Godfrey Mungal

Stanford University | SU · Department of Mechanical Engineering

Compact and efficient energy absorption is desirable for numerous applications including manufacturing, transportation, and protective equipment. An ideal shock absorber is a smart material or structure that can adapt its force-displacement properties to minimize the peak impact force regardless of the impact energy. While traditional shock absorbe...

Single-excitation, dual-band-collection toluene planar laser-induced fluorescence (PLIF) is used to measure temperature and number density (or partial pressure) fields in non-uniform supersonic complex flows in the presence of mixing and compressibility. The study provides a quantitative evaluation of the technique in transverse jets in supersonic...

This manuscript describes continuous, high-repetition-rate (20 kHz) toluene planar laser-induced fluorescence (PLIF) imaging in an expansion tube impulse flow facility. Cinematographic image sequences are acquired that visualize an underexpanded jet of hydrogen in Mach 0.9 crossflow, a practical flow configuration relevant to aerospace propulsion s...

Combustion in the supersonic regime presents several challenges when compared to its low-speed counterpart. Here we review some of these challenges, and we describe some of the key features of one of the canonical flow fields in supersonic combustion: the reacting transverse jet in a supersonic crossflow (JISCF). From a practical standpoint, the ke...

The article focuses on secondary diaphragm thickness effects and improved pressure measurements in an expansion tube. Analog or digital signal processing can be used to filter some noise from the measurements, but at the expense of measurement bandwidth or accuracy; thus, the engineer must compromise between bandwidth and SNR to acquire reliable pr...

The characteristics of vortices induced by spanwise forcing using a streamwise-oriented dielectric barrier discharge (DBD) vortex generator (VG) are investigated with and without a boundary layer bleed slot, the flow of which is also driven by a DBD actuator. The velocity fields of the induced flows are characterized by ensemble averaged particle i...

A dielectric barrier discharge (DBD) actuator is used to stabilize the base of a lifted ethane turbulent jet diffusion flame by modifying the coflow velocity field. The velocity field and flame base are measured by particle image velocimetry and unfiltered flame chemiluminescence imaging, respectively. An axisymmetric DBD actuator, integrated onto...

This paper examines the stabilization of premixed and jet diffusion flames of methane, ethane, and propane by nanosecond repetitive pulsed plasma discharges. Combustion products are measured using gas chromatography while laser-induced breakdown spectroscopy (LIBS) is used to characterize the local equivalence ratios. We find that in premixed flame...

We revisit a problem first introduced by Francis Hama in his 1962 Physics of Fluids article 'Streaklines in a Perturbed Shear Flow.' Using a nascent computer, Hama calculated streaklines and pathlines for an inviscid shear flow containing a non-amplifying, sinusoidal, traveling-wave perturbation. He found that this simple flow field and perturbatio...

Toluene PLIF has been applied to image temperature in supersonic flowfields containing shock waves. Single- and dual-camera imaging schemes with a single excitation wavelength (266 nm) are presented, and the dual-camera scheme is optimized for imaging temperature from 300 to 900 K. The single-camera technique is implemented to verify the diagnostic...

Studies of nanosecond repetitively pulsed discharge plasma-assisted combustion were carried out on a laminar premixed methane/air flow in the vicinity of the lean flammability limit. Experimental results indicated that complete combustion is achieved when the equivalence ratio is above the known lean flammability limit (ϕ = 0.53) at high discharge...

Toluene PLIF has been applied to image temperature in supersonic flows generated in an expansion tube. Toluene has high fluorescence quantum yield and high temperature sensitivity compared to similar aromatic tracers, and these qualities of toluene make it ideal for imaging supersonic flows where large temperature gradients can exist. Toluene fluor...

We investigate the characteristics of vortices induced by spanwise forcing using a streamwise oriented dielectric barrier discharge (DBD) with and without a boundary layer suction channel, the flow of which is also driven by a DBD. The velocity field over various regions in the vicinity of the actuator is obtained by ensemble averaging particle ima...

Experimental investigation of the structure and effects of supersonic combustion in a compact, optically accessible inlet/scramjet combustor model is presented. The model was developed to investigate mixing, ignition, and combustion in supersonic flows under flow conditions realistic of scramjet combustors in a university-scale expansion tube. The...

