Miniature valveless pulse combustors ( O ( cm 3 ) ) are small-scale thermoacoustic engines that generate high-momentum periodic jets, making them promising actuators for active flow control. These devices function without moving parts, relying on thermoacoustic oscillations in which unsteady heat release is synchronized with the acoustic field to sustain a limit cycle. This study presents a low-order thermo-electro-acoustic network model that integrates the Rott-Swift viscothermal formulation with a Crocco n-tau flame transfer function. Monte Carlo screening is applied to uncertain flame parameters to identify geometries that maximize unsteady jet output. Model predictions are validated using a miniature combustor equipped with pressure and force sensors, enabling a coherence-weighted thrust analysis that isolates combustion-driven performance. The results indicate that effective operation is achieved when length-dependent electro-acoustic modes, including throat acoustic and chamber Helmholtz modes, converge with the dominant intrinsic thermoacoustic (ITA) family. This convergence reflects a balance of throat inertance, chamber compliance, and flame dynamics consistent with the Rayleigh criterion. Harmonic and sum-tone interactions further reinforce acoustic branches, while shear-layer tones remain secondary. For three throat diameters, the predicted amplification windows agree with the measured coherence-weighted thrust RMS peaks and the eigen-map coincidence regions.

This study examines a Helmholtz-type pulse combustor as an active flow control strategy to reduce drag and manage flow separation around a circular cylinder. Experiments were conducted in a wind tunnel at Reynolds numbers ranging from 7.90 x 10(4) to 1.25 x 10(5). The tested configuration incorporated a pulse jet integrated into the cylinder, with controlled air and propane injection driving periodic combustion and momentum transfer of exhaust gases through two slots aligned with the cylinder's surface. Activating the pulse jet consistently reduced total drag, with the greatest improvement observed at lower reactant inflow rates. Increasing the reactant flows beyond this point yielded diminishing returns, suggesting a plateau effect. Load cell measurements indicated that pulse jet operation delayed flow separation and narrowed the cylinder's wake. Overall, these results highlight the promise of pulse combustion as a compact, energy-efficient means for active flow control.

In this study, an electromagnetic energy harvester for a low-frequency ocean wave was developed in a compact 3D-printed structure. Ocean wave energy conversion technologies exist, but maintaining them in the harsh marine environment is crucial for business. Friction increases maintenance costs. Therefore, magnetic levitation, being friction-free, is used for cost-effective, low-maintenance electromagnetic energy harvesting applications. Low-frequency oscillating energy is captured using repulsive magnetic levitation with a buoy and generating electricity using a permanent magnet and copper coil. A levitating magnet is repelled by a fixed one, inducing electricity as it passes through a coil. Experiments with a 0.1 Hz sine wave mimic the average frequency of ocean waves, showing successful voltage peaks at intervals. The output voltage and measured power from the harvester exhibit variations influenced by multiple parameters. The maximum output voltage observed was 3.4 V and an average of 99 mW of power was calculated. The experiment demonstrates the feasibility of using repulsive magnetic levitation for low-frequency wave energy harvesting and also encompasses various harvester configurations, including transfer magnet forces and top magnets.

This study investigates miniaturized pulsating combustion, explicitly focusing on harnessing resonant-based pulsed combustion in sub-inch-cubed combustion devices, to meet the reduced frequency demands inherent in active flow control mechanisms. The prevailing body of research in flow control has consistently underscored the efficacy of periodic momentum injection as a strategy for active flow control. This existing knowledge base catalyzes the current investigation, which seeks to leverage the inherently periodic nature of pulse combustor momentum output for flow control applications. in this study, miniature Helmholtz-type pulse combustor configurations, featuring combustion chamber volumes of $0.244\,\text{in}^3$ and $0.982\,\text{in}^3$ were investigated. The experimental framework involved designing and constructing combustion units with volumes, throat lengths, and orifice diameters selected to study variations in combustor output force and momentum associated with coincidence of combustion cavity chamber (Helmholtz) resonance and acoustic resonance of the combustion chamber exhaust pipe. The experimental setup entailed propane injection into a combustion chamber via a solenoid valve in parallel with steady inflow of air, driven by a compressor (flow rate hand-tuned to promote oscillatory combustion). The fuel-air mixture was initially ignited using a spark plug, however rapidly transitioned to a self-sustaining combustion process and spark plug use was discontinued. The resulting combustion products were expelled through a fume hood. This study presents time-resolved measurements of force, chamber pressure, static pressure at the jet exit, and the mass flow of reactants into the combustion chamber. Measured pulse jet operating frequencies and RMS force produced were superimposed on charts of device Helmholtz and acoustic modes to provide empirical insights and a potential explanation for the rare test cases that produced 4-6 times more force than other operating conditions. An efficiency metric was established by comparing the Root Mean Square (RMS) Force to the Fuel-Air Ratio (FAR) across varying lengths for each configuration. Complementing the quantitative data, Schlieren imagery was utilized to qualitatively illustrate the complex periodicity observed at the exit of the exhaust pipe to atmosphere, providing a visual representation of the pulsating combustion dynamics.

