scholarly journals MEASUREMENTS OF MASS TRANSPORT OVER A ROUGH BED

1984 ◽  
Vol 1 (19) ◽  
pp. 78 ◽  
Author(s):  
J.F.A. Sleath
Keyword(s):  
Reynolds Numbers ◽  
Mean Velocity ◽  
Low Reynolds Numbers ◽  
Average Value ◽  
Wave Flume ◽  
Roughness Elements ◽  
Rough Bed ◽  
Median Diameter ◽  
Bed Roughness ◽  
High Reynolds

Measurements have been made with a laser doppler anemometer of the time-mean velocity of the fluid close to the bed in a wave flume. Both a rough bed, consisting of gravel of median diameter 11 mm, and a smooth bed were investigated. With the rough bed the time-mean velocity at a given height was found to be strongly dependent on position relative to prominent roughness elements. At one point the time-mean drift at a given height might be in the direction of wave propagation while, at another, in the opposite direction. Significant variation in time-mean drift with horizontal position was observed at all values of Reynolds number tested. The effect of bed roughness on the average value of the time-mean velocity at a given height was found to be most marked at low Reynolds numbers: the maximum near bed value with this gravel bed was about 3 times that for a smooth bed at the lowest Reynolds numbers tested. At the highest Reynolds numbers there was no clear difference between the rough and smooth bed values even though the boundary layer over the rough bed was fully turbulent whereas that over the smooth bed was laminar. However, at these high Reynolds numbers both the rough and the smooth beds showed a reduction in drift velocity below that predicted by Longuet-Higgins (9) because of the increased importance of higher harmonics in the flow.

Comparisons of Pins/Dimples/Protrusions Cooling Concepts for a Turbine Blade Tip-Wall at High Reynolds Numbers

2010 ◽  
Author(s):  
Gongnan Xie ◽  
Bengt Sunde´n ◽  
Weihong Zhang
Keyword(s):  
Heat Transfer ◽  
Turbine Blade ◽  
Computational Models ◽  
Reynolds Numbers ◽  
Low Reynolds Numbers ◽  
Gas Leakage ◽  
Leakage Flows ◽  
Blade Tip ◽  
High Reynolds ◽  
The Cost

The blade tip region encounters high thermal loads because of the hot gas leakage flows, and it must therefore be cooled to ensure a long durability and safe operation. A common way to cool a blade tip is to design serpentine passages with 180° turn under the blade tip-cap inside the turbine blade. Improved internal convective cooling is therefore required to increase blade tip lifetime. Pins, dimples and protrusions are well recognized as effective devices to augment heat transfer in various applications. In this paper, enhanced heat transfer of an internal blade tip-wall has been predicted numerically. The computational models consist of a two-pass channel with 180° turn and arrays of circular pins or hemispherical dimples or protrusions internally mounted on the tip-wall. Inlet Reynolds numbers are ranging from 100,000 to 600,000. The overall performance of the two-pass channels is evaluated. Numerical results show that the heat transfer enhancement of the pinned tip is up to a factor of 3.0 higher than that of a smooth tip while the dimpled-tip and protruded-tip provide about 2.0 times higher heat transfer. These augmentations are achieved at the cost of an increase of pressure drop by less than 10%. By comparing the present cooling concepts with pins, dimples and protrusions, it is shown that the pinned-tip exhibit best performance to improve the blade tip cooling. However, when disregarding the added active area and considering the added mechanical stress, it is suggested that the usage of dimples is more suitable to enhance blade tip cooling, especially at low Reynolds numbers.

Reynolds Number Effects on the Freewheeling Behavior of a High Solidity Vertical River Kinetic Turbine

2010 ◽  
Author(s):  
Amir Hossein Birjandi ◽  
Eric Bibeau
Keyword(s):  
Reynolds Number ◽  
Reynolds Numbers ◽  
Speed Ratio ◽  
Stream Velocity ◽  
Blade Profile ◽  
Low Reynolds Numbers ◽  
Tip Speed Ratio ◽  
Reynolds Number Effects ◽  
High Reynolds ◽  
The University

A four-bladed, squirrel-cage, and scaled vertical kinetic turbine was designed, instrumented and tested in the water tunnel facilities at the University of Manitoba. With a solidity of 1.3 and NACA0021 blade profile, the turbine is classified as a high solidity model. Results were obtained for conditions during freewheeling at various Reynolds numbers. In this study, the freewheeling tip speed ratio, which relates the ratio of maximum blade speed to the free stream velocity at no load, was divided into three regions based on the Reynolds number. At low Reynolds numbers, the tip speed ratio was lower than unity and blades were in a stall condition. At the end of the first region, there was a sharp increase of the tip speed ratio so the second region has a tip speed ratio significantly higher than unity. In this region, the tip speed ratio increases almost linearly with Reynolds number. At high Reynolds numbers, the tip speed ratio is almost independent of Reynolds number in the third region. It should be noted that the transition between these three regions is a function of the blade profile and solidity. However, the three-region behavior is applicable to turbines with different profiles and solidities.

