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Aerodynamics of Low Pressure Steam Turbine Exhaust Systems
The low pressure (LP) exhaust system presents a promising avenue for improving the performance of large steam turbines. For this reason, LP exhaust systems have attracted the attention of the research community for decades. Nevertheless, we still lack understanding of the flow physics and loss mechanisms in the exhaust system, especially at part-load conditions. It is also unclear how the exhaust system should be designed when its required operating range widens. This thesis provides solutions to these aerodynamic issues through experimental and numerical investigations, and provides tools that could contribute to better designs of LP exhaust systems.
Firstly, the Computational Fluid Dynamics (CFD) solver ANSYS CFX was validated against experiments performed on a scaled test rig under representative part-load flow conditions. This validation exposed the weakness of Reynolds-averaged Navier–Stokes (RANS) CFD when there is a highly swirling flow and large separation regions in the exhaust diffuser.
To facilitate the numerical studies, a series of tools were also developed. A design suite, ExhaustGen, was used to automate the pre- and post-processing of CFD calculations. The exhaust diffuser was parametrised using "Minimum Energy Curves", which reduce the dimension of parameter space. Further, a suitable stage-hood interface treatment (Multiple Mixing Planes) was chosen to predict the circumferentially non-uniform flow in the exhaust hood at low computational cost.
Numerical investigation of the baseline geometry provided insights into the key flow features and loss mechanisms in the exhaust system, over a wide range of operating conditions. In particular, the bearing cone separation was identified as a key source of loss at part-load conditions. The effect of stage-hood interaction on the performance and design of the exhaust system was studied by varying the rotor blade design, which can positively influence system performance.
Finally, a global sensitivity study was performed to identify the most influential design parameters of the exhaust hood. These findings allow, for the first time, LP exhaust hood performance maps to be constructed, so that the benefits of choosing a suitable hood geometry and blade design can be revealed. The thesis also offers contribution towards formulating LP exhaust system design guidance for a wide operating range
How would Stance Detection Techniques Evolve after the Launch of ChatGPT?
Stance detection refers to the task of extracting the standpoint (Favor,
Against or Neither) towards a target in given texts. Such research gains
increasing attention with the proliferation of social media contents. The
conventional framework of handling stance detection is converting it into text
classification tasks. Deep learning models have already replaced rule-based
models and traditional machine learning models in solving such problems.
Current deep neural networks are facing two main challenges which are
insufficient labeled data and information in social media posts and the
unexplainable nature of deep learning models. A new pre-trained language model
chatGPT was launched on Nov 30, 2022. For the stance detection tasks, our
experiments show that ChatGPT can achieve SOTA or similar performance for
commonly used datasets including SemEval-2016 and P-Stance. At the same time,
ChatGPT can provide explanation for its own prediction, which is beyond the
capability of any existing model. The explanations for the cases it cannot
provide classification results are especially useful. ChatGPT has the potential
to be the best AI model for stance detection tasks in NLP, or at least change
the research paradigm of this field. ChatGPT also opens up the possibility of
building explanatory AI for stance detection
Multi-modal Facial Affective Analysis based on Masked Autoencoder
Human affective behavior analysis focuses on analyzing human expressions or
other behaviors to enhance the understanding of human psychology. The CVPR 2023
Competition on Affective Behavior Analysis in-the-wild (ABAW) is dedicated to
providing high-quality and large-scale Aff-wild2 for the recognition of
commonly used emotion representations, such as Action Units (AU), basic
expression categories(EXPR), and Valence-Arousal (VA). The competition is
committed to making significant strides in improving the accuracy and
practicality of affective analysis research in real-world scenarios. In this
paper, we introduce our submission to the CVPR 2023: ABAW5. Our approach
involves several key components. First, we utilize the visual information from
a Masked Autoencoder(MAE) model that has been pre-trained on a large-scale face
image dataset in a self-supervised manner. Next, we finetune the MAE encoder on
the image frames from the Aff-wild2 for AU, EXPR and VA tasks, which can be
regarded as a static and uni-modal training. Additionally, we leverage the
multi-modal and temporal information from the videos and implement a
transformer-based framework to fuse the multi-modal features. Our approach
achieves impressive results in the ABAW5 competition, with an average F1 score
of 55.49\% and 41.21\% in the AU and EXPR tracks, respectively, and an average
CCC of 0.6372 in the VA track. Our approach ranks first in the EXPR and AU
tracks, and second in the VA track. Extensive quantitative experiments and
ablation studies demonstrate the effectiveness of our proposed method
Multiscale characterization of thermoacoustic response and fatigue failure of aerospace structures
