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Daniel Guggenheim School of Aerospace Engineering

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College of Engineering

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Aerospace Design Group

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Aerospace Systems Design Laboratory (ASDL)

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Air Transportation Laboratory

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Cognitive Engineering Center

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Space Systems Design Laboratory (SSDL)

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Publication Search Results

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Permuted proper orthogonal decomposition for analysis of advecting structures

(Georgia Institute of Technology, 2024-04-27) Ek, Hanna Maria

This work is motivated by the large and ever-increasing amounts of data from studies in experimental and computational fluid dynamics, and the desire to extract and analyze coherent structures from such datasets. Specifically, this thesis is concerned with vortex patterns in turbulent shear flows, which appear as advecting structures in planar measurements or slices through three-dimensional computational domains. Space-only proper orthogonal decomposition (POD) is one of the most widely used techniques for the analysis of coherent structures and decomposes mean-subtracted data into the space-time separated form q^' (x,t)=∑_j =〖a_j (t) ϕ_j (x) 〗. This method is optimal in the spatial inner product and targets high energy spatial structures, but it is sensitive to input data alignment and cannot effectively handle translations. This work applies a re-orientation of the space-time coordinates in the POD framework, and the modified POD method, referred to as permuted POD (PPOD), is the focus of this thesis. PPOD decomposes data as q^' (x,t)=∑_j =〖a_j (n) ϕ_j (s,t) 〗, where x=(s,n) is a general spatial coordinate system, s is the coordinate along the bulk advection direction in curvilinear space, and n=(n_1,n_2 ) are the mutually-orthogonal directions normal to s. PPOD is optimal in the s,t inner product and, thus, targets advecting structures via their s,t correlations. Specifically, the PPOD modes, ϕ_j (s,t), portray advection as diagonal features in s,t space, where the slope of the features corresponds to the phase speed. Hence, these speeds are a natural output of the decomposition and can vary in an arbitrary and dispersive manner along the s coordinate. Generally, the PPOD modes have arbitrary s,t dependences, and a single mode can describe a broadband or multi-frequency disturbance, as well as time-varying characteristics, such as transient and intermittent dynamics. Additionally, one- and two-dimensional Fourier transforms of the PPOD modes provide useful alternative ways to portray the modal characteristics. For example, the wavenumber-frequency spectrum provides a compact visualization of disturbance advection velocity or dispersion. The PPOD properties are considered through the analysis of data from three high Reynolds number advection-dominated flows: an acoustically forced reacting wake, a swirling annular jet, and a jet in cross flow (JICF), and the results are compared with those from space-only POD. In the wake and swirling jet cases, the leading PPOD and space-only POD modes focus on similar features: advecting shear layer structures. However, low-rank approximations of the wake flow, which is characterized by a broad range of spectral and wavenumber content, show clear differences in the methods’ ability to capture the spatial and temporal information. For equal low-rank approximations, space-only POD provides higher-fidelity spatial reconstructions, while PPOD provides higher-order frequency content. In contrast, the leading PPOD and space-only POD modes for the JICF datasets capture different types of flow structures: advecting shear layer vortices (SLVs) and bulk jet flapping, respectively, while the SLVs are spread over lower energy modes in the case of space-only POD. This shows that the s,t inner product allows the PPOD method to directly target the SLVs, despite them containing a smaller fraction of the energy compared to the jet flapping. Additionally, the leading PPOD mode captures key characteristics of the SLV dynamics for each of the JICF cases, including those typical of convectively and globally unstable JICF, as well as intermittent characteristics and minor time-dependent differences or shifts in the dynamics. On the other hand, higher-order space-only POD approximations are required for comparable descriptions of these dynamics, and the rank depends on the operating conditions and stability characteristics of the JICF.
Assessment and Analysis of Turbulent Flame Speed Measurements of Hydrogen-Containing Fuels

