We analyze the sensitivity to image defects of the processing algorithm proposed by Salgado Sánchez et al. (Microgravity Sci. Technol. 37, 12, 2025) to evaluate melting bridge experiments in the context of the MarPCM microgravity project (Porter et al., Acta Astronaut. 210, 212–223, 2023). The algorithm uses the projection of input images onto the first m singular vectors (modes), obtained via Singular Value Decomposition (SVD), of the original (non-defective) image database. The resulting set of m amplitudes is then used as input for an Artificial Neural Network (ANN) that is trained to give the corresponding liquid fraction as an output. For the analysis presented here, the images are modified to generate a new database that includes rotated images, which represent optical misalignment, overexposed and underexposed images, which represent incorrect exposure time and/or aperture settings in the camera, noisy images and gappy images, which model the presence of dead pixels, bubbles and large reflections that compromise certain regions of the image. The results suggest that only relatively large defects are a concern for processing the experiment and that the most critical case is that of gappy images. Data repair algorithms based on SVD can be used to correct the defective images and reconstruct the missing information, which then allows for accurate processing.

This paper describes a laboratory set-up designed to support hands-on learning of heat transfer principles in aerospace engineering education. Developed within the framework of experiential and project-based learning, the set-up enables students to experimentally characterize the convective coefficient of a cooling fan and the thermo-optical properties of aluminum plates with different surface coatings, specifically their absorptivity and emissivity. A custom-built, LED-based radiation source (the ESAT Sun simulator) and a calibrated temperature acquisition system are used to emulate and monitor radiative heating under controlled conditions. Simplified physical models are developed for both the ESAT Sun simulator and the plates that capture the dominant thermal dynamics via first-order energy balances. The laboratory workflow includes real-time data acquisition, curve fitting, and thermal model inversion to estimate the convective and thermo-optical coefficients. The results demonstrate good agreement between the model predictions and observed temperatures, which supports the suitability of the set-up for education. The proposed activities can strengthen the student’s understanding of convective and radiative heat transport in aerospace applications while also fostering skills in data analysis, physical and numerical reasoning, and system-level thinking. Opportunities exist to expand the material library, refine the physical modeling, and evaluate the long-term pedagogical impact of the educational set-up described here.

Both Singular Value Decomposition (SVD) and Artificial Neural Networks (ANNs) can be powerful tools for image processing. Here they are applied in the context of the “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” (MarPCM) microgravity experiment [Porter et al. (Acta Astronautica 210, 212–223, 2023)], which investigates the use of thermocapillary (Marangoni) convection to expedite melting of organic Phase Change Materials (PCMs) in cuboidal and cylindrical domains. The processing of the cylindrical “melting bridge” experimental images is particularly challenging due to the converging lens effect caused by the curved interface and the refractive index of the liquid PCM. A combination of SVD and ANNs is used to propose an algorithm to process these images. The network is trained on a set of synthetic images of the melting bridge, generated via ray-tracing [Martinez et al. (Advances in Space Research 72, 1915–1928, 2023)] then projected onto the eigenmodes associated with the largest singular values of the image database, which includes snapshots of the melting process in all representative cases. Two optimal algorithm architectures are described, characterized by the number of SVD modes considered in the projection and the hyperparameters of the ANN. The performance of the algorithm is analyzed in terms of its ability to associate images with the correct liquid fraction. The processing strategy is tested by applying it to images obtained from ground experiments using the scientific prototype of the MarPCM cuboidal cell.

