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This paper focuses on rotating detonation combustor (RDC) experiments involving two configurations, a hollow and flow-through RDC, using hydrogen–air and ethylene–air mixtures. Two air injection areas with gaps of 1.778 and 3.81 mm were used at different air mass flow rates of 0.2, 0.3, 0.4, and 0.5 kg/s and equivalence ratios from 0.6 to 2 in 0.2 increments. Results show repeatable detonations for hydrogen–air mixtures in both configurations; however, the detonative range is wider, stronger, and faster in the hollow configuration. Speeds exceeding the Chapman–Jouguet velocity and pressures matching the ideal model were observed. Sustained detonations for ethylene–air mixtures were not attained with the flow-through configuration. Installing a back plate to convert the configuration into a hollow combustor promoted rotating detonations with the ethylene–air mixture at select operating points. The results of this study showcase the strong promise of producing and sustaining rotating detonation in nonannular RDC configurations.

Quantify differences in acoustics and intraglottal flow fields between Thyroplasty Type 1 (TT1) with and without arytenoid adduction (AA) using excised canine larynx model.Basic science experiments using excised larynges.Surgical procedures were implemented in eight excised canine larynges. Acoustics and intraglottal flow measurements were taken at low and high subglottal pressures in each experimental setup.In all larynges, vocal efficiency (VE) and cepstrum peak prominence (CPP) were higher, and the mean phonatory flow rate was lower in TT1 with AA than without AA. The glottal asymmetry is reduced with AA and promotes the formation of stronger vortices in the glottal flow during the closing phase of the vibrating folds.Findings suggest a clear acoustic and aerodynamic benefit to the addition of AA when performing TT1. It shows significant improvement in CPP, translating to decreased breathiness and dysphonia and increased VE, leading to easier and more sustainable phonation. Stronger intraglottal vortices are known to be correlated with the loudness of voice produced by phonation.N/A Laryngoscope, 2022.

Surgical treatment of obstructive sleep apnea (OSA) in children requires knowledge of upper airway dynamics, including the closing pressure (Pcrit), a measure of airway collapsibility. We applied a flow-structure interaction (FSI) computational model to estimate Pcrit in patient-specific upper airway models obtained from magnetic resonance imaging (MRI) scans. We sought to examine the agreement between measured and estimated Pcrit from FSI models in children with Down syndrome. We hypothesized that the estimated Pcrit would accurately reflect measured Pcrit during sleep and therefore reflect the severity of OSA as measured by the obstructive apnea-hypopnea index (AHI). All participants (n = 41) underwent polysomnography and sedated sleep MRI scans. We used Bland–Altman plots to examine the agreement between measured and estimated Pcrit. We determined associations between estimated Pcrit and OSA severity, as measured by AHI, using regression models. The agreement between passive and estimated Pcrit showed a fixed bias of −1.31 [confidence interval (CI) = −2.78, 0.15] and a nonsignificant proportional bias. A weaker agreement with active Pcrit was observed. A model including AHI, gender, an interaction term for AHI, and gender and neck circumference explained the largest variation (R(2) = 0.61) in the relationship between AHI and estimated Pcrit (P < 0.0001). Overlap between the areas of the airway with the lowest stiffness, and areas of collapse on dynamic MRI, was 77.4 ± 30% for the nasopharyngeal region and 78.6 ± 33% for the retroglossal region. The agreement between measured and estimated Pcrit and the significant association with AHI supports the validity of Pcrit estimates from the FSI model. NEW & NOTEWORTHY We present a noninvasive method for estimating critical closing pressure (Pcrit) using fluid-structure interaction (FSI) simulations and magnetic resonance imaging (MRI) scans in patients with obstructive sleep apnea (OSA). We used patient-specific stiffness measures in our FSI model to account for any individual variability in the elasticity of soft tissues surrounding the upper airway. We validated this model by measuring the degree of agreement between measured and estimated Pcrit.

