| 1. |
- Guo, Meng, 1988-
(författare)
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Hybrid Control of Multi-robot Systems under Complex Temporal Tasks
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Autonomous robots like household service robots, self-driving cars and dronesare emerging as important parts of our daily lives in the near future. They need tocomprehend and fulfill complex tasks specified by the users with minimal humanintervention. Also they should be able to handle un-modeled changes and contingentevents in the workspace. More importantly, they shall communicate and collaboratewith each other in an efficient and correct manner. In this thesis, we address theseissues by focusing on the distributed and hybrid control of multi-robot systemsunder complex individual tasks.We start from the nominal case where a single dynamical robot is deployed in astatic and fully-known workspace. Its local tasks are specified as Linear TemporalLogic (LTL) formulas containing the desired motion. We provide an automatedframework as the nominal solution to construct the hybrid controller that drives therobot such that its resulting trajectory satisfies the given task. Then we expand theproblem by considering a team of networked dynamical robots, where each robot hasa locally-specified individual task also as LTL formulas. In particular, we analyzefour different aspects as described below.When the workspace is only partially known to each robot, the nominal solutionmight be inadequate. Thus we first propose an algorithm for initial plan synthesis tohandle partially infeasible tasks that contain hard and soft constraints. We designan on-line scheme for each robot to verify and improve its local plan during runtime, utilizing its sensory measurements and communications with other robots. Itis ensured that the hard constraints for safety are always fulfilled while the softconstraints for performance are improved gradually.Secondly, we introduce a new approach to construct a full model of both robotmotion and actions. Based on this model, we can specify much broader robotic tasksand it is used to model inter-robot collaborative actions, which are essential for manymulti-robot applications to improve system capability, efficiency and robustness.Accordingly, we devise a distributed strategy where the robots coordinate theirmotion and action plans to fulfill the desired collaboration by their local tasks.Thirdly, continuous relative-motion constraints among the robots, such as collision avoidance and connectivity maintenance, are closely related to the stability,safety and integrity of multi-robot systems. We propose two different hybrid controlapproaches to guarantee the satisfaction of all local tasks and the relative-motionconstraints at all time: the first one is based on potential fields and nonlinear controltechnique; the second uses Embedded Graph Grammars (EGGs) as the main tool.At last, we take into account two common cooperative robotic tasks, namelyservice and formation tasks. These tasks are requested and exchanged among therobots during run time. The proposed hybrid control scheme ensures that the real-time plan execution incorporates not only local tasks of each robot but also thecontingent service and formation tasks it receives.Some of the theoretical results of the thesis have been implemented and demonstrated on various robotic platforms.
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| 2. |
- Johansson, Anna L V
(författare)
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Pregnancy and breast cancer : risk patterns, tumour characteristics and prognosis
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Pregnancy-associated breast cancer (PABC) is commonly defined as a breast cancer (BC) diagnosed during pregnancy or within one or two years after delivery. The risk of BC is lower than expected during pregnancy with a small rebound within 2 years after delivery, compared to non-pregnant women of the same age. Although a rare event, there is evidence that women with PABC have a poorer prognosis compared to women with BC diagnosed not near a pregnancy (non-PABC). It is important to improve the understanding of underlying mechanisms and identify factors that may influence outcome, including the role of detection and tumour biology. The possible influence of pregnancy exposures on detection and tumour biology may be time-varying, with different effects during the pregnancy period compared to time windows further away from delivery. Since a transient risk has been reported 5-10 years post-delivery, it is also of interest to compare pregnancy-associated tumours to tumours