Measurements of the fluid velocity and the occurrence and velocity of individual sediment grains are possible with the instrumentation developed in this study. The data demonstrate the shortcomings of the continuum approach to the mechanics of sediment suspension. This work is the foundation of an ongoing experimental program.1 of direct measurements of the fine-scale, time-varying characteristics of sediment-laden flows.
CHAPTER 1 INTRODUCTION
The chapter provides an overview of theoretical and experimental research on the basic mechanics of sediment transport. A discussion of the importance of fluid turbulence and an approach to the mechanics of sediment transport from the kinetics of individual sediment grains is given in Chapter 3. The use of laser Doppler velocimetry in sediment-laden streams is described in detail in Chapter 4.
CHAPTER 2 LITERATURE REVIEW
Furthermore, the experiments showed that suspended sediment in alluvial concentrations affects the turbulent characteristics of the flow. This analysis includes a decrease in mean sediment size with increase in suspension due to Peaks in the velocity autocorrelation due to collision-induced vibrations of the probe were also reported.
CHAPTER 3
The sediment transport rate can be considered independent of the fluid discharge or suspended sediment. 3.2.2) Note that the concentration is not only a function of the sediment transport, but also of the fluid volume flux. The term v'c' is the correlation between the vertical fluid velocity fluctuations and the sediment concentration fluctuations.
CHAPTER 4
The output current of the photodetector is proportional to the square of the intensity of the incident light. The larger cross-sectional angle of the beams also reduces the probability of detecting light scattered on sediment grains that pass through the beams just outside the measurement volume. The major diameter of the volume of the intersection of the rays is therefore about five times larger than the minor diameter.
CHAPTER 5
The probability density functions of the fluid velocity and the sediment grain velocity are determined. The probability density functions of the sediment grain inter-arrival time data are calculated for each data record. The standard deviation of the sediment grain inter-arrival times will be equal to the mean.
CHAPTER 6
PIVOT SUPPORT @ SURFACE DAMPING BOARD IOln.. VENTURI METERS FOR WHEEL SUSPENSION WHEEL WINDOW ® ADJUSTABLE JACK SUPPORT @) EXHAUST SECTION SECTION A-A Figure 6 .1.1 13-meter chute schematic drawing. Two precision stainless steel round bars mounted on top of the gutter sidewall beams serve as rails for a metal tool cart. The gear of the instrument mount on the carriage allows positioning of the mounted instrument to within 0.2 mm -in the vertical or transverse directions.
At the flue inlet, a series of scrapers were used to damp the large-scale turbulence and secondary currents generated in the return pipe and in the inlet and outlet sections. Two rectangular grids made of glued lucite strips (strip width=1.27 cm; opening=1.27 cm) were placed horizontally in the vertical part of the inlet section. The material is one of the size fractions obtained by Taylor (1971) in a falling velocity partition of a natural alluvial sand.
The maximum error in the discharge measurement was assessed to be approximately three percent. Due to the Froude number of the flow used in this study, Fr=0.75, there were often small standing waves on the surface. Samples were extracted at the local flow velocity (isokinetic) using a siphon arrangement as described by Brooks (1954).
A second copper sampling tube, shown in Figure 6.3.2, is suspended vertically in the throat of the exhaust section.
PLASTIC TUBING
TUBE SUPPORT COLLAR
SAND BED
IMPELLER
TO SAMPLER COLLECTION
SEDIMENT DISCHARGE
SAMPLER
CHAPTER 7
A general description of the laboratory flow in which the velocity results were obtained is given in this chapter. The mean of the samples is indicated by the plot symbol; sample range by solid line. The observed variation is of the order of that spiral only reported in similar laboratory measurements.
A total of twenty-four velocity data records were acquired at six different vertical locations in the water column. Twelve data records were retained for later analysis; seven data records were discarded due to the questionable accuracy of the velocity measurement data and five data records were discarded due to potential ambiguities in the identification of fluid and sediment grain measurements. Several short exploratory test data records were created with different combinations of the bandpass filter setting, the Doppler burst threshold level, and the preset number of zero crossings.
It is necessary to obtain and screen sample data records before a valid combination of the above instrumentation settings can be guaranteed. Some duplicate data records, records with identical instrumentation settings, were taken to check the temporal variability of the data acquisition. During subsequent analysis, a data record was discarded if it was found to contain an abnormally high percentage of invalid data events or if the probability density function of the Doppler burst signal frequency was significantly skewed with respect to the frequency passband of the filter.
