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Sustainable aviation fuels are a major candidate to reduce pollutant emissions in future aeronautical engines. Recently, the use of hydrogen as a fuel has gained a high interest partly because its combustion is free from carbon dioxide, a greenhouse gas, and produces few pollutants, mainly nitrogen oxides (NOx). Over the last decades, efforts on numerical methods for combustion simulation in aero-engines have largely been focused on kerosene-air combustion. However, the current transition may have a significant impact on the computational methodologies for combustor design. Hydrogen defines novel modeling issues and challenges the current state of art on numerical methodologies. The current study presents a numerical investigation of a hydrogen-air burner using large-eddy simulations (LES) with a focus on NOx prediction. The considered configuration is a two-staged combustor, similar to the well-known RQL (Rich-Quench-Lean) technology, supplied by a single coaxial injector characterized experimentally. Two combustion models are investigated: (i) tabulated chemistry based on premixed flamelets (ii) transported chemistry description by using a 21-species chemical scheme. Numerical results are compared with experimental data (NOx concentrations, temperature distributions, pressure losses). A focus on model predictions is carried out. Results show a good agreement to predict the main flow characteristics and the premixed flame position over different operating points and geometries for both frameworks. In contrast, NOx emissions are more sensitive: while the overall trend is well captured, the quantification is more scattered. Finally, an in-depth analysis is proposed to link NOx production with the nonpremixed flame size. [ABSTRACT FROM AUTHOR]

The predictive capability of the lattice-Boltzmann very large eddy simulation (LBM-VLES) methodology was evaluated for industrial gas turbine exhaust diffuser-collector applications. The effort focused on evaluating the accuracy of performance predictions against experimental rig data gathered at engine representative conditions including the Reynolds number, Mach number, and inlet flow conditions. The performance prediction capability of LBM-VLES simulations was also compared against Reynolds-averaged Navier-Stokes (RANS) simulations. Evaluations were completed for two distinct exhaust rig geometries at conditions indicative of their respective design points, and the performance prediction capability was also evaluated at conditions depicting two off-design cases. The off-design conditions were represented by an increase in inlet swirl angle and associated incidence on the diffuser struts. Overall, the authors found that LBM-VLES simulations were a suitable approach for predicting the performance of gas turbine exhaust diffuser-collector systems at both on- and off-design conditions. The LBM-VLES simulation accuracy was substantially better than that achieved by RANS simulations, with 66% lower RMS pressure recovery error between computational fluid dynamics (CFD) and experiments for the four cases studied, and an 89% reduction in error for the furthest off-design case. The computational cost of the transient LBM-VLES simulations was roughly the same as the RANS simulations performed using best practices for that solver. [ABSTRACT FROM AUTHOR]

Direct dry-cooled power plants typically operate numerous large-diameter axial flow fans in air-cooled condensers (ACCs) to facilitate the condensation of steam in the plant's thermodynamic cycle. Enhanced fan efficiency may, at scale, lead to significant parasitic power reductions and subsequent increases in plant power output. The performance of these fans may be increased by adding blade-tip modifications, which act to control blade-tip leakage flow. Previous studies have shown that manipulating the tip leakage vortex (TLV) increases the blade-to-air momentum transfer and stabilizes the air flow structures as it traverses the fan blade. This paper takes a step toward the development of improved tip vortex manipulators for an ACC fan. Three-dimensional computational fluid dynamics (CFD) simulations, and experimental testing of the modified and unmodified fan within an ISO test facility are performed. The results are then utilized to assess the accuracy of the simulation model, visualize the manipulated flow structures at the blade tip, and ultimately quantify the effect of the endplate designs on fan performance. Excellent correlation between numerical and test results for the unmodified fan is observed, and the benefit of blade-tip modification on fan performance is again confirmed experimentally. The simulation model, however, fails to predict improvements in fan performance under modification, and simulated tip leakage flows are investigated for clarification. It is hypothesized that deflection of tip endplates under operation account for differences between simulation and experiment, and that fluid-structure interaction analyses could potentially resolve discrepancies. [ABSTRACT FROM AUTHOR]

Individuals with transtibial amputation (TTA) experience asymmetric lower-limb loading which can lead to joint pain and injuries. However, it is unclear how walking over unexpected uneven terrain affects their loading patterns. This study sought to use modeling and simulation to determine how peak joint contact forces and impulses change for individuals with unilateral TTA during an uneven step and subsequent recovery step and how those patterns compare to able-bodied individuals. We expected residual limb loading during the uneven step and intact limb loading during the recovery step would increase relative to flush walking. Further, individuals with TTA would experience larger loading increases compared to able-bodied individuals. Simulations of individuals with TTA showed during the uneven step, changes in joint loading occurred at all joints except the prosthetic ankle relative to flush walking. During the recovery step, intact limb joint loading increased in early stance relative to flush walking. Simulations of able-bodied individuals showed large increases in ankle joint loading for both surface conditions. Overall, increases in early stance knee joint loading were larger for those with TTA compared to able-bodied individuals during both steps. These results suggest that individuals with TTA experience altered joint loading patterns when stepping on uneven terrain. Future work should investigate whether an adapting ankle-foot prosthesis can mitigate these changes to reduce injury risk. [ABSTRACT FROM AUTHOR]

In an internal combustion engine, interactions of fuel droplets and heated walls can significantly affect the combustion process and engine performance. The formation and characteristics of secondary droplets from drop-wall interactions are functions of various factors such as fuel properties, impact velocity, ambient conditions, and wall temperature. Understanding the impact behavior is important to optimize the distribution of the fuel-air mixture for efficient and clean combustion and to develop a comprehensive spray-wall interaction model. In this study, three-dimensional smoothed particle hydrodynamics (SPH) simulations are performed to investigate the interactions of fuel droplets with a heated wall at atmospheric and elevated pressures over a range of Weber numbers (We). The SPH model is validated using available experimental data. Secondary atomization is characterized by using size distributions for different fuels. The resulting droplets vary in size, where secondary droplets are mostly below 7 µm in diameter. Following these cases, this paper qualitatively describes the impact process and proposes empirical correlation relating the mean secondary droplet size to ambient pressure in the film-boiling regime. Postimpingement vaporization characteristics are also analyzed and compared for fuels with drastically different vapor pressures. [ABSTRACT FROM AUTHOR]

A multiyear power system R&D program was completed with the objective of developing an off-road hybrid heavy duty diesel engine with front end accessory drive-integrated energy storage. This system was validated to deliver 10.5-25.6% reduction in fuel consumption over current Tier 4 Final-based 18L diesel engines, over various off-road machine application cycles. The power system consisted of a downsized heavy-duty diesel 13L engine containing advanced combustion technologies, capable of elevated peak cylinder pressures and thermal efficiencies, thermal barrier coatings, exhaust waste heat recovery via SuperTurbo™ turbocompounding, and hybrid energy assisting and recovery through both mechanical and electrical systems. Following the concept definition, design, and analysis phases of the program, the final phase focused on building and validating the performance and efficiency in laboratory tests. While aspects of the system such as start/stop and reduced off-road cooling package energy losses were only analytically evaluated, the main 13L concept engine with full hybrid system was successfully built and tested in steady-state and in transient certification and real-world application cycles. Extensive simulations in Caterpillar's DYNASTY™ software environment utilized the validation test data to assess performance more fully and confidently over varied cycles and strategies. An average fuel consumption reduction of 17.9% was realized, and the majority (~13%) of the benefit stemmed from the core concept 13L engine. To conclude, a total cost of ownership analysis provides context to commercial viability and where adoption focus should be placed. [ABSTRACT FROM AUTHOR]

