bluuebunny/crossref_metadata_2025 huggingface dataset:
A Holistic Optimization of Convecting-Radiating Fin Systems
Authors: Sasikumar M. , Balaji C.
Published: December 2002

A convecting-radiating fin array, which stands vertically outside of a horizontal rectangular duct, has been analyzed for various design constraints. Fully developed turbulent flow is considered inside the duct. This study takes into account the variation of fluid temperature along the duct, which has been ignored in most of the earlier studies. The one-dimensional governing equation for temperature distribution along the fin is solved for all the fins of the fin array and the total heat transfer rate per unit system mass, total entropy generation rate and optimum fin height based on maximum heat transfer rate per unit system mass are evaluated from the derived temperature profiles. These quantities are then correlated as functions of geometric and flow parameters for three types of fin profile. Optimum solutions are generated based on (i) maximum heat dissipation rate per unit system mass and (ii) minimum entropy generation rate. A procedure to combine these two optima in order to obtain a “holistic” optimum is also discussed.
A Numerical Study of Buoyant Plane Parallel Jets
Authors: Spall Robert E.
Published: December 2002

A numerical study was performed to assess the influence of buoyancy on plane, parallel jets. Results indicate that, relative to isothermal jets, the location along the vertical symmetry plane at which the two jets merge (the merge point) decreases with increasing jet inlet temperature. This decrease is attributed to higher entrainment rates for the heated jet relative to the isothermal jet. It was also found that for sufficiently high values of the Archimedes number, the merge point becomes nearly independent of the initial jet spacing.
Simulation of Laminar Slot Jets Impinging on a Moving Surface
Authors: Chattopadhyay Himadri , Saha Sujoy K.
Published: December 2002

Laminar flow and heat transfer on a moving surface due to a bank of impinging slot jets have been numerically investigated. Two types of jet, namely axial and knife-jet with an exit angle of 60 deg were considered. The surface velocity up to two times the jet velocity at the nozzle exit was imposed on the impinging surface. It has been observed that while with increasing velocity of the impinging surface, the total heat transfer reduces; the distribution pattern becomes more uniform. For the same amount of mass and momentum flux at the nozzle exit, heat transfer from the axial jet is considerably higher than that from the vectored jets at all surface velocities considered. It was found that the local heat transfer over the surface for the case of the axial jet and the knife-jet scales with Re0.5 and Re0.55, respectively.
Modeling and Direct Simulation of Velocity Fluctuations and Particle-Velocity Correlations in Sedimentation
Authors: Cunha F. R. , Abade G. C. , Sousa A. J. , Hinch E. J.
Published: December 2002

In this paper we present direct numerical simulations of monodisperse and polydisperse suspensions of non-Brownian particles sedimenting at low Reynolds number. We describe a scheme to generate ergodic ensembles of random particulate systems and a numerical procedure for computing interactions among spherical particles based on Ewald summation technique for hydrodynamic mobility tensors. From the generation process truly random both monodisperse and multimodal size distributions of particles were obtained for dilute and moderate densities based on a minimum energy criterion. Concerned with computations of the Ewald sums our numerical procedure drastically reduces the CPU simulation time providing results of the hindered settling function in good agreement with available experimental data and asymptotic results for ordered and random periodic arrays of particles. We show new computer simulations with no flux boundary perpendicular to gravity and periodic boundary conditions in horizontal direction. The simulations reproduce the experimental correlation-time and anisotropy of the velocity fluctuations, but have the magnitude of these fluctuations increasing proportional to the size of the system.
Structure Inclination Angle in a Turbulent Adverse Pressure Gradient Boundary Layer
Authors: Krogstad Per-A˚ge , Kaspersen Jon Harald
Published: December 2002

The effect of a strong adverse pressure gradient on the turbulent structure in an equilibrium boundary layer was studied using two-point space-time correlations. The ρuu correlations from the adverse pressure gradient layer were found to be inclined at a considerably higher angle with respect to the wall in the inner region than in the corresponding zero pressure gradient reference case. A wavelet analysis of the correlation data showed that differences in the inclination angle exist at all length scales larger than the boundary layer thickness. The inclination angle for the conventionally averaged correlation contours was found to be in close agreement with the inclinations derived from the wavelet filtered contours corresponding to the most energetic wave numbers.
Multidimensional Diagnostics of Turbine Cavitation1
Authors: Bajic Branko
Published: December 2002

A novel technique for vibro-acoustical diagnostics of turbine cavitation is introduced and its use demonstrated on a Francis turbine. The technique enables identification of different cavitation mechanisms functioning in a turbine and delivers detailed turbine cavitation characteristics, for each of the mechanisms or for the total cavitation. The characteristics specify the contribution of every critical turbine part to the cavitation intensity. Typical diagnostic results: (1) enable optimization of turbine operation with respect to cavitation erosion; (2) show how a turbine’s cavitation behavior can be improved; and (3) form the basis for setting up a high-sensitivity, reliable cavitation monitoring system.
Thermal Damping in Cavitating Nozzle Flows
Authors: Delale Can F.
Published: December 2002

