological capability which can provide greater insight into microchannel flow phenomena in the slug-flow regime.

Methane–air explosion characteristics with different obstacle configurations

To investigate the flame and overpressure characteristics of methane–air explosion with different obstacle configurations, an experimental study has been conducted, taking account of the number of obstacles, obstacle distance from ignition source, and stream-wise and cross-wise obstacle positions. The results show that the flame speed and peak overpressure increase with the increasing number of obstacles, while the time to reach the peak is not fully determined by it. And the configuration having the farthest obstacle produces a higher overpressure and takes a longer time to reach the peak, but a slower flame propagation speed is obtained. Similar explosion characteristics are also observed in the configurations with two obstacles fixed at different stream-wise positions. Furthermore, the experimental results demonstrate that the peak overpressures and flame speeds in configurations with central or staggered obstacles are relatively higher, which should to be avoided in practical processes to minimize the risk associated with methane–air explosion.

Interaction of an off-surface cylinder with separated flow from a bluff body leading edge

Time resolved PIV measurements are performed on a cylinder of diameter D placed in the vicinity of the leading edge of an elongated bluff body at ReD=2.6×104. The off-surface cylinder is placed at three different wall-normal positions and two streamwise positions, upstream and downstream the bluff body leading edge. Topological, statistical and transient analyses are performed on the resulting flowfield. The streamwise position of the cylinder relative to the bluff body leading edge significantly affects the mean flow topology and the vortex shedding dynamics of both the cylinder and the bluff body separation bubble. When the cylinder is positioned upstream of the bluff body leading edge, the cylinder shedding becomes phase-locked to the bluff body bubble shedding, while the low frequency bubble fluctuation is not significantly affected. When the cylinder is positioned downstream the bluff body leading edge, the pressure gradient eliminates the separation bubble and regular cylinder vortex shedding occurs. The StD number of the cylinder shedding is reduced when the separation bubble is eliminated as the latter causes blockage that reduces the effective cylinder diameter. POD analysis captures the separation bubble and cylinder shedding dynamics, demonstrating that when the cylinder is placed near the wall, low frequency fluctuation modes associated with the separation bubble dominate over the cylinder shedding modes.

1522 矩形ダクト流における過渡的乱れの寿命

Lifetime of transient turbulence in a rectangular-duct with the cross-sectional aspect ratio A = 5 is measured experimentally by using a multi-PIV (Particle Image Velocimetry) system. The survival probability of transient turbulence is observed to decay exponentially as a function of its streamwise position. The comparison of the mean lifetime obtained from exponential fitting of the probability for A = 5 with the former experimental data for A = 3 shows that the mean lifetime significantly depends on the aspect ratio. For a fixed value of the bulk Reynolds number, the mean lifetime in the wider duct with A = 5 is shorter than that in the narrower duct with A = 3.

Measuring Large Scale Flow Structures Behind a Bluff Body Using a Hot-wire Rake

Wake behind a D-shaped bluff body were studied using experimental method. The forcing amplitude was Cμ = 0.1%. A rake of 14 hot-wire probes were used to measure the instantaneous velocity at different stream-wise position simultaneously. It was found when two actuators act in phase at a forcing frequency S tA of 0.16, the anti-symmetric motion (mode 1) in the wake was suppressed and the periodic changes in the size of the separation bubble (mode 2) were promoted.

Analysis of pressure perturbation sources on a generic space launcher after-body in supersonic flow using zonal turbulence modeling and dynamic mode decomposition

