Numer: Autor: Fraza:

Vol. 59, No. 3-4 (2012)

Structure of turbulent vortices in a compound channel

Alfonsas Rimkus

Water Research Institute of Lithuanian Aleksandras Stulginskis University, LT-58102, Vilainiai, Kedainiai
Lithuania, e-mail: alfonsas.rimkus@hidro.lzuu.lt

Abstract

In spite of many investigations performed on turbulent flows, their structure has not yet been sufficiently explored. The difficulty is that, when a detailed picture of the velocity field is necessary, the widely employed Particle Image Velocity (PIV) method can provide photos covering only a short interval of flow, which cannot include the largest structures of turbulent flow, and consequently these structures cannot be investigated. In this study, the author tried to obtain necessary data about the processes occurring in the flow by analyzing instantaneous velocity measurements carried out by 3D means. A measurement at the points of a flow cross-section takes at least 1 minute. During this time all vortex structures, including the largest, occur repeatedly many times and can be studied. The analysis of such measurements was the aim of this article. The process of the generation of vortices at the bottom and their further development, including the conditions of the development of the largest vortices, has been investigated. The results of these investigations are discussed in this article.

Key words: hydraulic investigations, turbulent vortices, measurement analysis, turbulent structure.

Full text link: http://www.ibwpan.gda.pl/docs/ahem/ahem59str113.pdf


On flow-altering countermeasures for scour at vertical-wall abutment

Alessio Radice1, Oskars Lauva2

1Assistant Professor, Dept. I.I.A.R., Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano,
Italy, Corresponding author, e-mail: alessio.radice@polimi.it

2M.Sc. student in Heat, Gas and Water Technology, Faculty of Civil Engineering, Riga Technical
University, Azenes street 16/20, Riga, LV-1048, Latvia

Abstract

Results are presented for clear-water scour experiments at a vertical-wall abutment where the obstacle was modified with slots or roughening elements as flow-altering countermeasures against the erosion process. The laboratory campaign comprehended an initial experiment with an unprotected obstacle, two experiments with slots above and beneath the non-scoured bed level, respectively, and one experiment with a roughened abutment. The repeatability of the experiments was checked and found satisfactory. The measured data set consisted of (i) scour depth with time; (ii) geometry of the erosion hole; and (iii) sediment motion pattern at several evolution stages of the process. A novel feature of the work was the attempt to combine evidence on the scour depth and that on sediment motion so as to shed light on the mechanism of scour reduction by the countermeasures tested. It is argued that this strategy might furnish guidelines for future extensive investigations of scour countermeasures, aimed at finding optimal design solutions.

Key words: local scour, abutment, process dynamics, flow-altering countermeasures, sediment transport

Full text link: http://www.ibwpan.gda.pl/docs/ahem/ahem59str137.pdf


Stream division by a channel bottom orifice

Jerzy M. Sawicki, Michał Siebert

Gdańsk University of Technology, Faculty of Civil and Environmental Engineering, ul. Narutowicza 11/12,
80-233 Gdańsk, Poland, e-mails: jsaw@pg.gda.pl, michalsiebert@gmail.com

Abstract

A particularly important application of devices dividing a stream of liquid is in storm water drainage systems. Dividers of rainwater sewage streams have in recent years aroused considerable interest among producers of separators. It is therefore important to develop mathematically simple and, if possible, physically precise methods of sizing such structures. It is all the more important given that there are few such methods in the literature on hydraulics. This work is meant to partially remedy this deficiency by elaborating on the calculation methodology and by verifying it empirically. A circular orifice at the bottom of the supplying channel is technically the simplest type of stream divider (which makes it higly practical and particularly appealing to producers). A procedure for dimensioning the structure under consideration is proposed here. It is formally simple, being meant as a practical designing tool. This proposal has been verified experimentally. The degree of the division of the stream was measured in a laboratory model. The main issues were the stability of the characteristics of the lateral outflow and the possibility of applying the classical equations of hydraulics to size the type of divider considered here. In both cases the results obtained may be regarded as positive. Admittedly, the output of the lateral outflow exceeds the nominal value for greater inflows Qp, but it is at most double the nominal value. From a technical point of view, this may be regarded as a good result, considering the simplicity of the system. The answer to the second issue proved even more satisfactory. It turns out that the formally simple hydraulics relationships provide a technically consistency of theory with measurement results. Thus the designing of objective devices does not require the application of difficult, time-consuming and costly methodology.