(Georgia Institute of Technology, 2023-08-14) Johnson, Henderson

Global efforts to reduce greenhouse gas emissions and achieve a carbon-neutral economy have spurred the exploration of integrating hydrogen into various aspects of the global energy infrastructure. This can involve incorporating hydrogen into existing power generation applications or utilizing fuels with significant hydrogen content, such as syngas. However, the introduction of hydrogen poses significant challenges due to its potential to greatly impact the combustion process, with many aspects of its behavior not yet fully understood under practical gas turbine operating conditions. This thesis aims to investigate the influence of thermodynamic, fluid mechanic, and fuel factors on the turbulent global consumption speed, ST,GC, across different fuel types containing up to 90% hydrogen. This parameter represents the average rate of conversion of reactants to products relative to a specific iso-surface. The presented database encompasses three distinct fuel types: H2/CO, H2/CO/CH4/N2, and H2/CH4¬, which represent fuels that are either commonly encountered in practical applications or are of interest for future applications. The latter two fuels are new to the overall Georgia Tech database of turbulent flame speed measurements which increase the amount of high pressure data (up to 20 atm), and add data at preheat temperatures up to 500 K. The addition of this data is of great importance as it allows for further exploration of thermodynamic and fuel effects on ST,GC¬. The analysis of this database reveals several key findings. Firstly, regardless of whether the unstretched laminar flame speed, SL,0, is held constant, higher pressures lead to an increase in ST,GC across all fuel types. The preheat temperature is also shown to increase ST,GC, but when normalized by the laminar flame speed, it demonstrates a decrease. Moreover, the effects of hydrogen addition in H2/CO and H2/CO/CH4/N2 fuel blends are more pronounced compared to those in H2/CH4 fuels. Building upon prior studies that link these observations to mixture stretch sensitivity, the database is analyzed within the framework of a quasi-steady leading points concept model. In this framework, the maximum stretched laminar flame speed, SL,max, serves as the normalizing parameter. This approach proves effective for the H2/CO fuels discussed in this work, as it captures fuel effects at a fixed pressure and preheat temperature. However, a notable limitation arises in its inability to account for systematic differences in pressure and preheat temperature, indicating the need for a second correlating parameter. To identify this second parameter, a systematic investigation of three additional dimensionless numbers, namely the turbulent Reynolds number, Ret, time scale ratio, and acceleration ratio, is presented. Each of these numbers represents a different physical phenomenon that could potentially account for the observed variation in the data reported. The addition of Ret was considered in prior work; however, we identify that is insufficient as an appropriate scaling number due to its inconsistent correlation with preheat temperature. The acceleration ratio was introduced as a novel means of attempting to capture the ability of a flame to accelerate relative to the flow field. Similar to the Reynolds number, this approach showed limited ability to capture both pressure and preheat temperature effects; nevertheless, it does offer a new way to think about turbulence-flame interactions. Ultimately, the time scale ratio emerges as the optimal second correlating parameter due to its lesser degree of scatter compared to the acceleration ratio. This finding is significant, as it aligns with prior analyses that incorporated the time scale ratio to quantify non-quasi-steady chemistry effects at the leading point and demonstrates its promise as an appropriate scaling approach across a wide variety of conditions.
Nonlinear Dynamics of Coupled Thermoacoustic Modes in the Presence of Noise

(Georgia Institute of Technology, 2022-07-30) John, Tony

This thesis investigates the dynamics of nonlinearly coupled thermoacoustic modes in the presence of noise. The dynamics of a single linearly unstable thermoacoustic mode has been extensively studied in literature. Typically, practical combustion systems consist of multiple thermoacoustic modes that are linearly stable or unstable at a wide range of frequencies. These modes can express independently or can interact with each other. The interactions between different modes is a strong function of the frequency spacing between them amongst other parameters such as its linear growth/decay rate, mode shapes etc. Studies have shown that, in a configuration with two linearly unstable modes, these modal interactions could lead to the suppression of one of the modes, and under certain conditions the more unstable mode (higher growth rate) can be suppressed. Frequency spacing between the modes particularly influenced the stability and existence of potential limit cycle solutions. In this work, the earlier studies are extended to include the effects of noise in the system, studying how deterministic dynamics change with the addition of noise and the impact of frequency spacing (i.e., closely or widely spaced) on the results. Noise can broaden the distribution of amplitudes (”diffusion”), change both the average limit cycle amplitudes (”drift”), and alter the bifurcation characteristics of the limit cycle solutions. In order to identify these noise-induced features, a local asymptotic analysis is performed to characterize the diffusive effects in the limit of low noise intensity. The width of the distribution is observed to be sensitive to frequency spacing and the variation in width along the limit cycle can be significant for widely spaced modes. Drift effects of noise are characterized by quantifying the shift in the averaged solutions from the deterministic values and the sensitivity of this shift to frequency spacing is explored. Further, bifurcation scenarios that exist due to symmetric/asymmetric coupling as well as those introduced by noise are identified. Examples are presented that show the dynamics of the system in its ensemble averaged state space and numerically obtained probability density functions (PDFs) are used to support the observations in the ensemble averaged state space. For certain frequency spacing and low noise intensity, two fixed points can be observed in the phase space and the most probable solution can be identified from the PDFs as well as by visually observing the domain of attraction for the fixed points. As the noise intensity is increased, changes in the qualitative features of the system are evident in the ensemble averaged state space and PDFs.
Shear layer dynamics of a reacting jet in a vitiated crossflow

