Plane Jet Research Articles

Accounting for the effects of buoyancy on the turbulent scalar fluxes

The paper reports on the development of an explicit, algebraic model for the turbulent scalar fluxes which properly reflects the dependence of these fluxes on the gradients of mean velocity and on gravitational acceleration. Such dependencies are required by the exact equations governing the conservation of the turbulent fluxes but are absent from models which are based on the notion of eddy diffusivity and constant Prandtl or Schmidt number. In the present contribution, tensor representation theory is used to express the scalar fluxes in terms of its vector and tensor dependencies and then by applying a few assumptions to arrive at a model that includes the proper dependencies while being sufficiently compact and robust to be of use in practical applications. Model calibration was accomplished by reference solely to data from Large-Eddy Simulations of homogeneous turbulence in neutral and stable stratification while model performance was assessed by comparisons with experimental data from two-dimensional heated plane and free jets and buoyant plumes. In all cases, the model’s performance was found to be better than an alternative implicit algebraic model, and on par with that of a differential scalar-flux transport closure.

Turbulent/non-turbulent interfaces in temporally evolving compressible planar jets

Turbulent/non-turbulent interfaces (TNTIs) in compressible jets are studied with direct numerical simulations of temporally evolving compressible planar jets with jet Mach numbers MJ of 0.6, 1.6, and 2.6 ejected with a jet initial pressure equal to the ambient pressure. The flow properties near the TNTI are investigated with statistics computed on the local interfacial coordinate. The layer thicknesses are about 10-13η for the TNTI layer, 3η for the viscous superlayer, and 7-10η for the turbulent sublayer (TSL), where η is the Kolmogorov scale on the jet centerline. The TSL thickness divided by η decreases from 10 to 7 as MJ increases. The turbulent fluid is characterized with lower density, higher temperature, and lower pressure than the non-turbulent fluid, where these properties sharply change within the TNTI layer. The rate of change in internal energy near the TNTI is proportional to the initial kinetic energy of the jet, where the internal energy at the outer edge of the TNTI layer changes because of the diffusive/dilatational effects. The movement of entrained fluid is similar in compressible and incompressible jets. Compressibility affects the total entrainment rate via the total surface area of the TNTI, where the surface area of the TNTI per unit area of the plane perpendicular to the cross-streamwise direction decreases from 9.5 to 7.0 as MJ increases. Strongly compressive waves appear in the non-turbulent region at a high Mach number, where the imprints of these waves are found within the TNTI layer as strong pressure/temperature correlation and large values of pressure skewness.

On new scaling laws in a temporally evolving turbulent plane jet using Lie symmetry analysis and direct numerical simulation

A temporally evolving turbulent plane jet is studied both by direct numerical simulation (DNS) and Lie symmetry analysis. The DNS is based on a high-order scheme to solve the Navier–Stokes equations for an incompressible fluid. Computations were conducted at Reynolds number $\mathit{Re}_{0}=8000$, where $\mathit{Re}_{0}$ is defined based on the initial jet thickness, $\unicode[STIX]{x1D6FF}_{0.5}(0)$, and the initial centreline velocity, $\overline{U}_{1}(0)$. A symmetry approach, known as the Lie group, is used to find symmetry transformations, and, in turn, group invariant solutions, which are also denoted as scaling laws in turbulence. This approach, which has been extensively developed to create analytical solutions of differential equations, is presently applied to the mean momentum and two-point correlation equations in a temporally evolving turbulent plane jet. The symmetry analysis of these equations allows us to derive new invariant (self-similar) solutions for the mean flow and higher moments of the velocities in the jet flow. The current DNS validates the consequence of Lie symmetry analysis and therefore confirms the establishment of novel scaling laws in turbulence. It is shown that the classical scaling law for the mean velocity is a specific form of the current scaling (which has a more general form); however, the scaling for the second and higher moments (such as Reynolds stresses) has a completely different structure compared to the classical scaling. While the failure of the classical scaling for the second moments of the fluctuating velocities has been noted from the jet data for many years, the DNS results nicely match with the present self-similar relations derived from Lie symmetry analysis. Key ingredients for the present results, in particular for the scaling laws of the higher moments, are symmetries, which are of a purely statistical nature. i.e. these symmetries are admitted by the moment equations, however, they are not observed by the original Navier–Stokes equations.

