We propose an active flow control technique using fluidic injectors to manipulate plume structures in plume surface interaction (PSI) phenomena during soft landing on planetary surfaces. While fluidic injectors are mainly proposed for noise reduction in aircraft engines, this study focuses on their application under planetary environments to modulate plumes extended by high nozzle pressure ratios. To investigate the performance of this proposed jet-jet interaction technique, which transverse supersonic jets penetrate a main hypersonic jet, we conduct both numerical simulations and experiments using four Mach 2.5 injector nozzles arranged around a Mach 5.8 main nozzle, under Martian ambient surface pressure. The jet-jet interaction reduces a Mach number of an impinging main jet on a perpendicular plate and expands its jet radius. These effects depend on the mass flow rate ratio between the main jet and the injector. Due to the reduced shock cell length, the highest mass flow rate ratio significantly decreases the stagnation pressure at the jet impingement centreline on the plate by approximately 90% compared to the case without jet-jet interaction, whereas a pressure rise occurs in the radial direction due to jet expansion. These pressure changes may still alter regolith erosion. Furthermore, a wall shear stress associated with erosion dynamics is reduced by the jet-jet interaction. Although we successfully demonstrate the effectiveness of the jet-jet interaction using fluidic injectors for plume modulation under Martian ambient pressure, further investigations of the jet impingement on a soil bed are required to clarify the effects of this technique on erosion dynamics.

Within the ESA-funded activity, LUNAR In-Situ LANding Structures (LUNAR ISLANDS), the feasibility of in-situ manufactured structures is explored. These aim to decrease the exposure of surrounding infrastructure and personnel to high-velocity particles of lunar regolith, which result from the interaction of the exhaust plume of the lander and the surface, which is covered by loose regolith. For the numerical investigation of Plume Surface Interaction (PSI), a hybrid modelling strategy is employed, combining a Navier-Stokes (NS)-based Computational Fluid Dynamics (CFD) solver for the continuum regions of the nozzle and plume with a Direct Simulation Monte Carlo (DSMC) solver for the rarefied regions. This contribution describes the automated identification of the breakdown interface and necessary division of the mesh based on that interface to allow for the use of a single mesh.

The traditional methods for modelling the electronic excitation in direct simulation Monte Carlo (DSMC) simulations treat each internal mode individually and disregard any interaction between modes. To address this, a vibronic model was developed by Civrais et al. (Phys. Fluids 36:086112, 2024) allowing each electronic excited state to excite its unique vibrational quantum levels. This work expands on previous work by providing complementary physical insights into the modelling of the vibrational quantum levels of electronic states. The fundamental differences between the uncoupled and coupled approaches in terms of the internal energies and distribution functions are outlined. Then, the scalability of the vibronic model against the traditional approach is demonstrated for elementary operations. It is shown that the coupled approach demonstrates a comparable runtime as the uncoupled approach. Finally, the traditional uncoupled and coupled approaches are applied to the modelling of the electronic excited states on the forebody of the Space Shuttle at an altitude of 99.49 km, demonstrating the inaccuracy of the traditional assumption of the electronic excited states being distributed according to the Boltzmann statistics. It is also shown that the population of the electronic excited states and their vibrational quantum levels strongly deviate between the two approaches, suggesting a significant impact on the line intensity of molecular species’ absorption spectrum.

This paper presents uniGasFoam, an open-source particle-based solver for multiscale rarefied gas flow simulations, which has been developed within the well-established OpenFOAM framework, and is an extension of the direct simulation Monte Carlo (DSMC) solver dsmcFoam+. The developed solver addresses the coupling challenges inherent in hybrid continuum-particle methods, originating from the disparate nature of finite-volume (FV) solvers found in computational fluid dynamics (CFD) software and DSMC particle solvers. This is achieved by employing alternative stochastic particle methods, resembling DSMC, to tackle the continuum limit. The uniGasFoam particle-particle coupling produces a numerical implementation that is simpler and more robust, faster in many steady-state flows, and more scalable for transient flows compared to conventional continuum-particle coupling. The presented framework is unified and generic, and can couple DSMC with stochastic particle (SP) and unified stochastic particle (USP) methods, or be employed for pure DSMC, SP, and USP gas simulations. To enhance user experience, optimise computational resources and minimise user error, advanced adaptive algorithms such as transient adaptive sub-cells, non-uniform cell weighting, and adaptive global time stepping have been integrated into uniGasFoam. In this paper, the hybrid USP-DSMC module of uniGasFoam is rigorously validated through multiple benchmark cases, consistently showing excellent agreement with pure DSMC, hybrid CFD-DSMC, and literature results. Notably, uniGasFoam achieves significant computational gains compared to pure dsmcFoam+ simulations, rendering it a robust computational tool well-suited for addressing multiscale rarefied gas flows of engineering importance.

