Counter-rotating Flows Research Articles

Inter-disks inversion surfaces

We consider a counter-rotating torus orbiting a central Kerr black hole (BH) with dimensionless spin a, and its accretion flow into the BH, in an agglomerate of an outer counter-rotating torus and an inner co-rotating torus. This work focus is the analysis of the inter-disks inversion surfaces. Inversion surfaces are spacetime surfaces, defined by the condition uϕ=0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$u^{\\phi }=0$$\\end{document} on the flow torodial velocity, located out of the BH ergoregion, and totally embedding the BH. They emerge as a necessary condition, related to the spacetime frame-dragging, for the counter-rotating flows into the central Kerr BH. In our analysis we study the inversion surfaces of the Kerr spacetimes for the counter-rotating flow from the outer torus, impacting on the inner co-rotating disk. Being totally or partially embedded in (internal to) the inversion surfaces, the inner co-rotating torus (or jet) could be totally or in part “shielded”, respectively, from the impact with flow with auϕ<0\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$a u^{\\phi }<0$$\\end{document}. We prove that, in general, in the spacetimes with a<0.551\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$a<0.551$$\\end{document} the co-rotating toroids are always external to the accretion flows inversion surfaces. For 0.551<a<0.886\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.551<a<0.886$$\\end{document}, co-rotating toroids could be partially internal (with the disk inner region, including the inner edge) in the flow inversion surface. For BHs with a>0.886\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$a>0.886$$\\end{document}, a co-rotating torus could be entirely embedded in the inversion surface and, for larger spins, it is internal to the inversion surfaces. Tori orbiting in the BH outer ergoregion are a particular case. Further constraints on the BHs spins are discussed in the article.

Prediction of Coaxial Rotor Hub Flow Using Mercury Framework

Rotor hubs are predominantly responsible for the parasitic drag encountered by high-speed rotorcraft. To gain a deeper understanding of the fluid dynamics around rotor hubs, simulations of counter-rotating coaxial rotor-hub flows were conducted, which was subsequently followed by a corroborative experiment at Pennsylvania State University. The simulation process employed a computational fluid dynamics framework termed “Mercury,” which utilizes an unstructured/Cartesian multimesh paradigm. This process incorporated Spalart???Allmaras delayed detached eddy simulation turbulence modelling for an accurate representation of the flow. The investigation into the coaxial hub flow physics was conducted at two different advance ratios: 0.25 and 0.6, constructed by building up the hub components. The interaction between the hub and fairing components led to an increase in the average hub drag and caused unsteady harmonics in the hub and fairing drags. Furthermore, it was noted that different advance ratios affected the drag and wake structures. The complete hub model was then simulated in a water tunnel with an advance ratio of 0.25. Predictions of mean and unsteady drag, as well as mean wake velocity fields, were compared with the experimental results. Overall, the mean wake velocity field from the simulation qualitatively aligned with the experimental results, especially in the near-wake region. Additionally, the buildup model analysis significantly aided in understanding the intricate wake structures surrounding the hub.

Analytical solutions for time-dependent kinematic three-dimensional magnetic reconnection.

Magnetic reconnection is a process that can rapidly convert magnetic field energy into plasma thermal energy and kinetic energy, and it is also an important energy conversion mechanism in space physics, astrophysics and plasma physics. Research related to analytical solutions for time-dependent three-dimensional magnetic reconnection is extremely difficult. For decades, several mathematical descriptions have been developed regarding different reconnection mechanisms, in which the equations based on magnetohydrodynamics theory outside the reconnection diffusion region are widely accepted. However, the equation set cannot be analytically solved unless specified constraints are imposed or the equations are reduced. Based on previous analytical methods for kinematic stationary reconnection, here the analytical solutions for time-dependent kinematic three-dimensional magnetic reconnection are discussed. In contrast to the counter-rotating plasma flows that existed in steady-state reconnection, it is found that spiral plasma flows, which have never been reported before, can be generated if the magnetic field changes exponentially with time. These analyses reveal new scenarios for time-dependent kinematic three-dimensional magnetic reconnection, and the deduced analytical solutions could improve our understanding of the dynamics involved in reconnection processes, as well as the interactions between the magnetic field and plasma flows during magnetic reconnection.

