Bulletin of the American Physical Society, Nov 16, 2015
Angeles-The process of laser-induced breakdown for diagnostics contains a multitude of physical p... more Angeles-The process of laser-induced breakdown for diagnostics contains a multitude of physical phenomena. An understanding of the interaction between the laser and the plasma is crucial for many applications. In the current work, we developed a collisional-radiative (CR) model for laser-produced argon plasmas. The model is constructed from the LANL database, 2 which includes all the relevant collisional and radiative processes for all the ionic stages of argon. The laser is coupled to the plasma via multiphoton ionization and inverse Bremsstrahlung; these processes are important for electron production and heating. The use of the CR model allows us to identify dominant mechanisms responsible for initial breakdown of the gas and thermal equilibriation processes. The results are compared with experimental data from laser-induced breakdown experiments. 3 1 Research supported by the AFOSR.
Bulletin of the American Physical Society, Nov 26, 2013
The jet in crossflow, or transverse jet, is a canonical flowfield that has relevance to engineeri... more The jet in crossflow, or transverse jet, is a canonical flowfield that has relevance to engineering systems ranging from dilution jets and film cooling for gas turbine engines to thrust vector control and fuel injection in high speed aerospace vehicles to environmental control of effluent from chimney and smokestack plumes. Over the years, our UCLA Energy and Propulsion Research Lab's studies on this flowfield have focused on the dynamics of the vorticity associated with equidensity and variable density jets in crossflow, including the stability characteristics of the jet's upstream shear layer. A range of different experimental diagnostics have been used to study the jet's upstream shear layer, whereby a transition from convectively unstable behavior at high jet-to-crossflow momentum flux ratios to absolutely unstable flow at low momentum flux and/or density ratios is identified. These differences in shear layer stability characteristics have a profound effect on how one employs external excitation to control jet penetration, spread, and mixing, depending on the flow regime and specific engineering application. These control strategies, and challenges for future research directions, will be identified in this presentation.
Public reporting burden for this collection of information is estimated to average 1 hour per res... more Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
Bulletin of the American Physical Society, Nov 24, 2020
Angeles-The present experiments investigate the response of multi-port gaseous jet diffusion flam... more Angeles-The present experiments investigate the response of multi-port gaseous jet diffusion flames to applied transverse acoustic forcing corresponding to a standing wave at a resonant frequency of 332 Hz. Microjet flames with jet Reynolds numbers in the range of 20 to 100 were explored and high speed visible imaging enabled time-resolved quantification of intensity-based flame response. The oscillatory flames were analyzed via proper orthogonal decomposition (POD) to extract spatial modes and their corresponding phase trajectories via POD coefficient plots. Different regimes of flame response were identified based on the degree of forcing, including weakly oscillatory combustion, transition to multi-mode periodic liftoff, and highly perturbed periodic liftoff and reattachment of the flame preceding extinction. Sparse mode distribution and symmetries in the phase portraits at low amplitude excitation suggested that a reduced order model could be used to predict the observed transitions in the combustion process, and this will be described in a separate study [Alves, et al., APS DFD 2020].
Bulletin of the American Physical Society, Nov 20, 2017
(LES) provides the opportunity to reduce the interaction of numerical/modeling errors and offers ... more (LES) provides the opportunity to reduce the interaction of numerical/modeling errors and offers the chance to carry out grid-converged assessments 2 , important for model development. By utilizing a quasi a priori evaluation methodwherein the LES is assisted by closures derived from a fully-resolved computation 3it then becomes possible to understand the combined impacts of filter construction (e.g., filter width, spectral sharpness) and discretization choice on the solution accuracy. The present work looks at calculations of the compressible LES Navier-Stokes system and considers discrete filtering formulations in conjunction with high-order finite differencing schemes. Accuracy of the overall method construction is compared to a consistently-filtered exact solution, and lessons are extended to a posteriori (i.e., non-assisted) evaluations 4 .
