scholarly journals Effect of Mean Velocity-to-Critical Velocity Ratios on Bed Topography and Incipient Motion in a Meandering Channel: Experimental Investigation

Water ◽  
2021 ◽  
Vol 13 (7) ◽  
pp. 883
Author(s):  
Nargess Moghaddassi ◽  
Seyed Habib Musavi-Jahromi ◽  
Mohammad Vaghefi ◽  
Amir Khosrojerdi
Keyword(s):  
Experimental Investigation ◽  
Critical Velocity ◽  
Velocity Ratio ◽  
Scour Depth ◽  
Incipient Motion ◽  
Mean Velocity ◽  
Straight Path ◽  
Meandering Channel ◽  
Bed Topography ◽  
The Mean

As 180-degree meanders are observed in abundance in nature, a meandering channel with two consecutive 180-degree bends was designed and constructed to investigate bed topography variations. These two 180-degree mild bends are located between two upstream and downstream straight paths. In this study, different mean velocity-to-critical velocity ratios have been tested at the upstream straight path to determine the meander’s incipient motion. To this end, bed topography variations along the meander and the downstream straight path were addressed for different mean velocity-to-critical velocity ratios. In addition, the upstream bend’s effect on the downstream bend was investigated. Results indicated that the maximum scour depth at the downstream bend increased as a result of changing the mean velocity-to-critical velocity ratio from 0.8 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 by, respectively, 1.5, 2.5, 5, 10, 12, and 26 times. Moreover, increasing the ratio increased the maximum sedimentary height by 3, 10, 23, 48, 49, and 56 times. The upstream bend’s incipient motion was observed for the mean velocity-to-critical velocity ratio of 0.89, while the downstream bend’s incipient motion occurred for the ratio of 0.78.

scholarly journals Effect of Mean Velocity to Critical Velocity Ratios on Bed Topography and Incipient Motion in a Meandering Channel: Experimental Investigation

2021 ◽  
Author(s):  
Nargess Moghaddassi ◽  
Seyed Habib Musavi-Jahromi ◽  
Mohammad Vaghefi ◽  
Amir Khosrojerdi
Keyword(s):  
Experimental Investigation ◽  
Critical Velocity ◽  
Velocity Ratio ◽  
Scour Depth ◽  
Incipient Motion ◽  
Mean Velocity ◽  
Straight Path ◽  
Meandering Channel ◽  
Bed Topography ◽  
The Mean

As 180-degree meanders are observed in abundance in nature, a meandering channel with two consecutive 180-degree bends was designed and constructed to investigate bed topography variations. These two 180-degree mild bends are located between two upstream and downstream straight paths. In this study, different mean velocity to critical velocity ratios have been tested at the upstream straight path to determine the meander's incipient motion. To this end, bed topography variations along the meander and the downstream straight path were addressed for different mean velocity to critical velocity ratios. In addition, the upstream bend's effect on the downstream bend has been investigated. Results indicated that the maximum scour depth at the downstream bend has increased as a result of changing the mean velocity to critical velocity ratio from 0.8 to 0.84, 0.86, 0.89, 0.92, 0.95, and 0.98 by respectively 1.5, 2.5, 5, 10, 12, and 26 times. Moreover, increasing the ratio increased the maximum sedimentary height by 3, 10, 23, 48, 49, and 56 times. The upstream bend's incipient motion was observed for the mean velocity to critical velocity ratio of 0.89, while the downstream bend was equal to 0.78.

Initial Region of Subsonic Coaxial Jets of High Mean-Velocity Ratio

1981 ◽  
Vol 103 (2) ◽  
pp. 335-338 ◽  
Author(s):  
N. W. M. Ko ◽  
H. Au
Keyword(s):  
Experimental Investigation ◽  
Turbulence Intensity ◽  
Velocity Ratio ◽  
Velocity Profiles ◽  
Initial Region ◽  
Mean Velocity ◽  
Coaxial Jets ◽  
Single Jet ◽  
The Mean

This paper describes an experimental investigation of the initial region of subsonic coaxial jets of three different mean-velocity ratios λ higher than unity. Detailed measurements have found similarity of the mean velocity and turbulence intensity profiles within the three zones: initial merging, intermediate, and fully merged zone. Similarity with single jet results has been found. In the inner mixing region, however, only the similarity of the mean velocity profiles has been found.

