More Like this

1. Exit time asymptotics for dynamical systems with fast random switching near an unstable equilibrium

   https://doi.org/10.1142/S0219493720500264  

   Bakhtin, Yuri; Gaál, Alexisz (August 2020, Stochastics and Dynamics)

   null (Ed.)

   We consider the exit problem for a one-dimensional system with random switching near an unstable equilibrium point of the averaged drift. In the infinite switching rate limit, we show that the exit time satisfies a limit theorem with a logarithmic deterministic term and a random correction converging in distribution. Thus, this setting is in the universality class of the unstable equilibrium exit under small white-noise perturbations. 

   more » « less
2. Modeling of jet noise: a perspective from large-eddy simulations

   https://doi.org/http://dx.doi.ord/10.1098/rsta.2019.0081  

   Bres, G. A.; Lele, S. K. (August 2019, Philosophical transactions)

   In the last decade, many research groups have reported predictions of jet noise using high-fidelity large-eddy simulations (LES) of the turbulent jet flow and these methods are beginning to be used more broadly. A brief overview of the publications since the review of Bodony and Lele (2008, AIAA J., Vol. 56, 346-380) is undertaken to assess the progress and overall contributions of LES towards a better understanding of jet noise. In particular, we stress the meshing, numerical and modeling advances which enable detailed geometric representation of the nozzle shape variations intended to impact the noise radiation, and sufficiently accurate capturing of the turbulent boundary layer at the nozzle exit. Examples of how LES is currently being used to complement experiments for challenging conditions (such as highly heated pressure mismatched jets with afterburners) and guide jet modeling efforts are highlighted. Some of the physical insights gained from these numerical studies are discussed, in particular on crackle, screech and shock-associated noise, impingement tones, acoustic analogy models, wave packet dynamics and resonant acoustic waves within the jet core. We close with some perspectives on the remaining challenges and upcoming opportunities for future applications. This article is part of the theme issue `Frontiers of aeroacoustic research: theory, computation and experiment'. 

   more » « less
3. Tipping mechanisms in a carbon cycle model

   https://doi.org/10.1063/5.0241550  

   Slyman, Katherine; Fleurantin, Emmanuel; Jones, Christopher_K_R T (May 2025, Chaos: An Interdisciplinary Journal of Nonlinear Science)

   Rate-induced tipping (R-tipping) occurs when a ramp parameter changes rapidly enough to cause the system to tip between co-existing, attracting states, while noise-induced tipping (N-tipping) occurs when there are random transitions between two attractors of the underlying deterministic system. This work investigates R-tipping and N-tipping events in a carbonate system in the upper ocean, in which the key objective is understanding how the system undergoes tipping away from a stable fixed point in a bistable regime. While R-tipping away from the fixed point fits the framework of an established scenario, N-tipping poses challenges due to a periodic orbit forming the basin boundary for the attracting fixed point of the underlying deterministic system. Furthermore, for N-tipping, we are interested in the situation where noise is away from the small noise limit as it is more appropriate for the application. We postulate that two key points on the basin boundary are critical to understanding the noisy behavior: the exit point of what we find to be the most probable escape path (MPEP), which is determined by the Onsager–Machlup functional, and the pivot point, a point identified through the Maslov index, which appears as an obstacle to the movement of the escape region of noisy trajectories through the periodic orbit as noise increases. 

   more » « less
4. Exploring the potential of exit tickets as formative assessments of student affect

   https://doi.org/10.1080/0969594X.2025.2510206  

   Shehzad, Umar; Recker, Mimi; Clarke-Midura, Jody (March 2025, Assessment in Education: Principles, Policy & Practice)

   This study investigates the use of exit tickets as formative assessments in maths-integrated computer science (CS) lessons for grade 5 students. Exit tickets are brief surveys administered immediately after instructional activities. Using structural equation modelling (SEM), we analysed data from 1,067 students to examine the reliability and validity of exit tickets in predicting summative pre/post survey results. The study found that the exit ticket responses consistently assessed student affect at two administration points, meeting strict measurement invariance criteria (χ2(21) = 1.34, p = 1.00). Confirmatory factor analysis revealed that exit tickets predicted student self-efficacy and interest in CS, which are key educational outcomes. These findings suggest that exit tickets can be valuable tools for enhancing instructional practices and supporting student learning and engagement in CS education. The study concludes with recommendations for effectively implementing exit tickets in educational settings. 

   more » « less
5. Ephemeral Exit Bridges for Tor

   Zhang, Zhao; Vaidya, Tavish; Subramanian, Kartik; Zhou, Wenchao; Sherr, Micah (July 2020, 50th IEEE/IFIP International Conference on Dependable Systems and Networks (DSN 2020))

   This paper examines an existential threat to Tor— the increasing frequency at which websites apply discriminatory behavior to users who arrive via the anonymity network. Our main contribution is the introduction of Tor exit bridges. Exit bridges, constructed as short-lived virtual machines on cloud service providers, serve as alternative egress points for Tor and are designed to bypass server-side censorship. Due to the proliferation of managed cloud-based desktop services (e.g., Amazon Workspaces), there is already a surprisingly large fraction of web requests that originate in the cloud. Trivially disrupting exit bridges by blocking requests from the cloud would thus lead to significant collateral damage. Our experiments demonstrate that exit bridges effectively circumvent server-side blocking of Tor with low overhead. Ad- ditionally, we perform a cost-analysis of exit bridges and show that even a large-scale deployment can be done at low cost. 

   more » « less