The melting of a phase change material (PCM) in a cuboidal domain under microgravity conditions is investigated numerically. The upper surface of the PCM is free (in contact with air, for example) and variations in its surface tension drive thermocapillary convection in the liquid phase, which significantly enhances heat transfer and accelerates melting. Furthermore, the change in liquid fraction during melting is associated with transitions among various modes of thermocapillary dynamics, including an oscillatory instability to hydrothermal waves. While the characteristics of PCM melting and thermocapillary dynamics have previously been investigated in this system using a two-dimensional model, the current work examines the important question of transverse dynamics and their effect on the melting process. Careful quantitative comparisons are made between the three- and two-dimensional models in terms of melting times, solid/liquid interface evolution, thermal fields, and spectrograms. The results show that transverse modes are often, but not always, reflection symmetric about the midplane and that their influence on melting and PCM performance is relatively minor in most cases. Thus, two-dimensional models may be used to reduce computational costs while still providing a reasonable approximation of the melting process for high Prandtl number materials, especially when compared to the midplane of the full cuboidal domain.

We experimentally confirm the efficacy of submerged oscillating baffles for sloshing mitigation in a partially filled open container of water. As with the more typical applications of tuned mass dampers, when the natural frequency of the baffle is close to that of the first sloshing mode, the main resonance peak is split into two peaks of lower amplitude. The reduction in resonant sloshing amplitude is superior to that of a fixed baffle.

We numerically investigate the effect on sloshing in microgravity of systems with passive, moving baffles whose motion is restricted by linear springs. The liquid is assumed to have physical properties similar to those of 5cSt silicone oil while the baffle is made of aluminium. Two kinds of numerical models are developed depending on whether the allowed baffle motion is translational or rotational. Simulations show that moving baffles significantly improve sloshing mitigation compared to fixed baffles, with decreases of up to 48% in the decay time and 23% in the fluid kinetic energy following excitation by a certain pulse-like acceleration. In all configurations considered, there exists an optimal spring stiffness that minimises the kinetic energy or decay time. Frequency analysis reveals two peaks in the range of 0.1–2Hz whose amplitudes vary as a function of spring stiffness, analogous to the behaviour of Tuned Mass Dampers (TMDs). The effectiveness of the moving baffle system is then assessed using a real microgravity perturbation measured during a reboosting manoeuvre of the International Space Station (ISS) and a significant reduction in decay time relative to the fixed baffle case is found: to 1.3s from 4s.

The results of a numerical investigation of the melting of a PCM occupying an axisymmetric volume in the presence of gravity are presented. The PCM is held between two circular supports maintained at different temperatures. The melting process, which is analyzed for n-octadecane, is affected by a combination of thermocapillary and natural convection. If the PCM is heated from above, the convective motion driven by the thermocapillary force is opposed by the buoyant force, which reduces the heat transfer rate. If the PCM is heated from below, natural convection acts in the same sense as thermocapillary convection and the heat transfer rate is increased. The volume 𝒱 of the PCM relative to an ideal cylinder, which selects the shape of the PCM/air interface, is found to play an important role. The overall effect of natural convection on heat transfer is characterized by the ratio of the melting time in microgravity to that of the same system with gravity. This gain factor is greater (less) than unity when heating from below (above) and depends strongly on 𝒱 , particularly for smaller PCM volumes.

Present as well as future challenges of space exploration point to the need for improved thermal control systems. The “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” experiment, which is approved by ESA for execution on board the International Space Station, aims to contribute directly to current knowledge and basic understanding of heat and mass transport in phase change materials (PCMs) that incorporate a free surface in reduced gravity. The experiment will apply fixed temperatures to opposite ends of PCM samples held in cuboidal and cylindrical containers in order to drive controlled melting and solidification cycles that will be observed by means of optical cameras. The recorded images will be complemented by thermal measurements at key positions along the samples, which will allow different thermocapillary flow regimes to be distinguished according to their temporal dynamics. It is anticipated that thermal Marangoni (thermocapillary) convection will increase the heat transfer rate in these PCM devices by a significant factor (on the order of two or more) compared to melting governed by thermal diffusion (conduction). If the PCM designs prove robust, the experiment results can be expected to lead to substantial improvements in future designs for passive PCM applications in space missions.