scholarly journals Natural logarithms

2013 ◽  
Vol 718 ◽  
pp. 1-4 ◽  
Author(s):  
B. J. McKeon
Keyword(s):  
Turbulent Flows ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Wall Turbulence ◽  
Significant Advance ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Logarithmic Scaling ◽  
The Mean ◽  
High Reynolds

AbstractMarusic et al. (J. Fluid Mech., vol. 716, 2013, R3) show the first clear evidence of universal logarithmic scaling emerging naturally (and simultaneously) in the mean velocity and the intensity of the streamwise velocity fluctuations about that mean in canonical turbulent flows near walls. These observations represent a significant advance in understanding of the behaviour of wall turbulence at high Reynolds number, but perhaps the most exciting implication of the experimental results lies in the agreement with the predictions of such scaling from a model introduced by Townsend (J. Fluid Mech., vol. 11, 1961, pp. 97–120), commonly termed the attached eddy hypothesis. The elegantly simple, yet powerful, study by Marusic et al. should spark further investigation of the behaviour of all fluctuating velocity components at high Reynolds numbers and the outstanding predictions of the attached eddy hypothesis.

Asymptotic scaling in turbulent pipe flow

2007 ◽  
Vol 365 (1852) ◽  
pp. 771-787 ◽  
Author(s):  
B.J McKeon ◽  
J.F Morrison
Keyword(s):  
Asymptotic Behaviour ◽  
Pipe Flow ◽  
Physical Space ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Turbulent Pipe Flow ◽  
Inertial Subrange ◽  
Velocity Spectrum ◽  
Mean Velocity ◽  
High Reynolds

The streamwise velocity component in turbulent pipe flow is assessed to determine whether it exhibits asymptotic behaviour that is indicative of high Reynolds numbers. The asymptotic behaviour of both the mean velocity (in the form of the log law) and that of the second moment of the streamwise component of velocity in the outer and overlap regions is consistent with the development of spectral regions which indicate inertial scaling. It is shown that an ‘inertial sublayer’ in physical space may be considered as a spatial analogue of the inertial subrange in the velocity spectrum and such behaviour only appears for Reynolds numbers R + >5×10 3 , approximately, much higher than was generally thought.

A Study on Turbulent Boundary Layers on a Smooth Flat Plate in an Open Channel

2001 ◽  
Vol 123 (2) ◽  
pp. 394-400 ◽  
Author(s):  
Ram Balachandar ◽  
D. Blakely ◽  
M. Tachie ◽  
G. Putz
Keyword(s):  
Flat Plate ◽  
Froude Number ◽  
Boundary Layers ◽  
Open Channel ◽  
Reynolds Numbers ◽  
Turbulent Boundary Layers ◽  
Wall Region ◽  
Mean Velocity ◽  
Number Range ◽  
High Reynolds

An experimental study was undertaken to investigate the characteristics of turbulent boundary layers developing on smooth flat plate in an open channel flow at moderately high Froude numbers (0.25<Fr<1.1) and low momentum thickness Reynolds numbers 800<Reθ<2900. The low range of Reynolds numbers and the high Froude number range make the study important, as most other studies of this type have been conducted at high Reynolds numbers and lower Froude numbers (∼0.1). Velocity measurements were carried out using a laser-Doppler anemometer equipped with a beam expansion device to enable measurements close to the wall region. The shear velocities were computed using the near-wall measurements in the viscous subregion. The variables of interest include the longitudinal mean velocity, the turbulence intensity, and the velocity skewness and flatness distributions across the boundary layer. The applicability of a constant Coles’ wake parameter (Π=0.55) to open channel flows has been discounted. The effect of the Froude number on the above parameters was also examined.

scholarly journals Model-based scaling of the streamwise energy density in high-Reynolds-number turbulent channels

2013 ◽  
Vol 734 ◽  
pp. 275-316 ◽  
Author(s):  
Rashad Moarref ◽  
Ati S. Sharma ◽  
Joel A. Tropp ◽  
Beverley J. McKeon
Keyword(s):  
Reynolds Number ◽  
Stokes Equations ◽  
Reynolds Numbers ◽  
Low Rank ◽  
Navier Stokes ◽  
Mean Velocity ◽  
Navier Stokes Equations ◽  
Self Similar ◽  
The Mean ◽  
High Reynolds

