Combustion devices are prone to combustion instabilities. They arise from a constructive coupling between the unsteady heat release rate of the flame and the resonant acoustic modes of the entire system. The occurence of such instabilities can pose a threat to both performance and integrity of combustion systems. Although these phenomena have been known for more than a century, avoiding their appearance in industrial engines is still challenging. The objective of this thesis is threefold: (1) study the dynamics of the resonant acoustic modes, (2) investigate the flame response of a liquid rocket engine under unstable conditions using Large Eddy Simulation(LES) and (3) derive, use and study Time Domain Impedance Boundary Conditions (TDIBCs), i.e. boundary conditions modeling complex acoustic impedances.

No description available.

A modelagem do campo acústico gerado por um transdutor ultra-sônico retangular é importante porque sua geometria é usada freqüentemente por transdutores multielementos (arrays). O conhecimento do campo acústico pode otimizar alguns parâmetros de projeto de transdutores ultra-sônicos, tais como: a sua geometria, a profundidade de foco, a largura do feixe acústico, bem como os parâmetros de focalização de um array (leis de retardos e de apodização). O modelo implementado neste trabalho calcula a pressão acústica e o potencial de velocidade gerados por transdutores de geometria retangular em um conjunto de pontos do espaço. A abordagem usa o método da integral de convolução e da resposta espacial impulsiva, cuja solução computacional do problema é exata e relativamente simples, o que normalmente não ocorre com outros métodos que apresentam um alto custo computacional (tempo de processamento). A resposta em pressão do transdutor em cada ponto do espaço é obtida com o modelo implementado em Matlab e é verificada experimentalmente. São realizadas medidas experimentais do campo de pressão gerado por transdutores retangulares monoelementos na faixa de freqüências de 400 kHz a 2,2 MHz e um array na freqüência de 1 MHz, em um tanque de imersão, utilizando um hidrofone pontual e um sistema computadorizado de varredura de campo. A comparação entre os resultados simulados e experimentais mostra uma boa concordância, bem como as suas limitações. / The simulation of the acoustic field generated by a rectangular ultrasonic transducer is important because its geometry is frequently used in arrays. The knowledge of the acoustic field can optimize some design parameters of ultrasonic transducers, such as: geometry, focus depth, acoustic beam width, as well as the delay laws and the apodization of an array. Such simulation is implemented in this work, using a model that calculates the acoustic pressure in a space point. The approach uses the convolution integral and the spatial impulsive response methods. This computational solution is exact and relatively simple. That does not usually happen to other methods with a higher computational time. The acoustic field simulations obtained with the model implemented in Matlab are verified experimentally. Measurements of the pressure field generated by rectangular transducers in 400 kHz to 2.2 MHz and by an 1 MHz array were made in an immersion tank, using a punctual hidrophone and a computerized system for acoustic field measurements. The comparison between the simulated and experimental results shows good agreement.

No description available.