In this study, a magnetorheological fluid (MRF) was encapsulated in a 3D printed cellular elastomeric encapsulant. Mechanical properties, such as stiffness and damping, of the encapsulant containing MRFs can be controlled via applied magnetic field. Such tunability can be exploited for adaptive vibration control or energy absorption systems. Here, MRF prepared with silicon oil (40% volume fraction) was injected into the 3D printed thermoplastic polyurethane (TPU) cellular encapsulant. The wall adjacent to the sealing layer had a circular opening and a TPU membrane was bonded to its perimeter so that the pressurized MRF could flow through the opening and be accumulated in the resulting pocket. An external magnetic field (0–350 mT) was applied to the sample at the time of uniaxial dynamic testing with 5% pre-strain. The MRF-TPU composites were characterized via cyclic force-displacement tests (1 Hz) under displacement amplitudes. The area of the force-displacement curve of the system with an accumulator is about 26% greater than one without an accumulator. The effect of the accumulator on the mechanical properties of the MRF-TPU composites was studied with strain amplitude and magnetic fields.

Compared to smooth surfaces, droplet spreading on porous surfaces is more complex and has relevance in many engineering applications. In this work, we investigate the infiltration dynamics of molten sand droplets on structured porous surfaces using the multiphase many-body dissipative particle dynamics (mDPD) method. We carry out three-dimensional simulations with different equilibrium contact angles and surface porosities. The temporal evolution of the radius of the wetted area follows a power law, as in the case of a smooth surface. The infiltration rate on the other hand is dictated by the competition between spreading and capillary inhibition of the pores. Additionally, the temporal evolution of the droplet height and the contact angle on the porous surface is also presented.

In this study, a magnetorheological fluids (MRF) was encapsulated in a compact 3D-printed fluid damper, and the field-dependent mechanical properties, such as stiffness and damping, of the MRF damper were experimentally evaluated. To this end, an MRF prepared with 40% volume fraction of iron particles and silicone oil was injected into the 3D-printed Acrylonitrile Butadiene Styrene (ABS) encapsulant with elastic thermoplastic polyurethane (TPU) compression layer. A fluid channel was fabricated between the top and bottom reservoirs and a field-induced magnetorheological (MR) effect was controlled in the channel, resulting in enhanced stiffness and damping. The bottom side adjacent to the bottom reservoir has a circular opening and a membrane was bonded to its perimeter so that the pressurized MRF could flow and be accumulated in the lower reservoir. An external magnetic field (i.e., 0 and 32 kA/m) was applied to the 3D-printed MRF damper at the time of a uniaxial dynamic test. The 3D-printed MRF damper was characterized via force-displacement tests at a 2.5 mm/sec displacement rate. The effect of the channel and accumulator on the mechanical properties of the 3D-printed MRF damper was investigated.

Ingestion of sand particles into gas turbine engines has been observed to cause damage to engine components and in some cases leads to catastrophic failure. One such mechanism responsible for engine failure occurs through the deposition of molten particles on the turbine blades in the hot-section of the engine. The deposited material reacts chemically and penetrates the thermal barrier coating (TBC) on the turbines blades eventually damaging them. In this work, we investigate the deposition of sand particles on a solid substrate using two-way coupled Euler-Lagrange simulations. In these simulations, hot gas at 1700 K is issued from a circular inlet at Mach 0.3. Simultaneously, spherical mono-dispersed sand particles, modeled after the Calcia-Magnesia-Alumino-Silicates(CMAS), are injected at a constant mass flow rate of 1 gram per minute. The deposition of these particles on a solid substrate, placed 20 cm away from the inlet along the axial direction, is investigated. Simulations are carried out for three different synthetic sand particles CMAS, AFRL 02 and AFRL 03. The effect of Stokes number on particle properties such as number of particle depositions, rebound velocity and coefficient of restitution are investigated.

