Terrain Aware Traverse Planning for Mars Rovers

NASA is proposing a Mars Sample Return mission, to be completed within one Martian year, that will require enhanced autonomy to perform its duties faster, safer, and more efficiently. With its main purpose being to retrieve samples possibly tens of kilometers away, it will need to drive beyond line-of-sight to get to its target more quickly than any rovers before. This research proposes a new methodology to support a sample return mission and is divided into three compo-nents: map preparation (map of traversability, i.e., ability of a terrain to sustain the traversal of a vehicle), path planning (pre-planning and replanning), and terrain analysis. The first component aims at creating a better knowledge of terrain traversability to support planning, by predicting rover slip and drive speed along the traverse using orbital data. By overlapping slope, rock abundance and terrain types at the same location, the expected drive velocity is obtained. By combining slope and thermal data, additional information about the experienced slip is derived, indicating whether it will be low (less than 30%) or medium to high (more than 30%). The second component involves planning the traverse for one Martian day (or sol) at a time, based on the map of expected drive speed. This research proposes to plan, offline, several paths traversable in one sol. Once online, the rover chooses the fastest option (the path cost being calculated using the distance divided by the expected velocity). During its drive, the rover monitors the terrain via analysis of its experienced wheel slip and actual speed. This information is then passed along the different pre-planned paths over a given distance (e.g., 25 m) and the map of traversability is locally updated given this new knowledge. When an update occurs, the rover calculates the new time of arrival of the various paths and replans its route if necessary. When tested in a simulation study on maps of the Columbia Hills, Mars, the rover successfully updates the map given new information drawn from a modified map used as ground truth for simulation purposes and replans its traverse when needed. The third component describes a method to assess the soil in-situ in case of dangerous terrain detected during the map update, or if the monitoring is not enough to confirm the traversability predicted by the map. The rover would deploy a shear vane instrument to compute intrinsic terrain parameters, information then propagated ahead of the rover to update the map and replan if necessary. Experiments in a laboratory setting as well as in the field showed promising results, the mounted shear vane giving values close to the expected terrain parameters of the tested soils

Thermal imagery for rover soil assessment using a multipurpose environmental chamber under simulated Mars conditions

Planetary rover missions on Mars have suffered entrapments and serious mobility incidents due to soil assessment limitations of stereo RGB cameras, which cannot characterize relevant physical phenomena such as thermal behavior that depend on granularity and cohesion. In particular, thermal inertia estimations are already being used to assess geophysical properties from 1-D low-resolution measurements by onboard thermopiles. However, no high-resolution measurements are currently available to characterize Martian soils for safer navigation in future missions, so new experimental methods are required to capture and analyze thermal images with planetary conditions in Earth-based experiments. In this work, we propose a novel measurement system configuration and experimental methodology to capture thermal images using isolated multipurpose environmental chambers (MECs) to replicate the temperature and pressure conditions of Mars. Furthermore, the system has allowed to measure diurnal cycles for four soil types of known physical characteristics under Martian and Earth pressures to perform a unique quantitative analysis and comparison of thermal behavior and thermal inertia for soil assessment. Even if no actual Martian infrared (IR) images are available for comparison, results indicate a correlation between granularity and thermal inertia that is consistent with available thermopile measurements recorded by rover’s onsite. Furthermore, the set of measurements acquired in the experiments has been made available to the scientific community.Funding for open access charge: Universidad de Málaga/CBUA. This work was supported in part by the Andalusian Regional Government through the Project Intelligent Multimodal Sensor for Identification of Terramechanic Characteristics in Off-Road Vehicles (IMSITER) under Grant P18-RT-991

The physics of wind-blown sand and dust

The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols. This article presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars. Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices. We also discuss the physics of wind-blown sand and dune formation on Venus and Titan.Comment: 72 journal pagers, 49 figure

