Velocity profiles assessment in natural channels during high floods

2011 ◽  
Vol 42 (2-3) ◽  
pp. 162-170 ◽  
Author(s):  
Tommaso Moramarco ◽  
Carla Saltalippi ◽  
Vijay P. Singh
Keyword(s):  
Flow Velocity ◽  
Maximum Flow ◽  
Velocity Profiles ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Flow Area ◽  
The Mean ◽  
Logarithmic Law ◽  
Maximum Flow Velocity

The accuracy of three different approaches for velocity profiles assessment during high floods, when the velocity points sampling is carried out only in the upper portion of the flow area, has been investigated. The first two methods assume the classical logarithmic law with additional terms, to take account of the dip-phenomenon in the velocity profile. The third one is based on the entropy theory and uses the maximum flow velocity occurring in the flow area. A sample of velocity measurements carried out at Pontelagoscuro gauged section (Po River, Italy), has been considered for the analysis. Six flood events have been selected and the accuracy of the investigated methods has been evaluated in terms of mean error in estimating both the mean velocity along each sampled vertical and the mean flow velocity. For high floods, the logarithmic law and the entropic approach were found quite accurate; however, the ability of the latter in reproducing the velocity profiles only by sampling the maximum flow velocity has been shown. Therefore, a procedure for velocity measurements based on the entropic approach has been proposed. The procedure allows one to both to shorten remarkably the time of the velocity sampling and to quickly estimate the discharge.

scholarly journals Evolution and structure of sink-flow turbulent boundary layers

2001 ◽  
Vol 428 ◽  
pp. 1-27 ◽  
Author(s):  
M. B. JONES ◽  
IVAN MARUSIC ◽  
A. E. PERRY
Keyword(s):  
Boundary Layers ◽  
Velocity Profiles ◽  
Turbulent Boundary Layers ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Mean Velocity ◽  
Flow Measurements ◽  
Law Of The Wall ◽  
The Mean ◽  
Logarithmic Law

An experimental and theoretical investigation of turbulent boundary layers developing in a sink-flow pressure gradient was undertaken. Three flow cases were studied, corresponding to different acceleration strengths. Mean-flow measurements were taken for all three cases, while Reynolds stresses and spectra measurements were made for two of the flow cases. In this study attention was focused on the evolution of the layers to an equilibrium turbulent state. All the layers were found to attain a state very close to precise equilibrium. This gave equilibrium sink flow data at higher Reynolds numbers than in previous experiments. The mean velocity profiles were found to collapse onto the conventional logarithmic law of the wall. However, for profiles measured with the Pitot tube, a slight ‘kick-up’ from the logarithmic law was observed near the buffer region, whereas the mean velocity profiles measured with a normal hot wire did not exhibit this deviation from the logarithmic law. As the layers approached equilibrium, the mean velocity profiles were found to approach the pure wall profile and for the highest level of acceleration Π was very close to zero, where Π is the Coles wake factor. This supports the proposition of Coles (1957), that the equilibrium sink flow corresponds to pure wall flow. Particular interest was also given to the evolutionary stages of the boundary layers, in order to test and further develop the closure hypothesis of Perry, Marusic & Li (1994). Improved quantitative agreement with the experimental results was found after slight modification of their original closure equation.

scholarly journals Application of entropic approach to estimate the mean flow velocity and Manning roughness coefficient in a high-curvature flume

2016 ◽  
Vol 48 (3) ◽  
pp. 634-645 ◽  
Author(s):  
D. Termini ◽  
T. Moramarco
Keyword(s):  
Flow Velocity ◽  
Maximum Flow ◽  
Roughness Coefficient ◽  
Mean Flow ◽  
Mean Velocity ◽  
Bed Deformation ◽  
Side Walls ◽  
The Mean ◽  
Manning Roughness ◽  
Mean Flow Velocity

The entropy-based approach allows the estimation of the mean flow velocity in open channel flow by using the maximum flow velocity. The linear relationship between the mean velocity, umax, and the mean flow velocity, um, through the dimensionless parameter Φ(M), has been verified both in natural rivers and in laboratory channels. Recently, the authors of this study investigated the reliability of the entropy-based formula in a straight channel and under different bed and side-walls' roughness conditions. The present study aims to further validate the entropy-based approach and to explore the effectiveness of entropy-based formula in high curvature channels. Results show that as the effect of the downstream variation of the channel's curvature the value of the parameter Φ(M) varies along the bend. When the bed deformation is evident, the variation of the parameter Φ(M) is strongly reduced compared to that obtained in absence of bed deformation. Results also show that the Manning's roughness coefficients determined through entropy-based formula are in agreement with those estimated by applying other literature's expressions but, unlike the latter, through the parameter Φ(M) the entropy-based formula could account for the effects due to the advective momentum transport by cross-circulation along the strongly curved reaches of the channel.