The fluctuating velocity field of a forced planar impinging gas jet

ABSTRACTThe fluctuating velocity field of a planar impinging gas jet is studied using particle image velocimetry, while anti-symmetrically forcing the jet column at the nozzle exit using planar synthetic jets. The jet exit velocity is set to = 11 m/s providing a jet Reynolds number of , and the impingement distance is held constant at eight times the nozzle width W. Under forcing conditions near unstable frequencies, both the stream-wise and cross-stream fluctuating velocity fields of the jet have very different characteristics than that of the unforced jet. The fluctuating velocity fields reported here were also found to have qualitatively similar features to that of a high-speed self-excited impinging jet. For the highly periodic flow conditions tested, a triple decomposition, and a low order reconstruction of the fluctuating velocity field using proper orthogonal decomposition is employed. These methods allow for the extraction of the coherent velocity and turbulence fields from the forced planar impinging gas jet.

Picosecond x-ray radiography of microjets expanding from laser shock-loaded grooves

Material ejection upon the breakout of a shock wave at a rough surface is a key safety issue for various applications, including pyrotechnics and inertial confinement fusion. For a few years, we have used laser driven compression to investigate microjetting from calibrated grooves in the free surface of shock-loaded specimens. Fast transverse optical shadowgraphy, time-resolved measurements of planar surface and jet tip velocities, and post-shock analysis of some recovered material have provided data over ranges of small spatial and temporal scales, short loading pulses (ns-order), and extremely high strain rates. In the new experiment reported here, picosecond laser irradiation of a thin copper wire generates an ultrashort x-ray burst which is used to radiograph the microjets expanding from plane wedged-shape grooves in tin and copper samples shock-loaded by a longer, nanosecond laser pulse. Such ultrafast radiography provides estimates of the density gradients along the jets and of the total ejected mass at different times after shock breakout. Furthermore, it reveals regions of low density inside the samples deep beneath the grooves, associated with subsurface damage due to tension induced by the interaction of rarefaction waves. Thus, combining this x-ray probe with our former experimental techniques provides a more complete insight into the physics of microjetting at very high loading rates and the ballistic properties of the resulting ejecta.

Combined effects of groups of vortices generated by a pulsating turbulent plane jet impinging on a semi-cylinder: Effects of the forcing frequency

A submerged pulsating plane turbulent jet was simulated using Large Eddy Simulation at different forcing frequencies. Based on a previous work, interesting phenomena, related to the interaction of coherent primary vortices with the dynamical and thermal fields along the target curved wall, were elucidated for a forcing frequency equal to 600 Hz. As a systematic continuation, the present work extends the study to the effects of more forcing frequencies. The main objective remains the same which is investigating the time-dependent responses of Nusselt number, friction coefficient and pressure profiles to the passage of the coherent structures along the curved impingement wall. The distance s, along the target wall, is measured from the impact point and normalized by the jet exit width W.It was found that for a forcing frequency equal to 200 Hz, coherent forced primary vortices induced by the pulsations are separated by less organized vortices naturally induced as in the unforced jet. The results showed clearly that the natural vortices have moderate effects on the boundary layer development on the impingement surface starting at relatively short distances from the stagnation point, s/W < 3. On the other hand, the forced vortices seem to have a more influential role on the wall jet behavior but further downstream.Increasing the forcing frequency to 400 Hz reduces the distance separating successive forced vortices causing the pairing phenomenon to occur at s/W = 6 along the target wall. Increasing the forcing frequency further makes the pairing phenomenon followed by vortex breakdown to occur at shorter distances along the target wall. The smaller forcing frequencies such as 400 Hz generate large and strong distant vortices that affect the dynamic field noticeably with an important deterioration of heat transfer due to their strong mixing effect and entrainment of cold air from the surroundings towards the target wall. On the other hand, high frequencies (1500 Hz and 2200 Hz) generate smaller closely successive vortices, with a weaker effect, that do not disturb the boundary layer on the target wall considerably up to a distance of s/W = 4 where the lowest heat transfer is observed. In fact, the cold air from the surroundings is not entrained and does not approach the target wall over a long distance which generates the highest heat transfer away from the impingement region.

