11115 research outputs found
Sort by
Varying Fidelity Comparative Analysis of Space Shuttle (STS-1) Reentry Dynamics
No full textThis paper describes concepts, modeling decisions, and engineering approximations that can be used to develop reentry simulations at various degree(s)-of-freedom (DOF). Included in this paper is a methodology for simulating the inaugural flight of the Space Shuttle (STS-1) from 3DOF to 6DOF. This paper uniquely quantifies trajectory state error to consolidate information that quantifies simulation error due to various design choices. Utilizing this methodology, it is demonstrated that the maximum error incurred by a Newtonian panel-method is O(10−3). Additionally, in 6DOF, various tracking control solutions including first and second-order attitude controllers are implemented to compare various estimation methods for tracking a specified control solution. Error analysis for each DOF simulation show that 4DOF modeling accurately captures the desired translational state trajectory; however, 6DOF modeling is necessary to properly consider the control torque. This paper demonstrates how various challenges to 6DOF modeling can be overcome to produce accurate reconstructions of a complex reentry scenario
The Relative Effects of Internal Reynolds Number and Advective Capacity Ratio on the Coolant Warming Factor
No full textConjugate heat transfer experiments to predict turbine component temperatures involve matching the Biot number of the experimental condition to that of the engine condition. Done properly, such an experiment could yield an overall effectiveness distribution that is relevant to the engine condition. However, the underlying theory suggests that the coolant warming factor, χ, must also be matched to achieve the desired effect, and the requirements to do so have been neglected in the literature. Additionally, little success has been achieved in determining the theoretical requirements to match χ. In this work, we develop these requirements, apply them for when coolant flow is scaled by the Reynolds number ratio and advective capacity ratio, and test them by comparing computational results against experimental data. The findings from this study indicate a strong influence of the thermal conductivity of the coolant. Interestingly, a thermal conductivity inappropriately large will have opposite effects on the coolant warming factor depending on which coolant flow rate parameter is used to characterize the coolant flow. Knowledge of the subtle requirements to properly replicate the coolant warming factor in an experiment will allow turbine designers to achieve more accurate surface temperature predictions through properly designed experiments
Computational Investigation of Flow in a Triangular Cavity
No full textComputational fluid dynamics simulations were conducted on a triangular prism cavity with L/D = 4.5 in M∞ = 2.9 flow to investigate the frequency content and flow features. Computational solutions were obtained using the Kestrel solver with improved delayed detached eddy simulation and Menter shear stress transport model for turbulence. This work found that the triangular prism cavity seems to dampen sound pressure level significantly when compared to similar rectangular cavity data. Additionally, the triangular prism cavity shares some flow features with rectangular cavities including a turbulent shear layer, which impacts the back wall causing a large recirculation vortex in the cavity, as well as streamwise vortex shedding from the sides of the cavity. But in contrast, the triangular cavity does not have apparent front and rear corner vortices, but a large, weak recirculation region in the front of the cavity, which rotates in the opposite direction of the primary recirculation vortex. Abstract © AIA
Optimal quality design of smart technologies for port digitalization: A game theoretical approach under digitalization synergy
No full textExcerpt: Smart ports improve operational efficiency through innovative technologies and data-driven solutions. A key strategic decision in the digitalization process is the quality level of smart technologies adopted. This paper examines optimal quality design under the landlord port model framework, where a port authority and a terminal operator jointly influence digital transformation. Abstract © Elsevier
Impact of amorphous pockets on displacement damage evolution in silicon
No full textExcerpt: Silicon has long been known to exhibit amorphization in response to heavy particle bombardment. For doses below the total amorphization threshold, partial amorphization is observed in the form of scattered amorphous pockets. While extensive research has gone into modeling the formation and evolution of amorphous pockets in response to irradiation, no studies yet investigate their impact on the evolution of other damage such as interstitial supersaturation and clustering. In this study, we survey the impact of amorphous pockets on defect evolution in silicon when treated as static sinks. Abstract © Elsevier.