Recent investigations of bubbly cavitating nozzle flows using the polytropic law for the partial gas pressure have shown flow instabilities that lead to flashing flow solutions. Here, we investigate the stabilizing effect of thermal damping on these instabilities. For this reason we consider the energy equation within the bubble, assumed to be composed of vapor and gas, in the uniform pressure approximation with low vapor concentration. The partial vapor pressure is fixed by the vapor saturation pressure corresponding to the interface temperature, which is evaluated by assuming the thin boundary layer approximation within the liquid. Consequently, the partial gas pressure is evaluated by its relation to the heat flux through the interface in the uniform pressure approximation. The model is then coupled to the steady-state cavitating nozzle flow equations replacing the polytropic law for the partial gas pressure. The instabilities found in steady cavitating nozzle flows are seen to be stabilized by thermal damping with or without the occurrence of bubbly shock waves.
Effect of Swirl on Rotordynamic Forces Caused by Front Shroud Pump Leakage
Authors: Hsu Yun , Brennen Christopher E.
Published: December 2002

Unsteady forces generated by fluid flow through the impeller shroud leakage path of a centrifugal pump were investigated. The effect of leakage path inlet swirl (pump discharge swirl) on the rotordynamic forces was re-examined. It was observed that increasing the inlet swirl is destabilizing both for normal and tangential rotordynamic forces. Attempts to reduce the swirl within the leakage path using ribs and grooves as swirl brakes showed benefits only at low leakage flow rate.
Mach Number Influence on Reduced-Order Models of Inviscid Potential Flows in Turbomachinery
Authors: Epureanu Bogdan I. , Dowell Earl H. , Hall Kenneth C.
Published: December 2002

An unsteady inviscid flow through a cascade of oscillating airfoils is investigated. An inviscid nonlinear subsonic and transonic model is used to compute the steady flow solution. Then a small amplitude motion of the airfoils about their steady flow configuration is considered. The unsteady flow is linearized about the nonlinear steady response based on the observation that in many practical cases the unsteadiness in the flow has a substantially smaller magnitude than the steady component. Several reduced-order modal models are constructed in the frequency domain using the proper orthogonal decomposition technique. The dependency of the required number of aerodynamic modes in a reduced-order model on the far-field upstream Mach number is investigated. It is shown that the transonic reduced-order models require a larger number of modes than the subsonic models for a similar geometry, range of reduced frequencies and interblade phase angles. The increased number of modes may be due to the increased Mach number per se, or the presence of the strong spatial gradients in the region of the shock. These two possible causes are investigated. Also, the geometry of the cascade is shown to influence strongly the shape of the aerodynamic modes, but only weakly the required dimension of the reduced-order models.
Reduced-Order Modeling of Unsteady Flows About Complex Configurations Using the Boundary Element Method
Authors: Esfahanian V. , Behbahani-nejad M.
Published: December 2002

An approach to developing a general technique for constructing reduced-order models of unsteady flows about three-dimensional complex geometries is presented. The boundary element method along with the potential flow is used to analyze unsteady flows over two-dimensional airfoils, three-dimensional wings, and wing-body configurations. Eigenanalysis of unsteady flows over a NACA 0012 airfoil, a three-dimensional wing with the NACA 0012 section and a wing-body configuration is performed in time domain based on the unsteady boundary element formulation. Reduced-order models are constructed with and without the static correction. The numerical results demonstrate the accuracy and efficiency of the present method in reduced-order modeling of unsteady flows over complex configurations.
The Approximate Deconvolution Model for Large-Eddy Simulation of Compressible Flows With Finite Volume Schemes
Authors: von Kaenel R. , Adams N. A. , Kleiser L. , Vos J. B.
Published: December 2002

The approximate deconvolution model for large-eddy simulation is formulated for a second-order finite volume scheme. With the approximate deconvolution model, an approximation of the unfiltered solution is obtained by repeated filtering, and given a good approximation of the unfiltered solution, the nonlinear terms of the Navier-Stokes equations are computed directly. The effect of scales not represented on the numerical grid is modeled by a relaxation regularization involving a secondary filter operation. A turbulent channel flow at a Mach number of M=1.5 and a Reynolds number based on bulk quantities of Re=3000 is selected for validation of the approximate deconvolution model implementation in a finite volume code. A direct numerical simulation of this configuration has been computed by Coleman et al. Overall, our large-eddy simulation results show good agreement with our filtered direct numerical simulation data. For this rather simple configuration and the low-order spatial discretization, differences between approximate deconvolution model and a no-model computation are found to be small.
Coarse Resolution Turbulence Simulations With Spectral Vanishing Viscosity—Large-Eddy Simulations (SVV-LES)
Authors: Kirby Robert M. , Karniadakis George Em
Published: December 2002