Chiral flows can induce neck formation in viscoelastic surfaces

The cell cortex is an active viscoelastic self-deforming sheet at the periphery of animal cells. It constricts animal cells during cell division. For some egg cells, the actomyosin cortex was shown to exhibit counter-rotating chiral flows along the axis of division. Such chiral surface flows were shown to contribute to spatial rearrangements and left-right symmetry breaking in developing organisms. In spite of this prospective biological importance, the effect of chiral forces on the flows and emergent shape dynamics of a deformable surface are completely unknown. To shed a first light on that matter, we present here a numerical study of an axisymmetric viscoelastic surface embedded in a viscous fluid. We impose a generic counter-rotating force field on this surface and study the resulting chiral flow field and shape dynamics for various surface mechanical parameters. Notably, we find that the building of a neck, as is observed during cell division, occurs if the surface contains a strong shear elastic component. Furthermore, we find that a large areal relaxation time results in flows towards the equator of the surface. These flows assist the transport of a surface concentration during the formation of a contractile ring. Accordingly, we show that chiral forces by themselves can drive pattern formation and stabilise contractile rings at the equator. These results provide first mechanistic evidence that chiral flows can play a significant role to orchestrate cell division.

Investigating mixing patterns of suspended sediment in a river confluence using high-resolution hyperspectral imagery

In a river confluence, the fluvial process is complex because of the merging of different flows from rivers with distinct morphologies. However, the mixing of suspended sediment in river confluences has been inadequately investigated owing to the low resolution of conventional techniques for measuring suspended sediment. In this study, we investigated the mixing of suspended sediment at the confluence of the Hwang and Nakdong Rivers in South Korea by analyzing the spectral characteristics of suspended sediment from high-resolution hyperspectral data. We retrieved the suspended sediment concentration (SSC) from high-resolution hyperspectral imagery using the conversion technique, which is a cluster-based machine learning regression with optical variability. Hyperspectral clustering was first applied to classify the water regions of the main river (Nakdong) and its tributary (Hwang). The clustering result at a near-field after the confluence point was strongly related to the mixing degree of flow and sediments from two different rivers. Through segmentation, regressors with hyperspectral clusters enabled accurate SSC measurement. In particular, they precisely retrieved the concentrations adjacent to the mixing layer, even when the relationship between the spectral data and suspended sediment concentrations of the main river and tributary was substantially different. Using a detailed SSC map, we compared two cases of mixing patterns by calculating the variation in sediment concentration along the downstream river confluence. We found that the mixing pattern was different because of the flow velocity and a large sandbar near the stagnant area even though river discharge was similar for both cases. In the slow mixing case, the sandbar caused a velocity-deficit with a wake that reduced the variance of SSC due to the irregular flow near the mixing layer. In this case, dual counter-rotating secondary flow cells also limited mixing across the post-confluence reach. However, when the wake and dual counter-rotating secondary flows weakened due to the erosion of the sandbar and lower flow velocity, the transverse mixing coefficient increased by approximately 40 % compared to the slow mixing case. This study demonstrated that the hyperspectral approach can be used to investigate the mixing of suspended sediment in greater detail in complex river systems, as it provides high-resolution information that conventional measurement methods cannot explore.

Effect of Counter- and Co-Swirl on Low-Frequency Combustion Instabilities of Jet A-1 Spray Flames

Abstract The present article experimentally investigates the influence of pilot swirling directions on low-frequency combustion instabilities of pilot diffusion flames in a laboratory-scale combustor with jet A-1 fuel and air at atmospheric pressure. Airblast atomization nozzles with either counter-rotating (CTR) or corotating (COR) pilot swirl flows were examined using nonlinear time-series analyses and high-speed flame imaging measurements under idle and subidle operating conditions. We show that while the amplitude and frequency of limit cycle oscillations are observed to be similar for both cases, detailed examinations of measured experimental data reveal marked differences in stabilization mechanisms and pressure-heat release coupling processes. The spray flame dynamics subjected to counter-rotating swirl flows are governed by large-amplitude pressure oscillations, even under the influence of destructive pressure-heat release rate interference. The mechanism of destructive interference is closely related to the interactions between a spiral diffusion flame and a periodically detached reaction zone. Nonpremixed liquid-fueled flames involving corotating swirl, on the other hand, feature a more compact and intense reaction zone.

