bluuebunny/crossref_metadata_2025 huggingface dataset:
Research on the Application of Genetic Algorithm in Long-Distance Solid-Liquid Transportation Pumping Systems
Authors: Xu Yun , Chen Ping , Shi Wei , Luo Xianwu
Published: September 2008

This paper treats the operation optimization for the LQS two-phase flow pump dredging system including three parallel pumps and two serial pumps by using the genetic algorithm. The Yellow River is well known for its large concentration sand-laden-water. In order to stop the increasing river bed by the depositing sand, a bank-strengthening project has been put into action in Shangdong province since 1998. In the project, very large amount of the Yellow River sand at river bed is dredged up and pumped to several miles away to strengthen the back side of the river bank. The LQS two-phase flow pump dredging system, which is a recently developed apparatus for this project, is applied in the sand transportation system. The main target of the present work is to conduct the operation optimization computation for the LQS two-phase flow pump dredging system by using the genetic algorithm. The genetic algorithm introduces the mechanism of the heredity and selection by the nature into Mathematics, and presents a new optimization algorithm. Based on the operation characteristics of the LQS dredging system, the mathematic models with hydraulic constraints were established, and the numerical expression of genetic algorithm for operation optimization was given. Selecting the minimum power consumption for pumping the same amount of sand as the objective function and making additive multiple constraints, we developed a kind of new genetic algorithm, where the searching range is reduced gradually so as to strengthen the searching ability for the algorithm. The computation results indicate that by applying the modified genetic algorithm for the global optimal solution, the present operation saved much more power than the previous one. The developed method has been applied in the actual project, and verified to be very effective.
Flow and Particle Deposition in Asymmetric Human Airways
Authors: Tian L. , Ahmadi G. , Hopke P. K. , Cheng Y.-S.
Published: September 2008

Transport and deposition of particles in human upper thoracic airways is important to understand their toxicology and the effect of exposure to environmental particulate matter as well as in the design of inhalation drug delivery devices. In the past, limited studies have employed 3-D asymmetric models to study the airflow through human lung, although tracheobronchial branching is generally asymmetric. Also limited work has been devoted to the study of particle deposition in upper airways where turbulence is important to particle depositions. It is also known that the asymmetry of the airways has a profound effect on the airflow fields and particle transport and deposition. The present study is concerned with providing a more accurate computational model for lung deposition. A realistic 3-D asymmetric bifurcation representation of human upper tracheobronchial tree has been developed to simulate the airflow field characterizing the inspiratory flow conditions using a turbulence Reynolds stress transport model. A Lagrangian particle trajectory analysis was also used for analyzing particle transport and deposition patterns in the upper tracheobronchial tree.
The Effect of an Active Travelling Wave Wall Motion on the Structure of a Turbulent Boundary Layer
Authors: Vlachos Pavlos P. , Etebari Ali , Pierrakos Olga
Published: September 2008

The topic of drag reduction (DR) in wall-bounded turbulent flows has received great attention over the past thirty years. This effort is motivated by the notion that the internal near wall structure of a turbulent flow can be substantially affected if a force is exerted on the fluid by a traveling wave propagating transversely on an active compliant surface. The work presented here supports this notion by demonstrating that the active skin results in organizing the structures that populate the near wall flow. For the first time, this mechanism is shown to significantly reduce the contribution of the Reynolds stresses to the viscous drag. The modification of the turbulent near wall flow as a function of actuation frequency is investigated and the persistence of the flow control is explored. In addition, the performance of the mechanical active skin actuator is characterized and a first order estimate of its energetics is achieved.
Modeling of Particulate Pressure in the Frame of Mesoscopic Eulerian Formalism for Compressible Reactive Dispersed Two-Phase Flows
Authors: Simoes M. , Simonin O.
Published: September 2008

In space propulsion, compressible reactive dispersed two-phase flows are investigated in order to predict the behavior of solid or liquid rocket motors. In the frame of full Eulerian approach, physical modeling of aerodynamic flows in such motors is performed resolving unsteady compressible Navier-Stokes equations for both phases. However, numerical simulations performed on a simple axisymmetric motor have pointed out a flaw of this basic Eulerian approach. Indeed, the variance of the particle velocity distribution is not accounted for, leading to unrealistic accumulations of particles in some specific flow region. To correct this shortcoming, we have developed an advanced Eulerian model based on a statistical approach in the framework of the Mesoscopic Eulerian Formalism (MEF).
Numerical Simulations of Unsteady Wet Steam Flows With Non-Equilibrium Condensation in the Nozzle and the Steam Turbine
Authors: Senoo Shigeki , White Alexander J.
Published: September 2008

