Reverse flow in a circular duct with an obstruction at the entry

In this paper the reverse flow in a duct (diameter D) with an obstruction at the front (which is a disc), is investigated using PIV. The gap g between the obstruction and the entry to the duct is systematically varied and it is found that maximum reverse flow occurs at a g/D value of 0.5. The flow is stagnant around g/D of 1.25 and forward flow occurs for g/D values of 1.5 and above. The vortex formation length behind the disc is smaller than that of the plate and the variation of the magnitude of reverse flow with the gap g between the axisymmetrical circular duct and two-dimensional channel differs considerably. The reverse flow phenomena in the circular duct with an obstruction at the entry can be explained by the vortex formation length and low pressure behind the obstruction.     

A Study of the flow phenomenon of water in a channel with flat plate obstruction geometry at the entry

The flow ina parallel walled test channel, when obstructed with a geometry at the entrance, can be forward, reverse and stagnant depending on the position of the obstruction. This interesting flow phenomenon has potential benefit in the control of energy and various flows in the process industry. In this experiment, the flat plate obstruction geometry was used as an obstruction at the entry of the test channel. The parameters that influence the flow inside and around the test channel were the gap (g) between the test channel and the obstruction geometry, the length (L) of the test channel and the Reynolds number (Re). The effect of the gap to channel width ratio (g/w) on the magnitude of the velocity ratio (V₁/V₀ : velocity inside/ velocity outside the test channel) was investigated for a range of Reynolds numbers. The maximum reverse flow observed was nearly 20% to 60% of the outside velocity for Reynolds number ranging from 1000 to 9000 at g/w ratio of 1.5. The maximum forward velocity inside the test channel was found 80% of the outside velocity at higher g/w ratio of 8. The effect of the test channel length on the velocity ratio was investigated for different g/w ratios and a fixed Reynolds number of 4000. The influence of the Reynolds number on the velocity ratio is also discussed and presented for different gap to width ratio (g/w). The flow visualisation photo-graphs showing fluid motion inside and around the test channel are also presented and discussed.     

Turbulent heat transfer of supercritical carbon dioxide in square cross-sectional duct flow

Numerical simulations are performed to develop a new heat transfer coefficient correlation applicable to the gas cooler design of a trans-critical carbon dioxide air-conditioner. Thermodynamic and transport properties of the supercritical gas cooling process change dramatically and significantly vary heat transfer coefficients to be much different from those of single or two phase flows. In the present study, the elliptic blending second moment turbulent closure precisely reflecting the effects of these thermo-physical property variations on the turbulent heat transfer is employed to model the Reynolds stresses and turbulent heat fluxes in the momentum and energy equations. Computational results related to the development of turbulent heat transfer during in-duct cooling of supercritical carbon dioxide were used to establish a new heat transfer coefficient correlation that would be widely applicable to a gas cooler design involving turbulent heat transfer of supercritical carbon dioxide in square cross-sectional duct flows. This paper was recommended for publication in revised form by Associate Editor Kyung-Soo Yang Seong Ho, Han received a B.S. degree in Mechanical Engineering from Kookmin University in 2003. He then went on to receive his M.S. degree from Korea University in 2005. He is currently in a Ph. D. course at Mechanical Engineering at Korea University in Seoul, Korea. His research interests are in the area of hydrogen energy, polymer electrolyte membrane fuel cell.     

Radiation-laminar free convection in a square duct with specular reflection by absorbing-emitting medium

The purpose of this work is to study the effects of specularly reflecting wall under the combined radiative and laminar free convective heat transfer in an infinite square duct. An absorbing and emitting gray medium is enclosed by the opaque and diffusely emitting walls. The walls may reflect diffusely or specularly. Boussinesq approximation is used for the buoyancy term. The radiative heat transfer is evaluated using the direct discrete ordinates method. The parameters under considerations are Rayleigh number, conduction to radiation parameter, optical thickness, wall emissivity and reflection mode. The differences caused by the reflection mode on the stream line, and temperature distribution and wall heat fluxes are studied. Some differences are observed for the categories mentioned above if the order of the conduction to radiation parameter is less than order of 10^-3 for the range of Rayleigh number studied. The differences at the side wall heat flux distributions are observed as long as the medium is optically thin. As the top wall emissivity decreases, the differences between these two modes are increased. As the optical thickness decreases at the fixed wall emissivity, the differences also increase. The difference of the streamlines or the temperature contours is not as distinct as the side wall heat flux distributions. The specular reflection may alter the fluid motion.     