The stability of countercurrent mixing layers in circular jets

1991 ◽  
Vol 227 ◽  
pp. 309-343 ◽  
Author(s):  
P. J. Strykowski ◽  
D. L. Niccum
Keyword(s):  
Shear Layer ◽  
Critical Velocity ◽  
Velocity Ratio ◽  
Mixing Layer ◽  
Mixing Layers ◽  
Mean Velocity ◽  
Axisymmetric Vortex ◽  
Circular Jets ◽  
Axisymmetric Shear ◽  
The Stability

A spatially developing countercurrent mixing layer was established experimentally by applying suction to the periphery of an axisymmetric jet. A laminar mixing region was studied in detail for a velocity ratio R = ΔU/2U between 1 and 1.5, where ΔU describes the intensity of the shear across the layer and U is the average speed of the two streams. Above a critical velocity ratio Rr = 1.32 the shear layer displays energetic oscillations at a discrete frequency which are the result of very organized axisymmetric vortex structures in the mixing layer. The spatial order of the primary vortices inhibits the pairing process and dramatically alters the spatial development of the shear layer downstream. Consequently, the turbulence level in the jet core is significantly reduced, as is the decay rate of the mean velocity on the jet centreline. The response of the shear layer to controlled external forcing indicates that the shear layer oscillations at supercritical velocity ratios are self-excited. The experimentally determined critical velocity ratio of 1.32, established for very thin axisymmetric shear layers, compares favourably with the theoretically predicted value of 1.315 for the transition from convective to absolute instability in plane mixing layers (Huerre & Monkewitz 1985).

Turbulent Mixing in the Developing Region of Coaxial Jets

1973 ◽  
Vol 95 (3) ◽  
pp. 467-473 ◽  
Author(s):  
D. Dura˜o ◽  
J. H. Whitelaw
Keyword(s):  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Velocity Ratio ◽  
Jet Flows ◽  
Normal Stresses ◽  
Downstream Location ◽  
Mean Velocity ◽  
Coaxial Jets ◽  
Developing Region ◽  
The Mean

Measurements of mean velocity, the three normal stresses and Reynolds shear stress are reported in the developing region of coaxial jet flows. The measurements were obtained with three velocity ratios, i.e., values of the ratio of maximum initial pipe velocity to maximum initial annulus velocity of 0, 0.23, and 0.62 and at downstream distances up to 17 outer diameters. The results show that coaxial jets tend to reach a self-preserving state much more rapidly than axisymmetric single jets; that the mean velocity, normal stresses, and Reynolds shear stress attain this state at a similar downstream location; and that, for the particular geometry investigated, a velocity ratio of around 0.15 corresponds to the slowest rate of development. Relationships between mean velocity gradient, Reynolds shear stress, and turbulent kinetic energy are examined to assess their ability to characterize the present flow: the results indicate the need to take account of the normal stresses in any proposed mathematical model.

Organized motions in a jet in crossflow

2001 ◽  
Vol 444 ◽  
pp. 117-149 ◽  
Author(s):  
A. RIVERO ◽  
J. A. FERRÉ ◽  
FRANCESC GIRALT
Keyword(s):  
Velocity Field ◽  
Velocity Ratio ◽  
Mean Field ◽  
Vortex Pair ◽  
Leeward Side ◽  
Mean Velocity ◽  
Jet In Crossflow ◽  
Planar Laser ◽  
The Mean ◽  
Counter Rotating Vortex Pair

