Converging-diverging Channel Research Articles

Vortical Structures in Wall-Bounded Turbulent Flow with Recirculation

Hairpin or horse-shoe vortices are a widely-accepted feature of the wall-bounded flows. These vortical structures have mostly been studied in canonical flows. Relatively few studies have been conducted on the characteristics of the vortical structures in wall-bounded flows with adverse pressure gradient and still fewer on the detached flows with recirculation. In the present contribution, vortices have been educed using a DNS database of incompressible flow over a 2-dimensional surface bump in a converging-diverging channel at a Reynolds number Reτ of 617, based on the friction velocity at inlet. Vortices have been educed from the instantaneous velocity field in streamwise/wall-normal and spanwise/wall-normal planes using the signed swirling strength criterion. Vortex validation is done through a fit of the vortex velocity field to the Oseen vortex model. The effects of a strong adverse pressure gradient and flow reciruclation on the population density and sizes of the streamwise and spanwise-oriented vortices have been studied. It has been found that a strong adverse pressure gradient and flow recirculation leads to the generation of a new near-wall peak of small spanwise prograde vortex population. Furthermore, this peak of vortex density has been found to coincide and hence relate to the outward movement of the peak of streamwise rms velocity fluctuations typical of adverse pressure gradient wall-bounded turbulent flows.

Viscoelastic medium flows in convergent-divergent channels

The problem of viscoelastic fluid flow in a convergent-divergent channel is considered. A mathematical model and the results of numerical investigations are presented. A model of the differential type is used to describe the rheological properties of viscoelastic fluids. A comparative analysis of the flow parameters of generalized Newtonian and viscoelastic fluids in the channels considered is carried out on the basis of the results of numerical calculations.

Critical behaviour of the MHD flow in convergent-divergent channels

The effect of external Magnetohydrodynamic (MHD) field on the steady two-dimensional nonlinear flow through Convergent-Divergent Channels of a viscous incompressible electrically conducting fluid is investigated. We compute the critical behaviour of the solution govern by the equation. Our approach uses the power series in order to observe the instability of the problem. The series is then summed by using various generalizations of the approximants. We find the critical values of various parameters and type of the principal singularity for different choice of MHD effect.DOI: 10.3329/jname.v7i2.5635

Tunable transmission and enhanced emission in ordered metallic nanostructures having varying channel shape

Extraordinary transmission spectra for one-dimensional (1D) gratings and two-dimensional (2D) metallic hole arrays change with the hole channel shape. In this paper, a converging-diverging channel (CDC) design was introduced. The transmission spectra corresponding to CDC-embedded nanostructures of 1D grating, circular and rectangular holes (2D hole arrays) are analyzed using three-dimensional (3D) finite-element method. Tuning of optical transmission by changing the CDC structure has been investigated. In addition, a cavity composed of a CDC metallic grating and a 1D photonic crystal (PhC) can lead to an enhanced emission. Large coherence length of the emission can be achieved by exploiting coherent properties of surface waves in grating and PhC.

Numerical simulations of the flow in a converging-diverging channel with control through a spanwise slot

The flow in a converging-diverging channel at Reτ = 180 is studied by Direct Numerical and Large Eddy Simulations. Continuous and pulsed jets are applied normally to the wall through a spanwise slot to delay the separation. The influence of the position is first analysed and it is shown that the control efficiency is closely linked to the location of the actuation within the channel. A parametric study on both the intensity and pulsating frequency for the optimal position shows that the separation can be drastically reduced and even vanishes with a proper choice of parameters. It is additionally reported that pulsed jets yield better results than continuous jets at an equivalent mass-flux. Moreover, low frequencies are shown to be more efficient than high frequencies. Aphysical analysis finally demonstrates that the efficiency of the low frequency pulsed jets is mainly due to the creation of spanwise structures, that does not occur in other cases.

Tuning the extraordinary transmission in a metallic/dielectric CDC hole array by changing the temperature

Tunable extraordinary transmission via changing temperature of a porous metallic layer on top of a thin layer of dielectric strontium titanate (STO) was studied. The metallic layer has a through-hole array and each hole has a circular converging-diverging channel (CDC) shape, which induces the excitation of surface plasmon polaritons (SPPs) and then results in a controllable extraordinary optical transmission in the terahertz (THz) frequency range. We used a three-dimensional (3D) finite element method to analyze the transmission characteristics of the structure. Location and magnitude of the transmission peaks can be adjusted by hole size, converging angle, and thicknesses of metal and STO layers. Remarkably, the suggested structure presents a strong transmission dependency on temperature, which offers a new approach to actively and externally tune the transmission. This new design could lead to a family of temperature-sensitive devices working in the THz frequency range, promising in many applications including photonics, nanolithography, imaging, and sensing.