Turbofan engines exhibit pronounced nonlinearity due to component characteristics and thermal processes, necessitating linearization to connect engine control with linear control theory. Off-equilibrium linearization offers excellent transient tracking and controller performance, making it ideal for military turbofan engines with rapid acceleration and deceleration. Hardware-in-Loop (HIL) experiments are crucial for verifying controller performance, but time delays in the HIL platform can cause oscillations in state variables when using this linearization method. To address this issue, this paper introduces an off-equilibrium linearization-based control strategy. This strategy employs an off-equilibrium linearized model to approximate the nonlinear time-delay system, followed by designing an H8 controller for the linear time-delay system. The effectiveness of this approach applied to turbofan engines, including its antidelay and robust tracking capabilities, is validated through simulations, HIL experiments, and semiphysical experiments. [ABSTRACT FROM AUTHOR]

Although precombustion chambers, or prechambers, have long been employed for improving large-bore two-stroke natural gas engine ignition and combustion stability, their design predates modern analysis techniques. Employing the latest computational fluid dynamics (CFD) modeling techniques, this study investigates the importance of temperature and chemistry for ignition of the main chamber, with an emphasis on eliminating unburned methane. The sensitivity of the ignition and complete combustion to main chamber air/fuel mixture homogeneity was also explored. This study compares the effect of purely thermal ignition, purely chemical ignition, and how their interplay can influence the complete combustion of methane in typical mixtures and in homogeneous distributions of fuel in the combustion chamber. The CFD results demonstrated that temperature and chemistry are equally important in the ignition mechanism, and combining the two phenomena is effective at igniting the main chamber. Reduction of residual methane in the main combustion chamber (MCC) is most effective when chemical intermediates and thermal ignition are combined. A rudimentary analysis of the effect of fuel/air stratification was also conducted, and it demonstrated that a dramatic reduction in methane emissions is observed for homogeneous mixtures. The flow field in the main combustion chamber was shown to create detrimental stratification of the fuel/air mixture, which inhibited complete combustion of the methane in the main chamber. By contrast, in the extreme case of a perfectly homogeneous distribution of both chemical intermediates and fuel in the combustion chamber, it is possible to completely eliminate unburned methane in the main combustion chamber. [ABSTRACT FROM AUTHOR]

In this investigation, we explore the profound impact of adiabatic partitions on heat transport within a square Rayleigh-Bénard (RB) convection enclosure characterized by surface roughness on the lower hot and upper cold plates. The surface irregularities take the form of a rectangular base and a triangular apex. Our study employs two-dimensional direct numerical simulations spanning the Rayleigh number (Ra) range of -106-108 and a Prandtl number (Pr) of 1. Adiabatic partitions, strategically positioned between successive roughness, serve as the focal point of our exploration. Remarkably, our findings reveal a substantial enhancement in heat transport with the introduction of a partition board between consecutive roughness elements. As we escalate the number of partitions from one to four, the heat flux experiences a notable augmentation, reaching 2.3 times that of the classical square RB configuration. This enhancement is attributed to the breakdown of large-scale rolls into multiple rolls, a phenomenon intensified by the increased partition height. Further intriguing observations unfold as we investigate the interplay of surface roughness and partitions. Configurations featuring roughness with partitions exhibit an impressive 2.7 times heat flux enhancement compared to the classical square RB setup. The complex interplay of heat transport improvement is closely connected to optimizing the distance between the conduction plate and the partition. Through meticulous analysis, we identify that the optimal gap facilitates heightened local velocity, effectively thinning the thermal boundary layer and consequently augmenting the overall heat flux. In essence, our study sheds light on the synergistic effects of adiabatic partitions and surface roughness in the context of RB convection. The observed enhancements in heat transport underscore the potential for tailored design strategies involving partitions and surface modifications to optimize thermal performance in diverse applications. [ABSTRACT FROM AUTHOR]

This study focuses on improving the thermal performance of a solar air heater (SAH) using a single-pass spiral-shaped ducts. The SAH is designed and tested under prevailing weather conditions of Gabes, Tunisia (33°52.8876' N,10°5.892' E). The experimental measurements are carried out over 4 days. Similarly, a computational fluid dynamics (CFD) model was developed to study the fluid flow and the heat transfer inside the SAH using the commercial software ansys fluent 2021 R1". The discrete ordinate (DO) radiation model and the k-ω shear stress transport (SST) turbulence model are used to study the radiative heat transfer and the turbulent flow in the SAH, respectively. The numerical model is validated against experimental data, and the average error does not exceed 3.6%. To improve the heat transfer phenomena, the ratio of horizontal baffle spacing "d" to vertical baffle spacing "p" (d/p) is numerically investigated. Moreover, the highest air outlet temperature during the test days reached 81.1 °C under a mass flowrate of 0.0077 kg/s. The maximum efficiencies are 57%, 54%, 49%, and 46% for the configurations d/p = 1.5, d/p = 2, d/p = 1, and d/p = 0.5 under a mass flowrate of 0.02 kg/s, respectively. The SAH design with d/p = 1.5 is about 4-10% more efficient than the standard design with d/p = 1 under a mass flowrate ranging from 0.0077 kg/s to 0.025 kg/s. [ABSTRACT FROM AUTHOR]

For organometallic halide solar cells (OHSC), it is expected that their performance in hot climates is to be challenged by high operating temperature conditions typical of these regions. This study explores, for the first time, the performance of formamidinium tin iodide (FASnI3) solar cells under variations of seasonal and climatic conditions in Nigeria using a non-steady-state thermal model. From the thermal analysis, results show that the air temperature in the location of the solar cell under study played a significant role in the increase and decrease of the rate of the overall heat transfer coefficient of the OHSC. However, the cell temperature depended on the rate of heat loss and the solar radiation absorbed by the OHSC. The electrical analysis was based on the numerical simulation of a FASnI3 solar cell with the aid of a Solar Cell Capacitance Simulator (SCAPS). A decrease in the power conversion efficiency (PCE) as the cell temperature increased was observed. Overall, while the OHSC suffered losses in efficiency in all locations during the hot season, the wet season saw an improvement in the PCE, especially in Twon-Brass (0.5% increase) where the most heat loss and least insolation were recorded. This shows that the power conversion efficiency of an operating OHSC is temperature-dependent, rather than the abundance of solar irradiance. [ABSTRACT FROM AUTHOR]

This paper proposes an investigation of a novel design of receiver absorber tube (circular-trapezoidal shaped) for parabolic trough concentrator (PTC) system aiming at catching a part of the lost (reflected) solar rays due to effects related to the incidence angle deviation and thus improving the PTC's thermal performance. Although they are always present in PTC, the effects of the deviation angle are often not considered in the literature. These effects are considered here in view of presenting a better and more useful focal area, the optimal circular-trapezoidal absorber tube shape is determined according to the maximal limit of the deviation angle that allows catching the maximum amount of solar rays. The corresponding model is established considering it as a two-dimensional problem and the derived deviation angle coefficient is compared to that obtained for a traditional circular-shaped tube. Moreover, the energy balance model is adapted to simulate the thermal efficiency of the considered tubes as a function of the deviation angle with some assumptions. The obtained results prove that from a deviation angle value of 0.1 deg, the gain in efficiency becomes more significant; it can reach 5% and even be higher for a deviation angle value of 0.5 deg. Finally, comsol multiphysics software package was used to determine the pressure drops, temperature, and velocity field distributions, considering a deviation angle value of 0.2 deg. The results confirm that the proposed tube absorbs more heat than the traditional one. [ABSTRACT FROM AUTHOR]

