Off-center Peaks Research Articles

Effect of aspect ratio on mean flow characteristics of cruciform orifice jet

PurposeThe purpose of this study reports the effects of aspect ratio (AR) on mean flow characteristics of the cruciform orifice jet.Design/methodology/approachThe aspect ratio is the height-to-width ratio of the lobe of the cruciform shape. The aspect ratios considered are 0.25, 0.5, 0.75, 1, 2, 3 and 4. The turbulent jet flow is issued through an orifice being fitted to the jet tunnel facility. The velocity measurements are recorded with the help of pitot-static tube connected to a digital manometer setup. The Reynolds number calculated using the equivalent diameter 50.46 × 10–3 m and exit velocity 51.23 m/s was 1.75 × 105. Based on the experimental data, the streamline velocity decay plots, the potential core length (PCL), mean velocity profiles and velocity half widths were plotted, and discussions were made based on the measured data. A smoke-based flow visualization was carried out at moderate Reynolds number 5396.FindingsThe PCL remains almost constant for the aspect ratio 0.25:1 and then starts decreasing for the aspect ratio 1:4. The decrease in PCL indicates improved mixing. The off-center peaks are found along the major axis in mean velocity profiles for almost all cruciform jets. More than one axis switching occurs and can be identified by the crossover points. The location of the first crossover point shifts forward, and the second crossover point shows an oscillating trend. The flow visualization exhibits the jet evolution, and the distance up to which the jet maintains the cruciform shape is increased with the aspect ratio.Research limitations/implicationsThe experiments are limited to air in air jet under isothermal conditions.Social implicationsThe cruciform orifices can be used as fuel injectors and in air-conditioning systems, thereby improving efficiency and energy usage.Originality/valueThe aspect ratio effects on PCL and axis switching are used to explain the mixing characteristics. Flow visualization was also used to support the discussion.

Flame and heat transfer characteristics of a micro combustor with multihole baffle-induced inflows for a thermo-photovoltaic application

For an effective thermo-photovoltaic application, the inflow effects on the flame and heat transfer characteristics of micro power generators are numerically investigated in a multihole baffled micro combustor. The flow inside the baffle hole is considered as a circular pipe flow. Various inlet profiles of the micro combustor are obtained by varying the baffle thickness corresponding to the length of the baffle hole. The velocity distribution of the flow past the baffle plate attains an M-shaped (also called saddle-back), a parabolic, or a fully developed profile according to the baffle thickness. For a baffle thickness less than the fuel hole diameter, the inflow of the combustion chamber has an M-shaped velocity profile with two off-center peaks. Vortical motions and lobe-shaped flame are promoted by this type of inlet condition. Combustion characteristics, including flame length, combustion efficiency, heat loss ratio, and preheating effect, are comprehensively examined to evaluate the possibility as an thermo-photovoltaic device. The results indicate that a micro combustor with a thin baffle (bt ≤ 0.1Df ) increases the combustion efficiency and wall heat flux. This suggests that the current baffled combustor can be effectively applied in both micro power generation systems and micro thermal emitters.

The thermal and hydrodynamic behaviour of confined, normally impinging laminar slot jets

Jet impingement cooling is an effective means of inducing high convective heat transfer coefficients for applications such as the cooling of gas turbine surfaces, drying of textiles, and the tempering of metal and glass. Slot jets can be used for applications which require cooling over a line or strip, and these can be realized at the micro-scale if size is constrained. In this paper the application of these jets is of specific interest for high-density photonic integrated circuits (PICs), which can generate device-level heat fluxes as high as 1kW/cm2. In this context, an understanding of the thermal and hydrodynamic behaviour of low Reynolds number, submerged, confined and normally impinging slot jets is currently unavailable in the literature and would be beneficial for the design of micro-scale cooling systems. This investigation utilized the isoflux heated foil technique and Particle-Image Velocimetry (PIV), to measure the single-phase convective heat transfer and velocity fields associated with confined liquid slot jets. The aspect ratios of the jets were varied from 1 to 8, with a constant hydraulic diameter, in order to determine the influence of aspect ratio on the measured parameters. The study was carried out over a jet exit Reynolds number range of 100–500, and a fixed confinement ratio of H/Dh=1. The results showed that, enhancements of up to 68% in area-averaged heat transfer could be achieved through increasing slot jet aspect ratio, with a corresponding 12% reduction in head loss coefficient. The appearance of off-center peaks in the velocity and Nusselt number distributions were also observed. These peaks were concluded to be as a result of the stagnation zone fluid dynamics and local flow entrainment.

