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EN
In this work, we study a one-equation turbulence k-epsilon model that governs fluid flows through permeable media. The model problem under consideration here is derived from the incompressible Navier-Stokes equations by the application of a time-averaging operator used in the k-epsilon modeling and a volume-averaging operator that is characteristic of modeling unsteady porous media flows. For the associated initial- and boundary-value problem, we prove the existence of suitable weak solutions (average velocity field and turbulent kinetic energy) in the space dimensions of physics interest.
EN
Underwater wireless optical communication is the best alternative for many applications especially for high bandwidth data communication between underwater objects and vehicles. The implementation of coding scheme along with advanced modulation technique and equalisation methods is identified as a key research scope for enhancing the performance of the system. In this paper, the coded generalised frequency division multiplexing (GFDM) technology is employed to provide high-data rates and less out-of-band emission. The Bose-Chaudhuri-Hocquenghem (BCH) and Reed-Solomon (RS) coding schemes along with equalisation techniques namely normalised least mean square (NLMS)-based decision feedback equalisers (DFE), minimum mean square error (MMSE) and zero forcing (ZF) are utilized to reduce inter symbol interference (ISI). The bit error rate (BER) performance is evaluated in the presence of pointing error (PE) and turbulence using Monte Carlo channel modelling simulations. The results showed that RS coding with NLMS-DFE outperforms other techniques and achieves a BER of roughly 10⁻⁵ with a signal-to-noise ratio levels below 20 dB. The simulation results demonstrate that RS code with 15 total symbols per code word and 3 data symbols, i.e., RS (15, 3) and BCH code with 31 total symbols in a code word and 6 data symbols, i.e., BCH (31, 6) provided the best error performance among other coding schemes employed. It is inferred that RS (15, 3) coded 2 × 2 multiple input multiple output systems with NLMS-DFE achieved a BER value of 1.1925 × 10⁻⁵ at 11 dB which is 16 dB less than uncoded system. Thus, the coded GFDM improves overall BER performance and has the potential to provide higher reliability for internet of underwater things (IoUT) applications.
EN
The relevance of this study is explained by the growing interest in increasing heat transfer by the development of high-performance thermal systems. Increasing the thermal characteristics of heat-exchanger systems is necessary for the efficient use of an energy source. The purpose of this study is to review the existing methods of heat-transfer intensification and examine the mathematical model of such an increase in efficiency when using petal turbulators. This study is based on a high-quality, reliable combination of proven theoretical methods (analysis, synthesis, concretization, generalization, modelling), and empirical methods. It is the introduction of turbulators into the flow channel that is one of the best methods of increasing passive heat exchange through such advantages as ease of manufacture and operation in combination with low operating and production costs. This study contains both passive and active methods of heat-exchange intensification that have been extensively investigated over the past decade. For this purpose, the newest studies of mainly authors from other countries were used, their detailed analysis was conducted and the results were summed up. In addition, a mathematical model of increasing the thermal efficiency of convective heating surfaces in a bundle of smooth pipes using petal turbulators was investigated, the results of which were tested on an experimental installation. The paper may interest a circle of readers interested in the problem of improving the thermal characteristics of heat exchangers, including researchers, teachers and students of higher educational institutions in the field of heat-power engineering.