Numerical techniques for non-equilibrium condensing flows are presented. Conservation equations for homogeneous gas-liquid two-phase compressible flows are solved by using a finite volume method based on an approximate Riemann solver. The phase change consists of the homogeneous nucleation and growth of existing droplets. Nucleation is computed with the classical Volmer-Frenkel model, corrected for the influence of the droplet temperature being higher than the steam temperature due to latent heat release. For droplet growth, two types of heat transfer model between droplets and the surrounding steam are used: a free molecular flow model and a semi-empirical two-layer model which is deemed to be valid over a wide range of Knudsen number. The computed pressure distribution and Sauter mean droplet diameters in a convergent-divergent (Laval) nozzle are compared with experimental data. Both droplet growth models capture qualitatively the pressure increases due to sudden heat release by the non-equilibrium condensation. However the agreement between computed and experimental pressure distributions is better for the two-layer model. The droplet diameter calculated by this model also agrees well with the experimental value, whereas that predicted by the free molecular model is too small. Condensing flows in a steam turbine cascade are calculated at different Mach numbers and inlet superheat conditions and are compared with experiments. Static pressure traverses downstream from the blade and pressure distributions on the blade surface agree well with experimental results in all cases. Once again, droplet diameters computed with the two-layer model give best agreement with the experiments. Droplet sizes are found to vary across the blade pitch due to the significant variation in expansion rate. Flow patterns including oblique shock waves and condensation-induced pressure increases are also presented and are similar to those shown in the experimental Schlieren photographs. Finally, calculations are presented for periodically unsteady condensing flows in a low expansion rate, convergent-divergent (Laval) nozzle. Depending on the inlet stagnation subcooling, two types of self-excited oscillations appear: a symmetric mode at lower inlet subcooling and an asymmetric mode at higher subcooling. Plots of oscillation frequency versus inlet sub-cooling exhibit a hysteresis loop, in accord with observations made by other researchers for moist air flow.
Experimental Visualization in a Cyclone Separation System
Authors: Gutie´rrez-Torres C. del C. , Tolentino-Eslava R. , Jime´nez-Bernal J. A.
Published: September 2008

Cyclones are one of the most used equipment for gas cleaning. These devices are widely used because of their simple construction, low energy requirements and their ability to work with high temperature and pressure levels. In this work, a flow visualization of a cyclone separation system is carried out. For that purpose, smoke particles and a laser beam were used to show the flow patterns within the separation system. The objective of this work is to visualize the flow within a cyclone separation system to elucidate the effect of the flow patterns in the gas-particle separation and the optimization of the cyclone.
Onset of Gas Blowthrough During Melt Expulsion From a Hole in a RPV: Numerical Analysis Using the SIMMER Code
Authors: Kretzschmar Frank
Published: September 2008

In the case of a severe accident in a nuclear power plant there is a residual risk, that the Reactor Pressure Vessel (RPV) does not withstand the thermal attack of the molten core material, of which the temperature can be about 3000 K. For the analysis of the processes governing melt dispersal and heating up of the containment atmosphere of a nuclear power plant in the case of such an event, it is important to know the time of the onset of gas blowthrough during the melt expulsion through the hole in the bottom of the RPV. In the test facility DISCO-C (Dispersion of Simulant Corium-Cold) at the FZK /6/, experiments were performed to furnish data for modeling Direct Containment Heating (DCH) processes in computer codes that will be used to extrapolate these results to the reactor case. DISCO-C models the RPV, the Reactor Coolant System (RCS), cavity and the annular subcompartments of a large European reactor in a scale 1:18. The liquid type, the initial liquid mass, the type of the driving gas and the size of the hole were varied in these experiments. We present results for the onset of the gas blowthrough that were reached by numerical analysis with the Multiphase-Code SIMMER. We compare the results with the experimental results from the DISCO-C experiments and with analytical correlations, given by other authors.
Visualization of the Thermal Flow Around a Thin Fin on the Hot Wall of a Differentially Heated Rectangular Cavity
Authors: Xu F. , Patterson J. C. , Lei C.
Published: September 2008

The transient flow around a thin fin on the sidewall in a differentially heated cavity is visualized using a shadowgraph technique. The transition of the flow from sudden heating to a quasi-steady state may be classified into three stages: an early stage, a transitional stage and a quasi-steady stage. Such classifications are similar to those in the case with a smaller square fin on the sidewall (Xu et al., 2006). However, there are distinct differences of the flow features between the cases with a small square fin and a larger thin fin. In the early stage, a lower intrusion front forms below the fin which induces strong turbulent mixing downstream of the fin and is convected away. In the transitional stage, as the fluid in the cavity is stratified, the flow around the thin fin separates and oscillates. The oscillations break the double-layer structure of the downstream thermal boundary layer adjacent to the sidewall in the quasi-steady stage. It is found that the separation and oscillations of the flow around the thin fin are dependent on the geometry of the fin. The frequency of oscillations has a good linear correlation with the Rayleigh number, but is lower than that of unstable travelling waves in the thermal boundary layer for the case without a fin (approximately 1/2).
3D Acoustic Lagrangian Velocimetry
Authors: Bourgoin M. , Baudet C. , Cartellier A. , Gervais P. , Gagne Y.
Published: September 2008