Summary The greatest rate of change in the glottal flow rate during phonation is a rapid decrease that occurs during the latter part of the glottal closing. Previous works showed that intraglottal flow separation vortices form in a divergent glottis, produce negative gauge pressures (below atmospheric) during closing. It is hypothesized here that flow separation vortices contribute to the rapid closing mechanism of the true vocal folds during phonation. Four idealized static models (M5) of the human larynx were investigated using large eddy simulation: 2 models featured parallel folds that did not enable flow separation in the glottis and 2 models involved a divergent glottis. The influence of the ventricular gap (narrow/wide) is evaluated. An unsteady pressure inlet representing a voicing cycle was applied to the sub-glottal region to mimic the time-varying glottal flow. Intraglottal vortex structures formed downstream of the separation point in a divergent glottis. Their existence caused a higher closing force that was applied onto the vocal folds. A narrow ventricular gap strengthens this effect. Strength of the intraglottal vortices increased with the maximum flow declination rate. Therefore, a more divergent shape of the glottis during glottal closing will be one of the main contributors to the voice quality.

The three-dimensional steady incompressible numerical results of an isothermal flowfield resulting from the interaction of two swirl-stabilized air nozzles in a multiple lean direct injection combu...

Previous models have theorized that, during phonation, skewing of the glottal waveform (which is correlated with acoustic intensity) occurred because of inertance of the vocal tract. Later, we reported that skewing of the flow rate waveform can occur without the presence of a vocal tract in an excised canine larynx. We hypothesized that in the absence of a vocal tract, the skewing formed when dynamic pressures acted on the glottal wall during the closing phase; such pressures were greatly affected by formation of intraglottal vortices. In this study, we aim to identify how changes in false vocal folds constriction can affect the acoustics and intraglottal flow dynamics. The intraglottal flow measurements were made using particle image velocimetry in an excised canine larynx where a vocal tract model was placed above the larynx and the constriction between the false vocal folds was varied. Our results show that for similar values of subglottal pressures, the skewing of the glottal waveform, strength of the intraglottal vortices, and acoustic energy increased as the constriction between the false vocal folds was increased. These preliminary findings suggest that acoustic intensity during phonation can be increased by the addition of a vocal tract with false fold constriction.

The effect of the intraglottal vortices on the glottal flow waveform was explored using flow-structure-interaction (FSI) modeling. These vortices form near the superior aspect of the vocal folds during the closing phase of the folds' vibration. The geometry of the vocal fold was based on the well-known M5 model. The model did not include a vocal tract to remove its inertance effect on the glottal flow. Material properties for the cover and body layers of the folds were set using curve fit to experimental data of tissue elasticity. A commercially available FSI solver was used to perform simulations at low and high values of subglottal input pressure. Validation of the FSI results showed a good agreement for the glottal flow and the vocal fold displacement data with measurements taken in the excised canine larynx model. The simulations result further support the hypothesis that intraglottal vortices can affect the glottal flow waveform, specifically its maximum flow declination rate (MFDR). It showed that MFDR occurs at the same phase when the highest intraglottal vortical strength and the negative pressure occur. It also showed that when MFDR occurs, the magnitude of the aerodynamic force acting on the glottal wall is greater than the elastic recoil force predicted in the tissue. These findings are significant because nearly all theoretical and computational models that study the vocal fold vibrations mechanism do not consider the intraglottal negative pressure caused by the vortices as an additional closing force acting on the folds.

Rotating detonation combustors (RDCs) are studied with significant interest in recent years, due to their promulgated benefits over other pressure gain combustion systems. Although notable strides ...

This paper investigates the effect of external expansion ramps on supersonic flow emitted from high-aspect-ratio rectangular nozzles with a focus on additional sources of noise due to ramps as well...

The objective of this study was to perform finite element analysis (FEA) of cuff inflation within an anatomically accurate model of an adult trachea in four different cuffed-tracheostomy tube designs. The leakage quantified by the distance between the cuff and trachea was largest for the Tracoe cuff and smallest for the Portex cuff. The smooth muscle stresses were greatest for the Portex and least for the Distal cuff, respectively. The proposed FEA model offers a promising approach to virtually evaluate the sealing efficacy of cuffed-tracheostomy tubes and the tracheal wall stresses induced by cuff inflation, prior to application.

