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The workshop addresses challenges in experimental and numerical simulation of flows in micro-gravity. It aims at assessing the current experimental and numerical capabilities for technically relevant flows in micro-gravity, particularly for capillary flow conditions and applications related to fuel storage and refuelling in orbit.
Topics of interest:
Ongoing preparations and results from experiments in a genuine micro-gravity environment
Activities, results and remaining challenges for ground facilities that can provide a micro-gravity environment
Status of developments for flight demonstrators, strategy and status of design and preparations
Numerical methods for capillary flows, e.g. assessing the current validation level and need for validation data
Status of empirical models for application of CFD methods to capillary flows in micro-gravity
Please see the timeline under the heading "Calendar of Events".
Registration is free of charge but mandatory for participation and normally restricted to organisations from ESA Member States and Associate Member States. Please contact one of the organisers in case of any questions.
SPHERES TETHER SLOSH combines fluid dynamics equipment with robotic
capabilities aboard the International Space Station ISS to investigate automated strategies
for steering passive cargo. A liquid-filled tank is maneuvered inside the ISS by two
Synchronized Position Hold, Engage, Reorient, Experimental Satellites (SPHERES). The
two satellites are attached to a passive sloshing tank by Kevlar tethers. The satellites shall
maneuver the passive tank both with open loop guidance as well as in closed loop control. A
better understanding on how to deorbit e.g. a stranded satellite shall be goal of the study.
Furthermore the data was used to benchmark Computational Fluid Dynamics (CFD)
coupled analysis software which may be used for future mission analysis. The experiment
was performed both with a fluid filled tank as well as with a rigid body tank of the same
mass and inertia. This allows better understanding the impact of the sloshing liquid. At the
present time tests were performed only with the liquid filled tank in open loop (noncontrolled). Comparisons showing the impact of the sloshing liquid were performed with
CFD. The maneuvers consider translation as well as rotation of the passive tank similar to
Δv and repointing maneuvers. Two experiment sessions were carried out January 17th 2018
and April 4th 2018. The experimental layout as well as experiment results are
presented.
This talk presents an overview of recent and ongoing activities at the von Karman Institute dedicated to the experimental analysis of flow configurations relevant to space propellant management. Specifically, the talk will present the results from Sloshing PARabolic flight Experiment (SPARGE) campaigns, the last Contact Angle Sloshing Experiment (CASE) campaign and describe the NT-SPARGE (Non-isoThermal) experiment currently in preparation. All these campaigns took (or will take) place during ESA Parabolic Flight Campaigns organized in collaboration with NOVESPACE.
The SPARGE experiments are dedicated to the experimental characterization of isothermal liquid sloshing in partially filled reservoirs subjected to a microgravity environment. This is a configuration of fundamental importance to launchers and satellites alike because liquid sloshing can potentially hamper the supply of engine feedlines and challenge Guidance, Navigation and Control (GNC) systems to cope with unexpected accelerations. The SPARGE campaign provided a comprehensive characterization, including high-speed visualization, interface tracking and Particle Image Velocimetry (PIV) of water and HFE7200 during the transition from hypergravity (~1.8g) to microgravity. The different properties of the two liquids play a role in the dynamic response of the system, showing different reaction time during the step to microgravity.
The CASE experiment was dedicated to the dynamics of capillary tubes in microgravity, with a focus on the impact of interface re-orientation and dynamics contact angles. Capillary driven flows in microgravity are essential in capillary fluidic systems that include gas-liquid separators, urine collection and processing, condensing heat exchangers and liquid sorbent CO2 scrubbers, to mention but a few. The CASE experiment reproduced the capillary driven flow in tubes of varying diameters, thus promoting various levels of velocity and acceleration, and exploring a broad range of operating conditions. High-speed visualization and PIV have been used to calibrate dynamic models for capillary flows for each of these.