The improvement of the lean blowout limit of bluffbody stabilized methane flames is investigated. The flame configuration consists of a hybrid of partially and fully premixed flames, which is produced by injecting methane jets from a streamline-shaped bluffbody into a fully premixed methane/air crossflow. Additional geometric modifications from the...

It is demonstrated that the boundary layer conditions of supersonic model inlet flows strongly affect the unstart that is induced by a transverse jet injection. Planar laser Rayleigh scattering from condensed CO2 particles is utilized to visualize flow features. Studies conducted over a range of inlet configurations reveal that relatively thick tur...

OH PLIF is used to investigate the flame structure in an inlet/scramjet combustor model tested in the Stanford 6-Inch expansion tube. The model is characterized by a compression inlet that generates a shock train confined within the constant area combustor. The inlet flow is maintained at M=2.8, p=40 kPa and T˜1250 K. Hydrogen is injected through a...

Laser-induced fluorescence (LIF) imaging using toluene as a tracer molecule has been developed for high-speed, low-to-moderate enthalpy conditions in the Stanford 6-inch Expansion Tube. The approach is demonstrated on three canonical compressible flow configurations: (i) supersonic flow over a 20 wedge, (ii) around a cylinder, and (iii) a supersoni...

Gas chromatographic and temperature measurements have been carried out to investigate the extent of premixed methane/air combustion with the application of nanosecond repetitive pulsed discharges around the lean flammability limit for laminar flows. The results show that the discharges lead to the complete combustion when the equivalence ratio is a...

A transverse jet is injected into a supersonic model inlet flow to induce unstart. Planar laser Rayleigh scattering from condensed CO2 particles is used to visualize flow dynamics during the unstart process, while in some cases, wall pressure traces are simultaneously recorded. Studies conducted over a range of inlet configurations reveal that the...

Afterburners (or augmentors) are used to increase thrust in aircraft engines. Static flame stability, or the robustness to flame blowoff, is an important metric in the performance assessment of combustion in aircraft engine afterburners, where bluff-body-type flame holders are typically used to stabilize the flame. The design of such flame holders...

In this work, the development, characterization and effects of fuel injection on the flowfield of a compact model scramjet inlet/combustor is presented. The inlet/combustor model has been developed with the aim of investigating mixing, ignition, and combustion in supersonic flows under flow configurations realistic of scramjet combustors. The model...

A transverse air jet injected into a Mach 5 model inlet flow generated by an in-draft supersonic wind tunnel, is used to induce inlet unstart. Planar Laser Rayleigh Scattering (PLRS) from condensed CO2 particles in the tunnel flow is used to visualize the unsteady flow during unstart. Simultaneously, pressure traces along the wind tunnel are record...

The work presented here summarizes our efforts on the development of a simplified model scramjet com-bustor to investigate supersonic mixing and combustion in the Stanford Expansion Tube Facility. The system is designed to replicate the conditions and the flow features found in a generic scramjet engine of a hyper-sonic vehicle. The initial concept...

A model scramjet combustor has been developed to reproduce the main flow features that are present in a scramjet engine on a typical hypersonic vehicle; these flow features include shock/boundary layer interactions, shock/jet interactions, and supersonic mixing and combustion. The model combustor has been designed with optical access on three sides...

A nanosecond pulsed plasma discharge located between two fuel jets is used to ignite and hold jet flames in supersonic crossflows, without the use of additional devices (e.g., cavities or backsteps) for flame holding. The fuel injection nozzles and discharge electrodes are mounted flush with the surface of the flat wall adjacent to the freestream f...

The work aims at investigating near-wall ignition and flame structure in transverse underexpanded hydrogen jets in high-enthalpy supersonic crossflows generated in an expansion tube. Crossflow conditions are held fixed at M=2.4, p=40 kPa and T 1400 K, while jet-to-crossflow momentum flux ratios J in the range 0.3-5.0 are considered. Schlieren and O...

A nanosecond pulsed plasma discharge located within a wall cavity is used to ignite jet flames (hydrogen and ethylene) in supersonic crossflows. The nonequilibrium plasma is produced by repetitive pulses of 15 kV peak voltage, 20 ns pulse width and 50 kHz repetition rate. Sonic fuel jets are injected into free stream air of Mach numbers Ma = 1.7 to...

In this study, the improvement of liftoff height of bluffbody-stabilized, partially premixed methane flames and the change of flow field in the recirculation zone of bluffbodies, of variously modified base geometries, are investigated in a high temperature (~1,315K) vitiated flow. The basic geometry of the bluffbody consists of a two-dimensional re...