The understanding of transport and mixing in fluids in the presence and in the absence of external fields and reactions represents a challenging topic of strategic relevance for space exploration. Indeed, mixing and transport of components in a fluid are especially important during long-term space missions where fuels, food and other materials, needed for the sustainability of long space travels, must be processed under microgravity conditions. So far, the processes of transport and mixing have been investigated mainly at the macroscopic and microscopic scale. Their investigation at the mesoscopic scale is becoming increasingly important for the understanding of mass transfer in confined systems, such as porous media, biological systems and microfluidic systems. Microgravity conditions will provide the opportunity to analyze the effect of external fields and reactions on optimizing mixing and transport in the absence of the convective flows induced by buoyancy on Earth. This would be of great practical applicative relevance to handle complex fluids under microgravity conditions for the processing of materials in space.

A ray-tracing algorithm is developed to obtain synthetic images for tracking the melting of the phase change material (PCM) noctadecane in a liquid bridge configuration. The translucent nature of the solid n-octadecane is modelled through a sigmoid function which provides a distance-dependent illumination intensity throughout the solid. The code is validated against two types of experiments: front shapes made of 3D-printed PLA and cast front shapes of solid n-octadecane. Comparison with these experiments allows the numerical constants involved in the model to be tuned to appropriate values. For application to the MarPCM microgravity experiment (Porter et al., 2023), the current study provides the expected images of the solid/liquid front obtained by the optical diagnosis module for three different values of the dimensionless volume (defined with respect to a perfect cylinder), V = 0:9, 1, and 1.1, and two applied temperature differences, DT = 5 and 30 K. The solid/liquid front shapes are obtained numerically from a two-dimensional axisymmetric code as in Varas et al. (2021). The results reveal a strong dependence of image quality on the liquid fraction L. Visual inspection leads to the conclusion that, below L  60%, an accurate determination of the front location may be difficult, particularly for the relatively shallow layer of liquid near the thermocapillary interface.& COPY; 2023 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).

Numerical simulations are used to analyze the dynamics of a free surface excited by thermal modulations at the lateral boundaries that generate a time-dependent thermocapillary flow. Fluid parameters are selected to be representative of 5 cSt silicone oil. Following the work of Gligor et al. [“Thermocapillary-driven dynamics of a free surface in microgravity: Response to steady and oscillatory thermal excitation,” Phys. Fluids 34, 042116 (2022)], the response of the free surface to oscillatory thermal excitation is characterized by the displacement of the contact points, and a frequency sweep is used to obtain a Bode-type diagram that reveals a resonance peak in the vicinity of the first sloshing mode. The ability of the thermocapillary flow to excite this sloshing mode suggests a control strategy that uses thermal modulations to dampen sloshing motion. After the response of the isothermal surface to a generic pulse-like inertial perturbation is measured, a classical proportional integral derivative control is implemented and the effect of its gains is considered separately. The efficacy of the controller is characterized by the decay time of the contact point oscillations and by a cost function. The effect of possible delays in the control loop is accounted for. Finally, a controller with a derivative gain is selected and used to dampen the motion induced by a reboosting maneuver of the International Space Station.

A numerical analysis of the thermocapillary-driven dynamics of a free surface in microgravity is presented for an open container of liquid subjected to steady or oscillatory thermal excitation. The response to this forcing is analyzed for parameters representative of common silicone oils. In contrast to previous investigations, we permit large-scale unconstrained motion of the contact points and deformation of the free surface, which allows us to study the interaction between free surface dynamics and thermocapillary flow. First, the response of the free surface to steady thermal excitation is considered and characterized by the asymmetry of the contact points. Linear dependence of this asymmetry on the applied Marangoni number is found, which is amplified by the vibroequilibria effect when supplemental (high-frequency) vibrations are introduced. In low-viscosity liquids, the transient dynamics of the free surface includes sloshing modes, suggesting that thermal modulation may be used to excite them. The free surface response to oscillatory thermal excitation is then studied for a wide range of parameters, including variations in contact angle β, viscosity ν, container length L, and fluid height H. We perform a frequency analysis and obtain Bode-type diagrams for the contact point oscillations, characterizing the low-frequency response by its amplitude and phase with respect to the thermal forcing, and demonstrate a resonance peak corresponding to the principal sloshing mode. Overall, these results indicate the potential of oscillatory thermal excitation for fluid control in microgravity.