We present a new implementation of the spectral vanishing viscosity method appropriate for alternative formulations of large-eddy simulations. We first review the method and subsequently present results for turbulent incompressible channel flow.
Ice Block Melting Into a Binary Solution: Coupling of the Interfacial Equilibrium and the Flow Structures
Authors: Mergui Sophie , Geoffroy Sandrine , Be´nard Christine
Published: December 2002

Melting of a vertical ice block in a cavity filled with an aqueous solution is studied experimentally. Local interfacial temperatures and front velocities are measured allowing for a quantitative study of the coupling of the fluid motion, thermal equilibrium and the local heat transfers at the melting front. It is found that the front equilibrium shift is correlated to the classical parameter of the phase change process, that is the Stefan number, while the local heat transfer at the interface is correlated to the parameters characterizing thermosolutal natural convection. Quantitative results about the time evolution of the double-diffusive multi-layers structure in the fluid phase are obtained. The formation of the first thermosolutal layer is analyzed with the help of numerical simulations. It is found that the mechanism responsible for the onset of this layer is due to a well known double-diffusive instability.
Theoretical Prediction of the Soil Thermal Conductivity at Moderately High Temperatures
Authors: Gori Fabio , Corasaniti Sandra
Published: December 2002

The theoretical model of the present paper assumes the unit cell of the porous medium as composed of a cubic space with a cubic solid particle at the center. The thermal conductivity is evaluated by solving the heat conduction equation with the assumption of parallel isotherms within the cubic space. The liquid water in the porous medium is distributed around the solid particle according to the phenomena of adsorption and capillarity. The thermal conductivity of the gas present within the pores takes into account the thermal conductivity of the water vapor and dry air, without enhanced vapor diffusion. The model simplifies the variation of the relative humidity, from dryness to the field capacity, with a linear increase. The predicted results, compared to experimental data, show the agreement is very good at the temperatures in the range (30–50°C) and acceptable at 70°C. At high temperature (90°C) the predictions are higher than the experiments and a better agreement could be obtained by decreasing the thermal conductivity of the gas phase. Besides, the trend of the theoretical predictions is in good agreement with the experiments also at high temperatures.
The Application of an Inverse Formulation in the Design of Boundary Conditions for Transient Radiating Enclosures
Authors: Ertu¨rk Hakan , Ezekoye Ofodike A. , Howell John R.
Published: December 2002

This study considers the design of thermal systems that are built to radiatively heat objects from a specified initial condition to a specified steady state following a prescribed temperature history. The enclosure housing the object, the object itself, and the heaters all have thermal capacity. The necessary power input distributions for the heaters during the heating process are sought to satisfy the design specifications. The problem is thus a transient inverse boundary condition estimation problem, where the geometry and the properties of the surfaces are specified and the boundary condition on the heater wall is to be found by making use of the information provided at the design surface for each time step. The boundary condition estimation problem requires the solution of a set of Fredholm equations of the first kind. Such a problem is known to be ill-posed. The introduction of the transient nature makes the inverse problem nonlinear and even more interesting, challenging, and realistic. A solution algorithm is proposed and used to produce a solution for a sample problem. In order to model radiative heat transmission, the Monte Carlo method is used, which enables us to handle specularly reflecting surfaces and blockage effects. The inverse problem is solved by the conjugate gradient method, which provides smooth and accurate results after the first few steps.
Heat Transfer Enhancement by Delta-Wing-Generated Tip Vortices in Flat-Plate and Developing Channel Flows
Authors: Gentry M. C. , Jacobi A. M.
Published: December 2002

Using delta wings placed at the leading edge of a flat plate, streamwise vortices are generated that modify the flow; the same wings are also used to modify a developing channel flow. Local and average measurements of convection coefficients are obtained using naphthalene sublimation, and the structure of the vortices is studied using flow visualization and vortex strength measurements. The pressure drop penalty associated with the heat transfer enhancement of the channel flow is also investigated. In regions where a vortex induces a surface-normal inflow, the local heat transfer coefficients are found to increase by as much as 300 percent over the baseline flow, depending on vortex strength and location relative to the boundary layer. Vortex strength increases with Reynolds number, wing aspect ratio, and wing attack angle, and the vortex strength decays as the vortex is carried downstream. Considering the complete channel surface, the largest spatially averaged heat average heat transfer enhancement is 55 percent; it is accompanied by a 100 percent increase in the pressure drop relative to the same channel flow with no delta-wing vortex generator.
Combined Radiation and Conduction in Glass Foams
Authors: Varady Mark J. , Fedorov Andrei G.
Published: December 2002

