Quantitative laser-based diagnostics like Raman spectroscopy are essential for studying high-temperature processes, but their application in intensely luminous and transient environments such as plasma torches is severely limited by overwhelming background emission. This study focuses on the quantitative thermometry of a 7 kW atmospheric air plasma jet, an environment where such measurements are notoriously difficult. To enable these measurements, a Polarization Lock-In Filtering (PLF) Raman technique is used to suppress the intense and fluctuating plasma background. The method successfully yields high-quality N2 ro-vibrational spectra along the jet’s central axis. Model-based fitting of these spectra produces a detailed axial temperature profile, showing a decay from over 3700 K near the nozzle. Furthermore, the high signal quality enabled the detection of singly ionized nitrogen (N2+) in the plasma core, providing direct evidence of its ionized state. These results represent the first application of PLF for thermometry in a plasma torch and provide critical experimental data for validating magnetohydrodynamic simulations. 

Low Reynolds number flows are central to the performance of airfoils used in small unmanned aerial vehicles (UAVs), micro air vehicles (MAVs), and aerodynamic platforms operating in rarefied atmospheres. Consequently, a deep understanding of airfoil behavior and accurate prediction of aerodynamic performance are essential for the optimal design of such systems. The present study employs Computational Fluid Dynamics (CFD) simulations to analyze the aerodynamic performance of a cambered plate at a Reynolds number of 10,000. Two Reynolds-Averaged Navier–Stokes (RANS) turbulence models, 𝛾–𝑅𝑒𝜃 and 𝑘-𝑘𝐿-𝜔, are utilized, along with the Unsteady Navier–Stokes (UNS) equations. The simulation results are compared against experimental data, with a focus on lift, drag, and pressure coefficients. The models studied perform moderately well at small angles of attack. The 𝛾–𝑅𝑒𝜃 model yields the lowest lift and drag errors (below 0.17 and 0.04, respectively), while the other models show significantly higher discrepancies, particularly in lift prediction. The 𝛾–𝑅𝑒𝜃 model demonstrates good overall accuracy, with notable deviation only in the prediction of the stall angle. In contrast, the 𝑘-𝑘𝐿-𝜔 model and the UNS equations capture the general flow trend up to stall but fail to provide reliable predictions beyond that point. These findings indicate that the 𝛾–𝑅𝑒𝜃 model is the most suitable among those tested for low Reynolds number transitional flow simulations.

In a non-transferred plasma torch, the working gas becomes ionized and forms plasma as it interacts with the electric arc at the cathode tip. However, in certain cathode shapes, particularly flat ones, and under specific conditions, the gas flow can separate at the cathode tip, forming a vortex region. While this flow separation is influenced by geometric factors, it occurs in the critical zone where plasma is generated. Understanding the causes of this separation is essential, as it may significantly impact torch performance. If the separation proves detrimental, it is important to identify ways to mitigate it. This paper presents a computational analysis of a non-transferred plasma torch to investigate the physics behind flow separation. The results highlight the location and causes of the separation, as well as its potential advantages and disadvantages. Finally, the paper explores theoretical approaches to address flow separation in plasma torches, offering practical insights for enhancing their design and efficiency.

The plasma jet produced by a non-transferred plasma torch may initially appear steady and laminar, but it undergoes significant turbulence as it interacts with the surrounding atmosphere. Within the plasma torch, the jet begins as laminar; however, upon exiting, it transitions into a turbulent flow, extending into a long, wavy structure as it develops. This paper explores the complexities of computational modeling for non-transferred plasma torches, focusing on the challenges of simulating the multiphysics and multiphase interactions at the outlet and tracing the evolution of the plasma jet. The computational analysis uses COMSOL Multiphysics software on a 2D axisymmetric geometry, with steady-state simulations incorporating various turbulence models. A comparative assessment of the results from each turbulence model is provided, highlighting their respective strengths and limitations. Although the diffusion of the turbulent jet at the outlet is presented, the turbulence models employed in this study only offer time-averaged values, rather than a detailed breakdown of the complete jet structure. The paper concludes by validating the computationally obtained velocity magnitudes against experimental data, ensuring the accuracy and reliability of the simulation results.

Labyrinth seals are commonly used in space mechanisms to reduce evaporative losses of lubricant molecules and limit the transport of contaminants. Analytical models and numerical simulations for predicting mass flow through these seals typically assume smooth, idealized surfaces, neglecting the effects of realistic surface roughness. This study systematically investigates the impact of surface roughness on the transmission probability (TP) of oil molecules using Monte Carlo simulations under free molecular flow conditions. Key geometric and surface parameters including average roughness (Ra), corridor length, and seal width are varied to evaluate their influence on molecular transport. The results demonstrate that surface roughness significantly reduces TP and molecular flux, especially in narrow and elongated geometries. Furthermore, increasing surface roughness by an order of magnitude enables a reduction in channel length or an increase in gap width by approximately 35–40 % while maintaining equivalent transmission probability. Based on these findings, a correction model is proposed to improve prediction accuracy and is validated against experimentally measured oil evaporative losses. This work highlights the potential of controlled surface texturing as a design strategy to both enhance sealing effectiveness and enable geometric reductions for improved compactness and manufacturability.

This study evaluates labyrinth seals for space mechanisms to enhance lubricant retention and reduce contamination under vacuum conditions. It examines the influence of seal geometry including length, width, surface roughness, and stepped features through analytical models, numerical simulations, and experimental validation. Surface roughness and geometric complexity strongly affect molecular transmission, while electrostatic fields and rotational dynamics further improve sealing performance. Experimental evaporation measurements align closely with simulations and validate correction models that incorporate surface roughness effects. These results guide the design of labyrinth seals for space applications.

Labyrinth seals, extensively used in space applications, serve to prevent the loss of liquid lubricants and shield satellite subsystems from contamination. These seals are essential for the reliable functioning of bearings and for protecting satellite subsystems from contamination. This study compares analytical predictions of lubricant loss against experimental measurements and computer simulations to optimize labyrinth seal configurations. Analytical models tend to overestimate mass loss by 5–8 times compared to experimental data, indicating limited reliability for complex seal geometries. Simulations using MolFlow+ and COMSOL Multiphysics align closely with experimental results, providing accurate mass loss predictions. Key findings highlight that labyrinth length, width, and surface roughness are critical factors in minimizing evaporative mass loss. Notably, stepped labyrinth seals with relief grooves and optimized step positioning effectively reduce molecular beaming effects and improve sealing performance compared to straight geometries. Effective sealing not only reduces mission failures but also helps to minimize space debris, thereby promoting safer satellite missions.