Bulletin of the American Physical Society, Nov 24, 2014
Prior work by our group demonstrates the use of a collisional-radiative model in reproducing the ... more Prior work by our group demonstrates the use of a collisional-radiative model in reproducing the correct steady-state shock structure of ionizing shocks in monatomic gases. 2 In this presentation, we report on time dependent calculations of ionizing shock flows, which reveal additional physical phenomena arising from the unsteadiness and nonlinear wave coupling between convection and kinetics. The observed phenomena are similar to instabilities often seen in gaseous detonations. 3 The present model also takes into account radiative heat losses and radiation transport, which result in a reduction in the shock velocity and precursor effects. The latter phenomena may be important at high shock velocities, and are being investigated in detail.
Bulletin of the American Physical Society, Nov 20, 2012
Angeles-A detailed model of electronic excited states is essential in capturing all the nonequili... more Angeles-A detailed model of electronic excited states is essential in capturing all the nonequilibrium processes of a partially ionized plasma by means of collisional and radiative interactions. This collisional-radiative (CR) model allows us to consider deviations from equilibrium distribution of the internal states, and is now more commonly used in the study of plasma discharges. Prior studies by Kapper and Cambier 2 and Panesi et al. 3 suggest that this level of detail is needed for an accurate prediction of the flow field, and it is particularly relevant to plasma-combustion interactions. The required number of excited states needed to be included in the CR model is often prohibitively large due to the nonequilibrium condition of the plasma. The consequence is a large system of ODE's which needs to be solved at each time step. A reduced mechanism for the CR model can be attained by grouping the upper states of the atomic state distribution (ASDF) into a pseudo-level in which the population is characterized either by a uniform distribution or a Boltzmann distribution. This talk presents both detailed and reduced models for an ionizing shock in Argon.
The present experiments focused on the response of burning gaseous fuel jets to prescribed transv... more The present experiments focused on the response of burning gaseous fuel jets to prescribed transverse acoustic excitation as a means of exploring the coupling of reactive, acoustic, and flow processes relevant to combustion instabilities. Single reactive methane microjets were exposed to transverse resonant standing wave disturbances within an acoustic waveguide for which a range of amplitudes of excitation were applied. Temporal flame response to acoustic excitation was studied via simultaneous phase-locked OH* chemiluminescence and visible imaging, and additionally via high speed, temporally resolved visible imaging. Quantification of flame dynamics was achieved for phase-locked data via the Rayleigh and temporal flame distortion measurements, and for time-resolved data via proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). Characteristic signatures associated with various types of flame response were identified, including weakly oscillatory combustion, full-scale flame coupling and lock-in to excitation, and multi-mode flame dynamics preceding flame extinction. It was found that modal decomposition of flame dynamics via POD and DMD analysis revealed important additional dynamical features and signatures in metrics characteristic of transitions in the flame-acoustic coupling.
The dominant non-dimensional parameter for isodensity transverse jet flow is the mean jet-to-cros... more The dominant non-dimensional parameter for isodensity transverse jet flow is the mean jet-to-crossflow velocity ratio,R. In Part 1 (Megerianet al.,J. Fluid Mech., vol. 593, 2007, p. 93), experimental results are presented for the behaviour of transverse-jet near-field shear-layer instabilities for velocity ratios in the range 1 4, using two different base flows for the transverse jet. The first analysis assumes the flow field to be described by a modified version of the potential flow solution of Coelho & Hunt (J. Fluid Mech., vol. 200, 1989, p. 95), in which the jet is enclosed by a vortex sheet. The second analysis assumes a continuous velocity model based on the same inviscid base flow; this analysis is valid for the larger values of Strouhal number expected to be typical of the most unstable disturbances, and allows prediction of a maximum spatial growth rate for the disturbances. In both approaches, results are obtained by expanding in inverse powers ofRso that the free-jet results are obtained asR→∞. The results from both approaches agree in the moderately low-frequency regime. Maximum spatial growth rates and associated Strouhal numbers extracted from the second approach both increase with decreasing velocity ratioR, in agreement with the experimental results from Part 1 in the range 4
This experimental study explores the effects of acoustic excitation on burning droplets of liquid... more This experimental study explores the effects of acoustic excitation on burning droplets of liquid ethanol loaded with reactive aluminum nanoparticles (nAl). Continuously fed fuel droplet combustion and flame extinction (blowout) experiments were conducted in the vicinity of a pressure node (or velocity antinode) created in a closed acoustic waveguide, with a range of applied resonant forcing frequencies, pressure or velocity perturbation amplitudes, and particle loading concentrations. Simultaneous phase-locked OH* chemiluminescence and visible images were taken to quantify the influence of nanoparticle concentration on burning rate constant (K) and flameacoustic coupling dynamics. In the presence of velocity perturbations, nAl-laden droplets were observed to burn for longer periods of time than they burned in the absence of acoustics, likely resulting from suppression of particle agglomerate formation and expulsion. It was found that at higher forcing frequencies, there were relatively greater enhancements in K values of nAl-loaded droplets than for lower frequencies. Interestingly, while at higher forcing amplitudes, the burning rate constant increased for a given loading concentration, that increase tended to be reduced at higher loading concentrations. Phase-locked imaging demonstrated that the presence of nAl increased the Rayleigh index (G) as compared with neat fuels under the same forcing conditions. Higher amplitude excitation leading to extinction showed that the addition of nAl could significantly increase the mean extinction strain rate, in some cases by up to 44%.