고정밀 실험 및 해석에 기반한 원자로 강수부 상부에서의 액막 견인 모델링

학위논문(박사) -- 서울대학교대학원 : 공과대학 원자핵공학과, 2021.8. 조형규.본 연구의 목적은 고정밀 실험과 CFD 해석으로부터 얻은 국소 유동 변수들을 기반으로 원자로 강수부 상부에서 발생할 수 있는 액막 견인 현상을 모델링하는 것이다. 먼저 안전주입수 우회 현상에서의 액막 거동에 대한 이해를 제고하고, CFD 해석 검증에 적합한 고정밀 실험 데이터를 확보하기 위해 1/10 스케일로 축소된 강수부에서 실험 연구가 수행되었다. 실험에서의 주요 측정 변수는 액막 두께였으며, 액막 두께 분포를 효율적으로 측정할 수 있는 전기적 기법의 액막 센서가 본 연구에서 개발되었다. 개발 센서는 폴리이미드(polyimide) 필름 기반의 연성회로기판을 이용하여 제작되었기 때문에 강수부와 같은 곡면부에도 쉽게 부착될 수 있었다. 공기 유동에 따라서 변하는 액막 거동에 대해서 시간 평균 액막 두께를 센서로 측정하고 이를 관측 결과와 비교하였다. 공기 유속이 증가할수록 액막이 퍼지는 폭은 파단부 중심으로 좁아졌으며, 두꺼운 액막 경계에서 액적 이탈이 발생하는 것이 관측되었다. 또한 공기 유속이 큰 경우에 파단부 주위에서의 두꺼운 액막으로부터 wisp가 생성되어 파단부로 견인되는 것이 확인되었다. 이와 같은 관측을 통해 액막 센서 측정에 대한 양적, 질적 평가를 수행할 수 있었다. CFD 해석은 3차원 형상 효과가 수반되는 이상유동의 거동을 정확히 예측하고자 할 때 유용하게 활용될 수 있다. 그러나 전통적인 이상유동 모델은 분산상과 연속상이 혼재해 있는 이상유동의 거동을 적절하게 다룰 수 없기 때문에 CFD 해석의 활용에 제한이 있었다. 본 연구에서는 VOF 모델과 Mixture 모델이 결합된 형태의 VOF-slip 모델을 활용하여 강수부에서의 액막견인 현상을 상용 CFD 코드인 STAR-CCM+로 해석하였다. VOF-slip 기법에서는 액막과 wisp와 같은 large-scale interface는 VOF로 해석하고, 액적과 같은 subgrid-scale interface는 Mixture 모델로 해석하는 것이 가능하다. 본 연구에서는 VOF-slip 모델을 활용할 때, 액적 직경값과 계면난류댐핑 계수가 해석 결과에 주요한 영향을 주는 파라미터임을 확인할 수 있었다. 민감도 분석을 통해 합리적인 액막 폭 및 우회율을 도출할 수 있는 액적 직경과 계면난류댐핑 계수를 결정하였다. 고정밀 실험 및 해석연구로부터 액막 견인 현상은 기상 및 액상의 유량과 액막 경계의 위치에 의해 결정된다는 것을 확인할 수 있었다. 따라서 이들의 특성을 대변할 수 있는 파라미터들을 Re_la, We, 그리고 R_fb와 같이 정의하고 정규화된 액막견인률을 예측할 수 있는 상관식을 개발했다. 개발 모델에서는 파단 저온관을 중심으로 off-take 볼륨이 두 섹션(LEFT, RIGHT)으로 구분된다. 그리고 각 섹션에서 예측된 우회율을 합산하여 전체 우회율이 도출된다. 개발된 액막견인률 모델은 MARS-multiD 코드에 삽입되었으며, SNU 실험과 KAERI에서 수행된 DIVA 실험 (1/5 스케일) 해석을 통해 검증되었다. 검증 결과, 개발 모델이 기존 MARS 해석보다 전체적으로 개선된 우회율 예측 성능을 보였다. 추가적으로 개발 모델의 활용이 증기-물 조건으로 확장될 수 있는지 확인해보기 위해 MIDAS 실험에 대한 해석을 수행하였다. 증기-물 조건에서는 모델이 개선될 여지가 있음이 확인되었으며, 모델 개선을 위해서는 응축 현상이 수반된 실험 연구가 추후에 수행되어야 할 것이다. 본 연구에서 수행된 원자로 강수부에서의 액막 견인 현상에 대한 고정밀 실험 및 해석, 그리고 상관식 개발 및 MARS-multiD 코드 적용은 향후 원자로 냉각재상실사고 조건 시, 안전주입수 우회량 예측에 유용하게 사용될 수 있다. 특히, 개발 모델은 재관수 기간 동안 우회율에 따른 노심의 재가열을 예측하는데 중요한 역할을 할 것으로 기대된다.This study focuses on the modelling of the film off-take phenomenon in a reactor vessel downcomer based on local flow parameters obtained from experiment and computational fluid dynamics (CFD) analysis. Experiments are conducted in the reduced-scale downcomer annulus of a nuclear reactor pressure vessel to investigate the liquid film behaviors under emergency core coolant (ECC) bypass conditions and to obtain high-fidelity data for the validation of two-phase flow CFD codes. The main instrumentation is an electrical conductance sensor for measuring the local liquid film thickness, which is developed in this study. The fabrication of the electrodes on a flexible printed circuit board enabled the installation of the sensor on the curved surface. The developed sensor is used to measure the time-averaged liquid film thickness, which shows the influence of the lateral air flow on the liquid film flow, and the results are compared with visual observations. As the air velocity increased, a droplet that was created in the thick part of the liquid film appeared, and the wisps generated near the broken cold leg could be observed. In the experiment, qualitative and quantitative analyses of the measurement results showed the reliability of the developed sensor, and helped to understand the liquid film behavior in the ECC bypass phenomenon. Furthermore, the measured film thickness could contribute to film off-take modelling and to validating the CFD codes, which have not been validated sufficiently because of the absence of local measurement data. Recent advances in computational power have resulted in the application of CFD to nuclear reactor safety analyses, which require accurate predictability for two-phase flow with three-dimensional (3D) geometrical effects. Even though the different flow regimes can exist simultaneously in the real flow, the traditional two-phase CFD models have a disadvantage with respect to regime dependency. Therefore, the