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Terminal velocity formula for spheres in a viscous fluid
No full textVarious attempts have been made to develop a general expression for the terminal velocity of spheres in a viscous fluid (Stokes, Prandtl, Oseen, Rubey, etc.: see Bogardi, 1974 and Vanoni, 1975). All of these formulae show a lack of accuracy and/or are restrict ed to a relatively small range of Reynolds numbers. Since sieving is a time consuming method for determining particle size the interest in the use of settling tubes has increased as a mean of particle-size analysis. A measure of the size of an irregular shaped particle is the sedimentation diameter defined as the diameter of the sphere that has the same density and the same terminal settling velocity as the given particle in the same sedimentation fluid. It is therefore very convenient to have a general and accurate expression for the conversion from settling velocity to sedimentation diameter. In this note new formulae for the terminal velocity as well as the sedimentation diameter are derived having a high accuracy and a wide range of validity. A relatively simple analytical expression is derived for the terminal velocity of sphere that depends on the diameter, the density and the kinematic viscosity, due to an acting force. The accuracy of the derived formula is better than 2% for Reynolds numbers up to 2000.Hydraulic EngineeringCivil Engineering and Geoscience
Valkolom: Intern voortgangsrapport
No full textDe valkolom, geconstrueerd door de Centrale Werkplaats, naar aanleiding van een onderzoek door Rietdijk, is bedoeld voor de analyse van zandmonsters. Met de valkolom kan men de valsnelheidsverdeling van zandmonsters bepalen, waaruit, desgewenst, een korrelgrootteverdeling afgeleid kan worden. Een andere manier om de korrelgrootteverdeling te bepalen is bijv. zeven. Dit is echter een tijdrovende analysemethode vergeleken met bezinking. De eisen welke aan het weegsysteem gesteld worden zijn 1) de responsiettijd van het weegsysteem moet verwaarloosbaar zijn t.o.v. de bezinktijd van de snelste zandkorrels. 2) de responsie van het weegsysteem op een stapvormig ingangssignaal mag geen overshoot (c.q. escillaties) vertonen, d.w.z. het systeem moet kritisch (of iets overkritisch) gedempt zijn. 3) bij een bepaalde pijpdiameter behoort een maximaal zandmonstergewicht (bij te grote zandmonsters treedt onderlinge beïnvloeding van de zandkorrels op: wolkvorming. etc.) Voor dit maximale monstergewicht moet de signaal/ruisverhouding minstens 40dB (= 102) zijn voor een redelijke registratie van de bezinkkromme. CONCLUSIES Wat betreft de eisen: 1) Ruimschoots aan de gestelde eis voldoen; haalbaar is een responsietijd van 20 ms. 2) Om dit te bereiken bij een mechanischeveerconstante van 100 N/m moet door proportionele tegenkoppeling de veerconstante een factor 100 vergroot worden. 3) In principe aan de gestelde eis voldaan. Het is zelfs mogelijk om voor een responsietijd van 65 ms een signaal/ruisverhouding van 60dB (= 103) te verkrijgen wat de electronische ruis betreft. Men moet dan geen proportionele tegenkoppeling toepassen om de veerconstante te vergroten, maar alleen tegenkoppeling voor de demping.Hydraulic EngineeringCivil Engineering and Geoscience
An improved settling tube system
No full textIn this report a settling tube system is described for the measurement of the settling velocity distribution of sand samples with particle diameters ranging from 0.1 to 2 mm and sample weights ranging from 0.1 to 20 gf(10^-3 to 0.2 N). The time of arrival of the particles is detected by means of an underwater balance. The response, stability and accuracy of this balance are greatly improved by utilizing the well-known principle of feedback (Slot & Geldof, 1979). The accuracy (systematic part of the error) of the measurenlent of the settling velocity turns out to be better than 2%, i.e. with the exception of the error due to the concentration effect. The error due to the concentration effect is dependent on the sample weight and particle size. It is possible to choose a sample weight (for sand particles in the range from 0.1 to 2 mm) for which the error due to the concentration effect is less than 1% and the signal-noise ratio is better than 72 dB (~4000). This means that the overall accuracy is better than 3%. The precision (random part of the error) of the measurements turns out to be better than 4% (95% confidence). A rapid analysis of the samples is accomplished by connecting the settling to be to a micro computer. Results are presented by means of a hard copy yielding plots of the cumulative distribution and the calculated moments (mean, standard deviation and skewness) of the settling velocity as well as the sedimentation diameter.Hydraulic EngineeringCivil Engineering and Geoscience
Optische Sedimentconcentratiemeter