Understanding of heat transfer in glass foams and the development of theoretical tools for predicting heat transfer properties of glass foams is critical to improving the efficiency of glass manufacturing. In this paper, combined radiation and conduction heat transfer in a semitransparent glass foam layer is analyzed. The foam layer is thin and of the uniform thickness, bounded by hot combustion gases on top and glass melt on bottom, and exposed to isotropic radiation originating from hot refractories. Heat transfer is assumed to be one-dimensional perpendicular to the plane-parallel foam layer. A previously developed model is used to calculate effective extinction coefficients and scattering phase function of the foam layer using a void size distribution and assuming all voids to be spherical. These radiation properties are then used along with a Schuster-Schwarzchild two-flux approximation to solve the radiative transfer equation (RTE). A method for obtaining the effective thermal conductivity of the foam layer is also presented. The RTE and the energy conservation equations are simultaneously solved using a numerical iteration procedure. The effect of foam thickness and bubble size on the temperature distribution in the foam layer is studied.
A Fractal Model for Nucleate Pool Boiling Heat Transfer
Authors: Yu Boming , Cheng Ping
Published: December 2002

In this paper, a fractal model for nucleate pool boiling heat transfer is developed based on the fractal distribution of sites (areas) of nucleation sites on boiling surfaces. Algebraic expressions for the fractal dimension and area fraction of nucleation sites are derived, which are shown to be a strong function of wall superheat. The predicted fractal dimension is shown in good agreement with those determined by the box-counting method. The fractal model for nucleate boiling heat transfer is found to be a function of wall superheat, the contact angle of the fluid and the heater material, and physical properties of the fluid with a minimum number of empirical constants. The predicted total heat flux from a boiling surface based on the present fractal model is compared with existing experimental data. An excellent agreement between the model predictions and experimental data is found, which verifies the validity of the present fractal model.
The Relationship Between Information, Sampling Rates, and Parameter Estimation Models
Authors: Emery A. F. , Blackwell B. F. , Dowding K. J.
Published: December 2002

To estimate parameters from experiments requires the specification of models and each model will exhibit different degrees of sensitivity to the parameters sought. Although experiments can be optimally designed without regard to the experimental data actually realized, the precision of the estimated parameters is a function of the sensitivity and the statistical characteristics of the data. The precision is affected by any correlation in the data, either auto or cross, and by the choice of the model used to estimate the parameters. An informative way of looking at an experiment is by using the concept of Information. An analysis of an actual experiment is used to show how the information, the optimal number of sensors, the optimal sampling rates, and the model are affected by the statistical nature of the signals. The paper demonstrates that one must differentiate between the data needed to specify the model and the precision in the estimated parameters provided by the data.
Cavitation in Hydraulic Tools Based on Thermodynamic Properties of Liquid and Gas
Authors: Iben U. , Wrona F. , Munz C.-D. , Beck M.
Published: December 2002

The simulation of cavitation phenomena plays an important role for development of modern hydraulic tools and injection systems. Cavitation leads to a reduction of mass flow and influences the wave motion in hydraulic components significantly. The article deals with the simulation of a homogeneous cavitation model based on thermodynamic properties of the liquid and steam to understand basic physical phenomena.
Air Entrainment Effects on the Pressure Transients of Pumping Systems With Weir Discharge Chamber
Authors: Lee T. S. , Ngoh K. L.
Published: December 2002

This paper presents a new model for the study of air entrainment on the extreme pressure surges in pumping systems. For the present fluid system considered with a weir discharge chamber, numerical investigations showed that, with the proposed model of the air entrainment, reasonable predictions of transient pressures with proper phasing and attenuation of pressure peaks can be obtained. The results obtained are consistent with observations from field measurements made when the pumps were operating at low pump cutout levels where air entrainment due to attached surface vortex and falling jets from the inflow near the pump intake were present. Further studies were also made on the design characteristics of the weir discharge chamber on the extreme pressure transients for the unsteady flow in the pipeline system with various degrees of air entrainment.
Subgrid-Scale Modeling of Turbulent Convection Using Truncated Navier-Stokes Dynamics
Authors: Domaradzki J. A. , Radhakrishnan S.
Published: December 2002

Using concepts from the subgrid-scale estimation modeling we develop a procedure for large-eddy simulations which employs Navier-Stokes equations truncated to an available mesh resolution. Operationally the procedure consists of numerically solving the truncated Navier-Stokes equation and a periodic processing of the small scale component of its solution. The modeling procedure is applied to simulate turbulent Rayleigh-Be´nard convection.
Pressure Drops of Water Flow Through Micromachined Particle Filters
Authors: Hsiai Tzung K. , Kwon Cho and Sung , Mo Yang Joon , Yang and Xing , Tai Yu-Chong , Ho Chih-Ming
Published: December 2002

When the particle is in the order of microns, flow through the small opening produces a large velocity gradient, leading to high viscous dissipation. Understanding the flow field is critical in determining the power requirement. In this paper, we studied water flow through filters fabricated by micro-electro-mechanical system (MEMS) techniques. The pressure drop calculated by a three-dimensional numerical code of the Navier-Stokes equations is in a resonable agreement with the experimental data if the diameter and the side wall profile of the holes are measured with high accuracy.
Implicit Turbulence Modeling for High Reynolds Number Flows
Authors: Margolin L. G. , Smolarkiewicz P. K. , Wyszogrodzki A. A.
Published: December 2002