A sparsity promoting dynamic mode decomposition (DMD) combined with a classical data-based statistical analysis is applied to the turbulent wake of a generic axisymmetric configuration of an Ariane 5-like launcher at Ma∞ = 6.0 computed via a zonal Reynolds-averaged Navier-Stokes/large-eddy simulation (RANS/LES) method. The objective of this work is to gain a better understanding of the wake flow dynamics of the generic launcher by clarification and visualization of initially unknown pressure perturbation sources on its after-body in coherent flow patterns. The investigated wake topology is characterized by a subsonic cavity region around the cylindrical nozzle extension which is formed due to the displacement effect of the afterexpanding jet plume emanating from the rocket nozzle (Mae = 2.52, pe/p∞ = 100) and the shear layer shedding from the main body. The cavity region contains two toroidal counter-rotating large-scale vortices which extensively interact with the turbulent shear layer, jet plume, and rocket walls, leading to the shear layer instability process to be amplified. The induced velocity fluctuations in the wake and the ultimately resulting pressure perturbations on the after-body feature three global characteristic frequency ranges, depending on the streamwise position inside the cavity. The most dominant peaks are detected at SrD r3 = 0.85 ± 0.075 near the nozzle exit, while the lower frequency peaks, in the range of SrD r2 = 0.55 ± 0.05 and SrD r1 = 0.25 ± 0.05, are found to be dominant closer to the rocket’s base. A sparse promoting DMD algorithm is applied to the time-resolved velocity field to clarify the origin of the detected peaks. This analysis extracts three low-frequency spatial modes at SrD = 0.27, 0.56, and 0.85. From the three-dimensional shape of the DMD modes and the reconstructed modulation of the mean flow in time, it is deduced that the detected most dominant peaks of SrD r3 ≈ 0.85 are caused by the radial flapping motion of the shear layer, while the middle-frequency range of SrD r2 ≈ 0.55 is found to be associated with its swinging motion. The less intensive peaks of SrD r1 ≈ 0.25 pronounced on the base wall are caused by the low-frequency longitudinal pumping of the two toroidal large-scale vortices inside the cavity.

1515 円管内オリフィス下流における熱伝達構造の非定常挙動

A technique using high-speed infrared thermography was used to measure the spatio-temporal heat transfer to a turbulent water pipe flow around an orifice plate (bore ratio: d/D = 0.49, Re_D ≈ 13000). The spatio-temporal distribution of the heat transfer coefficient was evaluated based on the temperature fluctuation of a heated thin-foil measured using infrared thermography. In this work, we tried to evaluate the convection velocity of the heat transfer structure on the heated surface, which is considered to be associated with the convection of the vortical structure in the near-wall region. As a result, it was confirmed that the maximum position of the heat transfer in the time-averaged distribution was not coincide with the time-averaged flow reattachment position, as reported in the literature. In order to investigate the mechanism of this difference, ensemble-averaged heat transfer coefficients were calculated on the condition that instantaneous reattachment appeared at specific streamwise positions. The ensemble-averaged heat transfer distribution shows a noticeable peak at the flow reattachment position, comparing with the time-averaged distribution. Also, the peak value of the ensemble-averaged heat transfer distribution tended to decrease toward downstream. This trend indicates that reattachment at upstream contributes to the heat transfer enhancement more significant than that at downstream. This is the reason why the maximum position of the time-averaged heat transfer exists upstream of the time-averaged flow reattachment position.

Stirring and scalar transfer by grid-generated turbulence in the presence of a mean scalar gradient

AbstractThe stirring of a passive scalar by grid-generated turbulence in the presence of a mean scalar gradient is studied by direct numerical simulations (DNS) for six different grids: one fractal square grid with three fractal iterations, one fractal square grid with four fractal iterations, one fractal I grid and three different regular grids. Our results can be summarised as follows. (i) For all these grids, the turbulence intensity averaged over time and over a plane parallel to the grid takes its peak value when the streamwise position of this plane is between $0.75M_{eff}$ and $1.5M_{eff}$ where $M_{eff}$ is the effective mesh size introduced by Hurst & Vassilicos (Phys. Fluids, vol. 19, 2007, 035103). (ii) Downstream of the location of this peak, the turbulence intensity averaged in this way is greatly enhanced by the fractal grids relative to the regular grids even though the fractal grids have comparable or even lower blockage ratios. The novelty of this result lies in the fact that it concerns turbulence intensities averaged over lateral planes (as well as time). (iii) The pressure drop is about the same across grids of the same blockage ratio whether fractal or not, but the pressure recovery is longer for the fractal grids. (iv) Even so, the fractal grids enhance turbulent scalar fluxes by up to an order of magnitude in the region downstream of the aforementioned peak and they also greatly enhance the streamwise growth of the fluctuating scalar variance in that region. (v) We demonstrate on a simple planar model problem that the cause of this phenomenon lies in the fractality of the grids. (vi) The turbulence scalar flux coefficient is constant far enough downstream of all the present grids and is significantly dependent on the nature and details of the turbulence-generating grid.