PL
Znaczenie tego badania wynika z rosnącego zainteresowania zwiększeniem wymiany ciepła poprzez rozwój wysokowydajnych systemów termicznych. Zwiększenie charakterystyki cieplnej układów wymienników ciepła jest niezbędne do efektywnego wykorzystania źródła energii. Celem niniejszej pracy jest przegląd istniejących metod intensyfikacji wymiany ciepła oraz zbadanie modelu matematycznego takiego wzrostu wydajności przy zastosowaniu turbulatorów płatkowych. Niniejsze opracowanie opiera się na wysokiej jakości, rzetelnym połączeniu sprawdzonych metod teoretycznych (analiza, synteza, konkretyzacja, uogólnienie, modelowanie) oraz metod empirycznych. Ponieważ to właśnie wprowadzenie zawirowywaczy do kanału przepływowego jest jedną z najlepszych metod zwiększenia biernej wymiany ciepła poprzez takie zalety jak łatwość wykonania i eksploatacji w połączeniu z niskimi kosztami eksploatacji i produkcji. Niniejsze opracowanie obejmuje zarówno pasywne, jak i aktywne metody intensyfikacji wymiany ciepła, które były szeroko badane w ciągu ostatniej dekady. W tym celu wykorzystano najnowsze badania, głównie autorów z innych krajów, dokonano ich szczegółowej analizy i podsumowano wyniki. Ponadto zbadano model matematyczny zwiększania sprawności cieplnej konwekcyjnych powierzchni grzewczych w wiązce rur gładkich za pomocą turbulatorów płatkowych, którego wyniki przetestowano na instalacji doświadczalnej. Artykuł może zainteresować grono czytelników zainteresowanych problematyką poprawy właściwości cieplnych wymienników ciepła, w tym naukowców, nauczycieli i studentów wyższych uczelni z zakresu elektroenergetyki.
4
Content available remote Wake flow field of a wall-mounted pipe with spoiler on a rough channel bed
EN
This research work focuses on the wake flow region of a cylinder with a spoiler on a rough bed under steady flow conditions. The acoustic Doppler velocimetry was used for the measurement of three-dimensional velocity data for two Reynolds numbers in a fully developed turbulent flow around the cylinder with a spoiler. The mean flow velocities, second-order turbulence structures, and conditional statistics were investigated in the wake region of the spoilered cylinder. The flow was separated from the spoiler with the formation of two shear layers between free surface flow and recirculating flow. It is observed that the flow is reattaching to the bed at 11D irrespective of the Reynolds number. Downstream of the cylinder, the mean velocity distributions are asymmetric due to the wall-wake effect, and the point of inflection is observed for each velocity profile at z = 0.40ẟ. The turbulence intensities, Reynolds stresses, and TKE are highly enhanced in the wake region of the cylinder as compared to their respective upstream values for both runs. The turbulence intensities, Reynolds normal stresses, Reynolds shear stresses, and turbulent kinetic energy are attaining peaks at z = 0.4ẟ for all the streamwise locations, and the peaks are found to be highest at x = 10D. The quadrant analysis results indicate that the sweeps are dominating bursting events in the inner and intermediate layers, while ejections are dominating in the outer layer of the wake region. As the hole size, H increases ejections stress fraction rises as compared to that of the sweeps in the wake region for z = 0.2-0.7 h.
5
Content available remote Problemy dotyczące modelowania obciążenia wiatrem obiektów budowlanych
PL
Im bardziej skomplikowany kształt obiektu budowlanego, tym trudniej prognozować, jak będą na niego oddziaływały masy powietrza podczas przepływu. Problem jest istotny z uwagi na niezawodność obiektów budowlanych. Analizy numeryczne oraz badania doświadczalne pozwolą w przyszłości zmodyfikować wytyczne normowe, przyczyniając się do poprawy bezpieczeństwa obiektów budowlanych.
EN
The more complex the shape of a building object, the more difficult it is to predict how air masses will affect it during flow. The problem is important for the reliability of buildings. Numerical analyses and experimental studies will allow future modification of the standard guidelines, contributing to the improvement of the safety of buildings.
EN
Underwater Wireless Optical Communication (UWOC) offers significant research prospective with major challenges in the design and implementation. UWOC is capable of providing high rate of data transmission across large distances. This paper attempts to focus on the intricacies of practical implementations and open research issues of UWOC systems. Critical advances and progresses made in the field, modelling techniques and link design challenges are summarised. The purpose of this review is to give suggestions towards feasible and reliable UWOC design with improved performance. Finally the major points are summarized so that it will assist the future research in UWOC.
EN
Local wind conditions can vary strongly depending on the landmark and vegetation, as well as on the skyline of the buildings in an urban surrounding. Weather, season and time of day influence the yield of electric power. In order to promote the use of small wind turbines as an alternative to photovoltaic power generation, design optimization for location-optimized small wind turbines was carried out. In this work, we want to concentrate on vertical axis wind turbines. Experimental studies, as well as numerical simulations, have been conducted. On the one hand, bionically optimized core structures will be integrated and implemented in the hybrid material of the turbine blades. Several optimization attempts have been examined for single blades. Detailed simulative investigations with large eddy simulations improve the aerodynamic behaviour of the new rotor design. Finally, based on the results of the studies and investigations, a new rotor will be manufactured and tested experimentally in the wind tunnel. A comparison with the reference system from the first part of the paper shows the improvements and effectiveness of the measures and processes investigated.