Pulsating duct flow in the presence of an orifice plate

A one-dimensional flow model is used to evaluate the coefficient of discharge and the coefficient of pressure drop across an orifice plate in a pulsating flow of air. It is found that the coefficient of pressure drop continues to have the same value as in steady flow but the coefficient of discharge is modified.     

Numerical analysis of turbulent flow and heat transfer in a square sectioned U-bend duct by elliptic-blending second moment closure

A second moment turbulence closure using the elliptic-blending equation is introduced to analyze the turbulence and heat transfer in a square sectioned U-bend duct flow. The turbulent heat flux model based on the elliptic concept satisfies the near-wall balance between viscous diffusion, viscous dissipation and temperature-pressure gradient correlation, and also has the characteristics of approaching its respective conventional high Reynolds number model far away from the wall. Also, the traditional GGDH heat flux model is compared with the present elliptic concept-based heat flux model. The turbulent heat flux models are closely linked to the ellipticblending second moment closure which is used for the prediction of Reynolds stresses. The predicted results show their reasonable agreement with experimental data for a square sectioned U-bend duct flow field adopted in the present study.     

Experimental characterization of the flow field of square prism with a detached splitter plate at high reynolds number

The characteristics of the flow field of a square prism with detached splitter plate in its wake were investigated by measuring the fluid force on the prism and by visualizing the flow field through particle image velocimetry (PIV) with a high Reynolds number (Re = 10,000). The experimental parameters included the ratios of the splitter and prism widths (H/B = 0.5–1.5) as well as the gap ratios (G/B = 0–2) between the prism and the splitter plate at a high Reynolds number (Re = 10,000). The drag reduction rate of the square prism increased with increasing H/B for the same G/B; meanwhile, it increased and then decreased with increasing G/B for the same H/B. When the detached splitter place was installed, vortices rotating in opposite directions were generated on its upper and lower sides. Reverse flow was caused by the vortices in the wake region of the square prism, and the prism drag was decreased by the reverse flow.     

Control of flow around a square cylinder at incidence by using a splitter plate

Passive control of vortex shedding behind a square cylinder at incidence has been conducted experimentally by using a stationary splitter plate for the Reynolds numbers of 3.0×10⁴. The splitter plate was located at the center of the rear face of the square cylinder in tandem. The width of the cylinder and the plate were both chosen to be 30 mm and the incidence angle of the square cylinder was rotated between 0° to 45°. In this study, the combined effects of the splitter plate and angle of incidence on the pressure distributions and vortex-shedding phenomenon were investigated. Vortex shedding frequency was obtained from velocity measurements and aerodynamic force coefficients acted on the cylinder were calculated from pressure distributions. Characteristics of the vortex formation region and location of the flow attachments, reattachments and separation were observed by using the smoke–wire flow visualization technique. For the case with the plate, there is a sudden jump in the Strouhal number in the vicinity of 13° which corresponds to a minimum value of the drag coefficient. At zero angle of incidence, Strouhal number and a drag coefficient of the square cylinder decreased about 20% by means of the splitter plate. Drag reduction was minimum at about 13° and reached its maximum value at about 20°.     

Turbulent natural convection flow and heat transfer in an inclined square enclosure

Numerical analysis was performed for the two-dimensional turbulent natural convection in an inclined enclosure. The enclosure has two walls which one is heated and the other cooled, and has the other two walls of the linear temperature distributions. The inclined angle is equal to zero when the wall of linear temperature was horizontal and increases counter-clockwise. The mean continuity, mean momentum and mean energy equations have been obtained by using the conventional time-averaging procesure. The turbulent model has been applied ak-ε two equation model of turbulence similar to the one proposed by the Launder and Spalding. Numerical results were studied for a series of inclined angle, ranging from 0° to 60° and for a Grashof number range of 6×10⁶∼10⁸. The average heat transfer rate on hot wall is shown maximum value at 30° regardless of Grashof number taken here. When Gr≥5×10⁷ and θ≥45°, the flow region of whole enclosure became a significant turbulence. This paper was presented at the International Symposium on the Refined Flow Modeling and Turbulent Measurement. Iowa City, Iowa, U.S.A., 1985     