An experimental study is conducted on bluff-body stabilized premixed flames in a curved, square cross-section duct. High flow velocities coupled with a small radius of curvature of the duct induce high centrifugal acceleration normal to the flame sheet. A cylindrical flame holder spans the width of the square cross-section and is positioned at the channel mid-height. Flame shear layers are stabilized on the radially inward (upper) and outward (lower) edges of the flame holder. Side-view high-speed Schlieren images and high-speed pressure measurements are captured. Static stability, overall pressure loss, and statistics and velocimetry results from the Schlieren images are reported, and results are compared to a straight configuration with no centrifugal acceleration. Two bluff-body diameters are studied to show the effect of flame holder diameter on static stability. For the curved configuration, blowout velocities are higher for the smaller bluff-body diameter, likely due to flow acceleration effects. Blowout velocities are lower for the curved configuration compared to the straight configuration which may be due to the destabilizing Rayleigh-Taylor (RT) effect on the upper flame layer. Overall pressure loss is slightly higher for the curved configuration than the straight configuration. High-speed Schlieren results show centrifugal acceleration causes significant structural and velocimetric asymmetry in the bluff-body wake. In the curved configuration, the upper flame layer displays destabilizing RT instabilities, and the lower flame layer displays stabilizing RT effects. The upper flame shows vigorous RT instabilities which broaden the flame brush and sustain a flame leading edge independent of inlet Reynolds number or velocity. Conversely, the lower flame exhibits suppression of Kelvin-Helmholtz and flame-generated instabilities in the wake, which confines the flame brush and significantly reduces transverse flame velocities. The lower flame edge profile moves toward the channel centerline with increasing inlet Reynolds number. The upper flame in the curved configuration shows higher flame edge velocities than the straight configuration while the lower flame shows velocities closer to zero. The empirical constant to the power law relation for upper flame edge velocities agrees with RT-dominated flame growth theory for this experimental scale and agrees with other RT-dominated flame studies.

This work investigates the stabilization mechanisms of two types of longitudinal pulsations in rotating detonation combustors. The first type is linked to operating modes with two counter-rotating waves in combustors with open outlets and appears as a minor peak in the pressure spectrum. The second type is observed as pulsed operation of the combustor when the outlet is restricted. Different combustor lengths are studied and the susceptibility to these longitudinal pulsations is investigated. Pressure measurements along the length of the combustor and around the perimeter are used to identify the operating mode and to describe the propagation and stabilization mechanisms of the two longitudinal modes. The results show that both modes are linked to the longitudinal acoustic resonance of the combustor. The length-to-perimeter ratio and the mass flux are identified as the driving parameters for the existence of these longitudinal modes. The first mode is shown to be an acoustic resonance supported by the intersections of counter-rotating waves. The second mode is controlled by the reflection of an explosion induced shock wave propagating through a high velocity bulk flow.

During vocal fold vibration, there may be a mucosal wave in the superior-inferior (vertical) direction, resulting in a convergent shape during opening and a divergent shape during closing. Most of our understanding of the converging/diverging shape of the glottis has come from studies in a hemilarynx model. Previous work has shown that vibratory patterns in the full excised larynx are different than the hemilarynx. This study characterized the dynamics of the medial glottal wall geometry during vibrations in the full excised canine larynx model. Using particle image velocimetry, the intraglottal geometry was measured at the midmembranous coronal plane in an excised canine larynx model. Measurements of the glottal area were taken simultaneously using high-speed imaging. The results show that skewing of the glottal area waveform occurs without the presence of a vocal tract and that the phase-lag of the superior edge relative to the inferior edge is smaller than reported and depends on the subglottal pressure. In addition, it shows that the glottal divergence angle during closing is proportional to the magnitude of the acoustic intensity and the intraglottal negative pressure. This preliminary data suggests that more studies are needed to determine the important mechanisms determining the relationship between intraglottal flow, intraglottal geometry, and acoustics.

The phenomenon of onset time after ignition in atmospheric and backpressurized rotating detonation combustors is dealt with in the current Paper. By altering the airflow and fuel flow rates, plenum...

The spectral proper orthogonal decomposition method, which extracts spatially and temporally coherent modes in stationary flowfields, is used in the current paper to analyze high-speed broadband ch...