<p>The primary goal of this dissertation was to assess the feasibility of transforming an electromagnetic (EM) shock wave lithotripter with an acoustic lens as its focusing device from the original axisymmetric pressure distribution to a non-axisymmetric steerable acoustic field. This work was motivated by the desire to better match the distribution of effective acoustic pressure and pulse energy with the trajectory and anatomical features around renal and ureteral calculi during clinical shock wave lithotripsy (SWL). The acoustic field transformation was accomplished by the design of a fan-shaped acoustic barrier (mask) placed on top of the lithotripter acoustic lens to selectively reduce the source aperture along the direction of the barrier axis, therefore effectively broadening the beam width (<italic>BW</italic>) of the lithotripter field in this preferred direction. Moreover, the geometry of the original lens (L₁) was modified so that the acoustic focus of the new lens (L₂) at high output voltages (necessitated by the incorporation of the mask) is closely aligned with the lithotripter focus. The mask was further driven by a motor-controlled gear system to rotate around the lithotripter axis, generating a steerable and non-axisymmetric acoustic field. In this dissertation project, a linear acoustic model was first used for parametric studies to assess the effects of mask geometry (opening angle and thickness) on beam elongation and peak pressure reduction. Based on this analysis, two mask geometries (L₂+M₈₀₂₅ and L₂+M₉₀₃₀) were selected for modest and maximum beam elongation within the acceptable output range of the shock wave source. The acoustic and cavitation fields of the new lens with masks, as well as the corresponding field produced by the original lens, were characterized using fiber optical probe hydrophone measurements and stereoscopic high-speed imaging. Different output voltage settings were used for each lens configuration (i.e., 14 kV for L₁, 15.8 kV for L₂+M₈₀₂₅, and 17 kV L₂+M₉₀₃₀) to produce equivalent acoustic pulse energy of 45 mJ in all setups, measured in the lithotripter focal plane. Under this condition, L₂+M₈₀₂₅ and L₂+M₉₀₃₀ generate lower peak pressure (38.2 and 36.8 MPa) with a significantly broadened BW_y (11.4 and 14.3 mm) along the y-axis (head-to-toe direction of the patient), which is aligned with the mask axis, compared to the high peak pressure (44.1 MPa) and moderate <italic>BW</italic> (7.5 mm) of L₁. It is worth noting that L₂+M₈₀₂₅ and L₂+M₉₀₃₀ produce a <italic>BW</italic>_x (7.6 and 7.5 mm) in the orthogonal direction to the mask axis, which is also comparable to L₁. Similarly, the beam width of the cavitation field was broadened from 8.1 to 12.2 mm for L₂+M₈₀₂₅, and from 10.9 to 17.9 mm for L₂+M₉₀₃₀, compared to the range of 8.8 to 9.4 mm measured from L₁. In comparison, L₂+M₈₀₂₅ produces a denser and narrower bubble cloud along the y-axis than L₂+M₉₀₃₀. In vitro stone comminution (<italic>SC</italic>) tests in a tube holder (Diameter = 14 mm) have demonstrated that L₂+M₈₀₂₅ and L₂+M₉₀₃₀ are more effective at off-axis positions and during simulated respiratory motion along the elongated beam direction. The results of <italic>SC</italic> also confirmed the correlation between <italic>SC</italic> and the average peak pressure, p_+(avg), and effective acoustic pulse energy, E_eft, delivered to the stone, as shown in previous studies. Furthermore, a ureter model was developed and used to assess the performance of L₂+M₉₀₃₀, which has the maximally elongated <italic>BW</italic> under various static and simulated respiratory motion conditions. The results suggest that L₂+M₉₀₃₀ can produce significantly better <italic>SC</italic> than L₁ when the elongated beam is effectively aligned with the stone/fragments in the ureter or with their motion trajectory during the course of SWL treatment. Altogether, the results of this dissertation work have demonstrated <italic>in vitro</italic> that a non-axisymmetric and steerable acoustic field can significantly enhance stone comminution under clinically relevant SWL conditions. Future work is warranted to optimize the mask design and steering protocol to maximize the benefit of such an adaptable and versatile design to improve the performance and safety of clinical EM lithotripters.</p> / Dissertation

Cette thèse présente une étude des instabilités haute-fréquence dans les moteurs-fusées. Ce phénomène, qui a posé de nombreux problèmes dans les programmes de développement de moteur, est abordé de trois façons complémentaires : expérimentalement, théoriquement et numériquement. Premièrement, des expériences sont menées afin d’identifier les principaux processus et d’apporter les mécanismes ayant lieu lorsque le moteur devient instable. Pour parvenir à ce stade, un nouveau modulateur (VHAM), capable de créer des ondes acoustiques représentatives de ce qui se produit dans un moteur réel, est conçu et caractérisé. La deuxième partie concerne l’analyse théorique. Deux modèles (FAME, SDM) sont développés en suivant les principales conclusions de la campagne expérimentale : les oscillations de dégagement de chaleur sont dues au mouvement transverse des flammes, et le phénomène est déclenché lorsque des gouttelettes deviennent suffisamment petites pour être convectées par le champ acoustique. En utilisant ces modèles comme base de référence, un code numérique (STAHF) est présenté. Son but est de rendre compte des mécanismes déjà identifiés pour un coût de calcul faible. Il est ensuite montré qu’il peut être utilisé pour étudier des moteurs-fusées grandeur nature. La LES compressible est choisie pour étudier l’interaction entre l’acoustique et la combustion numériquement. Un nouveau modèle de combustion pour flammes non-prémélangées basé sur une hypothèse de chimie infiniment rapide est présenté et validé sur une flamme bien documentée (H3). Il est ensuite utilisé pour étudier l’interaction entre une onde acoustique transverse et la flamme H3. Une comparaison entre le terme source de Rayleigh calculé à partir de la simulation et celui prédit par le modèle théorique FAME est finalement menée. / This thesis presents a study of high frequency instabilities in rocket engines. This issue, which has plagued many engine development programs, is approached by three complementary viewpoints: experimental, theoretical, and numerical. First, experiments are carried out to identify the main processes involved and bring forth mechanisms taking place when an engine becomes unstable. To achieve this stage, a new modulator (the VHAM), capable of creating acoustic waves representative of what occurs in an actual engine, is designed and characterized. The second part of this thesis concern theoretical analysis. Two models are developed following the main conclusions of the experimental campaign: heat release oscillations are due to the transverse flames’ motion, and the phenomenon is triggered when droplets become small enough to be convected by the acoustic field. Using these models as a baseline, a numerical code (STAHF) is presented. Its purpose is to account for mechanisms identified previously for little computational cost. This code is validated on particularly responding situations observed during experiments. It is then shown that it can be used to study real scale rocket engines. The third point of view adopted to address the problem is numerical simulation. Full compressible LES is chosen to study the interaction between acoustics and combustion. A new combustion model for non-premixed flames with infinitely fast chemistry is presented and validated on a well documented flame (H3). It is then used to study the interaction between a transverse acoustic wave and the H3 flame. A comparison between the Rayleigh source term computed from the simulation and the one predicted by the theoretical model FAME is conducted eventually.

