Flow Field Properties Research Articles

Time resolved flowfield, flame structure and acoustic characterization of a staged multi-injection burner

The present work details the analysis of staging and multipoint injection effects on the flow field, flame structure and acoustic properties of a lean swirled injector, representative of a gas turbine burner. The study is carried out for a constant power and global equivalence ratio while the staging parameter α between pilot and multipoint stages is varied. Using PLIF-OH, it is shown that the flame structure consists in a large central recirculation zone of burnt gases surrounded by an annular conical jet of fresh gases. The OH spatial distribution indicates that increasing α significantly modifies the flame structure, which becomes more compact and more organized while the mean flame front angle increases. Using High Frequency PIV in the reactive situation, a temporal analysis of the velocity field is carried out, indicating that large coherent structures appear periodically within the combustion chamber. The structure frequencies are determined and compared with acoustic measurements. This comparison indicates that for α = 0%, the vortex frequency is slightly lower than the first eigenmode of the chamber, while for α = 30%, it corresponds to a secondary peak at a higher frequency of the spectrum for both p′ and q′, with dramatic consequences on the burner behavior in terms of stability. When α increases starting from zero, the level of p′ and q′ increases to reach a maximum value for α = 20% before decreasing for higher staging. This acoustic behavior can be compared with the evolution of the temperature in the injector, that also reaches a maximum value for α = 20%, proving that strong instabilities are associated with flame stabilization within the injector. Increasing the staging factor makes it possible to decrease the flame instability level while keeping the flame compact and robust.

Hybrid Particle-Continuum Simulations of Hypersonic Flow Over a Hollow-Cylinder-Flare Geometry

A modular particle-continuum numerical method is used to simulate steady-state hypersonic flow over a hollow-cylinder-flare geometry. The resulting flowfield involves a mixture of rarefied nonequilibrium flow and high-density continuum flow. The hybrid particle-continuum method loosely couples direct simulation Monte Carlo and Navier-Stokes methods, which operate in different regions, use different mesh densities, and are updated using different-sized time steps. Hybrid numerical results are compared with full particle and full continuum simulations as well as with experimental data. The hybrid particle-continuum simulations are demonstrated to reproduce experimental and full particle simulation results for surface and flowfield properties including velocity slip, temperature jump, thermal nonequilibrium, heating rates, and pressure distributions with high accuracy. The hybrid method, which uses particle simulation next to the surface, is also shown to predict accurate heating rates even when a highly dissipative numerical scheme is used for the continuum solver. For this particular flow, a hybrid simulation is obtained with modest computational savings over full particle simulation.

Geometrical Properties of the Resolved-Scale Velocity and Temperature Fields Predicted using Large-Eddy Simulation

In this paper, local geometrical properties of the velocity and temperature fields of combined forced and natural convection in a vertical slot are studied using large-eddy simulation based on both numerical and analytical approaches. Previous studies on turbulence geometrical statistics appearing in the literature have primarily focused on either isothermal or isotropic turbulent flows; whereas in this work, we extend the scope of research to investigation of a wall-bounded thermal flow. In particular, we focus on studying the resolved helicity, enstrophy generation, local vortex stretching, and a variety of characteristic geometrical alignment patterns between the resolved velocity, vorticity, temperature gradient, subgrid-scale heat flux and the eigenvectors of the resolved strain rate tensor. In order to quantify the effect of buoyancy on the geometrical properties of the thermal flow field, a systematic comparative analysis has been conducted based on three different flow regimes (i.e., viscous sublayer, buffer layer and logarithmic layer) in both the hot and cold wall regions. The near-wall restriction on the geometrical property of the thermal flow field has been analyzed and some interesting wall-limiting geometrical alignment patterns in the form of Dirac delta functions are also reported.

Assessment of a film-cooling flow structure by large-eddy simulation

Large-eddy simulation results using the approximate-deconvolution subgrid-scale model (ADM) are reported for a jet-in-crossflow configuration related to the film-cooling of turbine blades. Here, relatively cool air is ejected from a large isobaric plenum through an oblique round nozzle into a hotter turbulent boundary layer. The parameters are chosen in accordance with typical film-cooling conditions of turbines. For computational economy, a steady mean-turbulent inflow profile is used for the crossflow. The adequacy of this simplification is evaluated. The properties of the mean flow field are investigated and the mean film-cooling efficiency is compared to results from the literature. Special attention is paid to the flow structure inside the jet nozzle, which is considered to have a key influence on the mixing process. The physical understanding of film cooling is deepened with an illustration of the instantaneous flow field and its connection to the findings for the mean flow.