A detailed numerical investigation of pattern selection for thermocapillary flow in rectangular containers in microgravity is presented. These dynamics are studied for liquid n-octadecane, an alkane with high Prandtl number (Pr = 52.53), due to its relevance to recent microgravity experiments. Pattern selection is analyzed in terms of the aspect ratio, I', and the applied Marangoni number, Ma. In short containers, the bifurcation picture is characterized by a transition from steady thermocapillary flow to a standing wave (SW) oscillatory mode as Ma is varied. This transition takes the form of a primary subcritical Hopf bifurcation accompanied by a secondary saddle node; these two bifurcations delimit a region of bistability. In large containers, the dynamics is characterized by a supercritical Hopf bifurcation that marks the transition from steady flow to a traveling wave (TW) mode. The critical Ma for this transition increases with I'. In intermediate containers, a complex pattern selection scenario is found, where both steady and oscillatory convection, in the form of either TWs or SWs, can appear depending on I' and Ma. Finally, we apply this bifurcation analysis to help explain recent results on thermocapillary flows during the melting of phase change materials in microgravity [Salgado S??nchez et al., A20 (2021)]. The temporal evolution of the phase change is characterized by an effective I' and Ma in the liquid phase. We find very good agreement between the flow transitions observed during melting and those predicted for the equivalent rectangular containers over the explored range of 1.5 I' 16.

A numerical analysis of the melting of n-octadecane in microgravity is presented for a small aspect ratio rectangular container. The container is bounded above by an air layer that exchanges heat with the phase change material (PCM) and supports thermocapillary convection in the liquid phase. The air temperature is assumed to match the applied temperatures at the lateral walls and to change linearly between them. The effect of key dimensionless parameters is investigated including the Marangoni number (Ma), which quantifies the heat transport due to the thermocapillary flow, and the Biot (Bi) number, which characterizes the heat exchanged across the PCM/air interface. Several different dynamic regimes are distinguished according to whether the flow is quasi-steady or oscillatory; the latter may be characterized by an oscillatory standing wave (OSW), a hydrothermal traveling wave, or a novel type of thermal traveling wave (TTW). The results are summarized with a stability map in terms of Bi and Ma. Notably, there are parameters where the flow undergoes transitions between distinct regimes during melting, including a transition between the TTW and OSW modes and other regions where the oscillatory flow undergoes a homoclinic bifurcation. The effect of Bi on heat transport is also investigated and shown to be particularly relevant for small Ma.

The results of numerical simulations investigating the influence of natural and thermocapillary convection on the solid/liquid phase transition of n-octadecane in rectangular containers are presented. The melting process is modelled using an enthalpy-porosity formulation of the Navier-Stokes equations. A systematic analysis is performed by varying key dimensionless parameters including the container aspect ratio (Gamma), the Rayleigh (Ra) and Marangoni (Ma) numbers, which quantify the strength of natural and thermocapillary convection, and the dynamic Bond number (Bo(dyn)), which measures their relative importance. In large containers with Gamma >> 1 and Bo(dyn) << 1, thermocapillary convection is shown to significantly accelerate the melting process, enhancing the heat transfer rate of the system by as much as a factor of 20 at large applied Ma. For Gamma less than or similar to 8, this enhancement factor takes reduced values of 1-3 and exhibits relatively weak dependence on Ma. In short containers with Gamma less than or similar to 3 and Bo(dyn) similar to O(1), the thermocapillary effect is detrimental on average and increases the total melting time. The presence of normal vertical gravity is seen to stabilise the dynamics of the flow, delaying the appearance of oscillatory convection beyond the range of parameters considered here. By reducing Bo(dyn), we examine the transition to microgravity and determine the critical value Bo(dyn)(cr) for oscillatory flow at large Ma for the representative aspect ratios 1.5 and 12. (C) 2020 Elsevier Ltd. All rights reserved.