Planar laser Rayleigh scattering (PLRS) from condensed CO2 particles is used to visualize flow structure in a Mach 5 wind tunnel undergoing unstart. Detailed flow features such as laminar/turbulent boundary layers and shockwaves are readily illustrated by the technique. A downstream transverse air jet, inducing flow unchoking downstream of the jet,...

This paper describes a mechanism for the stabilization of ultra lean premixed methane/air flames by pulsed nonequilibrium plasma enhancement. It is shown that the pulsed discharge plasma produces a cool (∼500–600 K) stream of relatively stable intermediate species including hydrogen (H2) and carbon monoxide (CO), which play a central role in enhanc...

The drag-reducing action of dilute solutions of long-chain polymers in a flat-plate turbulent boundary layer is studied using particle imaging velocimetry (PIV) and planar laser induced fluorescence (PLIF). The results are used to characterize and quantify the spatial distribution of the injected polymer solution and the downstream development of t...

In this paper, our recent studies of five different flame configurations which are assisted by nanosecond pulsed discharge (NPD) are summarized. The flow configurations are: methane jets in subsonic coflow and cross flow, supersonic hydrogen jets in cross flow with cavity and unmodified (flat plate) geometries, and laminar fully premixed methane fl...

Recent experiments have reconstructed the D structure of the combustion regions around burning jets-in-supersonic-crossflow using OH-PLIF in multiple orthogonal planes. A sonic hydrogen jet with a momentum-flux ratio of J = 4 was injected normally into high temperature air crossflows at two freestream conditions: (a) T = 1250 K, M = 2.7, P = 25 kPa...

A nanosecond pulsed plasma discharge is used to ignite jet (hydrocarbon and hydrogen) flames in supersonic air cross flows (Ma = 1.7 to 3.0). The flow pattern and shockwaves induced by the fuel jets and flow disturbances are characterized by Schlieren imaging. Planar laser induced fluorescence and emission spectroscopy are employed for imaging the...

We report on a study of the use of repetitive ultrashort pulsed plasma discharges in stabilizing a lifted methane jet diffusion flame in elevated temperature (855 K-975 K) vitiated coflow. CH chemiluminescence images are used to record the flame liftoff height, which serves as a measure of the flame stability. The results show that, for the same re...

A short-pulse repetitive discharge is used to ignite hydrogen jet flames in supersonic crossflows. Nonequilibrium plasma is produced by repetitive pulses of 7-kV peak voltage, 20-ns pulsewidth, and 50-kHz repetition rate. Sonic or subsonic hydrogen jets are injected into a pure-oxygen supersonic free-stream flow of Mach numbers M = 1.7-2.3. The fue...

In this work we investigate the use of high repetition rate pulsed plasma sources as a means to enhance the stability of jet diffusion flames for application to practical combustion devices. Such plasma sources have recently become popular owing to their low power requirements and their proven abilities to ignite leaner mixtures and hold stable fla...

In this paper, a numerical and experimental assessment of the Damköhler number similarity is performed. Three bluff-body stabilized flames are computed using large-eddy simulations (LES) and the flamelet progress variable (FPV) model. The vitiated inflow and the bluff-body shape mimic the arrangement in an augmentor system. The geometry of the bluf...

Repetitively pulsed, nonequilibrium plasma discharges are studied for their ability to ignite and stabilize methane jet flames in cross flow air. The placement of the discharge is surveyed to optimize the flame duty cycle (stability). We find that flame ignition is not achieved when the discharge is located in the potential core region of the fuel...

This work reviews some evidence for the existence of large-scale structures in turbulent diffusion flames, at low and high Reynolds numbers, for free jets and jets in crossflow over a range of buoyancy conditions. The striking differences between the velocity field and the scalar field will be highlighted and implications for mixing and reaction ar...

Zero-pressure-gradient turbulent boundary-layer drag reduction by polymer injection has been studied with particle image velocimetry. Flow fields ranging from low to maximum drag reduction have been investigated. A previously developed technique – the (1 − y/δ) fit to the total shear stress profile – has been used to evaluate the skin friction, dra...