Friction stir welding (FSW) produces inhomogeneous mechanical and metallurgical properties in the weld, which further require post-weld processing to control the heterogeneity. In the present study, the heterogeneity in the weld is reduced through counter-variable rotation friction stir welding (CVRFSW). The material flow and temperature distribution significantly affect the inhomogeneity of the FSWed properties which has been studied by developing a three-dimensional Lagrangian method-based viscoplastic model. The material flow, strain rate, and temperature distribution in conventional FSW (CFSW) and CVRFSW are studied quantitatively. The study revealed that CVRFSW improved joint strength and reduced the inhomogeneity of temperature, strain, and hardness. At a 10% lower shoulder speed than a pin, the weld strength improved by 16%. The simulation predicted that the temperature difference between the advancing side (AS) and the retreating side (RS) was 36 °C in CFSW, which was reduced to 8 °C in CVRFSW. Material deformation in CVRFSW occurred at a strain rate more than twice that of CFSW, and the asymmetry of strain rate between AS and RS reduced to one-fifth. Microstructures and their orientations of the welds were studied in detail. These findings contribute to the understanding of CVRFSW processes for enhanced weld quality and mechanical performance for industrial applications. [ABSTRACT FROM AUTHOR]

With air traffic expected to grow 5% annually until the year 2030, alternative fuels such as hydrogen are being investigated in order to tackle the current environmental crisis. Due to safety concerns, future hydrogen combustion chambers will require new designs of injection systems and are expected to operate under multimode combustion regimes. From a large-eddy-simulation (LES) perspective, a prerequisite for the shift toward new hydrogen combustion chamber technologies is a robust turbulent combustion model capable of functioning in non-premixed conditions. Turbulent combustion modeling using flame front filtering is a well-developed strategy in premixed combustion (filtered-tabulated chemistry for large-Eddy-simulation (F-TACLES)). This approach has been extended to non-premixed flames however, it suffers from high flame filter size sensitivity. Moreover, thin hydrogen flame fronts will result in lower resolution on the LES grid, potentially amplifying this issue. In order to address the feasibility of the non-premixed F-TACLES model applied to hydrogen fuel, simple one-dimensional and two-dimensional laminar counterflow diffusion flames are computed. The model is then tested on the three-dimensional Sandia hydrogen jet flame with a Reynolds number of 10,000. Simulations and a priori tests show that tabulated subgrid-scale correction terms are stiff and can result in nonphysical results, however the model is capable of correctly reproducing non-premixed flame structures for small filter sizes. [ABSTRACT FROM AUTHOR]

The roughness geometry has been introduced to improve the rate of heat transfer in a solar air heater duct. In the current work, circular and triangular shape geometries are used as roughness elements in the rectangular channel to enhance the thermal performance of reversed flow solar air heater (RFSAH). The important parameters selected for the research are Reynolds number (Re = 5000-18,000 (5 values)), pitch ratio (P/e = 4-12 (5 values)), and height ratio (e/D = 0.0392-0.1571 (4 values)). A 2D-computational fluid dynamics (CFD) model was developed using ansys (fluent 2022r1), and simulation was performed using the (RNG) turbulence model and validated with one set of experimental results for smooth duct and previous research. The findings revealed that the highest value of heat transfer was augmented about 2.18 times and 2.35 times for circular and triangular roughness geometry, respectively, as compared to the smooth channel at a Reynolds number of 12,000. The thermohydraulic performance factor (TPF) is 1.58 and 1.7 at pitch ratios of 6 and 5 for circular and triangular roughness geometry respectively, at Re of 12,000. [ABSTRACT FROM AUTHOR]

The next generation of ultrahigh bypass ratio civil aero-engines promises notable engine cycle benefits. However, these benefits can be significantly eroded by a possible increase in nacelle weight and drag due to the typical larger fan diameters. More compact nacelles, with shorter intakes, may be required to enable a net reduction in aero-engine fuel burn. The aim of this paper is to assess the influence of the design style of short intakes on the unsteady interaction under crosswind conditions between fan and intake, with a focus on the separation onset and characteristics of the boundary layer within the intake. Three intake designs were assessed, and a hierarchical computational fluid dynamics (CFD) approach was used to determine and quantify primary aerodynamic interactions between the fan and the intake design. Similar to previous findings for a specific intake configuration, both intake flow unsteadiness and the unsteady upstream perturbations from the fan have a detrimental effect on the separation onset for the range of intake designs. The separation of the boundary layer within the intake was shock driven for the three different design styles. The simulations also quantified the unsteady intake flows with an emphasis on the spectral characteristics and engine-order signatures of the flow distortion. Overall, this work showed that is beneficial for the intake boundary layer to delay the diffusion closer to the fan and reduce the preshock Mach number to mitigate the adverse unsteady interaction between the fan and the shock. [ABSTRACT FROM AUTHOR]

For discrete time nonlinear networked control systems, a novel self-triggered adaptive model predictive control (MPC) strategy is developed. Different from the existing self-triggered MPC methods that determine the triggering instants based on the difference between the optimal and real states at one single instant, the proposed approach updates the MPC system according to the differential form of the state error of two consecutive sampling moments to effectively reduce the computation and communication burden while maintaining the ideal control performance. In addition, this paper introduces a new adaptive prediction horizon mechanism to the self-triggered MPC, so that the amplitude of prediction horizon contraction is sufficiently large to further reduce the computational burden of the MPC method. Finally, the recursive feasibility and robust stability of this proposed strategy are proved strictly by theoretical analysis, and the simulation comparison results are shown to verify the proposed framework. [ABSTRACT FROM AUTHOR]

Thermal energy storage (TES) devices use phase change materials (PCMs) to store and release thermal energy. Control-oriented models are needed to predict the behavior of TES devices and experimental validation is necessary to demonstrate the predictive capabilities of these models. This paper presents an experimental validation of a switched moving boundary (MB) approach for modeling TES devices, where the dynamics of the device are captured with fewer states than traditional models. A graph-based modeling approach is used to model heat flow, while the moving boundary captures the time-varying liquid and solid regions of the TES. The model uses a finite state machine (FSM) to switch between four modes of operation based on the state-of-charge (SOC) of the TES. Results show that the switched MB approach has similar accuracy and lower computational cost compared to traditional modeling approaches when predicting the SOC of an experimental TES device. [ABSTRACT FROM AUTHOR]

An unsteady Reynolds-averaged Navier-Stokes model on body-fitted meshes in a commercial package (SIMERICSMP+) with a mismatched grid interface is used to study fluid dynamics around a ducted wind turbine. The model is validated by studying turbulent flow past a marine propeller. The nondimensional thrust and torque coefficients are compared against experimental data and results from a large eddy simulation model. Both coefficients are found to be within 3% of experimental results. Following this validation, the impact of different tip speed ratios on the ducted wind turbine's fluid dynamics is assessed. The optimal tip speed ratio is found to be the design value of 3.93 with a maximum power coefficient of 0.465 based on the duct exit area. The corresponding thrust coefficient is found to be 1.02 based on the rotor area. Lower tip speed ratios experience larger flow separation on the duct interior. Higher tip speed ratios decrease the size of the low-velocity region behind the hub. The ducted wind turbine's performance at design conditions is compared to an open rotor. The ducted wind turbine increases the power coefficient by 96% over the open rotor. The impact of hub size on the ducted wind turbine is also studied by simulating a smaller hub with 77% diameter. At the design tip speed ratio, the smaller hub has a power coefficient of 0.417. The maximum power coefficient is found to be 0.446 at a higher tip speed ratio of 4.5. [ABSTRACT FROM AUTHOR]