方管中中性悬浮颗粒惯性迁移现象的数值研究

针对方管中中性悬浮颗粒(颗粒与流体密度等于1)的惯性迁移现象问题,采用并行虚拟区域方法(DF/FD)进行了完全直接数值模拟。选取了周期性管长(L=2H)和3种不同粒径大小的球形颗粒,模拟研究了颗粒在Re为100~1 500范围内的方腔管道中的惯性迁移过程,确定了颗粒的迁移轨迹和平衡位置,以及粒径对颗粒惯性迁移现象的影响,并与圆管、槽道流结果进行了对比。研究结果表明:颗粒迁移后的平衡位置主要分为方管对角线和边线中间两种;随着雷诺数的增加,对角线上的平衡位置愈靠近角落,颗粒粒径越小,离角落越近;边线中间的平衡位置则先靠近壁面,当Re增加到800左右,颗粒的平衡位置开始远离壁面向方管中心迁移,大颗粒即粒径比a/H=0.15表现的最为明显,会形成类似于圆管中的内部平衡位置。     

Supersonic flow of hydrogen and air in a duct with variable area

A chemically nonequilibrium supersonic flow of hydrogen and air has been investigated in a duct with conically divergent or convergent walls. Elementary reaction schemes of radicals involved in reaction of hydrogen-air have been considered and solved through the CHEMKIN code. The aim was to promote an understanding of characteristics of chemically nonequilibrium supersonic flow by introducing a simple mathematical formulation. The temperature, pressure, and density all were found to decrease for divergent ducts as the flow was accelerated, whereas they increased for a slightly convergent duct or a constant cross-sectional area duct. For the divergent nozzle with a greater degree the flow became chemically frozen. But it was quite necessary to take account of the effect of chemical nonequilibrium in a moderately expanded or all convergent conical ducts. As was expected, it was found that the temperature, pressure and Mach number were reduced for a fuel-lean mixture.     

Experimental study of the flow between triangular ribs in a square channel

An experimental study of the flow features of a channel with triangular ribs was carried out using particle image velocimetry technique. The experiments were performed for three Reynolds numbers, including 2900, 8400 and 15000, and were based on the hydraulic diameter of the channel and the mean velocity. The mean and instantaneous velocity fields and turbulence statistics were also depicted in detail.

     

Effect of Reynolds numbers on flow past four square cylinders in an in-line square configuration for different gap spacings

In this paper two-dimensional (2-D) numerical investigation of flow past four square cylinders in an in-line square configuration are performed using the lattice Boltzmann method. The gap spacing g = s/d is set at 1, 3 and 6 and Reynolds number ranging from Re = 60 to 175. We observed four distinct wake patterns: (i) a steady wake pattern (Re = 60 and g = 1); (ii) a stable shielding wake pattern (80 ≤ Re ≤ 175 and g = 1); (iii) a wiggling shielding wake pattern (60 ≤ Re ≤ 175 and g = 3) and (iv) a vortex shedding wake pattern (60 ≤ Re ≤ 175 and g = 6). At g = 1, the Reynolds number is observed to have a strong effect on the wake patterns. It is also found that at g = 1, the secondary cylinder interaction frequency significantly contributes for drag and lift coefficients signal. It is found that the primary vortex shedding frequency dominates the flow and the role of secondary cylinder interaction frequency almost vanish at g = 6. It is observed that the jet between the gaps strongly influenced the wake interaction for different gap spacing and Reynolds number combination. To fully understand the wake transformations the details vorticity contour visualization, power spectra of lift coefficient signal and time signal analysis of drag and lift coefficients also presented in this paper.     

Measurements of the flow fields around two square cylinders in a tandem arrangement

The velocities, turbulence intensities, Reynolds shear stresses, and turbulent kinetic energies of the flow fields around two square cylinders in a tandem arrangement were investigated using particle image velocimetry (PIV). The experiments were made for the spacing between the two cylinders ranging from s/D = 0.5 to 10.0 and two Reynolds numbers of 5,300 and 16,000. The results showed that the flow patterns at s/D≤2.0 were drastically different from those at s/D≥2.5 for both Reynolds numbers. The sudden change in the flow patterns depended on the reattachment of the shear layer separated from the upstream cylinder.     

Reynolds number effect on turbulent secondary flow in a duct

Numerical simulations of fully-developed turbulent flow through a straight square duct at three Reynolds numbers of 190, 300, and 550 based on the friction velocity averaged over the duct perimeter and duct half width are reported. The effect of Reynolds number on the mean and turbulence statistics and secondary flow is investigated. The mean streamwise-velocity profiles along the wall bisector are found to obey a logarithmic scaling when they are normalized by the friction velocity at the mid-wall. Magnitudes and spatial distributions of the peak production and diffusion terms in the mean streamwise-vorticity equation normalized by the wall units are found to be unaffected by the Reynolds-number variation when they are considered in wall-unit coordinates.     