Experimental Study on the Aerodynamic Sealing of Air Curtains

Controlling the air quality is of the utmost importance in today’s buildings. Vertical air curtains are often used to separate two different climatic zones with a view to reduce heat transfer. In fact, this research work proposes an air curtain aimed to ensure a proper separation between two zones, a clean one and a contaminated one. The methodology of this research includes: (i) small-scale tests on water models to ensure that the contamination does not pass through the air curtain, and (ii) an analytical development integrating the main physical characteristics of plane jets. In the solution developed, the airflow is extracted from the contaminated compartment to reduce the curtain airflow rejected to the exterior of the compartment. In this research work, it was possible to determine the minimum exhaust flow necessary to ensure the aerodynamic sealing of the air curtain. This article addresses the methodology used to perform the small-scale water tests and the corresponding results.

Effects of Asymmetric Gas Distribution on the Instability of a Plane Power-Law Liquid Jet

As a kind of non-Newtonian fluid with special rheological features, the study of the breakup of power-law liquid jets has drawn more interest due to its extensive engineering applications. This paper investigated the effect of gas media confinement and asymmetry on the instability of power-law plane jets by linear instability analysis. The gas asymmetric conditions mainly result from unequal gas media thickness and aerodynamic forces on both sides of a liquid jet. The results show a limited gas space will strengthen the interaction between gas and liquid and destabilize the power-law liquid jet. Power-law fluid is easier to disintegrate into droplets in asymmetric gas medium than that in the symmetric case. The aerodynamic asymmetry destabilizes para-sinuous mode, whereas stabilizes para-varicose mode. For a large Weber number, the aerodynamic asymmetry plays a more significant role on jet instability compared with boundary asymmetry. The para-sinuous mode is always responsible for the jet breakup in the asymmetric gas media. With a larger gas density or higher liquid velocity, the aerodynamic asymmetry could dramatically promote liquid disintegration. Finally, the influence of two asymmetry distributions on the unstable range was analyzed and the critical curves were obtained to distinguish unstable regimes and stable regimes.

Alkali metal emissions in an early-stage pulverized-coal flame: DNS analysis of reacting layers and chemistry tabulation

The intricate coupling between coal pyrolysis, gas phase combustion and the emissions of alkali metal, such as sodium, is studied in the early stage of a temporally evolving three-dimensional planar turbulent jet carrying pulverized-coal particles. Complex chemistry is used to account for both the combustion of volatile hydrocarbons and the sodium containing species. The response of the sodium chemistry is analyzed in the mixture fraction space, along with the topology of the reactions zones. Combustion is found to start preferentially in partially premixed flames, which then evolve toward diffusion-like reactive layers and reach chemical equilibrium. From the direct numerical simulation (DNS) database, the possibility of modeling the dynamics of sodium species using one-dimensional premixed flamelet generated manifolds (FGM) is investigated. A chemical lookup table is constructed for the combustion of the partially premixed volatiles and an additional three-dimensional simulation is performed to compare the tabulated sodium species against their reference counterparts with complex chemistry. Quantitative analysis of the performance of the developed chemistry tabulation confirms the validity of the approach. Perspectives for the modeling of sodium emissions in pulverized-coal furnaces and boilers are finally drawn.