Sustained hypersonic flight has presented a complex problem to researchers and structural designers in recent decades as it has been seen to induce failure of thin aerospace panels in modes that had not been previously accounted for. These new and unaccounted for failure modes are attributed to the extreme and coupled loading conditions of the thermoacoustic environment prevalent in hypersonic flight. Prior research has highlighted the resonant behavior of simple structures in a combined loading environment of vibration and heating. The effects of various heating distributions on pre-thermally and post-thermally buckled plates have been evaluated in theoretical and experimental work. However, this understanding has not yet found its way into advanced thermomechanical coupled simulations, in part because fatigue failure caused by in-plane thermal gradients from localized heating, vibration, and mechanical boundary conditions has not been sufficiently addressed in the laboratory setting to validate such complex simulations. The present work seeks to add to our current understanding of this topic with a series of experiments which investigate structural response and failure at multiple length scales. Non-contact optical methods for displacement and strain measurement were used to study the resonance, broadband excitation response, and thermal loading response of structures with varying boundary conditions. Thin aerospace-type beams and plates made of a nickel super-alloy, Hastelloy X, Al 1100-O, and Al 1100-H14 were subjected to forced vibration initially at room temperature and subsequently with localized heating to examine the effects of thermal stress gradients on structural response. Coarse-grained specimens were then produced by annealing aluminum Al 1100-O (commercially pure Al) to explore the role of microstructural phenomena in the thermoacoustic environment and their influences on global behavior. Using oligocrystal samples in this fashion made the grain scale effects occur at the same scale as the sample size and thus both effects could be investigated simultaneously. The microstructural heterogeneity of coarse-grained beams was shown to have significant effect on plastic hinging behavior at the beam root. Finally, fatigue experiments were performed in a combined loading environment to assess behavior beyond the linear elastic regime and promote plasticity and failure. Although fatigue failure was suppressed in thin beams and panels, adding a stress concentrator, such as a notch near the beam root, promoted fatigue crack nucleation
TrTr: A Versatile Pre-Trained Large Traffic Model based on Transformer for Capturing Trajectory Diversity in Vehicle Population
Understanding trajectory diversity is a fundamental aspect of addressing
practical traffic tasks. However, capturing the diversity of trajectories
presents challenges, particularly with traditional machine learning and
recurrent neural networks due to the requirement of large-scale parameters. The
emerging Transformer technology, renowned for its parallel computation
capabilities enabling the utilization of models with hundreds of millions of
parameters, offers a promising solution. In this study, we apply the
Transformer architecture to traffic tasks, aiming to learn the diversity of
trajectories within vehicle populations. We analyze the Transformer's attention
mechanism and its adaptability to the goals of traffic tasks, and subsequently,
design specific pre-training tasks. To achieve this, we create a data structure
tailored to the attention mechanism and introduce a set of noises that
correspond to spatio-temporal demands, which are incorporated into the
structured data during the pre-training process. The designed pre-training
model demonstrates excellent performance in capturing the spatial distribution
of the vehicle population, with no instances of vehicle overlap and an RMSE of
0.6059 when compared to the ground truth values. In the context of time series
prediction, approximately 95% of the predicted trajectories' speeds closely
align with the true speeds, within a deviation of 7.5144m/s. Furthermore, in
the stability test, the model exhibits robustness by continuously predicting a
time series ten times longer than the input sequence, delivering smooth
trajectories and showcasing diverse driving behaviors. The pre-trained model
also provides a good basis for downstream fine-tuning tasks. The number of
parameters of our model is over 50 million.Comment: 16 pages, 6 figures, under reviewed by Transportation Research Board
Annual Meeting, work in updat
Correlation Filters for Unmanned Aerial Vehicle-Based Aerial Tracking: A Review and Experimental Evaluation
Aerial tracking, which has exhibited its omnipresent dedication and splendid
performance, is one of the most active applications in the remote sensing
field. Especially, unmanned aerial vehicle (UAV)-based remote sensing system,
equipped with a visual tracking approach, has been widely used in aviation,
navigation, agriculture,transportation, and public security, etc. As is
mentioned above, the UAV-based aerial tracking platform has been gradually
developed from research to practical application stage, reaching one of the
main aerial remote sensing technologies in the future. However, due to the
real-world onerous situations, e.g., harsh external challenges, the vibration
of the UAV mechanical structure (especially under strong wind conditions), the
maneuvering flight in complex environment, and the limited computation
resources onboard, accuracy, robustness, and high efficiency are all crucial
for the onboard tracking methods. Recently, the discriminative correlation
filter (DCF)-based trackers have stood out for their high computational
efficiency and appealing robustness on a single CPU, and have flourished in the
UAV visual tracking community. In this work, the basic framework of the
DCF-based trackers is firstly generalized, based on which, 23 state-of-the-art
DCF-based trackers are orderly summarized according to their innovations for
solving various issues. Besides, exhaustive and quantitative experiments have
been extended on various prevailing UAV tracking benchmarks, i.e., UAV123,
UAV123@10fps, UAV20L, UAVDT, DTB70, and VisDrone2019-SOT, which contain 371,903
frames in total. The experiments show the performance, verify the feasibility,
and demonstrate the current challenges of DCF-based trackers onboard UAV
tracking.Comment: 28 pages, 10 figures, submitted to GRS
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