Simulation of vortex shedding from a square cylinder in oscillating channel flow

Laminar vortex shedding of a square cylinder mounted between two parallel walls are numerically simulated by using a finite volunme method based on a linear upwind differencing scheme and SIMPLER algorithm. The unsteady terms are approximated by a two-step fully-implicit backward scheme. The lock-on phenomena in an oscillating flow as well as the vortex shedding in a steady flow are investigated. The values of Strouhal number in the steady uniform flow are reasonably predicted with accuracy by the present numerical method. The frequency range of the incoming flow for the lock-on appear is well simulated. When the flow is loked-on, the shedding frequency is half of the incoming flow frequency. The structural characteristics of shedding vortex in the lock-on range is discussed in the present paper.     

Analysis of turbulent flow through a square-sectioned duct with installed 90-degree elbow

This paper presents the results of a theoretical and experimental investigation of the flow of air through a square-sectioned duct with installed bend. A special test stand was built which allowed testing two different bends with dimensionless mean radius values 2.01 and 1.5. To measure the volumetric flow rate, a measuring orifice plate was installed for control purposes. Additionally, numerical integration of the continuity equation, equations of motion, and equations of selected turbulence models was carried out using FLUENT. As a result of the numerical calculations, time-averaged velocity fields, pressures, and components of the turbulent stress tensor in a developing hydrodynamic flow through a square-sectioned duct with installed bend were determined. The numerical results obtained were subjected to detailed verification by comparison with experimental results obtained on the constructed test stand and those available in the literature.     

Drag reduction for flow past a square cylinder through corner chamfering

This study investigates the unsteady incompressible flow around a square cylinder with different chamfer ratios (CRs) using a commercial finite volume code, ANSYS Fluent. CR ranges from 0.0 (sharp square cylinder) to 0.5 (diamond cylinder) with variable increments. Detailed analysis of flow characteristics is conducted at Reynolds number (Re) = 2100. Additionally, simulation is extended to cover Re, i.e., Re = 100, 500, and 10000. The simulation results show that cylinder with CR = 0.1 outperforms all other cases by enabling a drag reduction of about 60 % at Re = 10⁴. Drag has an inverse relationship with the wake closure length. Time-averaged coefficient of pressure, streamlines, and vorticity contours are also discussed to better understand near-wake features and the physics of drag reduction.

     

A study of the flow characteristics of developing turbulent pulsating flows in a curved duct

Flow characteristics of turbulent pulsating flows in a square-sectional curved duct were experimentally investigated. Experimental studies for air flow were conducted to measure axial velocity profiles, secondary flow and pressure distributions in a square-sectional 180° curved duct by using an LDV system with a data acquisition and processing system which includes a Rotating Machinery Resolve (RMR) and PHASE software. Measurements were made at the seven cross-sections from the inlet (ø=0°) to the outlet (ø=180°) of the duct with 30° intervals. Pressure was measured by using a magnetic differential pressure gage. The experiment was conducted in nineteen sections from the inlet to the outlet of the duct at 10° intervals.Velocity profiles for turbulent pulsating flows were large at the outer wall for a bend angle of ø=30° because of the centrifugal force. The velocity profiles were similar to those of turbulent steady flows. The secondary flow of the turbulent pulsating flow had a positive value at a bend angle of 150° without regarding the phase. The dimensionless value of the secondary flow became gradually weak and approached to zero in the region of a bend angle of 180° regardless of the ratio of velocity amplitude. The pressure difference of turbulent pulsating flows was the largest near the region of a bend of angle of 90° in the case of the middle region and became small beyond 90.     

Determining the flow correlation for an orifice with a non-dimensional number

Traditional devices like orifice meters play a crucial function as a flow measuring device because there is inaccuracy in the measurement of the flow measuring device concern. The pressure drop (Δp) between the upside and downsides of the orifice-pipe flow passage is calculated using Bernoulli's principle. Orifice meter produces errors and uncertainty in the downstream of the flow because of wake or backflow. The proposed study provides the procedure to calculate the Δp and flow characteristics for a circular orifice for a compressible fluid (Air) with CFD analysis. The numerical study was carried out by considering combined parameters such as area ratio (σ) and space ratio (s) as geometrical parameters and Reynolds number as flow parameters to minimize the errors of the numerical calculation. The input parameter σ varies from 0.2 to 0.6, and the s varies from 0.1 to 0.9. Whereas the Reynolds number (Re) varies from 10000 to 100000. A non-dimensional number is defined by the combined effect of σ and s to generated correlations with accuracy which is enhanced predicted results of the work. The correlation will make a significant contribution to the flow monitoring device design.