EN
In the present work, the effect of three insoluble additives densities on reducing the drag of crude oil was investigated. The objective of the present work is to evaluate the effect of the insoluble additive’s densities on their drag reduction efficiency in hydrocarbon flow medium. Three powders with different densities are chosen, namely carbon powder, glass powder, and copper powder, with a density of 1710 kg/m3 , 2550 kg/m3 , and 8950 kg/m3 , respectively. The turbulence flow environment was created in a custom-made rotating disc apparatus with a maximum rotation speed of 300 rpm. To evaluate the effect of the powder density, the particle's size was chosen to be 100 µm. All the solutions were tested at the exact operating conditions with a rotation speed ranging between 200 to 2200 rpm. The experimental results showed a clear effect of the powder density on the drag reduction performance. The glass powders showed the highest drag reduction effect, while the copper and carbon powders were lower. The effect of the degree of turbulence on the drag reduction performance of the powders was clear, where the interaction between the powders and the turbulence structures (eddies) governed the turbulence-suppression efficiency of the additives.
EN
Acoustic Doppler velocimetry profilers (ADVPs) are widely used in both experimental and field studies because of their robustness in velocity measurements. The acquired measurements do not only offer estimates of the local and instantaneous flow velocity at the interrogated measurement volume, but can also be further processed for the estimation of the bed surface shear stresses, thus they are finding a wide range of applications ranging from water engineering to geomorphology and ecohydraulics. This study aims to evaluate the performance of an ADVP in obtaining hydrodynamics measurements under fixed flow conditions, with various probe configurations. To this goal, a robust search is conducted where ADVP probe settings are sequentially altered. A number of assessment criteria are used including qualitative observations, such as checking the shape of the velocity profile, as well as quantitative error metrics, including signal-to-noise ratio, correlations and number of spikes. Further, estimation of the bed shear stresses computed by means of using the log Law of the Wall and turbulent kinetic energy, allow obtaining a better understanding of the uncertainties involved and the importance of making a better informed choice with respect to the probe configuration settings. Thus, the methodology and performance metrics provided herein, although presented for a given flow, can generally be applied from practitioners and researchers alike.
10
Content available remote Hydrodynamics and turbulence anisotropy for complex flow in a sinuous channel
EN
Sinuous channel flows are the most natural form of alluvial streams. The complex flow in the channel bends has been the main focus of the study. This paper examines the flow velocity and the three-dimensional velocity fluctuations in a pure sinuous channel. The main focus of the study is on the characterization of turbulence anisotropy along the sinuous bend. Experiments were conducted in a sinuous channel of a rectangular cross-section to identify the turbulence present in the flow. Secondary flow, Reynolds shear stress, turbulent kinetic energy, and anisotropy turbulence were evaluated in the sinuous bend. The bend apex is composed of a large circulation cell at the center of the bend section. The maximum Reynolds shear stress (RSS) is located at the bend apex with the streamwise-transverse and transverse-vertical components of RSS showing high peaks of positive and negative values. This fact is in support of the secondary flow observed in this study. Anisotropic stress tensors were estimated at different bend sections and are shown to have greater contribution toward streamwise and transverse direction. Anisotropic invariant map (AIM) identified the turbulence at bend sections and varying flow depth. Two dimensional, cigar-shaped, and pancake-shaped turbulence was observed at the bend upstream and downstream. Isotropic turbulence was observed at the bend apex. Near the bed (z∕h ≤ 0.2) and away from the bed (z∕h ≥ 0.4), pancake-shaped and cigar-shaped turbulence was observed.