diagnosed further away, say up to ten years, from delivery with respect to tumour biology and prognosis. Having a relative with breast cancer is an established risk factor for breast cancer, in particular at a young age. Women with a family history of breast cancer may be more likely to have pre-malignant breast cells at a young age, and could be more susceptible to pregnancy-related exposures, such as elevated levels of endogenous hormones. Study III assessed the risk pattern for breast cancer during pregnancy and up to ten years after delivery in relation to family history of breast cancer using a cohort of 3.5 million women aged 15-44 years identified in the Swedish Multi-Generation Register and the Swedish Cancer Register. In total, 15,548 women had a recorded BC diagnosis of which 1,208 were PABC. To reduce the comparison population and improve computational efficiency, a case-cohort design was utilised. The results showed that a family history of BC did not affect the risk pattern around pregnancy and up to 10 years post-delivery, indicating that pregnancy-related exposures do not interact with the familial pre-disposition for BC in these women. Results from several studies have suggested that BC tumours diagnosed around delivery have adverse tumour characteristics, such as advanced TNM stage and triple-negative subtype, but it remains unclear if adverse tumour characteristics can fully explain the worse prognosis in women with PABC. Studies II and IV assessed TNM stage in cohorts of patients diagnosed with BC during pregnancy and up to ten years post delivery using information from the Swedish Cancer Register for years 2002-2009 (study II) and the Swedish Breast Cancer Quality Register for years 1992-2009 (study IV). Study IV also assessed histological grade and tumour biology, such as hormone receptor status. The results showed that tumours detected during pregnancy and within one year of delivery were more often advanced, of high grade and hormone receptor negative (triple-negative and non-luminal Her2 positive subtypes) compared to nulliparous women. These associations were most pronounced in tumours diagnosed during the first six months post-delivery. In combination with the previously observed lower risk during pregnancy, these findings could reflect diagnostic delays, resulting in more advanced tumours at diagnosis, or a suppression of hormone responsive tumours around delivery. With the aim to investigate prognosis in women with PABC, two partly over-lapping cohorts of BC patients were identified in the Swedish Cancer Register for years 1963-2002 (study I) and in the Breast Cancer Quality Register for years 1992-2009 (study IV), including 1,110 and 778 women with PABC, respectively. The association between PABC and mortality was analysed using Cox regression and flexible parametric models. The results from study I and IV corroborated previous findings that women with PABC have a poorer prognosis compared to women with non-PABC. The mortality was highest in women diagnosed 0-6 months after delivery, and remained elevated up to eight years after diagnosis, in comparison to women diagnosed not near a delivery. However, following adjustment for tumour characteristics, the difference in mortality between women with PABC and women with non-PABC was attenuated and non-significant (study IV). These findings suggest that although PABC is associated with more advanced and aggressive tumours, the outcome is similar to that in women with non-PABC when taking tumour characteristics into account. For comparison, we also assessed tumour characteristics and prognosis in women diagnosed with BC 2-10 years after delivery (study IV). A transient risk of BC around 5-10 years after delivery has been reported previously. Compared to nulliparous women, women diagnosed 2-5 years post-delivery did not differ in tumour size, grade and ER status, but more often presented with spread to lymph nodes, negative PR status and non-luminal Her2 positive subtype. Women with BC diagnosed 2-5 years post-delivery also exhibited a poorer prognosis compared to nulliparous women, which remained significant after adjustment for T/N stage and hormone receptor status. These findings could indicate different mechanisms for risk, detection and prognosis in women diagnosed 2-5 years after delivery compared to women with PABC. In conclusion, the findings indicate that pregnant and lactating women exhibit different patterns of BC risk, tumour biology and prognosis compared to other pre-menopausal women with BC. The poorer prognosis observed in women with PABC can largely be explained by adverse tumour characteristics at diagnosis.