This study combines experimental and numerical methods to explore an innovative application of dielectric barrier discharge (DBD) plasma flow control in a low-speed S-duct. S-ducts are widely used in modern aerospace and turbine equipment; however, their complex geometries make them susceptible to flow separation and stall. Therefore, enhancing internal flow stability is always a critical research focus. As an emerging active flow control technique, DBD plasma flow control generates wall jets or vortices to improve flow characteristics, offering advantages such as no mechanical wear, low power consumption, and rapid response compared to traditional control methods. However, constrained by an incomplete understanding of the physical properties of plasma to date, its application has been widely limited to simple geometries. This study aims to expand the applicability of DBD plasma flow control to more complex structures. The results indicate that, at a flow velocity in the duct of approximately 20 m/s, the plasma actuator, acting as a wall jet, can effectively modify the velocity distribution within the duct. However, this effect is relatively limited when reflected in pressure variations. This research demonstrates the potential of DBD plasma actuators in complex flow environments and provides essential experimental and simulation references for future studies.

A new Micro-Macro-Surrogate (MMS) hybrid method is presented that couples the Direct Simulation Monte Carlo (DSMC) method with Computational Fluid Dynamics (CFD) to simulate low-speed rarefied gas flows. The proposed MMS method incorporates surrogate modelling instead of direct coupling of DSMC data with the CFD, addressing the limitations CFD has in accurately modelling rarefied gas flows, the computational cost of DSMC for low-speed and multiscale flows, as well as the pitfalls of noise in conventional direct coupling approaches. The surrogate models, trained on the DSMC data using Bayesian inference, provide noise-free and accurate corrections to the CFD simulation enabling it to capture the non-continuum physics. The MMS hybrid approach is validated by simulating low-speed, force-driven rarefied gas flows in a canonical parallel-plate system and shows excellent agreement with DSMC benchmark results. A comparison with the typical domain decomposition DSMC-CFD hybrid method is also presented, to demonstrate the advantages of noise-avoidance in the proposed approach. The method also inherently captures the uncertainty arising from micro-model fluctuations, allowing for the quantification of noise-related uncertainty in the predictions. The proposed MMS method demonstrates the potential to enable multiscale simulations where CFD is inaccurate and DSMC is prohibitively expensive.

This study investigates the formation and evolution of craters under near-vacuum conditions due to supersonic jet impingement, with a focus on regolith particle ejection dynamics. Experiments were conducted at the Plume-Regolith Facility of the University of Glasgow, where controlled jet pulses (0.1, 0.5 and 1.0 s) were directed to high- and low-density regolith simulants. Using Particle Image Velocimetry (PIV) and infrared imaging, the crater morphology and ejecta distribution were analysed over multiple sequential jet pulses. The results indicate that the high-density regolith forms compact, steep-walled U-shaped craters, while the lowdensity regolith produces larger, sloped V-shaped craters. Longer pulse durations result in increased crater expansion and higher ejection angles. In particular, steep-walled craters promote high-angle ejection near the nozzle axis, posing potential risks to spacecraft landers. Regardless of regolith density, expelled particles exhibit ejection angles between 40 degrees and 50 degrees, suggesting long-distance travel in low-gravity environments. These findings improve our understanding of plume-regolith interactions and contribute to safer planetary landing strategies by informing numerical models and spacecraft design considerations.

This study investigates the flow characteristics within an S-shaped duct at low speeds and examines the impact of varying lengths of upstream and downstream extensions on the internal flow field. The experimental setup includes an S-shaped duct model and an axial fan, with the fan positioned downstream to regulate flow velocity within the duct. Taking the S-duct throat as the reference point, the velocity range under investigation spans from 0.06 to 0.15 Mach. Measurements are taken in a region between the S-shaped duct and the fan using a traverse system to capture aerodynamic interface plane flow parameters. Additionally, wall static pressure measurements are utilized to characterize the flow field within the duct. Results indicate that although flow velocity has a relatively minor effect on the flow pattern inside the S-duct, it can cause a significant fluctuation of the wall static pressure coefficient. In contrast, the influences of the upstream and downstream extensions on the flow pattern inside the S-duct are more critical. Specifically, the upstream extension directly impacts the stall region at the duct's bottom and significantly increases the inlet boundary thickness, while the downstream extension determines the flow modes of the secondary flow on the exit. When both upstream and downstream extensions are sufficiently long, the influence of flow velocities on the flow pattern inside the S-duct can be almost eliminated.