Time-resolved heat transfer characteristics for periodically pulsating turbulent flows with time varying flow temperatures

Time-resolved heat transfer characteristics for periodically pulsating turbulent channel flows with time varying flow temperatures have been experimentally investigated. Fast response thin-film surface thermocouples were used to measure the test plate surface temperature history at two stream-wise positions. Time-resolved heat transfer coefficients were determined with the Cook-Felderman method. Correction methods were applied for the direct measurements of time varying flow temperature and surface heat flux to eliminate the influences of thermal inertia effect of the thermocouple. Corresponding relationships in time between time-resolved heat transfer coefficient and time-resolved flow velocity and temperature are presented and discussed in this paper. Generally the heat transfer coefficient pulsates in phase with the flow velocity. But as shown in the currently tested cases, the temporal behaviour of heat transfer coefficient under unsteady aero-thermal conditions is not only determined by the unsteady flow velocity. Due to the coupled effect of unsteady flow velocity and unsteady flow temperature on the convective heat transfer which should be in a nonlinear way according to the data analysis, the unsteady flow temperature also plays an effect, which makes the measured heat transfer coefficients have unexpected features in the temporal waveform and deviate from the data calculated based on quasi-steady hypothesis. The coupled effect of unsteady flow velocity and temperature could be influenced by their frequency, phase relationship and waveform to produce heat transfer coefficients with different temporal waveforms for different unsteady aero-thermal conditions.

Structure of Turbulent Channel Flow Perturbed by a Wall-Mounted Cylindrical Element

The current study reports structural modifications imposed in fully developed, turbulent channel flow by an isolated wall-mounted circular cylinder. The cylinder extended into the logarithmic layer of the flow, perturbing the larger flow scales that embody a significant fraction of the turbulent kinetic energy and Reynolds shear stress. Hot-wire measurements were made in the wake of the wall-mounted circular cylinder at multiple wall-normal and streamwise positions. Besides observing the expected mean velocity deficit in the wake of the cylinder, a secondary peak in streamwise Reynolds normal stress away from the wall was observed, coupled with suppression of the near-wall peak native to the incoming unperturbed, turbulent flow. These observations are similar to those made by Ryan et al. (“Effects of Simple Wall-Mounted Cylinder Arrangements on a Turbulent Boundary Layer,” AIAA Journal, Vol. 49, No. 10, 2011, pp. 2210–2220) for a wall-mounted cylinder protruding into the logarithmic layer of a turbulent boundary layer. Premultiplied velocity spectra elaborated on these energy modifications, specifically the occurrence of two distinct energy peaks at two-thirds of the cylinder height and an attenuation of energy associated with larger flow scales close to the wall. All of these perturbations were found to decay with streamwise distance downstream as the flow relaxed toward the unperturbed state. A clear persistence of the structures at the aforementioned peak at two-thirds cylinder height, similar in scale to the very large-scale motions in canonical wall turbulence, suggests an environment preferring structures of such scale. The influence of the cylinder aspect ratio on the characteristics of the perturbed flow is evaluated, and a distinction in wake structure is identified.

Influence of turbulence on the drop growth in warm clouds, Part I: comparison of numerically and experimentally determined collision kernels

This study deals with the comparison of numerically and experimentally determined collision kernels of water drops in air turbulence. The numerical and experimental setups are matched as closely as possible. However, due to the individual numerical and experimental restrictions, it could not be avoided that the turbulent kinetic energy dissipation rate of the measurement and the simulations differ. Direct numerical simulations (DNS) are performed resulting in a very large database concerning geometric collision kernels with 1470 individual entries. Based on this database a fit function for the turbulent enhancement of the collision kernel is developed. In the experiments, the collision rates of large drops (radius > 7.5μm$> 7.5\,\text{\textmu{}m}$) are measured. These collision rates are compared with the developed fit, evaluated at the measurement conditions. Since the total collision rates match well for all occurring dissipation rates the distribution information of the fit could be used to enhance the statistical reliability and for the first time an experimental collision kernel could be constructed. In addition to the collision rates, the drop size distributions at three consecutive streamwise positions are measured. The drop size distributions contain mainly small drops (radius < 7.5μm$< 7.5\,\text{\textmu{}m}$). The measured evolution of the drop size distribution is confronted with model calculations based on the newly derived fit of the collision kernel. It turns out that the observed fast evolution of the drop size distribution can only be modeled if the collision kernel for small drops is drastically increased. A physical argument for this amplification is missing since for such small drops, neither DNSs nor experiments have been performed. For large drops, for which a good agreement of the collision rates was found in the DNS and the experiment, the time for the evolution of the spectrum in the wind tunnel is too short to draw any conclusion. Hence, the long-time evolution of the drop size distribution is presented in a submitted paper by Riechelmann et al.