Dissipative particle dynamics simulation of macromolecular solutions under Poiseuille flow in microchannels

macromolecular solutions under Poiseuille flow in microchannels are investigated using the dissipative particle dynamics (DPD) approach. The results show that the macromolecular solutions are non-Newtonian fluids which can be described by power-law fluids, and the power-law index decreases with the increase of the macromolecular concentration. The DPD simulations show that the hydrodynamic interaction between the macromolecular chains and the wall, and the gradient of Brownian diffusivity of the chains govern the cross-stream migration of the macromolecules. However, the chain-wall hydrodynamic interaction may not be fully developed and are partly screened in conventional DPD approach. Hence, the chains migrate toward the wall during flow. Simulation results also indicate that the migration toward the wall increases with the increase of the driving force. The competition between the unscreened chain-wall hydrodynamic interaction and Brownian diffusivity leads to two symmetric off-center peaks and a local minimum in the channel centerline in the chain center-of-mass distribution. Under strong confinement, the chain-wall hydrodynamic interaction may be fully screened and the Brownian motion is weak, thus the chains weakly move toward the wall for channel of small width.

Simulation of gas–solid two-phase flow in the annulus of drilling well

This paper investigates the hydrodynamic behavior of gas–solid two-phase flow in the annular space of an air drilling well under different arrangements by using three-dimensional approach. Two-fluid model is used to solve the governing equations in the Eulerian–Eulerian framework. Effect of eccentricity and drill pipe rotation on the pressure drop, volume fraction and velocity profile are examined. The results are compared with available data in the literature and good agreement is found. The results show that the presence of solid particles in the annulus change the air velocity profile significantly and create two off-center peaks velocity close to the walls instead of one peak velocity in the middle. Eccentricity of drill pipe makes more accumulation of the cuttings in the smaller space of the annulus. Increasing the eccentricity increases pressure drop due to impact of particles with annulus wall and also particles collision with each other. Rotation of the drill pipe shifts maximum air velocity location toward smaller space of the annulus which results more uniform cutting distributions in the annulus and improvement in their transportations. Pressure drop in the annulus increases as eccentricity and rotation of drill pipe increase.

Migration and fractionation of deformable particles in microchannel

The complexity of the coupling between soft particle deformation and fluid perturbation has limited studies of soft particle hydrodynamics to dilute suspensions. A hybrid Brownian dynamics-lattice Boltzmann method is presented that models nondilute soft spherical deformable particle (DP) suspensions in flow. Dependences on particle size and density are investigated for suspensions with over 100 DP. Multi-DP interactions lead to complex dependence of particle distributions on concentration and flow rate. Flow-induced DP migration toward channel center for DP in narrow channels is found. In wide channels, off-center peaks in the center of mass distribution for DP are found. The migration of DP leads to faster average speed of DP than the flow, which can be exploited for fractionating DPs of different sizes.