The purpose of this computational study was to investigate the effects of neonate-focused clinical delivery maneuvers on brachial plexus (BP) during shoulder dystocia. During shoulder dystocia, the anterior shoulder of the neonate is obstructed behind the symphysis pubis of the maternal pelvis, postdelivery of the neonate's head. This is managed by a series of clinical delivery maneuvers. The goal of this study was to simulate these delivery maneuvers and study their effects on neonatal BP strain. Using madymo models of a maternal pelvis and a 90th-percentile neonate, various delivery maneuvers and positions were simulated including the lithotomy position alone of the maternal pelvis, delivery with the application of various suprapubic pressures (SPPs), neonate in an oblique position, and during posterior arm delivery maneuver. The resulting BP strain (%) along with the required maternal delivery force was reported in these independently simulated scenarios. The lithotomy position alone served as the baseline. Each of the successive maneuvers reported a decrease in the required delivery force and resulting neonatal BP strain. As the applied SPP force increased (three scenarios simulated), the required maternal delivery force and neonatal BP strain decreased. A further decrease in both delivery force and neonatal BP strain was observed in the oblique position, with the lowest delivery force and neonatal BP strain reported during the posterior arm delivery maneuver. Data obtained from the improved computational models in this study enhance our understanding of the effects of clinical maneuvers on neonatal BP strain during complicated birthing scenarios such as shoulder dystocia. [ABSTRACT FROM AUTHOR]

Sensitivity coefficients are used to understand how errors in subject-specific musculoskeletal model parameters influence model predictions. Previous sensitivity studies in the lower limb calculated sensitivity using perturbations that do not fully represent the diversity of the population. Hence, the present study performs sensitivity analysis in the upper limb using a large synthetic dataset to capture greater physiological diversity. The large dataset (n = 401 synthetic subjects) was created by adjusting maximum isometric force, optimal fiber length, pennation angle, and bone mass to induce atrophy, hypertrophy, osteoporosis, and osteopetrosis in two upper limb musculoskeletal models. Simulations of three isometric and two isokinetic upper limb tasks were performed using each synthetic subject to predict muscle activations. Sensitivity coefficients were calculated using three different methods (two point, linear regression, and sensitivity functions) to understand how changes in Hill-type parameters influenced predicted muscle activations. The sensitivity coefficient methods were then compared by evaluating how well the coefficients accounted for measurement uncertainty. This was done by using the sensitivity coefficients to predict the range of muscle activations given known errors in measuring musculoskeletal parameters from medical imaging. Sensitivity functions were found to best account for measurement uncertainty. Simulated muscle activations were most sensitive to optimal fiber length and maximum isometric force during upper limb tasks. Importantly, the level of sensitivity was muscle and task dependent. These findings provide a foundation for how large synthetic datasets can be applied to capture physiologically diverse populations and understand how model parameters influence predictions. [ABSTRACT FROM AUTHOR]

A lumped-parameter dynamic performance model for a single-spool turbojet engine is presented in this paper. This model can handle pre and poststall transients under forward and reverse-flow conditions. The inter-component volume technique is employed instead of the standard matching technique to be able to handle high-frequency transients and reverse-flow conditions. Inspired by Greitzer's lumped-parameter surge model, momentum (duct) and volume elements are placed within the flow path to handle surge dynamics. Compressor and turbine maps are extended to low-flow and reverse-flow regions using a combination of the guidelines presented by Kurzke, the cubic axisymmetric characteristics of Moore and Greitzer, and a quadratic function guess for in-stall characteristics. Combustor efficiency, stability limits, and delay are taken from the literature. Poststall behavior of the model is validated using the data available in the literature for a Rolls-Royce Viper engine. A good match is observed with a correct prediction of poststall behaviors, which transition from surge after locked stall to multiple surge cycles around 80% speed and multiple surge cycles to surge after flameout around 95% speed. The effects of different modeling choices and modeling parameters on the obtained results are discussed. The produced model can be calibrated for a specific engine with surge tests, and it can be used for hard-to-test scenarios like surge after shaft breakage. Different surge/stall-causing events, such as fuel spiking, in-bleeding, and shaft breakage, are simulated to see the capabilities of the model. [ABSTRACT FROM AUTHOR]

This research presents a detailed transient experimental and computational study of heat transfer and airflow in a scaled room linked with a double-duct vertical roof solar chimney (SC). The analysis was made in the coupled system, varying the heat flux supplied to the roof SC absorber during the day. Experimental temperature profiles were obtained at six different depths and heights, the empirical heat transfer coefficient was computed for the SC absorber, and the variation of air changes per hour was determined. A good agreement between experimental and numerical temperature profiles and average Nusselt number (Nut) is shown in the latter of average difference of 3.41%. The validated computational model was used to analyze the effect of the transient heating of the SC on temperature fields and flow patterns in the thermal system. Almost symmetrical temperature and y-velocity profiles are formed in the ducts at different hours of the day, with thermal boundary layers of about 4 mm. Correlations of the Nusselt number and air changes per hour (ACH) are provided as a function of the modified Rayleigh number. [ABSTRACT FROM AUTHOR]

This paper investigates a solar air heater's thermohydraulic and thermogeometric performances with an artificially roughened absorber plate with Joukowski airfoil ribs. The rib height and shape have a bearing on the overall performance of the air heater. Joukowski airfoil ribs of different sizes are generated from a circular rib of a radius of 1 mm using conformal mapping. Simulations are performed for the turbulent flow of air through the roughened duct in the Reynolds number (Re) range of 4000 ≤ Re ≤ 15,000 using ANSYS FLUENT version 2020. The renormalization group kinetic energy-turbulence dissipation rate (RNG κ - ε) model with enhanced wall treatment (EWT) has been employed to model the turbulent flow. The grid refinement study is performed to optimize the mesh size and estimate the numerical solution error. The proposed rib design is tested for both headwind and tailwind flow arrangements. The tailwind performance is better for smaller-size Joukowski ribs. However, the medium- and large-size Joukowski ribs perform better in headwind configurations. At low Re, heat transfer is more dominant than friction leading to higher thermohydraulic performance, whereas, at high Re, the reverse trend is observed. [ABSTRACT FROM AUTHOR]

The expected annual energy output of vertical bifacial solar panel arrays was modeled with an eye on how array design attributes affect the output. We considered module height, cell density (single- or double-high racking), inter-row spacing, and inverter connection (rows of modules wired together or separately), and the inclusion of bypass diodes. We observed that these design choices have a substantial impact on the annual energy yield on a per-module basis and per-acre basis. We modeled the instantaneous brightness and shading based on the position of the sun and adjacent rows of modules, which caused nonuniform irradiance due to inter-row shading effects. Based on the irradiance, we calculated current, voltage, and power values throughout a year for different design strategies. Double-high racking, which uses two landscape-oriented modules stacked vertically, offers noteworthy power gains per acre with only a modest increase of inter-row shading. When bypass diodes are included in the module design and improved inverter wiring is used, much of the loss due to inter-row shading is mitigated, and the total power output per acre is nearly doubled, with modules seeing an 80% power increase per acre for 20 ft row spacing, and over 90% power increase per acre for 40 ft spacing. [ABSTRACT FROM AUTHOR]