(Georgia Institute of Technology, 2020-12-01) Nair, Vedanth

The jet in crossflow (JICF) is a canonical shear flow that is present in a number of practical configurations including industrial gas turbines. Its complex flow topology, heavily influenced by underlying hydrodynamic instabilities, makes it an attractive configuration to implement when the mixing performance is critical. Past studies analyzing the behavior of non-reacting jets have noted that the overall performance of JICF configurations can be tied to the behavior of the shear layer, which influences both near-field and far-field jet dynamics. As a result, techniques used to manipulate jet mixing and penetration, such as active jet modulation, require an understanding of the dominant instability characteristics of the shear layer. Although this configuration finds extensive use in reacting applications, the hydrodynamics of reacting flows are often fundamentally different from non-reacting flows, and few studies have analyzed the influence of heat release and reactions on JICF dynamics. In addition to varying the momentum flux ratio (J) and the density ratio (S) this study presents a novel method of systematically moving the flame position with respect to the shear layer to gauge its impact on shear layer stability. High speed optical diagnostics including Stereoscopic PIV, OH-PLIF and OH* chemiluminescence were used to quantify the flowfield and infer the behavior of the reaction zone. Moving the flame inside the shear layer was observed to significantly change the jet topology as the shear layer vortices (SLV) were completely suppressed. This was further quantified through a growth rate defined based on tracking the swirling strength of SLV structures. Other structural characteristics including the location of mixing transition were shown to be highly correlated with this extracted growth rate. Time-resolved velocity data was further used to quantify the shear layer spectrum by extracting the dominant instability frequencies and classify the instability behavior as convectively and globally unstable. In order to explain the observed instability behavior, the counter current shear layer (CCSL) model was used to extract an analogous stratification parameter (S’), which along with the counter current velocity (Λ) ratio was shown to capture the stability behavior of both non-reacting as well as reacting configurations.
Lean blowout sensitivities of complex liquid fuels

(Georgia Institute of Technology, 2019-05-20) Rock, Nicholas

Lean blowout is a process whereby a previously stable flame is either extinguished or convected out of its combustor. In aviation applications, blowout is a direct threat to passenger safety and it therefore sets operational limits on a combustor. Understanding the blowout problem is a key prerequisite to the deployment of alternative aviation fuels, as these fuels are expected to have comparable flame stability characteristics as traditional jet fuels. The objective of this work is to identify the fuel properties that govern lean blowout and to characterize their effect on the physics involved in the blowout process. The blowout performance of 18 different liquid fuels were experimentally compared in an aircraft relevant combustor. These experiments were repeated at 3 different air inlet temperatures, 300 K, 450 K, and 550 K, in order to vary the effect of fuel physical properties. Custom fuels were introduced that were specifically designed to decouple interrelated fuel properties and to accentuate the significance of preferential vaporization on lean blowout. The methodology that was used clearly demonstrated differences in the equivalence ratio at blowout between fuels, and a multiple linear regression analysis was performed to determine the relative contributions of each of the fuel properties. This work also characterizes the effect of fuel composition on the processes that precede blowout of the flame, thereby providing an explanation for why certain fuel properties govern lean blowout boundaries. By quantifying the time variation of flame luminosity and extinction “events” as a function of blowout proximity, it was demonstrated that local extinction processes are operative in and lead to blowout in spray flames. In addition, high speed imaging was used to analyze the space-time evolution of the most upstream point of the flame near blowout. Fast motion of these points upstream relative to the flow velocity was interpreted as flame re-ignition. These re-ignition processes become manifest when the stability of the flame is severely threatened by local extinction and often allow for recoveries that extend flame burning. Fuel composition was shown to have a clear effect on a flame’s propensity for extinction and re-ignition.
Turbulence-chemistry interactions for lean premixed flames