Development of magnetorheological elastomer (MRE) materials suitable for fused filament fabrication (FFF)-type additive manufacturing processes is performed. Filament materials are thermoplastic polyurethane (TPU) pellets and Fe 3 O 4 magnetic particles. A trickling method is employed to introduce the materials into the filament extruder at a constant rate, after which they are mixed in a hopper and extruded at 160 °C. The trickling rate of Fe 3 O 4 was controlled to produce different volume concentrations [0%–13% (v/v)]. Micrograph analysis on the cross section of the filaments indicates that magnetic particles are uniformly dispersed in the TPU matrix. As the volume concentration of particles in the matrix is increased over 10% (v/v), the size distribution of magnetic particles is slightly increased, indicating formation of aggregation of magnetic particles. Micropores are observed in the filaments, and the number of pores seem to increase with increasing volume concentration of magnetic particles. A commercial off the shelf (COTS) FFF 3-D printer is successfully used to produce cylindrical MRE samples with different magnetic particle volume concentrations. Magnetorheological (MR) properties of the printed MRE samples are characterized by uniaxial compression testing under different applied magnetic field. Material elastic modulus was found to increase with the intensity of magnetic field for a given magnetic particle volume concentration. The relative MR effect varies from −1.3% to 60% within the studied volume concentrations and applied fields. The samples with 5% (v/v) show the highest MR effect when compared with samples with higher magnetic particle volume concentrations. The results of these experiments confirm that the equivalent energy dissipation of the material rises with the increase in the Fe 3 O 4 concentration of MRE.

In this study, magnetorheological fluid (MRF) was successfully encapsulated in a 3-D printed elastomeric cellular structure. To this end, an MRF, which was composed of (40% volume fraction) carbonyl iron particles (6– $10~\mu \text{m}$ in diameter) suspended in silicone oil, was encapsulated in a thermoplastic polyurethane (TPU) elastomeric cellular structure. A 3-D printer was used to print a TPU elastomer with a rectangular cellular structure in the shape of a circular cylinder. The MRF was injected into the rectangular voids within the TPU cellular structure (hereinafter MRF-TPU elastomeric composite), and then sealed into the composite by 3-D printing a capping or sealing layer on top. The mechanical stiffness and damping properties of the MRF-TPU elastomeric composite with respect to external magnetic fields (0, 2, and 7 kG) and excitation frequencies (1, 5, and 10 Hz) were measured via uniaxial dynamic mechanical testing. Also, the effects of excitation and prestrain amplitude on the mechanical properties of the MRF-TPU elastomeric composite were investigated in these experiments. The complex stiffness and dissipated energy measured via dynamic mechanical testing were used as the performance index.

Upon coming into contact with a solid surface, a liquid droplet spreads rapidly during the early moments due to inertial/capillary effects before the viscous dissipation slows it down. The temporal evolution of the spreading radius depends on the viscosity of the liquid drop. For low-viscosity liquids, the spreading radius follows a power-law, whereas for higher viscosity liquids it scales linearly with time with additional logarithmic corrections. In this work, the spreading dynamics of molten sand is investigated at isothermal conditions. The molten sand is a mixture of Calcia, Magnesia, Alumina, and Silicate, commonly referred to as CMAS, and is characterized by large viscosity, density, and surface tension. The multiphase many-body dissipative particle dynamics (mDPD) model is carefully parame-terized to simulate a highly viscous molten CMAS droplet at 1260 oC. Three-dimensional (3D) simulations were carried out at different initial drop sizes and equilibrium contact angles. Despite its unique properties, the spreading behavior of molten CMAS is in good agreement with theory and experiments of viscous coalescence of drops. Importantly, the two distinct spreading regimes are observed in the mDPD simulations. Due to the large vis-cosity, a slower but a nonunique spreading rate is observed in the inertial regime. However, the spreading rate in the viscous regime is in agreement with Tanner's law. The spreading radius remains unaffected by the initial drop size and collapses onto a master curve under viscous time scaling in agreement with theory and experiments. For different equilibrium angles, the spreading rate is observed to be nearly identical in the inertial regime. This indicates a universal spreading behavior during the early stages of spreading unaffected by both the initial drop size and the equilibrium contact angle. The contact line velocity was also computed to assess its relation with the dynamic contact angle. The dynamic contact angle data collapse when plotted as a function of the capillary number, displaying a remarkable agreement with Hoffman's description of dynamic contact angle evolution.