We report Lagrangian measurements obtained with an acoustic Doppler velocimetry technique. From the Doppler frequency shift of acoustic waves scattered by tracer particles in a turbulent flow, we are able to measure the full three-component velocity of the particles. As a first application, we have studied velocity statistics of Lagrangian tracers in a turbulent air jet at Rλ∼320 and at various distances from the nozzle. The choice of an air jet is motivated by the fact that jets produce a well characterized high level tubulence and open air flows are well suited to simultaneaously achieve classical hot wire Eulerian measurements. Therefore, we are also able to explicitly address the question of the differences between Eulerian and Lagrangian statistics. As Lagrangian tracers we use soap bubbles inflated with Helium which are neutrally buoyant in air and can be assimilated to fluid particles. Velocity statistics are analysed. We show that the Lagrangian autocorrelation decays faster in time than its Eulerian counterpart. Finally we present Lagrangian time velocity increments statistics which, as already reported by previous work, exhibits stronger intermittency than Eulerian velocity increments.
Turbulence Structures in Horizontal Two-Phase Flows Under Counter-Current Conditions
Authors: Sta¨bler Thomas D. , Meyer Leonhard , Schulenberg Thomas , Laurien Eckart
Published: September 2008

In order to improve the multi-dimensional numerical simulation of horizontal two-phase flows, the knowledge of local turbulent quantities is of great importance. In horizontal stratified flows, the denser (first) phase flows as a film beneath the other (second) phase. Under counter-current conditions, the second phase flows into the opposite direction of the first phase. In the present investigations a liquid film flows counter-currently to a gas flow. According to the flow rates of both phases, different flow regimes set in. In supercritical flows (Fr>1), the height of the liquid film increases in flow direction, while it decreases in subcritical flows (Fr<1). For sufficiently high gas flow rates the upper part of the liquid film flows into direction of the gas flow, while the lower part still flows into its initial direction opposite to the gas flow. Only a reduced amount of water reaches the end of the test section. This flow regime is referred to as partially reversed flow. The presented local measurements provide not only the mean and rms-velocities of the liquid film, but also the corresponding Reynolds stresses. Local measurements are carried out at two different positions along the test section for various boundary conditions. Furthermore, the liquid injection height has been varied. The kinematic and turbulent structures of the different flow patterns are presented and compared.
Measuring Flow Velocity and Flow Pattern of a Suspension-Gas Mixture Inside a Twin-Fluid Atomizer
Authors: Schmidt Florian , Mewes Dieter , Lo¨rcher Marc
Published: September 2008

Twin-fluid atomizers are widely used for spray-drying application. The suspension to be dried can be atomized very efficiently if the atomizer is operated at critical conditions. The three-phase flow containing solids, gas and liquid is accelerated inside the atomizer due to a pressure gradient. If the upstream pressure is sufficiently high, a maximum possible mass flow rate is achieved. This operating condition is called “critical”. The velocity of the three-phase flow and the flow pattern in the exit cross section has a major impact on the jet break-up and thus on the spray characteristics. In this experimental work the flow velocity and flow pattern inside the nozzle of the atomizer is measured. A laser-sensor is used to determine the flow velocity via cross-correlation at different operating conditions and positions inside the nozzle. The same sensor is used to measure the flow pattern by analyzing the time dependent laser light absorption of of the flow. The influence of various compositions of the suspension concerning gas volume flow rate and particle concentration on the measured velocities and flow patterns are derived. Higher gas volume flow rates increase the velocities and higher particle concentration have a decreasing influence. For a pure gas-liquid flow the obtained results are in good agreement with a theoretical model. In the exit cross-section plug flow and annular flow is observed depending on the gas volume flow fraction. The particles in the suspension have no significant influence on the flow pattern.
Numerical Simulation of Sand Erosion Phenomena in Rotor/Stator Interaction of Compressor
Authors: Suzuki Masaya , Inaba Kazuaki , Yamamoto Makoto
Published: September 2008

Sand erosion is a phenomenon where solid particles impinging to a wall cause serious mechanical damages to the wall surface. This phenomenon is a typical gas-particle two-phase turbulent flow and a multi-physics problem where the flow field, particle trajectory and wall deformation interact with among others. On the other hand, aircraft engines operating in a particulate environment are subjected to the performance and lifetime deterioration due to sand erosion. Especially, the compressor of the aircraft engines is severely damaged. The flow fields of the compressor have strongly three dimensional and unsteady characters. In order to estimate the deterioration due to sand erosion, the sand erosion simulation for the compressor is required under the consideration of the rotor-stator interaction. In the present study, we apply our three dimensional sand erosion prediction code to a single stage axial flow compressor. We numerically investigate the change of the flow field, the particle trajectories, and the eroded wall shape in the compressor, to clarify the effects of sand erosion.
Experiments and Simulations With Large Gas Bubbles in Mercury Towards Establishing a Gas Layer to Mitigate Cavitation Damage
Authors: Wendel Mark , Felde David , Karnowski Thomas , Riemer Bernard , Ruggles Arthur
Published: September 2008