Valved pulsejet combustor geometries of different chamber and tail pipe lengths are tested experimentally. High-speed pressure and ionization data from various opaque engines are complemented with high-speed visualization data (from transparent engines of the same geometric sizing) acquired from three cameras, with the first resolving the broadband luminosity in the device, the second ascertaining the reed valves’ opening time and the third capturing the pulsejet exhaust flow using background oriented Schlieren. The resulting information presents a detailed description of the valved pulsejet mechanics that has been lacking in literature. The rapid combustion event powering every cycle is attributed to multiple auto-ignitions occurring mostly in tandem. An important precursor to this combustion event is the “whipping” produced by the fast closure of the reed valves, which creates a buffer region of unburned reactants ready to be consumed simultaneously. By cross-sectionally averaging the broadband luminosity over different cycles, x-t plots of combustion and fluid dynamics inside a pulsejet combustor are presented, which along with other acquired data, result in the conclusion that Helmholtz resonance is the mode of operation as opposed to the widely claimed quarter-wave oscillatory behavior. By drawing from analogues seen in compression ignition engines, it is argued that valved pulsejet combustors behave very closely to the latter in terms of the method of sustained operation—rapid combustion caused by multiple auto-ignitions events—brought forth by a periodic compression of the reacting mixtures enabled by the fluidic piston (as opposed to a mechanical one in automotive engines) in the tail pipe. In this sense, it is argued that compression ignition, rather than the much broader and less clear “resonant combustion” might be a better descriptor for pulsejet behavior.

In this work, the effect of different injector geometries and different outlet restrictions on the operating modes of a hydrogen-air Rotating Detonation Combustor (RDC) is investigated. The different operating modes are identified based on pressure measurements in the combustor annulus and the reactant supply, combined with high-speed video from the aft end of the combustor. The pressure frequency spectra are analyzed to determine the global operating mode in terms of number, direction, and speed of waves. The results explore the ability of the RDC to establish rotating, counter-rotating, and longitudinal waves, as well as their superpositions. A good agreement between longitudinal modes and the acoustic resonance frequencies of the RDC annulus was found. Overall, operation was found to be highly injector and outlet restriction dependent. Adding an outlet restriction helped to suppress counter-rotating waves, which is a prerequisite to stabilize single detonation waves. However, it was also shown to prompt high frequency pulsed operation. Apart from the total reactant supply pressure, the relative strength of the injectors was identified as a key factor for stable RDC operation. Pressure feedback into the reactant supply was observed to be dependent on the reactant supply pressure and the RDC operating mode. The study further revealed the presence of transverse resonance modes in the fuel plenum, however these oscillations were weak relative to the injector pressures and do not appear to influence the combustion.

The current study discusses the experimental results of the effect of two types of nozzle internal geometries. The first configuration uses a common inlet that diverges the flow into two separate channels that feed twin rectangular nozzles, and the second configuration has the plug flow type system with twin rectangular nozzles drawing air from a common plenum. The nozzles are converging diverging type with design Mach number of 1.5. Flow conditions encompassing overexpanded, design, and underexpanded conditions are tested with acoustic data collected in the farfield and nearfield domains. High-speed Schlieren imaging is used to visualize the effect of internal geometry on the development of the jet shock cell structure and twin jet interaction. The variations in the Overall Sound Pressure Level (OASPL), frequency spectra, and jet phase coupling are used to quantify the effect of the internal geometries. This is supplemented by Spectral Proper Orthogonal Decomposition (SPOD) results obtained from the Schlieren images to quantify the variations in flow development.

Twin jet configurations are prone to a wide range of interactions due to flow instabilities driven by jet self-excitation and cross-excitation of one jet on another. These instability modes and associated phases are sensitive to many parameters chief among which is the exit profile of the nozzle. Extensive studies on Twin Circular nozzles and Large Aspect Ratio (AR) Twin rectangular nozzles have shown various mode coupling but, not much is known about twin jets with moderate aspect ratios (AR

The flow field of supersonic jets with micro vortex generators (MVGs) is investigated using Particle Image Velocimetry (PIV) to shed light on the noise reduction mechanisms associated with such devices. Recently, MVG nozzles have been developed as a new noise reduction technology through the collaboration between the University of Cincinnati and the Naval Research Laboratory. MVGs were implemented on model scale nozzles representative of GE F404 engine nozzles, and noise reductions up to −6 dB have been observed in both microphone measurements and LES simulations. The PIV measurements of the flow field reveal that the MVGs alter the shock cell spacing and strengths in the jet plume and substantially reduce the mean axial velocity at the jet axis. In addition, streamwise vortices generated by MVGs enhance shear layer mixing and redistributes turbulence along the shear layer to narrower regions. The measured flow field quantities are extracted to explain flow field changes on the noise reductions observed in the acoustic far field.