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| 3. |
- Montecchia, Matteo, 1990-
(författare)
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Numerical and modelling aspects of large-eddy and hybrid simulations of turbulent flows
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this study, the explicit algebraic sub-grid scale (SGS) model (EAM) has been extensively validated in wall-resolved large-eddy simulations (LES) of wall-bounded turbulent flows at different Reynolds numbers and a wide range of resolutions. Compared to eddy-viscosity based models, the formulation of the EAM is more consistent with the physics and allows to accurately capture SGS anisotropy,which is relevant especially close to walls.The present work aims to extend the validation of the EAM to larger Reynolds numbers using codes with different orders of numerical accuracy.The first simulations, performed by using a pseudo-spectral code, show that the use of the EAM, compared to the dynamic Smagorinsky model (DSM), leads to significant improvements in the prediction of the first-and second order statistics of turbulent channel flow.These improvements are observed from relatively low to reasonably high Reynolds numbers and with coarse grids.The evaluation of the EAM was continued by implementing and testing of the EAM in the general-purpose finite-volume code OpenFOAM.Several tests of LES of turbulent channel flow have shown thatthe use of the Rhie and Chow (R&C) interpolation in OpenFOAM induces significant numerical dissipation.A new custom-built solver has been utilized in order to minimize the dissipation without generating significant adverse effects. The use of the EAM, together with the new solver, gives a substantially improved prediction of the mean velocity profiles as compared to predictions using the DSM, resulting in roughly 50% reduction in the grid point requirements to achieve a given degree of accuracy. In periodic hill flow, LES with the EAM agreed reasonably well with the reference dataat different bulk Reynolds numbers and reduced the misprediction of the first- and second order statistics observed in LES with DSM.The reduction of the R&C filter dissipation was also shown to be beneficial for the prediction of the mean quantities. An analysis of the skin friction along the lower wall reveals spanwise-elongated, almost axi-symmetric vortical structures generated by the Kelvin-Helmholtz instability. The structures introduced a significant amount of anisotropy.The last part of the study involved the development of a novel hybrid RANS-LES model where explicit algebraic Reynolds stress modelling is applied in both RANS and LES regions.Validations have been conducted on turbulent channel and periodic hill flows at different Reynolds numbers.The explicit algebraic Reynolds stress model for improved-delayed-detached-eddy simulation (EARSM-IDDES) gives reasonable predictions of the mean quantities and Reynolds stresses in both the geometries considered.The use of EARSM-IDDES, compared to the k-omega SST-IDDES model, improves the estimation of the quantities close to the wall.The present work has proven that the use of EAM in wall-resolved LES of wall-bounded flows in simple and complex geometries leads to a substantial reduction of computational requirements both in high-accuracy and general-purpose codes, compared to the use of eddy-viscosity models.In hybrid simulations the EARSM-IDDES shows a clear potential in capturing the physics of wall-bounded flows.
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| 4. |
- Pouransari, Zeinab, 1982-
(författare)
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Numerical studies of turbulent flames in wall-jet flows
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The present thesis deals with the fundamental aspects of turbulent mixing and non-premixed combustion in the wall-jet flow, which has a close resemblance to many industrial applications. Direct numerical simulations (DNS) of turbulent wall-jets with isothermal and exothermic reactions are performed. In the computational domain, fuel and oxidizer enter separately in a nonpremixed manner and the flow is compressible, fully turbulent and subsonic. The triple “turbulence-chemistry-wall” interactions in the wall-jet flow have been addressed first by focusing on turbulent flow effects on the isothermal reaction, and then, by concentrating on heat-release effects on both turbulence and flame characteristics in the exothermic reaction. In the former, the mixing characteristics of the flow, the key statistics for combustion and the near-wall effects in the absence of thermal effects are isolated and studied. In the latter, the main target was to identify the heat-release effects on the different mixing scales of turbulence. Key statistics such as the scalar dissipation rates, time scale ratios, two-point correlations, one and two-dimensional premultiplied spectra are used to illustrate the heat release induced modifications. Finer small mixing scales were observed in the isothermal simulations and larger vortical structures formed after adding significant amounts of heat-release. A deeper insight into the heat release effects on three-dimensional mixing and reaction characteristics of the turbulent wall-jet flow has been gained by digging in different scales of DNS datasets. In particular, attention has been paid to the anisotropy levels and intermittency of the flow by investigating the probability density functions, higher order moments of velocities and reacting scalars and anisotropy invariant maps for different reacting cases. To evaluate and isolate the Damkohler number effects on the reaction zone structure from those of the heat release a comparison between two DNS cases with different Damkohler numbers but a comparable temperature rise is performed. Furthermore, the wall effects on the flame and flow characteristics, for instance, the wall heat transfer; the near-wall combustion effects on the skin-friction, the isothermal wall cooling effects on the average burning rates and the possibility of formation of the premixed mode within the non-premixed flame are addressed. The DNS datasets are also used for a priori analysis, focused on the heat release effects on the subgrid-scale (SGS) statistics. The findings regarding the turbulence small-scale characteristics, gained through the statistical analysis of the flow have many phenomenological parallels with those concerning the SGS statistics. Finally, a DNS of turbulent reacting wall-jet at a substantially higher Reynolds number is performed in order to extend the applicability range for the conclusions of the present study and figuring out the possible differences.
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