Implicit turbulence modeling is the numerical simulation of high Reynolds fluid flow using nonoscillatory finite volume (NFV) schemes without any explicit subgrid scale model. Here we investigate the ability of a particular NFV scheme, MPDATA, to simulate decaying turbulence in a triply periodic cube for a variety of viscosities, comparing our results to analogous pseudo-spectral studies. In the regime of direct numerical simulation, MPDATA is shown to agree closely with the pseudo-spectral results. As viscosity is reduced, the two model results diverge. We study the MPDATA results in the inviscid limit, using a combination of mathematical analysis and computational experiment. We validate these results as representing the turbulent flow in the limit of very high Reynolds number.
Turbulence and Phase Distribution in Bubbly Pipe Flow Under Microgravity Condition
Authors: Chahed J. , Colin C. , Masbernat L.
Published: December 2002

The role of the turbulence in the void fraction distribution in bubbly pipe flow under microgravity condition is evaluated on the basis of numerical simulations using a Eulerian-Eulerian two-fluid model. In microgravity, the average relative velocity is weak and the void fraction distribution is mainly governed by the turbulence. The simulations show that the turbulent contributions of the added mass force play an important role in the phase distribution phenomenon. It is clearly proved that the turbulence acts on the bubbles distribution not only by the pressure term but also by the turbulent correlations obtained by averaging the added mass force.
Effect of Gap Size on Tip Leakage Cavitation Inception, Associated Noise and Flow Structure
Authors: Gopalan Shridhar , Katz Joseph , Liu Han L.
Published: December 2002

This paper focuses on the onset of tip-leakage cavitation on a fixed hydrofoil. The objectives are to investigate the effect of gap size on the flow structure, conditions of cavitation inception, the associated bubble dynamics and cavitation noise. The same hydrofoil with three tip gap sizes of 12%, 28%, and 52% of the maximum tip thickness are studied. Controlled cavitation tests are performed after de-aerating the water in the tunnel and using electrolysis to generate cavitation nuclei. The experiments consist of simultaneously detecting cavitation inception using a 2000 fps digital camera (visual) and two accelerometers (“acoustic”) mounted on the test section windows. Good agreement between these methods is achieved when the visual observations are performed carefully. To obtain the time-dependent noise spectra, portions of the signal containing cavitation noise are analyzed using Hilbert-Huang transforms. Rates of cavitation events as a function of the cavitation index (σ) for the three gap sizes are also measured. The cavitation inception index decreases with increasing gap sizes. The experiments demonstrate that high-amplitude noise spikes are generated when the bubbles are distorted and “shredded”—broken to several bubbles following their growth in the vortex core. Mere changes to bubble size and shape caused significantly lower noise. High-resolution particle image velocimetry (PIV) with a vector spacing of 180 μm is used to measure the flow, especially to capture the slender tip vortices where cavitation inception is observed. The instantaneous realizations are analyzed to obtain probability density functions of the circulation of the leakage vortex. The circulation decreases with increasing gap sizes and minimum pressure coefficients in the cores of these vortices are estimated using a Rankine model. The diameter of the vortex core varied between 540–720 μm. These coefficients show a very good agreement with the measured cavitation inception indices.
Turbulence Control With Particle Image Velocimetry in a Backward-Facing Step1
Authors: Piirto Mika , Saarenrinne Pentti , Eloranta Hannu
Published: December 2002

A new concept of particle image velocimetry (PIV) for turbulence control is introduced. The PIV method is used to measure the flow and supply information on the turbulence quantities on-line for a control station connected to an actuator for flow manipulation. With this “closed-loop” system of PIV and PID controllers, it is possible to control increase or decrease turbulence quantities or length scales. Special techniques for the on-line measurement are developed. The mean velocity is computed with a moving time average operator and the Reynolds decomposition is applied for the calculation of the Reynolds stresses, velocity fluctuations, or other instantaneous turbulence quantities. The control concept is tested in a backward-facing step with a DC-motor based actuator for mixing of the near wall flow. A length-scale estimate similar to the integral scale and the Reynolds shear stress are calculated on-line. The results show it is possible to control the turbulence and for example to compensate the disturbance on the Reynolds shear stress caused by a manual change in flow velocity. The control frequency is quite slow (e.g., 0.1–100 Hz), limited primarily by the image-grabbing operations and the computation of the velocity vectors in the PIV station. For this reason the method is applicable for slow processes, e.g., to steer the mixing processes or more generally to manage the turbulence level or the length scales.
Recent Progress on MILES for High Reynolds Number Flows
Authors: Grinstein F. F. , Fureby C.
Published: December 2002