A graphical abstract of this work is openly available at the DOI link
Machine learning approach to synthetic data generation: Uncertainty generative model with neural attention
No full textData scarcity undermines the precision of empirical and analytical research by limiting sample sizes and reducing statistical power. In domains such as business operations, financial management, and information systems, failure data often arise from rare events, introducing substantial aleatoric and epistemic uncertainty. Existing synthetic data generation methods, including interpolation‐based oversampling and generative models, face persistent challenges. They often fail to capture rare events, preserve temporal dependencies, or model multiple sources of uncertainty, leading to unrealistic samples and degraded performance in downstream tasks. This study introduces the uncertainty generative model with neural attention (UGMNA), a synthetic data generation approach that integrates attentive neural processes, the Heston stochastic volatility model, and stochastic differential equations within a continuous‐time latent framework. UGMNA addresses data scarcity by generating synthetic samples that emulate the distributional characteristics of original datasets while explicitly modeling both aleatoric and epistemic uncertainty. Its design enhances statistical power by augmenting limited datasets and ensures that synthetic data reflect key patterns, temporal dynamics, and complex distributions encountered in real‐world scenarios. Experimental results across multiple case studies demonstrate that UGMNA reduces both types of uncertainty while preserving essential data patterns. Compared with conventional baselines and state‐of‐the‐art generators, UGMNA consistently improves predictive accuracy, ranking performance, and model calibration in data‐scarce, high‐variance environments. These findings establish UGMNA as a robust framework for generating reliable synthetic data, offering practical utility for research and decision‐making in contexts where data scarcity and uncertainty hinder model development
Target Defense Using a Turret and Mobile Defender Team
No full textA scenario is considered wherein a stationary, turn constrained agent (Turret) and a mobile agent (Defender) cooperate to protect the former from an adversarial mobile agent (Attacker). The Attacker wishes to reach the Turret prior to getting captured by either the Defender or Turret, if possible. Meanwhile, the Defender and Turret seek to capture the Attacker as far from the Turret as possible. This scenario is formulated as a differential game and solved using a geometric approach. Necessary and sufficient conditions for the Turret-Defender team winning and the Attacker winning are given. In the case of the Turret-Defender team winning equilibrium strategies for the min max terminal distance of the Attacker to the Turret are given. Three cases arise corresponding to solo capture by the Defender, solo capture by the Turret, and capture simultaneously by both Turret and Defender
The Effects of proton irradiation on the current–voltage and capacitance–voltage characteristics of GeSn/Si photodiodes
No full text
Global Ionospheric F Region Parameters From GNSS-POD Limb Measurements: Evaluations and Comparisons With Two Empirical Models - IRI-2020 and NeQuick-2
No full textAn optimal estimation (OE) technique has recently been developed for F region electron density (Ne) using Global Navigation Satellite System (GNSS) limb sounding on low Earth orbit (LEO) satellites (COSMIC-2, Spire, and FengYun-3). This method provides unprecedented spatiotemporal sampling for global monthly Ne climatology within 100–500 km in 2 hr intervals. The global dataset, collected during mid to moderately high solar activity, is compared with leading models: IRI-2020 and NeQuick-2. Diurnal variations in summer, winter, and equinoctial months are examined for the F2-layer peak, as well as the topside and bottomside of the F region. The observed and modeled NmF2 and hmF2 show good agreement during the daytime, but discrepancies appear with NeQuick-2 at night. The OE-retrieved dataset reveals distinct interhemispheric differences in topside scale height between the summer and winter hemispheres, which are not adequately captured by models. The estimated topside scale heights in IRI-2020 are ~20–30 km higher than observations on regional scale, but this difference decreases to ~12–20 km on global scale. In the bottomside, the agreement between observations and models varies significantly between daytime and nighttime conditions. During the daytime, the global bottomside thicknesses derived from OE-retrieved profiles agree within 10 km with the IRI-2020, but they are ~10–15 km higher than NeQuick-2. The nighttime thicknesses differ substantially, with deviations reaching up to ~30 km compared to IRI-2020 and ~45 km compared to NeQuick-2. As models face challenges due to lack of reliable measurements, especially in the topside and bottomside, improvements in GNSS-LEO observing techniques can provide more accurate and comprehensive data to characterize the global ionosphere