Procoagulant Properties of Flow Fields in Stenotic and Expansive Orifices

In the United States, over 125,000 mechanical heart valves (MHVs) are implanted each year. Flow through the MHV hinge can cause thromboemboli formation. The purpose of this study was to examine various orifice geometries representing the MHV hinge region and how these geometries may contribute to platelet activation and thrombin generation. We also characterized these flow fields with digital particle image velocimetry (DPIV). Citrated human blood at room temperature was forced through the orifices (400 and 800 microm ID) with a centrifugal bypass pump, continuously infusing calcium chloride to partially reverse the citrate anticoagulant. Blood samples were tested for the presence of thrombin-antithrombin complex (TAT) and platelet factor 4 (PF4). Velocity and shear stress were measured with DPIV using a blood analog fluid seeded with fluorescent microbeads. The results indicate that small changes in geometry, although they do not affect the bulk flow, change the coagulation propensity as blood flows through the orifices. A more abrupt geometry allows more stagnation to occur resulting in more thrombin generation. PF4 measurements indicated similar levels of platelet activation for all orifices. DPIV showed differences in the jets with respect to entrainment of stagnant fluid. These results help to pinpoint the important parameters that lead to flow stasis and subsequent thrombus formation.

Effects of flow field and diffusion layer properties on water accumulation in a PEM fuel cell

Water is the main product of the electrochemical reaction in a proton exchange membrane (PEM) fuel cell. Where the water is produced over the active area of the cell and how it accumulates within the flow fields and gas diffusion layers, strongly affects the performance of the device and influences operational considerations such as freeze and durability. In this work, the neutron radiography method was used to obtain two-dimensional distributions of liquid water in operating 50 cm 2 fuel cells. Variations were made of flow field channel and diffusion media properties to assess the effects on the overall volume and spatial distribution of accumulated water. Flow field channels with hydrophobic coating retain more water, but the distribution of a greater number of smaller slugs in the channel area improves fuel cell performance at high current density. Channels with triangular geometry retain less water than rectangular channels of the same cross-sectional area, and the water is mostly trapped in the two corners adjacent to the diffusion media. It was also found that cells constructed using diffusion media with lower in-plane gas permeability tended to retain less water. In some cases, large differences in fuel cell performance were observed with very small changes in accumulated water volume, suggesting that flooding within the electrode layer or at the electrode-diffusion media interface is the primary cause of the significant mass transport voltage loss.

Upscaling Hydraulic Properties and Soil Water Flow Processes in Heterogeneous Soils: A Review

This review covers, in a comprehensive manner, the approaches available in the literature to upscale soil water processes and hydraulic parameters in the vadose zone. We distinguish two categories of upscaling methods: forward approaches requiring information about the spatial distribution of hydraulic parameters at a small scale, and inverse modeling approaches requiring information about the spatial and temporal variation of state variables at various scales, including so‐called “soft data”. Geostatistical and scaling approaches are crucial to upscale soil water processes and to derive large‐scale effective fluxes and parameters from small‐scale information. Upscaling approaches include stochastic perturbation methods, the scaleway approach, the stream‐tube approach, the aggregation concept, inverse modeling approaches, and data fusion. With all upscaling methods, the estimated effective parameters depend not only on the properties of the heterogeneous flow field but also on boundary conditions. The use of the Richards equation at the field and watershed scale is based more on pragmatism than on a sound physical basis. There are practically no data sets presently available that provide sufficient information to extensively validate existing upscaling approaches. Use of numerical case studies has therefore been most common. More recently and still under development, hydrogeophysical methods combined with ground‐based remote sensing techniques promise significant contributions toward providing high‐quality data sets. Finally, most of the upscaling literature in vadose zone research has dealt with bare soils or deep vadose zones. There is a need to develop upscaling methods for real world soils, considering root water uptake mechanisms and other soil–plant–atmosphere interactions.