Phase locked particle image velocimetry (PIV) is used to study flow alteration by dielectric barrier discharge actuators mounted on a bluff body. Images from these PIV experiments confirm the importance of negative ions in the actuation process and demonstrate cross-talk interactions between adjacent actuators.

The structure of a plasma-assisted laminar premixed flame is studied numerically. The initial radical yield generated by a nonequilibrium discharge serves as the boundary condition for a one-dimensional flame code predicting the formation of a cool flame which pilots the premixed methane/air combustion. The ignition of the surrounding unactivated m...

This article provides a review of recent progress in understanding and predicting polymer drag reduction (DR) in turbulent wall-bounded shear flows. The reduction in turbulent friction losses by the dilute addition of high-molecular weight polymers to flowing liquids has been extensively studied since the phenomenon was first observed over 60 years...

Phase-locked particle image velocimetry is used to study the mechanism of induced flow in the near field of a rf dielectric barrier discharge actuator mounted in the separated flow region of a bluff body. Flow actuation is found to be asymmetric, with suction toward the buried downstream electrode when it is biased positively relative to the upstre...

A mapping of regimes of stable, unstable, and no combustion in scramjet combustor conditions and other similar conditions has been undertaken in the recently constructed 6 inch expansion tube at Stanford University. The mapping investigated the effects of stagnation temperature, static temperature, and jet-to-freestream momentum flux ratio. The res...

A fast response, porous pressure-sensitive paint (PSP) technique was used to measure the surface pressure field resulting from a 2mm sonic jet injected into supersonic crossflow. These measurements were carried out in an expansion tube facility providing a freestream Mach number of 3.4 for a total test time of 400 μs. Initial work focused on valida...

A pulsed plasma source is used to stabilize lean premixed methane flames. Nitric oxide (NO) production is measured using probe sampling and chemiluminescence analysis. While the discharge is a potential source of NO, it is observed that the flame also consumes some NO. The NO production in the flame is modeled by use of a modified opposed diffusion...

Surface dielectric barrier discharges (DBDs) are used to delay flow separation behind a bluff body in atmospheric pressure air. The bluff body is made by connecting a flat plate to a half cylinder tangentially as a round-cornered trailing edge. Four DBD plasma actuators are attached along the surface of the half cylinder. Particle Image Velocimetry...

The authors examine three different types of plasma discharges in their ability to stabilize a lifted jet diffusion flame in coflow. The three discharges include a single-electrode corona discharge, an asymmetric dielectric-barrier discharge (DBD), and a repetitive ultrashort-pulsed discharge. The degree of nonequilibrium of this pulsed discharge i...

The friction drag in polymer drag reduced turbulent boundary layer flow is decomposed into four dynamical contributions. It is found that drag reduction is achieved by either, or both, an attenuation of the Reynolds stress and a reduction in the total stress gradient near the wall. Particle image velocimetry (PIV) measurements made in a turbulent d...

Particle image velocimetry and smoke visualization are used to study the alteration of the flow field in the wake of a bluff body by use of an alternating current (AC) surface dielectric barrier discharge. Staggered, surface, and buried electrodes were positioned on the downstream side of circular cylinders at conditions of Re D = 1 × 104−4 × 104...

A new expansion tube facility has been constructed at Stanford University to enable research on scramjet combustor design and performance. The facility is designed to allow the duplication of combustor entrance conditions over a range of conditions covering vehicle flight Mach numbers of 4 to 9. The range can be extended to Mach 15 if the simulated...

Two methods of recovering the entire total shear stress profile from incomplete velocity data in turbulent boundary layers are presented and validated for both DNS simulations and experimental measurements. The first method, an exponential–polynomial curve fit, recovers the whole total shear stress profile using the data from the outer part of the...

We report an experimental investigation that reveals significant differences in the near-flowfield properties of hydrogen and ethylene jets injected into a supersonic crossflow at a similar jet-to-freestream momentum flux ratio. Previously, the momentum flux ratio was found to be the main controlling parameter of the jet's penetration. Current expe...

Simultaneous planar-laser induced fluorescence (PLIF) and particle image velocimetry (PIV) provide a comprehensive view of the molecular mixing and velocity fields in the stabilization region of turbulent, lifted jet diffusion flames. The Mie scattering medium for PIV is a glycerol–water fog, which evaporates at elevated temperatures and allows inf...