No full textVoor het bestuderen van sedimenttransport onder invloed van golven en stroming is meetapparatuur noodzakelijk waarmee de sedimentconcentratiemomentaan gemeten kan worden. De te meten concentraties zullen uoorgaans liggen tussen 100 en 10000 ppm. Het nauwkeurig meten van ue lage concentraties zal i.h.a. een probleem vormen i.v.m. de stabiliteit van het meetsysteem. De reeds op de markt verschenen concentratiemeter ISCHS ("Iowa Seuiment Concentration l1easuring System") vertoont nogal wat gebreken, o.a. i.v.m. de stabiliteit. De Iscas werkt volgens het principe van lichtextillctie, waarbij de afgeschermde hoeveelheid licht een maat voor de concentratie is. Hiervoor zijn een lichtbron en een fotodetector nodig, welke bij de ISCHS in het uiteinde van de probe (bij het meetvolume) zijn aangebracht. Vanwege de kleine afmetingen zijn halfgeleiderfotocomponenten gebruikt. Deze halfgeleiderfotocomponenten zijn echter sterk temperatuurgevoelig, zodat watertemperatuur en watersnelheid (volgens de weg van King) een drift in het meetsysteem veroorzaken. Tevens is de gemeten concentratie snelheidsafhankelijk (zie [1], [2], [3] en [4]), hetgeen een gevolg is van de foutief uitgevoerde demodulatieschakeling. De plaatsing van de fotocomponenten in de probe (c.q. in het water) geeft uus de nodige problemen i.v.m. watertemperatuur en watersnelheid. Om deze problemen te voorkomen is er naar een mogelijkheid gezocht om de fotocomponenten buiten het water te plaatsen. In februari 1976 is aan de CED opdracht gegeven om een sedimentconcentratie meter volgens het zelfde principe te ontwikkelen, waarbij echter nu het licht via glasfiber naar het meetvolume geleid wordt. Het voordeel hiervan is dat men nu de mogelijkheid heeft om een andere meetmethode toe te passen. In plaats van één fotodetector worden er nu twee gebruikt, waarbij de tweede fotodetector dienst doet als referentie. Hen heeft dan het voordeel dat bij identieke fotodetectoren de omgevingstemperatuur weinig invloed heeft op het verschilsignaal van de twee fotodetectoren. Het licht van de lichtbron moet nu via een deelspiegel gesplitst worden, zodat beide fotodetectoren evenveel licht ontvangen. De CED heeft helaas twee lichtbronnen gebruikt, waa rdoor het voordeel van de balansmethode weer gedeeltelijk verloren gaat hierdoor voldoet het door de CED geleverde apparaat dan ook niet aan de gestelde eisen. In 1973 is in het Laboratorium voor Vloeistofmechanica aan de bouw van een apparaat begonnen waarin één lichtbron gebruikt wordt, waarvan het licht m.b.v. een deelspiegel gesplitst wordt. De verbetering blijkt echter toch nog te klein te zijn, zodat in april 1980 een onderzoek gestart is naar de oorzaken hiervan. CONCLUSIE De gestelde eis voor de resolutie van het meetsysteem, hoofdzakelijk bepaald door de maximaal toegestane drift van 0,0025%/°C, is haalbaar, waarbij de volgende punten essentieel zijn: 1) speciale temperatuurcompensatiemethode voor de fototransistoren 2) meetmethode volgens de balansmeetmethode 3) goede keuze van de electronische componenten 4) Köhlerse belichtingsmethode voor homogene belichting 5) temperatuurstabilisatie van lichtbron (met Peltier-element).Hydraulic EngineeringCivil Engineering and Geoscience
Settling Tube Analysis of Sand
No full textFor various reasons particle-size analysis of sediment is used in many fields of science and technology, a.o. earth sciences, agricultural and civil engineering. Relatively coarse-grained sediment like sand, with dimensions ranging from 0.06 till 2 mm, is analyzed almost exclusively by sieving. The size of particles in the silt and clay range (0.002 - 0.06 mm resp. smaller than 0.002 mm) commonly is derived from their settling velocity using Stokes' law; in this connection the important studies by Odén (1915, 1925) and Fisher/Odén (924) are recalled. Since the 1930's attempts have been made to remove this difference in analyzing technique by application of fall velocity analysis to sand-size sediment as well, using an empirical relation between size and fall velocity. A second and more important motive for the development of settling velocity analysis of sand is the surmise that the fall velocity of a particle might be a more fundamental property than its size as far as its behaviour in a stream is concerned. Hence, in studies on depositional environment of recent and ancient sediments settling velocity analysis is sometimes given preference to sieving (Reed et al., 1975; Emery, 1978). In this respect it is noted that in an alluvial stream the initiation of motion as well as the concentration distribution of suspended sediment with height are dependent on the ratio between shear velocity and fall velocity. Thirdly, in some cases the amount of material available to sampling is very small, e.g. in flume experiments on selective transport and in studies on sedimentary structures (Emery, 1978; Grace et al., 1978). Unlike sieving settling velocity analysis permits -- and sometimes necessitates the use of small samples. Fourthly, in settling velocity analysis the measurement process is continuous by nature. Therefore, more information is available than in the case of sieving, which is a discontinuous method. In addition, the application of a modern detection system and electronic data processing enables rapid analysis of a large number of samples in a short time. CONCLUSIONS 1. The specifications of the detection method applied in a settling tube are closely related to the characteristics of the sediment, the sedimentation fluid and the settling tube dimensions. 