Dissipative particle dynamics simulation of depletion layer and polymer migration in micro- and nanochannels for dilute polymer solutions

The flows of dilute polymer solutions in micro- and nanoscale channels are of both fundamental and practical importance in variety of applications in which the channel gap is of the same order as the size of the suspended particles or macromolecules. In such systems depletion layers are observed near solid-fluid interfaces, even in equilibrium, and the imposition of flow results in further cross-stream migration of the particles. In this work we employ dissipative particle dynamics to study depletion and migration in dilute polymer solutions in channels several times larger than the radius of gyration (Rg) of bead-spring chains. We compare depletion layers for different chain models and levels of chain representation, solvent quality, and relative wall-solvent-polymer interactions. By suitable scaling the simulated depletion layers compare well with the asymptotic lattice theory solution of depletion near a repulsive wall. In Poiseuille flow, polymer migration across the streamlines increases with the Peclet and the Reynolds number until the center-of-mass distribution develops two symmetric off-center peaks which identify the preferred chain positions across the channel. These appear to be governed by the balance of wall-chain repulsive interactions and an off-center driving force of the type known as the Segre-Silberberg effect.

NUMERICAL STUDY ON FLOW AND HEAT TRANSFER OF A 3D LAMINAR IMPINGING SQUARE JET

The flow and heat transfer characteristics of a semi-confined laminar impinging square jet are investigated numerically by using the solution of the steady-state three-dimensional Navier-Stokes and energy equations for incompressible flows. Numerical results are presented to analyze the mechanism of the formation of the four streamwise velocity off-center peaks in jet. The effects of inlet Reynolds number and plate-to-plate spacing on jet flow-field structure and heat transfer characteristics are examined in the laminar range. The calculated results show that heat transfer performance of the impinging jet is dominated by the flow structure. It is the for- mation of the four off-center velocity peaks at the horizontal cut planes near to the impingement surface that results in the formation of the four temperature off-center minima at the corresponding cut planes and the formation of the four off-center peaks of the local Nusselt number at the impingement surface.

Virgo High-Resolution CO Survey: V. Circumnuclear Elliptical Ring in NGC 4569

Abstract We present high-resolution (${1\rlap {.}{}^{\mathrm {\prime \prime }}8}$–${4\rlap {.}{}^{\mathrm {\prime \prime }}5}$) CO data of the Virgo spiral galaxy NGC 4569, obtained using the Nobeyama Millimeter Array. We found that the molecular gas is highly concentrated in the circumnuclear region with two off-center peaks. A CO image with the highest angular resolution of $ {2\rlap {.}{}^{\mathrm {\prime \prime }}0} \times {1\rlap {.}{}^{\mathrm {\prime \prime }}8}$ shows that six blobs likely form a circumnuclear elliptical ring (CER) with a semimajor axis of 720 pc. The CER shows a strongly twisted velocity field, and the Position–velocity diagram shows significant forbidden velocity components. These kinetic features are understood as being non-circular motion due to a bar-potential. We found that the CER coincides with the $\mathrm{H}\alpha$ bright central core and that the mass ratio of the molecular gas to the dynamical mass is about 18%. These results support a gaseous inflow scenario induced by a weak bar potential and self-gravity of the gas.

Three-Dimensional Simulation of Laminar Rectangular Impinging Jets, Flow Structure, and Heat Transfer

The flow and heat transfer characteristics of impinging laminar jets issuing from rectangular slots of different aspect ratios have been investigated numerically through the solution of three-dimensional Navier-Stokes and energy equations in steady state. The three-dimensional simulation reveals the existence of pronounced streamwise velocity off-center peaks near the impingement plate. Furthermore, the effect of these off-center velocity peaks on the Nusselt number distribution is also investigated. Interesting three-dimensional flow structures are detected which cannot be predicted by two-dimensional simulations.

Comparison of spike production in pp and π +p/K +p interactions at 205–360 GeV/c

Spike production is studied in pp and π +p/K +p collisions in the beam-momentum range of 205–360 GeV/c. The pseudorapidity distribution of spike centers exhibits two narrow peaks in pp interactions, while having one wide bump in πp and Kp interactions. The position of the peaks is consistent with the expectation from a model of coherent gluon radiation at finite length. The interference between the quark color amplitudes obtained within this model causes two off-center peaks in pp data, but only one central peak in πp and Kp data.