This paper presents the development and validation of hybridDCFoam, a coupled DSMC/Navier–Stokes–Fourier solver, for simulating steady-state multiscale rarefied gas flows. The developed solver is based on the domain decomposition method, while the state-based information exchange approach is implemented between the particle and continuum solvers. The solver is developed within the OpenFOAM framework and couples the dsmcFoam+ and rhoCentralFoam solvers in order to handle the particle and continuum flow regions. hybridDCFoam can tackle arbitrary 3D geometries and supports parallel simulations using the MPI domain decomposition available in OpenFOAM. The developed solver is validated for flow over a flat plate, flow past a cylinder, pressure-driven flow through a slit, and expansion through a conical nozzle. The presented benchmarks yield an excellent agreement between hybrid and pure DSMC simulations. Moreover, hybridDCFoam is able to accurately recover both numerical and experimental results reported in the literature. Notably, hybridDCFoam achieves significant computational gains compared to pure dsmcFoam+ simulations, rendering it well-suited for addressing multiscale rarefied gas flows of engineering importance.

This article introduces a novel model for describing the electronic excited states in the direct simulation Monte Carlo (DSMC) technique. The model involves the coupling the vibrational and electronic modes of molecular species, enabling each electronic excited state to excite its unique vibrational quantum levels. Numerical techniques are developed for equilibrium and post-collision sampling, as well as for measuring the internal temperature. The DSMC results demonstrate excellent agreement with theoretical predictions, providing verification of the successful implementation in a DSMC solver. For important thermophysical properties of molecular oxygen, such as the specific heat capacity, it is shown that the new model provides a better prediction than a compilation of past studies in comparison to the standard uncoupled approach in DSMC. The model is then applied to simulate a canonical nonreactive oxygen hypersonic flow past a cylindrical body. The population distribution of electronic excited states exhibit significant deviation from the standard approach typically used in the coupling between DSMC and radiation transport solvers.

This work proposes an extended version of the quantum-kinetic chemistry models, aiming to accurately reproduce experimental measurements and high-fidelity calculations in both thermal equilibrium and non-equilibrium. The extension involves the development of new formulations, incorporating a set of tunable parameters obtained from a non-linear least squares fit on the dataset. The newly derived analytical expressions are implemented in a direct simulation Monte Carlo (DSMC) solver. These formulations are applied to the 19 most representative chemical reactions of an air mixture involving dissociation and exchange reactions. The DSMC reaction rates demonstrate excellent agreement with the newly derived analytical expressions, providing verification of the successful implementation in the DSMC solver. The study demonstrates excellent reproduction of the baseline dataset for both thermal equilibrium and non-equilibrium. Furthermore, the new formulations are applied to simulate the surface heat flux during the second space transport system (STS-II) mission at an altitude of 92.35 km.

An extension to the normal shock relations for a thermally perfect, calorically imperfect gas, modelling the vibrational excitation with an anharmonic oscillator model and including the influence of electronic modes, is derived and studied. Such additional considerations constitute an extension to the work achieved in the past, which modelled the caloric imperfections with a harmonic oscillator for vibrational energy and did not consider the effect of electronic energy. Additionally, the newly derived expressions provide physical insights into the limitations of experimentation for replicating flight conditions, which is demonstrated through providing solutions at different upstream temperatures. The results are compared with direct simulation Monte Carlo simulations for nitrogen and air, with the extent of the caloric imperfection of the gas showing excellent agreement. For low upstream temperatures, the extended relations are found to be in good agreement with the original normal shock wave expressions, but the results diverge for higher upstream temperatures that would be more representative of real flows. The results show that the new expressions depart from ideal gas theory for Mach numbers in excess of 4.9 at wind-tunnel conditions and for any Mach number above 3.0 at flight conditions. It is also shown that the traditional harmonic oscillator model and the anharmonic oscillator model begin to diverge at Mach number 3.0 for molecular oxygen gas and at Mach number 5.0 for an air mixture at flight conditions.