In this paper, we introduce a novel direct maximum power point tracking (MPPT) approach that combines the backstepping controller (BSC) and the super-twisting algorithm (STA). The direct backstepping super-twisting algorithm control (BSSTAC) MPPT was developed to extract the maximum power point (MPP) produced by a photovoltaic (PV) generator connected to the battery through a boost DC-DC converter. To reduce the number of sensors required for the BSSTAC implementation, a high gain observer (HGO) was proposed to estimate the value of the state of the PV storage system from measurements of the PV generator voltage and current. The suggested technique is based on the quadratic Lyapunov function and does not employ a standard MPPT algorithm. Results show that the suggested control scheme has good tracking performance with reduced overshoot, chattering, and settling time as compared to the prevalent MPPT tracking algorithms such as perturb and observe (P&O), conventional sliding mode control (CSMC), BSC, and integral backstepping controller (IBSC). Finally, real-time findings using the dSPACE DS 1104 software indicate that the generator PV can accurately forecast the MPP, as well as the efficacy of the suggested MPPT technique. The provided approach's effectiveness has been validated by a comprehensive comparison with different methods, resulting in the greatest efficiency of 99.88% for BSSTAC. [ABSTRACT FROM AUTHOR]

Abdominal aortic aneurysm can exhibit transitional flow characteristics in laminar flow regimes. To report transitional flow characteristics, we examined the convergence of phase-averaged solutions by executing blood flow simulations of a patient-specific abdominal aortic aneurysmal model for 257 cardiac cycles with periodic, pulsatile boundary conditions. The phase-averaged solutions were computed by averaging the solutions over various numbers of cardiac cycles and compared against the ones averaged over 124 cycles. The phase-averaged solutions reported small differences when they were averaged over a large number of cardiac cycles. The instantaneous solutions, however, failed to exhibit fluctuations reported in the phase-averaged solutions. To study transitional blood flows in the aneurysmal region, we need to report phase-averaged solutions as they exhibit nonperiodic, disturbed flow characteristics. Additionally, when reporting phase-averaged solutions, it is preferred to compute an average over a large number of cardiac cycles to be able to represent flow structures of the converged phase-averaged solutions. [ABSTRACT FROM AUTHOR]

Rheumatic heart disease (RHD) is a neglected tropical disease despite the substantial global health burden. In this study, we aimed to develop a lower cost method of modeling aortic blood flow using subject-specific velocity profiles, aiding our understanding of RHD's consequences on the structure and function of the ascending aorta. Echocardiography and cardiovascular magnetic resonance (CMR) are often used for diagnosis, including valve dysfunction assessments. However, there is a need to further characterize aortic valve lesions to improve treatment options and timing for patients, while using accessible and affordable imaging strategies. Here, we simulated effects of RHD aortic valve lesions on the aorta using computational fluid dynamics (CFD). We hypothesized that inlet velocity distribution and wall shear stress (WSS) will differ between RHD and non-RHD individuals, as well as between subject-specific and standard Womersley velocity profiles. Phase-contrast CMR data from South Africa of six RHD subjects with aortic stenosis and/or regurgitation and six matched controls were used to estimate subject-specific velocity inlet profiles and the mean velocity for Womersley profiles. Our findings were twofold. First, we found WSS in subject-specific RHD was significantly higher (p < 0.05) than control subject simulations, while Womersley simulation groups did not differ. Second, evaluating spatial velocity differences (⁠ΔSV ⁠) between simulation types revealed that simulations of RHD had significantly higher ΔSV than non-RHD (p < 0.05), these results highlight the need for implementing subject-specific input into RHD CFD, which we demonstrate how to accomplish through accessible methods. [ABSTRACT FROM AUTHOR]

Hydraulic turbines are operated more frequently at no-load conditions, also known as speed-no-load (SNL), to provide a spinning reserve that can rapidly connect to the electrical grid. As intermittent energy sources gain popularity, turbines will be required to provide spinning reserves more frequently. Previous studies show vortical flow structures in the vaneless space and the draft tube and rotating stall between the runner blades of certain axial turbines operating at SNL conditions. These flow phenomena are associated with pressure pulsations and torque fluctuations which put high stress on the turbine. The origin of the instabilities is not fully understood and not extensively studied. Moreover, mitigation techniques for SNL must be designed and explored to ensure the safe operation of the turbines at off-design conditions. This study presents a mitigation technique with independent control of each guide vane. The idea is to open some of the guide vanes to the best efficiency point (BEP) angle while keeping the remaining ones closed, aiming to reduce the swirl and thus avoid the instability to develop. The restriction is to have zero net torque on the shaft. Results show that the flow structures in the vaneless space can be broken down, which decreases pressure and velocity fluctuations. Furthermore, the rotating stall between the runner blades is reduced. The time-averaged flow upstream of the runner is changed while the flow below the runner remains mainly unchanged. [ABSTRACT FROM AUTHOR]

With the introduction of an ever-larger share of renewable but intermittent energy sources on electrical grids, hydraulic turbines are more often used as network stabilizers. In such a role, they are generally operated in off-design operations like speed-no-load (SNL). No energy is extracted from the flow at SNL operation, but the runner rotates at the synchronous speed linked to the electrical grid. The flow inside the runner of low-head turbines operating at SNL is often dominated by a columnar vortex array that may induce damaging pressure fluctuations. This paper presents the study of a control device to mitigate those vortices. At SNL, the small guide vane opening leads to a high swirl in the runner generating secondary flows such as columnar vortices and backflows. The proposed concept is to move SNL operation toward a higher guide vane opening and hence lower swirl, preventing the formation of a columnar vortex array. Lowering the input swirl of SNL is accomplished by opening up the guide vanes while using a control device to limit the discharge. The control device, like a spoiler on an aircraft wing, is introduced on the guide vanes to generate added head losses, significantly decreasing the discharge in high guide vane angles. This paper compares the hydrodynamics of the flow in a propeller turbine with different spoiler geometries. The study is based on both Reynolds-averaged Navier-Stokes (RANS) and unsteady RANS (URANS) flow simulations. It highlights how such devices can successfully mitigate columnar vortices and their associated pressure fluctuations on runner blades. [ABSTRACT FROM AUTHOR]

Previous computer models have successfully predicted cardiac growth and remodeling in adults with pathologies. However, applying these models to infants is complicated by the fact that they also undergo normal, somatic cardiac growth and remodeling. Therefore, we designed a computational model to predict ventricular dimensions and hemodynamics in healthy, growing infants by modifying an adult canine left ventricular growth model. The heart chambers were modeled as time-varying elastances coupled to a circuit model of the circulation. Circulation parameters were allometrically scaled and adjusted for maturation to simulate birth through 3 yrs of age. Ventricular growth was driven by perturbations in myocyte strain. The model successfully matched clinical measurements of pressures, ventricular and atrial volumes, and ventricular thicknesses within two standard deviations of multiple infant studies. To test the model, we input 10th and 90th percentile infant weights. Predicted volumes and thicknesses decreased and increased within normal ranges and pressures were unchanged. When we simulated coarctation of the aorta, systemic blood pressure, left ventricular thickness, and left ventricular volume all increased, following trends in clinical data. Our model enables a greater understanding of somatic and pathological growth in infants with congenital heart defects. Its flexibility and computational efficiency when compared to models employing more complex geometries allow for rapid analysis of pathological mechanisms affecting cardiac growth and hemodynamics. [ABSTRACT FROM AUTHOR]