One option that shows promise for protecting solid surfaces from cavitation damage in liquid metal spallation targets involves introducing an interstitial gas layer between the liquid metal and the containment vessel wall. Several approaches toward establishing such a protective gas layer are being investigated at the Oak Ridge National Laboratory including large bubble injection and methods that involve stabilization of the layer by surface modifications to enhance gas hold-up on the wall or by inserting a porous media. It has previously been reported that using a gas layer configuration in a test target showed an order-of-magnitude decrease in damage for an in-beam experiment. Video images that were taken of the successful gas/mercury flow configuration have been analyzed and correlated. The results show that the success was obtained under conditions where only 60% of the solid wall was covered with gas. Such a result implies that this mitigation scheme may have much more potential. Additional experiments with gas injection into water are underway. Multi-component flow simulations are also being used to provide direction for these new experiments. These simulations have been used to size the gas layer and position multiple inlet nozzles.
Swirling Flow Through Venturi Tubes of Convergent Angle 10.5° and 21°
Authors: Gibson Jeff , Reader-Harris Michael
Published: September 2008

Computational Fluid Dynamics (CFD) was used to compute the effect of two bends in perpendicular planes on the performance of 4-inch Venturi tubes with β = 0.4, 0.6 and 0.75 for water at a Reynolds number of 350,000 and at various distances from the bend. Two types of Venturi tubes were analysed, the first having a standard convergent angle of 21°, the second having a non-standard convergent angle of 10.5°. Good agreement with experiment was obtained. Swirling axisymmetric flows were computed to help interpret experimental data.
Simulation of Open Channel Network Using Finite Element Approach
Authors: Hamedi M. H. , Ghassemi M.
Published: September 2008

This paper presents a finite element method (FEM) for simulating open channel network flows. Using a recursive formula derived from element equations, a relationship for channel junctions is established and a system equation is obtained. The system equation is much smaller in size in comparison with the simultaneous solution approach and its solution provides boundary conditions for single channels. The FEM is applied to solve individual channels, and a double sweep method is employed to save computation time. To demonstrate the potential of the approach, two channel networks were computed. It is shown that the results obtained with the proposed FEM are very close to those given by the widely used Preissmann scheme, indicating that it is an effective method to model flows in open channels.
Natural Convection in a Horizontal Cavity Partially Occupied by a Porous Media Saturated by a Coolant Liquid
Authors: Oueslati Fakhreddine S. , Bennacer Rachid , Sammouda Habib , Belghith Ali
Published: September 2008

The natural convection is studied in a cavity witch the lower half is filled with a porous media that is saturated with a first fluid (liquid), and the upper is filled with a second fluid (gas). The horizontal borders are heated and cooled by uniform heat fluxes and vertical ones are adiabatic. The formulation of the problem is based on the Darcy-Brinkman model. The density variation is taken into account by the Boussinesq approximation. The system of the coupled equations is resolved by the classic finite volume method. The numerical results show that the variation of the conductivity of the porous media influences strongly the flow structure and the heat transfer as well as in upper that in the lower zones. The effect of conductivity is conditioned by the porosity which plays a very significant roll on the heat transfer. The structures of this flow show that this kind of problem with specific boundary conditions generates a complex flow structure of several contra-rotating two to two cells, in the upper half of the cavity.
Deformation of Nitrogen Bubbles in DC Electric Fields
Authors: Chen Feng , Peng Yao , Song Yaozu , Chen Min
Published: September 2008

The deformation of nitrogen bubbles in transformer oil with various DC electric fields was studied experimentally and theoretically. The bubble deformation was visualized by a high-speed digital camera. The major axis of the bubble was elongated along the direction parallel to the applied electric field, with the elongation increasing as the electric field magnitude increased. The electrical Weber number (We) was used to correlate the electric field magnitude and the electric permittivity of the working fluid to the bubble aspect ratio (AR). The experimental results indicate that the bubble aspect ratio increases with increasing We. The total electrical stresses were calculated on an actual bubble shape including the electrostriction stresses.
Upstream Potential Propagation Effects of Unsteady Stator-Rotor Interaction in an Axial Flow Blower: Numerical Analysis and Experimental Validation
Authors: Ferna´ndez Oro J. M. , Argu¨elles Di´az K. M. , Santolaria Morros C. , Ballesteros Tajadura R.
Published: September 2008