A detailed numerical investigation of the melting of phase change materials (PCMs) in microgravity is presented. We consider the high Prandtl number alkane n-octadecane and quantify the effect of ther-mocapillary convection on the heat transfer observed during axisymmetric melting with a liquid bridge geometry. The phase change is studied by varying key dimensionless parameters including Marangoni (Ma) and Stefan (Ste) numbers, which are selected by the applied temperature difference between the circular supports, and geometry, which is characterised by the aspect ratio I" = L/R and the dimension -less volume V. The case of cylindrical geometry with V = 1 is considered first, and detailed analyses of the effect of both Ma (Ste) and I" on heat transport and the type of oscillatory flow that appears in the liquid phase are provided. The results are compared with those available in the literature for rectangu-lar geometry, demonstrating an improvement of approximately 50% for the cylindrical case. We further extend the analysis to noncylindrical configurations with V = 1 that correspond to stable liquid bridges when fully melted. (c) 2021 Elsevier Ltd. All rights reserved.

We describe a dynamical state observed shortly above onset of the frozen wave instability. The transition to drifting waves, which are repeatedly created and destroyed, is a marked departure from the usual behavior of frozen waves, which are generally understood to remain motionless (on average) in the reference frame of the vibrating container. The spatial inhomogeneity of the underlying base flow, due both to the presence of the lateral walls and to the associated vibroequilibria effect, provides the driving mechanism. Energy arguments are used to understand the initial outward drift and the existence of a critical threshold which is estimated from the dependence of the drift velocity on the applied forcing. The dependence on container aspect ratio Γ is investigated, and drifting is seen to occur only when 1.5≲Γ≲3.5.

We give a brief review of several prominent fluid instabilities representing transitions driven by gravity, surface tension, thermal energy, and applied motion/acceleration. Strategies for controlling these instabilities, including their pattern formation properties, are discussed. The importance of gravity for many common fluid instabilities is emphasized and used to understand the sometimes dramatically different behavior of fluids in microgravity environments. This is illustrated in greater detail, using recent results, for the case of the frozen wave instability, which leads to large columnar structures in the absence of gravity. The development of these highly nonlinear states is often complex, but can be manipulated through an appropriate choice of forcing amplitude, container length and height, initial inclination of the surface, and other parameters affecting the nonlinear and inhomogeneous growth process. The increased opportunity for controlling fluids and their instabilities via small forcing or parameter changes in microgravity is noted.

Recent microgravity experiments have demonstrated that Faraday waves can arise in a secondary instability over the primary columnar patterns that develop after the frozen wave instability. While some numerical studies have investigated this phenomenon, theoretical analyses are only found in the works of Shevtsova et al. (2016) [1] and Lyubimova et al. (2019) [2]. Here, we extend these efforts by analysing the stability of a three-layer system, and derive the critical onset of Faraday waves, which appear via Hopf bifurcation. Numerical simulations — based on a model that reproduces the frozen wave mode with lowest wavenumber — are carried out to test this result and to analyse the character of the bifurcation. The predicted Hopf bifurcation is confirmed, which constitutes the first observation of modulated secondary Faraday waves. The abrupt growth of these modulated waves above onset indicates that the primary bifurcation is subcritical and is accompanied by a saddle-node bifurcation of periodic orbits that stabilises the (branch of) unstable solutions created in the subcritical Hopf bifurcation. Further above onset, these modulated waves are destroyed via a saddle-node heteroclinic bifurcation. Results for an N-layer configuration, which represents a more general frozen wave pattern, are also presented and compared with the three-layer case.