The NT-SPARGE campaign in preparation will target the experimental analysis of the thermohydraulic of a partially filled reservoir undergoing thermal disturbances and sloshing. This project extends the previous SPARGE campaigns in isothermal conditions to a non-isothermal condition with phase change. With sufficient long holding time, a partially filled reservoir tends to reach slightly subcooled conditions in the liquid and slightly superheated conditions in the ullage gas. Heat leakages or refilling operations can further exacerbate this thermal gap and promote thermal stratification in the liquid. In these conditions, the sudden enhancement of heat and mass transfer coefficients, due to a sloshing event, can result in sudden condensation or evaporation phenomena which, in turn, produce significant (and undesired) pressure fluctuations. NT-SPARGE aims at characterizing the thermal response of a partially filled reservoir during sloshing in microgravity and use the available data to calibrate simplified models for the tank thermodynamics.
Storage and transfer of propellants in space will become a primary requirement for future long-range long-term space missions. A propellant depot can store the propellants in space and thereby help in refilling the tanks of the docked spacecrafts. It is challenging to understand the flow physics related to storage and refuelling in orbit. While in normal gravity the free surface remains flat for a tank filled with liquid, in microgravity capillary forces dominate and lead to a new configuration of the free surface. Propellant management devices help in controlling the shape and position of the free surface in microgravity. Loss of propellant can be reduced and self-pressurization of tanks can be controlled if the stability of the free surface in microgravity during the filling of a tank is known.
In this work, the experimental results from the drop tower experiments that were carried out at the ZARM drop tower with a microgravity time of 9 seconds are presented. An experiment tank is filled with test liquid HFE-7500 to an initial fill height. The behaviour of the free surface and its interaction with an incoming liquid during the filling of the tank is investigated for different volumetric inflow rates. After an initial period of liquid reorientation inside the tank, the filling process is started. The free surface behaviour is tested for two inlet configurations of the experiment tank, which are the central and the lateral inlet. The incoming liquid jet directly touches the free surface when using the central inlet. For the lateral inlet, the presence of a velocity control plate (VCP) dissipates the linear momentum of the incoming liquid into the bulk liquid.
The behaviour of the free surface can be classified into three regimes of subcritical, critical and supercritical flow. The non-dimensional Weber number can be used to represent the flow regimes. For the central inlet tests, the incoming liquid jet causes a perturbation of the free surface and forms a geyser in the subcritical range. The geyser that grows with time and disintegrates into droplets is observed in the critical range. For the supercritical regime, the liquid jet touches the outlet port at the top of the tank. For the lateral inlet tests, the free surface remains unperturbed during the filling, enabling a quiescent and faster filling of the tank. Different flow regimes during the filling of a tank in microgravity can be identified from these results so that depending on the application the corresponding regime can be chosen. Furthermore, based on these drop tower experiment results, a space station experiment can be designed and planned.
Phase separation is critical for the supply of gas-free liquid propellant from the tank outlet to the engine of a spacecraft. In a microgravity environment, surface tension and contact angle become the governing mechanism for phase separation and dictate the position of the liquid-gas interface. Liquids with zero-degree contact angles tend to adhere to the tank wall, and gas stays in the center. Therefore, to maintain a constant supply of liquid to the outlet of the tank, a liquid acquisition device (LAD) is essential. Screen channel liquid acquisition devices (SC-LAD) are a type of LAD that work on the principles of capillary action. Liquid enters the channel through a porous screen but the entry of gas is blocked as long as the pressure difference across the screen is below its bubble point.
In this project, the experiment is designed to test the phase separation in a microgravity environment with the help of a screen channel liquid acquisition device SC-LAD. For this purpose, a supply tank has been designed with a SC-LAD inside it. The screen used in the SC-LAD is DTW 200x1400. The liquid is removed from the supply tank with the help of a gear pump and a combination of valves in the liquid pipeline. A total of 22 drop tower tests are performed with 9.1 seconds of microgravity each. The analysis of the sensor data and the images obtained by the high-speed cameras shows a successful separation of phases through the SC-LAD in subcritical conditions and ingestion of bubbles at the critical condition. A combination of various complex phenomena and their effects on one another could be also observed visually during the experiments. The phenomena observed are reorientation of the free surface under microgravity, capillary rise of liquids between parallel plates, flow through screen pressure loss due to applied removal flow rate and bubble point breakthrough of the screen.