The potential effect of the inlet guide vanes blockage is predominant in an axial one-stage configuration when the upstream flow field is considered. In the same way, rotor downstream, the main unsteadiness is provoked by the rotor wakes mixing-out at the machine discharge. Nevertheless, if the gap between the rows is significantly reduced, the stator wakes are not allowed to be mixed out before impinging the rotor blades, so a chopping effect overcomes, stretching and tilting them, and generating wake-wake interactions and new loss sources at the exit. On a similar trend, it is expected that a reduced axial gap allows the potential unsteadiness of the rotor blockage between the blades to be propagated upstream, modulating the flow conditions at the stator passages, and even at vanes leading edge locations. In this paper, the evolution of the rotor potential interaction within the stator passages and up to the vanes leading edge is analyzed. The main goal is placed on the analysis of the propagation, relating the axial distance with the attenuation of those potential mechanisms. A numerical 3D simulation of a complete single stage axial flow blower has been developed and executed using a commercial code that resolves the URANS set of equations. The axial gap between the 13-IGVs stator and the 9-blade rotor has been modified in order to evaluate its influence on the potential distortion propagated upstream of the stator. For the closing of turbulence, a LES scheme with a Smagorinsky-Lilly model is used in the computations. Finally, due to the LES characteristics, a phase-averaged procedure has to be introduced for the simulation post-processing. Complementary, experimental measurements have been carried out over a test rig with modifiable axial gap between the fixed and rotating blade rows. As a matter of fact, pressure transducers were placed all along the machine shroud to capture pressure fluctuations related to potential sources radiated from the rotor blades. These measurements have been analyzed using frequential analysis, which is essential to identify the origin of the flow inlet distortions. The final objective is to complete the rotor-stator interaction scenario both downstream and upstream the stage. Previous works were focused on the downstream conditions and now the upstream potential propagation is studied in detail.
Numerical and Experimental Investigation of the Turbulent Flow and Mixing in a Static T-Mixer
Authors: Schwertfirm Florian , Manhart Michael , Gradl Johannes , Peukert Wolfgang , Christoph-Schwarzer Hans
Published: September 2008

Turbulent mixing plays an important role in chemical engineering, especially when the chemical reaction is fast compared to the mixing time. In this context a detailed knowledge of the flow field, the distribution of turbulent kinetic energy (TKE) and its dissipation rate is important, as these quantities are used for many mixing models. For this reason we conduct two direct numerical simulations (DNS) of a confined impinging jet reactor (CIJR) at Re = 500 and Sc = 1, with and without modelling the feeding pipes. The data is compared with particle image velocimetry (PIV) measurements and it is shown that only the simulation which includes the feeding pipes gives the correct flow field. This flow field is dominated by a stable vortex in the main mixing duct. High intensities of turbulent kinetic energy and dissipation are found in the impingement zone and decrease rapidly towards the exit of the CIJR. In the whole CIJR the turbulence is not in equilibrium. The mixing of the scalar is rapid in the impingement zone, but due to the fast decrease of the turbulent intensities and due to the relaminarisation of the flow the PDFs of the scalar distributions in cross sections of the CIJR take a bimodal form after the impingement zone.
Molecular Simulation of Viscous Flow
Authors: Hamedi M. H. , Ghassemi M.
Published: September 2008

Sometimes the continuum equations of fluid mechanics must be completed by information about processes occurring at microscopic seals. For example, we can consider the formulation of boundary conditions, the resolution of singular solutions of the governing equations, and the determination of constitutive relations. In such situations, molecular dynamics calculations can be simulated at the atomic scale and provide the missing information. We review the numerical procedure and then discuss several recent applications of this technique. We consider the validity of lubrication theory at very short distances, the smoothing of corner singularities for both Newtonian and non-Newtonian fluids, and some molecular aspects of wetting phenomena, particularly contact angle hysteresis.
Homogeneous Nucleation of the Homologous Series of n-Alkanes
Authors: Ghosh David , Bergmann Dirk , Wo¨lk Judith , Strey Reinhard , Tanimura Shinobu , Wyslouzil Barbara E.
Published: September 2008

We measured homogeneous nucleation rates J = N/Δt for the homologous series of n-alkanes from n-heptane to n-decane in the temperature range of 150 K to 220 K using a supersonic Laval nozzle. For each compound we conducted two sets of experiments under identical conditions. First, we obtained the characteristic time 1·10−5 ≤ Δt/s ≤ 4·10−5, the time during which nucleation occurs, by carrying out static pressure trace measurements along the axis of the nozzle. In the second set of experiments, we determined the particle number density 1011 ≤ N/cm−3 ≤ 1012 using Small Angle X-ray Scattering. Consequently, we achieve nucleation rates of 2.5·1015 ≤ J/cm−3s−1 ≤ 1017 in our experiments. Hale’s scaled nucleation model [1,2] shows that our results are consistent with the low pressure measurements carried out by Peters et al. [3], Luitjens et al. [4] and the scaling factors shown by Rusyniak and El-Shall [5]. All condensable species are treated as thermally perfect, calorically imperfect vapors in this work.
Mixed Convection in a Differentially Rotating Annulus
Authors: Lepiller Vale´rie , Yoon Dong-Hyeog , Prigent Arnaud , Yang Kyung-Soo , Mutabazi Innocent
Published: September 2008

We investigate experimentally and numerically the influence of a radial temperature gradient in a vertical rotating cylindrical annulus. The geometrical parameters are fixed with aspect ratio and radii ratio respectively equal to 114 and 0.8. After imposing a Grashof number, the convection cell is destabilized by inertial forces due to the increasing of the rotation of the inner cylinder. Above a critical value of the control parameter, the Reynolds number, a spiral pattern occurs giving rise to a finite extent propagating pattern through a supercritical Hopf bifurcation.
Detached Eddy Simulation of Atmospheric Flow About a Surface Mounted Cube at High Reynolds Number
Authors: Wilson Robert P. , Haupt Sue Ellen , Peltier L. Joel , Kunz Robert F.
Published: September 2008

