Turbulent Jet Mixing Research Articles

Toward Smart Control of Turbulent Jet Mixing and Combustion

Recent progress in active control of jet mixing and combustion is introduced. Miniature electromagnetic flap actuators are mounted on the periphery of an axisymmetric nozzle exit. It is demonstrated that even weak disturbances introduced into the initial shear layer by these actuators can significantly modify the large-scale vortical structures. This control technique is extended to the control of methane/air mixing and diffusion combustion by using a coaxial jet nozzle with the same flap actuators. As a result, the flame characteristics can be much improved in terms of stability and emission. Direct numerical simulation of a confined coaxial jet control has also been carried out. Although the distributed actuators are modeled somewhat ideally, DNS clearly demonstrates enormous effects of the present control scheme on the initial shear layer dynamics and concentration mixing.

Mixing and coherent vortices in turbulent coaxial jets

Direct numerical simulations associated with mixing in constant-density round coaxial jets are performed. They are validated by comparison against laboratory experiments. The mixing process is studied by seeding a passive tracer first in the outer annular jet, then in the inner jet. We demonstrate the important role played by coherent vortices in the mixing mechanisms. The turbulent mixing exhibits an intermittent character as a consequence of fluid ejections caused by the counter-rotating streamwise vortices. We quantify also the domination of the outer jet and show that the fluid issuing from the central jet remains confined. To cite this article: G. Balarac, M. Si-Ameur, C. R. Mecanique 333 (2005).

Turbulent heat mixing of a heavy liquid metal flow within the MEGAPIE window geometry: The heated jet experiments

The use of heavy liquid metals (HLM) serving both as coolant and as spallation source facilitates simple beam window geometries. An adequate cooling of the beam window requires a conditioning of the flow. Within the MEGAPIE window this is realized in a cylindrically shaped geometry, in which the main flow is guided in an annular gap downwards and then u-turned close to a hemispherical shell into a riser tube. In order to avoid stagnating fluid domains leading to unacceptably high window temperatures a jet flow is injected in direction of the lower shell. In this study the turbulent mixing of hot jet into a cold main flow is investigated both experimentally and numerically for the MEGAPIE geometry on a 1:1 scale. The experiments have been conducted in the Karlsruhe Lead Laboratory (KALLA). In parallel a numerical simulation has been performed. Close to the technically most interesting positions in the lower shell, a reasonably good agreement between numerical and experimental data has been found. Here, a sufficient description of the turbulent heat transfer in a lead–bismuth flow was obtained. However, as the flow proceeds downstream not only qualitative but quantitative differences appear, which have to be analyzed in more detail in the future.

Comparative study of turbulent mixing in jet in cross-flow configurations using LES

Mixing processes in turbulent fluid motion are of fundamental interest in many situations in engineering practice. Due to its practical importance in a vast number of applications, the generic configuration of the jet in cross-flow has been studied extensively in the past. Recently, the question has received a lot of attention, whether the unsteady behavior of the jet in cross-flow can be influenced by either active or passive means in order to control and enhance the mixing process. In the present paper, we use the large eddy simulation (LES) methodology to investigate how turbulent mixing can be enhanced by varying the angle between the jet and the oncoming cross-flow. After validating the computations against measurements by Andreopoulos and Rodi, we analyze qualitatively and quantitatively the mixing process for three configurations with different angles. It is shown that the inclination influences the characteristics of vortical structures and secondary motion which in turn have an effect on the mixing process. Besides a PDF of the passive scalar and a scalar energy spectrum, a mixedness parameter is used to provide information with respect to the quality and rate of mixing.

Analysis of a turbulent jet mixing flow by using a PIV-PLIF combined system

In the present paper, the development of a high-resolution PIV-PLIF combined system for the simultaneous measurements of velocity and passive scalar concentration fields is described. The high-resolution PIV-PLIF combined system is used to perform the simultaneous whole-field measurements of velocity and concentration in the near field of a turbulent jet mixing flow. The characteristics of the mass transfer process and momentum transfer process in the near field of the jet mixing flows are discussed in the terms of ensemble-averaged velocity and concentration, turbulent intensity, concentration standard deviation and the correlation terms between the fluctuating velocities and concentration.

円形噴流の制御に関する研究 : PVDFフィルムアクチュエータによる軸方向励起(S22-1 噴流の制御-1,S22 流れ制御の技術)

Turbulent mixing of a circular jet, which is commonly used in many gas turbine applications, has been studied from the viewpoint of control. The static pressure fluctuation is a basic flow parameter in turbulence. In this study, the vortex structure of the free shear layer in the circular jet is excited by the piezo film actuators. The effect of axial excitation on the turbulent mixing is clarified by simultaneous measurement using hot wire anemometer and static pressure probe.