CFD study used VOF-slip, which is a hybrid model combining volume of fluid (VOF) and mixture model offered by STAR-CCM+ 15.04 was used. This approach enables the large-scale interface to be treated using the VOF method and the subgrid-scale interface to be treated with a mixture model that accounts for a phase slip via the drag law. The key parameters of the VOF-slip model for the film off-take phenomenon were the droplet diameter and the interface turbulence damping coefficient. Therefore, the sensitivity analyses are conducted by varying droplet diameter and damping coefficient and a suitable value was determined based on the film spreading width and ECC bypass fraction. The droplet diameter was determined to be 150 μm for all simulation cases. The interface turbulence damping coefficients ranged from 0 to 30 and mesh-independent damping term ranged from 2.7×10-5m to 5.7×10-5m. From experiment and CFD analysis studies, it was confirmed that the liquid film off-take phenomenon is governed by the air flow rate, water flow rate, and the film boundary position. Considering these three parameters, the normalized film off take rate was correlated with Rela, We, Rfb, and Bo. The concept of the model was to divide the off-take volume into two sections (LEFT and RIGHT) through a virtual boundary so that the model could evaluate the film off-take rate in each section differently. The developed film off-take model was implemented in MARS-multiD, and it was validated with the SNU experiment (1/10 scale) and DIVA test (1/5 scale). The validation results showed that the newly developed film off-take model could improve the predictability of the bypass fraction. In addition, the MIDAS test with steam-water flow was simulated using the developed model, and the results was confirmed that the phenomenon accompanied by condensation should be experimentally investigated in future study to accurately predict the film off-take in the steam-water condition.Chapter 1 Introduction 1 1.1 Background and Motivation 1 1.1.1 Liquid film off-take in reactor vessel downcomer 1 1.1.2 Challenges with multi-dimensional system code simulation 3 1.2 Literature Review 5 1.2.1 ECC bypass experiment 5 1.2.2 Liquid film thickness sensor 7 1.2.3 CFD analysis 9 1.2.4 Modelling 11 1.3 Objectives and Scopes 12 Chapter 2 Liquid Film Thickness Sensor 18 2.1 Sensor Design 18 2.1.1 Features with flush-mounted electrode 18 2.1.2 Electrode design 19 2.1.3 Circuitry design 22 2.2 Sensor Calibration 23 2.2.1 Calibration method 23 2.2.2 Calibration result 24 Chapter 3 Experiment for Two-phase Film Flow 40 3.1 Scaling for ECC bypass phenomenon 40 3.2 Experimental Setup and Conditions 42 3.2.1 Experiment facility 42 3.2.2 Test matrix 43 3.3 Experimental Results 45 3.3.1 Time-averaged film thickness 45 3.3.2 Fluctuation of film thickness 51 3.3.3 ECC bypass fraction 53 Chapter 4 CFD Analysis 71 4.1 Two-phase CFD Models 71 4.1.1 VOF model 71 4.1.2 Mixture model 73 4.1.3 Two-fluid model 74 4.2 CFD Modelling 75 4.2.1 VOF-slip model 75 4.2.2 Interface turbulence damping 77 4.2.3 Computational domain and simulation cases 79 4.3 Simulation Results 79 4.3.1 No air flow conditions 79 4.3.2 Determination of droplet diameter 80 4.3.3 Effect of interface turbulence damping 83 Chapter 5 Modelling of Film Off-take 104 5.1 Difficulties Associated with Simulating Film Off-take Phenomenon 104 5.2 Development of Film Off-take Model 106 5.2.1 Strategy for model development 106 5.2.2 Definition of modelling parameters 108 5.2.3 Development of film off-take model 114 5.3 Validation of Developed Film Off-take Model 115 5.3.1 SNU experiment 115 5.3.2 DIVA experiment 116 5.3.3 MIDAS experiment 119 5.4 Applicability of Developed Film Off-take Model 121 Chapter 6 Conclusions 147 6.1 Summary 147 6.2 Recommendations 149 Nomenclature 150 References 154 Appendix A Uncertainty Analysis 162 Appendix B Implementation of Model in MARS 166 국문 초록 169박