Spacecraft using cryogenic fuels are currently in development to extend the duration of missions from hours to weeks and months. To mitigate the pressure rise caused by the transfer of thermal energy into the tank from the environment, venting maneuvers are a possible operation. Depressurization maneuvers will introduce a superheat in the bulk liquid, that was previously at the saturation temperature of the initial pressure, and thus might lead to large-scale phase change occurring in the tank both at the free surface and at favorable nucleation sites at the tank wall. The results shown here aim to investigate the depressurization process on the scale of a single nucleation site.
A series of three experiments was performed using liquid methane in the Drop Tower at the University of Bremen which offers 4.74 s of compensated gravity. The experimental setup consists of a single species system of liquid and gaseous methane. Inside the glass cylinder, a polished stainless steel structure is used to introduce a single, artificial nucleation site into the liquid. The setup is equipped with temperature and pressure sensors, and optical access was provided by two endoscopes. The liquid was brought into an isothermal state at the starting pressure. After that, the experimental setup was released into free fall and the system experienced microgravity. Once the initial sloshing motion of the free surface was damped, a valve was opened for 140 ms. The decrease in ullage pressure uniformly superheated the liquid. This led to an immediate generation of vapor at the artificial nucleation site. During the remaining time of the experiment, the generated vapor formed a single bubble above the cavity. The superheat of the liquid decreased due to evaporation at the interface of bulk liquid and the ullage, which led to rising ullage pressure.
The image data was evaluated for the radius of the vapor bubble. This experiment was replicated four additional times and the results indicated good reproducibility. The spatially uniform superheat makes the experimental results well suited for comparison against analytical models. A variety of analytical models that are valid for spherical symmetry and a constant superheat. The absence of buoyancy under compensated gravity conditions allows increased observation time and for the establishment of a temperature distribution in the liquid which is close to the analytical solution. Deviations from the analytical model are introduced because a constant superheat is not given in the presented experiments. Therefore, models were applied using both the highest and lowest measured superheat of 3.5 K and 1.5 K. The analytical solutions thus should envelop the experimental data and act as upper and lower bounds for the expected bubble size, which they do favorably. The use of a cryogenic fluid allows for a range of scaling parameters that is close spaceflight applications.
Cryogenic propellant management in micro-gravity environment has become of a primordial importance for the development of future space transportation systems. Upper stages are required to operate during long coasting phases, in order to allow for multiple payloads injections. Even small overheats can induce nucleate boiling in the saturated liquid, thus inducing overpressures in the tank, obliging to vent the gas and thus inducing a waste of propellant. Moreover, in order to sub-cool the propellant prior to engine ignition, a common approach consists in reducing the pressure in the tank, thus inducing cavitation in the liquid, that is bubble creation and growth induced by the pressure decrease. The same issues are encountered for in space depot that are expected to play an important role for in space exploration. Cryogenic propellants must be stored for several weeks/months, and pressure cycles could be employed to sub-cool the propellant before the fuel transfer. There is clearly a necessity to study the phenomena associated with phase change induced by temperature and pressure variations in a micro-gravity environment in order to increase the understanding and develop suitable guidelines allowing a proper tank design.
The peculiarity of the problem is that phase change phenomena are driven by temperature gradients at the interfaces and capillary effects at the contact line: small scales drive bubbles growth and their impact at the tank scale in terms of heat transfer and pressure increase. From a numerical point of view, direct numerical simulation (DNS) solvers are needed in order to study these phenomena at the bubble scale. The goal of this presentation is to show some recent developments that have been carried on the DIVA code for the DNS of bubbles growth induced either by overheat or pressure decrease [1-2].
First, a parametric study of a bubble nucleated in a subcooled liquid, under micro-gravity conditions will be presented. The simulation in the fluid domain is coupled with a temperature field resolution in the solid wall. Contact angles and Jacob numbers have been varied in order to extract a correlation for the diameter and the Nusselt number. However, these simulations have been limited to high contact angles (greater than 30°) and small wall thermal conductivity because of the lack of a model for the micro-region. Indeed, cryogens are wettable fluids characterized by very low microscopic contact angles. For a bubble nucleated over a superheated wall, micro-regions are expected to be formed at the contact line for such fluids: over an extension of the order of hundreds of nanometers, the contact angle varies between the microscopic and apparent values. This induces a variation of the local interface temperature and has a strong impact on the phase change mass flow rate. The micro-region cannot be simulated and need to be modeled. The numerical challenge consists in introducing a proper sub-grid model for the micro-region while maintaining suitable numerical properties of the solver, including numerical convergence and energy balances.