The demand for quality dried products necessitates cost effective and innovative drying techniques that will improve its market value. The slow drying rate, weather dependency, and moisture reabsorption have been identified as the major challenges of solar drying operation. To address these shortcomings, hybrid solar drying systems have been recommended for the drying of various agricultural materials and other porous products. Designing a better drying system to accommodate thermal storage materials requires detailed analysis, which could be achieved through numerical simulation. Therefore, the numerical simulation of heat and mass transfer in a forced convection solar drying system integrated with black-coated firebrick sensible thermal storage materials (STSM) for the cocoa beans, locust beans, cereal grains, etc., was investigated under no-load conditions. The equations governing the fluid flow for a three-dimensional solar drying system were solved using the finite volume method with the aid of ansys, the computational fluid dynamics software to comprehend the dynamic and thermal behavior of the airflow within the dryer. The experimental maximum temperature values of 96.9 °C and 77.3 °C for the collector and drying chamber were in agreement with the simulated maximum collector and drying chamber temperatures of 116.9 °C and 80 °C respectively. The designed solar drying system with the incorporated STSM showed the capacity of raising the temperature of the air within the drying chamber to 3-37 °C above ambient temperature between 01:00 p.m. and 10:00 p.m. The agreement of the simulated dryer model with the experimental one is an indication that the developed dryer is suitable for drying cocoa, locust beans, fish, cereal grains, and some other agricultural products within an acceptable period based on the previous studies and therefore, the drying system is recommended to avoid the shortcomings associated with traditional/open sun drying. [ABSTRACT FROM AUTHOR]

Heat collection performance simulations using the system advisor model (SAM) with typical meteorological year weather data from four geographic locations are used to investigate (a) the optimum overall tilt of piecewise-focusing (PWF) collectors, and (b) PWF collector performance in comparison to the SAM default central receiver system. Results show that the overall tilt angle is not critical, but values up to 50 deg are best at non-tropical latitudes, even when output in summer is more valuable than in winter. For tropical latitudes, 40 deg of tilt is sufficient. Increasing PWF collector width relative to height is advantageous. Depending on location, PWF collector performance is 66-90% better than for the SAM default 100 MWe central receiver system, per square meter of reflector or heliostat. Using SAM's detailed control over system parameters, it is shown that the PWF collector's superior performance is derived mainly from better geometry (smaller cosine losses), but the near-absence of atmospheric attenuation and the smaller receiver heat losses are also significant. [ABSTRACT FROM AUTHOR]

This study investigates the thermal performance of an evacuated U-tube solar collector (ETSC) using different tube geometrical configurations. The effect of tube geometry on the overall collector efficiency is numerically analyzed and compared with experimental results. Three different ETSC configurations made of copper viz., Model 1 (M1) having one inlet and two outlets, Model 2 (M2) having one inlet and three outlets, and Model 3 (M3) having one inlet and four outlets are considered. An overall rise in temperature of heat transfer fluid at the outlets for each model is predicted and compared with a conventional U-tube (CT) for different mass flowrates and solar insolations to evaluate the collector performance. In comparison with the CT, the outlet temperature of the M3 and M1 is higher by 46.2% and 40.3%, respectively. M2 gives a nearly similar fluid outlet temperature as M1. A maximum of 35.4% enhancement in heat gain compared to the CT is observed for M3 (which is best among modified configurations) under similar operating conditions. However, at 788 W/m² solar insolation and a constant mass flowrate of 0.0167 kg/s, the estimated thermal efficiency of M1 is higher among the three models which is 33.5% higher than the CT. The experimental results closely approximate the numerical predictions with a deviation of ± 1.1 °C. From the economic evaluation of the modified collectors, a minimum payback period of 2.5 years was observed for Model 1 which is the shortest among the investigated ETSC systems. [ABSTRACT FROM AUTHOR]

The Compound Parabolic Concentrator (CPC), when coupled with the photovoltaic system, namely the Concentrated Photovoltaic Thermal System (CPVT), makes utilizing solar energy efficient. The major challenge that hinders the electrical and thermal performance of the CPC-CPVT system is the non-uniform heat flux distribution on the absorber surface. In the present paper, detailed ray-tracing simulations have been carried out to understand the heat flux distribution characteristics of CPC with different geometrical conditions, and those are concentration ratio, truncation ratio, incident angle, and average heat flux on the absorber surface. To have a thorough understanding, the analysis has been carried out in multiple steps. First, it is performed by analyzing the effect of concentration ratio and incident angle on heat flux distribution characteristics at a fixed truncation ratio. Second, investigations have been carried out to understand the heat flux distribution characteristics at different truncation ratios and different incident angles by keeping the concentration ratio constant. Local concentration ratio and non-uniformity index have been employed to quantify the non-uniformity of heat flux distribution on the absorber surface. It has been observed that the 0-deg incidence angle is the most effective angle to achieve uniform heat flux distribution on the absorber surface. This paper sheds insight into the heat flux distribution characteristics on the absorber surface of a CPC-CPVT system which can be used by the research community for designing an effective CPVT system from the perspective of uniform heat flux distribution on the absorber surface. [ABSTRACT FROM AUTHOR]

Recently, many studies have reported that a curved solar air heater (CSAH) performs better than a conventional flat SAH without using any extra material. It only requires geometrical modification. The present investigation is a two-dimensional numerical study of flow, heat transfer, and entropy generation characteristics of a CSAH having a sinusoidal profile absorber plate. Reynolds number (Re) and relative roughness pitch (λ/a) have been varied from 3800 to 18,000 and 7.143 to 17.857, respectively, while keeping the value of relative roughness height (a/Dh) at 0.042. The finite volume method (FVM) and SST k-ω model have been used to solve the governing equations. The average Nusselt number and average friction factor have been calculated to find the thermo-hydraulic performance parameter (THPP), which further helped determine the optimal arrangement of the number of sinusoidal waves in the absorber plate of the SAH. The maximum value of THPP developed with the proposed setup was found to be 5.9778. Turbulent flow features have been represented in the form of contours. Correlations have also been developed for Nuavg_r and favg_r as a function of Re and λ/a. Entropy generation per unit length due to heat transfer and fluid friction has been graphically represented. [ABSTRACT FROM AUTHOR]

A simulative method for quantifying the discharge process of cold gas airbag inflators is presented. The pressure, mass flow and the influences of the flow field are relevant to a robust and predictive airbag deployment. Simulations in this regard are compared and validated with experimental data. It turns out that simulated mean pressures inside the inflator deviate by 5-10% from measured data. A complex and highly turbulent flow field with supersonic and subsonic flow emerges. An influential longitudinal vortex forms in the cold gas inflator, leading to a highly dynamic discharge process. This vortex would not be found with the current state-of-the-art methods, such as the simple tank test or analytical models. It is shown that a simple turbulence model such as the k-ω shear stress transport predicts the flow field with sufficient accuracy in comparison with the large eddy simulation. Real gas effects must be taken into account inside the high-pressure reservoir, leading to a faster discharge compared to the ideal gas, due to faster moving expansion waves in the reservoir. Real gas effects outside the high-pressure reservoir seem to be negligible. A simplified simulation model was developed that uses only part of the whole cold gas inflator model and serves as a good practical approach for airbag deployment simulations, with less computational effort. Thus, the method presented here can provide high-quality inflow data for airbag deployment simulations. [ABSTRACT FROM AUTHOR]

