Low Reynolds Number Open Channel Flows Over a Backward Facing Step

2012 ◽  
Author(s):  
Afua A. Ampadu-Mintah ◽  
Mark F. Tachie
Keyword(s):  
Reynolds Number ◽  
Open Channel ◽  
Reynolds Shear Stress ◽  
Low Reynolds Number ◽  
Reynolds Numbers ◽  
Backward Facing Step ◽  
Closed Channel ◽  
Freestream Velocity ◽  
Reynolds Number Effects ◽  
Low Reynolds

Low Reynolds number effects on turbulent flows over a backward facing step (BFS) in an open channel were investigated. The Reynolds numbers based on momentum thickness (θ) and step height (h) are in the range 590 ≤ Reθ ≤ 1950 and 950 ≤ Reh ≤ 2900, respectively. The Froude number based on the approach water depth and freestream velocity varied from 0.12 to 0.37. A particle image velocimetry technique was used to measure the velocity field. The flow patterns in the reattachment and redevelopment regions are qualitatively similar for all the three Reynolds numbers studied. The mean velocity profiles in outer coordinates do not exhibit significant Reynolds number effects downstream of the BFS. On the contrary, the turbulence intensities and Reynolds shear stress do not show Reynolds number similarity. As expected, similarity with the upstream profile improves with increasing streamwise distance from the reattachment point. Data obtained in this study were also compared with previous measurements made over backward facing step in a closed channel to study free surface effects. The results showed that deviation of flow over BFS in open channel from flow over BFS in a closed channel is more significant in the immediate vicinity of the step.

Low Reynolds Number Effect on Open Channel Flow Over a Rib

2014 ◽  
Author(s):  
Ebenezer E. Essel ◽  
Kathryn Atamanchuk ◽  
Samuel d’Auteuil ◽  
Mark F. Tachie
Keyword(s):  
Reynolds Number ◽  
Channel Flow ◽  
Open Channel ◽  
Low Reynolds Number ◽  
Open Channel Flow ◽  
Recirculation Region ◽  
Particle Image Velocimetry Technique ◽  
Mean Velocity ◽  
Freestream Velocity ◽  
Low Reynolds

An experimental study was conducted to investigate low Reynolds number effects on open channel flow over a transverse square rib. Particle image velocimetry technique was used to perform detailed velocity measurement in the upstream and recirculation region of a square rib of height, h = 12 mm. The Reynolds number based on the freestream velocity and rib height, Reh = 1510, 2650 and 3950 and the ratio of the boundary layer thickness to step height, δ/h = 2.5 ± 0.2. The results showed that the reattachment length of Reh = 2650 and 3950 increased by 5.7% compared with corresponding value of Reh = 1510. The mean velocities were independent of Reynolds number in the recirculation region but at the reattachment point, Reh = 3650 reduced the streamwise mean velocity and enhanced the wall-normal mean velocity in the region adjacent to the wall. The turbulent kinetic energy beyond the center of the recirculation region increased with increasing Reynolds number.

Low-Reynolds-number effects in a fully developed turbulent channel flow

1992 ◽  
Vol 236 ◽  
pp. 579-605 ◽  
Author(s):  
R. A. Antonia ◽  
M. Teitel ◽  
J. Kim ◽  
L. W. B. Browne
Keyword(s):  
Reynolds Number ◽  
Channel Flow ◽  
Low Reynolds Number ◽  
Turbulent Channel Flow ◽  
Reynolds Numbers ◽  
Wall Region ◽  
Low Reynolds Numbers ◽  
Reynolds Number Effects ◽  
Low Reynolds ◽  
Inner Region

Low-Reynolds-number effects are observed in the inner region of a fully developed turbulent channel flow, using data obtained either from experiments or by direct numerical simulations. The Reynolds-number influence is observed on the turbulence intensities and to a lesser degree on the average production and dissipation of the turbulent energy. In the near-wall region, the data confirm Wei & Willmarth's (1989) conclusion that the Reynolds stresses do not scale on wall variables. One of the reasons proposed by these authors to account for this behaviour, namely the ‘geometry’ effect or direct interaction between inner regions on opposite walls, was investigated in some detail by introducing temperature at one of the walls, both in experiment and simulation. Although the extent of penetration of thermal excursions into the opposite side of the channel can be significant at low Reynolds numbers, the contribution these excursions make to the Reynolds shear stress and the spanwise vorticity in the opposite wall region is negligible. In the inner region, spectra and co-spectra of the velocity fluctuations u and v change rapidly with the Reynolds number, the variations being mainly confined to low wavenumbers in the u spectrum. These spectra, and the corresponding variances, are discussed in the context of the active/inactive motion concept and the possibility of increased vortex stretching at the wall. A comparison is made between the channel and the boundary layer at low Reynolds numbers.

Vortex Detection and Characterization in Low Reynolds Number Separation

2007 ◽  
Author(s):  
Daniel R. Morse ◽  
James A. Liburdy
Keyword(s):  
Reynolds Number ◽  
Low Reynolds Number ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Chord Length ◽  
Separation Flow ◽  
Time Resolved ◽  
Freestream Velocity ◽  
Vortex Detection ◽  
Low Reynolds

This study focuses on the detection and characterization of vortices in low Reynolds number separation flow over the elliptical leading edge of a flat plate airfoil. Velocity fields were obtained using Time Resolved Particle Image Velocimetry (TRPIV). The Reynolds number based on chord length ranged from 14,700 to 66,700. Experiments were performed for velocities of 1.1, 2.0 and 5.0 m/s and angles of attack of 14°, 16°, 18° and 20°. These velocities correspond to chord length Reynolds numbers of 1.47×104, 2.68×104, and 6.70×104, respectively. A local swirl calculation was used to determine regions of high circulation, and the convection of the centers of these regions was used to determine convective velocities of these vortical structures. The streamwise convective velocity normalized by the freestream velocity is observed to range from approximately 0.4 to 0.65 over the range of angles of attack, with slightly increasing values as the angle of attack increases.