Electrodeposition process modeling using continuous and discrete scales

Chemical bath electro deposition process is used in many industrial applications to obtain a thin layer material on a target surface. Numerous metal, magnetic or semi-conductor materials, such as oxide, chalcogenure or alloys are obtained in electrochemical cells at laboratory scale. Some of these materials are interesting to be produced at industrial scale, for example for electronics, fuel cells or photovoltaic applications. In industrial electrochemical cells, dimension is larger and many parameters such as hydrodynamics or electro active specie transport are heterogeneous. There are many industrial electrochemical techniques in which the electrode moves with respect to the solution. These systems, like the rotating electrodes, are called hydrodynamic electrochemical processes. It is also interesting to notice that micronic structure, such as roughness, columnar structure or porosity of material deposit is local flow dependent. Then, it appears, that the material deposit composition and structural quality need integrated information from the micronic scale to the industrial scale, using, of course, the laboratory scale measurements. The aim of the present work is to model and to numerically simulate the hydrodynamics, electrochemical and chemical coupled phenomena, which are occurring during the chemical bath electro deposition process for laboratory and industrial configurations. Experimental measurements obtained at laboratory scale with zinc oxide thin layer deposit are used to identify transport or kinetic data input which are conserved during the scale-up. Flow and chemical species concentration field properties are calculated in the working electrode surface vicinity taking into account homogeneous and heterogeneous reactions. The numerical method used is the finite volume method. In addition, using a Monte Carlo method, micronic information is calculated such as the roughness and the porosity of the thin layer material obtained.

Exergetic aspects of high‐speed boundary layers

AbstractExergy is an established concept for evaluating heat cycles. It quantifies the available work that can be extracted from a system. The extension of the classical definition to non‐uniform flow field properties is applied here to high‐speed boundary layers. The destruction of exergy is quantified by the corresponding loss thickness and is evaluated locally. Turbulent fluctuations in this approach are described by their turbulent exergy, which is a generalisation of the concept of turbulent kinetic energy for incompressible flow. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of the contact model on the onset of sprag‐slip

AbstractIn systems with sliding‐friction often strong self‐excited vibrations do occur. One of the possible underlying mechanisms is the so‐called sprag‐slip instability. In the present work the onset of sprag‐slip is investigated by a simple model in which an inclined elastic beam slides over a rigid belt moving with constant velocity. For a Coulomb friction law and a contact model with constant contact stiffness for a certain range of parameters the system loses its static solution corresponding to the steady sliding state. Simultaneously with this loss of existence of the static solution the qualitative properties of the system's flow field in phase space change, resembling a transition from stable to unstable behavior. To investigate the influence of contact models and related parameters on the details of this onset of sprag‐slip also Hertz theory of elastic contact is applied. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modeling coupled aerodynamics and vocal fold dynamics using immersed boundary methods

The penalty immersed boundary (PIB) method, originally introduced by Peskin (1972) to model the function of the mammalian heart, is tested as a fluid-structure interaction model of the closely coupled dynamics of the vocal folds and aerodynamics in phonation. Two-dimensional vocal folds are simulated with material properties chosen to result in self-oscillation and volume flows in physiological frequency ranges. Properties of the glottal flow field, including vorticity, are studied in conjunction with the dynamic vocal fold motion. The results of using the PIB method to model self-oscillating vocal folds for the case of 8 cm H20 as the transglottal pressure gradient are described. The volume flow at 8 cm H20, the transglottal pressure, and vortex dynamics associated with the self-oscillating model are shown. Volume flow is also given for 2, 4, and 12 cm H2O, illustrating the robustness of the model to a range of transglottal pressures. The results indicate that the PIB method applied to modeling phonation has good potential for the study of the interdependence of aerodynamics and vocal fold motion.

Active Regulation of Speed during a Simulated Low-Altitude Flight Task

This study examined active regulation of speed during a low-altitude flight task as a function of global optical flow rate, speed, and the presence or absence of a concurrent altitude disturbance. The results showed that altitude clearly had an impact on speed control. Control of speed was much more difficult when altitude disturbances were present. Even in the no altitude disturbance conditions, performance tended to be best at lower altitudes. Consistent with previous research, the results suggest that speed and altitude changes have additive effects on speed judgments. This is inconsistent with the simple global optical flow rate hypothesis that had suggested multiplicative effects. However, it is consistent with the general notion that judgments of self motion are based on properties of optical flow fields (i.e. angles and angular rates) that depend on distance and motion relative to textured surfaces.

