Vortex Detachment and Reverse Flow in Pulsatile Laminar Flow Through Axisymmetric Sudden Expansions

1999 ◽  
Vol 121 (3) ◽  
pp. 574-579 ◽  
Author(s):  
S. Tavoularis ◽  
R. K. Singh
Keyword(s):  
Reynolds Number ◽  
Recirculation Zone ◽  
Reverse Flow ◽  
Flow Region ◽  
Vortex Rings ◽  
Mean Velocity ◽  
Zone Length ◽  
Deceleration Phase ◽  
Pulsatile Flows ◽  
Flow Through

Incompressible, steady and pulsatile flows in axisymmetric sudden expansions with diameter ratios of 1:2.25 and 1:2.00 have been simulated numerically over the ranges of time-averaged bulk Reynolds number 0.1 ≤ Re ≤ 400 and Womersley number 0.1 ≤ W ≤ 50. For steady flow, the calculated recirculation zone length increased linearly with an increase in Re, in good agreement with earlier experiments. For pulsatile flows, particularly at higher values of W, the recirculation zone length correlated strongly with the acceleration of the flow and not with the instantaneous Reynolds number; it increased during the deceleration phase and decreased during the acceleration phase. The computed mean velocity and reattachment length were in general agreement with published experimental data. At relatively low W, the computed near-wall, reverse flow region extended along the full domain over part of the cycle, similarly to that in the experiments. At low values of W, the vortex rings created at the expansion remained attached and oscillated back and forth; for an intermediate range of W, they detached and moved downstream; at relatively high W, these vortices became, once more, attached.

Aerodynamic Behavior of Asymmetric Jets in 3-D Furnaces

1994 ◽  
Author(s):  
F. M. El-Mahallawy ◽  
M. A. Hassan ◽  
M. A. Ismail ◽  
H. Zafan
Keyword(s):  
Numerical Simulation ◽  
Flow Pattern ◽  
Numerical Experiments ◽  
Recirculation Zone ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Flow Region ◽  
Stabilization Method ◽  
Flow Features

The purpose of this paper is to present and evaluate numerical experiments illustrating the flow features in a 3-D furnace utilizing unconventional asymmetrical jet that creates natural recirculation zone. The numerical simulation of this aerodynamic stabilization method have unveiled the three-dimensional nature of the flow pattern which possesses a quite large reverse flow region. The size and strength of the built recirculation zone would be capable of stabilizing the burning of low-quality fuels.

CHARACTERISTICS OF FLOW THROUGH RIGID, EMERGENT AND SPARSE VEGETATION

2016 ◽  
Vol 78 (10) ◽  
Author(s):  
Suraya Sharil ◽  
Wan Hanna Melini Wan Mohtar ◽  
Siti Fatin Mohd Razali
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Turbulence Intensity ◽  
Transverse Velocity ◽  
Flow Region ◽  
Vegetation Density ◽  
Flow Depth ◽  
Flow Through ◽  
Fast Flow

This paper looks into the flow profiles in terms of longitudinal and transverse velocities, turbulence intensity and turbulent kinetic energy in relation to the vegetation density, flow depth and stem Reynolds number. An experimental study was conducted in a fully vegetated flume, whereby a control volume was selected for detailed velocity measurement using Acoustic Doppler Velocimeter (ADV). This research considered 0.97%, 3.90% and 7.80% vegetation density or solid volume fractions (SVF) which are categorised as sparse in the lab work. Series of experiments were conducted in uniform flow condition with stem Reynolds number, Red ranging between 1300 and 3000. Experimental results managed to capture the wake area (velocity deficit; < 1) and fast flow region (velocity enhance; > 1). The boundary between the wake area and fast flow region is reflected by the highest magnitude of the normalised longitudinal turbulence intensity and turbulent kinetic energy. Positive normalised transverse velocity represents the flow diversion away from the vegetation and the negative normalised transverse velocity indicates flux towards the centre of the wake. Both turbulence intensity and turbulent kinetic energy display no observable relation with the flow depth. This is probably because the characteristic length for turbulent flow through vegetation is the stem diameter.  

Local Pressure Measurement of Gaseous Flow Through Microchannels

1999 ◽  
Author(s):  
Stephen E. Turner ◽  
Hongwei Sun ◽  
Mohammad Faghri ◽  
Otto J. Gregory
Keyword(s):  
Reynolds Number ◽  
Pressure Distribution ◽  
Friction Factor ◽  
Knudsen Number ◽  
Flow Region ◽  
Reynolds Numbers ◽  
Helium Flow ◽  
Local Pressure ◽  
Pressure Losses ◽  
Flow Through

Abstract This paper presents an experimental investigation on nitrogen and helium flow in microchannels etched in silicon with hydraulic diameters of 9.7, 19.6, and 46.6 μm, and Reynolds numbers ranging from 0. 2 to 1000. The objectives of this research are (1) to measure the pressure distribution along the length of a microchannel; and (2) to determine the friction factor within the fully developed region of the microchannel. The pressure distribution is presented as absolute local pressure plotted against the distance from the microchannel inlet. The friction factor results are presented as the product of friction factor and Reynolds number plotted against Reynolds number with the outlet Knudsen number, Kn, as a curve parameter. The following conclusions have been reached in the present investigation: (1) Pressure losses at the microchannel entrance can be significant; (2) the product, f*Re, when measured sufficiently far away from the entrance and exit is a constant in the laminar flow region; and (3) the friction factor decreases as the Knudsen number increases.