EN
Scour downstream of smooth and rough rigid aprons under wall jets has been studied experimentally. Effect of apron roughness on scour has been investigated, and quantification of reduction in the scour depth due to rough apron has been performed. Characteristics of velocity and turbulence over smooth and rough aprons as well as within the scour hole have been analyzed to study the behavior of the jet and its interaction with the rough apron. Results show that there is a significant reduction in the equilibrium scour depth due to roughness. A maximum of 82.8% and a minimum of 31.1% reduction in the equilibrium scour depth was observed due to inducing roughness over the rigid apron. Velocity characteristics establish the cause of reduction in the equilibrium scour depth, which is due to reduction in the erosive capacity of the jet as it moves over the rough apron. The potential core of the jet gets consumed at a much lesser length due to roughness over the apron than over a smooth apron, as the boundary layer develops at a smaller distance. Further, it was observed that it takes a smaller length for the flow to get fully developed under the rough apron as compared with the smooth apron. Based on the results of the present analysis, recommendation can be made for use of roughness over the apron to restrict scour due to wall jets.
EN
Stochastic airfoil flutter in an unsteady flow is discussed using the stochastic P-bifurcation method, taking into account potential effects of the longitudinal and vertical turbulent flow. The critical conditions of stochastic P-bifurcation are deduced by stochastic singularity analysis in order to discuss stochastic P-bifurcation phenomena. The results of parameter analysis show that as the turbulent intensity increases, the critical flutter velocity for sharp stochastic airfoil flutter decreases. And the large amplitude vibration comes earlier; an increase in the turbulent scale causes an earlier appearance of the critical velocity for large amplitude stochastic flutter.
EN
This paper presents the results of experimental testing of parameters of the flow of an agitated liquid in a stirred tank with an eccentrically positioned shaft and with a Rushton turbine. The investigations were focused on the impact of the stirrer shaft shift in relation to the stirred tank vertical axis on the agitated liquid mean velocities and the liquid turbulent velocity fluctuations, as well as on the turbulence intensity in the tank. All the experiments were carried out in a stirred tank with the inner diameter of 286 mm and a flat bottom. The adopted values of the shaft eccentricity were zero (central position) and half the tank radius. The liquid flow instantaneous velocities were measured using laser Doppler anemometry.
EN
This paper presents a numerical analysis on turbulent flow and forced-convection characteristics of rectangular solar air heater tube fitted with staggered, transverse, V-shape, modern obstacles on the heated walls. Air, whose Prandtl number is 0.71, is the working fluid used, and the Reynolds number considered equal to 6×10 3. The governing flow equations are solved using a finite volume approach and the semi-implicit pressure linked equation (SIMPLE) algorithm. With regard to the flow characteristics, the quadratic upstream interpolation for convective kinetics differencing scheme (QUICK) was applied, and a second-order upwind scheme (SOU) was used for the pressure terms. The dynamic thermo-energy behavior of the V-shaped baffles with various flow attack angles, i.e., 50°, 60°, 70°, and 80° are simulated, analyzed, and compared with those of the conventional flat rectangular baffles with attack value of 90°. In all situations, the thermal transfer rate was found to be much larger than unity; its maximum value was around 3.143 for the flow attack angle of 90° and y = H/2.
EN
This work aims to find the influence of the liquid viscosity on the shape of an air Taylor bubble, rising up in a pipe column which contains the liquid under conditions that the liquid is stagnant and the Froude number is approximately equal to 0.35. Five liquid viscosities (from 0.001 to 0.01 Pa · s) were selected for being computationally investigated. An appropriate shape of a Taylor bubble, corresponding to each selected viscosity, was obtained by considering a pressure distribution of the air inside the bubble. Simulation results showed that the Taylor bubble shape would be thicker if the liquid viscosity was decreased. This could be explained by using the theory of the log-law velocity profile.