Effetti dell'ipocapnia e dell'ipercapnia sul flusso liquorale acqueduttale e sul flusso ematico nel seno sagittale superiore. Studio quantitativo con Cine-RM contrasto di fase in soggetti normali

1997 ◽  
Vol 10 (2_suppl) ◽  
pp. 43-45
Author(s):  
M. Santoni ◽  
M. Mascalchi ◽  
M. Cellerini ◽  
A. Cuttano ◽  
G. Dal Pozzo
Keyword(s):  
Flow Velocity ◽  
Cardiac Cycle ◽  
Mean Flow ◽  
Csf Flow ◽  
Flow Area ◽  
Sagittal Sinus ◽  
Cine Mr ◽  
Plastic Bag ◽  
The Mean ◽  
Hydrodynamic Equilibrium

The aqueductal CSF and superior sagittal sinus (SSS) blood flow were quantified in 9 healthy volunteers using a Phase Contrast Cine MR technique with retrospective cardiac gating and velocity of encoding of 10 and 30 cm/s. All subjects were examined in basal conditions, during hypocapnia determined by sustained hyperventilation and during hypercapnia (>5%) obtained with rebreathing in a plastic bag. In basal conditions, the aqueductal CSF flow area showed a mean increase of 24% synchronous with the cerebral systole, whereas variations of the SSS flow area during the cardiac cycle were negligible (>9%). The mean flow velocity and volume in the SSS were 11.7 cm/s and 236 ml/min. Mean peak systolic and diastolic aqueductal CSF velocity were + 2 cm/s and −2 cm/s. Hypocapnia determined in all but one subjects a decrease of the SSS blood and aqueductal CSF velocities. During hypocapnia a decrease of the aqueductal CSF flow area was also observed. Hypercapnia was associated with a decrease of the SSS flow velocity in 3 subjects and to minor changes in the remaining 6 subjects. Hypercapnia determined an increase (up to 30%) in the peak systolic and diastolic aqueductal CSF flow velocity in 3 subjects, negligible changes in 5 subjects and a decrease of the same parameters in one subject. The aqueductal CSF flow area was decreased in 8 subjects during hypercapnia. Hypocapnia and hypercapnia are relatively simple tests which can be used to modify the intracranial hydrodynamic equilibrium in healthy subjects. Further investigations are however needed before their application to the study of intracranial hydrodynamics in patients with hydrocephalus.

Experimental Study on Mean Velocity Profiles Under Wave-Current Interactions

2008 ◽  
Author(s):  
Jinhai Zheng ◽  
Tongfei Li ◽  
Yixin Yan ◽  
Jinchun Hu
Keyword(s):  
River Estuary ◽  
Maximum Velocity ◽  
Velocity Profiles ◽  
Yangtze River Estuary ◽  
Current Profile ◽  
Linear Superposition ◽  
Mean Velocity ◽  
The Mean ◽  
Logarithmic Law ◽  
A Current

A series of experiments are carried out in a laboratory flume for combined wave-current flows, to investigate the characteristics of vertical structure of current profile in a wave-current co-existing water areas. Changes induced in the mean velocity profiles are considered for a range of wave heights, wave periods, water depths and flow velocities which are based on those typical data in the Yangtze River Estuary, China. Preliminary tests are conducted on the unidirectional current and on the wave alone. These show that the current mean velocity profiles agree well with the logarithmic law, and that the waves are approximated closely by the Stokes’ second-order theory. For the combined wave and current tests, the mean velocity profiles generally differ from those suggested by a linear superposition of wave and current velocities. In the case of waves following a current, the velocity distributions exhibit a relatively greater velocity near the bed and a smaller velocity above a certain depth relative to the logarithmic law, and the maximum velocity is observed at a lower location. In the case of wave opposing a current, the velocity distributions depart from the logarithmic law with a relatively smaller velocity near the bed and a greater velocity above a certain depth. Experimental data can be used to validate the theoretical or mathematical models associated with the combined wave-current motions.

scholarly journals Experimental Study of Mean Flow Characteristics in the Near Field of Wedge-Shaped Swirling Jets

