arXiv subjects

Kecheng Xu

Publications and source records attributed to Kecheng Xu.

7 recordsLinked to original sources

arXiv2024

Role of Duty Cycle in Burst-Modulated Synthetic Jet Flow Control

The effect of duty cycle (DC) and blowing ratio on synthetic jet flow control over a stalled NACA 0025 airfoil at Re_c=10^5 was investigated experimentally. A finite-span microblower array operating with burst modulation was tested across a wide range of control parameters to assess aerodynamic performance, power consumption, and flow stability. Flow reattachment was achieved once a threshold momentum coefficient was met via increasing either the DC or blowing ratio. Control effectiveness increased sharply upon reattachment, with additional momentum providing incremental improvements in lift, spanwise control, and flow stability, though these effects eventually saturated. Substantial lift improvements are observed at DCs as low as 5%, indicating that brief, high-momentum bursts were the most power-efficient for achieving reattachment. However, flow stability was reduced at low DCs due to the inconsistent streamwise dissipation of spanwise vortices responsible for flow control. Higher DC control strategies resulted in more consistent boundary layer control. These results provide a framework for selecting control strategies that balance aerodynamic performance and stability with power efficiency.

physics.flu-dyn↗

arXiv2024

Spanwise Control Authority of Synthetic Jets on a Stalled Airfoil

This study investigates the aerodynamic effects of low- and high-frequency synthetic jet control strategies on a National Advisory Committee for Aeronautics (NACA) 0025 airfoil. Visualizations and measurements are employed to assess the stability of the flow, focusing on the shear layer and wake dynamics under two forcing frequencies. High-frequency actuation is found to induce steadier flow reattachment and more favorable aerodynamic characteristics compared to low-frequency control. Flow structures resulting from high-frequency actuation, notably vortex rings, are identified and their significance in flow control is evaluated. Furthermore, the spanwise control authority is analyzed, revealing variations in aerodynamic stability away from the midspan. Insights from modal analysis provide additional understanding of flow structures and their evolution across different spanwise planes.

physics.flu-dyn↗

arXiv2024

On the Role of the Wall Curvature in the Development of Flows Reattached over an Airfoil through Unsteady Blowing

An array of twelve circular-orifice synthetic jet actuators (SJAs) was used to provide the unsteady forcing required for flow separation control over a National Advisory Committee for Aeronautics (NACA) 0025 airfoil at a chord-based Reynolds number of 100000 and an angle of attack of 10°. Two distinct high- and low-forcing frequencies corresponding to the shear layer and wake instabilities were used at an identical blowing strength for flow control. Particle image velocimetry (PIV) was used to measure the velocity fields at the centerline of the airfoil. The results showed the presence of a turbulent shear layer stretching from the edge of the reattached boundary layer to the irrotational flow with an invariant mean spanwise vorticity in the wall-normal direction. It was revealed that the coherent structures for the high-frequency controlled case are advected along the boundary of the rigid-body rotation shear layer and the irrotational flow, whereas for the low-frequency actuation, some structures directly pass through the rigid-body rotation region, disrupting the wall-normal balance of vorticity. Analytical expressions were derived for the variation of the mean spanwise vorticity in the rigid-body rotation region and the curvature-multiplied mean angular momentum in the irrotational flow region based on order-of-magnitude analysis and semi-empirical grounds. The resulting patterns showed an excellent agreement with the measured experimental data.

physics.flu-dyn↗

arXiv2023

Visualizing Three-Dimensional Effects of Synthetic Jet Flow Control

This study investigates the three-dimensionality of synthetic jet flow control over a NACA 0025 profile wing using horizontal and vertical smoke wire visualization. The stalled flow in the baseline case is visualized, providing insights into the shear layer roll-up process, the transition to turbulence, and vortex shedding in the wake. In the controlled flow study, two actuation frequencies, $F^+=1.18$ and $F^+=11.76$, are investigated, with a focus on spanwise control authority and the role of coherent structures in flow reattachment. The results indicate that while the control is effective at the midspan over the entire chord length, its effect diminishes with increasing distance from the midspan. Both control cases result in significant spanwise velocities, observed by a contraction of the flow towards midspan. Lastly, the high-frequency actuation results in unique small-scale structures at the shear layer-freestream interface.

physics.flu-dyn↗

arXiv2021

Emergency Vehicles Audio Detection and Localization in Autonomous Driving

Emergency vehicles in service have right-of-way over all other vehicles. Hence, all other vehicles are supposed to take proper actions to yield emergency vehicles with active sirens. As this task requires the cooperation between ears and eyes for human drivers, it also needs audio detection as a supplement to vision-based algorithms for fully autonomous driving vehicles. In urban driving scenarios, we need to know both the existence of emergency vehicles and their relative positions to us to decide the proper actions. We present a novel system from collecting the real-world siren data to the deployment of models using only two cost-efficient microphones. We are able to achieve promising performance for each task separately, especially within the crucial 10m to 50m distance range to react (the size of our ego vehicle is around 5m in length and 2m in width). The recall rate to determine the existence of sirens is 99.16% , the median and mean angle absolute error is 9.64° and 19.18° respectively, and the median and mean distance absolute error of 9.30m and 10.58m respectively within that range. We also benchmark various machine learning approaches that can determine the siren existence and sound source localization which includes direction and distance simultaneously within 50ms of latency.

cs.SD↗

arXiv2021

Lane Attention: Predicting Vehicles' Moving Trajectories by Learning Their Attention over Lanes

Accurately forecasting the future movements of surrounding vehicles is essential for safe and efficient operations of autonomous driving cars. This task is difficult because a vehicle's moving trajectory is greatly determined by its driver's intention, which is often hard to estimate. By leveraging attention mechanisms along with long short-term memory (LSTM) networks, this work learns the relation between a driver's intention and the vehicle's changing positions relative to road infrastructures, and uses it to guide the prediction. Different from other state-of-the-art solutions, our work treats the on-road lanes as non-Euclidean structures, unfolds the vehicle's moving history to form a spatio-temporal graph, and uses methods from Graph Neural Networks to solve the problem. Not only is our approach a pioneering attempt in using non-Euclidean methods to process static environmental features around a predicted object, our model also outperforms other state-of-the-art models in several metrics. The practicability and interpretability analysis of the model shows great potential for large-scale deployment in various autonomous driving systems in addition to our own.

cs.LG↗

arXiv2020

Data Driven Prediction Architecture for Autonomous Driving and its Application on Apollo Platform

Autonomous Driving vehicles (ADV) are on road with large scales. For safe and efficient operations, ADVs must be able to predict the future states and iterative with road entities in complex, real-world driving scenarios. How to migrate a well-trained prediction model from one geo-fenced area to another is essential in scaling the ADV operation and is difficult most of the time since the terrains, traffic rules, entities distributions, driving/walking patterns would be largely different in different geo-fenced operation areas. In this paper, we introduce a highly automated learning-based prediction model pipeline, which has been deployed on Baidu Apollo self-driving platform, to support different prediction learning sub-modules' data annotation, feature extraction, model training/tuning and deployment. This pipeline is completely automatic without any human intervention and shows an up to 400\% efficiency increase in parameter tuning, when deployed at scale in different scenarios across nations.

cs.RO↗