Inverse filtering is a signal processing technique used to estimate the glottal waveform from the speech signal. Although this technique is regularly used in research studies, it has never been formally validated because of the difficulties in obtaining direct measurements of the glottal airflow. The objective of this study is a first step towards validating this technique by comparing its estimated glottal waveform with flow measurements taken simultaneously at the glottal exit. The setup is based on synthetic vocal folds connected to a vocal tract model. Direct measurements of the volume flow at the glottal exit are taken using time-resolved tomographic particle image velocimetry. A circumferentially vented pneumotachograph (i.e., Rothenberg) mask is connected to the vocal tract and used with the Glottal Enterprise system to calculate the glottal waveform using inverse filtering. Effect of varying subglottal pressures and effect of near-field (i.e., the minimal gap between the false vocal folds) and far-field (i.e., oral opening) constrictions were also investigated. Results show that overall using inverse filtering gives a good approximation of the glottal flow waveform, but its accuracy can change depending on the constrictions in the vocal tract. The clinical implication of these findings will be further discussed.

Objective: The loud and severely distorting form of audible nasal emission (commonly known as nasal turbulence or nasal rustle) typically occurs with a small velopharyngeal opening during production of pressure-sensitive consonants. The purpose of this study was to determine whether bubbling of the secretions, which commonly occurs on the superior aspect of the velopharyngeal port when there is a small opening, is a periodic process that can generate sound in the nasal cavity. Participants: Ten pediatric patients were included in the study. All participants had normal articulation and resonance but exhibited audible nasal emission characterized as nasal rustle. Measures: For each participant, high-speed video (HSV) nasopharyngoscopy and acoustic signals were recorded simultaneously. The acoustic recordings were captured in a manner similar to nasometry using nasal and oral microphones connected to a separation plate. Spectral analysis of the audio recordings and the HSV images was used to determine correlation between the acoustic and visual measurements. Results: This study showed that secretion bubbling is a periodic process and its frequency, measured from the HSV data, was also captured by the acoustic measurements. The nasal acoustic signal correlated more strongly with the video of bubbling than the oral acoustic signal in the majority of the cases where bubbling occurred. Conclusion: These findings are strong evidence that secretion bubbling plays a significant role in the mechanism that generates undesired sound in the nasal cavity. Further work is needed to determine whether this sound is perceived as nasal rustle.

In streamlined airplane configurations, additional noise sources can be created from interactions between the jet flow and surfaces on an aircraft’s body. During takeoff and landing procedures the ground is close enough to already cause jet-surface interference. To assess this interaction, the presence of a flat plate impinging on a supersonic jet of a low aspect ratio (2:1) rectangular nozzle of equivalent exit diameter, De = 20.65 mm, is studied from the minor and major axis orientation. The impact of the plate, 30De in length to resemble an aircraft carrier deck, is studied at supersonic nozzle pressure ratios (NPRs) of 2.5–4.5 for a low jet temperature ratio of TR = 1.1. Streamwise particle image velocimetry (PIV) data was taken to extract average velocity and turbulence kinetic energy (TKE) of the flow. Plate offset (h) distances of h/De = 0, 1, 2, and 3 from the nozzle lip are studied to assess trends related to shock cell spacing, potential core length, and shear layer development relative to a free jet configuration. The surface offset from the nozzle is shown to vary flow properties and even increase screech tones particularly at h/De = 1 and 3, while the in between distance of h/De = 2 is shown to reduce them. These off-design conditions causing screech are of particular interest in this study.

This study considers the steady-state, non-reacting mixing of fuel and air within the hydrogen-air Rotating Detonation Combustor (RDC) currently in use at TU Berlin. The interaction of reactants occurs in a confined jet-in-crossflow (JIC) configuration with an axially injected fuel jet and an air stream entering radially inwards. The investigation of the baseline flow case provided three flow characteristics primarily responsible for affecting the process of mixing: supersonic shock patterns, the existence of two major recirculation zones, and a counter-rotating vortex pair (CVP) structure. In a parametric study with nine different flow configurations, attained by the variation of reactant inlet flow rates, the effect on mixing behavior and performance was analyzed in order to determine the most impactful parameter for the RDC refill process. The air mass flow rate was identified as the primary parameter with respect to the general flow field due to the interaction of a dominant air barrel shock with the fuel jet. The low flow rate cases allowed the greater fuel and air jet interaction in the near injection region of the combustor, whereas in the far field the higher flow rate configurations attained comparable mixing quality despite more complicated fuel and air jet shock structures.