The present experiments explored the dynamical character of the gaseous jet injected flush into c... more The present experiments explored the dynamical character of the gaseous jet injected flush into cross-flow for variable jet-to-cross-flow momentum flux ratios$J$(5, 12 and 41) and density ratios$S$(0.35 and 1.0). Contoured nozzle and straight pipe injectors were studied here, with the jet Reynolds number fixed at 1900 as other flow parameters were varied. Simultaneous acetone planar laser-induced fluorescence (PLIF) imaging and stereo particle image velocimetry (PIV) were used to study the relationships between scalar and velocity/vorticity fields, with a special focus on comparing PLIF-based extraction of scalar dissipation rates and local strain rates with PIV-based local strain rates in the upstream and downstream shear layers of the jet. There was remarkable similarity between the scalar and vorticity fields for the jet in cross-flow, spanning conditions for absolutely unstable upstream jet shear layers at low$J$or$S$values to conditions for convectively unstable shear layers for larger$J$, equidensity conditions (Megerianet al.,J. Fluid Mech., vol. 593, 2007, pp. 93–129; Getsingeret al.,Exp. Fluids, vol. 53, 2012, pp. 783–801). Proper orthogonal decomposition applied to both scalar and velocity fields revealed strengthening instabilities in both the upstream shear layer and in the jet’s wake as$J$was reduced. The simultaneous measurements allowed PLIF-extracted scalar dissipation rates and strain rates to be determined via a flamelet-like model and compared with PIV-extracted strain rates, each in the diffusion layer-normal direction. There was generally very good qualitative and quantitative agreement for these metrics in both the jet upstream and downstream shear layers for most flow conditions, with excellent correspondence to locations of shear layer vorticity roll up, although downstream shear layer strain rates in some cases showed lesser correspondence between PLIF- and PIV-based data. Such differences are shown to potentially result from diffusion and resolution effects as well as the influence of three-dimensional and transient effects which can be more significant in the lee side of the jet. Nevertheless, the present results reveal interesting dynamics and demonstrate the importance of strain fields in enhanced diffusion and transport phenomena.
An analytical/numerical model for the deflection and mixing of a single gaseous jet in a superson... more An analytical/numerical model for the deflection and mixing of a single gaseous jet in a supersonic crossflow is presented. The jet cross-section is described in terms of the compressible vortex pair resulting from viscous and impulsive forces acting at the jet periphery, and the vortex pair data are combined with data for the mass and momentum balance along the jet axis in order to model the trajectory and mixing of the injected fluid. A numerical technique is employed to solve for the inviscid outer flow and the position of the bow shock which envelopes the jet. The model is shown to be capable of predicting overall jet penetration (for perfectly or slightly underexpanded jets) to within 10 percent of experimental findings, while requiring only a few seconds of computer time.
