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Theoretical description of PIV measurement errors

Cameron Stuart Murray

Acta Geophysica

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2022

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Vol. 70, no 5

2379--2387

EN

Expressions for particle image velocimetry (PIV) mean error and error variance are derived for iterative deformation method algorithms. The analytical expressions explicitly account for the role of in- and out-of-plane displacements, displacement gradients, particle image diameter, fill factor of the imaging sensor, image noise, light sheet intensity distribution, seeding particle concentration, the interpolation function used to deform PIV images, and the interrogation window size and weighting window. The newly derived analytical expressions show good agreement with errors estimated using synthetic image sets.

2

Time-averaged Turbulent Flow Characteristics over a Highly Spatially Heterogeneous Gravel-Bed

Sarkar S.

Acta Geophysica

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2016

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Vol. 64, no. 5

1797--1824

EN

The present study focuses on the time-averaged turbulence characteristics over a highly spatially-heterogeneous gravel-bed. The timeaveraged streamwise velocity, Reynolds shear and normal stresses, turbulent kinetic energy, higher-order moments of velocity fluctuations, length scales, and the turbulent bursting were measured over a gravel-bed with an array of larger gravels. It was observed that the turbulence characteristics do not vary significantly above the crest level of the array as compared to those below the array. The nondimensional streamwise velocity decreases considerably with a decrease in depth below the array. Below the array, the Reynolds shear stress (RSS) deviates from the gravity-law of RSS distributions. Turbulence intensities reduce below the crest level of the gravel-bed. The third-order moments of velocity fluctuations increase below the crest level of the gravel-bed and give a clear indication of sweeps as the predominating event which were further verified with the quadrant analysis plots. The turbulent length scales values change significantly below the crest level of the gravel-bed.

3

Scour Downstream of Grade Control Structures under the Influence of Upward Seepage

Shafai-Bejestan M., Nabavi S. M. R., Dey S.

Acta Geophysica

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2016

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Vol. 64, no. 3

694--710

EN

The installation of free falling jet grade control structures has become a popular choice for river bed stabilization. However, the formation and development of scour downstream of the structure may lead to failure of the structure itself. The current approaches to scour depth prediction are generally based on studies conducted with the absence of upward seepage. In the present study, the effects of upward seepage on the scour depth were investigated. A total of 78 tests without and with the application of upward seepage were carried out using three different sediment sizes, three different tailwater depths, four different flow discharges, and four different upward seepage flow discharge rates. In some tests, the three-dimensional components of the flow velocity within the scour hole were measured for both the cases with and without upward seepage. The scour depth measured for the no-seepage results compared well with the most accurate relationship found in the literature. It was found that generally the upward seepage reduced the downward velocity components near the bed, which led to a decrease in the maximum scour depth. A maximum scour depth reduction of 49% was found for a minimum tailwater depth, small sediment size, and high flow discharge. A decay of the downward velocity vector within the jet impingement was found due to the upward seepage flow velocity. The well known equation of D’Agostino and Ferro was modified to account for the effect of upward seepage, which satisfactorily predicted the experimental scour depth, with a reasonable average error of 10.7%.

4

Koncepcja stanowiska do badania wypływu zanieczyszczeń z kanałów burzowych

Rząsa M. R., Boguniewicz-Zabłocka J.

Zeszyty Naukowe Wydziału Elektrotechniki i Automatyki Politechniki Gdańskiej

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2015

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Nr 47

155--158

PL

Pomiary przepływu mieszaniny polidyspersyjnej są jednymi z najtrudniejszych do zrealizowania w warunkach technicznych. W artykule opisano koncepcję systemu pomiarowego, który umożliwi obserwację przepływającej mieszaniny oraz wyznaczenia podstawowych parametrów jej ruchu. W pracy zaproponowano sposób opisu wydmy przesuwającego się piasku. Wymiary wydmy zdefiniowano za pomocą czterech charakterystycznych wymiarów. Zaproponowano stanowisko laboratoryjne umożliwiające badanie przepływu mieszaniny w kanale otwartym przy różnym nachyleniu kanału i różnym składzie granulometrycznym frakcji stałej. Stanowisko wyposażono w system rejestracji obrazu. Obrazy zarejestrowane kamerą poddawane są procesowi komputerowej analizy obrazu i na tej podstawie możliwe jest określenie podstawowych parametrów ruchu mieszaniny. Praca zawiera opis koncepcji stanowiska laboratoryjnego oraz ideę metody analizy obrazu.

EN

A storm water system with open channels for the discharge of rainwater exists in most urbanised areas. The flow in these kind of channels is generally multiphase flow, in which there are solids and liquids and in some places there may be an additional gas phase. Therefore the ability to describe such flow is reduces to the description of the motion of the polydisperse mixture. Polydisperse mixture flow measurements are among the most difficult to achieve in technical terms. The article describes the concept of a measurement system that allows observation of the poly-dispersed flow and determine the basic parameters of its motion. Concept of experimental laboratory set-up was proposed to measure the parameters of the polydispersed mixture flow process in open channel with different slope of the channel and various granulometry of the solid fraction. In this work description of sand dunes with four typical parameters was proposed. A digital camera is used to interrogate the flow. Images recorded on the camera will be subjected to a process of image analysis, and from this the basic parameters of the movement are determined. The work includes a description of the experimental set-up with the idea of image analysis methods.