Heat transfer of non-Newtonian liquid flows in convergent-divergent channels with fourth-kind thermal boundary conditions

In the present paper, we consider the heat transfer of non-Newtonian liquid flows moving in convergent- divergent channels with common boundary. The case of flows moving in opposite directions was addressed. At the common boundary, continuity conditions for temperature and heat flow were adopted. A mathematical model, a calculation algorithm, and simulation data are reported. Plots of numerical data characterizing the channel flows are presented. A comparative analysis of heat transfer in smooth and convergent-divergent channels is given.

Large-eddy simulations of control of a separated flow over a 2D bump by means of pulsed jets

A numerical study of active flow control of a separated converging-diverging channel flow is presented. Several geometrical and operating parameters of the continuous and pulsed jets are investigated using large-eddy simulations (LES), which are highly resolved in the actuation region. The LES without control is validated with a direct numerical simulation of the same flow. Configurations with both counter-rotating and co-rotating jets are tested. The main statistical results are in general agreement with the literature. Thanks to the unsteady highly resolved simulation, both temporal and spatial organizations of the generated vortices are investigated. An optimal frequency associated with the convective time as well as an optimal duty cycle are obtained, leading to a control efficiency comparable to the cases with continuous jets and the same operating velocity and, consequently, to a significant saving in mass flow rate.

Turbulent Flow in Wavy Channels Simulated with Nonlinear Models and a New Implicit Formulation

This work examines the performance of linear and nonlinear eddy-viscosity models when used to predict the turbulent flow in periodically sinusoidal-wave channels. Two geometries are investigated, namely a converging-diverging channel and a channel with concave-convex walls. The numerical method employed for the discretization of the equations is the control-volume method in a boundary-fitted nonorthogonal coordinate system. The SIMPLE algorithm is used for correcting the pressure field. The classical wall function and a low Reynolds model are used to describe the flow near the wall. Comparisons between those two approaches using linear and nonlinear turbulence models are done. Here, a new implicit numerical treatment is proposed for the nonlinear diffusion terms of the momentum equations in order to increase the robustness. Results show that by decomposing and treating terms as presented, solutions using nonlinear models and the high Reynolds wall treatment, which combine accuracy and economy, are more stable and easier to be obtained.

Pressure-driven transport of particles through a converging-diverging microchannel

Pressure-driven transport of particles through a symmetric converging-diverging microchannel is studied by solving a coupled nonlinear system, which is composed of the Navier-Stokes and continuity equations using the arbitrary Lagrangian-Eulerian finite-element technique. The predicted particle translation is in good agreement with existing experimental observations. The effects of pressure gradient, particle size, channel geometry, and a particle's initial location on the particle transport are investigated. The pressure gradient has no effect on the ratio of the translational velocity of particles through a converging-diverging channel to that in the upstream straight channel. Particles are generally accelerated in the converging region and then decelerated in the diverging region, with the maximum translational velocity at the throat. For particles with diameters close to the width of the channel throat, the usual acceleration process is divided into three stages: Acceleration, deceleration, and reacceleration instead of a monotonic acceleration. Moreover, the maximum translational velocity occurs at the end of the first acceleration stage rather than at the throat. Along the centerline of the microchannel, particles do not rotate, and the closer a particle is located near the channel wall, the higher is its rotational velocity. Analysis of the transport of two particles demonstrates the feasibility of using a converging-diverging microchannel for passive (biological and synthetic) particle separation and ordering.

Tunable and polarization-selective THz range transmission properties of metallic rectangular array with a varying hole channel shape

This paper proposes a metallic hole array of a rectangular converging-diverging channel (RCDC) shape with extraordinary transmission. We use a three-dimensional (3D) finite element method to analyze the transmission characteristics of two-dimensional metallic hole arrays (2D-MHA) with RCDC. For a straight channel MHA, when the aperture size is reduced, the transmission peaks have a blue-shift. The same result is observed for a smaller gap throat for the RCDC structure. For the rectangular holes with a high length-width ratio, a similar blue-shift in the transmission peaks as well as a narrower full width at half maximum (FWHM) are observed. The asymmetry from the rectangular shape gives this structure high selectivity for light with different polarizations. Furthermore, the RCDC shape gives extra degrees of geometrical variables to 2D-MHA for tuning the location of the transmission peak and the FWHM. The tunable transmission property of this structure shows promise for applications in tunable filters, photonic circuits, and biosensors.