(Georgia Institute of Technology, 2018-10-09) Dasgupta, Debolina

Turbulent combustion, particularly premixed combustion has great practical importance due to their extensive industrial usage in gas turbines, internal combustion engines etc. However, the physics governing the inherent multi- scale interactions of turbulence, flow-field and chemistry is not yet well established. A complete understanding of each of these interactions and their coupling is essential for the development of models that can aid simulations of realistic engines (using Large Eddy Simulations (LES) or Reynolds averaged Navier-Stokes equations (RANS). Particularly, understanding the flame structure and its stabilization requires an understanding of the turbulence-chemistry interactions. This can manifest itself in many different forms. For example, flame wrinkling gives rise to flame stretch that can modify the local temperature and species concentrations in turn altering the local chemistry. Also, the smaller eddies in a turbulent flow can penetrate into the preheat and reaction zones changing the species’ gradients within the flame. The influence of turbulence on chemistry can be analyzed in two different ways: firstly, a “global” analysis which investigates the direct impact of turbulence on the chemical pathways (a series of elementary reactions involved in the fuel oxidation process) and secondly, a “local” analysis which investigates the influence of turbulence on the chemical flame structure (i.e. species and reaction rate profiles). To understand these influences of turbulence, this work performs Direct Numerical Simulations (DNS) for lean premixed flames involving three fuels: hydrogen, methane and n-dodecane. A “global” analysis using different metrics such as heat release and species consumption/production is performed to quantify the changes in the chemical pathways. This analysis is performed for the metrics averaged over the entire flame and conditioned on local flame features such as fuel consumption, curvature etc. The results are also compared and contrasted with simple laminar flame models such as unstretched flames, stretched flames and perfectly stirred reactors. In general, the laminar models provide a good estimate for the chemical pathways for these key metrics suggesting turbulence does not have a significant impact on the fuel oxidation pathways. However, this is not true for the reaction rate and species profiles across the flame. Conditional means of these quantities are calculated to identify the “local” influence of turbulence on chemistry. These conditional means are also compared with laminar unstretched and stretched flames to identify regions of good agreement and deviation. The laminar calculations are performed using two different transport models; firstly, the mixture-averaged transport wherein every species diffuses into the mixture with its molecular diffusivity and secondly, Le=1 transport wherein the mass diffusivity of every species is equal to the thermal diffusivity of the mixture eliminating effects of preferential and differential diffusion. Le=1 is considered the theoretical limit of transport where turbulent mixing governs the transport process opposed to molecular diffusivity. For lean hydrogen/air flames (Le<1), the behavior of the profiles is similar to the evolution of the laminar profiles with increasing stretch. However, for the lean methane/air flames (Le~1), with increasing turbulence intensity, the flame profiles deviate from the evolution of laminar profiles with increasing stretch and align more closely with the Le=1 transport model laminar flame profiles. For n-dodecane/air flames (Le>1), the evolution of the turbulent flame profiles, with slight increase in turbulence intensity, replicates the behavior of stretched flames. However, with a further increase, a deviation is seen from the stretched flame profiles. Additionally, these profiles significantly deviate from the Le=1 transport model suggesting the inadequacy of stretched flames and a simple Le=1 model to replicate the behavior of stretched flames. In order, to identify the effect of increased diffusivity due to turbulence, a new transport model is implemented for unstretched and stretched flames wherein a constant is added to the mass diffusivity of the species obtained from the mixture-averaged transport. This constant covers multiple orders of magnitude mimicking the effect of increased turbulence diffusivity. For the lean hydrogen flames(Le<1), the turbulent flame profiles are seen to evolve similar to the laminar profiles with increasing stretch and not similar to the laminar profiles with increasing diffusivity. This suggests mixtures containing a highly diffusive fuel does not need the aid of turbulence to enhance transport. For the lean methane flames (Le~1), the turbulent flame profile evolution is similar to the effect of increasing diffusivity for unstretched flames suggesting a significant effect of diffusivity on the flame structure. For the lean n-dodecane flames (Le>1), the turbulent flame profiles evolve similar to the effect of increased diffusivity on stretched flames. This further emphasizes the necessity to include diffusivity in laminar models used to replicate turbulent flame structure. Overall, this work helps identify the key players in turbulence-chemistry interactions which need to be considered for modeling real combustors.
Experimental investigation of transverse acoustic instabilities

