Numerical and experimental modeling of turbulent flows in furnace chambers

Results are presented from an experimental and numerical modeling of two-dimensonal turbulent flows in furnace chambers. The article follows the evolution of the flow patterns with a change in the configuration of the chamber. The range of validity of the study results are discussed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 6, pp. 948–956, December, 1990.     

Comparison of the results of modeling convective heat transfer in turbulent flows with experimental data

The results of simulation of natural turbulent convection in a square air cavity measuring 0.75 × 0.75 m and having isothermal vertical and highly heat-conducting horizontal walls are compared with the experimental data obtained for this cavity at a Rayleigh number equal to 1.58⋅10⁹. In carrying out numerical investigations, a two-dimensional, low-turbulence, two-parameter k–ε model known as the low-Reynolds-number k–ε turbulence model was used. The results of investigations are presented for the distributions of the velocity and temperature components, as well as local and average values of the Nusselt number. The model was also used in calculating forced turbulent convection in a low-velocity channel with a backward facing step. The results of modeling are compared with experimental data on heat transfer in a turbulent separation flow downstream of the step. In both cases, a satisfactory agreement of the measured values with those predicted by the k–ε turbulence model is obtained.     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow. Another example concerns the radial distribution of the bubbles in pipe flow, which control the wall heat and momentum transfers. This distribution is very sensitive to gravity. On earth it is mainly controlled by the action of the lift force due to the bubble drift velocity. In micro gravity in absence of bubble drift, the bubbles are dispersed by the turbulence of the liquid and the classical model fails in the prediction of the bubble distribution. The first results of experiments and numerical simulations on isolated bubbles in normal and micro gravity conditions are presented. They should allow in the future improving the modelling of the turbulent bubbly flow in micro gravity but also on earth.     

Two-phase bubbly flows in microgravity: Some open questions

Several studies on gas-liquid pipe flows in micro gravity have been performed. They were motivated by the technical problems arising in the design of the thermohydraulic loops for the space applications. Most of the studies were focused on the determination of the flow pattern, wall shear stress, heat transfer and phase fraction and provided many empirical correlations. Unfortunately some basic mechanism are not yet well understood in micro gravity. For example the transition from bubbly to slug flow is well predicted by a critical value of the void fraction depending on an Ohnesorge number, but the criteria of transition cannot take into account the pipe length and the bubble size at the pipe inlet. To improve this criteria, a physical model of bubble coalescence in turbulent flow is used to predict the bubble size evolution along the pipe in micro gravity, but it is still limited to bubble smaller than the pipe diameter and should be extended to larger bubbles to predict the transition to slug flow.     

Aero-optic effects in turbulent flows

Large-scale vortices and related aero-optic effects in a turbulent boundary layer, a free mixing layer, and an underwater stream have been mathematically simulated. The results are used for the analysis of distortions in the wavefront-phase function of a coherent light beam, which are induced by turbulent fluctuations in the parameters of the medium. The results of numerical calculations are compared to the experimental data and to a solution of the Reynolds equations.     

Mathematical models for the numerical study of turbulent flows

Abstract

This paper presents (1) a brief overview of the mathematical models used in the numerical study of turbulent flows; (2) a K‐? model of turbulence; and (3) extensions of the K‐? model to account for some of the effects of compressibility, low Reynolds number, streamline curvature, and preferential stress dissipation.     

A mathematical modeling approach to resource allocation for railroad-highway crossing safety upgrades

State Departments of Transportation (S-DOT's) periodically allocate budget for safety upgrades at railroad-highway crossings. Efficient resource allocation is crucial for reducing accidents at railroad-highway crossings and increasing railroad as well as highway transportation safety. While a specific method is not restricted to S-DOT's, sorting type of procedures are recommended by the Federal Railroad Administration (FRA), United States Department of Transportation for the resource allocation problem. In this study, a generic mathematical model is proposed for the resource allocation problem for railroad-highway crossing safety upgrades. The proposed approach is compared to sorting based methods for safety upgrades of public at-grade railroad-highway crossings in Tennessee. The comparison shows that the proposed mathematical modeling approach is more efficient than sorting methods in reducing accidents and severity.     