Vortex loops are compact toroidal structures wherein fluid rotation forms a closed loop around a fictitious axis, manifest in many natural occurrences. These phenomena result from brief impulses through vents or apertures in fluid systems, such as in caves, volcanic crusts, downbursts, or the descent of liquid droplets. The majority of naturally occurring and laboratory-generated vortex loops, studied for fundamental research on their formation, growth, instability, and dissolution, are classified as incompressible. This categorisation denotes negligible alterations in thermodynamic properties within the vortex loop. However, a distinct category of vortex loops emerges from processes involving artillery, shock tubes, explosions, chemical interactions, and combustion. This class primarily constitutes compressible vortex loops. Their presence in flow fields spans over a century, and they have been observed since the application of open-ended shock tubes to explore phenomena like diffracting shock waves, blast wave interactions with objects, and noise mitigation. The study and comprehension of compressible vortex loops and their interactions have historically relied heavily on optical techniques, lacking comprehensive insight into the intricate flow dynamics. Nevertheless, the advancements in flow visualisation tools and computational capabilities in the 21st century have significantly aided scientists in scrutinising and characterising these vortex loops and their interactions in intricate detail. Unfortunately, a comprehensive review of the literature addressing compressible vortex loops originating from shock tubes, their evolution, and interactions with shockwaves and various objects, including walls, appears lacking. This review article aims to address this gap.

This work presents a new formulation of the quantum-kinetic (QK) chemistry models, in which the vibrational excitation is modeled with an anharmonic oscillator model. The new formulations are applied to some of the most representative dissociation reactions occurring during an Earth re-entry. The newly derived analytical expressions are implemented in a direct simulation Monte Carlo (DSMC) solver. The DSMC reaction rates demonstrate excellent agreement with the newly derived analytical expressions, verifying the successful implementation in the DSMC solver. The new models suggest that dissociation reactions are more likely to occur than with the original QK models. Furthermore, the new formulations are compared against experimental measurements, high-fidelity calculations, and well-established chemistry models for both thermal equilibrium and non-equilibrium conditions, presenting reasonable agreement with the baseline database. Additionally, the limitations of the new formulations are assessed for thermal non-equilibrium conditions where an excessive utilization of the relative translational energy and insufficient utilization of the pre-collision vibrational energy to promote dissociation reactions is found.

An experiment to track and measure the transient phenomenon of plume-liberated regolith in near-vacuum conditions was performed in a dedicated plume-regolith facility housed at the University of Glasgow. This facility with a total volume of around 82 m3 can simulate a soft or hard landing event on “extraterrestrial” sub-atmospheric pressures. Particle image velocimetry method was used to estimate the ejection velocity and ejected angle of regolith particles, and its limitations are discussed. Glass microspheres that are matched with the size of the Lunar and Martian moon “Phobos” surface regoliths are used as simulants. With an exit Mach number of 6.6, a heated convergent–divergent nozzle represents the lander nozzle. Preliminary results capture ejecta development up to 30 ms from plume impingement. Flow visualization reveals the initial moments of plume boundary growth and regolith ejection. The vector images indicate a triangular-shaped sheet of particles sweeping from the regolith bed at a positive inclination with a local maximum velocity close to 100 m/s. The low-density “Phobos” simulant advances at a higher speed, reaches higher elevations, and covers a larger spatial area compared to a higher-density “Lunar” simulant. Observation of the crater formation reveals the difference in cohesive forces between the selected simulants. A higher inclination of particle ejection of more than 50° adjacent to the jet indicates particle entrainment originating from the interior of the crater. Stream traces reveal the deflection of ejected particles upon impingement on the lander surface at close proximity.

At hypersonic speeds, vehicles are subjected to extreme aerothermodynamic conditions, which require accurate predictions of the internal degrees of freedom of molecules. The harmonic oscillator (HO) model is commonly used to model the vibrational excitation of molecules in the framework of direct simulation Monte Carlo. Although it benefits from a simple computation, the HO model is known to have serious shortcomings as the temperature becomes significantly high. To address these limitations, an alternative model, called an anharmonic oscillator (aHO), has been developed and implemented in the dsmcFoam+ solver. The current work aims at evaluating an aHO model for an air mixture at Mach 6 and quantifying its influence on a pure oxygen gas flow at Mach 16. The gas mixture study suggests the predictions for the two vibrational models agree very well for four flow properties along the stagnation streamline. The pure oxygen gas study indicates a different topology of the vibrational temperature flow fields when calculated with an aHO model, which can be attributed to the increased Mach number relative to the air mixture case. When an aHO model is considered, the internal modes along the stagnation streamline result in a lower peak temperature that is closer to the surface of the cylinder. The inclusion of the electronic mode of the molecules amplifies this behaviour and results in a significantly lower temperature in the thermal equilibrium state. The surface heat flux and surface pressure appear to be less impacted by the two vibrational models.