Axisymmetric Inlet Design for Combined-Cycle Engines

Performance considerations for a turbine-based combined-cycle engine inlet are presented using the inlet of the Lockheed SR-71 as a baseline. The methodology incorporated is to modify the SR-71 inlet to expand its operational envelope to higher speeds. A numerical model is developed using the axisymmetric method of characteristics to perform full inviscid flow analysis, including any internal shock reflections. Starting characteristics are quantified based on the Kantrowitz limit. The original SR-71 inlet is analyzed throughout the designed flight regime, beginning at Mach 1.7 and ending with the shock-on-lip condition at Mach 3.2. A series of modifications are then considered for their ability to extend the range of the inlet into the hypersonic flight regime. The results show that two modifications, the widened shoulder centerbody and the variable cone with reextension, have the ability to remain started into the Mach 6‐7 range and have similar compressive performance to each other across the entire Mach spectrum. Nomenclature A = area M = Mach number m = mass flow P = pressure r = radial location T = temperature x = axial location γ = ratio of specific heats χ = flowfield property

Experiments on swirl stabilized non-premixed natural gas flames in a model gasturbine combustor

This study reports on experiments in an optically accessible model gas turbine combustor equipped with a single generic nozzle. Laser Doppler velocimetry (LDV) and planar laser-induced fluorescence (LIF) are applied to generate an experimental data base that allows for an improved understanding of turbulent combustion under elevated pressure important for gas turbine technology and suitable for validation of numerical simulations. Flow field properties such as mean and fluctuations of all velocity components at various locations including the nozzle exit profiles are reported for an operational point at 2 bar in addition to coherent motions of the flow that are superimposed to the turbulence. Flame lift-off height, flame brush location, mean reaction progress, and topological quantities such as flame surface density are studied under various pressures ranging from 2 to 6 bar.

General Aviation Propeller-Airframe Integration Simulations

The interference flowfields of propellers when installed on general aviation aircraft is investigated. The interaction is known to be a significant source of cabin noise and is likely an efficiency loss mechanism due to standard blade designs not completely accounting for installation interference. The aerodynamic model used in the simulations incorporates state-of-the-art unsteady three-dimensional Navier-Stokes solutions through the use of a high-resolution approximate Riemann solver utilizing the Baldwin-Lomax turbulence model. The software used for simulation purposes is a derivative of a software package developed for NASA John. H. Glenn Research Center at Lewis Field. Several configurational arrangements were studied and compared to one another. The propellers tested include a standard general aviation propeller and a modern propeller design. Both are mounted on various fuselage configurations including geometries involving engine cooling inlets and short and long spinners to adjust propeller/cowl spacing. A detailed examination of flowfield properties provides insight into the interaction between the propeller and the cowl immediately downstream.

Measurement of Zeta-Potential at Microchannel Wall by a Nanoscale Laser Induced Fluorescence Imaging

A nanoscale laser induced fluorescence technique was proposed by using fluorescent dye and evanescent wave with total internal reflection of laser beam. The present study focused on the two-dimensional measurement of zeta-potential at the microchannel wall, which is an electrostatic charge of the wall surface and a characteristic property of an electroosmotic flow field. The evanescent wave, which decays exponentially from the wall, was used as an excitation light of the fluorescent dye. The fluorescent intensity in the vicinity of wall detected by a CCD camera is closely related to the zeta-potestial. Two kinds of fluorescent dye solution with different ionic concentration were injected into a T-shaped microchannel, and formed a mixing flow field in the junction area. The two-dimensional distribution of zeta-potential was measured at the microchannel wall in a pressure driven flow field. A transverse gradient of zeta-potential was observed in the mixing field, and the two-dimensional distribution of zeta-potential was changed by the difference in the averaged velocity. To understand the ion motion in the mixing flow field, the three-dimensional structure of flow field was analyzed by velocity measurement using micron-resolution particle image velocimetry and numerical simulation. It is concluded that the two-dimensional distribution of zeta-potential at the microchannel wall was dependent on the ion motion in the flow field, which was governed by convection and molecular diffusion.