Flow in a Channel With Longitudinal Ribs

1994 ◽  
Vol 116 (2) ◽  
pp. 233-237 ◽  
Author(s):  
C. Y. Wang
Keyword(s):  
Reynolds Number ◽  
Eigenfunction Expansion ◽  
Complex Geometry ◽  
Parallel Plates ◽  
Match Method ◽  
Mean Velocity ◽  
Longitudinal Ribs ◽  
Flow Through ◽  
The Mean ◽  
Wetted Perimeter

The laminar, viscous flow between parallel plates with evenly spaced longitudinal ribs is solved by an eigenfunction expansion and point-match method. The ribs on both plates may be symmetrically placed or staggered. For a given pressure gradient, the mean velocity is plotted as a function of the geometric parameters. We find the wetted perimeter and the friction factor—Reynolds number product are unsuitable parameters for the flow through ducts of complex geometry.

Shear-Layer Flow Regimes and Wave Instabilities and Reattachment Lengths Downstream of an Abrupt Circular Channel Expansion

1972 ◽  
Vol 39 (3) ◽  
pp. 677-681 ◽  
Author(s):  
L. H. Back ◽  
E. J. Roschke
Keyword(s):  
Reynolds Number ◽  
Shear Layer ◽  
Reverse Flow ◽  
Flow Region ◽  
Flow Regimes ◽  
Circular Channel ◽  
Number Range ◽  
Channel Expansion ◽  
Wave Instabilities ◽  
Layer Flow

An experimental investigation of water flow through an abrupt circular-channel expansion is described over a Reynolds number range between 20 and 4200. The shear layer between the central jet and the reverse flow region along the wall downstream behaved differently in the various flow regimes that were observed. With increasing Reynolds number these regimes changed progressively from a laminar flow to an unstable vortex sheetlike flow and then to a more random fluctuating flow. The distance between the step and the reattachment location downstream correspondingly increased, reached a maximum, and then decreased. Of particular significance are the shear layer wave instabilities observed in the shear flow and their relationship to rettachment which apparently has not received much attention previously. Visual observations aided in understanding the results.

Laminarization and Reversion to Turbulence of Low Reynolds Number Flow Through a Converging to Constant Area Duct

1986 ◽  
Vol 108 (3) ◽  
pp. 325-330 ◽  
Author(s):  
Hiroaki Tanaka ◽  
Hirotaka Yabuki
Keyword(s):  
Reynolds Number ◽  
Parallel Plate ◽  
Streamwise Velocity ◽  
Parallel Plates ◽  
Test Channel ◽  
Mean Velocity ◽  
Reynolds Number Flow ◽  
Constant Area ◽  
Number Flow ◽  
Flow Through

Airflow in fully developed turbulent state between two parallel plates was accelerated through a linearly converging section, and then it flowed into a parallel-plate channel again. The Reynolds number 2hum/ν was 10,000 and the acceleration parameter K in the accelerating section was 8 × 10−6. Fluctuations of streamwise velocity as well as time-mean velocity profiles were measured at ten traversing stations located along the test channel by a hot-wire anemometer. It was found that the flow, partly laminarized in the accelerating section, continued to laminarize in the first part of the downstream parallel-plate section and then the reversion to turbulence occurred in the way similar to the case of natural transition in a pipe, where the transition proceeds through a regime of the so-called turbulent slug flow.

Comparison of Flying-Hot-Wire and Stationary-Hot-Wire Measurements of Flow Over a Backward-Facing Step

1999 ◽  
Vol 121 (2) ◽  
pp. 441-445 ◽  
Author(s):  
O. O. Badran ◽  
H. H. Bruun
Keyword(s):  
Reverse Flow ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Flow Region ◽  
Hot Wire ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
Hot Wire Anemometry ◽  
Blunt Leading Edge

This paper is concerned with measurements of the flow field in the separated flow region behind a backward-facing step. The main instrument used in this research was Flying X Hot-Wire Anemometry (FHWA). Stationary (single normal) Hot-Wire Anemometry (SHWA) was also used. Comparative measurements between the SHW probe and the FHW system were conducted downstream of the step (step height H = 120 mm) and results are presented for axial locations of 1H and 2H. Two step configurations were considered; (i) a blunt leading edge with flow underneath (Case I) and (ii) a blunt leading edge with no flow underneath (Case II). It is observed from the results presented that the two Hot-Wire methods produce significantly different mean velocity and turbulence results inside the separation bubble. In particular, the SHWA method cannot detect the reverse flow velocity direction, while the Flying Hot-Wire clearly identifies the existing reverse flow. Also, in the shear flow region, the results presented indicate that measurements with a SHW probe must be treated with great caution.