A promising large-eddy simulation (LES) approach is monotonically integrated LES (MILES) which involves solving the Navier-Stokes equations using high-resolution monotone algorithms. In MILES, the subgrid scale (SGS) flow physics is provided by intrinsic, nonlinear, high-frequency filters built into the discretization and implicit SGS models. Mathematical and physical aspects of implicit SGS modeling using nonlinear flux-limiters are addressed using a formalism based on the modified LES equations approach. Detailed properties of the implicit subgrid model are related to the flux limiter, which in turn depends on the specifics of the numerical scheme; we illustrate how the latter properties can directly affect their potential in the MILES framework. Major unresolved issues relevant to LES of complex practical turbulent flows are discussed in this context, including some aspects of boundary condition modeling and overall computational model validation.
On Homogenization-Based Methods for Large-Eddy Simulation
Authors: Persson L. , Fureby C. , Svanstedt N.
Published: December 2002

The ability to predict complex engineering flows is limited by the available turbulence models and the present-day computer capacity. In Reynolds averaged numerical simulations (RANS), which is the most prevalent approach today, equations for the mean flow are solved in conjunction with a model for the statistical properties of the turbulence. Considering the limitations of RANS and the desire to study more complex flows, more sophisticated methods are called for. An approach that fulfills these requirements is large-eddy simulation (LES) which attempts to resolve the dynamics of the large-scale flow, while modeling only the effects of the small-scale fluctuations. The limitations of LES are, however, closely tied to the subgrid model, which invariably relies on the use of eddy-viscosity models. Turbulent flows of practical importance involve inherently three-dimensional unsteady features, often subjected to strong inhomogeneous effects and rapid deformation that cannot be captured by isotropic models. As an alternative to the filtering approach fundamental to LES, we here consider the homogenization method, which consists of finding a so-called homogenized problem, i.e. finding a homogeneous “material” whose overall response is close to that of the heterogeneous “material” when the size of the inhomogeneity is small. Here, we develop a homogenization-based LES-model using a multiple-scales expansion technique and taking advantage of the scaling properties of the Navier-Stokes equations. To study the model simulations of forced homogeneous isotropic turbulence and channel flow are carried out, and comparisons are made with LES, direct numerical simulation and experimental data.
Large-Eddy Simulation of Supersonic Flat-Plate Boundary Layers Using the Monotonically Integrated Large-Eddy Simulation (MILES) Technique
Authors: Yan H. , Knight D. , Zheltovodov A. A.
Published: December 2002

A supersonic flat-plate boundary layer at a Reynolds number of 2×104 based on the inflow boundary layer thickness is investigated at different Mach numbers (M=2.88 and 4) using the monotonically integrated large-eddy simulation (MILES) technique. The inherent numerical dissipation is taken as an implicit subgrid scales (SGS) model to close the Favre-filtered compressible Navier-Stokes (NS) equations. A finite volume method with second-order accuracy in time and space is implemented for the solution of the Navier-Stokes equations on an unstructured grid of tetrahedra. The heat transfer coefficient is predicted by simulating both adiabatic and isothermal cases. The mean flowfield and turbulent stresses are in good agreement with experiment. The relationship between the predicted skin friction coefficient and heat transfer coefficient is in close agreement with the Reynolds analogy factor. The variation of turbulent Prandtl number cross the boundary layer falls within the experimental envelope. These are the first LES predictions of adiabatic and isothermal supersonic flat plate boundary layers using the MILES technique.
Visualization of Shear Stress With Micro Imaging Chip and Discrete Wavelet Transform
Authors: Kimura Motoaki , Takei Masahiro , Ho Chih-Ming , Saito Yoshifuru , Horii Kiyoshi
Published: December 2002

The two-dimensional low-speed structure of a turbulent boundary layer has been clearly visualized by a combination of a shear stress sensor using micro electro mechanical systems and the discrete wavelet transform. The application of two-dimensional discrete wavelet transforms to the visualization of wall shear stress data obtained using the micro shear stress imaging chip is described. The experiment was carried out under various Reynolds number conditions. It is shown that it is possible to visualize the low-speed streak structure as contours of two-dimensional wavelet level corresponding to spanwise wave number as a function of Reynolds number.
A Study of the Effect of Surface Metalization on Thermal Conductivity Measurements by the Transient Thermo-Reflectance Method
Authors: Burzo Mihai G. , Komarov Pavel L. , Raad Peter E.
Published: December 2002

This work is a numerical and experimental investigation of the effect of the use of a metallic absorption layer on the laser-based measurements of the thermal conductivity of dielectric, semiconductor, and highly-conductive materials. The specific experimental studies, which were carried out on silicon dioxide samples, were used to validate the numerical approach and to support the findings of this investigation. The numerical and supporting experimental results reveal the presence of behaviors associated with thermally thin and thermally thick absorption layers, depending on the ratio between the thickness of the absorption layer and the heat penetration depth. It is concluded that the TTR method performs optimally when the thickness of the metalization layer falls in the transition range between the identified thermally thin and thermally thick layers.
Heat Transfer Coefficients in Concentric Annuli
Authors: Dirker Jaco , Meyer Josua P.
Published: December 2002