Tunable transmission at 100 THz through a metallic hole array with a varying hole channel shape

Extraordinary optical transmission spectrum for a two-dimensional metallic hole array (2D-MHA) changes with the hole channel shape. In this paper a new converging-diverging channel (CDC) shape is proposed. A three-dimensional (3D) finite element method is utilized to analyze the transmission characteristics of the 2D-MHA with CDC. The transmission peaks are blue-shifted when the gap at the throat of CDC is reduced. Similar blue-shift in the transmission peaks are observed for a straight channel MHA when the aperture size is reduced. The transmission for the straight channel MHA is not sensitive to the metal film thickness. But, for a CDC MHA the transmission varies with the metal film thickness. Also, the CDC shape gives an extra degree of geometrical variable to 2D-MHA for tuning the transmission peak location with potential applications in nanolithography, imaging and biosensing.

Extraordinary Transmission and Enhanced Emission with Metallic Gratings Having Converging-Diverging Channels

Transmission metallic gratings having the shape of converging-diverging channel (CDC) give an extra degree of freedom to exhibit enhanced transmission resonances. By varying the gap size at the throat of CDC, the spectral locations of the transmission resonance bands can be shifted close to each other and have high transmittance in a very narrow energy band. Hence, the CDC shape metallic gratings can lead to almost perfect transmittance for any desired wavelength by carefully optimizing the metallic material, gap at the throat of CDC, and grating parameters. In addition, a cavity surrounded by the CDC shaped metallic grating and a one-dimensional (1D) photonic crystal (PhC) can lead to an enhanced emission with properties similar to a laser. The large coherence length of the emission is achieved by exploiting the coherence properties of the surface waves on the gratings and PhC. The new multilayer structure can attain the spectral and directional control of emission with onlyp-polarization. The resonance condition inside the cavity is extremely sensitive to the wavelength, which would then lead to high emission in a very narrow wavelength band. Such simple 1D multilayer structure should be easy to fabricate and have applications in photonic circuits, thermophotovoltaics, and potentially in energy efficient incandescent sources.

Monochromatic polarized coherent emitter enhanced by surface plasmons and a cavity resonance

In this paper we propose and analyze a design of multilayer nanostructure that could be used as a tunable monochromatic polarized thermal emitter in the visible region for any direction with large temporal coherence and spatial coherence extending into the far field. The thermal emitter has a cavity that is surrounded by a thin silver grating having converging-diverging channel on one side and by a one-dimensional (1D) photonic crystal (PhC) on the other side. The large coherence length is achieved by making use of the coherence properties of the surface waves. Due to the nature of surface waves the new multilayer structure can attain the spectral and directional control of thermal radiation with only $p$ polarization. Finite element method was used for analyzing the emission properties of the thermal emitter. The resonance condition inside the cavity is extremely sensitive to the wavelength, which would then lead to high emission in a very narrow wavelength band. Such simple 1D multilayer structure should be easy to fabricate and have applications in photonic circuits, thermophotovoltaics and potentially in energy efficient incandescent sources.

Extraordinary transmission in a narrow energy band for metallic gratings with converging-diverging channels

Transmission metallic gratings with narrow and deep slits having the shape of converging-diverging channel (CDC) can exhibit enhanced transmission resonances for wavelengths larger than the periodicity of the grating. Using finite element method, the authors show that, by varying the gap size at the throat of CDC, the spectral locations of the transmission resonance bands can be shifted close to each other and have high transmittance in a very narrow energy band. Also, the proposed shape can lead to almost perfect transmittance for any desired wavelength by carefully optimizing the metallic material, gap at the throat of CDC, and grating parameters.

Hermite–Padé approximation approach to MHD Jeffery–Hamel flows

In this paper, we studied the problem of nonlinear flow of an incompressible conducting viscous fluid in convergent–divergent channels under the influence of an externally applied homogeneous magnetic field. The governing equations is obtained and solved using a special type of Hermite–Padé approximation semi-numerical approach. This technique offers some advantages over solutions obtained by using traditional methods such as finite differences, spectral method, shooting method, etc. It reveals the analytical structure of the solution function and the important properties of overall flow structure including velocity field, flow reversal control and bifurcations are discussed.