(Georgia Institute of Technology, 2017-11-09) Smith, Travis Edward

This work presents 5 kHz stereo PIV and OH PLIF measurements as well as OH* and CH* chemiluminescence measurements of transversely forced swirl flames. The presence of transverse forcing on this naturally unstable flow both influences the natural instabilities, as well as amplifies disturbances that may not necessarily manifest themselves during natural oscillations. By manipulating the structure of the acoustic forcing field, both axisymmetric and helical modes are preferentially excited away from the frequency of natural instability. Additionally, forced and self-excited transverse acoustic instability studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that it is not the transverse disturbances themselves, but rather the induced axial acoustic disturbances, that control the bulk of the heat release response. The work first presents a method for spatially interpolating the phase locked r-z and r-θ planar velocity and flame position data, extracting the full three-dimensional structure of the helical disturbances. These helical disturbances are also decomposed into symmetric and antisymmetric disturbances about the jet core, showing the subsequent axial evolution (in magnitude and phase) of each of these underlying disturbances. Then experiments performed with essentially the same transverse acoustic wave field, but with and without axial acoustics, show that significant heat release oscillations are only excited in the former case. The results show that the axial disturbances at the nozzle exit are the dominant cause of the heat release oscillations. These observations support the theory that the key role of the transverse motions is to act as the “clock” for the instability, setting the frequency of the oscillations while having a negligible direct effect on the actual heat release fluctuations. They also show that transverse instabilities can be damped by either actively canceling the induced axial acoustics in the nozzle (rather than the much larger energy transverse combustor disturbances), or by passively tuning the nozzle impedance to drive an axial acoustic velocity node at the nozzle outlet.
Flow characterization of lifted flames in swirling, reacting flows

(Georgia Institute of Technology, 2017-05-15) Chterev, Ianko Pavlov

Swirl stabilized combustors are commonly used in gaseous fueled land-based gas turbines and liquid fueled aerospace combustors to achieve simultaneously high efficiency, low emissions, wide operability limits, and low thermal and mechanical hardware loadings. Flame shape and location are critical to successful design, and are, therefore, the general focus of this work. In premixed swirl combustion aerodynamically stabilized flames are sometimes observed and desirable as they potentially reduce hardware heat loadings. However, their understanding is largely phenomenological and geometry specific. First, aerodynamically stabilized flames are subject to flow perturbations such as a precessing vortex core (PVC), and therefore, this thesis studies how a precessing flow field affects time-averaged quantities such as flame location. Second, in swirling flowfields with no interior time-averaged stagnation point, flames are sometimes aerodynamically stabilized by instantaneous stagnation points created by large scale structures such as the PVC. Since this places the flame in a time-averaged reverse flow, natural questions are what the flame and flow characteristics are at the flame stabilization location, such as flame stretch, and why the flame does not flash back. Experiments in high pressure, multi-phase, hydrocarbon fueled, reacting flows are highly complex, and quantities such as liquid and gas phase fuel distribution, heat release and flowfield are difficult to obtain. Thus, another focus of this work is experimental development to study the internal physics. First, this thesis finds that precession in radial-axial planar measurements can result in the time-averaged stagnation point to be located in a highly negative region of the flow. Since the time-averaged flow field is often used to determine the flame location, these findings indicate that time-averaged treatments may lead to erroneous results. Precession can also alter the general flow field topology by inducing asymmetries and can cause time-averages to converge slower. Second, the local flow field of a flame aerodynamically stabilized by instantaneous stagnation points is characterized using planar velocity and flame location measurements, conditioned using a line-of-sight technique to capture the flame global leading in the imaging plane. The flame stretch is measured, indicating that the stretch the flame experiences has a high dependence on nozzle velocity. However, the scaling is not understood, and further study is proposed. The time-averaged flame stretch is much higher than opposed diffusion flame extinction stretch rate calculations, which also requires further study. Furthermore, the stretch is not correlated strongly with location and flow velocity. Last, a simultaneous stereo-PIV and fuel/OH-PLIF technique is developed using a single PLIF laser (and a PIV laser) to characterize the spray distribution, flame shape and location, and dual phase flow field for two different jet fuels, at pressures from 2 to 5 bar. Two different flame shapes are observed, with a stability behavior different than with gaseous fuel. Furthermore, the flames extends into the annular jet core, a phenomenon not observed in premixed systems, and mentioned as needing verification in the liquid fueled combustion literature.
Dynamics of Harmonically Forced Nonpremixed Flames