Flow about a surface mounted blunt body has been a classic test-bed for computational fluid dynamics (CFD). It demonstrates separation, vortex shedding, and stationary vortical structures, all of which are difficult to model well. There has been recent interest in pushing such CFD computations to higher Reynold’s number (Re) that more closely match atmospheric flows for studies involving wind loading on structures, dispersion of contaminant around buildings, and characterizing flow in an urban environment. To that end, this study strives to carefully model flow at atmospheric Re using high fidelity grid resolution and best and state-of-the-art techniques, then to systematically relax the grid and modeling assumptions to characterize the fidelity necessary to reproduce certain features of the high Re flow.
Destabilization of the Taylor Vortex Flow by a Radial Temperature Gradient
Authors: Lepiller Vale´rie , Hwang Jong-Yeon , Prigent Arnaud , Yang Kyung-Soo , Mutabazi Innocent
Published: September 2008

Both experimental and numerical studies have shown that the Taylor vortices are destabilized by a weak radial temperature gradient and transit to spiral vortices with a small inclination. For a large radial temperature gradient, from Taylor vortices emerges a disordered pattern with some windows of spiral vortices. Spatio-temporal characteristics of resulting pattern are presented.
Characterisation of Flow and Heat Transfer Regimes in Various Horizontal Ducts Submitted to Mixed Convection
Authors: Abid C. , Medale M. , Koffi F. , Papini F. , Benderradji A.
Published: September 2008

The emphasis of this communication is to make a synthesis of several results we have obtained in various mixed convection configurations. This study has been conducted for circular or rectangular ducts submitted to different ways of heating (vertical or horizontal thermal gradient in the rectangular case and combined vertical and horizontal in the circular case). The bibliography is rather poor for mixed convection in liquids, so the chosen working fluid used here is water. Moreover, a wide range of forced fluid flow and heat flux rates has been considered spreading from laminar to turbulent flow. The characterization of fluid flow and heat transfer regimes is based on temporal recording of temperature measurements obtained in several locations by means of thermocouples or infrared thermography. The analysis of these temperature signals highlights several regimes depending on control parameters. The flow structure in the cases of uniformly heated circular duct and the rectangular one heated from below is constituted of two longitudinal rolls and we notice only one roll in the case of the rectangular duct submitted to the horizontal thermal gradient. For low Reynolds and Rayleigh Numbers, the behavior of all these configurations is stable, however the increasing of these parameters induces thermal instability in the case of circular and rectangular ducts heated from below. That means that the thermal vertical gradient is responsible of the occurring of the thermal instability. This result shows that the horizontal thermal gradient is a stabilizing gradient while the vertical one is a destabilizing one. As this instability enhances heat transfer, it will be very helpful to characterize and to identify the domain where it is occurring in order to prevent or to provoke it depending on the expected performance of the heat exchanger. In this paper, we propose to establish a diagram showing the domain of occurrence of this instability for the various cases cited above and to describe the fluid flow and heat transfer associated to these configurations.
Hydrodynamics of a Fluorine Electrolyser
Authors: Caire J. P. , Espinasse G. , Nicolas F. , Peyrard M.
Published: September 2008

Two dimensional modelling of the gas-liquid free convection in a fluorine electrolyser was made with use of the Estet-Astrid finite volume code. The model takes into account the specific behaviour of hydrogen and fluorine bubbles evolving at electrodes in free convection in a KF-2HF electrolyte. The numerical results give a satisfactory representation of the velocity field which could not be visualized or measured in such an aggressive medium.
Simulation of the Flow Condition Near the Intake of a Pump Sump
Authors: Liu Shuhong , Li Yong , Luo Xianwu , Wu Yulin
Published: September 2008

It is known that the flow condition in the pump sump is very complicated, which usually performs as several types of vortex, water wave of free surface, vibration, noise, and etc. To make clear the flow condition experimentally and numerically is very important to develop the performance and operating stability of the pump station. As one of the projects with Hitachi Industries ltd. Co., Japan, the investigation experimentally on the internal flow condition of the pump sump has been carried out in Tsinghua University. In this paper, we introduce the simulation results, which can show more detail information near the intake of the pump sump. The simulation is focused on the area near the intake, together with the extension of up-stream and down-stream. The calculation research includes two parts: steady simulation and unsteady simulation by VOF model, which is provided by the commercial software of Fluent. Through the steady simulation, the distribution of free water surface (water height) near the intake, as well as the flow condition inside the intake, were obtained. Comparing with the experimental data, a good agreement was observed. After analyzing the unsteady calculation results by VOF, four stages of the development of air-entraining vortex, and the wave characteristic of free water surface, were obtained, which were in accord with the experiment visualizing.
Numerical 3D Simulation of a Longitudinal Ventilation System: Memorial Tunnel Case
Authors: Galdo-Vega Monica , Ballesteros-Tajadura Rafael , Santolaria-Morros Carlos
Published: September 2008