Counter-rotating Taylor-Couette flows with radial temperature gradient

In the present work, counter-rotating turbulent Taylor-Couette flows with radial temperature gradient have been studied as an extension of previously studied simple-rotating turbulent Taylor-Couette flows. The rotation axis is orthogonal to the gravity vector. Direct Numerical Simulations (DNS) have been carried out for Reynolds number ranging from 1000 to 5000 and Richardson number varying from 0 to 0.4. The effect of variation of Reynolds number and Richardson number on the flow statistics, flow dynamics, and near-wall coherent structures are studied. For neutrally buoyant cases, near-wall coherent structures exist near the inner and the outer wall, with the core being almost vortex-free. With an increase in Richardson number, more dense and finer vortical structures spread out in the core in half the circumferential domain only. This behavior is due to the spatial stable or unstable stratification in the azimuthal direction, leading to the generation of turbulence in the lower half of the domain and mitigation of turbulence in the upper half of the domain. Further, the turbulent kinetic energy (TKE) budget analysis reveals an increase in turbulence on heating the outer cylinder wall due to an increase in the production term. Heating the outer cylinder wall leads to a slight decrease in skin friction for the inner cylinder.

Investigation of unsteady secondary flows and large-scale turbulence in heterogeneous turbulent boundary layers

Following the findings by Wangsawijaya et al. (J. Fluid Mech., vol. 894, 2020, A7), we re-examine the turbulent boundary layers developing over surfaces with spanwise heterogeneous roughness of various roughness half-wavelengths $0.32 \leq S/\bar {\delta } \leq 3.63$, where $S$ is the width of the roughness strips and $\bar {\delta }$ is the spanwise-averaged boundary-layer thickness. The heterogeneous cases induce counter-rotating secondary flows, and these are compared with the large-scale turbulent structures that occur naturally over the smooth wall. Both appear as meandering elongated high- and low-momentum streaks in the instantaneous flow field. Results based on the triple decomposed velocity fluctuations suggest that the secondary flows are spanwise-locked turbulent structures, with $S/\bar {\delta }$ governing the strength of the turbulent structures and the efficacy of the surface in locking the structures in place (most effective when $S/\bar {\delta } \approx 1$). In terms of unsteadiness, we find additional evidence from conditional averages of the fluctuating velocity fields showing that the secondary flows exhibit maximum unsteadiness (or meandering) when $S/\bar {\delta } \approx 1$. The conditional averages of both spanwise heterogeneous and smooth-wall cases result in structures that are reminiscent of those proposed for the streak-vortex instability model for the inner cycle of wall-bounded turbulence. However, in this case these structures are larger and do not necessarily share the same formation mechanism with the inner cycle. Secondary flows and large-scale structures coexist in the limits where either $S/\bar {\delta } \gg 1$ or $S/\bar {\delta } \ll 1$, where the secondary flows scale on $\delta$ or $S$, respectively. When $S/\bar {\delta } \gg 1$, the secondary flows are locked about the roughness transition, while relatively unaltered large-scale structures occur further from the transition. In the case where $S/\bar {\delta } \ll 1$, $S$-scaled secondary flows are confined close to the surface, coexisting with unaltered larger-scale turbulent structures that penetrate much deeper into the layer.

Fouling Mitigation via Chaotic Advection in a Flat Membrane Module with a Patterned Surface.

Fouling mitigation using chaotic advection caused by herringbone-shaped grooves in a flat membrane module is numerically investigated. The feed flow is laminar with the Reynolds number () ranging from 50 to 500. In addition, we assume a constant permeate flux on the membrane surface. Typical flow characteristics include two counter-rotating flows and downwelling flows, which are highly influenced by the groove depth at each . Poincaré sections are plotted to represent the dynamical systems of the flows and to analyze mixing. The flow systems become globally chaotic as the groove depth increases above a threshold value. Fouling mitigation via chaotic advection is demonstrated using the dimensionless average concentration () on the membrane and its growth rate. When the flow system is chaotic, the growth rate of drops significantly compared to that predicted from the film theory, demonstrating that chaotic advection is an attractive hydrodynamic technique that mitigates membrane fouling. At each Re, there exists an optimal groove depth minimizing and the growth rate of . Under the optimum groove geometry, foulants near the membrane are transported back to the bulk flow via the downwelling flows, distributed uniformly in the entire channel via chaotic advection.

Cell lineage-dependent chiral actomyosin flows drive cellular rearrangements in early Caenorhabditis elegans development.

Proper positioning of cells is essential for many aspects of development. Daughter cell positions can be specified via orienting the cell division axis during cytokinesis. Rotatory actomyosin flows during division have been implied in specifying and reorienting the cell division axis, but how general such reorientation events are, and how they are controlled, remains unclear. We followed the first nine divisions of Caenorhabditis elegans embryo development and demonstrate that chiral counter-rotating flows arise systematically in early AB lineage, but not in early P/EMS lineage cell divisions. Combining our experiments with thin film active chiral fluid theory we identify a mechanism by which chiral counter-rotating actomyosin flows arise in the AB lineage only, and show that they drive lineage-specific spindle skew and cell reorientation events. In conclusion, our work sheds light on the physical processes that underlie chiral morphogenesis in early development.