A modelagem do campo acústico gerado por um transdutor ultra-sônico retangular é importante porque sua geometria é usada freqüentemente por transdutores multielementos (arrays). O conhecimento do campo acústico pode otimizar alguns parâmetros de projeto de transdutores ultra-sônicos, tais como: a sua geometria, a profundidade de foco, a largura do feixe acústico, bem como os parâmetros de focalização de um array (leis de retardos e de apodização). O modelo implementado neste trabalho calcula a pressão acústica e o potencial de velocidade gerados por transdutores de geometria retangular em um conjunto de pontos do espaço. A abordagem usa o método da integral de convolução e da resposta espacial impulsiva, cuja solução computacional do problema é exata e relativamente simples, o que normalmente não ocorre com outros métodos que apresentam um alto custo computacional (tempo de processamento). A resposta em pressão do transdutor em cada ponto do espaço é obtida com o modelo implementado em Matlab e é verificada experimentalmente. São realizadas medidas experimentais do campo de pressão gerado por transdutores retangulares monoelementos na faixa de freqüências de 400 kHz a 2,2 MHz e um array na freqüência de 1 MHz, em um tanque de imersão, utilizando um hidrofone pontual e um sistema computadorizado de varredura de campo. A comparação entre os resultados simulados e experimentais mostra uma boa concordância, bem como as suas limitações. / The simulation of the acoustic field generated by a rectangular ultrasonic transducer is important because its geometry is frequently used in arrays. The knowledge of the acoustic field can optimize some design parameters of ultrasonic transducers, such as: geometry, focus depth, acoustic beam width, as well as the delay laws and the apodization of an array. Such simulation is implemented in this work, using a model that calculates the acoustic pressure in a space point. The approach uses the convolution integral and the spatial impulsive response methods. This computational solution is exact and relatively simple. That does not usually happen to other methods with a higher computational time. The acoustic field simulations obtained with the model implemented in Matlab are verified experimentally. Measurements of the pressure field generated by rectangular transducers in 400 kHz to 2.2 MHz and by an 1 MHz array were made in an immersion tank, using a punctual hidrophone and a computerized system for acoustic field measurements. The comparison between the simulated and experimental results shows good agreement.