Boundary layer laminarization by convex curvature and acceleration

Particle image velocimetry boundary layer measurements are presented for flows over convex surfaces, and subject to flow acceleration. These mechanisms are present in gas reactor core designs, which is of concern because it is known that they can cause boundary layer laminarization which can have a negative effect on surface heat transfer. A wind tunnel was constructed where flow acceleration and curvature effects were applied separately, and simultaneously to investigate the separate and combinative influence of these mechanisms. Applied acceleration for the different test cases varied from K=1.6×10−6 to 1×10−5. Curvature values of δ/R=0.015 to δ/R=0.05 were tested. The applied acceleration spans across the threshold value when laminarization is expected to occur for flat plate geometries.Measurements were taken using particle image velocimetry. Mean flow parameters including mean velocity profiles, boundary layer thickness, and shape factor are presented as a function of streamwise position during the boundary layer response as it is subjected to the laminarization causing mechanisms. Velocity profiles are normalized by both outer and inner variables.It is observed that as the flow is subjected to these mechanisms, the boundary layer mean flow parameters diverge from their turbulent values and approach laminar characteristics. The linear viscous sublayer increases in thickness, the overall boundary layer thickness is reduced, and the shape factor increases. The response in the boundary layer mean flow parameters is more pronounced and occurs more quickly when subject to both mechanisms simultaneously.

Combustion characteristics of a dual-mode scramjet injecting liquid kerosene by multiple struts

In order to optimize fuel allocation between multiple injection struts in scramjet combustor, experimental investigation has been carried out to study the combustion characteristics of a dual-mode scramjet, in which liquid kerosene was injected from three staggered arranged struts, and the effect of fuel allocation between the three struts on the combustion characteristics has been analyzed. The results show strong influence of the quantity and streamwise position of struts on the wall pressure distribution and both the equivalence ratio limits of inlet unstarting and lean blowout. To obtain a wide stable operation range, just one strut is needed for injecting most of the fuel in the combustors with configuration and size as in this work, and a streamwise position closer to the downstream divergent segment is a better injection location. Lower lean blowout limit can be obtained by injecting the overall much lean fuel from just one strut with a relatively closer distance to the downstream flameholder.

Experimental investigation of polymer diffusion in the drag-reduced turbulent channel flow of inhomogeneous solution

Spatial polymer diffusion in the drag-reduced turbulent channel flow of an inhomogeneous polymer solution was investigated by simultaneously measuring velocity and concentration fields using particle imaging velocimetry and planar laser-induced fluorescence techniques. The polymer solution was dosed into the turbulent channel flow from the surface of one-side of the channel wall. The Reynolds number (based on channel height, bulk velocity and solvent viscosity) was set as 4.0×104 and the weight concentrations of dosed polymer solution were set to 25, 50 and 100ppm. The measurements were obtained in the streamwise wall-normal (x–y) plane at three streamwise positions along the dosing wall. The detailed statistical analyses consisting of concentration distribution, turbulence modification, turbulent mass flux, and eddy diffusivities of momentum and of mass are presented. The results show that the polymer diffusion, which has a close relationship with the local polymer concentration and drag reduction in the drag-reduced turbulent channel flow, is suppressed due to the inhibited turbulence other than the diffusion of passive scalar in ordinary turbulence. Two characteristic regions exist in the near-wall region according to the diffusion characteristics and altered motions in the wall-normal direction. The wall-normal turbulent fluxes that control the transport of mass are reduced significantly in the near-wall region for the drag-reduced flow when compared with the case of dosing water. With the increase of local polymer concentration in the “effective position”, the corresponding drag reduction rate (DR) increases. The turbulent Schmidt number (ScT), which represents the relative intensities of the eddy diffusivities of momentum and of mass, is also found to increase with increasing DR. The mean value of ScT for the drag-reduced flow can rise to 2.9, while it is 1.2 for the case of dosing water in the present measurements.

Viscous Flows Driven by Uniform Shear over a Porous Stretching Sheet in the Presence of Suction/Blowing

An analysis is carried out to study the steady two-dimensional flow of an incompressible viscous fluid past a porous deformable sheet, which is stretched in its own plane with a velocity proportional to the distance from the fixed point subject to uniform suction or blowing. A uniform shear flow of strain rate β is considered over the stretching sheet. The analysis of the result obtained shows that the magnitude of the wall shear stress increases with the increase of suction velocity and decreases with the increase of blowing velocity and this effect is more pronounced for suction than blowing. It is seen that the horizontal velocity component (at a fixed streamwise position along the plate) increases with the increase in the ratio of shear rate β and stretching rate (c) (i.e., β/c) and there is an indication of flow reversal. It is also observed that this flow reversal region increases with the increase in β/c.

Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight

Many species travel in highly organized groups. The most quoted function of these configurations is to reduce energy expenditure and enhance locomotor performance of individuals in the assemblage. The distinctive V formation of bird flocks has long intrigued researchers and continues to attract both scientific and popular attention. The well-held belief is that such aggregations give an energetic benefit for those birds that are flying behind and to one side of another bird through using the regions of upwash generated by the wings of the preceding bird, although a definitive account of the aerodynamic implications of these formations has remained elusive. Here we show that individuals of northern bald ibises (Geronticus eremita) flying in a V flock position themselves in aerodynamically optimum positions, in that they agree with theoretical aerodynamic predictions. Furthermore, we demonstrate that birds show wingtip path coherence when flying in V positions, flapping spatially in phase and thus enabling upwash capture to be maximized throughout the entire flap cycle. In contrast, when birds fly immediately behind another bird--in a streamwise position--there is no wingtip path coherence; the wing-beats are in spatial anti-phase. This could potentially reduce the adverse effects of downwash for the following bird. These aerodynamic accomplishments were previously not thought possible for birds because of the complex flight dynamics and sensory feedback that would be required to perform such a feat. We conclude that the intricate mechanisms involved in V formation flight indicate awareness of the spatial wake structures of nearby flock-mates, and remarkable ability either to sense or predict it. We suggest that birds in V formation have phasing strategies to cope with the dynamic wakes produced by flapping wings.

Experimental study of monodisperse granular flow through an inclined rotating chute

In blast furnaces, particles like coke, sinter and pellets enter from a hopper and are distributed on the burden surface by a rotating chute. Such particulate flows suffer occasionally from particle segregation during transportation caused by differences in density or size. To get a more fundamental insight into these effects, we started an experimental study to investigate the effect of rotation on such particulate flows. Here, as a first step, we present an experimental study of granular flow of monodisperse 3 mm spherical glass particles flowing with a constant mass rate through a rotating smooth rectangular chute, which is inclined at a fixed angle. Experiments are performed for a sufficiently long time to study steady (but streamwise accelerating) flow. Particle image velocimetry (PIV), electronic ultrasonic height sensors, and a dynamic weighing scale are used to measure the surface velocity of the particle stream, the particle bed height and mass flow rate in the chute, respectively. The influence of rotation speed and angle of inclination of the chute is studied. We find an interesting interplay between the Coriolis force, which pushes the granular flow to the sidewall of the chute and tends to diminish the acceleration of the flow, and centrifugal forces that accelerate the flow. The velocity components display a complex dependence on the spanwise and streamwise position in the chute. The bed height in the chute is also influenced by these effects of system rotation. These measurements provide a well-defined set of observations for refining and validating computer simulations of granular flows, and point out some important limitations of physical experiments. We present preliminary three-dimensional discrete particle simulations, which show that the experimental measurements of bed height and surface particle velocity in a chute inclined at 30° can be predicted nearly quantitatively both without and with rotation of the chute.

Draft tube discharge fluctuation during self-sustained pressure surge: fluorescent particle image velocimetry in two-phase flow

Hydraulic machines play an increasingly important role in providing a secondary energy reserve for the integration of renewable energy sources in the existing power grid. This requires a significant extension of their usual operating range, involving the presence of cavitating flow regimes in the draft tube. At overload conditions, the self-sustained oscillation of a large cavity at the runner outlet, called vortex rope, generates violent periodic pressure pulsations. In an effort to better understand the nature of this unstable behavior and its interaction with the surrounding hydraulic and mechanical system, the flow leaving the runner is investigated by means of particle image velocimetry. The measurements are performed in the draft tube cone of a reduced scale model of a Francis turbine. A cost-effective method for the in-house production of fluorescent seeding material is developed and described, based on off-the-shelf polyamide particles and Rhodamine B dye. Velocity profiles are obtained at three streamwise positions in the draft tube cone, and the corresponding discharge variation in presence of the vortex rope is calculated. The results suggest that 5–10 % of the discharge in the draft tube cone is passing inside the vortex rope.