Mean Flow Properties Research Articles

A GENERALIZED MODEL OF ACOUSTIC RESPONSE OF TURBULENT PREMIXED FLAME AND ITS APPLICATION TO GAS-TURBINE COMBUSTION INSTABILITY ANALYSIS

ABSTRACT An analytical model is developed to study the combustion response of turbulent premixed flames to acoustic oscillations. The analysis is based on a level-set flamelet model, and accommodates spatial variations in chamber geometry and mean-flow properties. All known factors affecting the flame response to local flow disturbances are analyzed. A triple decomposition technique, which expresses each flow variable as the sum of a long-time-averaged, a periodic, and a turbulent component, is used to examine the interactions between acoustic and turbulent motions and their collective influence on the flame dynamics. As specific examples, both a simple and an enveloped flame commonly observed in a swirl-stabilized combustor are studied. The resultant flame response is incorporated into a three-dimensional acoustic analysis to determine the stability characteristics of a model gas-turbine combustor. Results are consistent with experimental observations and numerical simulations in terms of the stability boundary and acoustic wave properties. In particular, the enveloped flame tends to be resonantly coupled with the acoustic velocity oscillation, leading to large excursions of combustion oscillations.

Unsteady flow evolution in swirl injectors with radial entry. II. External excitations

Our previous study on turbulent flows in a gas-turbine swirl injector was extended to explore the effects of externally impressed excitations on the unsteady flow evolution. Three-dimensional large-eddy simulations were conducted to investigate the responses of the injector flowfield by imposing periodical oscillations of the mass flow rate at the entrance over a wide range of frequencies. Results show that the impressed disturbances are decomposed and propagate in two different modes because of their distinct propagating mechanisms in swirl injectors. The flow oscillation in the streamwise direction travels in the form of acoustic wave, whereas the oscillation in the circumferential direction is convected downstream with the local flow velocity. The vortex breakdown is mainly controlled by the dynamics in the core region near the axis, not so much by the excitation in the main flow passage surrounding the central recirculation zone. External excitations only exert minor influences on the mean flow properties due to the broadband characteristics of the injector flow. One exception is in the outer shear-layer region when the forcing frequency matches the intrinsic frequency of vortex shedding, and the mixing process of two counter-rotating swirl flows is considerably enhanced. The dynamic response of the injector flow, however, depends significantly on the forcing frequency in terms of the acoustic admittance and the mass transfer functions. Energy can be transferred among the various structures in the flowfield under external excitations, causing highly nonuniform spatial and temporal distributions of the oscillatory flow properties at the injector exit. The mass transfer function between the injector exit and entrance at the forcing frequency could be substantially greater than unity when the disturbance resonates with the injector flow. The injector essentially acts as an amplifier under this condition.

Response of a spatially developing turbulent boundary layer to a spanwise oscillating electromagnetic force

Direct numerical simulations were performed to investigate the physics of a spatially developing turbulent boundary layer flow subjected to a spanwise oscillating electromagnetic force. The electromagnetic force was applied beneath a finite length of the flat plate. A fully implicit fractional step method was employed to simulate the flow. The mean and turbulent flow properties were obtained to analyze the spatial evolution of the near-wall vortical structure. It was found that the skin friction drag is significantly reduced for an oscillation period of T + = 100. Imposition of electromagnetic forcing leads to a decrease in turbulence production, which in turn causes a reduction in the turbulent kinetic energy. Instantaneous flow visualization techniques were used to observe the responses of the streamwise vortices and streak structures. The visualization results show that imposition of the spanwise oscillation suppresses and weakens the streak structures. Downstream of the region where the electromagnetic force is applied the flow eventually relaxes back to a two-dimensional equilibrium boundary layer.

The counter-rotating core of a swirling turbulent jet issued from a rotating pipe flow

Axially rotating turbulent pipe flow is an example where rotation strongly affects the turbulence and thereby the Reynolds stresses and mean flow properties. The present Letter reports new measurements where a rotating pipe flow is used to establish a swirling jet. The measurements in the jet show that at some distance downstream (approximately 6 nozzle diameters) the central part of the jet starts to rotate in the opposite direction as compared to the rotation of the pipe. This effect is explained by the influence of the cross flow Reynolds stress originating in the pipe flow.