Key words: flow regulator, overfall, storm sewer

Full text link: http://www.ibwpan.gda.pl/docs/ahem/ahem59str155.pdf


A study on modelling the plain strain behaviour of sand and its stability

Sawicki Andrzej, Justyna Sławińska

Institute of Hydro-Engineering, Polish Academy of Sciences, ul. Kościerska 7, 80-328 Gdańsk, Poland,
e-mails: as@ibwpan.gda.pl, stynaju@ibwpan.gda.pl

Abstract

The plane strain behaviour of sand is studied using, previously proposed, incremental model describing its pre-failure deformations. Original model has been formulated for the tri-axial configuration, and then generalized for 3D conditions. This 3D model was subsequently adapted to study deformations of sand in the plane strain conditions, in the x1, x3 plane. There are three unknowns in such a configuration, namely the principal strains ε1, ε3 and the principal stress σ2. Respective equations were derived, and then applied to study deformations of sand for chosen stress paths. The governing incremental equations were integrated numerically, and it was shown, for some loading paths, that σ2 depends linearly on the other principal stresses, so introduction of apparent Poisson’s ratio is justified, as a kind of approximation. Subsequent analysis of deformations of sand was performed using this concept, as well as using full system of governing equations.

Key words: sand deformations, liquefaction, instability, plane strain compression

Full text link: http://www.ibwpan.gda.pl/docs/ahem/ahem59str085.pdf


Estimation of stresses in a dry sand layer tested on shaking table

Andrzej Sawicki1, Marek Kulczykowski1, Robert Jankowski2

1Institute of Hydro-Engineering, Polish Academy of Sciences, ul. Kościerska 7, 80-328 Gdańsk, Poland,
e-mails: as@ibwpan.gda.pl, marek@ibwpan.gda.pl (corresponding author)

2Gdańsk University of Technology, Narutowicza 11/12, 80-233 Gdańsk, Poland,
e-mail: robert.jankowski@wilis.pg.gda.pl

Abstract

Theoretical analysis of shaking table experiments, simulating earthquake response of a dry sand layer, is presented. The aim of such experiments is to study seismic-induced compaction of soil and resulting settlements. In order to determine the soil compaction, the cyclic stresses and strains should be calculated first. These stresses are caused by the cyclic horizontal acceleration at the base of soil layer, so it is important to determine the stress field as function of the base acceleration. It is particularly important for a proper interpretation of shaking table tests, where the base acceleration is controlled but the stresses are hard to measure, and they can only be deduced. Preliminary experiments have shown that small accelerations do not lead to essential settlements, whilst large accelerations cause some phenomena typical for limit states, including a visible appearance of slip lines. All these problems should be well understood for rational planning of experiments. The analysis of these problems is presented in this paper. First, some heuristic considerations about the dynamics of experimental system are presented. Then, the analysis of boundary conditions, expressed as resultants of respective stresses is shown. A particular form of boundary conditions has been chosen, which satisfies the macroscopic boundary conditions and the equilibrium equations. Then, some considerations are presented in order to obtain statically admissible stress field, which does not exceed the Coulomb-Mohr yield conditions. Such an approach leads to determination of the limit base accelerations, which do not cause the plastic state in soil. It was shown that larger accelerations lead to increase of the lateral stresses, and the respective method, which may replace complex plasticity analyses, is proposed. It is shown that it is the lateral stress coefficient K0 that controls the statically admissible stress field during the shaking table experiments.

Key words:

Full text link: http://www.ibwpan.gda.pl/docs/ahem/ahem59str101.pdf