An experimental study to identify the structures present in a jet in crossflow has been carried out at a jet-to-crossflow velocity ratio U/Ucf = 3.8 and Reynolds number Re = UcfD/v = 6600. The hot-wire velocity data measured with a rake of eight X-wires at x/D = 5 and 15 and flow visualizations using planar laser-induced fluorescence (PLIF) confirm that the well-established pair of counter-rotating vortices is a feature of the mean field and that the upright, tornado-like or Fric's vortices that are shed to the leeward side of the jet are connected to the jet flow at the core. The counter-rotating vortex pair is strongly modulated by a coherent velocity field that, in fact, is as important as the mean velocity field. Three different structures – folded vortex rings, horseshoe vortices and handle-type structures – contribute to this coherent field. The new handle-like structures identified in the current study link the boundary layer vorticity with the counter-rotating vortex pair through the upright tornado-like vortices. They are responsible for the modulation and meandering of the counter-rotating vortex pair observed both in video recordings of visualizations and in the instantaneous velocity field. These results corroborate that the genesis of the dominant counter-rotating vortex pair strongly depends on the high pressure gradients that develop in the region near the jet exit, both inside and outside the nozzle.

scholarly journals Adjoint shape optimization of a duct for a wall jet film cooling setup

2021 ◽  
Vol 2119 (1) ◽  
pp. 012018
Author(s):  
N N Kozyulin ◽  
M S Bobrov ◽  
M Y Hrebtov
Keyword(s):  
Heat Flux ◽  
Shape Optimization ◽  
Film Cooling ◽  
Cooling System ◽  
Velocity Ratio ◽  
Optimization Method ◽  
Wall Jet ◽  
Mean Velocity ◽  
Jet Cooling ◽  
The Mean

Abstract The paper presents the results of optimization of the geometric parameters of the simplified wall jet cooling system using a modified Adjoint Shape optimization method for algebraic systems of equations (Discrete Adjoint Optimization). The modification consists in using a linearized discrete system of equations with the replacement of derivatives by their finite-volume approximations. The jet flowed through a duct and out from a nozzle. The duct was inclined at an angle of 35 degrees to the cooled wall. The mean velocity ratio between the jet and the main flow was set to 2. The total heat flux on the cooled wall was taken as a cost function. The problem was considered in a two-dimensional stationary turbulent formulation (RANS). As a result of optimization, the shape of the duct changed significantly, affecting the flow inside it. The optimization led to the disappearance of the recirculation zone and reattaching of the jet to the cooled wall. As a result of the optimization performed, the heat flux at the wall increased by 20%.

scholarly journals Effects of different mean velocity ratios on dynamics characteristics of a coaxial jet

2008 ◽  
Vol 12 (2) ◽  
pp. 49-58 ◽  
Author(s):  
Ali Mergheni ◽  
Toufik Boushaki ◽  
Jean-Charles Sautet ◽  
Gille Godard ◽  
Ticha Ben ◽  
...  
Keyword(s):  
Shear Stress ◽  
Velocity Ratio ◽  
Diameter Ratio ◽  
Outer Diameter ◽  
Mean Velocity ◽  
Potential Core ◽  
Zero Crossing ◽  
Core Length ◽  
The Mean ◽  
Outer Potential

The flow field of a coaxial jet configuration having inner and outer diameter ratio Di /Do = 0.33 is studied for four values of the velocity ratios and m = Ui /Uo = 5.17, 1.13, 0.77, and 0.54. The profiles of the mean axial velocity, of the axial turbulence intensities, and of the shear stress are described for the initial and fully zones. The obtained results show the inner potential core length of the coaxial jet strongly depends on the velocity ratio while the outer potential core for jets having velocity ratios greater than unity seems to be insensitive to the velocity ratio. As expected, the inner jet core length is seen to decrease with decreasing velocity ratio; jets with velocity less than unity develop faster than those with m greater than unity and the Reynolds stress show a zero-crossing in the near-region. .

scholarly journals A Comparison of Critical Velocity Estimates to Actual Velocities in Predicting Simulated Rowing Performance

2000 ◽  
Vol 25 (4) ◽  
pp. 223-235 ◽  
Author(s):  
Michael D.J. Kennedy ◽  
Gordon J. Bell
Keyword(s):  
Oxygen Consumption ◽  
Critical Velocity ◽  
Critical Speed ◽  
Critical Power ◽  
Aerobic Power ◽  
Maximal Oxygen Consumption ◽  
Mean Velocity ◽  
Rowing Performance ◽  
The Mean ◽  
The Relationship