A strategy for Earth science from space in the 1980s. Part 1: Solid earth and oceans

The report develops a ten-year science strategy for investigating the solid earth and dynamics of world oceans from Earth orbit. The strategy begins from the premise that earth studies have proceeded to the point where further advances in understanding Earth processes must be based on a global perspective and that the U.S. is technically ready to begin a global study approach from Earth orbit. The major areas of study and their fundamental problems are identified. The strategy defines the primary science objectives to be addressed and the essential measurements and precision to achieve them

Workshop on the Martian Surface and Atmosphere Through Time

The purpose of the workshop was to bring together the Mars Surface and Atmosphere Through Time (MSATT) Community and interested researchers to begin to explore the interdisciplinary nature of, and to determine the relationships between, various aspects of Mars science that involve the geological and chemical evolution of its surface, the structure and dynamics of its atmosphere, interactions between the surface and atmosphere, and the present and past states of its volatile endowment and climate system

Solitary waves on falling liquid films in the inertia-dominated regime

We offer new insights and results on the hydrodynamics of solitary waves on inertia-dominated falling liquid films using a combination of experimental measurements, direct numerical simulations (DNS) and low-dimensional (LD) modelling. The DNS are shown to be in very good agreement with experimental measurements in terms of the main wave characteristics and velocity profiles over the entire range of investigated Reynolds numbers. And, surprisingly, the LD model is found to predict accurately the film height even for inertia-dominated films with high Reynolds numbers. Based on a detailed analysis of the flow field within the liquid film, the hydrodynamic mechanism responsible for a constant, or even reducing, maximum film height when the Reynolds number increases above a critical value is identified, and reasons why no flow reversal is observed underneath the wave trough above a critical Reynolds number are proposed. The saturation of the maximum film height is shown to be linked to a reduced effective inertia acting on the solitary waves as a result of flow recirculation in the main wave hump and in the moving frame of reference. Nevertheless, the velocity profile at the crest of the solitary waves remains parabolic and self-similar even after the onset of flow recirculation. The upper limit of the Reynolds number with respect to flow reversal is primarily the result of steeper solitary waves at high Reynolds numbers, which leads to larger streamwise pressure gradients that counter flow reversal. Our results should be of interest in the optimisation of the heat and mass transport characteristics of falling liquid films and can also serve as a benchmark for future model development