In this work, a numerical 3D simulation of a longitudinal ventilation system is developed to analyze the fire behavior inside a road tunnel. Recent disasters, like crashes in the Mont Blanc tunnel (France, 1999) or San Gottardo (Italy, 2001), have shown the need for better integral actions during possible fire incidents. The minimum delay time, required for starting the jet fans, or the evolution of the smoke patterns inside the tunnel are critical issues when rescue plans are designed. Some methods to study the smoke propagation during a fire are: pseudo-thermal scale models, full scale test and numerical models. Several contributions using the first method can be found in references [1], [2] and [3]. However it is very difficult to extrapolate the results from this kind of models. The second method (full scale test) is the most expensive of all and only two of them have been conducted recently: EUREKA Project [4] and the Memorial Tunnel Fire Ventilation Test Program [5]. The last method (numerical models) it is now under development. The objective of this work is to validate a numerical model, to predict the behavior of the smoke generated during a fire incident inside a road tunnel, comparing its results with previous experimental data collected in the Memorial Tunnel Project. In addition, a good agreement was achieved, so a methodology to predict the performance of a longitudinal ventilation system in case of fire was accurately established.
Modeling Flow Across a Race Car Air Filter
Authors: Binns Robert , Shollenberger Kim A.
Published: September 2008

Air entering a NASCAR stock car travels through an air filter enclosed in an “airbox” before being mixed with fuel. To maximize the mass of air/fuel mixture delivered to the cylinders, it is desired to minimize airflow restrictions. This paper presents a preliminary model to predict pressure drop and airflow across an air filter in a simplified geometry. The model includes experimental measurements and numerical simulations using a three-dimensional finite-volume package. The simplified geometry is a 1.52-m long flow channel with 12.7 cm by 7.97 cm cross-section. Measurements of pressure drop versus air velocity were made for separate and combined elements of the filter. A curve fit to the data is used to calculate coefficients for a porous-jump boundary condition used to model flow through porous media. Numerical simulations were run using this model for each filter element in the simplified geometry to verify that measured and calculated pressure drops agreed. Finally, flow visualization techniques were used to reveal streaklines in the flow. Good agreement was found between calculated streamlines and experimental observations. Thus, the porous jump boundary condition is a valid model for the filter material. Future work will include three-dimensional, transient numerical simulations of airflow within the airbox.
Comparison Between Euler-Euler and Euler-Lagrange Computations of Gas-Solid Turbulent Flow in a Horizontal Channel With Different Wall Roughness
Authors: Konan N. A. , Lai´n S. , Simonin O. , Sommerfeld M.
Published: September 2008

This paper presents practical eulerian rough wall boundary conditions for particle mean velocity and fluctuating components. The model derives from a pdf approach and the Sommerfeld particle-wall rebound theory [1]. We have implemented these boundary conditions in 2D Euler/Euler code “Me´lodif” by neglecting the “shadow effect” [2] and assuming an half-gaussian distribution for wall-normal incident particle velocities. Lagrangian calculations in which the particle-wall collisions are treated following Sommerfeld [1], are carried out. Low and high roughness cases are simulated and both simulations results are compared with Sommerfeld experimental database.
Heat and Mass Transfers in a Heated Concrete Element: 20 to 600°C
Authors: Noumowe A. , de Morais M. V. G. , Kanema M. , Gallias J. L. , Cabrillac R.
Published: September 2008

The aim of this work is to carry out a numerical study on the coupled heat and mass transfers in a concrete element exposed to elevated temperature in order to explain the behaviour observed during experimental studies. Comparisons were drawn on numerical and experimental results on the thermo-hydrous behavior of a concrete element. Parametric analyses were carried out in order to underline main parameters involved in concrete behavior at high temperature. The numerical and experimental results included thermal gradient, water vapor pressure, relative humidity, concrete mass losses due to dehydration, water content for a concrete element heated from 20 to 600 °C. The results show high thermal gradients and high vapor pressure in the concrete element in addition to the damage due to concrete chemical transformations at high temperature.
Rough Wall Boundary Condition Derivation for Particle Continuum Equations: Validation From LES/DPS of Gas-Solid Turbulent Channel Flow
Authors: Konan N. A. , Simonin O. , Squires K. D.
Published: September 2008

This paper presents a theoretical approach leading to the establishment of practical rough wall boundary conditions within the framework of continuum approach for the dispersed phase simulation in two-phase flows. The study is based on a similar statistical description of the rebounds on the wall adopted by Sakiz et al. [1] and take into account “the shadow effect” as proposed by Sommerfeld et al [2]. The validity of these eulerian rough wall boundary condition formulations is checked by carrying out LES of carrier phase flow in channel and coupled with DPS, with a given wall roughness standard deviation.
Circulation of Vortex Rings Formed From Tube and Orifice Openings
Authors: Krueger Paul S.
Published: September 2008