Finally, the presentation will end with an overview of the compressible solver development allowing the simulation of phase change induced either by temperature or pressure variation. Indeed, when it comes to phase change induced by pressure variations, the numerical challenge consists in including the proper thermodynamic description of the system (liquid, vapour and interface), coupling the pressure and temperature variations, and being able to describe both rapid and slow pressure variations. This will be shown on some examples of bubbles cavitation under different pressure temporal variation rates.
[1] Urbano, Tanguy and Colin, Int. J. of Heat and Mass Trans.,143, pp 118521 (2019)
[2] Urbano, Bibal and Tanguy, J. Comp. Fluids, 456, pp. 111034 (2022)
Understanding the collision and mixing of droplets is of importance in many applications such as fuel combustion, spray painting or icing phenomena on flight systems. The parameters determining the coalescence of two droplets also determine the dynamics and mixing during the droplet coalescence, as well as the threshold criteria for drop rebound or coalescence regimes. The coalescence is influenced by interfacial tensions, the ambient gas atmosphere, evaporation rate, the flow and stability of the thin gas layer between both droplets, Marangoni effects and the dynamics of the approaching droplets. Further drop mixing is determined by the internal flow, induced by the drop coalescence. Computational Fluid Dynamics (CFD) simulations of such processes are extremely complicated since very different length scales are involved in the problem.
The objective of this experimental study is the characterization of the coalescence and mixing of two different liquid drops under the terrestrial and microgravity conditions during parabolic flight campaign. The coalescence of two liquid convex surfaces of relatively large radii of curvature is studied in this work. The deformation of the surfaces, caused by the dynamics of the gas or vapor flow in the gap and by the Marangoni stresses, is observed using a high-speed video system. Additionally, a high-speed micro-PIV system is employed to characterize the internal flow during the drop coalescence and the influence of fluid properties, ambient conditions and collision properties. Longer lasting observations of the drop coalescence are planned under microgravity conditions on the International Space Station (ISS).
This study is performed in the framework of the Lufo IV “DropCoal” project.
In this talk, we present a preliminary investigation of cryogenic tanks of the AS-203 low gravity cryogenic experiment.
A 3D full FEM thermal model was generated to predict the temperature distribution in the different components of the upper stage of the launcher in orbit. The structure is analysed with the commercially-available OOFELIE::Multiphysics engineering software solution. This thermal model accounts for radiation heat input from the Sun, Earth Albedo, and Deep Space & Earth Infra-Red. Different materials are involved in the heat conduction in the different structural components, and the considered fluids (LOX & LH2) are at cryogenic conditions. Initial conditions are not computed since launch-time (ad-initio) but are deduced from measurements on different surfaces of the spacecraft. Therefore, the definition of a dedicated strategy to compute this initial temperature distribution is exposed. Dynamic simulation over a period of ~20.000 seconds is considered in this investigation. Computed temperature distributions and pressure rates are in good agreement with test data.
This first thermal FEM model contains some limitations/simplifications. Some of them could be removed in more advanced models. However, it already provides valuable insights for pre-design/ pre-sizing activities.