This review article examines the last decade of studies investigating solid, molten, and liquid particle interactions with one another and with walls in heterogeneous multiphase flows. Such flows are experienced in state-of-the-art and future-concept gas turbine engines, where particles from the environment, including volcanic ash, runway debris, dust clouds, and sand, are transported by a fluid carrier phase and undergo high-speed collisions with high-temperature engine components. Sand or volcanic ash ingestion in gas turbine engines is known to lead to power-loss and/or complete engine failure. The particle-wall interactions that occur in high-temperature sections of an engine involve physics and intrinsic conditions that are sufficiently complex that they result in highly disparate and transient outcomes. These particles, which often times are made up of glassy constituents called calcium–magnesium–alumino–silicate (CMAS), are susceptible to phase change at combustor temperatures (1650°), and can deposit on surfaces, undergo elastic and plastic deformation, rebound, and undergo breakup. Considerable research has been put into developing empirical and physics-based models and numerical strategies to address phase interactions. This article provides a detailed account of the conceptual foundation of physics-based models employed to understand the behavior of particle-wall interaction, the evolution of numerical methods utilized for modeling these interactions, and challenges associated with improving models of particle-particle and particle-wall interactions needed to better characterize multiphase flows. It also includes description of a testbed for acquiring canonical data for model validation studies.

The objective of this research is to develop new thermal/environmental barrier coatings (T/EBCs) that exhibit greater durability and CMAS (Calcia-Magnesia-Alumino-Silicates) resistance than any of the current state-of-the-art rotorcraft turbine engine coatings. Commercial/Military aircraft engines, especially helicopter engines undergo severe damage to critical components when they need to operate over sandy terrains or volcanic zones. Typical high pressure turbine vanes/blades with current coatings undergo damages that include blade coating wear, sand glazing, Calcia-Magnesia-Alumina-Silicates (CMAS) attack, oxidation, plugged cooling holes, all of which can cause rapid engine performance loss and in severe cases ending up in loss of aircraft. Design of novel T/EBCs for high temperature operation is presented in this paper based on ongoing work in understanding the fundamental governing parameters affecting CMAS adhesion, build-up, and chemical attack. The paper intends to report specific objectives and findings obtained thus far from an ambitious T/EBC research program funded by OSD’s Strategic Environmental Research and Development Program (SERDP). Systematic sand-phobic development research efforts and methodologies from modeling to engine relevant high-temperature environmental test evaluations are described in this paper to innovate improved T/EBCs for both Ni-superalloy based substrates and emerging SiC-SiC Ceramic Matrix Composite (CMC) based substrates.

The objective of this work is to conduct non-intrusive uncertainty quantification in large eddy simulation (LES) of entrained particle deposition and associated heat transfer, focusing on parameter uncertainty. Specifically, we examine the effects of uncertainty in the particle convective Stokes number and mass-density on specific quantities of interest. To conduct this analysis 150 LES simulations are performed using the Monte Carlo approach to build a statistically significant sample space. This enables a global sensitivity analysis and forward propagation of uncertainty providing histograms, probability density functions, and Pearson correlation coefficients on the quantities of interest. The results provide insights of the underlying deposition behavior in LES simulations when considering Calcia-Magnesia-Alumina-Silicates (CMAS) particulate entrainment and transport over a linear cascade nozzle guide vane previously studied experimentally at the Von Karman Institute of Fluid Dynamics. The results demonstrate a strong sensitivity and negative correlation to particle Stokes number capturing a 5 fold increase in deposition count. Analysis of the surface heat flux shows a 1.25 fold increase in heat flux for the smallest particles considered with the PDF showing a Gaussian distribution. The results provide useful insights towards the behavior of CMAS impact statistics that will be ultimately useful to guide future design processes of more durable coatings. The research conducted in this paper reports specific objectives and findings to inform our T/EBC research program sponsored in part by DoD’s Strategic Environmental Research and Development Program (SERDP) and the DEVCOM Army Research Laboratory Propulsion Sciences program.