2020 ◽  
Vol 5 (10) ◽  
pp. 1199-1203
Author(s):  
Md. Mosharrof Hossain ◽  
Muhammed Hasnain Kabir Nayeem ◽  
Dr. Md Abu Taher Ali
Keyword(s):  
Flow Field ◽  
Near Field ◽  
Flow Characteristics ◽  
Velocity Profiles ◽  
Mean Flow ◽  
Mean Velocity ◽  
Potential Core ◽  
Swirling Jets ◽  
The Mean ◽  
Sinusoidal Flow

In this investigation experiment was carried out in 80 mm diameter swirling pipe jet, where swirl was generated by attaching wedge-shaped helixes in the pipe. All measurements were taken at Re 5.3e4. In the plain pipe jet the potential core was found to exist up to x/D=5 but in the swirling jet there was no existence of potential core. The mean velocity profiles were found to be influenced by the presence of wedge-shaped helixes in the pipe. The velocity profiles indicated the presence of sinusoidal flow field in the radial direction existed only in the near field of the jet. This flow field died out after x/D=3 and the existence of jet flow diminished after x/D=5.

PIV-POD Investigation of the Wake of a Sharp-Edged Flat Bluff Body Immersed in a Shallow Channel Flow

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Arindam Singha ◽  
A.-M. Shinneeb ◽  
Ram Balachandar
Keyword(s):  
Velocity Field ◽  
Channel Flow ◽  
Bluff Body ◽  
Maximum Flow ◽  
Velocity Fields ◽  
Central Plane ◽  
Mean Flow ◽  
Mean Velocity ◽  
Near Surface ◽  
The Mean

This paper reports particle-image velocimetry measurements of instantaneous velocity fields in the wake of a sharp-edged bluff body immersed vertically in a shallow smooth open channel flow. The maximum flow velocity was 0.19 m/s and the Reynolds number based on the water depth was 18,270. The purpose of the present study is to show the vertical variation of the velocity field in the near region of a shallow wake. Measurements of the flow field in the vertical central plane and in the horizontal near-bed, mid-depth, and near-surface planes were taken. Then, the mean flow quantities such as the mean velocity, turbulence intensity, and Reynolds stress fields were investigated. In addition, the proper orthogonal decomposition technique was used to reconstruct the velocity fields to investigate the energetic vortical structures. The results showed that the largest recirculation zone in the mean velocity fields occurred in the mid-depth velocity field, while the smallest one occurred near the bed. Also, the fluid was entrained from the sides toward the wake central plane in the three horizontal velocity fields but with different rates. This behavior was attributed to the existence of quasi-streamwise vortices near the boundaries. In addition, patterns of ejection and sweep events near the free surface similar to the features commonly observed near the wall-bounded flows were observed.

Experimental investigation of turbulent shear flow with quadratic mean-velocity profiles

1976 ◽  
Vol 73 (1) ◽  
pp. 165-188 ◽  
Author(s):  
H. K. Richards ◽  
J. B. Morton
Keyword(s):  
Flow Direction ◽  
Correlation Coefficients ◽  
Velocity Profiles ◽  
Turbulent Shear ◽  
Turbulent Shear Flow ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Mean Velocity ◽  
Quadratic Mean ◽  
The Mean

Three turbulent shear flows with quadratic mean-velocity profiles are generated by using an appropriately designed honeycomb and parallel-rod grids with adjustable rod spacing. The details of two of the flow fields, with quadratic mean-velocity profiles with constant positive mean-shear gradients ($\partial^2\overline{U}_1/\partial X^2_2 >0$), are obtained, and include, in the mean flow direction, the development and distribution of mean velocities, fluctuating velocities, Reynolds stresses, microscales, integral scales, energy spectra, shear correlation coefficients and two-point spatial velocity correlation coefficients. A third flow field is generated with a quadratic mean velocity profile with constant negative mean-shear gradient ($\partial^2\overline{U}_1/\partial X^2_2 < 0$), to investigate in the mean flow direction the effect of the change in sign on the resulting field. An open-return wind tunnel with a 2 × 2 × 20 ft test-section is used.

scholarly journals Effects of Mean Flow Profiles on the Instability of a Low-Density Gas Jet Injected Into a High-Density Gas

Volume 1 ◽  
2004 ◽  
Author(s):  
Nanda Kishore Vedantam ◽  
Ramkumar N. Parthasarathy
Keyword(s):  
Shear Layer ◽  
Froude Number ◽  
Density Ratio ◽  
Critical Density ◽  
Velocity Profiles ◽  
High Density ◽  
Low Density ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