The most accurate critical velocity (CV) estimate for the prediction of velocity during a simulated 2,000-m rowing race and the relationship to aerobic power were studied. Sixteen male rowers completed randomized maximal exertion trials (200, 400, 600, 800, 1,000, and 1,200 m), a maximal oxygen consumption [Formula: see text] on a Concept II rowing machine, and an actual 2,000-m simulated rowing race. Three mathematical models were applied to 4 rowing distance combinations producing 12 CV estimates. Seven of the 12 possible CV estimates were not significantly different from actual 2,000-m velocity. Comparison of the 3 CV models using all 6 trial distances revealed that the nonlinear model produced a CV estimate lower than the 2 linear CV models. CV was significantly correlated to [Formula: see text] (r = 0.91) and the mean velocity achieved during the 2,000-m simulated rowing race (r = 0.97). [Formula: see text] was significantly correlated to 2,000-m simulated rowing race velocity (r = 0.93). Key words: maximal oxygen consumption, critical power, critical speed

Pressure Measurements of Coaxial Jet of High Mean-Velocity Ratio

1983 ◽  
Vol 7 (2) ◽  
pp. 51-57 ◽  
Author(s):  
H. Au ◽  
N.W.M. Ko
Keyword(s):  
Experimental Investigation ◽  
Coherent Structures ◽  
Velocity Ratio ◽  
Pressure Fluctuations ◽  
Intermediate Zone ◽  
Pressure Measurements ◽  
Spectral Measurements ◽  
Initial Region ◽  
Mean Velocity ◽  
Single Jet

This paper describes an experimental investigation of the initial region of a subsonic cold coaxial jet at a mean-velocity ratio λ, outer to inner, of 1.25. Detailed measurements in the initial region have shown that similarity of the pressure intensity profiles exists in the three zones: the initial merging zone, the intermediate zone and the fully merged zone. Spectral measurements of the pressure fluctuations confirm the existence of coherent structures in the outer mixing region. Comparison of the coaxial jet results with those of the single jet has been attempted.

Wall region of a relaxing three-dimensional incompressible turbulent boundary layer

1978 ◽  
Vol 85 (1) ◽  
pp. 33-56 ◽  
Author(s):  
K. S. Hebbar ◽  
W. L. Melnik
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Velocity Ratio ◽  
Three Dimensional ◽  
Cross Flow ◽  
Wall Region ◽  
Mean Flow ◽  
Mean Velocity ◽  
Flow Angle ◽  
The Mean

An experimental investigation was conducted at selected locations in the wall region of a three-dimensional turbulent boundary layer relaxing in a nominally zero external pressure gradient behind a transverse hump (in the form of a 30° swept, 5 ft chord, wing-type model) faired into the side wall of a low-speed wind tunnel. The boundary layer (approximately 3·5 in. thick near the first survey station, where the length Reynolds number was 5·5 × 106) had a maximum cross-flow velocity ratio of 0·145 and a maximum cross-flow angle of 21·9° close to the wall. The hot-wire data indicated that the apparent dimensionless velocity profiles in the viscous sublayer are universal and that the wall influence on the hot wire is negligible beyond y+= 5. The existence of wall similarity in the relaxing flow field was confirmed in the form of a log law based on the resultant mean velocity and resultant friction velocity (obtained from the measured skin friction).The smallest collateral region extended from the point nearest to the wall (y+≈ 1) up to y+= 9·7, corresponding to a resultant mean velocity ratio (local to free-stream) of 0·187. The unusual feature of these profiles was the presence of a narrow region of slightly decreasing cross-flow angle (1° or less) that extended from the point of maximum cross-flow angle down to the outer limit of the collateral region. A sublayer analysis of the flow field using the measured local transverse pressure gradient slightly overestimated the decrease in cross-flow angle. It is concluded that, in the absence of these gradients, the skewing of the flow could have been much more pronounced practically down to the wall (limited only by the resolution of the sensor), implying a near-wallnon-collateralflow field consistent with the equations of motion in the neighbourhood of the wall.The streamwise relaxation of the mean flow field based on the decay of the cross-flow angle was much faster in the inner layer than in the outer layer. Over the stream-wise distance covered, the mean flow in the inner layer and the wall shear-stress vector relaxed to a two-dimensional state in approximately 10 boundary-layer thicknesses whereas the relaxation of the turbulence was slower and was not complete over the same distance.

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