Flow of Newtonian and Non-Newtonian Fluids in a Concentric Annulus With Rotation of the Inner Cylinder

1994 ◽  
Vol 116 (4) ◽  
pp. 821-827 ◽  
Author(s):  
J. M. Nouri ◽  
J. H. Whitelaw
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Drag Coefficient ◽  
Cross Flow ◽  
Flow Region ◽  
Newtonian Fluids ◽  
Rossby Number ◽  
Shear Stresses ◽  
Mean Velocity ◽  
Flow Components

Mean velocity and the corresponding Reynolds shear stresses of Newtonian and non-Newtonian fluids have been measured in a fully developed concentric flow with a diameter ratio of 0.5 and at a inner cylinder rotational speed of 300 rpm. With the Newtonian fluid in laminar flow the effects of the inner shaft rotation were a uniform increase in the drag coefficient by about 28 percent, a flatter and less skewed axial mean velocity and a swirl profile with a narrow boundary close to the inner wall with a thickness of about 22 percent of the gap between the pipes. These effects reduced gradually with bulk flow Reynolds number so that, in the turbulent flow region with a Rossby number of 10, the drag coefficient and profiles of axial mean velocity with and without rotation were similar. The intensity of the turbulence quantities was enhanced by rotation particularly close to the inner wall at a Reynolds number of 9,000 and was similar to that of the nonrotating flow at the higher Reynolds number. The effects of the rotation with the 0.2 percent CMC solution were similar to those of the Newtonian fluids but smaller in magnitude since the Rossby number with the CMC solution is considerably higher for a similar Reynolds number. Comparison between the results of the Newtonian and non-Newtonian fluids with rotation at a Reynolds number of 9000 showed similar features to those of nonrotating flows with an extension of non-turbulent flow, a drag reduction of up to 67 percent, and suppression of all fluctuation velocities compared with Newtonian values particularly the cross-flow components. The results also showed that the swirl velocity profiles of both fluids were the same at a similar Rossby number.

Turbulence Measurements Over a Heated Two-Dimensional Hill

2007 ◽  
Author(s):  
T. Houra ◽  
M. Tagawa ◽  
Y. Nagano
Keyword(s):  
Reverse Flow ◽  
Reynolds Shear Stress ◽  
Low Frequency ◽  
Flow Region ◽  
Outer Edge ◽  
Heated Wall ◽  
Leeward Side ◽  
Two Dimensional ◽  
Mean Velocity ◽  
Turbulent Characteristics

Experimental investigation has been made on the flow and thermal fields over a heated two-dimensional hill with a cosine-squared shape. The detailed turbulent characteristics were measured by a backscatter-type two-component LDV, a PIV system, a fine thermocouple and a cold-wire probe. In the reverse-flow region on the leeward side of the hill, the turbulence intensities and the Reynolds shear stress show much larger values than that in a canonical wall-bounded shear flow. The mean temperature maintains a relatively high value below the location where the horizontal mean velocity rapidly decreases. At the outer edge of the reverse-flow region, there exists a second maximum intensity of temperature fluctuations. The instantaneous temperature waveforms near the heated wall show very large amplitude consisting of high-frequency fluctuations superimposed on the low-frequency motions.

Numerical Calculations of the Turbulent Flow Through a Controlled Diffusion Compressor Cascade

1995 ◽  
Vol 117 (2) ◽  
pp. 223-230 ◽  
Author(s):  
Shin-Hyoung Kang ◽  
Joon Sik Lee ◽  
Myung-Ryul Choi ◽  
Kyung-Yup Kim
Keyword(s):  
Reynolds Number ◽  
Control Volume ◽  
Low Reynolds Number ◽  
Turbulence Models ◽  
Compressor Cascade ◽  
Mean Velocity ◽  
Flow Angle ◽  
Controlled Diffusion ◽  
Flow Through ◽  
Low Reynolds

The viscous flow through a controlled diffusion (CD) compressor cascade was calculated and compared with the measured data for two different test conditions. A control volume method was used, which has been developed for a generalized nonorthogonal coordinate system. The discretized equations for the physical covariant velocity components were obtained by an algebraic manipulation of the discretized equations for the Cartesian velocity components. Low Reynolds number k–ε turbulence models were used to obtain the eddy viscosity. The numerical scheme using the low Reynolds number k–ε turbulence model reasonably predicted the general performance, i.e, mean outlet flow angle and loss coefficients. The development of the shear layer along the pressure and suction sides was well estimated, and the physical features found in the experiment were reasonably well confirmed in the simulation. However, the calculated profiles of mean velocity and turbulent kinetic energy in the near wake show considerable disagreement with the measured values.

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