Theoretical and Computational Fluid Dynamics, Apr 1, 1994
The essentially nonoscillatory (ENO) shock-capturing scheme for the solution of hyperbolic equati... more The essentially nonoscillatory (ENO) shock-capturing scheme for the solution of hyperbolic equations is extended to solve a system of coupled conservation equations governing two-dimensional, time-dependent, compressible chemically reacing flow with full chemistry. The thermodynamic properties of the mixture are modeled accurately, and stiff kinetic terms are separated from the fluid motion by a fractional step algorithm. The methodology is used to study the concept of shock-induced mixing and combustion, a process by which the interaction of a shock wave with a jet of low-density hydrogen fuel enhances mixing through streamwise vorticity generation. Test cases with and without chemical reaction are explored here. Our results indicate that, in the temperature range examined, vorticity generation as well as the distribution of atomic species do not change significantly with the introduction of a chemical reaction and subsequent heat release. The actual diffusion of hydrogen is also relatively unaffected by the reaction process. This suggests that the fluid mechanics of this problem may be successfully decoupled from the combustion processes, and that computation of the mixing problem (without combustion chemistry) can elucidate much of the important physical features of the flow.
*† ‡ § This experimental study explores the nature and control of shear layer instabilities assoc... more *† ‡ § This experimental study explores the nature and control of shear layer instabilities associated with the single jet in crossflow or transverse jet, a flowfield widely used in propulsive devices. These studies take advantage of prior experimental findings which suggest that the character of the jet’s nearfield shear layer instabilities can be significantly different for the transverse jet as compared with the free jet, and that the instabilities transition further when the jet-to-crossflow velocity ratio R is reduced below approximately 3.5. The differences in the stability characteristics of the jet in crossflow suggest the necessity of a “two-pronged” approach to the control of transverse jet penetration and spread, depending on the values of R, which is explored in this paper. For the transverse jet where R > 3.5, the instabilities are initiated beyond one diameter of the jet orifice, with a type of spatial evolution that indicates a convectively unstable flow. Such instabilities are affected by even low level sinusoidal forcing of the jet, which can then be used to control jet penetration and spread. For the case where R lies below 3.5, however, there is a rapid initiation of strong, distinct modes and harmonics near the jet exit that do not evolve spatially and are not affected by even strong external sinusoidal forcing of the jet. In this instance, strong jet forcing with a prescribed time scale is required to impact jet behavior. It is demonstrated that when the applied jet forcing waveform is a square wave with a prescribed temporal pulsewidth, significant jet response can be obtained even under globally unstable conditions.
Bulletin of the American Physical Society, Nov 16, 2015
Angeles-The process of laser-induced breakdown for diagnostics contains a multitude of physical p... more Angeles-The process of laser-induced breakdown for diagnostics contains a multitude of physical phenomena. An understanding of the interaction between the laser and the plasma is crucial for many applications. In the current work, we developed a collisional-radiative (CR) model for laser-produced argon plasmas. The model is constructed from the LANL database, 2 which includes all the relevant collisional and radiative processes for all the ionic stages of argon. The laser is coupled to the plasma via multiphoton ionization and inverse Bremsstrahlung; these processes are important for electron production and heating. The use of the CR model allows us to identify dominant mechanisms responsible for initial breakdown of the gas and thermal equilibriation processes. The results are compared with experimental data from laser-induced breakdown experiments. 3 1 Research supported by the AFOSR.
Bulletin of the American Physical Society, Nov 26, 2013
The jet in crossflow, or transverse jet, is a canonical flowfield that has relevance to engineeri... more The jet in crossflow, or transverse jet, is a canonical flowfield that has relevance to engineering systems ranging from dilution jets and film cooling for gas turbine engines to thrust vector control and fuel injection in high speed aerospace vehicles to environmental control of effluent from chimney and smokestack plumes. Over the years, our UCLA Energy and Propulsion Research Lab's studies on this flowfield have focused on the dynamics of the vorticity associated with equidensity and variable density jets in crossflow, including the stability characteristics of the jet's upstream shear layer. A range of different experimental diagnostics have been used to study the jet's upstream shear layer, whereby a transition from convectively unstable behavior at high jet-to-crossflow momentum flux ratios to absolutely unstable flow at low momentum flux and/or density ratios is identified. These differences in shear layer stability characteristics have a profound effect on how one employs external excitation to control jet penetration, spread, and mixing, depending on the flow regime and specific engineering application. These control strategies, and challenges for future research directions, will be identified in this presentation.
Public reporting burden for this collection of information is estimated to average 1 hour per res... more Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.