Laboratory measurement and numerical simulation of flow and turbulence in a meandering-like flume with spurs

The paper reported herein presents the laboratory measurement of near-bed flow and turbulence induced by non-submerged spurs protruding from the bank of a meandering-like laboratory flume with smooth rigid bed. The flow property was measured using a 3D acoustic doppler velocimeter for various combinations and locations of spurs in order to assess their effect on mean flow field. Likewise, turbulent characteristics were computed from the measured data for one of the experimental cases. Furthermore, a 2D numerical model was developed for the simulation of mean flow property, turbulent intensities as well as vorticity field using cubic-interpolated pseudoparticle (CIP) numerical technique. The simulated down-stream and cross-stream mean flow property as well as turbulent intensities in shear layer was found to be in good agreement with the experimental results. The numerical simulation of vortices, generated from the tip of the spur, was seen to be reliable. In addition, the migration of small vortices was visualized in the experimental flume using a simple technique.

Dynamics of ring vortices impinging on planar shock waves

In the present paper the encounter of vortex rings with planar shock waves is studied in a three-dimensional environment, with the objective to assess the influence of shock and vortex strength and vortex orientation on the dynamics of the process. The study relies on numerical simulations performed by means of a state-of-the-art hybrid compact/WENO (weighted essentially nonoscillatory) shock-capturing algorithm to solve the Euler equations of gas dynamics. The study focuses on the characterization of the interaction in terms of the evolution of the geometrical parameters of the vortex and the mean flow properties such as kinetic energy and enstrophy. The vortex is compressed as it passes through the shock, and its characteristic dimensions decrease; at the same time its axis is deflected and the ring follows a complex dynamic evolution, even though a nearly steady state is reached for the global quantities. In particular, it has been found that the total vortex kinetic energy is generally nonincreasing after the interaction, while the enstrophy always increases. A simple theory is developed here to shed some light onto the physical phenomena involved, which is found to compare reasonably well with the results of the computations. Some final comments are made regarding the extension of the results reported in the present study to the analysis of shock-compressed turbulence.

Vortex-In-Cell and Probability Density Function Approach for a Passive Scalar Field in a Mixing Layer

A Reynolds-averaged simulation based on the vortex-in-cell (VIC) and the transport equation for the probability density function (PDF) of a scalar has been developed to predict the passive scalar field in a two-dimensional spatially growing mixing layer. The VIC computes the instantaneous velocity and vorticity fields. Then the mean-flow properties, i.e. the mean velocity, the root-mean-square (rms) longitudinal and lateral velocity fluctuations, the Reynolds shear stress, and the rms vorticity fluctuations are computed and used as input to the PDF equation. The PDF transport equation is solved using the Monte Carlo technique. The convection term uses the mean velocities from the VIC. The turbulent diffusion term is modeled using the gradient transport model, in which the eddy diffusivity, computed via the Boussinesq's postulate, uses the Reynolds shear stress and gradients of mean velocities from the VIC. The molecular mixing term is closed by the modified Curl model. The computational results were compared with two-dimensional experimental results for passive scalar. The predicted turbulent flow characteristics, i.e. mean velocity and rms longitudinal fluctuations in the self-preserving region, show good agreement with the experimental measurements. The profiles of the mean scalar and the rms scalar fluctuations are also in reasonable agreement with the experimental measurements. Comparison between the mean scalar and the mean velocity profiles shows that the scalar mixing region extends further into the free stream than does the momentum mixing region, indicating enhanced transport of scalar over momentum. The rms scalar profiles exhibit an asymmetry relative to the concentration centerline, and indicate that the fluid on the high-speed side mixes at a faster rate than the fluid on the low-speed side. The asymmetry is due to the asymmetry in the mixing frequency cross-stream profiles. Also, the PDFs have peaks biased toward the high-speed side.

Two-dimensional flow characteristics of wave interactions with a fixed rectangular structure

Wave interactions with a fixed rectangular structure were investigated experimentally using particle image velocimetry (PIV) to simulate the condition of a barge in a beam sea. The structure was mounted in a two-dimensional wave tank while regular waves were used. No overtopping occurred on the deck. Two-dimensional spatial velocity maps on both sides of the structure were measured at eight different phases. Phase average was used to extract the mean flow and turbulence property from the repeated instantaneous PIV velocity measurements. The mean velocity field was demonstrated along with the generation and evolution of vortices and turbulent kinetic energy on both sides of the structure. Each term in the turbulent kinetic energy budget was measured or estimated and the balance was discussed. Clear differences between the flow pattern and the turbulent structure at the two sides were also illustrated.