In spacecraft such as liquid-propellant rockets and satellites, the liquid propellant in the tank oscillates due to disturbances caused by engine thrust fluctuations and attitude control. This is called sloshing, and the change in the center of gravity of the liquid caused by sloshing interferes with guidance and control of spacecraft. As a conventional countermeasure against sloshing, baffle plates are employed to damp liquid surface oscillation. Together with thrusters and reaction wheels on the airframe side, center of gravity is controlled. However, it has been pointed out that when large-amplitude sloshing occurs in a microgravity environment, the liquid may collide the baffle plates and deteriorate the behavior of the aircraft. The force generated by large-amplitude sloshing cannot be absorbed by thrusters or reaction wheels, and there is a high risk that the aircraft will tip over. Therefore, technology is necessary to quantitatively predict the effects of large-amplitude sloshing in a tank equipped with baffle plates on propellant and aircraft attitude. The purpose of this study is to propose an improved mechanism to suppress liquid behavior while reducing the adverse effects on the aircraft attitude when large-amplitude sloshing occurs, through microgravity experiments and coupled liquid-fuselage analysis. This paper presents experiment and numerical computations for a hypothesis of free-fall and vertical landing of the aircraft on a microgravity astral body. The experiment was conducted in cooperation with JAXA at Uematsu Electric's drop tower. Sloshing in a microgravity environment was generated by moving the tank vertically upward during free fall, and the liquid surface deformation was recorded and data was collected. In the experiment, the tank move was performed after waiting 1 second after the start of the fall. As a result, it was observed that after the tank wall surface was wetted, the liquid column rose significantly and impacted the tank ceiling. Numerical computation was also performed under the same conditions as in the experiment. As a result, the tank wall surface is wetted and liquid column rose in the same way as the experiment, as well as the force exerted by the liquid on the tank is evaluated. In addition, Numerical computation about the tank with baffle plate was performed. It was found that the baffle plates prevent the liquid level from rising in a microgravity environment, thereby suppressing the behavior of the liquid and the forces exerted by the liquid on the tank walls. The above results are for the ideal case where the spacecraft lands vertically and all legs of the spacecraft are grounded at the same time. In a real case, it is possible that all the legs do not touch the ground at the same time and the aircraft can rotate. If this situation happens, the liquid can impact the baffle plates and adversely affect the aircraft's attitude. In the future, we plan to evaluate the case of rotation by calculating the moments.
To enable human space missions to mars and beyond, future payload mass needs to increase significantly. Cryogenic propellants with both high specific impulse and with the possibility of fuel production on mars make them an excellent choice for these types of missions. Using cryogenic propellants for missions longer than a few hours poses several challenges including the need to minimize liquid losses due to boil-off caused by low storage temperatures and to manage the low-g issues. So far there has not been a mission to investigate and demonstrate technologies for the storage of cryogenic fluids in space for propulsion applications. This demonstration is critical for cryogenic fluid phenomena under micro-gravity.
As such a collaborative study between NASA and DLR was conceived to develop a micro-gravity experiment of cryogenic fluids. In this study, the feasibility of a cryogenic fluid demonstration satellite is shown, called the Future-oriented Research platform for Orbital cryogenic Storage Technologies (FROST). FROST is investigated to fly as primary payload on the DLR CompactSat Bus. This bus offers a maximum total mass of 500kg, which is driven by testing capabilities at DLR Bremen and current launcher rideshare opportunities. These 500kg include the bus segment, so that the maximum payload mass is iterated with the bus. This was started in a CE-Study in May 2019, where a payload mass of ~115 kg without experiment fluid came out to fit within this restriction. The following phase 0-A study investigated this setup in more detail, demonstrating the feasibility. The achievable main topics are: the demonstration of long-term storage technologies, the demonstration of pressure control in cryogenic tank systems, the demonstration of the transfer of cryogenic fluids between tanks, investigating (the demonstration of) the general behavior of cryogenic liquids in micro-gravity conditions, and the demonstration of mass gauging methods for cryogenic fluids in micro-gravity. All those topics can be investigated under micro-gravity (bond-number <1) conditions as well as under small thrust for bond-numbers up to 10, provided by the bus. In this presentation the results of the study will be presented, discussing research possibilities on the proposed platform.
The tank pressure prediction is key for design of propulsion & GNC system, tank design, ground & mission & its 0g phases, but for its prediction several physical systems & disciplines needs to be strongly coupled.
The in-house tool TankPSB (Pressure, Stratification & Budget) estimates, with lower-cost & computational time, the thermal stratification in liquid, gas & wall, pressurization mass fluxes, tank pressure, mass budgets and fill level in a cryogenic tank, considering heat and mass conservation within fine stratification layers, for preselection of cost-intensive 3-d computation, parametric studies or the layout of complex test sequences.
The applicability of TankPSB for reduced gravity missions is discussed. Current model implementations for capillary dominated regimes is presented. Status & needs for implementation of boiling and cavitation effects for 0g application is outlined.