Bulletin of the American Physical Society, Nov 24, 2020
Angeles-The present experiments investigate the response of multi-port gaseous jet diffusion flam... more Angeles-The present experiments investigate the response of multi-port gaseous jet diffusion flames to applied transverse acoustic forcing corresponding to a standing wave at a resonant frequency of 332 Hz. Microjet flames with jet Reynolds numbers in the range of 20 to 100 were explored and high speed visible imaging enabled time-resolved quantification of intensity-based flame response. The oscillatory flames were analyzed via proper orthogonal decomposition (POD) to extract spatial modes and their corresponding phase trajectories via POD coefficient plots. Different regimes of flame response were identified based on the degree of forcing, including weakly oscillatory combustion, transition to multi-mode periodic liftoff, and highly perturbed periodic liftoff and reattachment of the flame preceding extinction. Sparse mode distribution and symmetries in the phase portraits at low amplitude excitation suggested that a reduced order model could be used to predict the observed transitions in the combustion process, and this will be described in a separate study [Alves, et al., APS DFD 2020].
Bulletin of the American Physical Society, Nov 20, 2017
(LES) provides the opportunity to reduce the interaction of numerical/modeling errors and offers ... more (LES) provides the opportunity to reduce the interaction of numerical/modeling errors and offers the chance to carry out grid-converged assessments 2 , important for model development. By utilizing a quasi a priori evaluation methodwherein the LES is assisted by closures derived from a fully-resolved computation 3it then becomes possible to understand the combined impacts of filter construction (e.g., filter width, spectral sharpness) and discretization choice on the solution accuracy. The present work looks at calculations of the compressible LES Navier-Stokes system and considers discrete filtering formulations in conjunction with high-order finite differencing schemes. Accuracy of the overall method construction is compared to a consistently-filtered exact solution, and lessons are extended to a posteriori (i.e., non-assisted) evaluations 4 .
Bulletin of the American Physical Society, Nov 24, 2014
Prior work by our group demonstrates the use of a collisional-radiative model in reproducing the ... more Prior work by our group demonstrates the use of a collisional-radiative model in reproducing the correct steady-state shock structure of ionizing shocks in monatomic gases. 2 In this presentation, we report on time dependent calculations of ionizing shock flows, which reveal additional physical phenomena arising from the unsteadiness and nonlinear wave coupling between convection and kinetics. The observed phenomena are similar to instabilities often seen in gaseous detonations. 3 The present model also takes into account radiative heat losses and radiation transport, which result in a reduction in the shock velocity and precursor effects. The latter phenomena may be important at high shock velocities, and are being investigated in detail.
Bulletin of the American Physical Society, Nov 20, 2012
Angeles-A detailed model of electronic excited states is essential in capturing all the nonequili... more Angeles-A detailed model of electronic excited states is essential in capturing all the nonequilibrium processes of a partially ionized plasma by means of collisional and radiative interactions. This collisional-radiative (CR) model allows us to consider deviations from equilibrium distribution of the internal states, and is now more commonly used in the study of plasma discharges. Prior studies by Kapper and Cambier 2 and Panesi et al. 3 suggest that this level of detail is needed for an accurate prediction of the flow field, and it is particularly relevant to plasma-combustion interactions. The required number of excited states needed to be included in the CR model is often prohibitively large due to the nonequilibrium condition of the plasma. The consequence is a large system of ODE's which needs to be solved at each time step. A reduced mechanism for the CR model can be attained by grouping the upper states of the atomic state distribution (ASDF) into a pseudo-level in which the population is characterized either by a uniform distribution or a Boltzmann distribution. This talk presents both detailed and reduced models for an ionizing shock in Argon.
The present experiments focused on the response of burning gaseous fuel jets to prescribed transv... more The present experiments focused on the response of burning gaseous fuel jets to prescribed transverse acoustic excitation as a means of exploring the coupling of reactive, acoustic, and flow processes relevant to combustion instabilities. Single reactive methane microjets were exposed to transverse resonant standing wave disturbances within an acoustic waveguide for which a range of amplitudes of excitation were applied. Temporal flame response to acoustic excitation was studied via simultaneous phase-locked OH* chemiluminescence and visible imaging, and additionally via high speed, temporally resolved visible imaging. Quantification of flame dynamics was achieved for phase-locked data via the Rayleigh and temporal flame distortion measurements, and for time-resolved data via proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). Characteristic signatures associated with various types of flame response were identified, including weakly oscillatory combustion, full-scale flame coupling and lock-in to excitation, and multi-mode flame dynamics preceding flame extinction. It was found that modal decomposition of flame dynamics via POD and DMD analysis revealed important additional dynamical features and signatures in metrics characteristic of transitions in the flame-acoustic coupling.