A Quantitative Comparison of Delta Wing Vortices in the Near-Wake For Incompressible and Supersonic Free Streams

When requiring quantitative data on delta wing vortices for design purposes, low-speed results have often been extrapolated to configurations intended for supersonic operation. This practice stems from a lack of database in high-speed flows due to measurement difficulties. In the present paper an attempt is made to examine this practice by comparing data from an incompressible flow experiment designed specifically to correspond to an earlier experiment in supersonic flows. The comparison is made for a 75° sweptback delta wing at angles of attack of 7° and 12°. For the incompressible flow, detailed flow-field properties including vorticity and turbulence characteristics are obtained by hot-wire and pressure probe surveys. The results are compared, wherever possible, with available data from the earlier Mach 2.49 experiment. The results indicate that quantitative similarities exist in the distributions of total pressure and swirl velocities. Qualitative similarities also exist in other properties, however, many differences are observed. The vortex core is smaller and rounded at low speed. At high speed, it is elongated in the spanwise direction near the trailing edge but goes through “axis switching” within a short distance downstream. The vortex is located farther outboard, i.e., the spacing between the two legs of the vortex pair is larger, at low speed. The axial velocity distribution within the core is significantly different in the two flow regimes. A “jet-like” profile, observed at low speed, either disappears or becomes “wake-like” at high speed. The axial velocity characteristics are examined in the light of an analytical model.

Aerothermodynamic Performance Analysis of Hypersonic Flow on Power Law Leading Edges

A numerical study is reported on power-law-shaped leading edges situated in a rarefied hypersonic flow. The work is motivated by interest in investigating the flowfield properties of power-law-shaped leading edges as possible candidates for blunting geometries of hypersonic leading edges because power-law leading-edge shapes have both blunt (infinite body slope at the nose) and sharp (zero radius of curvature at the nose) characteristics. The sensitivity of the stagnation point heating, total drag, and shock wave standoff distance to shape variations of such leading edges is calculated by using the direct simulation Monte Carlo method. Comparisons based on equivalent stagnation point heating, equivalent total drag, and equivalent shock standoff distance are made between power law shapes and round leading edges. For the flow conditions considered, round leading edges provide lower drag and smaller shock standoff distance than power law leading edges for equivalent stagnation point heating. It was also found that power law shapes have smaller shock standoff distance and larger stagnation point heating than the round leading edges for equivalent total drag.

Particle formation of ibuprofen–supercritical CO 2 system from rapid expansion of supercritical solutions (RESS): A mathematical model

A one-dimensional flow model has been proposed to describe the particle formation through the rapid expansion of supercritical solutions (RESS). This model includes the wall friction in the nozzle and heat exchange with the surrounding in the supersonic free jet region. The calculations were performed on the ibuprofen–supercritical CO 2 system in terms of the supersaturation, nucleation rate, critical nucleus size and number concentration of the critical nuclei. The results show that very high supersaturation (approximately 10 9), thus high nucleation rate, can be attained when the fluid leaves the nozzle exit to the supersonic free jet region. Simultaneously, the critical nucleus size, in the nanometer size, decreases, whereas the number concentration of critical nuclei increases. Besides, sensitivity analyses of pre-expansion pressures, pre-expansion temperatures and nozzle length have been carried out but found that they seem to not significantly influence the flow field properties.

Numerical simulations of underexpanded supersonic jet and free shear layer using WENO schemes

Numerical simulations of the flowfield structures and properties of underexpanded supersonic jet and planar shear layer are performed by solving the time-dependent, compressible Euler equations. The numerical code uses the high-order weighted essentially non-oscillatory (WENO) finite difference schemes with the fifth-order-accurate for spatial discretization and the fourth-order-accurate Runge–Kutta scheme for time integration. The present predictions are compared with the available experimental data and other numerical results using different numerical schemes. Good agreements between the predicted and experimental data are achieved for both cases. The results show that the near-field region of the jet and shear layer is influenced by compressibility, which reduces turbulent mixing rates and suppresses vortex roll-up and pairings at high-convective Mach number. Analyses of sound pressure level of the jet indicate that the broadband shock noise is generated from the shock cells and radiates in the direction of 40°–50° to the jet axis. Various forcing frequencies are applied at the inflow boundary to demonstrate mixing enhancement and shock cell destruction for the underexpanded jet. The computation of an underexpanded free shear layer indicates that the numerical turbulence model inherent with WENO algorithm can satisfactorily predict turbulence properties at a convective Mach number of 0.64 even without including subgrid-scale turbulence models.