Large Eddy Simulation of Planar Free Jet Flow Using the Space-Time Conservation Element/Solution Element Scheme

In order to investigate the characteristics of planar free jet flows, the Monotonically Integrated Large Eddy Simulation (MILES) method with the high-resolution space-time conservation element/solution element (CE/SE) scheme was used to simulate the two-dimensional free jet flows. For free jet flows within the Reynolds number range 35300~2200, the flow structures, averaged and instant velocity, turbulence intensity, velocity profiles of different cross-sections were examined respectively. The simulation results agree well with the experiment data. A region of constant height exists right after the jet outlet, in which the average centerline velocity and turbulence intensity maintains same with the jet outlet, and the instant fluctuation amplitudes are also small. For turbulent flows, the length of the undisturbed region is equal to the length of region of constant height. While near the laminar region, the length of undisturbed region is slightly larger than the length of zone of constant height. The vortex structures start to generate after the undisturbed region, develop coherently in the region of flow establishment, and break down gradually in the region of fully established flow region. As the Reynolds number increases, the length of zone of constant height, the length of undisturbed region and the length of potential core region all decrease correspondingly.

Aerodynamic noise reduction using dual-jet planar air curtains

In this paper, dual-jet planar air curtains are demonstrated to be able to successfully remove aerodynamic noise radiated from tandem rods in a crossflow. Single-jet air curtains have shown significant promise as a low noise technology but can introduce additional noise sources such as lip and mixing noise. In this work, dual-jet air curtains are shown to address these obstacles, achieving the same shielding height with a significantly lower overall system noise. Providing high amplitude tonal and broadband noise, tandem rods are chosen as the test case and the fluid mechanics and acoustics associated with their shielding by air curtains are examined both experimentally and numerically. Particle image velocimetry and flow visualisation methods allow the flow fields to be examined, and a numerical analysis with computational fluid dynamics to be validated. Barely examined in the literature to date, the trajectory of planar jets in crossflow are studied here and compared to a basic semi-empirical model. The accurate identification of the trajectory allows a method for an optimum shielding height and dual jet velocity combination to be proposed and tested. Acoustic beamforming and 1/3 octave band analysis are performed for diagnostic noise source localisation and comparative evaluation. It is demonstrated that a judicious choice of flow parameters can result in the complete removal of the aerodynamic noise with almost no acoustic penalty.

Numerical study of local entropy generation in a heated turbulent plane jet developing in a co-flowing stream

• Effect of co-flow and directed co-flow with positive and negative angles is studied. • Numerical results are found to be in good agreement with experimental data. • High entropy generation corresponds to lower co-flow. • High values of entropy generation are near the nozzle exit. • The Merit number increases with the co-flow velocity. The present paper numerically investigates the effects of a co-flowing stream on the mean and turbulent flow properties, air entrainment, mixing efficiency, entropy generation rate, and Merit and Bejan numbers of a heated turbulent plane jet emerging in a co-flowing stream. The angle of moving external stream (α) is varied between −10° and + 10°.The first order k-ε turbulence model was used and compared to the existing experimental data. The Finite Volume Method (FVM) was used to discretize the governing equations. The predicted results were consistent with the experimental data. The obtained results showed that the alfa angle directed away from the centerline enhances the mixing. Moreover, the entrainment hypothesis was introduced to describe the development of turbulent jets issuing in a co-flowing air stream. This hypothesis is related to the mass flow rate of the surrounding fluid entrained into the jet. So far, it is obvious that the co-flow has considerably decreased air entrainment.