Steady flow of one uniformly rotating fluid layer above another immiscible uniformly rotating fluid layer

The steady laminar flow of two immiscible, uniformly rotating fluid layers is studied and exact similarity solutions of the axisymmetric Navier-Stokes equations in cylindrical polar coordinates are found. The similarity solutions occur with a flat interface at z = 0 under the parameter restriction that σ²ρ = 1 where σ is the ratio of the fluid angular velocities at z = ±∞ and ρ is the density ratio of the two fluids. Under this restriction the problem reduces to one with two independent parameters, σ and μ, which is the viscosity ratio of the fluids. Numerical results of the resulting system of ODEs are found for selected values of μ and σ, and it is shown that similarity solutions exist for σc(μ)≤ σ ≤ 1 where σc(μ) ˂ 0 (i.e. counter-rotating flows). For σ ˂ 0 the lower fluid can become divided into two distinct recirculation regions between which fluid cannot transfer.

Steady Aerodynamic Characteristics of Two-Dimensional NACA0012 Airfoil for One Revolution Angle of Attack

Steady variations in aerodynamic forces and flow behaviors of two-dimensional NACA0012 airfoil were investigated using a numerical method for One Revolution Angle of Attack (AOA) at Reynolds number of $$10^{5}$$ . The profiles of lift coefficients, drag coefficients, and pressure coefficients were compared with those of the experimental data. The AERODAS model was used to analyze the profiles of lift and drag coefficients. Wake characteristics were given along with the deficit profiles of incoming velocity components. Both the characteristics of normal and reverse airfoil models were compared with the basic aerodynamic data for the same range of AOA. The results show that two peaks of the lift coefficients appeared at $$11.5{^{\circ }}$$ and $$42{^{\circ }}$$ and are in good agreement with the pre-stall and post-stall models, respectively. Counter-rotating vortex flows originated from the leading and trailing edges at a high AOA, which formed an impermeable zone over the suction surface and made reattachments in the wake. Moreover, the acceleration of inflow along the boundary of the vortex wrap appeared in the profile of the wake velocity. The drag profile was found to be independent of the airfoil mode, but the lift profile was quite sensitive to the airfoil mode.

Vortex-induced suspension of sediment in the surf zone

Abstract A major mechanism of sediment suspension by organized vortices produced under violent breaking waves in the surf zone was identified through physical and computational experiments. Counter-rotating flows within obliquely descending eddies produced between adjacent primary roller vortices induce transverse convergent near-bed flows, driving bed load transport to form regular patterns of transverse depositions. The deposited sediment is then rapidly ejected by upward carrier flows induced between the vortices. This mechanism of vortex-induced suspension is supported by experimental evidence that coherent sediment clouds are ejected where the obliquely descending eddies reach the sea bed after the breaking wave front has passed. In addition to the effects of settling and turbulent diffusion caused by breaking waves, the effect of the vortex-induced flows was incorporated into a suspension model on the basis of vorticity dynamics and parametric characteristics of transverse flows in breaking waves. The model proposed here reasonably predicts an exponential attenuation of the measured sediment concentration due to violent plunging waves and significantly improves the underprediction of the concentration produced by previous models.

Instability of counter-rotating stellar disks

We use an N-body simulation, constructed using GADGET-2, to investigate an accretion flow onto an astrophysical disk that is in the opposite sense to the disk's rotation. In order to separate dynamics intrinsic to the counter-rotating flow from the impact of the flow onto the disk, we consider an initial condition in which the counter-rotating flow is in an annular region immediately exterior the main portion of the astrophysical disk. Such counter-rotating flows are seen in systems such as NGC 4826 (known as the "Evil Eye Galaxy"). Interaction between the rotating and counter-rotating components is due to two-stream instability in the boundary region. A multi-armed spiral density wave is excited in the astrophysical disk and a density distribution with high azimuthal mode number is excited in the counter-rotating flow. Density fluctuations in the counter-rotating flow aggregate into larger clumps and some of the material in the counter-rotating flow is scattered to large radii. Accretion flow processes such as this are increasingly seen to be of importance in the evolution of multi-component galactic disks.

