Effects of sample size and concentration of seeding in LDA measurements on the velocity bias in open channel flow

Han, Yu; Yang, Shu-Qing; Dharmasiri, Nadeesha; Sivakumar, Muttucumaru

doi:10.1016/j.flowmeasinst.2014.05.008

Cited by 9 publications

(7 citation statements)

References 15 publications

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“…The characteristics of the LDV system used are summarised in Table 2. A more complete description of the LDV system used is found in [22] and [23].…”

Section: Determination Of Shear Velocity (U * )mentioning

confidence: 99%

“…In order to obtain mean velocity and other statistical data accurately it is important to choose the appropriate sample size [23,43,67]. Hence, throughout our current experiments the size of the sample chosen was 2000 which gives the likely relative error as only around 0.09% (< 2%) which can be considered to be suicient for the two component LDV in the current coniguration Han et al [23] and Peltier et al [43]. The sampling time used was 20 s and the acquisition frequency was 100 Hz.…”

Section: Determination Of Shear Velocity (U * )mentioning

confidence: 99%

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Re-examining log law velocity profile in smooth open channel flows

et al. 2020

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2020, Springer Nature B.V. A comprehensive investigation of velocity distribution is presented, and the log law is reexamined using experimental data from a smooth uniform open channel flow. It is widely reported that the coefficients of the log law in channel flows deviate from those obtained from circular pipe flows by Nikuradse (Laws of flow in rough pipes, 1933), but the mechanism is not clear and no theoretical formulae are available to express these deviations. A Laser Doppler Velocimetry system was used to measure velocity profiles at the centre of the channel. The data obtained support previous conclusions that the additive constant B of the log law in channel flows can no longer be considered to be 5.5. Interestingly, the experimental data also support Tracy and Lester's discovery that the shear velocities on both sides of the log law are different. Better agreement can be achieved if the global shear velocity (U*1) is used to normalize the measured velocity, and the local shear velocity (U*2) is used to normalize the distance from the wall. Other researchers' data in the literature also validates this new relationship. Based on this new relationship, a theoretical value of B is obtained, which agrees well with the observed B, thus a new form of log law for channel flow is suggested. The new relationship developed was verified with present experimental and past literature data suggesting its universality irrespective of wide or narrow open channels, or subcritical or super critical flow conditions. By using the developed relationship the large scatter associated with the additive constant B associated with the log law has been explained. It is found that the additive constant B in the log law is a function of channel aspect ratio. The developed relationship for B is validated from a wide range of data from the literature to confirm its universality.

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“…The characteristics of the LDV system used are summarised in Table 2. A more complete description of the LDV system used is found in [22] and [23].…”

Section: Determination Of Shear Velocity (U * )mentioning

confidence: 99%

Section: Determination Of Shear Velocity (U * )mentioning

confidence: 99%

Re-examining log law velocity profile in smooth open channel flows

et al. 2020

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“…Over the past century, the knowledge of velocity vertical distribution in an open channel cross-section received great attention by several research teams due to its importance in the understanding of numerous hydraulic phenomena such as flood control, pollutant dispersion, and sediment transport, and its various applications in the design and planning of hydropower plants and structures or infrastructures interacting with fluid flows [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. The sophisticated numerical modeling and advanced experimental techniques allowed for obtaining a good reconstruction of velocity profiles in fully-developed turbulent wide open channel flows, though they have yet not been able to well describe the dip of the maximum velocity below the free surface in narrow open channels due to the presence of secondary currents [ 9 , 10 , 11 , 12 , 13 ].…”

Section: Introductionmentioning

confidence: 99%

Entropy Wake Law for Streamwise Velocity Profiles in Smooth Rectangular Open Channels

Mirauda

Russo

2020

Entropy

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In narrow open channels, the three-dimensional nature of the flow and the transport momentum from the sidewalls to the central region cause the maximum longitudinal velocity to occur below the water surface. The entropy model is unable to accurately describe the velocities near the free surface when the dip phenomenon exists. The present paper proposes a new dip-modified entropy law for steady open channel flows, which consists of three additional terms: the first one similar to Coles’ function; the second one linearly proportional to the logarithmic distance from the free surface; and the third one depending on the cubic correction near the maximum velocity. The validity of the new model was tested on a set of laboratory measurements carried out in a straight rectangular flume with smooth boundaries and for different values of water discharge, bottom slope, and aspect ratio. A detailed error analysis showed good agreement with the data measured through the present research and a more accurate prediction of the velocity-dip-position compared with the one evaluated through the original entropy model. In addition, the modified entropy wake law matched very well with other literature data collected in rectangular cross-sections with different flow conditions.

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“…In addition, a number of models have been proposed to express the division lines within a divisible flow, for example, Daido's method (DM; [12]), Guo and Julien's method (GJM; [13]), 2 Mathematical Problems in Engineering and Yang and Lim's method (YLM; [14]). These models were subjected to detailed reviews by Yang et al [15] and Han et al [16,17], and these authors concluded that division lines could be located based on the existence of measured Reynolds shear stress values of zero below the free surface in rectangular channel flow. However, the Reynolds shear stress can only be measured using very sophisticated equipment, like laser Doppler anemometry (LDA), in controlled laboratory environments.…”

Section: Introductionmentioning

confidence: 99%

Flow Partitioning in Rectangular Open Channel Flow

Han

Yang

Sivakumar

et al. 2018

Mathematical Problems in Engineering

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Hydraulic engineers often divide a flow region into subregions to simplify calculations. However, the implementation of flow divisibility remains an open issue and has not yet been implemented as a fully developed mathematical tool for modeling complex channel flows independently of experimental verification. This paper addresses whether a three-dimensional flow is physically divisible, meaning that division lines with zero Reynolds shear stress exist. An intensive laboratory investigation was conducted to carefully measure the time-averaged velocity in a rectangular open channel flow using a laser Doppler anemometry system. Two innovative methods are employed to determine the locations of division lines based on the measured velocity profile. The results clearly reveal that lines with zero total shear stress are discernible, indicating that the flow is physically divisible. Moreover, the experimental data were employed to test previously proposed methods of calculating division lines, and the results show that Yang and Lim’s method is the most reasonable predictor.

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Effects of sample size and concentration of seeding in LDA measurements on the velocity bias in open channel flow

Cited by 9 publications

References 15 publications

Re-examining log law velocity profile in smooth open channel flows

Re-examining log law velocity profile in smooth open channel flows

Entropy Wake Law for Streamwise Velocity Profiles in Smooth Rectangular Open Channels

Flow Partitioning in Rectangular Open Channel Flow

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