The effects of surfactant on the multiscale structure of bubbly flows

It is well known that a bubble in contaminated water rises much slower than one in purified water, and the rising velocity in a contaminated system can be less than half that in a purified system. This phenomenon is explained by the so-called Marangoni effect caused by surfactant adsorption on the bubble surface. In other words, while a bubble is rising, there exists a surface concentration distribution of surfactant along the bubble surface because the adsorbed surfactant is swept off from the front part and accumulates in the rear part by advection. Owing to this surfactant accumulation in the rear part, a variation of surface tension appears along the surface and this causes a tangential shear stress on the bubble surface. This shear stress results in the decrease in the rising velocity of the bubble in contaminated liquid. More interestingly, this Marangoni effect influences not only the bubble's rising velocity but also its lateral migration in the presence of mean shear. Together, these influences cause a drastic change of the whole bubbly flow structures. In this paper, we discuss some experimental results related to this drastic change in bubbly flow structure. We show that bubble clustering phenomena are observed in an upward bubbly channel flow under certain conditions of surfactant concentrations. This cluster disappears with an increase in the concentration. We explain this phenomenon by reference to the lift force acting on a bubble in aqueous surfactant solutions. It is shown that the shear-induced lift force acting on a contaminated bubble of 1mm size can be much smaller than that on a clean bubble.     

A Lagrangian modeling approach with the direct simulation Monte-Carlo method for inter-particle collisions in turbulent flow

A Lagrangian modeling approach, which combines the direct simulation Monte-Carlo (DSMC) method and a Reynolds-averaged Navier-Stokes model to account for inter-particle collisions and turbulence characteristics of the carrier fluid, respectively, is proposed. The wall-bounded turbulent particle-laden flows in which the experimental data are available are chosen as the test problems for demonstration. Results obtained with the deterministic method accounting for inter-particle collisions are used as a basis for validating the proposed stochastic Lagrangian model. Good agreement between the predictions obtained separately with the deterministic and DSMC methods is achieved. The benefit of saving computational expenditure when using the DSMC method becomes more remarkable than the deterministic method as the number of particles loaded in the flow is increased. In addition, the study demonstrates that τ_P/τ_C is a proper parameter to monitor the role of inter-particle collisions in the physical processes of particle-laden flows.     

Bend Pressure Drop Predictions Using the Euler-Euler Model in Dense Phase Pneumatic Conveying

Pneumatic conveying of powdered and granular materials is a very common transport technology across a broad range of industries, for example, chemicals, cosmetics, pharmaceuticals, and power generation. As the demands of these industries for greater efficiency increases and to comply with environmental regulations there is a need for a more fundamental understanding of the behavior of materials in pneumatic conveying systems. The approach presented in this article is to develop a model of a section of pneumatic conveying line, a horizontal or vertical 90° bend, in the commercial CFD software package FLUENT and to describe the multiphase flow behavior by the mixture or Eulerian method. Models of this type have been used in the past to show qualitative and quantitative agreement between model and experiment. The model results presented were compared with experimental data gathered from an industrial-scale pneumatic conveying test system. Broad qualitative agreement in trends and flow patterns were found. Quantitative comparisons were less uniform, with predictions from around 10% to 90% different from experimental results, depending on conveying conditions and bend orientation.     

Scaling of turbulent separating flows

The present work investigates the scaling of the turbulent boundary layer in regions of adverse pressure gradient flow. For the first time, direct numerical simulation and experimental data are applied to the theory presented in Cruz and Silva Freire [Cruz, D. O. A., & Silva Freire, A. P. (1998). On single limits and the asymptotic behaviour of separating turbulent boundary layers. International Journal of Heat and Mass Transfer, 41, 2097-2111] to explain how the classical two-layered asymptotic structure reduces to a new structure consistent with the local solutions of Goldstein and of Stratford at a point of zero wall shear stress. The work discusses in detail the behaviour of an adaptable characteristic velocity (u_R) that can be used in regions of attached as well as separated flows. In particular, u_R is compared to velocity scales based on the local wall shear stress and on the pressure gradient at the wall. This is also made here for the first time. A generalized law of the wall is compared with the numerical and experimental data, showing good agreement. This law is shown to reduce to the classical logarithmic solution and to the solution of Stratford under the relevant limiting conditions.     

Classification of fluctuating turbulent flows

The existence is shown of five different forms of the appearance of superposed fluctuations on flow characteristics on the basis of an analysis of the mechanism of turbulence propagation in fluctuating flows.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 59, No. 5, pp. 725–735, November 1990.     