Variation in the rotational position of the aortic root relative to the left ventricle is present in normal trileaflet aortic valves. Its impact on the resulting fluid mechanics of blood flow in the thoracic aorta and structural mechanics in the aortic wall are unknown. We aimed to determine the regional hemodynamic and biomechanical differences in different rotational positions of the normal aortic root (clockwise, central, and counterclockwise positions). Cardiac magnetic resonance imaging (CMR) data was acquired from a normal pediatric patient. These were used for reconstruction of the aortic valve and thoracic aorta 3D model. Fluid–structure interaction (FSI) simulations were employed to study the influence of the root rotation with a central position as compared to observed extreme variations. Patient-specific phase-encoding CMR data were used to assess the validity of computed blood flow. The 3D FSI model was coupled with Windkessel boundary conditions that were tuned for physiological pressures. A grid velocity function was adopted for the valve motion during the systolic period. The largest wall shear stress level is detected in the clockwise positioned aortic root at the sinutubular junction. Two counter-rotating vortex cores are formed within the aortic root of both the central and extreme root configurations, however, in the clockwise root the vortex system becomes more symmetric. This also coincides with more entrainment of the valve jet and more turbulence production along the shear layer. A clockwise rotational position of the aortic root imparts an increased wall shear stress at the sinutubular junction and proximal ascending aorta in comparison to other root rotation positions. This may pose increased risk for dilation of the sinutubular junction and ascending aorta in the patient with a clockwise positioned aortic root compared to other normal positional configurations.

Open-loop control of self-excited flame pulsating oscillations and thermo-acoustic instability is considered in this work. The performance of the control strategy is numerically evaluated in a 2D Rijke-type combustor with a perforated pipe implemented. It is found that approximately 38 dB sound pressure level (SPL) reduction can be achieved by actively tuning the cooling flow through the perforated pipe. Furthermore, the vorticity-induced damping performance is contributing to the breaking up of flame-acoustics coupling. However, the shedding of vortices is not uniformly distributed along the perforated pipe. To apply the control strategy in practice and to validate the findings, experimental studies are performed on a customer-designed Rijke-type combustor with a perforated liner implemented. To mimic practical engines, a cooling flow generated by a centrifugal pump is provided to pass through the perforated pipe. Properly tuning the cooling flow rate is found to lead to the unstable combustor being successfully stabilized. SPL is reduced by approximately 35 dB at ω 1 / 2 π ≈ 245 Hz, and harmonic thermoacoustic modes are completely attenuating. Further study is conducted by suddenly removing the perforated pipe section. The combustion system is found to be associated with not only classical thermo-acoustic limit cycle oscillations with a dominant mode at 2.45 × 10 2 Hz, but also beating oscillations at 1.4 × 10 0 Hz. It is revealed that increasing acoustic losses by implementing the perforated pipe is another critical mechanism contributing to attenuating flame pulsating instability. The present work opens up an applicable means to attenuate both self-excited high-frequency thermoacoustic and low-frequency flame pulsating oscillations.

Rotating detonation combustors (RDC) are at the forefront of pressure gain combustion (PGC) research, utilizing one or more azimuthally spinning detonation waves, an intrinsically unsteady process, to effect a stagnation pressure rise across the device. The prospective step-increase in efficiency, simplicity of design without the requirement for mechanical actuations and the ease of assembly make it an especially promising technology that could be integrated into existing propulsion and power generation architectures. This is coupled with the significant complexity of the detonation-based multi-axis flow field and the associated combustion modes and coupling mechanisms. The current paper is an overview of the research done worldwide to address some of the challenges and questions pertaining to the physics of RDC operation. When appropriate, notable parallels are drawn to the phenomena of low and high frequency instabilities in solid and liquid rockets that have been recognized as the most severe hindrance to their operation.