scholarly journals The turbulent near wake of a flat plate at low Reynolds number

1990 ◽  
Vol 217 ◽  
pp. 93-114 ◽  
Author(s):  
A. Nakayama ◽  
B. Liu
Keyword(s):  
Reynolds Number ◽  
Flat Plate ◽  
Reynolds Shear Stress ◽  
Low Reynolds Number ◽  
Subsequent Development ◽  
Near Wake ◽  
Mean Velocity ◽  
Reynolds Number Effects ◽  
The Mean ◽  
Low Reynolds

Mean-velocity and turbulence measurements have been made in the turbulent near wake of a flat plate at various Reynolds numbers in order to investigate the low-Reynolds-number effects in this region. The results indicate that the low-Reynolds-number effects are significant enough to partially explain the discrepancies in the existing mean-velocity data. It has been found that, while the Reynolds-number-independent, inner-law similarity of the boundary layers continues to exist, the width of the inner wake that develops within the inner-law region scales with the outer variable. Therefore, the mean velocity near the wake centreline depends on the Reynolds number. It is conjectured that this is due to the influence of the large eddies of the outer layer on the spreading of the inner wake.Measured turbulence quantities indicate that sudden changes occurring just downstream of the trailing edge are independent of the Reynolds number, but the subsequent development of the turbulent stress profiles depends on the Reynolds number. The Reynolds shear stress and the mean-velocity profiles within the inner wake show approximate similarity.

Numerical Simulations of Resonant Heat Transfer Augmentation at Low Reynolds Numbers

2001 ◽  
Author(s):  
Miles Greiner ◽  
Paul F. Fischer ◽  
Henry Tufo
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Natural Frequency ◽  
Flow Rate ◽  
Low Reynolds Number ◽  
Reynolds Numbers ◽  
Spectral Element ◽  
Pumping Power ◽  
Number Systems ◽  
Low Reynolds

Abstract The effect of flow rate modulation on low Reynolds number heat transfer enhancement in a transversely grooved passage was numerically simulated using a two-dimensional spectral element technique. Simulations were performed at subcritical Reynolds numbers of Rem = 133 and 267, with 20% and 40% flow rate oscillations. The net pumping power required to modulate the flow was minimized as the forcing frequency approached the predicted natural frequency. However, mixing and heat transfer levels both increased as the natural frequency was approached. Oscillatory forcing in a grooved passage requires two orders of magnitude less pumping power than flat passage systems for the same heat transfer level. Hydrodynamic resonance appears to be an effective method of increasing heat transfer in low Reynolds number systems where pumping power is at a premium, such as micro heat transfer applications.

A Computational Study on Airfoils at a Low Reynolds Number

2000 ◽  
Author(s):  
Ajit Pal Singh ◽  
S. H. Winoto ◽  
D. A. Shah ◽  
K. G. Lim ◽  
Robert E. K. Goh
Keyword(s):  
Reynolds Number ◽  
Computational Analysis ◽  
Computational Study ◽  
Low Reynolds Number ◽  
Micro Air Vehicles ◽  
Reynolds Numbers ◽  
Aerodynamic Performance ◽  
Low Reynolds Numbers ◽  
Study Results ◽  
Low Reynolds

Abstract Performance characteristics of some low Reynolds number airfoils for the use in micro air vehicles (MAVs) are computationally studied using XFOIL at a Reynolds number of 80,000. XFOIL, which is based on linear-vorticity stream function panel method coupled with a viscous integral formulation, is used for the analysis. In the first part of the study, results obtained from the XFOIL have been compared with available experimental data at low Reynolds numbers. XFOIL is then used to study relative aerodynamic performance of nine different airfoils. The computational analysis has shown that the S1223 airfoil has a relatively better performance than other airfoils considered for the analysis.

On High-Performance Airfoil at Very Low Reynolds Number

2016 ◽  
Vol 28 (3) ◽  
pp. 273-285
Author(s):  
Katsuya Hirata ◽  
◽  
Ryo Nozawa ◽  
Shogo Kondo ◽  
Kazuki Onishi ◽  
...  
Keyword(s):  
Reynolds Number ◽  
Flat Plate ◽  
High Performance ◽  
Low Reynolds Number ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Aerodynamic Characteristics ◽  
Slow Flow ◽  
Low Reynolds Numbers ◽  
Low Reynolds

[abstFig src='/00280003/02.jpg' width=""300"" text='Iso-Q surfaces of very-slow flow past an iNACA0015' ] The airfoil is often used as the elemental device for flying/swimming robots, determining its basic performances. However, most of the aerodynamic characteristics of the airfoil have been investigated at Reynolds numbers Re’s more than 106. On the other hand, our knowledge is not enough in low Reynolds-number ranges, in spite of the recent miniaturisation of robots. In the present study, referring to our previous findings (Hirata et al., 2011), we numerically examine three kinds of high-performance airfoils proposed for very-low Reynolds numbers; namely, an iNACA0015 (the NACA0015 placed back to front), an FPBi (a flat plate blended with iNACA0015 as its upper half) and an FPBN (a flat plate blended with the NACA0015 as its upper half), in comparison with such basic airfoils as a NACA0015 and an FP (a flat plate), at a Reynolds number Re = 1.0 × 102 using two- and three-dimensional computations. As a result, the FPBi shows the best performance among the five kinds of airfoils.

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