Orientador: Vera Lúcia da Silveira Nantes Button / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação / Made available in DSpace on 2018-08-22T22:55:05Z (GMT). No. of bitstreams: 1 Nascimento_ValeriaMonteirodo_D.pdf: 6068450 bytes, checksum: 537e820daa45af6d8d50ccee4ad135e5 (MD5) Previous issue date: 2013 / Resumo: Para observar o efeito no campo acústico dos diversos parâmetros de configuração do projeto de um transdutor ultrassônico, foram simuladas várias configurações de transdutor de alta frequência (5 a 50 MHz), tipo array anular, com kerf e kerfless, de cerâmica PZT-5H e de filme de PVDF. Transdutores com configurações de três, quatro, cinco e seis anéis, em que os anéis possuíam a mesma largura foram simulados com elemento ativo de cerâmica. E transdutores com cinco, seis, sete, oito e dez anéis de mesma área, com separação física entre os elementos (kerf) e sem separação física entre os elementos (kerfless) foram simulados com elementos piezoelétricos de cerâmica PZT-5H e filme de PVDF. Também foram testados materiais de diferentes impedâncias acústicas nas camadas de retaguarda (epóxi, epóxi e Araldite, ferro, tungstênio, alumina e madeira), foram usadas diversas funções de excitação dos elementos piezoelétricos (Blackman, Wavelet, Gauss, Seno, Step) em frequências variadas, além de se acionar os diversos elementos anelares isoladamente, em grupos e com atraso temporal. O objetivo das simulações realizadas foi determinar a melhor configuração de um transdutor array anular, quanto ao número de anéis, espaçamento entre eles e necessidade ou não de separação física entre os elementos, levando-se em conta a complexidade de construção e as características do campo acústico gerado. Para isso, os parâmetros observados no campo acústico foram à amplitude do pico, a amplitude média de pressão, a profundidade do campo, a colimação do feixe principal e a presença de lóbulos laterais. Outros parâmetros observados foram à tensão e a carga nos elementos. Os resultados obtidos nas simulações dos transdutores arrays circulares, feitas com o programa PZFlex®, foram processados no Matlab® para visualização do campo acústico e extração de parâmetros. As funções de transferência do sinal de excitação e da resposta à estimulação foram calculadas com os dados obtidos. O transdutor kerfless foi simulado no PZFlex® e seus resultados processados no Matlab®, para comparar com os resultados do transdutor com kerf de mesma configuração. Os resultados das simulações mostraram que o campo acústico neste tipo de transdutor, array anular, tem a região de campo distante começando bem próximo à face do transdutor. A camada de retaguarda de epóxi e Araldite® apresentaram valores mais elevados de amplitude do campo acústico. A função de excitação foi Wavelet na frequência de 30 MHz devido às restrições da relação diâmetro / espessura da cerâmica PZT-5H. O transdutor kerfless apresentou um campo acústico com as mesmas características do transdutor com kerf, com a vantagem de sua construção ser mais simples / Abstract: In order to observe the effect of ultrasound transducers parameters configurations in an acoustic field, simulations were made in some different configuration of an annular array ultrasound transducer in high frequency (5 to 50 MHz), kerf and kerfless, with active element of PZT - 5H ceramic or PVDF. The transducers configurations simulated were three, four, five and six annulus, with the same width, and the active element was PZT-5H. And transducers with five, six, seven, eight and ten annulus, with the same area, with and without physical separation between the elements had been simulated with active element of PZT-5H ceramics or PVDF. The materials of backing layer (epoxy, epoxy and Araldite, iron, tungsten, alumina and wood dust) with different impedance were also be tested, and a variety of excited functions (Blackman, Wavelet, Gauss, Sine, Step), in different frequencies, and also with time delay in active elements. The objective of this simulation was to find out the better configuration of an annular array ultrasound transducer in high frequency, the number of annulus, spacing between them, the area, the transducer is kerf or kerfless, considering the build complexity and the characteristics of acoustic field. To do this, the peak amplitude, the depth and the average amplitude of the acoustic field was measured. The charge and the voltage in the elements were also observed. The results obtained in simulations in PZFlex® software were run in Matlab® to visualize the acoustic field and to extract parameter. The ultrasound kerfless transducer was simulated in PZFlex® and the data obtained runs in Matlab®, the results were compared with the results of a transducer with kerf in the same configuration. The simulation results showed that the acoustic field of this kind of transducer has Fraunhofer zone began near the transducer's face. The backing layer with epoxy and Araldite® showed high amplitude of acoustic field. The frequency 30 MHz was choice due to diameter/thickness relations. The excitation function was wavelet, as this present high values response in acoustic field. Kerfless transducer showed acoustic field characteristics the same as the kerf transducers, with the advantage of a simple construction / Doutorado / Engenharia Biomedica / Doutora em Engenharia Elétrica

No / The Helmholtz integral equation formulation is used to produce the solution for the acoustic field reflected from a finite, thin, poroelastic plate in a rigid baffle with simply supported edges. The acoustic properties of the porous material are predicted using the effective fluid assumption. The solutions for the displacement of the plate and for the loading acoustic pressures are given in the form of the sine transform. The sine transform coefficients are obtained from the solution of a system of linear equations resulting from three integral Helmholtz formulations which relate the displacement of the plate and the acoustic pressures on the front and on the back of the plate. The effect of an air gap behind the plate in the front of a rigid wall is also considered. A parametric study is performed to predict the effect of variations in the parameters of the poroelastic plate. It is shown that thin, light, poroelastic plates can provide high values of the acoustic absorption even for low frequency sound. This effect can be exploited to design compact noise control systems with improved acoustic performance.