The structure of fine-scale scalar mixing in gas-phase planar turbulent jets

Fine-scale scalar mixing in gas-phase planar turbulent jets is studied using measurements of three-component scalar gradient and scalar energy dissipation rate fields. Simultaneous planar Rayleigh scattering and planar laser-induced fluorescence, applied in parallel planes, yield the three-dimensional scalar field measurements. The spatial resolution is sufficient to permit differentiation in all three spatial directions. The data span a range of outer-scale Reynolds numbers from 3290 to 8330. Direct measurement of the thicknesses of scalar dissipation structures (layers) shows that the thicknesses scale with outer-scale Reynolds number as ) plays a significant role in the scalar dissipation process. The present data resolve a range of length scales from the dissipation scales up to nearly the jet full width, and thus can be used in a priori testing of subgrid models for scalar mixing in large-eddy simulations (LES). Comparison of two models for subgrid scalar variance, a scale-similarity model and a gradient-based model, indicates that the scale-similarity model is more accurate at larger LES filter sizes.

Modeling study of turbulent mixing over the continental shelf: Comparison of turbulent closure schemes

The sensitivity of model‐produced time‐dependent wind‐driven circulation on the continental shelf to the turbulent closure scheme employed is studied with a two‐dimensional approximation (variations across‐shelf and in depth) using the Princeton Ocean Model. The level 2.5 Mellor‐Yamada closure (MY), k‐ε closure, and K‐Profile Parameterization schemes are used to evaluate the mesoscale fields and the spatial and temporal variability of mixing. All three submodels produce similar features in the mesoscale circulation. They produce qualitatively similar eddy diffusivities and eddy viscosities, although the turbulent structure and the mixing intensities can differ quantitatively. The k‐ε length scale follows the buoyancy length scale when stratification is important. In contrast, the length scale produced by the q2l equation in the MY scheme deviates substantially from the buoyancy scale unless a stratification‐dependent limitation is imposed. During upwelling‐favorable winds, the majority of turbulent mixing occurs in the top and the bottom boundary layers and in the vicinity of the vertically and horizontally sheared coastal jet. Turbulent mixing in the coastal jet is primarily driven by shear‐production. The near‐surface flow on the inner shelf becomes convectively unstable as wind stress forces the upwelled water to flow offshore in the surface layer. During downwelling‐favorable winds, the strongest mixing occurs in the vicinity of the downwelling front. The largest turbulent kinetic energy and dissipation are found near the bottom of the front. Turbulence in the bottom boundary layer offshore of the front is concentrated between recirculation cells which are generated as a result of symmetric instabilities in the boundary layer flow.

Simultaneous velocity and concentration measurements of a turbulent jet mixing flow.

A method for the simultaneous measurement of velocity and passive scalar concentration fields by means of particle image velocimetry (PIV) and planar laser induced florescence (PLIF) techniques is described here. An application of the combined PIV-PLIF system is demonstrated by performing simultaneous velocity and concentration measurements in the near field of a turbulent jet mixing flow. The distributions of the ensemble-averaged velocity and concentration, turbulent velocity fluctuation, concentration standard deviation, and the correlation terms between the fluctuating velocities and concentration in the near field of the turbulent jet flow are presented as the measurement results of the simultaneous PIV-PLIF system.

Optimization of turbulent jet mixing

We introduce an approach for controlling jet mixing that combines direct numerical simulation of an incompressible jet flow with stochastic optimization procedures. The jet is excited with helical and combined helical and axial actuations at the orifice. An objective function that measures the spreading of the jet evaluates the performance of the actuation parameters. The optimization procedure searches for the best actuation by automatically varying the parameters and calculating their objective function value. Solutions that lead to a pronounced spreading of the jet are found within reasonable time, although the evaluation of the objective function, the DNS of the jet, is expensive. For a jet flow at low Reynolds number the performance of different search algorithms (simulated annealing and evolution strategies) is evaluated. We compare various objective functions based on radial velocity and the concentration of a passive scalar, including functions that penalize actuation with high amplitudes. We find that a combined axial and helical actuation is much more efficient with respect to jet mixing than a helical actuation alone.