EN
One of the mathematical tools to measure the generation rate of new patterns along a sequence of symbols is the Lempel-Ziv complexity (LZ). Under additional assumptions, LZ is an estimator of entropy in the Shannon sense. Since entropy is considered as a measure of randomness, this means that LZ can be treated also as a randomness indicator. In this paper, we used LZ concept to the analysis of different flow regimes in cold flow combustor models. Experimental data for two combustor’s configurations motivated by efficient mixing need were considered. Extensive computer analysis was applied to develop a complexity approach to the analysis of velocity fluctuations recorded with hot-wire anemometry and PIV technique. A natural encoding method to address these velocity fluctuations was proposed. It turned out, that with this encoding the complexity values of the sequences are well correlated with the values obtained by means of RMS method (larger/smaller complexity larger/smaller RMS). However, our calculations pointed out the interesting result that most complex, this means most random, behavior does not overlap with the “most turbulent” point determined by the RMS method, but it is located in the point with maximal average velocity. It seems that complexity method can be particularly useful to analyze turbulent and unsteady flow regimes. Moreover, the complexity can also be used to establish other flow characteristics like its ergodicity or mixing.
17
Content available remote Turbulent mixing and dispersion mechanisms over flexible and dense vegetation
EN
The present study investigates flow turbulence and dispersion processes in the presence of flexible and dense vegetation on the bed. The turbulent dispersion coefficients and the terms of the turbulent kinetic energy equation are determined by using data collected in a straight laboratory channel with living vegetation on the bed. Results show that the turbulent integral lengths assume an order of magnitude comparable to the stems’ characteristic dimension independently by the direction and the turbulence assumes an isotropic behavior. The coefficients of dispersion have a trend similar to that of the turbulent lengths and assume low values in the longitudinal, transversal and vertical directions. Results also show that, in the mixing layer, the shear and wake turbulence production terms balance the dissipation; the turbulent diffusion term also assumes low values and its sign varies along the vertical indicating a transport of turbulent energy both from the vegetation to the free surface and from the free surface to vegetation.
EN
Velocity profiles upstream and downstream of two aquatic plant species that are similar in morphology but differ in patch structures were measured in a natural river. Turbulence statistics were analyzed after thorough data filtering. In the wake of the M. alterniflorum, which was a slender, 0.3 m wide and 1.2 m long patch of aspect ratio 1:4, there were distinctive peaks in both, turbulence intensity and turbulent kinetic energy, which indicated increased lateral mixing. In contrast to the M. alterniflorum, turbulence statistics in the wake of the M. spicatum, which was the larger, 2 m wide and 2.4 m long patch of aspect ratio 1:1.5, indicated increased lateral shear of a greater magnitude. The turbulent kinetic energy was diminished in the closest layer to the bed downstream the both plants, although, in the case of M. alterniflorum, the observed values were similar to those upstream. The occurrence of the mixing layer below the height of M. spicatum was visible in the power spectral density plot. In both cases, ejections in the wake diminished in favor of other coherent structures. The shape and configuration of a patch are decisive factors governing the occurrence of flow instabilities downstream of the patch.
EN
The methodology of obtaining a logarithmic velocity profile describing the velocity distribution in the cross section of the boundary layer, which is based on the well-known equation of L. Prandtl, based on its semi-empirical turbulence theory, is considered. It is shown that the logarithmic velocity profile obtained in this way does not satisfy any boundary condition arising from the classical definition of such concept as the boundary layer. The perfect coincidence of this velocity profile with the experimental data of Nikuradze demonstrated in the world scientific literature is a consequence of making these profiles not in a fixed, but in a floating coordinate system. When rebuilding the velocity profiles obtained at different Reynolds numbers, all the profiles lose their versatility and do not coincide with the actual velocity profiles in cylindrical pipes.
EN
The aim of this work is to propose an empirical model for predicting shapes of a Taylor bubble, which is a part of slug flows, under different values of the surface tension in stagnant liquids by employing numerical simulations. The k - turbulence model was used in the framework of finite volume method for simulating flow fields in a unit of slug flow and also the pressure distribution on a Taylor bubble surface. Assuming that an air pressure distribution inside the Taylor bubble must be uniform, a grid search method was exploited to find an appropriate shape of a Taylor bubble for six values of surface tension. It was found that the shape of a Taylor bubble would be blunter if the surface tension was increased. This was because the surface tension affected the Froude number, controlling the flow around a Taylor bubble. The simulation results were also compared with the Taylor bubble shape, created by the Dumitrescu-and-Taylor model and former studies in order to ensure that they were consistent. Finally, the empirical model was presented from the simulation results.
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