The dominant non-dimensional parameter for isodensity transverse jet flow is the mean jet-to-cros... more The dominant non-dimensional parameter for isodensity transverse jet flow is the mean jet-to-crossflow velocity ratio,R. In Part 1 (Megerianet al.,J. Fluid Mech., vol. 593, 2007, p. 93), experimental results are presented for the behaviour of transverse-jet near-field shear-layer instabilities for velocity ratios in the range 1 4, using two different base flows for the transverse jet. The first analysis assumes the flow field to be described by a modified version of the potential flow solution of Coelho & Hunt (J. Fluid Mech., vol. 200, 1989, p. 95), in which the jet is enclosed by a vortex sheet. The second analysis assumes a continuous velocity model based on the same inviscid base flow; this analysis is valid for the larger values of Strouhal number expected to be typical of the most unstable disturbances, and allows prediction of a maximum spatial growth rate for the disturbances. In both approaches, results are obtained by expanding in inverse powers ofRso that the free-jet results are obtained asR→∞. The results from both approaches agree in the moderately low-frequency regime. Maximum spatial growth rates and associated Strouhal numbers extracted from the second approach both increase with decreasing velocity ratioR, in agreement with the experimental results from Part 1 in the range 4
This experimental study explores the effects of acoustic excitation on burning droplets of liquid... more This experimental study explores the effects of acoustic excitation on burning droplets of liquid ethanol loaded with reactive aluminum nanoparticles (nAl). Continuously fed fuel droplet combustion and flame extinction (blowout) experiments were conducted in the vicinity of a pressure node (or velocity antinode) created in a closed acoustic waveguide, with a range of applied resonant forcing frequencies, pressure or velocity perturbation amplitudes, and particle loading concentrations. Simultaneous phase-locked OH* chemiluminescence and visible images were taken to quantify the influence of nanoparticle concentration on burning rate constant (K) and flameacoustic coupling dynamics. In the presence of velocity perturbations, nAl-laden droplets were observed to burn for longer periods of time than they burned in the absence of acoustics, likely resulting from suppression of particle agglomerate formation and expulsion. It was found that at higher forcing frequencies, there were relatively greater enhancements in K values of nAl-loaded droplets than for lower frequencies. Interestingly, while at higher forcing amplitudes, the burning rate constant increased for a given loading concentration, that increase tended to be reduced at higher loading concentrations. Phase-locked imaging demonstrated that the presence of nAl increased the Rayleigh index (G) as compared with neat fuels under the same forcing conditions. Higher amplitude excitation leading to extinction showed that the addition of nAl could significantly increase the mean extinction strain rate, in some cases by up to 44%.
The present experiments explored the dynamical character of the gaseous jet injected flush into c... more The present experiments explored the dynamical character of the gaseous jet injected flush into cross-flow for variable jet-to-cross-flow momentum flux ratios$J$(5, 12 and 41) and density ratios$S$(0.35 and 1.0). Contoured nozzle and straight pipe injectors were studied here, with the jet Reynolds number fixed at 1900 as other flow parameters were varied. Simultaneous acetone planar laser-induced fluorescence (PLIF) imaging and stereo particle image velocimetry (PIV) were used to study the relationships between scalar and velocity/vorticity fields, with a special focus on comparing PLIF-based extraction of scalar dissipation rates and local strain rates with PIV-based local strain rates in the upstream and downstream shear layers of the jet. There was remarkable similarity between the scalar and vorticity fields for the jet in cross-flow, spanning conditions for absolutely unstable upstream jet shear layers at low$J$or$S$values to conditions for convectively unstable shear layers for larger$J$, equidensity conditions (Megerianet al.,J. Fluid Mech., vol. 593, 2007, pp. 93–129; Getsingeret al.,Exp. Fluids, vol. 53, 2012, pp. 783–801). Proper orthogonal decomposition applied to both scalar and velocity fields revealed strengthening instabilities in both the upstream shear layer and in the jet’s wake as$J$was reduced. The simultaneous measurements allowed PLIF-extracted scalar dissipation rates and strain rates to be determined via a flamelet-like model and compared with PIV-extracted strain rates, each in the diffusion layer-normal direction. There was generally very good qualitative and quantitative agreement for these metrics in both the jet upstream and downstream shear layers for most flow conditions, with excellent correspondence to locations of shear layer vorticity roll up, although downstream shear layer strain rates in some cases showed lesser correspondence between PLIF- and PIV-based data. Such differences are shown to potentially result from diffusion and resolution effects as well as the influence of three-dimensional and transient effects which can be more significant in the lee side of the jet. Nevertheless, the present results reveal interesting dynamics and demonstrate the importance of strain fields in enhanced diffusion and transport phenomena.