Understanding the dynamics of rocket plume impingement on flat surfaces is critical for designing effective and sustainable landing pads. The current near vacuum study measures the surface pressures and temperature profiles arising on a flat surface due to highly underexpanded, axisymmetric plume impingement. The experiments were conducted in a dedicated, large-volume plume-regolith facility situated at the University of Glasgow. A total of eight tests were conducted, comprising of both constant and pulsed firing modes. The impingement plate is located at stand-off distances equal to 4 and 16 times the nozzle exit diameters and plate inclinations of 0 degrees and 30 degrees. Reduced stand-off distance increases impingement pressure, with a transient peak in the early stages of impingement indicating the presence of a primary shock wave. Higher stand-off distance resulted in decreased impingement pressure without an initial peak, but with a spike at the end of impingement once the nozzle had stopped firing. For inclined plates, the centerline impingement pressure magnitude decreased by around 40% compared with the 0 degrees inclination. The measured pressures at lower stand-off height are then compared with the results of both a two-way coupled direct simulation Monte Carlo/Navier-Stokes-Fourier solver and an analytical theory. The results from all methods are in good agreement with each other, with the simulation and experimental centerline pressures being within 1% of each other. As the optical thickness of the gas is very low in the experimental case, the numerical method is used to generate a Schlieren image to analyze the shock structure.

This study investigates the flow field and turbulence characteristics within two types of S-shaped ducts through both experimental and numerical methods. The experiments include pressure measurements and constant temperature anemometry, while the numerical simulations employ the Reynolds-averaged Navier-Stokes (RANS) approach. To ensure the accuracy of the simulations, a mesh independence study was conducted, and the results were compared with experimental data. Additionally, three common turbulence models- k-& varepsilon;, k-omega, and the shear stress transport (SST)-were tested. After comparing the two S-shaped ducts, the configuration with superior performance was selected for further unsteady simulations. Modal analysis was subsequently performed using proper orthogonal decomposition, dynamic mode decomposition, and spectral proper orthogonal decomposition. The results show that the S-shaped diffuser performs better when the diffusion rate follows an S-shaped curve rather than a linear increase. In low-velocity experiments, a 1 kHz sampling frequency and 1-second period suffice for turbulence analysis, as higher values only yield minimal differences. Regarding the simulation, although RANS simulations can match the experiment, the discrepancies with experimental data can also not be ignored. Among the three turbulence models, the SST model best matches the experiment. However, modal analysis of the transient flow field reveals that RANS makes it hard to capture turbulence details in low-speed conditions, and only the k-& varepsilon; model has modes with clear dynamic patterns. Last, the functionalities and features of each modal analysis method are demonstrated.

Vehicles undergoing hypersonic speed experience extreme aerothermodynamic conditions. Real gas effects cannot be neglected, and thus internal degrees of freedom of molecules being partially/fully excited must be carefully predicted in order to accurately capture the physics of the flowfield. Within direct simulation Monte Carlo solvers, a harmonic oscillator (HO) model, where the quantum levels are evenly spaced, is typically used for vibrational energy. A more realistic model is an anharmonic oscillator (aHO), in which the energy between quantum levels is not evenly spaced. In this work, the Morse-aHO model is compared against HO. The Morse-aHO model is implemented in the dsmcFoam+ solver, and the numerical results are in excellent agreement with analytical and potential energy surface solutions for the partition function, mean vibrational energy, and degrees of freedom. A method for measuring the vibrational temperature of the gas when using the anharmonic model in a direct simulation Monte Carlo solver is presented, which is essential for returning macroscopic fields. For important thermophysical properties of molecular oxygen, such as the specific heat capacity, it is shown that the aHO and HO models begin to diverge at temperatures above 1000 K, making the use of HO questionable for all but low-enthalpy flows. For the same gas, including the electronic energy mode significantly improves the accuracy of the specific heat prediction, compared to experimental data, for temperatures above 2000 K. For relaxation from a state of thermal nonequilibrium, it is shown that the aHO model results in a slightly lower equilibrium temperature. When applied to hypersonic flow over a cylinder, the aHO model results in a smaller shock standoff distance and lower peak temperatures.