Effect of secondary flows on dispersion in finite-length channels at high Peclet numbers

We investigate the effects of secondary (transverse) flows on convection-dominated dispersion of pressure driven, open column laminar flow in a conduit with rectangular cross-section. We show that secondary flows significantly reduce dispersion (enhancing transverse diffusion) in Taylor-Aris regime [H. Zhao and H. H. Bau, “Effect of secondary flows on Taylor-Aris dispersion,” Anal. Chem. 79, 7792–7798 (2007)], as well as in convection-controlled regime. In the convection-controlled dispersion regime (i.e., laminar dispersion in finite-length channel with axial flow at high Peclet numbers) the properties of the dispersion boundary layer and the values of the scaling exponents controlling the dependence of the moment hierarchy on the Peclet number \documentclass[12pt]{minimal}\begin{document}$m^{(n)}_{\rm out} \sim Pe_{\rm eff}^{\theta _n}$\end{document}m out (n)∼Pe eff θn are determined by the local near-wall behaviour of the axial velocity. The presence of transverse flows strongly modify the localization properties of the dispersion boundary layer and consequently the moment scaling exponents. Different secondary flows, electrokinetically induced and independent of the primary axial flow are considered. A complete scaling theory is presented for the nth order moment of the outlet chromatogram as a function of the axial Peclet number, the secondary flow's pattern and intensity. We show that some secondary flows (the corotating and the counter-rotating cavity flows) significantly reduce dispersion and \documentclass[12pt]{minimal}\begin{document}$m^{(n)}_{\rm out} \sim Pe_{\rm eff}^{(n-1)/3}$\end{document}m out (n)∼Pe eff (n−1)/3. No significant dispersion reduction is obtained with the cavity cross-flow \documentclass[12pt]{minimal}\begin{document}$m^{(n)}_{\rm out} \sim Pe_{\rm eff}^{(n-1)/2}$\end{document}m out (n)∼Pe eff (n−1)/2. The best result is obtained with the two full-motion counter-rotating cross-flows because \documentclass[12pt]{minimal}\begin{document}$m^{(n)}_{\rm out}$\end{document}m out (n) saturates towards a constant value. Theoretical results from scaling theory are strongly supported by numerical results obtained by Finite Element Method.

3D simulation of laminar flow and heat transfer in V-baffled square channel

The article presents a numerical investigation on laminar flow and heat transfer characteristics in a three-dimensional isothermal wall square-channel fitted with inline 45° V-shaped baffles on two opposite walls. The computations based on the finite volume method with the SIMPLE algorithm have been conducted for the airflow in terms of Reynolds numbers ranging from 200 to 2000. The inline V-baffles with its V-tip pointing downstream and the attack angle (or half V-apex angle) of 45° relative to the flow direction are mounted repeatedly on the lower and upper walls. The baffled channel flow shows a fully developed periodic flow and heat transfer profile for BR = 0.2 at x/ D≈ 8 downstream of the inlet. Influences of different baffle height ratios ( BR) and pitch ratios, ( PR) on thermal behaviors for a fully developed periodic condition are investigated. It is apparent that the longitudinal counter-rotating vortex flows created by the V-baffle can induce impingement/attachment flows over the walls resulting in greater increase in heat transfer over the test channel. Apart from speeding up the fully developed periodic flow pattern, the rise of the BR leads to the increase in Nu/Nu 0 and f/f 0 values while that of the PR provides an opposite trend. The V-baffle performs better than the angled baffle at a similar condition. The V-baffle with BR = 0.2 and PR = 1.5 yields the maximum thermal performance of about 3.8 whereas the Nu/Nu 0 is some 14 times above the smooth channel at higher Re.

Cell movement during chick primitive streak formation

Gastrulation in amniotes begins with extensive re-arrangements of cells in the epiblast resulting in the formation of the primitive streak. We have developed a transfection method that enables us to transfect randomly distributed epiblast cells in the Stage XI–XIII chick blastoderms with GFP fusion proteins. This allows us to use time-lapse microscopy for detailed analysis of the movements and proliferation of epiblast cells during streak formation. Cells in the posterior two thirds of the embryo move in two striking counter-rotating flows that meet at the site of streak formation at the posterior end of the embryo. Cells divide during this rotational movement with a cell cycle time of 6–7 h. Daughter cells remain together, forming small clusters and as result of the flow patterns line up in the streak. Expression of the cyclin-dependent kinase inhibitor, P21/Waf inhibits cell division and severely limits embryo growth, but does not inhibit streak formation or associated flows. To investigate the role off cell–cell intercalation in streak formation we have inhibited the Wnt planar-polarity signalling pathway by expression of a dominant negative Wnt11 and a Dishevelled mutant Xdd1. Both treatments do not result in an inhibition of streak formation, but both severely affect extension of the embryo in later development. Likewise inhibition of myosin II which as been shown to drive cell–cell intercalation during Drosophila germ band extension, has no effect on streak formation, but also effectively blocks elongation after regression has started. These experiments make it unlikely that streak formation involves known cell–cell intercalation mechanisms. Expression of a dominant negative FGFR1c receptor construct as well as the soluble extracellular domain of the FGFR1c receptor both effectively block the cell movements associated with streak formation and mesoderm differentiation, showing the importance of FGF signalling in these processes.