Low-cost, high-performance energy storage solutions are in great demand for applications such as vehicle electrification and electricity generation from renewable sources. Lithium-based batteries have emerged as strong contenders due to their high energy density and stability. However, their reliance on scarce lithium reserves and high production costs makes them impractical for many applications. Sodium-based batteries (SBBs) are gaining traction as a more affordable option, with costs of $50 to $100 per kWh and an abundant resource base. Despite these advantages, SBBs still face many obstacles, primarily due to limited research on sodium-based chemistries. Additionally, sodium-based batteries have inherent limitations, including lower energy capacity and reduced cycle life, which restrict their viability for long-term use. This thesis addresses several critical challenges faced by SBBs and explores new strategies for enhancing their performance and viability for large-scale applications. First, a low-concentration, non-flammable electrolyte consisting of 0.3 M NaPF6 in a mixed solvent was formulated and tested in SBBs. This electrolyte significantly improves the cyclability and performance of SBBs across a wide temperature range, with high-capacity retention at both elevated and sub-zero temperatures. Molecular simulations reveal that the improved ion-pairing underpins the exceptional performance. This development addresses major challenges in SBBs by offering a safer, more cost-effective solution for large-scale applications. Second, sodium-sulfur (Na-S) batteries were explored to achieve high energy densities. An external acoustic field was implemented to enhance Na-S battery performance by inhibiting the shuttle effect and reducing dendrite growth, two key challenges in Na-S systems. This method offers a scalable, non-chemical solution to improve cycle life and efficiency, making Na-S batteries a more viable candidate for large-scale energy storage. This progress, along with the high theoretical capacity of Na-S batteries, helps address the limitations not resolved by the electrolyte engineering work of SBBs. Third, the mechanisms of Na2Sx (x≤2) precipitation in sodium-sulfur (Na-S) and sodium-oxygen-sulfur (Na/O2-S) systems were investigated. The results reveal that higher-order sodium polysulfides display the lowest current density, indicating a stronger driving force is needed to initiate their reaction. In Na/(O2)-S systems, the transition from high-order to low-order oxy-sulfur intermediates demands less energy compared to Na-S systems. The insights gained here help further optimize Na-S/Na/(O2)-S batteries to enhance their performance and cycle life. Together, the work in this dissertation addressed several critical needs in the development of SBBs and helped advance their commercialization. / Doctor of Philosophy / As the demand for affordable and efficient energy storage grows, especially to support electric vehicles and electricity generation from renewable sources, new types of batteries are needed. While lithium-based batteries are commonly used due to their high energy capacity, they rely on scarce and expensive lithium resources, making them impractical for large-scale applications like storing energy for electrical grids. This has led researchers to explore sodium-based batteries, which use sodium, a much more abundant and cost-effective element. However, sodium-based batteries still face challenges, such as lower energy capacity and shorter lifespan compared to their lithium counterparts. This thesis focuses on improving sodium-based battery performance to make them a more viable option for large-scale energy storage. The first project introduces a new, safer, low-cost electrolyte that significantly enhances the performance of sodium-based batteries, even in extreme temperatures. This solution improves the battery's ability to retain its energy over multiple charging cycles, addressing some of the key issues in current sodium-based battery technology. In the second project, the focus shifts to sodium-sulfur (Na-S) batteries, which offer the potential for higher energy storage due to sulfur's high capacity. To address common issues in Na-S batteries, such as the polysulfide shuttle effect and dendrite formation, a novel acoustic approach is explored. This method reduces the migration of polysulfides between the electrodes, which preserves capacity, and also suppresses the growth of dendrites on the sodium metal anode, improving both safety and battery lifespan. As a result, the efficiency and durability of Na-S batteries are significantly enhanced, bringing them closer to practical, large-scale applications. Despite these advancements, some fundamental questions remain about how sodium polysulfides form and behave inside Na-S batteries. The third project focuses on understanding the precipitation and interaction of these compounds under different conditions. Gaining insight into these processes is essential for further enhancing battery performance and ensuring long-term reliability. In conclusion, this thesis explores several innovative strategies to improve the design and performance of sodium-based batteries, which help make them more practical for large-scale energy storage applications and ultimately contribute to a more sustainable future.