Numerical Prediction of Two-Phase MHD Power Generation System Using Cavitating Flow of Electrically Conducting Magnetic Fluid

A new concept of a two-phase MHD power generation system using a cavitating flow of an Electrically Conducting Magnetic Fluid (ECMF) is proposed, and the driving and power generation performance of the system is numerically predicted. A typical computational model for cavitating flow of a mercury-based magnetic fluid is proposed and several flow characteristics, taking into account the strong nonuniform magnetic field, are numerically investigated to realize the further development and high performance of the proposed new type of two-phase fluid driving system using magnetic fluids. Based on numerical results, the two-dimensional structure of the cavitating flows as well as the cloud cavity formation of the ECMF through a vertical converging-diverging channel are shown in detail. The numerical results demonstrate that an effective two-phase electromagnetic driving force and fluid acceleration, and high power density can be obtained by the practical use of magnetization of the working fluid of ECMF. Also clarified is the cavitation number of ECMF in the case of a strong magnetic field with a larger value than that in the case of a nonmagnetic fluid. Further clarified is the precise control of the cavitating flow in ECMF that is possible by effective use of the magnetic body force that acts on cavitation bubbles.

Forced Convection in Micropolar Fluid Flow through a Wavy-Wall Channel

ABSTRACT Forced convection of micropolar fluids through a periodic array of wavy-wall channels has been analyzed by using a simple coordinate transformation method and the spline alternating- direction implicit method. The effects of the wavy amplitude, the micropolar parameter, and the Reynolds number on skin friction coefficient and Nusselt number have been examined in detail. Results show that the flow through a sinusoidally curved converging-diverging channel forms a strong forward flow and a reticular vortex within each wave for larger Reynolds number and larger wavy amplitudes. For the micropolar fluids, increasing the vortex viscosity causes an increase in the total viscosity of the fluid, thus the skin friction coefficient increases while the Nusselt number decreases. Also, the influence of vortex viscosity on the minimum cross section of the wavy-wall channel and on a tiny change of the maximum cross section is manifest. Moreover, both Reynolds number and wavy amplitude tend to enhance the total heat transfer rate, regardless of whether the fluids are Newtonian or micropolar fluids.

Accelerated Particle Electrophoretic Motion and Separation in Converging−Diverging Microchannels

Accelerated particle electrophoretic motions were visualized in converging-diverging microchannels on poly(dimethylsiloxane) chips. The accelerated particle electrophoretic separation is highly desirable in on-chip flow cytometry and high-speed electrophoresis. The effects of electric field, particle size, particle trajectory, and channel structure on the particle electrophoretic motion are examined. We find that the ratio of the particle velocity in the throat to that in the straight channel is significantly lower than their cross-sectional area ratio. This discrepancy may be attributed to the locally higher electric field around the two poles of a particle, as compared to other regions inside the microchannel. We also find that the particle velocity ratio is increased for smaller particles moving through symmetric converging-diverging channels under lower electric fields. These variations may be attributed to the negative dielectrophoretic force that is generated by the nonuniform electric field in the converging-diverging section. In addition, we find that particle trajectory has insignificant influences on the maximum velocity ratio obtained in the throat.

Stability characteristics of suspension flow through wavy-walled channels

A numerical study of the linear temporal stability characteristics of particulate suspension flow through a converging-diverging symmetric wavy-walled channel is considered. The basic flow is a superposition of plane channel flow of particulate suspension and periodic flow components arising due to the small amplitude sinusoidal waviness of the channel walls. The disturbance equations are derived within the framework of Floquet theory and solved using the spectral collocation method. The effects of small amplitude sinusoidal waviness of the channel walls and those of the presence of particles on the initial growth of the disturbances are examined. Two-dimensional stability calculations for particulate suspensions indicate the presence of fast growing unstable modes that arise due to the waviness of the walls. Neutral stability calculations are performed in the disturbances wavenumber-Reynolds number (αs−Re) plane, for the wavy channel with representative values of wavenumber (λ) and the wall amplitude to semi-channel height ratio (∈w) for different values of volume fraction density of the particles (C). It is observed that the critical Reynolds number for transition decreases with increase of ∈w and C. However, the flow can be modulated by suitable wall excitation which in turn can stabilize the flow.