The variability observed in the residual data is judged to be due to fluctuations in flow, not velocity data acquisition.
CHAPTER 8
VELCJC[TI
Figures 8.1.7 and 8.1.8 show the effects of the applied filter on the fluid velocity measurements in the previous figures. The velocity of some sediment grains appears to be very different from velocity measurements of the nearby filtered fluid. Profiles of mean velocity of fluid u and sediment grains u, g are shown in Figure 8.2.
The probability density function of the fluid velocity is given by the solid line; that of the sediment grains from the dashed line. At the remaining sites, the probability density functions of fluid velocity and sediment grain velocity are quite similar. The results of spectral calculations of velocity fluctuations of sediment grains are even less instructive.
The average sampling frequency of the sediment grain velocity in the data records shown is approximately 4 Hz. The expected error in the sediment grain velocity spectral calculations is larger than that of the fluid velocity spectral estimates due to the relatively smaller number of sediment grain velocity measurements. The effects of the bias correction procedures on the calculated mean and standard deviation of the fluid velocity are shown in Figure.
The results for one of the data records from each location are shown in Figure 8.2.12.
INTER-ARRIVAL TIME VS TIME
INTER-RRRI VR L TIME VS TIM E
GRAIN SIZE 9
I NT ER-R RRIVRL TIME VS TIME
CHAPTER 9
The implications of the obtained data are then examined for new insights into the mechanics of sediment suspension and entrainment. The main difficulty in this study is the possible conditional sampling of the flow velocity and is directly attributable to the laser Doppler technique. The primary source of electronic noise in devices used in this study is the statistical quantum noise of the photodetector.
In a sediment-laden fluid flow, multiparticle light scattering is particularly common near the beam intersection volume. In this study, more than half of the recorded data events were not valid events. The development of the laser-Doppler technique for use in sediment-laden streams is still in the experimental phase.
The sediment grain velocity variance is noticeably smaller than the fluid velocity variance. Some recorded grain velocity measurement events are caused by multiple grains passing through the volume simultaneously. Removal of the mobile sediment layer would allow experimental control over local sediment transport rates.
Velocity performance with different local sediment transport rates can be evaluated.
CHAPTER 10 SUMMARY
First, the amount of data needed to adequately describe all the processes present in the stream is quite large. Flow timescales range from small, relatively fast fluctuations in fluid turbulence to long-scale, slowly changing changes in sediment bedforms. These problems are due to the complex nature of sediment-laden flows and are independent of the instruments used to observe flow variables.
Despite the various difficulties, the data obtained in this study provide insight into the mechanics of sediment suspension and entrainment. The data also demonstrate the shortcomings of the continuum approach to sediment suspension mechanics. Direct observations of the turbulent structure of the fluid and the movements of the suspended sediment grains will contribute much to the knowledge of the small-scale, time-fluctuating characteristics of sediment-laden flows.
California Institute of Technology, Pasadena, California, in partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1954. Durst, F., Studies of particle motion by laser-Doppler technology, Proceedings of the Dynamic Flow Conference, I.M.S.T., Marseille ,. Chein, Second approximation to the solution of suspended load theory, Series 47, Issue 2, Engineering Research Institute, University of California, Berkeley, California, January 31, 1952.
Silverman, Alias-free sampling of random noise, Journai of the Society for Industrial and Applied Mathematics, vol.
SECTION I INTRODUCTION
SECTION II PROCESSOR SYSTEM DESCRIPTION II.1 Counter System Overview
Disclaimer: "Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author and do not necessarily reflect the views of the National Science Foundation." The response of a photodetector is slow with respect to the light frequency but not slow with respect to the light difference frequency. The compensation current is caused by the Gaussian light intensity of the laser light beams.
The ratio of the pedestal amplitude to the Doppler burst amplitude is a function of the optical geometry and scattering particle size. In sediment-laden flows, the sorting of signals generated by sand grains and fluid trace particles can be done by measuring the pedestal amplitude. By minimizing the small diameter and making it close to the .. sand diameter, the measurement volume becomes small enough to .. allow fluid velocity measurement because sand particles are sometimes absent.
The large beam angle also reduces the probability of a . grains of sand near the collected scattered light. However, if the angle becomes too large, the scattering efficiency of the .. smaller tracer particles decreases, making them quite difficult to detect. Note that the ratio of pedestal amplitude .. to Doppler burst amplitude can be as high as 100:1 as a result of transmitting optics and large sand grain size.
To accurately determine fluid and sand grain velocities, the signal processor must meet several criteria.
SECTION II
I PARTICLE SIZE I