Numerical simulations of buoyant reactive jets with sidewall effects

The near field dynamics of buoyant reactive jets with adjacent sidewalls is investigated by time-dependent three-dimensional direct simulations. The physical problem is a fuel jet issuing vertically into an oxidant ambient environment in a corner configuration with sidewall boundaries. Simulation results are presented for two cases with different jet nozzle geometries: a corner-round reactive jet and a corner-square reactive jet with the same cross-sectional area on the nozzle plane. Buoyancy-induced large vortical structures evolve spatially in the flow field and transition to turbulence occurs downstream. Calculation of the mean flow properties shows that entrain ment of the corner-round jet is stronger than that of the corner-square jet due to the stronger vortex deformation in the corner-round case.

自己保存条件を満足する粗面乱流境界層の平均量特性(G05-6 内部流れ・境界層(2),G05 流体工学部門一般講演)

自己保存条件を満足する粗面乱流境界層の平均量特性(G05-6 内部流れ・境界層(2),G05 流体工学部門一般講演)

SIMULATION OF SWIRLING TURBULENT FLOWS AND HEAT TRANSFER IN AN ANNULAR DUCT

The article presents a numerical simulation of swirling turbulent flows and heat transfer in an annular duct. The time-averaged governing equations are solved, which are closed by a new algebraic Reynolds stress model (ASM). The simulation is performed under different flow conditions. The calculated results of gas axial and tangential velocities, turbulent kinetic energy, temperature, and local heat transfer coefficients on the inner and outer walls of the annulus are provided. They illustrate the effect of swirl number, inlet axial velocity, and ratio of inner to outer radius on the mean flow and turbulence properties, as well as on enhancing heat transfer in the annular duct.

On non-linear hydrodynamic instability and enhanced transport in differentially rotating flows

In this paper we argue that differential rotation can possibly sustain hydrodynamic turbulence in the absence of magnetic field. We explain why the non-linearities of the hydrodynamic equations (i.e. turbulent diffusion) should not be neglected, either as a simplifying approximation or based on boundary counditions. The consequences of lifting this hypothesis are studied for the flow stability and the enhanced turbulent transport. We develop a simple general model for the energetics of turbulent fluctuations in differentially rotating flows. By taking into account the non-linearities of the equations of motions, we give constraints on the mean flow properties for the possible development of shear instability. The results from recent laboratory experiments on rotating flows show -- in agreement with the model -- that the pertinent parameter for stability appears to be the Rossby number Ro. The laboratory experiments seem to be compatible with Ro 1 in the inviscid or high rotation rates limit. Our results, taken in the inviscid limit, are coherent with the classical linear stability analysis, in the sense that the critical perturbation equals zero on the marginal linear stability curve. We also propose a prescription for turbulent viscosity which generalize the beta-prescription derived in Richard & Zahn 1999.

Determining friction coefficients for interrill flows: the significance of flow filaments and backwater effects

AbstractFriction coefficients in overland flows are customarily estimated from mean flow properties (depth, velocity, slope) that subsume spatial variations in flow arising from two major causes: microtopography and obstacles. This paper uses laboratory experiments in shallow flumes to examine the extent of non‐uniformity in flow conditions associated with each cause. Randomly placed emergent obstacles in a flume with a shallow axial channel generally yielded higher hydraulic roughness than the same pattern of obstacles on a planar flume, as well as greater variation in roughness as the obstacle locations were altered. In both flumes, hydraulic roughness fell with increasing Reynolds number for 10% obstacle cover, showed a flattening trend at 20% cover, and exhibited a convex‐downward trend at 30% obstacle cover. These results indicate the progressive onset of flow controls at narrow gaps in the obstacle field. In such flows, the use of mean flow properties conceals the existence of two main subdivisions of flow: flow filaments and backwater flows. In the experiments, flow filaments involved velocities more than twice the overall mean, whereas backwater flows were much slower than the mean. The existence of fast‐moving flow filaments may be significant in understanding soil transport in surface runoff, and backwater depths may modify splash detachment. Similarly, friction coefficients that fail to reflect these important non‐uniform flow components may not be optimal for hydraulic calculations or in erosion models. It is concluded that new approaches to observing and processing flow data may be required, in order to avoid the loss of important flow detail that is entailed in assuming uniform flow conditions. Copyright © 2003 John Wiley & Sons, Ltd.

LINEAR AND NONLINEAR EDDY-VISCOSITY TURBULENCE MODELS FOR A CONFINED SWIRLING COAXIAL JET

An investigation into the predictive performance of linear and nonlinear eddy-viscosity turbulence models for a confined swirling coaxial jet is presented. The computational results show that the k - l linear model performs very well in predicting the mean flow properties, while almost all nonlinear models overpredict the strength of recirculation and cannot provide satisfactory results. The cubic model of Apsley and Leschziner, derived from an algebraic stress model, yields better results than other cubic models. The deficiency in predicting scalar field is perhaps due to the assumption of constant turbulent Prandtl number and unsteady large-scale motions.