Unsteady flow evolution in swirl injectors with radial entry. II. External excitations

Our previous study on turbulent flows in a gas-turbine swirl injector was extended to explore the effects of externally impressed excitations on the unsteady flow evolution. Three-dimensional large-eddy simulations were conducted to investigate the responses of the injector flowfield by imposing periodical oscillations of the mass flow rate at the entrance over a wide range of frequencies. Results show that the impressed disturbances are decomposed and propagate in two different modes because of their distinct propagating mechanisms in swirl injectors. The flow oscillation in the streamwise direction travels in the form of acoustic wave, whereas the oscillation in the circumferential direction is convected downstream with the local flow velocity. The vortex breakdown is mainly controlled by the dynamics in the core region near the axis, not so much by the excitation in the main flow passage surrounding the central recirculation zone. External excitations only exert minor influences on the mean flow properties due to the broadband characteristics of the injector flow. One exception is in the outer shear-layer region when the forcing frequency matches the intrinsic frequency of vortex shedding, and the mixing process of two counter-rotating swirl flows is considerably enhanced. The dynamic response of the injector flow, however, depends significantly on the forcing frequency in terms of the acoustic admittance and the mass transfer functions. Energy can be transferred among the various structures in the flowfield under external excitations, causing highly nonuniform spatial and temporal distributions of the oscillatory flow properties at the injector exit. The mass transfer function between the injector exit and entrance at the forcing frequency could be substantially greater than unity when the disturbance resonates with the injector flow. The injector essentially acts as an amplifier under this condition.

Measurements and three-dimensional modeling of air pollutant dispersion in an Urban Street Canyon

The concentrations of gaseous pollutants carbon monoxide, nitrogen oxides (NO x =NO+NO 2 ) , and sulfur dioxide were measured, and the vehicle type and traffic flow rates in an urban street canyon with an aspect ratio of 0.8 and a length to width ratio of 3 were recorded. Three-dimensional (3D) airflow and dispersion of pollutants were modeled using the RNG k – ε turbulence model, which was solved numerically using the finite volume method. Vehicle emissions were estimated from the measured traffic flow rates and modeled as banded line sources along the street. Both measurements and simulations reveal that pollutant concentrations typically follow the traffic flow rate; they decline as the height increases, and are higher on the leeward side than on the windward side. 3D simulations reveal that the vortex line, joining the centers of cross-sectional vortices of the street canyon, meanders between street buildings, and the flows leave the canyon through the top. Entrainment of outside air reduces pollutant concentrations near the street outlet, where counter-rotating vortices or secondary flows are present. Numerical predictions agree reasonably well with measurements.

Visualization and Prediction of Natural Convection of Water Near Its Density Maximum in a Tall Rectangular Enclosure at High Rayleigh Numbers

An experimental and numerical investigation is presented concerning the natural convection of water near its maximum-density in a differentially heated rectangular enclosure at high Rayleigh numbers, in which an oscillatory convection regime may arise. The water in a tall enclosure of Ay=8 is initially at rest and at a uniform temperature below 4°C and then the temperature of the hot vertical wall is suddenly raised and kept at a uniform temperature above 4°C. The cold vertical wall is maintained at a constant uniform temperature equal to that of the initial temperature of the water. The top and bottom walls are insulated. Using thermally sensitive liquid crystal particles as tracers, flow and temperature fields of a temporally oscillatory convection was documented experimentally for RaW=3.454×105 with the density inversion parameter θm=0.5. The oscillatory convection features a cyclic sequence of onset at the lower quarter-height region, growth, and decay of the upward-drifting secondary vortices within counter-rotating bicellular flows in the enclosure. Two and three-dimensional numerical simulations corresponding to the visualization experiments are undertaken. Comparison of experimental with numerical results reveals that two-dimensional numerical simulation captures the main features of the observed convection flow.