A DNS evaluation of mixing and evaporation models for TPDF modelling of nonpremixed spray flames

In the present work, nonpremixed temporally evolving planar spray jet flames are simulated using both direct numerical simulation (DNS) and the composition transported probability density function (TPDF) method. The objective is to assess the performance of various mixing and evaporation source term distribution models which are required to close the PDF transport equation in spray flames. Quantities which would normally be provided to the TPDF solver by spray models and turbulence models are provided from the DNS: the mean flow velocity, turbulent diffusivity, mixing frequency, and cell-mean evaporation source term. Two cases with different Damköhler numbers (Da) are considered. The low Da case (Da-) features extinction followed by reignition while extinction in the high Da case (Da+) is insignificant. The TPDF modelling considers two mixing models: interaction by exchange with the mean (IEM) and Euclidean minimum spanning trees (EMST). Three models for distribution of the evaporation source terms are considered: EQUAL which distributes them in proportion to notional particles’ mass weight, NEW which creates new particles of pure fuel, and SAT which distributes the sources preferentially to notional particles close to saturation. It is found that the IEM model overpredicts the extinction when used with any evaporation model for both Da- and Da+ cases. The EMST model captures well the trend for extinction and reignition for the Da- case when it is coupled with the EQUAL evaporation model, but it overpredicts the extinction when coupled with the NEW or SAT evaporation model. For the Da+ case, all evaporation models reasonably capture the flame dynamics when coupled with EMST. The flame temperature in the mixture fraction space was examined to further assess the model performance. In general the EMST model results in narrow PDFs with little conditional fluctuation, while the IEM model produces bimodal PDFs with burning and partial extinction branches.

Comparative analysis of methods for heat losses in turbulent premixed flames using physically-derived reduced-order manifolds

Heat losses have the potential to substantially modify turbulent combustion processes, especially the formation of pollutants such as nitrogen oxides. The chemistry governing these species is strongly temperature sensitive, making heat losses critical for an accurate prediction. To account for the effects of heat loss in large eddy simulation (LES) using a precomputed reduced-order manifold approach, thermochemical states must be precomputed not only for adiabatic conditions but also over a range of reduced enthalpy states. However, there are a number of methods for producing reduced enthalpy states, which invoke different implicit assumptions. In this work, a set of a priori and a posteriori LES studies have been performed for turbulent premixed flames considering heat losses within a precomputed reduced-order manifold approach to determine the sensitivity to the method by which reduced enthalpy states are generated. Two general approaches are explored for generating these reduced enthalpy states and are compared in detail to assess any effects on turbulent flame structure and emissions. In the first approach, the enthalpy is reduced at the boundary of the one-dimensional (1D) premixed flame solution, resulting in a single enthalpy deficit for a single premixed flame solution. In the second approach, a variable heat loss source term is introduced into the 1D flame solutions by mimicking a real heat loss to reduce the post-flame enthalpy. The two approaches are compared in methane–air piloted turbulent premixed planar jet flames with different diluents that maintain a constant adiabatic flame temperature but experience different radiation heat losses. Both a priori and a posteriori results, as well as a chemical pathway analysis, indicate that the manner by which the heat loss is accounted for in the manifold is of secondary importance compared to other model uncertainties such as the chemical mechanism, except in situations where heat loss is unphysically fast compared to the flame time scale. A new theoretical framework to explain this insensitivity is also proposed, and its validity is briefly assessed.

Breakup modes of a planar liquid jet in a static electric field

Breakup process of a two-dimensional planar jet is analytically investigated when the jet is placed between two parallel sheath walls on which an external electric field is imposed. In the analysis, we consider the influence of finite relaxation time and convection of surface charges and tangential stress on the jet surfaces due to the parallel electric field. According to the leaky dielectric model and the slender jet approximation, nonlinear evolution equations for the jet are derived when the aerodynamic effect is neglected. The equations are solved numerically under the initial-boundary condition that the jet is emanating from a slit nozzle. It is found that there exist two types of breakup modes for symmetric deformations: (i) the surface tension dominant mode where a tip of the jet grows larger as the jet flows downward and (ii) the electric force dominant mode where the tip of the jet is accelerated and becomes thinner and thinner to the downstream. The existing regions of these two modes are mainly determined by two parameters (=charge relaxation time/time of fluid motion) and Λ (=electric pressure/fluid inertial). We find critical curves in the parameter space, across which the mode is transferred from (i) to (ii) with the increase of Λ and/or the decrease of . It is also found that antisymmetric deformations of the jet grow to the downward as long as the jet is thick, but the deformations are suppressed and stabilized as the jet becomes sufficiently thin.