(Georgia Institute of Technology, 2016-04-19) Magina, Nicholas A

This thesis describes the dynamics, both spatio-temporal and heat release, of harmonically excited non-premixed flames. Analytical, numerical, computational, and, experimental analyses were performed, along with combined analyses methods, to study excitation and evolution of wrinkles on the flame front. Explicit expressions for the dynamics were developed. Wrinkle convection at the mean axial flow speed, and wrinkle dissipation and dispersion were analytically identified in the Pe-->∞ and Pe>>1 limits, respectively. Altered inlet mixture fraction profiles and attachment point dynamics were shown to accompany axial diffusion effects. Some physical effects such as axial diffusion, forcing configuration, and anisotropic diffusion altered the wrinkle interference pattern/waveform characteristics, while others, such as confinement, dimensionality, and differential diffusion, altered the dynamics through modifying the mean flame location. Comparisons to established premixed flame dynamics were made throughout. Despite having similar space-time dynamics, the heat release dynamics of the two differed greatly, having different dominant contributions, as well as different asymptotic trends. Experimental results obtained validated previous findings as well as enabled advanced model development, revealing the importance of accurate mixture fraction field capture, particularly in the near burner exit region. Findings shed light onto model and predictive improvements for future works.
Dynamics of premixed flames in non-axisymmetric disturbance fields

(Georgia Institute of Technology, 2013-07-19) Acharya, Vishal Srinivas

With strict environmental regulations, gas turbine emissions have been heavily constrained. This requires operating conditions wherein thermo-acoustic flame instabilities are prevalent. During this process the combustor acoustics and combustion heat release fluctuations are coupled and can cause severe structural damage to engine components, reduced operability, and inefficiency that eventually increase emissions. In order to develop an engine without these problems, there needs to be a better understanding of the physics behind the coupling mechanisms of this instability. Among the several coupling mechanisms, the “velocity coupling” process is the main focus of this thesis. The majority of literature has treated axisymmetric disturbance fields which are typical of longitudinal acoustic forcing and axisymmetric excitation of ring vortices. Two important non-axisymmetric disturbances are: (1) transverse acoustics, in the case of circumferential modes of a multi-nozzle annular combustor and (2) helical flow disturbances, seen in the case of swirling flow hydrodynamic instabilities. With significantly less analytical treatment of this non-axisymmetric problem, a general framework is developed for three-dimensional swirl-stabilized flame response to non-axisymmetric disturbances. The dynamics are tracked using a level-set based G-equation applicable to infinitely thin flame sheets. For specific assumptions in a linear framework, general solution characteristics are obtained. The results are presented separately for axisymmetric and non-axisymmetric mean flames. The unsteady heat release process leads to an unsteady volume generation at the flame front due to the expansion of gases. This unsteady volume generation leads to sound generation by the flame as a distributed monopole source. A sound generation model is developed where ambient pressure fluctuations are generated by this distributed fluctuating heat release source on the flame surface. The flame response framework is used to provide this local heat release source input. This study has been specifically performed for the helical flow disturbance cases to illustrate the effects different modes have on the generated sound. Results show that the effects on global heat release and sound generation are significantly different. Finally, the prediction from the analytical models is compared with experimental data. First, a two-dimensional bluff-body stabilized flame experiment is used to obtain measurements of both the flow and flame position in time. This enables a local flame response comparison since the data are spatially resolved along the flame. Next, a three-dimensional swirl-stabilized lifted flame experiment is considered. The measured flow data is used as input to the G-equation model and the global flame response is predicted. This is then compared with the corresponding value obtained using global CH* chemilumenescence measurements.