This paper examines the use of plasma discharges in flame stabilization. Three different types of plasma discharges are applied to a lifted jet diffusion flame in a coflow configuration, and evaluated for their abilities to enhance flame stabilization. A single electrode corona discharge (SECD) between a platinum electrode and the flame base is fou...

Submitted to the Department of Mechanical Engineering. Copyright by the author. Thesis (Ph. D.)--Stanford University, 2006.

The addition of dilute polymer solutions to turbulent wall bounded flows can cause a significant reduction in the skin friction drag. This drag reducing effect of polymers, called the Tom's effect, has been well known for more than 50 years now. However, there is limited understanding of the physics behind this phenomenon of drag reduction. In our...

The drag reduction in a zero pressure gradient (ZPG) turbulent boundary layer (TBL) using a rigid rodlike polymer was experimentally and numerically investigated. Using injection of the rigid polysaccharide scleroglucan, drag reductions of approximately 10–15 % were observed, with three distinct drag reduction regimes: a non-Newtonian flow region n...

Simultaneous planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) yield measurements of two-dimensional jet fluid concentration and velocity fields, both the mean and fluctuating terms, in turbulent crossflowing jets. The jet-to-crossflow velocity ratio is r = 5.7 and the jet exit Reynolds number is approximately 5000. The...

Planar velocity fields of mixing-enhanced compressible planar shear layers are measured via particle image velocimetry (PIV) in order to investigate the mechanism of mixing enhancement by sub-boundary-layer triangular disturbances. The measurements are conducted at convective Mach numbers, $M_{{c}}$, of 0.62 and 0.24 to examine compressibility effe...

Stable combustion in a harsh environment, such as lean fuel concentration and/or low temperature conditions, is attractive because of its low emissions. However, such flames suffer from lack of stability. In our research, an AC plasma discharge is applied to a lifted jet flame to increase the stability of the flame. Two types of discharges are util...

Two methods of recovering the entire total shear stress profile from incomplete velocity data in turbulent boundary layers are presented and validated for both DNS simulations and experimental measurements. The first method, an exponential-polynomial curve fit, recovers the whole total shear stress profile well by using the data from the outer part...

Since the original observations of drag reduction by Toms (1948), the molecular mechanisms by which the drag in a turbulent channel flow is reduced have been the subject of a great deal of theory. The talk will focus on the mechanisms of turbulent drag reduction by polymer and fiber additives as elucidated from large scale numerical simulation of b...

The addition of dilute polymer solutions to turbulent wall bounded flows causes a significant reduction in the skin friction drag, the Tom's effect, and has been well known for more than 50 years now. However, there is limited understanding, at present, of the physics behind this phenomenon of drag reduction. This talk addresses the flow statistics...

The efficient mixing of fluids in compressible shear layers is still of fundamental importance to high-speed propulsion. To further understand the role of compressibility, measurements of mixing efficiency are performed in a convective Mach number=2.64 shear layer. The mean scalar field and estimated mixing efficiency are measured using a new “cold...

Simultaneous planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) yield measurements of two-dimensional jet fluid concentration and velocity fields in turbulent crossflowing jets. The jet-to-crossflow velocity ratio is r = 5.7 and the jet exit Reynolds number is approximately 5000. The measurements are focused on the develo...

We demonstrate and discuss a liquid pumping effect based on the interaction between periodically generated cavitation microbubbles and a millimeter diameter tube. The bubbles are generated by the heat of laser adsorption at the tip of an optical fiber immersed in liquid. When the bubbles are generated well away from the tube, a steady streaming flo...

Images of fluids in motion have served both scientific and artistic purposes at least since the time of Leonardo de Vinci over 500 years ago. The visualization of fluid flow has played a major role in the development of fluid dynamics and its technological and scientific applications, from the evolution of flight to the tracking of weather to under...

The structure of turbulence in a drag-reduced flat-plate boundary layer flow has been studied with particle image velocimetry (PIV). Drag reduction was achieved by injection of a concentrated polymer solution through a spanwise slot along the test wall at a location upstream of the PIV measurement station. Planes of velocity were measured parallel...

The scalar-field imaging of a hypersonic mixing flow is performed in a mixing facility that is shock tunnel driven. The instantaneous mixture-fraction field of a hypersonic two-dimensional mixing layer (M 1 = 5.1, M 2 = 0.3) is determined with a temperature-insensitive planar laser-induced fluorescence technique with nitric oxide (NO) as the tracer...