Effects of Forward Flight on Jet Mixing Noise from Fine-Scale Turbulence

It is known experimentally that a jet in forward flight radiates less noise than the samejetin a static environment. At a forward flight Mach number of 0.2, the noise reduction, depending on the jet operating conditions, could be as large as 4-5 dB in the sideline directions. In the past, a way to predict flight effects was to use the method of relative velocity exponent. Another method was to extrapolate measured static jet noise to the flight condition by means of scaling formulas. Both methods are semi-empirical. The fine scale turbulence jet mixing noise theory of Tam and Auriault (Tam, C. K. W., and Auriault, L., Jet Mixing Noise from Fine Scale Turbulence, AIAA Journal, Vol. 37, No. 2, 1999, pp. 145-153) is extended for application to jets in simulated forward flight. It will be shown that the calculated noise spectra at different simulated forward flight Mach numbers for both supersonic and subsonic jets compare well with experiments. The effects of forward flight on the sources of fine-scale turbulent jet mixing noise is also investigated. It is found that in the presence of forward flight the dominant noise sources move downstream. The turbulence intensity and the size of turbulent eddies responsible for noise emission are reduced.

Effects of forward flight on jet mixing noise from fine-scale turbulence

It is known experimentally that a jet in forward flight radiates less noise than the samejetin a static environment. At a forward flight Mach number of 0.2, the noise reduction, depending on the jet operating conditions, could be as large as 4-5 dB in the sideline directions. In the past, a way to predict flight effects was to use the method of relative velocity exponent. Another method was to extrapolate measured static jet noise to the flight condition by means of scaling formulas. Both methods are semi-empirical. The fine scale turbulence jet mixing noise theory of Tam and Auriault (Tam, C. K. W., and Auriault, L., Jet Mixing Noise from Fine Scale Turbulence, AIAA Journal, Vol. 37, No. 2, 1999, pp. 145-153) is extended for application to jets in simulated forward flight. It will be shown that the calculated noise spectra at different simulated forward flight Mach numbers for both supersonic and subsonic jets compare well with experiments. The effects of forward flight on the sources of fine-scale turbulent jet mixing noise is also investigated. It is found that in the presence of forward flight the dominant noise sources move downstream. The turbulence intensity and the size of turbulent eddies responsible for noise emission are reduced.

Numerical Simulation of Shock-Enhanced Mixing in Nonuniform Density Turbulent Jets

A numerical study of the interaction of weak normal shock waves with turbulent jets was conducted. The cone guration consisted of a planar jet of air, helium, or carbon dioxide situated on the centerline of a shock tube withanaircoe ow.Theshockstrengthswere Ms =1:2and1:4.Thenumericalmodelwasbasedon atime-dependent, Navier‐Stokes approach and a two-equation q‐! turbulence model. The results indicate that passage of a shock through low-density (helium)jets produces a vortexlike structure not seen for the case of the air or carbon dioxide jets. Helium jets exhibit a decrease in mean jet e uid concentration due to shock interaction of up to approximately 30% at a location 30 jet exit heights downstream of the jet exit for a shock strength of Ms =1:4. The amount of mixing enhancementincreases with increasing shock strength and decreases with increasing downstream distance. In comparison, the air and carbon dioxide jets show a signie cantly smaller degree of mixing enhancement. These results are qualitatively consistent with recent experimental results in axisymmetric, turbulent jets subject to normal shock passage.

Shock-induced mixing of nonhomogeneous density turbulent jets

An experimental study of the mixing enhancement and changes in flow structure arising from the interaction of weak normal shock waves with turbulent jets was conducted. The experimental configuration was an axisymmetric jet processed by weak normal shock waves propagating in a shock tube along the jet axis. Experiments involved three different jet gases: helium, air, and carbon dioxide, each in a coflowing air stream, with nominal jet fluid to ambient density ratios of 0.14, 1.00, and 1.52, respectively. The jet local Reynolds number was Reδ≈25 000 and the nominal oncoming shock Mach numbers were 1.23 and 1.45. Planar laser Mie light scattering from mineral oil smoke was utilized for flow visualization and for obtaining jet fluid concentration distributions across diametric planes of jets. Analysis of the spatial probability density function (pdf) of jet fluid concentration indicates that the average helium jet fluid concentration levels decrease and become more uniform in the regions processed by the shock waves. The degree of mixing enhancement increases with increasing shock strength, and amounts to nearly 30% for the stronger shock (M=1.45). The passage of a shock through low-density (helium) jets induces the formation of a flow structure that resembles a large-scale, toroidal vortex. The air and carbon dioxide jets exhibit neither a vortex-like structure or a significant change in mixing upon shock passage, unlike the helium jets. A comparison of the results for the helium and carbon dioxide jets indicates that the reversal of the density ratio between the jet and the surroundings, and the consequent change in the sign of baroclinic vorticity does not yield similar effects in terms of flow structure or mixing enhancement. The average concentration behind the shock wave decreases for both air and helium jets with increasing distance behind the shock. These features are explained qualitatively in terms of a simple characteristic time scale argument. The spatially averaged scalar dissipation calculated from the concentration data decreases for both the air and helium jets due to shock passage. The change is less marked for the air jet than for the helium jet for a given shock strength.