AbstractWe study the Reynolds-number scaling and the geometric self-similarity of a gain-based, low-rank approximation to turbulent channel flows, determined by the resolvent formulation of McKeon & Sharma (J. Fluid Mech., vol. 658, 2010, pp. 336–382), in order to obtain a description of the streamwise turbulence intensity from direct consideration of the Navier–Stokes equations. Under this formulation, the velocity field is decomposed into propagating waves (with single streamwise and spanwise wavelengths and wave speed) whose wall-normal shapes are determined from the principal singular function of the corresponding resolvent operator. Using the accepted scalings of the mean velocity in wall-bounded turbulent flows, we establish that the resolvent operator admits three classes of wave parameters that induce universal behaviour with Reynolds number in the low-rank model, and which are consistent with scalings proposed throughout the wall turbulence literature. In addition, it is shown that a necessary condition for geometrically self-similar resolvent modes is the presence of a logarithmic turbulent mean velocity. Under the practical assumption that the mean velocity consists of a logarithmic region, we identify the scalings that constitute hierarchies of self-similar modes that are parameterized by the critical wall-normal location where the speed of the mode equals the local turbulent mean velocity. For the rank-1 model subject to broadband forcing, the integrated streamwise energy density takes a universal form which is consistent with the dominant near-wall turbulent motions. When the shape of the forcing is optimized to enforce matching with results from direct numerical simulations at low turbulent Reynolds numbers, further similarity appears. Representation of these weight functions using similarity laws enables prediction of the Reynolds number and wall-normal variations of the streamwise energy intensity at high Reynolds numbers (${Re}_{\tau } \approx 1{0}^{3} {\unicode{x2013}} 1{0}^{10} $). Results from this low-rank model of the Navier–Stokes equations compare favourably with experimental results in the literature.

scholarly journals Turbulent flows in channels and pipes

2007 ◽  
Vol 29 (3) ◽  
pp. 385-396
Author(s):  
Khanh Le Chau
Keyword(s):  
Variational Principle ◽  
Viscous Fluid ◽  
Turbulent Flows ◽  
Good Accuracy ◽  
Reynolds Numbers ◽  
Low Reynolds Numbers ◽  
Pipe Flows ◽  
High Reynolds Numbers ◽  
Friction Factors ◽  
High Reynolds

A variational principle for channel and pipe flows of incompressible viscous fluid is proposed. For low Reynolds numbers this variational principle reduces to the principle of minimum dissipation. For high Reynolds numbers it enables one to calculate the velocity profiles and the corresponding friction factors with reasonably good accuracy.

Flow Beneath a Ship at Small Underkeel Clearance

2006 ◽  
Vol 50 (03) ◽  
pp. 250-258
Author(s):  
Tim Gourlay
Keyword(s):  
Couette Flow ◽  
Reynolds Numbers ◽  
Parallel Plates ◽  
Bulk Carrier ◽  
Low Reynolds Numbers ◽  
Sea Floor ◽  
Flow Models ◽  
High Reynolds Numbers ◽  
Close Proximity ◽  
High Reynolds

This article looks at the case of a large, flat-bottomed ship, such as a bulk carrier, moving in close proximity to a flat sea floor. It is shown that the flow beneath the ship can be modeled as a shear flow between two parallel plates, one of which is moving. The resulting flow can be represented using laminar Couette flow at low Reynolds numbers (possible at model scale) or the very different turbulent Couette flow at high Reynolds numbers (full scale). Implications of these flow models on squat and viscous resistance are discussed.

Turbulence in radial flow between parallel disks at medium and low Reynolds numbers

1987 ◽  
Vol 185 ◽  
pp. 483-502 ◽  
Author(s):  
M. Tabatabai ◽  
A. Pollard
Keyword(s):  
Radial Flow ◽  
Reynolds Numbers ◽  
Viscous Sublayer ◽  
Decay Process ◽  
Low Reynolds Numbers ◽  
Parallel Disks ◽  
Turbulence Decay ◽  
Low Reynolds ◽  
High Reynolds ◽  
Eventual Outcome

The radial flow of air between two closely spaced parallel disks is studied experimentally and the behaviour of the flow, especially the turbulence decay mechanism, is examined. At high Reynolds numbers the flow resembles fully developed turbulent two-dimensional channel flow. A quasi-laminar boundary layer is found to gradually replace the viscous sublayer as the Reynolds number decreases. At low Reynolds numbers, the turbulence decays and the flow gradually approaches a laminar-type profile. The decay process is shown to be very slow and indications of a weak turbulence-generating mechanism is observed even at very low Reynolds numbers. Relaminarization, rather than being an abrupt change in the state of the flow, is an eventual outcome of the turbulence decay process.

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