2. An underwater balance as applied in DUST is sufficiently accurate for fall velocity analysis of both fine and coarse sand. 3. Measurements in the test model of DUST have confirmed that the error due to settling convection is depending on particle size. For particles smaller than 0.2 mm the maximum value of the ratio between sample volume and settling tube volume, as estimated by the procedure of Kranenburg/Geldof (1974) is too large. 4. The reproducibility of the median fall velocity of samples of sieved sand, analyzed in the test model, is better than 2%. 5. Given a certain error in the measurement of the time of arrival of the particles at the level of measurement and given the maximum fall velocity in the samples to be analyzed, it can be shown that an optimum value of the delay time of the weighing system exists. The magnitude of the dynamic effect of the settling particles on the weighing system needs further study.Hydraulic EngineeringCivil Engineering and Geoscience
An improved settling tube system for sand
No full textGeneral design aspects of settling tube systems for fall velocity analysis of sand are reviewed, with emphasize being placed on detection methods for particle arrival at the lower end of the settling column and the various sources of errors. This is followed by a detailed discussion on the development of an improved settling tube system employing an underwater balance. In order to achieve a high degree of accuracy, stability and damping, the weighing system includes a feedback mechanism which consists of a solenoid-magnet unit and a differentiator; this yields a resolution of 10 micro newton. The overall accuracy of fall velocity is better than 3% (which takes into consideration concentration effect) and the precision is better than 4% (at a confidence level of 95%). Data acquisition and processing are performed by means of a microcomputer with a l2-bit AID converter, sampling in the velocity domain.Civil Engineering and Geoscience
Design aspects and performance of a settling tube system
No full textThe DUST (Delft University Settling Tube) is a settling tube system intended to analyse particle size (settling velocity) of sand ranging from 0.06 mm to 2 mm, with the sample mass varying from 0.5 g to 20 g. The main parts of the system are (see fig. 1): a. the sample introduction device (venetian blind with rotating lamellae) b. the settling tube c. the measuring device (underwater balance with feedback) All particles of a sample are released by the sample introduction device at the same time, after which they pass the settling tube. Finally they come at the balance, which measures more or less instantaneously the weight of the particles, thus yielding a cumulative settling velocity distribution. Dimensions and characteristics of the instrument and the measurement procedures have to be chosen such that the final error in the distribution function attains a minimum. Several of the relevant aspects are treated in the next sections. CONCLUSIONS a. Using an underwater balance with feedback, it is possible to measure the settling velocity distribution of (sand) particles ranging from 0.1 to 2 mm with an accuracy (systematic part of the error) better than 7%, in which 5% is due to the settling phenomenon itself (concentration effects). The precision (random part of the error) is better than 2%, including the error of sample splitting. The accuracy of the (relative) weight-measurement is better than 0.1% with a precision better than 0.5%. b. The impact of the particles can'be interpreted as a contribution to the delay time of the weighing system. As the impact partly compensates the delay time of the weighing system, the demands upon the delay time can be weakened. Given the possible velocity range, there is an optimum value of the delay time, for which the error will be smaller than 0.25 gy^2/L. c. Further research has to be done on the dependence of the settling phenomenon on the average particle diameter, the scatter in particle diameter, the diameter of the settling tube and the temperature of the sedimentation fluid.Hydraulic EngineeringCivil Engineering and Geoscience
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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