Development of a large-aspect-ratio rectangular turbulent free jet

This paper reports the results of detailed mean flow and turbulence measurements made with hot-wire anemometry in the flowfield of a turbulent free jet of air issuing from a sharp-edged rectangular slot of aspect ratio 20 into still air surroundings. All three components of the mean velocity vector, the three Reynolds normal stresses and the two Reynolds primary shear stresses were measured. The mean streamwise vorticity and the turbulence kinetic energy have been calculated from the relevant measured data. It was found that the cross section of the jet, in the near flowfield, is dominated by counter-rotating streamwise vortices that facilitate rapid nearfield mixing. Rapid mixing is also evidenced by a short potential core length and high values of the turbulence kinetic energy and the Reynolds primary shear stresses compared with those found in an axisymmetric jet at the corresponding streamwise locations. In addition, mean streamwise velocity off-center peaks, which may be the result of the self-induction of the counter-rotating streamwise vortices, were found in the near flowfield, and the jet was found to spread monotonically in the central plane of the slot minor axis and to contract initially in the central plane of the slot major axis.

Streamwise Evolution of a Square Jet Cross Section

Results of an experimental study of a turbulent free jet of air issuing from a sharp-edged square slot into still air surroundings are presented. The study was done to shed some light on the stream wise development of the jet cross section. The quantities measured directly, using hot-wire anemometry, include the three components of the mean velocity vector, the three Reynolds normal stresses, and the two Reynolds primary shear stresses. It was found that the near region of the jet is dominated by four sets of counter-rotat ing streamwise vortices. These vortices, which represent Prandtl's secondary flow of the first kind, are generated from distributed vorticity shed from the four corners of the slot by skewing of the shear layers as a result of the vena contracta effect. Mean streamwise velocity off-center peaks were also found in the very near region; such mean streamwise velocity off-center peaks may be the result of the self-induction of the streamwise vortices. Furthermore, the higher numerical values of the Reynolds normal and primary shear stresses in the present square jet, compared with those found in a round jet, indicate faster mixing of the square jet.

Effects of nonparallel exit flow on round turbulent free jets

Results of an experimental and numerical study of the effects of nonparallel exit flow on round turbulent free jets are presented. Two nozzle types with the same exit area were used: a sharp-edged slot machined into an aluminum plate and a smoothly contoured nozzle manufactured from Plexiglass. Hot-wire anemometry was used to obtain the mean acquired with a pitot-static tube and a pressure transducer. An elliptic, finite-volume-based procedure was used in the numerical study which predicted the mean velocity and the mean static pressure. The experimental results show the existence of distinct off-center peaks in the mean streamwise velocity in the very near flow field of the jet from the sharp-edged round slot, which had pronounced nonparallel exit flow. These mean streamwise velocity off-center peaks were absent in the smoothly contoured nozzle jet flow which was found to spread faster than the sharp-edged slot jet flow in the initial region. The agreement between the experimental results and the numerical predictions was found to be excellent for the mean streamwise velocity decay on the jet centerline and the spread of the jet beyond the initial region. Only qualitative agreement can be claimed for the other quantities.

Experimental and numerical study of a turbulent free square jet

Results of an experimental and numerical study of a turbulent free jet of air from a sharp-edged square slot are presented. The jet was treated as elliptic in the numerical study that predicted mean streamwise velocity, mean static pressure, and turbulence kinetic energy distributions. The experimental results include the mean velocity, the turbulent normal and shearing stresses obtained with hot-wire anemometry, and the mean static pressure acquired with a pitot-static tube in conjunction with a pressure transducer. The experimental results reveal the existence of pronounced mean streamwise velocity off-center peaks in the very near field, where positive mean static pressures were also found. Furthermore, the square jet is found to spread significantly faster in the near flow field than a jet from a round slot that has the same exit area as the square slot under identical test conditions. The general agreement between experimental results and numerical predictions is relatively good.