The geometric shape of a passage’s cross-section has an effect on its convective heat transfer capabilities. For concentric annuli, the diameter ratio of the annular space plays an important role. The purpose of this investigation was to find a correlation that will accurately predict heat transfer coefficients at the inner wall of smooth concentric annuli for turbulent flow of water. Experiments were conducted with a wide range of annular diameter ratios and the Wilson plot method was used to develop a convective heat transfer correlation. The deduced correlation predicted Nusselt numbers accurately within 3 percent of measured values for annular diameter ratios between 1.7 and 3.2 and a Reynolds number range, based on the hydraulic diameter, of 4 000 to 30,000.
A Thermocapillary Mechanism for Lateral Motion of Bubbles on a Heated Surface During Subcooled Nucleate Boiling
Authors: Sides Paul J.
Published: December 2002

Both thermocapillary flow and the concerted motion of bubbles toward each other in subcooled nucleate boiling have been mentioned in the literature on boiling phenomena, but never associated with each other. Also, it has been shown in previously unrelated contributions that thermocapillary flow around bubbles of sparingly soluble gas can cause those bubbles to aggregate on a warm surface. The conjunction of these observations leads to the hypothesis that mutual entrainment in thermocapillary flow might drive bubbles toward each other during nucleate boiling of a subcooled liquid. An approximate equation for estimating the observability of such motion is presented. The effect would be especially important in cases where the bubble release rate is low such as boiling on horizontal down-facing surfaces and boiling in microgravity.
Vapor Phase Measurements in Subcooled Boiling Flow
Authors: Kang S. , Roy R. P.
Published: December 2002

Measurements of vapor fraction and bubble axial velocity were carried out in subcooled boiling flow using a newly designed two-sensor fiber-optic probe. The sensors encountered the axial motion of the vapor bubbles essentially head-on. The new measurements were more repeatable and had less scatter in the outer low vapor fraction region of the boiling layer compared to our earlier measurements.
Laminar Fluid Flow and Heat Transfer in a Lid-Driven Cavity Due to a Thin Fin
Authors: Shi Xundan , Khodadadi J. M.
Published: December 2002

A finite-volume-based computational study of steady laminar flow and heat transfer (neglecting natural convection) within a lid-driven square cavity due to a single thin fin is presented. The lid moves from left to right and a fixed thin fin can be positioned perpendicular to any of the three stationary walls. Three fins with lengths equal to 5, 10, and 15 percent of the side, positioned at 15 locations were examined for Re=500, 1000, 2000, and Pr=1 (total of 135 cases). Placing a fin on the right wall brings about multi-cell recirculating vortices compared to the case without a fin that exhibits a primary vortex and two small corner cells. A fin slows the flow near the anchoring wall and reduces the temperature gradients, thus degrading heat transfer capacity. A fin positioned near the top right corner of the cavity can reduce heat transfer most effectively in cases with all three different Reynolds numbers and lengths. Regardless of the Reynolds number, placing a fin on the right wall—compared to putting a fin on the left and bottom walls—can always enhance heat transfer on the left wall and at the same time, reduce heat transfer on the bottom, right and top walls. A long fin has the most marked effect on the system’s heat transfer capabilities. Mean Nusselt number was successfully correlated to the Reynolds number, length of the fin and its position.
Numerical Simulations of Resonant Heat Transfer Augmentation at Low Reynolds Numbers
Authors: Greiner Miles , Fischer Paul F. , Tufo Henry
Published: December 2002

The effect of flow rate modulation on low Reynolds number heat transfer enhancement in a transversely grooved passage was numerically simulated using a two-dimensional spectral element technique. Simulations were performed at subcritical Reynolds numbers of Rem=133 and 267, with 20 percent and 40 percent flow rate oscillations. The net pumping power required to modulate the flow was minimized as the forcing frequency approached the predicted natural frequency. However, mixing and heat transfer levels both increased as the natural frequency was approached. Oscillatory forcing in a grooved passage requires two orders of magnitude less pumping power than flat passage systems for the same heat transfer level. Hydrodynamic resonance appears to be an effective method of increasing heat transfer in low Reynolds number systems, especially when pumping power is at a premium.
Heat Transfer Model for Evaporation of Elongated Bubble Flows in Microchannels
Authors: Jacobi Anthony M. , Thome John R.
Published: December 2002