An analytical/numerical model for the deflection and mixing of a single gaseous jet in a superson... more An analytical/numerical model for the deflection and mixing of a single gaseous jet in a supersonic crossflow is presented. The jet cross-section is described in terms of the compressible vortex pair resulting from viscous and impulsive forces acting at the jet periphery, and the vortex pair data are combined with data for the mass and momentum balance along the jet axis in order to model the trajectory and mixing of the injected fluid. A numerical technique is employed to solve for the inviscid outer flow and the position of the bow shock which envelopes the jet. The model is shown to be capable of predicting overall jet penetration (for perfectly or slightly underexpanded jets) to within 10 percent of experimental findings, while requiring only a few seconds of computer time.
Theoretical and Computational Fluid Dynamics, Apr 1, 1994
The essentially nonoscillatory (ENO) shock-capturing scheme for the solution of hyperbolic equati... more The essentially nonoscillatory (ENO) shock-capturing scheme for the solution of hyperbolic equations is extended to solve a system of coupled conservation equations governing two-dimensional, time-dependent, compressible chemically reacing flow with full chemistry. The thermodynamic properties of the mixture are modeled accurately, and stiff kinetic terms are separated from the fluid motion by a fractional step algorithm. The methodology is used to study the concept of shock-induced mixing and combustion, a process by which the interaction of a shock wave with a jet of low-density hydrogen fuel enhances mixing through streamwise vorticity generation. Test cases with and without chemical reaction are explored here. Our results indicate that, in the temperature range examined, vorticity generation as well as the distribution of atomic species do not change significantly with the introduction of a chemical reaction and subsequent heat release. The actual diffusion of hydrogen is also relatively unaffected by the reaction process. This suggests that the fluid mechanics of this problem may be successfully decoupled from the combustion processes, and that computation of the mixing problem (without combustion chemistry) can elucidate much of the important physical features of the flow.
*† ‡ § This experimental study explores the nature and control of shear layer instabilities assoc... more *† ‡ § This experimental study explores the nature and control of shear layer instabilities associated with the single jet in crossflow or transverse jet, a flowfield widely used in propulsive devices. These studies take advantage of prior experimental findings which suggest that the character of the jet’s nearfield shear layer instabilities can be significantly different for the transverse jet as compared with the free jet, and that the instabilities transition further when the jet-to-crossflow velocity ratio R is reduced below approximately 3.5. The differences in the stability characteristics of the jet in crossflow suggest the necessity of a “two-pronged” approach to the control of transverse jet penetration and spread, depending on the values of R, which is explored in this paper. For the transverse jet where R > 3.5, the instabilities are initiated beyond one diameter of the jet orifice, with a type of spatial evolution that indicates a convectively unstable flow. Such instabilities are affected by even low level sinusoidal forcing of the jet, which can then be used to control jet penetration and spread. For the case where R lies below 3.5, however, there is a rapid initiation of strong, distinct modes and harmonics near the jet exit that do not evolve spatially and are not affected by even strong external sinusoidal forcing of the jet. In this instance, strong jet forcing with a prescribed time scale is required to impact jet behavior. It is demonstrated that when the applied jet forcing waveform is a square wave with a prescribed temporal pulsewidth, significant jet response can be obtained even under globally unstable conditions.
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Papers by Ann Karagozian