This work studies a particle injection rig to understand how its design affects particle impingement and rebound from a target plate. The motivation behind this study is to understand how dust ingestion affects aviation gas-turbine engines. The particles are injected into a constant area duct upstream of the plate, and they exit through a converging nozzle. The major result concerns how particles respond differently to changes in the flow field based on their diameter. Near the plate, small particles follow the flow streamlines which causes them to both significantly slow down and to disperse in all directions. However, large particles move ballistically, so they impact the plate with nearly the same velocity and orientation they had at the duct exit. Reynolds-averaged Navier-Stokes (RANS) simulations are compared to large eddy simulation (LES). While RANS are capable of predicting mean particle impact statistics, they display narrower statistical variation than LES, suggesting that particle dispersion is underpredicted in RANS. [ABSTRACT FROM AUTHOR]

Latent thermal energy storage (TES) devices could enable advances in many thermal management applications, including peak load shifting for reducing energy demand and cost of HVAC or providing supplemental heat rejection in transient thermal management systems. However, real-time feedback control of such devices is currently limited by the absence of suitable state of charge estimation techniques, given the nonlinearities associated with phase change dynamics. In this paper, we design and experimentally validate a state-dependent Riccati equation (SDRE) filter for state of charge estimation in a phase change material (PCM)-based TES device integrated into a single-phase thermal-fluid loop. The advantage of the SDRE filter is that it does not require linearization of the nonlinear finite volume model; instead, it uses a linear parameter-varying system model which can be quickly derived using graph-based methods. We leverage graph-based methods to prove that the system model is uniformly detectable, guaranteeing that the state estimates are bounded. Using measurements from five thermocouples embedded in the PCM of the TES and two thermocouples measuring the fluid temperature at the inlet and outlet of the device, the state estimator uses a reduced-order finite volume model to determine the temperature distribution inside the PCM and in turn, the state of charge of the device. We demonstrate the state estimator in simulation and on experimental data collected from a thermal management system testbed to show that the state estimation error converges near zero and remains bounded. [ABSTRACT FROM AUTHOR]

Buoyant wakes encountered in the ocean environment are characterized by high Reynolds (Re) and Froude (Fr) numbers, leading to significant space-time resolution requirements for turbulence resolving CFD models (i.e., direct numerical simulations (DNS), large eddy simulations (LES)). Therefore, Reynolds-averaged Navier-Stokes (RANS) based models are attractive for these configurations. The inherently complex dynamics of stratified systems render eddy-viscosity-based modeling inappropriate. RANS second-moment closure (SMC) based modeling is more suitable because it accounts for flow anisotropy by solving the transport equations of important second-moment terms. Accordingly, eleven transport equations are solved at the SMC level, and a range of submodels are implemented for diffusion, pressure strain and scrambling, and dissipation terms. This work studies nonstratified and stratified towed wakes using SMC and DNS. Submodels in the SMC are evaluated in terms of how well their exact Reynolds averaged form impacts the accuracy of the full RANS closure. An ensemble average of 40 and 80-100 DNS realizations are required and conducted for these temporally evolving nonstratified and stratified wakes, respectively, to obtain converged higher-order statistics. SMC over-predicts wake height by over a factor of 2, and under-predicts defect velocity, wake width, and turbulent kinetic and potential energies by factors ranging from 1.3 to 3.5. Also, SMC predicts a near isotropic decay of normal Reynolds stresses (a33→-0.25) ⁠, in contrast to the anisotropic decay (a33→-0.64) returned by DNS. The DNS data also provide important insights related to the inaccuracy of the dissipation rate isotropy assumption and the non-negligible contribution of pressure diffusion terms. These results lead to several important recommendations for SMC modeling improvement. [ABSTRACT FROM AUTHOR]

Origami robot is a hotspot for research in the field of soft robot. However, there are still some major limitations to their application. This study proposed an origami robot based on spherical-linkage parallel mechanism (SLPM) for realizing some functions that cannot be accomplished by conventional robots. This study designed the manufacturing and assembling processes for the SLPM section according to the needs of practical applications, to explore the influence of flexible hinge on the resistance of SLPM section to lateral and torsional forces, the finite element simulation of SLPM section was performed, and the physical model of SLPM section was made to conduct a series of experiment. Also, an origami robot based on SLPM was also made, and the motion form of the robot was explored by adams. At last, through establishing a mathematical model, the relationship for conversion between the two control modes of the robot was deduced. Based on this, an experiment on the bending angle of the robot was carried out, and the simulation results were compared. This paper will promote the research of origami robot in structure design, motion control, etc. [ABSTRACT FROM AUTHOR]

This study presents a conceptual design for a concentrated solar power plant by using direct steam generation and a stand-alone power system based on a concentration of solar parabolic troughs. The system is located at the solar research site (SRS) of Universiti Teknologi PETRONAS in Ipoh, Malaysia. The model system uses an integrated turbine with 1.2 kW generators, and steam is generated by a flow loop powered by solar parabolic trough concentrators. An in-situ calculation of normal direct irradiance was conducted in SRS and used as an input for the mathematical model. The model was developed to assess the transient behavior of the system and the behavior of the projected power generation under seasonal variations and daily solar radiation. The parabolic trough power plant achieved the highest average output of 8.85% during the low rainfall season in March. [ABSTRACT FROM AUTHOR]

This paper reports on a parametric investigation of the thermal enhancement of a double-reflector parabolic trough collector when employing an in-line mixed V-shape (IMVS) ribbed absorber tube. Three heat transfer fluids (HTFs) are investigated, and a wide range of fluid inlet temperatures are studied. Various geometric parameters of the V-shape rib are analyzed to determine the optimum design of such a modification to the wall of the absorber tube. Results show that the HTF thermal oil Syltherm 800 is superior to the other HTFs that were studied. Results also show that a lower inlet temperature of the HTF leads to better thermo-hydraulic performance. The study provides a set of values for designing a V-shape ribbed absorber tube that produces optimum thermo-hydraulic performance. The optimum ribbed tube design shows a performance enhancement of about 64% compared to a smooth tube. [ABSTRACT FROM AUTHOR]

In this paper, we propose a globally stable adaptive controller for the human shank motion tracking problem that appears in neuromuscular electrical stimulation systems. The control problem is complicated by the fact that the mathematical model of the human shank dynamics is nonlinear and the parameters enter in a nonlinear and nonseparable form. To solve the problem, we first derive a nonlinearly parameterized regressor equation (NLPRE) that is used with a new parameter estimator specifically tailored for this NLPRE. This estimator is then combined with a classical feedback linearizing controller to ensure the tracking objective is globally achieved. A further contribution of the paper is the proof that parameter convergence, and consequent global tracking, is guaranteed with an extremely weak interval excitation requirement. A simulation study comparing the proposed adaptive controller with existing ones in the literature shows comparable human shank tracking performance but with fewer parameter estimates and without requiring knowledge of bounds for the unknown parameters. [ABSTRACT FROM AUTHOR]