One-dimensional turbulence modeling for cylindrical and spherical flows: model formulation and application

The one-dimensional turbulence (ODT) model resolves a full range of time and length scales and is computationally efficient. ODT has been applied to a wide range of complex multi-scale flows, such as turbulent combustion. Previous ODT comparisons to experimental data have focused mainly on planar flows. Applications to cylindrical flows, such as round jets, have been based on rough analogies, e.g., by exploiting the fortuitous consistency of the similarity scalings of temporally developing planar jets and spatially developing round jets. To obtain a more systematic treatment, a new formulation of the ODT model in cylindrical and spherical coordinates is presented here. The model is written in terms of a geometric factor so that planar, cylindrical, and spherical configurations are represented in the same way. Temporal and spatial versions of the model are presented. A Lagrangian finite-volume implementation is used with a dynamically adaptive mesh. The adaptive mesh facilitates the implementation of cylindrical and spherical versions of the triplet map, which is used to model turbulent advection (eddy events) in the one-dimensional flow coordinate. In cylindrical and spherical coordinates, geometric stretching of the three triplet map images occurs due to the radial dependence of volume, with the stretching being strongest near the centerline. Two triplet map variants, TMA and TMB, are presented. In TMA, the three map images have the same volume, but different radial segment lengths. In TMB, the three map images have the same radial segment lengths, but different segment volumes. Cylindrical results are presented for temporal pipe flow, a spatial nonreacting jet, and a spatial nonreacting jet flame. These results compare very well to direct numerical simulation for the pipe flow, and to experimental data for the jets. The nonreacting jet treatment overpredicts velocity fluctuations near the centerline, due to the geometric stretching of the triplet maps and its effect on the eddy event rate distribution. TMB performs better than TMA. A hybrid planar-TMB (PTMB) approach is also presented, which further improves the results. TMA, TMB, and PTMB are nearly identical in the pipe flow where the key dynamics occur near the wall away from the centerline. The jet flame illustrates effects of variable density and viscosity, including dilatational effects.

An Experimental Characterization of the Interaction Between Two Tandem Planar Jets in a Crossflow

This study reports an experimental investigation of two planar jets in a crossflow in a tandem arrangement. Tests were conducted in an open-jet wind tunnel facility using two-dimensional (2D)-particle imaging velocimetry (PIV) measurement. Using the terminology in the dual jets in a quiescent ambient, the mean flow field of the crossflow arrangement was divided into a converging region, a merging region, and a combined region. An approach to determining the range of these three regions was proposed based on the mean characteristics of horizontal velocity profiles of the flow field, validated by the experimental data. The momentum-dominated near field (MDNF) for the rear jet in the dual-jet configuration was recognized using the horizontal offset of mean jet trajectory, which accordingly gives a quantitative definition of the MDNF range. Discussions were made on the effects of different parameters on the three regions and MDNF. Finally, snapshot proper orthogonal decomposition (POD) analysis was conducted, characterizing the coherent structures of the flow field, particularly the large-scale vortices. It was observed that the large-scale vortices mainly occur in the shear layers of the jets and their occurrence is affected by the parameters of the jets. In addition, compared with the single-jet configuration, the introduction of the front jet was found to contribute to the occurrence and development of the large-scale vortices.