Gas turbine engines for fixed-wing or rotary-wing aircraft are operated in a variety of harsh weather environments ranging from arctic, volcanic zones, to desert conditions. Operation under these degraded conditions leads to the undesired entrainment of complex particulates resulting in drastic performance losses. Hence, there is a critical need to understand the governing mechanisms to inform the development of durable thermal and environmental barrier coatings. The objective of the current work is to present a novel multiscale physics-based approach to study two-phase flows that take into account the underpinning particle transport and deposition dynamics. Sessile droplet models are presented and used to compute the contact angle at high temperatures and compared with experiments. The study also investigates the sensitivity of deposition patterns to the Stokes number and the results identify local vulnerability regions. The analysis suggests that particle size distributions and the initial trajectories of the particles are critically important in predicting the final deposition pattern.

Sand ingestion is highly detrimental for gas turbines because it leads to erosion and corrosion of engine components, accelerating material fatigue and contributing to global engine failure. In this paper the high velocity impact of a molten sand particle onto a solid wall is investigated by means of the Smoothed Particles Hydrodynamics method where the three phases are taken into account. Nominal conditions are a 25 μm particle composed of molten sand (dynamic viscosity μl=11 Pa·s) impacting the wall at a velocity of 250 m/s. The influence of different parameters are explored such as the mechanical properties of the molten sand particle (density, viscosity, surface tension), the impact conditions (velocity magnitude, particle size and angle of impact) as well as the particle shape (sphere or cube with different geometrical features impacting the wall). It is found that the particles do not form a lamella during the impact but mostly conserve its initial shape. It is also confirmed that sharp features such as edges lead to a larger normal pressure at the impact location. Correlations to quantify (i) the spread factor, (ii) the maximum and mean impact force and impact pressure and (iii) the slip distance are derived for the first time based on the investigated parameters. The importance of these correlations is that they provide information needed to implement low-order models for studying impact and deposition of molten sand in engineering simulations.

The behavior of magnetostrictive nanowires and their potential for use in the design of devices for actuation and sensing applications is explored in this chapter. An introduction to magnetostrictive materials is presented first. This includes MOKE images of the response of magnetic domains in a magnetostrictive material to applied magnetic fields and to applied mechanical compression, as well as presentation of typical sensor and actuator characterization data from bulk samples of the magnetostrictive alloy Fe100−xGax (10 ≤ x ≤ ~35 atomic %) (also known as Galfenol or Fe-Ga). Next, models of magnetostriction at the macro- and microscale are presented. Micromagnetic simulations are used to visualize the significant role of shape anisotropy on magnetostriction in nanowires and to explain the challenges associated with achieving magnetic domain rotation in high aspect ratio nanowires. Methods for fabrication of Fe-Ga nanowires and Fe-Ga/X multilayer nanowires are discussed. Structural and magnetic characterization data are presented from Fe-Ga and Fe-Ga/Cu nanowires with of diameters of 100–200 nm and aspect ratios ranging from as low as 0.5 in some of the multilayer nanowire segments to over 100 in Fe-Ga single alloy nanowires. The last two sections of the chapter present experimental studies into use of the nanowires for actuation and sensing. This includes demonstration of magnetic domain rotation and magnetostriction in Fe-Ga/Cu multilayer nanowires in response to an applied magnetic field, as well as the use of a GMR sensor to detect magnetic domain rotation in response to application of a compressive mechanical load to an array of Fe-Ga/Cu nanowires.

deposition in turboshaft engines Luis G. Bravo1,a) , Nishan Jain2 , Prashant Khare3, Muthuvel Murugan1, Anindya Ghoshal1, Alison Flatau2 US Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, USA Department of Aerospace Engineering, University of Maryland, College Park, Maryland 20742, USA Department of Aerospace Engineering, University of Cincinnati, Cincinnati, Ohio 45221, USA Address all correspondence to this author. e-mail: luis.g.bravorobles.civ@mail.mil Received: 17 April 2020; accepted: 5 August 2020