Recent experimental studies of evaporation in microchannels have shown that local flow-boiling coefficients are almost independent of vapor quality, weakly dependent on mass flux, moderately dependent on evaporating pressure, and strongly dependent on heat flux. In a conventional (macrochannel) geometry, such trends suggest nucleate boiling as the dominant heat transfer mechanism. In this paper, we put forward a simple new heat transfer model based on the hypothesis that thin-film evaporation into elongated bubbles is the important heat transfer mechanism in these flows. The new model predicts the above trends and quantitatively predicts flow-boiling coefficients for experimental data with several fluids. The success of this new model supports the idea that thin-film evaporation into elongated bubbles is the important heat transfer mechanism in microchannel evaporation. The model provides a new tool for the study of such flows, assists in understanding the heat transfer behavior, and provides a framework for predicting heat transfer.
Large-Eddy Simulation on Curvilinear Grids Using Compact Differencing and Filtering Schemes
Authors: Visbal M. R. , Rizzetta D. P.
Published: December 2002

This work investigates the application of a high-order finite difference method for compressible large-eddy simulations on stretched, curvilinear and dynamic meshes. The solver utilizes 4th and 6th-order compact-differencing schemes for the spatial discretization, coupled with both explicit and implicit time-marching methods. Up to 10th order, Pade-type low-pass spatial filter operators are also incorporated to eliminate the spurious high-frequency modes which inevitably arise due to the lack of inherent dissipation in the spatial scheme. The solution procedure is evaluated for the case of decaying compressible isotropic turbulence and turbulent channel flow. The compact/filtering approach is found to be superior to standard second and fourth-order centered, as well as third-order upwind-biased approximations. For the case of isotropic turbulence, better results are obtained with the compact/filtering method (without an added subgrid-scale model) than with the constant-coefficient and dynamic Smagorinsky models. This is attributed to the fact that the SGS models, unlike the optimized low-pass filter, exert dissipation over a wide range of wave numbers including on some of the resolved scales. For channel flow simulations on coarse meshes, the compact/filtering and dynamic models provide similar results, with no clear advantage achieved by incorporating the SGS model. However, additional computations at higher Reynolds numbers must be considered in order to further evaluate this issue. The accuracy and efficiency of the implicit time-marching method relative to the explicit approach are also evaluated. It is shown that a second-order iterative implicit scheme represents an effective choice for large-eddy simulation of compressible wall-bounded flows.
A Methodology for Simulations of Complex Turbulent Flows
Authors: Fasel H. F. , Seidel J. , Wernz S.
Published: December 2002

A new flow simulation methodology (FSM) for computing turbulent shear flows is presented. The development of FSM was initiated in close collaboration with C. Speziale (then at Boston University). The centerpiece of FSM is a strategy to provide the proper amount of modeling of the subgrid scales. The strategy is implemented by use of a “contribution function” which is dependent on the local and instantaneous “physical” resolution in the computation. This physical resolution is obtained during the actual simulation by comparing the size of the smallest relevant scales to the local grid size used in the computation. The contribution function is designed such that it provides no modeling if the computation is locally well resolved so that the computation approaches a direct numerical simulation in the fine grid limit, or provides modeling of all scales in the coarse grid limit and thus approaches an unsteady RANS calculation. In between these resolution limits, the contribution function adjusts the necessary modeling for the unresolved scales while the larger (resolved) scales are computed as in traditional large-eddy simulations (LES). However, a LES that is based on the present strategy is distinctly different from traditional LES in that the required amount of modeling is determined by physical considerations, and that state-of-the-art turbulence models (as developed for Reynolds-averaged Navier-Stokes) can be employed for modeling of the unresolved scales. Thus, in contrast to traditional LES based on the Smagorinsky model, with FSM a consistent approach (in the local sense) to the coarse grid and fine grid limits is possible. As a consequence of this, FSM should require much fewer grid points for a given calculation than traditional LES or, for a given grid size, should allow computations for larger Reynolds numbers. In the present paper, the fundamental aspects of FSM are presented and discussed. Several examples are provided. The examples were chosen such that they expose, on the one hand, the inherent difficulties of simulating complex wall bounded flows, and on the other hand demonstrate the potential of the FSM approach.
Detached-Eddy Simulations and Reynolds-Averaged Navier-Stokes Simulations of Delta Wing Vortical Flowfields
Authors: Morton Scott , Forsythe James , Mitchell Anthony , Hajek David
Published: December 2002

An understanding of vortical structures and vortex breakdown is essential for the development of highly maneuverable vehicles and high angle of attack flight. This is primarily due to the physical limits these phenomena impose on aircraft and missiles at extreme flight conditions. Demands for more maneuverable air vehicles have pushed the limits of current CFD methods in the high Reynolds number regime. Simulation methods must be able to accurately describe the unsteady, vortical flowfields associated with fighter aircraft at Reynolds numbers more representative of full-scale vehicles. It is the goal of this paper to demonstrate the ability of detached-eddy Simulation (DES), a hybrid Reynolds-averaged Navier-Stokes (RANS)/large-eddy Simulation (LES) method, to accurately predict vortex breakdown at Reynolds numbers above 1×106. Detailed experiments performed at Onera are used to compare simulations utilizing both RANS and DES turbulence models.