Spacecraft/Rover Hybrids for the Exploration of Small Solar System Bodies

This study investigated a mission architecture that allows the systematic and affordable in-situ exploration of small solar system bodies, such as asteroids, comets, and Martian moons (Figure 1). The architecture relies on the novel concept of spacecraft/rover hybrids,which are surface mobility platforms capable of achieving large surface coverage (by attitude controlled hops, akin to spacecraft flight), fine mobility (by tumbling), and coarse instrument pointing (by changing orientation relative to the ground) in the low-gravity environments(micro-g to milli-g) of small bodies. The actuation of the hybrids relies on spinning three internal flywheels. Using a combination of torques, the three flywheel motors can produce a reaction torque in any orientation without additional moving parts. This mobility concept allows all subsystems to be packaged in one sealed enclosure and enables the platforms to be minimalistic. The hybrids would be deployed from a mother spacecraft, which would act as a communication relay to Earth and would aid the in-situ assets with tasks such as localization and navigation (Figure 1). The hybrids are expected to be more capable and affordable than wheeled or legged rovers, due to their multiple modes of mobility (both hopping and tumbling), and have simpler environmental sealing and thermal management (since all components are sealed in one enclosure, assuming non-deployable science instruments). In summary, this NIAC Phase II study has significantly increased the TRL (Technology Readiness Level) of the mobility and autonomy subsystems of spacecraft/rover hybrids, and characterized system engineering aspects in the context of a reference mission to Phobos. Future studies should focus on improving the robustness of the autonomy module and further refine system engineering aspects, in view of opportunities for technology infusion

Improved wind turbine monitoring using operational data

With wind energy becoming a major source of energy, there is a pressing need to reduce all associated costs to be competitive in a market that might be fully subsidy-free in the near future. Before thousands of wind turbines were installed all over the world, research in e.g. understanding aerodynamics, developing new materials, designing better gearboxes, improving power electronics etc., helped to cut down wind turbine manufacturing costs. It might be assumed, that this would be sufficient to reduce the costs of wind energy as the resource, the wind itself, is free of costs. However, it has become clear that the operation and maintenance of wind turbines contributes significantly to the overall cost of energy. Harsh environmental conditions and the frequently remote locations of the turbines makes maintenance of wind turbines challenging. Just recently, the industry realised that a move from reactive and scheduled maintenance towards preventative or condition-based maintenance will be crucial to further reduce costs. Knowing the condition of the wind turbine is key for any optimisation of operation and maintenance. There are various possibilities to install advanced sensors and monitoring systems developed in recent years. However, these will inevitably incur new costs that need to be worthwhile and retro-fits to existing turbines might not always be feasible. In contrast, this work focuses on ways to use operational data as recorded by the turbine's Supervisory Control And Data Acquisition (SCADA) system, which is installed in all modern wind turbines for operating purposes -- without additional costs. SCADA data usually contain information about the environmental conditions (e.g. wind speed, ambient temperature), the operation of the turbine (power production, rotational speed, pitch angle) and potentially the system's health status (temperatures, vibration). These measurements are commonly recorded in ten-minutely averages and might be seen as indirect and top-level information about the turbine's condition. Firstly, this thesis discusses the use of operational data to monitor the power performance to assess the overall efficiency of wind turbines and to analyse and optimise maintenance. In a sensitivity study, the financial consequences of imperfect maintenance are evaluated based on case study data and compared with environmental effects such as blade icing. It is shown how decision-making of wind farm operators could be supported with detailed `what-if' scenario analyses. Secondly, model-based monitoring of SCADA temperatures is investigated. This approach tries to identify hidden changes in the load-dependent fluctuations of drivetrain temperatures that can potentially reveal increased degradation and possible imminent failure. A detailed comparison of machine learning regression techniques and model configurations is conducted based on data from four wind farms with varying properties. The results indicate that the detailed setup of the model is very important while the selection of the modelling technique might be less relevant than expected. Ways to establish reliable failure detection are discussed and a condition index is developed based on an ensemble of different models and anomaly measures. However, the findings also highlight that better documentation of maintenance is required to further improve data-driven condition monitoring approaches. In the next part, the capabilities of operational data are explored in a study with data from both the SCADA system and a Condition Monitoring System (CMS) based on drivetrain vibrations. Analyses of signal similarity and data clusters reveal signal relationships and potential for synergistic effects of the different data sources. An application of machine learning techniques demonstrates that the alarms of the commercial CMS can be predicted in certain cases with SCADA data alone. Finally, the benefits of having wind turbines in farms are investigated in the context of condition monitoring. Several approaches are developed to improve failure detection based on operational statistics, CMS vibrations or SCADA temperatures. It is demonstrated that utilising comparisons with neighbouring turbines might be beneficial to get earlier and more reliable warnings of imminent failures. This work has been part of the Advanced Wind Energy Systems Operation and Maintenance Expertise (AWESOME) project, a European consortium with companies, universities and research centres in the wind energy sector from Spain, Italy, Germany, Denmark, Norway and UK. Parts of this work were developed in collaboration with other fellows in the project (as marked and explained in footnotes)