Two common configurations for generating vortex rings via jet pulses are the tube and orifice geometries. The orifice geometry forces the flow to contract as it approaches the jet exit plane, which can strongly affect vorticity flux and the circulation of the resulting ring. The author’s recent extension of the traditional slug model for vortex ring circulation (called the “pressure corrected” or PC model) accounts for the geometric differences between the tube and orifice cases, but model validation for the orifice geometry has been limited due to the lack of data for this configuration. The present study compares process of circulation generation by tube and orifice geometries using numerical simulations of finite duration jets from tube and orifice openings. Total jet slug length-to-diameter ratios (L/D) in the range of 0.5 to 3.5 and a jet Reynolds number of 2000 are considered. The numerical results confirm the underlying assumptions of the PC model. The model results for the tube geometry are within 14% of the numerical results. Incorporating the scaling of ring velocity with ejected jet length (X/D) obtained from the present numerical results improves the predictions for the orifice case, giving accuracy to within 20%. The overall geometry effect appears as a two-fold increase in circulation for the orifice case over the tube case at the same L/D.
Quantitative Investigation of Coherent Structures in a Free Jet Using PIV and POD
Authors: Shinneeb A.-M. , Balachandar R. , Bugg J. D.
Published: September 2008

This paper investigates an isothermal free water jet discharging horizontally from a circular nozzle (9 mm) into a stationary body of water. The jet exit velocity was 2.5 m/s and the exit Reynolds number was 22,500. Particle image velocimetry (PIV) measurements were made at the jet exit and in the far field of the jet. The large-scale structures in the far field were investigated by performing a proper orthogonal decomposition (POD) analysis on the fluctuating velocity fields. The number of modes used for the POD reconstruction of the velocity fields was selected to recover 40% of the turbulent kinetic energy. A vortex identification algorithm was then employed to quantify the size, circulation, and direction of rotation of the exposed vortices. Then, the identified vortex characteristics were analysed by performing a statistical study of their distributions. The results clearly reveal that the jet contains a substantial number of eddies of both rotation directions. Moreover, the number of vortices decreases in the axial direction while their size increases. The mean circulation magnitude is preserved in the axial direction. The results indicate that larger vortices have a higher value of circulation while smaller values of circulation are associated with smaller eddies.
Effects of the Inlet Boundary Layer Thickness on the Rotating Stall in an Axial Compressor
Authors: Choi Minsuk , Baek Je Hyun , Oh Seong Hwan , Ki Dock Jong
Published: September 2008

A three-dimensional numerical simulation is conducted to study an effect of the inlet boundary layer thickness on the rotating stall in an axial compressor. The inlet boundary layer thickness has significant effects on the hub-corner-separation in the junction of the hub and suction surface. As the load is increased, the size of the hub-corner-separation is increased dramatically for the thick inlet boundary layer but it is diminished to be indistinguishable from the wake of the blade for the thin inlet boundary layer. The difference induced by different inlet conditions at high load should have affected characteristics of the rotating stall such as the inception process and propagation speed of the stall cells. For two cases of different inlet boundary layers, the numerical simulation is progressed as the flow coefficient is decreased until the rotating stall begins and then effects of the inlet boundary layer thickness on the rotating stall are analyzed by using the axial velocity history and the rotary total pressure distribution. For the thick inlet boundary layer, a pre-stall disturbance arises firstly in the hub-corner-separation and then in the tip leakage flow as the load is increased. For the thin inlet boundary layer, however, an asymmetric disturbance is initially generated in the tip region because of the corner-separation in the junction of the casing and suction surface. The disturbance of the tip leakage flow grows to be a stationary stall cell which is adhered to the blade passage by throttling process in case of the thick inlet boundary layer. When the stationary stall cell reach a critical size, this cell moves along the blade row and becomes a short-length-scale rotating stall. However, the rotating stall is not found at a smaller flow rate for the thin inlet boundary layer because the flow in the tip region is more energetic than that of its counterpart. In addition, it is found that the inlet boundary layer thickness has an effect on the cause of the initial disturbance which collapses the axi-symmetric flow under high load and the internal flow with a thick boundary layer on the casing is susceptible to the rotating stall.
Acoustic Cavitation Thermodynamic Modeling in Transmission Pipelines by an Implicit Conservative High-Resolution Numerical Algorithm
Authors: Catania A. E. , Ferrari A. , Manno M.
Published: September 2008

A general treatment of acoustic cavitation for the case of one-dimensional pipe flows is presented, including both fluid dynamics instabilities, which can occur at cavitation inception, and non-equilibrium effects during bubble dynamics. Different approaches to cavitation modelling are also considered and compared. A novel barotropic cavitation model has been developed, based on the partial differential equations governing the mass conservation and momentum balance. The analytical expression for the vapour source term driving cavitation has been carried out by means of the energy conservation equation and a general formula for the sound speed in homogeneous bubbly flows has been used. A newly developed high-resolution, conservative, implicit, second-order accurate numerical scheme was applied to solve the Euler’s hyperbolic equations governing the pipe flow. It gave reduced oscillation problems at the discontinuities that were induced by cavitation. The resultant computational model was assessed through its application to a literature test-case, which involved a pipe connecting two constant-pressure reservoirs, water being the working fluid. The prediction outcomes were discussed so as to underline the most interesting fluid-dynamic phenomena, such as the dynamics of shock waves arising at cavitation collapse. The influence of the frequency-dependent friction model on the simulation of the pressure wave dynamics in the presence of cavitation was also analysed.