횡방향으로 회전하는 90도 정사각 단면 곡덕트에서 발달하는 난류유동의 측정

Mean flow and turbulence properties of developing turbulent flows in a 90 degree square bend with spanwise rotation were measured by a hot-wire anemometer. A slanted wire is rotated into 6 orientations and the voltage outputs from them are combined to obtain the mean velocity and Reynolds stress components. The combinative effects of the centrifugal and Coriolis forces due to the curvature and the rotation of bend on the mean motion and turbulence structures are investigated experimentally. Results show that the two body forces can either enhance or counteract each other depending on the flow direction in the bend.<br/>

Management of a Stronger Wall Jet by a Streamwise Vortex with Periodic Perturbation. Mean Flow Properties and Evolution of a Streamwise Vortex.

Effect of periodic perturbation was investigated experimentally on the management of a stronger wall jet with a streamwise vortex. A half-delta wing located on the wall generates the streamwise vorticity and the periodic perturbation is added by varying in angle of attack of the wing. Measurements of mean velocity, secondary velocity vectors, and streamwise vorticity were examined to see effect of the periodic perturbation on the evolution process of the wall jet and streamwise vortex. The periodic perturbation accelerates development of vortex radius in spanwise direction over the initial stage and increases relative strength of spanwise secondary velocity vector to the transverse velocity vector. The streamwise momentum flux is increased by interaction between streamwise vortex and wall jet. However, the periodic perturbation reduces increasing rate of the momentum flux.

MUTUAL BEHAVIORS OF BOTH PARTICLES AND FLUIDS IN NEAR-WALL REGION OF SMOOTH OPEN-CHANNEL FLOW

The particle-laden flow has been investigated extensively. The wall friction and sediment transport rate are affected by the motions of bed-load particles. Therefore, mean flow properties and turbulence characteristics may be changed from those of clear-water flows. In this study, “Particle-Tracking Velocimetry (PTV)”, which is one of visualization techniques, was used to measure the velocity of both fluids and particles in near-wall region including the viscous sublayer and buffer layer. It was found that the particle velocity is larger than the fluid velocity in the near wall-region (y+≤20). In contrast, these velocities are vise versa in the region of y+>20. The relative velocity between particles and fluids increases with an increase of the particle diameter and its density.

Particle image analysis of open-channel flow at a backward facing step having a trench

Characteristics of open-channel flow at a backward facing step with a trench section were experimentally investigated. The experiments were performed using a straight open channel flume by varying the trench length. Two-dimensional velocity measurements by the particle tracking velocimetry (PTV) were conducted at the trench section using a high-resolution video camera of 12 fps. Mean and turbulent flow properties along with the water surface profile were measured for various aspect ratios of the trench. It was also observed that the flow at the trench begins to oscillate periodically for a specific aspect ratio of the trench.

Turbulence statistics in a fire room model by large eddy simulation

Fire and smoke movement in a room is influenced by the turbulence characteristics (such as Reynolds stress, turbulent heat flux, etc.) of the flow and temperature fields. In order to accurately predict fire and smoke movement by computational fluid dynamics (CFD), it is necessary to verify these turbulence quantities. The purpose of this study is to predict the turbulence structure of the flow and temperature fields due to a fire in the compartment by large eddy simulation (LES) using detailed experimental data to verify the simulation results. The results show reasonably good agreement with experimental data for both the mean flow properties and the turbulence quantities with the exception of the region near ceiling. This study provides useful information for verifying LES technique when applied to compartment fires.

Multiple compaction of microcrystalline cellulose in a roller compactor

The effect of multiple roller compaction was investigated using microcrystalline cellulose as a model substance. Granules were prepared, examined and recompacted in a Gerteis 3 W-Polygran roller compactor up to ten times. Examinations were carried out for granule size distribution, density and flow properties. Ribbons were investigated for quality, and adhesion of ribbons to the rolls was traced. Finally tablets were produced from the granule samples and examined for their compression behaviour. Multicompression reduces the amount of fines, increases mean granule size and flow properties and also improves size distribution. Although roll adhesion diminishes with increasing cycles, this decrease is not sufficient enough to result in a visibly reduced gap variability. By multicompaction, bulk density increases which indicates that the porosity of granules decreased during the multiple compaction cycles. However, the ability of MCC to form bondings with neighbouring particles is diminished during various cycles which results in decreasing crushing forces of the subsequently prepared tablets.