Microbubble enhanced high intensity focused ultrasound (HIFU) is of great interest to tissue ablation for solid tumor treatments such as in liver and brain cancers, in which contrast agents/microbubbles are injected into the targeted region to promote heating and reduce prefocal tissue damage. A compressible Euler-Lagrange coupled model has been developed to accurately characterize the acoustic and thermal fields during this process. This employs a compressible Navier-Stokes solver for the ultrasound acoustic field and a discrete singularities model for bubble dynamics. To address the demanding computational cost relevant to practical medical applications, a multilevel hybrid message-passing interface (MPI)-open multiprocessing (OpenMP) parallelization scheme is developed to take advantage of both scalability of MPI and load balancing of OpenMP. At the first level, the Eulerian computational domain is divided into multiple subdomains and the bubbles are subdivided into groups based on which subdomain they fall into. At the next level, in each subdomain containing bubbles, multiple OpenMP threads are activated to speed up the computations of the bubble dynamics. For improved throughput, the OpenMP threads are more heavily distributed to subdomains where the bubbles are clustered. By doing this, MPI load imbalance issue due to uneven bubble distribution is mitigated by OpenMP speedup locally for those subdomains hosting more bubbles than others. The hybrid MPI-OpenMP Euler-Lagrange solver is used to conduct simulations and physical studies of bubble-enhanced HIFU problems containing a large number of microbubbles. The phenomenon of acoustic shadowing caused by the bubble cloud is then analyzed and discussed. Efficiency tests on two different machines with 48 processors are conducted and indicate 2-3 times speedup with the same hardware by introducing an OpenMP parallelization in combination with the MPI parallelization. [ABSTRACT FROM AUTHOR]

Parcel-based simulations are the most common method to simulate fuel sprays, especially for reacting conditions, due to their lower computational cost compared to more highly resolved simulations. It is important, however, to understand how spray boundary conditions are used to initialize the parcels that represent the atomizing fluid affect downstream conditions and overall simulation performance. Traditionally, when parcel simulations are used, the injector tip geometry is either significantly simplified or removed altogether due to resolution limits. Recent advances in computational power and numerical methods, however, have made it possible to resolve flow through these features. Previous work has shown some potential effects of even simple injector tip geometries, and this study investigates the effect of very detailed nozzle geometries on parcel-based simulations that have typically ignored these details. Four different parameters were investigated: whether a simulation includes a detailed injector tip geometry or a flat surface; whether parcels are initialized at the counterbore exit, which is more common, or at the nozzle exit; the use of an experimentally derived rate of injection or one-way coupling with a separate internal nozzle volume of fluid (VOF) simulation; the use of nominal or measured injector geometry. Simulations were compared using both global penetrations as well as local data near the injector. Spray penetration and other global measures showed limited sensitivity to boundary conditions/initialization procedure, but local data such as the local liquid volume fraction showed greater variation between the conditions, which may have an impact on mixing and combustion predictions in engine applications. [ABSTRACT FROM AUTHOR]

The lean direct injection (LDI) concept can eventually replace the current combustion systems in aviation engines because of its low NOx emissions without compromising other parameters. A novel multiswirl LDI burner with distributed fuel injection surrounded by air-flow through multiple hexagonal swirlers of vane angle 45 deg was developed. In this study, large eddy simulation (LES), using the dynamic Smagorinsky-Lilly subgrid model together with the flamelet generated manifold (FGM) combustion model, has been used to analyze the correlation between aerodynamics, mixing, and combustion characteristics of the burner. The agreement between the experimental data and the LES simulation was good, and it can accurately predict the trend of variables like velocity, temperature, and mixture fraction at different Reynolds number and equivalence ratios. The LES-FGM results demonstrated the rapid formation of a homogeneous fuel-air mixture over a short distance, which results in excellent flame stability, and it has ultralow NOx emission due to low flame temperature and negligible internal recirculation. It was discovered that going from a global equivalence ratio of 0.7 to 0.9 increases the flame liftoff distance because the high fuel volume flowrate causes the mixing of swirling air and fuel jet to become more and more delayed. The research in this paper provides an insight into the ability of the LES-FGM method to capture the complex flow field structures and the flame behavior globally of a novel low NOx multiswirl burner that might be a competitor in the combustors of commercial gas turbine engines. [ABSTRACT FROM AUTHOR]

Predicting the spray evolution using simulations requires accurate modeling of the turbulent gas-phase flow field. In this study, the high-order spectral-element method (SEM), implemented in the code Nek5000, was used to provide highly resolved solutions to the turbulent flow field. Spray modeling capabilities were implemented into the Nek5000 code. The spray is modeled in a Lagrangian-Eulerian (LE) framework, where the liquid is represented by discrete parcels of droplets. The method for coupling liquid and gas in the context of SEM is described, which allows for very fine meshes to be used without affecting the stability of the solution. Large-eddy simulations (LES) of the eight-hole ECN Spray G gasoline injector were conducted. Numerical results are compared against experimental data for liquid penetration, droplet size and gas velocity. The morphology of the multiplume spray is compared against experimental data. The effect of different spray injection inputs is analyzed. It was found that using a plume direction of 33 deg and an injection cone angle of 30 deg produced the best results overall. This work shows the applicability of SEM for spray modeling applications, where use of a high-order flow solver can help us understand the multiplume spray aerodynamics and how it leads to plume collapse under certain conditions. Results also highlight the need for tuning spray input parameters in the LE framework, even when high-fidelity gas flow solutions are possible. [ABSTRACT FROM AUTHOR]

The current work reports a numerical investigation of the water produced and thermal performance of a solar still (SS). Using a SS for desalination is a proposal for low-income remote communities needing potable water. The study deals with the SS under five different concentrations of salt (0, 5, 10, 20, and 35 g/kg). Previous experimental results reported in the literature indicate that the increase in salinity leads to a decrease in productivity, so phase change material (PCM) was added under the water basin to counter the reduction. The mathematical model and numerical methodology were validated by comparing them with experimental results reported in the literature. The relative difference between temperatures was less than 2%, and for water production, it was less than 3.5%. The present mathematical model has the novelty of utilizing the water properties as a function of temperature and salt concentration, contrary to other models that use pure water properties. The results show that daily productivity decreases when the salinity increases from 0 to 35 g/kg. For each case, the time evolution of hourly and cumulate productivity is presented, as well as water temperature and the temperature difference between water and glass. Also, the behavior of heat flux between water and PCM is analyzed. The overall efficiency is calculated for all the cases. [ABSTRACT FROM AUTHOR]

Laminar sub-layer formation in a smooth solar air heater (SAH) is one of the reasons for the low heat transfer coefficient. One of the most effective ways to overcome the problem and improve the heat transfer rate inside the SAH is to use artificial roughness in the form of ribs. The present investigation studies the consequence of inverted T-shaped ribs on the absorber plate of a CSAH. The absorber plate is exposed to a constant heat flux of 1000 W/m2 and is made up of aluminum. The investigation is done on the effect of Reynolds number (Re), relative roughness pitch (P/e), and relative roughness height (e/Dh) on entropy generation, fluid flow, and heat transfer characteristics of the system. A 2D fluid domain has been considered for the numerical analysis, and finite volume method is used to solve the equations of continuity, momentum, and energy. The governing equations are solved using the SST k-ɛ model. Thermo-hydraulic performance parameter (THPP) is also calculated using Nuavg_r and favg_r which further helped to determine the optimal arrangement of inverted T-shaped ribs on the absorber plate of the SAH. The maximum THPP of 4.7744 is found for P/e = 7.143 at Re = 18,000. Correlation for Nuavg_r and favg_r as a function of Re and P/e is developed. Entropy generation per unit length due to fluid friction and heat transfer has been graphically represented. [ABSTRACT FROM AUTHOR]