Numerical simulation of shock-enhanced mixing in nonuniform density turbulent jets

Numerical simulation of shock-enhanced mixing in nonuniform density turbulent jets

Dual emission laser induced fluorescence for direct planar scalar behavior measurements

In this paper, a new method of measuring scalar behavior in bulk aqueous fluid flows is presented. Using a simple ratiometric scheme, laser induced fluorescence from organic dyes can be normalized so that direct measurements of a scalar in the flow are possible. The technique dual emission laser induced fluorescence (DELIF) relies on normalizing the fluorescence emission intensity of one dye with the fluorescence emission intensity of a second dye. Since each dye fluoresces at a different wavelength, one can optically separate the emission of each dye. This paper contains an overview of the basic ratiometric technique for pH and temperature measurements as well as the spectral properties of nine water soluble dyes. It also covers the three most significant sources of error in DELIF applications. To demonstrate the technique, steady state turbulent jet mixing and temperature fields in a thermal plume were quantified. The accuracy was camera limited at under 3% of the fluorescence ratio which corresponds to 0.1 pH units or 1.8 °C.

Effects of micro-mixing in gas-phase turbulent jets

Abstract Scalar mixing in turbulent jets is modeled using a hybrid Monte Carlo PDF method which solves for the joint velocity-scalar PDF. Molecular scalar mixing is modeled by: Interaction by Exchange with the Mean, Coalescence/Dispersion (C/D), Binomial Langevin and Mapping Closure models. The concentration field statistics adopt a self-similar solution along rays emerging from the jets virtual origin. Predicted PDF's of the normalized concentration are compared to measurements. All mixing models yield good agreement with the measurements at all radial and axial positions. The C/D model gives the best results close to the centerline but at the edge of the jet, the model predicts unrealistic PDF shapes.

Flow and Separation Characteristics of Three-dimensional Negatively Buoyant Wall Jets

This paper reports a study of the behavior of cold air jets, specifically three-dimensional, incompressible, negatively buoyant, turbulent wall jets, and their separation from horizontal surfaces. With a cold-air distribution system, both the supply air temperature and flow rate are lower than in conventional systems for the same cooling load. The cold air is supplied to a zone at temperatures between 39°F and 49°F (3°C and 9°C) instead of the conventional 55°F (13°C). As the building's ventilation and cooling airflow supply temperature and flow rate are reduced below the conventional values, indoor thermal environment considerations become increasingly important. An analysis using integral forms of the momentum and energy equations is presented for determination of the separation point of buoyant turbulent wall jets. The analysis shows that the separation point is a function of the outlet Richardson number in addition to the geometry of the outlet diffuser or nozzle. Also reported in this paper is an experimental investigation of the mixing and separation of three-dimensional buoyant turbulent wall jets of air discharging into a large room. Velocity and temperature difference profiles and half-width growth rates for these jets are presented. The separation distances are also measured, and compared to the predicted results.

Scalar mixing in turbulent concentric round jets

AbstractThe scalar mixing field of a free, turbulent concentric round jet has been examined using marker nephelometry. The flow conditions included velocity ratios between the centre and annular jet of 0.188, 0.519 and 0.911. The correlation function between the concentration fluctuations in the two jet streams increased from –1.0 near the nozzle to + 1.0 further downstream indicating a tendency to complete mixing between the jets. The initial mixing behaviour between the jets was better for the lower velocity ratio between the jets (Ui/Uo = 0.188) although further downstream there was better transverse mixing between the jets with the higher velocity ratio (Ui/Uo = 0.911).

Turbulent mixing in a free jet issuing from a low aspect ratio contoured rectangular nozzle

AbstractThe flow field of a turbulent free jet issuing from a contoured rectangular nozzle of aspect ratio 2 has been studied experimentally, using hot-wire anemometry. The study was undertaken to gain some understanding of the mixing process within the jet. Results of the measured and derived mean flow and turbulence quantities are presented. The three components of the mean velocity vector, the three Reynolds normal stresses, the two Reynolds primary shear stresses, and the flatness factor of the streamwise fluctuating velocity were measured. Mass entrainment, turbulence kinetic energy, and the intermittency factor were derived from the mean streamwise velocity data, the Reynolds normal stress data, and the flatness factor data, respectively. The derived results and the Reynolds primary shear stress data indicate enhanced near-field mixing of the present rectangular jet compared to a round turbulent free jet. The mean streamwise velocity field changes from a rectangular to an oval and then to an approximately circular shape at about twenty equivalent nozzle diameters downstream of the nozzle exit plane.