The effects of the mean velocity profiles on the instability characteristics in the near-injector region of axisymmetric low-density gas jets injected vertically upwards into a high-density gas medium were investigated using linear inviscid stability analysis. The flow was assumed to be isothermal and locally parallel. Three velocity profiles, signifying different changes in the mean velocity in the shear layer, were used in the analysis. The effects of the inhomogeneous shear layer and the Froude number (signifying the effects of gravity) on the instability for each set of mean profiles were delineated. At a large Froude number (negligible gravity), a critical density ratio was found for the three profiles at which the jet became absolutely unstable. The critical density ratio for each velocity profile was increased as the Froude number was reduced. A critical Froude number was found for the three sets of profiles, below which the jet was absolutely unstable for all the density ratios less than unity, which demarcated the jet flow into the momentum-driven regime and the buoyancy-driven regime.

Size and effect on the mean flow of large-scale horizontal coherent structures in open-channel flows: an experimental studyThis paper is one of a selection of papers in this Special Issue in honour of Professor M. Selim Yalin (1925–2007).

2009 ◽  
Vol 36 (10) ◽  
pp. 1643-1655 ◽  
Author(s):  
Ana Maria Ferreira da Silva ◽  
Habib Ahmari
Keyword(s):  
Flow Velocity ◽  
Coherent Structures ◽  
Large Scale ◽  
Open Channel ◽  
Channel Flows ◽  
Flow Depth ◽  
Mean Flow ◽  
Velocity Measurements ◽  
Open Channel Flows ◽  
The Mean

The size of the largest horizontal coherent structures (HCSs) of turbulence in open-channel flows is investigated experimentally on the basis of three series of flow velocity measurements. These are further used to explore the dynamics and morphological consequences of HCSs. The flow velocity measurements were carried out in a 21 m long and 1 m wide channel, with a bed formed by sand with average grain size of 2 mm. The bed surface was flat. The turbulent and subcritical flow under investigation was uniform, with a flow depth of 4 cm. The bed slope of 0.0015 was such that, for the present flow depth, the bed shear stress acting on the bed was substantially below the threshold for initiation of motion, thus ensuring that the bed remained flat throughout the measurements. To the knowledge of the writers, this work is a first attempt to systematically investigate HCSs in open-channel flows. It should be viewed as an extension to the case of horizontal structures of work previously carried out by a number of authors on large-scale organized turbulence motion in open-channel flows, so far focusing exclusively on vertical coherent structures (VCSs). The horizontal burst length was found to be between five and seven times the flow width. A slight internal meandering of the flow caused by the superimposition of burst sequences on the mean flow was detectable. Both of these findings lend support to the longstanding belief expressed by many prominent researchers that the formation of large-scale river forms is directly related to the large-scale turbulence. In particular, the present measurements for the first time provide some direct evidence in support of hypotheses previously raised by Yalin and da Silva regarding the formation of alternate bars and meanders through the action of HCSs on the mean flow and the mobile bed and banks.

Experimental Study on Hydraulic Roughness of Submerged Grass in Ecological Riverbank

2013 ◽  
Vol 838-841 ◽  
pp. 1743-1748
Author(s):  
Dian Guang Ma ◽  
Chun Xin Zhong ◽  
Wu Ning ◽  
Qing Ye ◽  
Sheng Zhu
Keyword(s):  
Flow Velocity ◽  
Roughness Coefficient ◽  
Theoretic Analysis ◽  
Mean Flow ◽  
Normal Range ◽  
Obvious Effect ◽  
Hydraulic Roughness ◽  
Natural Turf ◽  
Scouring Process ◽  
The Mean

A model experiment about the hydraulic roughness of natural turf used in riverbank was carried out in flume. To examine the rationality of experimental design, the hydraulic roughness coefficient of plexiglass-flume was tested firstly. The result was 0.0085, which is quite normal. Then the tested hydraulic roughness caused by vegetation ranges from 0.020 to 0.090 for the chosen plants, which is also acceptable. Furthermore, the tested incipient velocities of krasnozem, and paddysoil had the range of 0.55~0.65m·s-1 and 1.0~1.1m·s-1, respectively. All these experimental results are in normal range, which means that the design of this experimental is rational. Experimental research illustrate that, the roughness coefficient of plant reduces with the increasing of flow velocity. When the mean flow velocity is over 3m·s-1, Mannings n values vary between 0.025 and 0.035. This phenomenon is accord with the theoretic analysis. During the scouring process, not only the flow velocity, but also the flow duration has an obvious effect on the coarseness of vegetative bed.

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