Various operating modes have been observed in the rotating detonation combustor (RDC) studied here, as well as by others. These modes can be generally categorized into two groups: one or more co-rotating waves and counter-rotating waves. The combustor is able to stabilize both types of operation, however the stabilization mechanism remains unclear. In order to better understand these cases, these operational modes are investigated with high-speed pressure transducers installed in the combustor annulus combined with simultaneous high-speed video imaging of the natural luminosity of the detonation wave from the aft end of the RDC. These results confirm the operation in both the steady single wave mode as well as the counter-rotating waves mode. The presence of this mode further demonstrates the complex dynamics inherent in the stabilization of the detonation wave. As each wave propagates, they initially begin to weaken followed by re-strengthening after collision which serves to stabilize the mode. Differences in the velocities of the waves was also observed to result in a beating phenomenon expressed as an amplitude modulation of the pressure traces. High-speed video imaging of the high-temperature emission from H2O in the RDC annulus was used in conjunction with the pressure traces for a range of flow rates and equivalence ratios. Processing of the images into the wave frame of reference allowed for the identification of the average luminosity profile, characterized by a steep increase in natural luminosity near the wave front, followed by a trailing tail extending approximately half the annulus perimeter.

This work describes an experimental study on the transition dynamics in a hydrogen-air RDC as the operating mode transitions from two steadily propagating, equally-fast counter-rotating waves to a single wave depending on the operating condition. The operating mode for a range of equivalence ratios and mass flow rates is investigated based on time-resolved measurements of the pressure in the combustor annulus at multiple azimuthal locations, as well as based on simultaneous measurements of the natural flame luminosity from the aft end of the RDC. Overall reactant mass flow and equivalence ratio, coupled with the plenum pressures, were confirmed as the driving parameters for wave mode transition. Further experimentation is necessary to decouple these effects, however the results illustrate complex transition dynamics, yielding an acceleration of the primary wave and the simultaneous deceleration of the secondary wave(s). At the same time the secondary wave number is observed to increase up to a triplet of counter-rotating waves. A non-dimensional wave dominance parameter is proposed, which links the relative wave speeds of the waves to thermodynamic properties of the combustor. It indicates regimes of constant wave mode that are independent of changing operating conditions and that extend far into the operating map for the given RDC geometry. The results further indicate a correlation between the operating mode with two counter-rotating waves and choked flames, as well as quasi-detonations observed in generic detonation experiments in the literature.

The cyclic propagation of detonation waves in the Rotating Detonation Combustor (RDC) make it an ideal candidate for using the cross-correlation technique. However, the potential for complicated operating modes combined with difficult diagnostic access introduces the possibility for ambiguous outcomes of the analysis. This work will first consider the specific details of the application of the technique to analyzing the features of RDC operation, including wave speed, direction, and operating mode. The accuracy and limitations of the technique will be quantified from a theoretical perspective. The second half of the work will focus on several example run conditions and operating modes in the RDC and will highlight differences in the analysis of variable combustion modes. From these cases studies, it is clear that a great deal of insight on the operation can be gleaned from analyzing the correlation. However, care must be taken in the experimental setup and analysis as a poorly designed approach can easily yield deceptive results. Finally, guidelines for preparing the experiment and conducting the analysis are provided to improve the quality of the results.

A novel application of fluidic injection was developed to investigate and understand the effects of discrete fluidic injection internal to the jet nozzle. Various injection locations, angles, and c...

A new supersonic jet noise reduction technology has been developed using Micro Vortex Generators (MVGs) by the collaboration between the University of Cincinnati and the Naval Research Laboratory. MVGs are used on model scale nozzles that are representative of GE F404 engine nozzles. Noise reductions up to −10 dB have been observed in both laboratory measurements and LES simulations at conditions related to take off in the overexpanded regime. Analysis of the acoustic field and flow field using Schlieren visualization reveal the noise reduction mechanisms associated with MVGs. Direct visualization of the changes in shock cell spacing, Large Coherent Structures (LCS) formation, and their convective velocity are identified and those changes modify the downstream propagating hydrodynamic waves and the upstream propagating acoustics waves. Spectral Proper Orthogonal Decomposition (SPOD) is utilized to examine the flow sources at frequencies associated with the noise components observed in the acoustic spectra to explain the noise reduction mechanisms of MVGs.