Reappraisal of the velocity derivative flatness factor in various turbulent flows

We first analytically show, starting with the Navier–Stokes equations, that the value of the derivative flatness is controlled by pressure diffusion of energy, viscous destructive effects and large-scale effects (decay and/or production). The latter two terms tend to zero when the Taylor-microscale Reynolds number$Re_{\unicode[STIX]{x1D706}}$is sufficiently large. We argue that the pressure-diffusion term should also tend to a constant at large$Re_{\unicode[STIX]{x1D706}}$. Available data for the velocity derivative flatness,$F$, in different turbulent flows are re-examined and interpreted in the light of the finite-Reynolds-number effect. It is found that$F$can differ from flow to flow at moderate$Re_{\unicode[STIX]{x1D706}}$; for a given flow,$F$may also depend on the initial conditions. The data for$F$in various flows, e.g. along the axis in the far field of plane and circular jets, and grid turbulence, show that it approaches a constant, with a value slightly larger than 10, when$Re_{\unicode[STIX]{x1D706}}$is sufficiently large. This behaviour for$F$is supported, at least qualitatively, by our analytical considerations. The constancy of$F$at large$Re_{\unicode[STIX]{x1D706}}$violates the refined similarity hypothesis introduced by Kolmogorov (J. Fluid Mech., vol. 13, 1962, pp. 82–85) to account for the intermittency of the energy dissipation rate. It is not, however, inconsistent with Kolmogorov’s original similarity hypothesis (Dokl. Akad. Nauk SSSR, vol. 30, 1941, pp. 299–303), although we contend that the power-law relation$F\sim Re_{\unicode[STIX]{x1D706}}^{\unicode[STIX]{x1D6FC}_{4}}$(Kolmogorov 1962), which is widely accepted in the literature, has in reality been almost invariably used to ‘model’ the finite-Reynolds-number effect for the laboratory data and has been strongly influenced by the weighting given to the atmospheric surface layer data. The inclusion of the latter data has misled previous investigations of how$F$varies with $Re_{\unicode[STIX]{x1D706}}$.

Direct Numerical Simulation Code Validation for Compressible Shear Flows Using Linear Stability Theory

In order to simulate compressible shear flow stability and aeroacoustic problems, a numerical code must be able to capture how a baseflow behaves when submitted to small disturbances. If the disturbances are amplified, the flow is unstable. The linear stability theory (LST) provides a framework to obtain information about the growth rate in relation to the excitation frequency for a given baseflow. A linear direct numerical simulation (DNS) should capture the same growth rate as the LST, providing a severe test for the code. In the present study, DNS simulations of a two-dimensional compressible mixing layer and of a two-dimensional compressible plane jet are performed. Disturbances are introduced at the domain inflow and spatial growth rates obtained with a DNS code are compared with growth rates obtained from LST analyses, for each baseflow, in order to verify and validate the DNS code. The good comparison between DNS simulations and LST results indicates that the code is able to simulate compressible flow problems and it is possible to use it to perform numerical simulation of instability and aeroacoustic problems.

Dynamic mode decomposition of a direct numerical simulation of a turbulent premixed planar jet flame: convergence of the modes

Dynamic Mode Decomposition (DMD) is a technique that enables investigation of unsteady and dynamic phenomena by decomposing data into coherent modes with corresponding growth rates and oscillatory frequencies. Because the method identifies structures unbiased by energy, it is particularly well suited to exploring dynamic processes having phenomena that span disparate temporal and spatial scales. In turbulent combustion, DMD has been previously applied to the analysis of narrowband phenomena such as combustion instabilities utilising both experimental and computational data. In this work, DMD is used as a tool to analyse broadband turbulent combustion phenomena from a three-dimensional direct numerical simulation of a low Mach number spatially-evolving turbulent planar premixed hydrogen/air jet flame. The focus of this investigation is on defining the metric of convergence of the DMD modes for broadband phenomena when both the temporal resolution and number of data snapshots can be varied independently. The residual is identified as an effective, even if imperfect, metric for judging convergence of the DMD modes. Other metrics – specifically, the convergence of the mode eigenvalues and the decay of the amplitudes of the modes – fail to capture convergence of the modes independently but do complete the information needed to evaluate the quality of the DMD analysis.