On the asymptotic layering of turbulent flows

The asymptotic structure of a turbulent boundary layer on a plate with a boundary-layer distributed suction, consisting of a suction zone, a viscous zone, a buffer zone, a velocity-defect zone, a Corrsin superlayer, and a irrotational-flow zone, has been determined. The analysis was carried out within the framework of the Reynolds equations with the use of the combined method of different scales and joined of asymptotic expansions. The Corrsin superlayer was interpreted as a discontinuity of turbulent stresses. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 81, No. 2, pp. 322–329, March–April, 2008.     

The use of the hybrid RANS/ILES approach for the investigation of three-dimensional separated turbulent flows in curvilinear diffusers

The hybrid RANS/ILES approach is used for the investigation of turbulent separated flow in curvilinear annular diffusers with the area ratio of 2.04 and 2.7. The effect of geometry on the loss of symmetry of flow in an axisymmetric annular diffuser is considered. The effect of pressure difference in the diffuser on the flow and on the characteristics of turbulence in this diffuser is investigated. The effect of nonuniform total pressure at the channel inlet on the distribution of parameters in the outlet section of the diffuser is determined. The loss of total pressure in diffusers is determined for all of the considered modes. The accuracy of the results is confirmed by comparison with the experimental data available for these diffusers.     

Current trends in modelling research for turbulent aerodynamic flows

The engineering tools of choice for the computation of practical engineering flows have begun to migrate from those based on the traditional Reynolds-averaged Navier-Stokes approach to methodologies capable, in theory if not in practice, of accurately predicting some instantaneous scales of motion in the flow. The migration has largely been driven by both the success of Reynolds-averaged methods over a wide variety of flows and the inherent limitations of the method itself. Practitioners, emboldened by their ability to predict a wide variety of statistically steady equilibrium turbulent flows, have now turned their attention to flow control and non-equilibrium flows, i.e. separation control. This review gives some current priorities in traditional Reynolds-averaged modelling research as well as some methodologies being applied to a new class of turbulent flow control problem.     

Bend Pressure Drop Predictions Using the Euler-Euler Model in Dense Phase Pneumatic Conveying

Pneumatic conveying of powdered and granular materials is a very common transport technology across a broad range of industries, for example, chemicals, cosmetics, pharmaceuticals, and power generation. As the demands of these industries for greater efficiency increases and to comply with environmental regulations there is a need for a more fundamental understanding of the behavior of materials in pneumatic conveying systems. The approach presented in this article is to develop a model of a section of pneumatic conveying line, a horizontal or vertical 90° bend, in the commercial CFD software package FLUENT and to describe the multiphase flow behavior by the mixture or Eulerian method. Models of this type have been used in the past to show qualitative and quantitative agreement between model and experiment. The model results presented were compared with experimental data gathered from an industrial-scale pneumatic conveying test system. Broad qualitative agreement in trends and flow patterns were found. Quantitative comparisons were less uniform, with predictions from around 10% to 90% different from experimental results, depending on conveying conditions and bend orientation.     

On nonlinear models for the pressure-strain rate correlations in turbulent flows

Simple inequalities are obtained that can be used for direct verification of the adequacy of nonlinear models describing a rapid part of the pressure-strain rate correlation tensor. Analysis of a quadratic model shows that such models cannot be accepted in a most part of the physical domain because of violation of the condition of positive definiteness of the spectral matrix of two-point correlations of the pulsation velocity. The boundaries of such “forbidden” zones can be even wider than those for the classical linear models.     

In-line holographic particle image velocimetry for turbulent flows

A holographic system has been developed to measure the velocity field in three-dimensional flow regions. The system records the position of small tracer particles on two in-line holograms of the flow obtained simultaneously. Two exposures are recorded on each hologram. The flow velocity is derived from the displacement of the particles between exposures. A general design procedure is described for selecting the particle diameter and the concentration on the basis of the configuration of the flow facility and the resolution characteristics of the holographic imaging system. The system was implemented in a 2 ft x 2 ft (1 ft = 30.48 cm) water channel to measure the velocity field in a turbulent free-surface jet. The spatial resolution of the system is 1 mm, and the field of view is 100 mm, approximately. Measurements performed with this system are compared with results reported in the literature and are found to be in good agreement.     

Features of the refractometry of turbulent gas flows

The features of the interference refractometry of turbulent gas flows, which are a special case of a randomly inhomogeneous medium, are considered. Recommendations are made on the quasi-optimal choice of the parameters of the refractometer system. Translated from Izmeritel’naya Tekhnika, No. 8, pp. 31–35, August, 1996.