Magazine of Civil Engineering, №6, 2013Брянская Ю.В. Уточнение кинематических характеристик турбулентного течения Уточнение кинематических характеристик турбулентного течения К.т.н., профессор Ю.В. Брянская, ФБГОУ ВПО «Московский государственный строительный университет» Ключевые слова: течение в трубах; теория турбулентности; профиль скорости; параметр Кармана; гидравлическое сопротивление Полуэмпирическая теория турбулентности Прандтля -Никурадзе, которая позволила получить распределение скоростей, согласующееся с закономерностями сопротивления, почти столетие сохраняет свою актуальность. Эта теория, являющаяся важным звеном в механике жидкости, основана на ряде гипотез, сформулированных Л. Прандтлем [1], среди которых основополагающей является гипотеза о длине пути перемешивания. Согласно этой гипотезе длина пути перемешивания масс жидкости в турбулентном потоке увеличивается пропорционально расстоянию от твердой границы потока с коэффициентом пропорциональности, называемым параметром Кармана, который считается постоянным. При постоянном значении параметра Кармана и так называемых «вторых констант турбулентности» достигается согласие закономерностей сопротивления и измеренных распределений скоростей в потоке, что считается важным достижением полуэмпирической теории турбулентности. Установленные разными авторами [2-4] постоянные и изменяющиеся значения параметра Кармана и «вторых констант турбулентности» приводят к необходимости модификации полуэмпирической теории турбулентности, возможность которой рассматривается в настоящей статье.
The role of fine sediments of technogenic origin in channel process is analyzed. The specific fea tures of sedimentation of particles with different density and shape and the issues of flocculation and consol idation of sediments in the bottom part of flow are discussed. Experimental data on the adhesion of particles are given and a relationship for its assessment is proposed. The class of sediments for which the adhesion determines the critical erosion velocity is determined. The processes of river water self purification and pre cipitation of fine sediments were compared to show that sedimentation processes play an important role in river water self purification.
For the solution of engineering problems require increasingly accurate estimates of the hydraulic characteristics of the water streams. To date, it is impossible to consider sufficiently complete theoretical and experimental justification of the main provisions of the theory of turbulence, hydraulic resistance, channel processes. The composition of tasks related to flows in wide channels, turbulence problems are of scientific and practical interest. Various interpretations of the determination of the critical Froude number in wide open water flows based on observations and theoretical transformations are considered. The conditions for the emergence of a critical regime of water flow in an open wide channel are analyzed in order to estimate the critical Froude number and critical depth. Estimates of the critical Froude number for laboratory and field conditions are given. The estimations allow us to consider the proposed approach acceptable for determining the conditions of occurrence of the critical flow regime. The General, physical interpretation of conditions of occurrence of the critical regime of water flow on the basis of phenomenological approach is specified. The results take into account the values of the components of the total specific energy of the section. This shows the estimated calculation. The results obtained theoretically make it possible to compare the above interpretations and determine their applicability, and the results of the analysis can be useful for the estimated calculations of flows in channels and river flows in rigid, undeformable boundaries and with minor channel deformations.
Introduction. To date, pipeline transport is an important, highly efficient and promising method of transporting water, gas, oil, etc. It is often necessary to overcome water barriers in the course of laying pipelines. At the design stage, many engineering problems can be solved by analyzing the velocity distribution and evaluating hydraulic resistances. The value of hydraulic resistances depends on the position of the pipeline crossing relative to the incoming flow. Materials and methods. Experimental studies of models of pipeline crossings in a wind tunnel were conducted to determine the values of coefficients of hydrodynamic resistance and the lifting force. Results. During the experiments, components of the flow force, acting on the pipeline crossing, were measured using the strain gauge balance at different Reynolds numbers applied to find values of coefficients of hydrodynamic resistance and the lifting force for a pipeline lying on a screen at different angles to the incoming flow. 1/2 and 1/3 of the pipeline diameter was buried in the riverbed and safeguarded by flexible concrete mats. Conclusions. The lowest values of coefficients of hydrodynamic resistance and the lifting force were obtained for a pipeline with 1/2 of the pipeline diameter buried in the riverbed and laid perpendicularly to the flow direction. Concrete mats are the optimal loading for non-buried pipelines.
Nonuniform steady-state movement is analyzed.An approach based on differential equations of energy conservation [1], the analytical integration of which for essentially critical cases gives rise to difficulties, and should be performed numerically, is usually employed for analysis of laws governing nonuniform movement. An example of this approach is Bakhmetev's solution [2], which is supported by a number of assumptions containing certain inaccuracies: constancy of the hydraulic exponent of the channel in a section of nonuniform movement, which in reality varies owing to variation in the relative width B/h of the channel [3]; and, determination of the hydraulic gradient with use of Chezy's formula, which can be used only for uniform movement, and also other formulas for calculation of the Chezy coefficient, which are derived for a uniform movement. Additional inaccuracies of the solution are associated with the assumption of constancy over the integration segment of the complex a c C i g B 2 (where á is the Coriolis coefficient, C is the Chezy coefficient, and B and ÷ are the width and wetted perimeter of the channel). It should be noted that the energy equation is reduced to the balance of gradients, which for flatland water courses are extremely small, and of the order of 10 -4 -10 -5 ; this requires high accuracy of the equation's solution, a fact inconsistent with the procedure proposed by Bakhmetev. The indicated inaccuracies may result in substantial errors in predicting flooding zones, particularly in flatland areas. These situations preserve the urgency of problems involving the hydraulics of nonuniform, smoothly varying, open flows. Let us examine the feasibility of a dynamic approach to analysis of nonuniform smoothly varying movement in a broad open prismatic channel. Figure 1 shows the variation in movement in an isolated volume between sections 1 -2 subject to applied forces in a projection in the direction of motion. The distance between sections 1 -2 is assumed equal to dx.During time dt, the mass of liquid is shifted and falls into position between sections 1 ¢ -2 ¢. Isolating three regions in the zone of movement, the amount of movement in the initial time can be conditionally written as(1)Here, the change in the amount of movement is expressed assince in segment b during time dt, no changes occur, i.e., MA(b) t + dt = MA(b) t . Since the movement is steady-state, it is obvious that the amount of movement in sections a and c will be independent of time t; the subscripts are therefore eliminated in expression (3).
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