المؤلفون: Cooke, Justin P., Jerolmack, Douglas J., Park, George I.

مصطلحات موضوعية: Physics - Fluid Dynamics, Physics - Atmospheric and Oceanic Physics

الوصف: Landforms such as sand dunes act as roughness elements to Atmospheric Boundary Layer (ABL) flows, triggering the development of new scales of turbulent motions. These turbulent motions, in turn, energize and kick-up sand particles, influencing sediment transport and ultimately the formation and migration of dunes -- with knock on consequences for dust emission. While feedbacks between flow and form have been studied at the scale of dunes, research has not examined how the development of an Internal Boundary Layer (IBL) over the entire dune field influences sediment-transporting turbulence. Here, we deploy large-eddy simulation of an ABL encountering a natural roughness transition: the sand dunes at White Sands National Park, New Mexico. We analyze turbulence producing motions and how they change as the IBL grows over the dune field. Frequency spectrum and Reynolds shear stress profiles show that IBL thickness determines the largest scales of turbulence. More, the developing IBL enhances the frequency, magnitude and duration of sweep and ejection events -- turbulence producing motions whose peaks systematically migrate away from the wall as the IBL thickens. Because sweep and ejection events are known to drive sediment transport, our findings provide a mechanism for coupling the co-evolution of the landscape and the ABL flow over it. More broadly, our results have implications for how roughness transitions influence the transport of pollutants, particulates, heat, and moisture.
Comment: 31 pages, 15 figures, 2 tables

الوصول الحر: http://arxiv.org/abs/2406.05163Test

المؤلفون: Austin, George I., Pe'er, Itsik, Korem, Tal

مصطلحات موضوعية: Statistics - Methodology, Computer Science - Machine Learning, Quantitative Biology - Quantitative Methods

الوصف: Cross-validation is a common method for estimating the predictive performance of machine learning models. In a data-scarce regime, where one typically wishes to maximize the number of instances used for training the model, an approach called "leave-one-out cross-validation" is often used. In this design, a separate model is built for predicting each data instance after training on all other instances. Since this results in a single test data point available per model trained, predictions are aggregated across the entire dataset to calculate common rank-based performance metrics such as the area under the receiver operating characteristic or precision-recall curves. In this work, we demonstrate that this approach creates a negative correlation between the average label of each training fold and the label of its corresponding test instance, a phenomenon that we term distributional bias. As machine learning models tend to regress to the mean of their training data, this distributional bias tends to negatively impact performance evaluation and hyperparameter optimization. We show that this effect generalizes to leave-P-out cross-validation and persists across a wide range of modeling and evaluation approaches, and that it can lead to a bias against stronger regularization. To address this, we propose a generalizable rebalanced cross-validation approach that corrects for distributional bias. We demonstrate that our approach improves cross-validation performance evaluation in synthetic simulations and in several published leave-one-out analyses.
Comment: 20 pages, 5 figures, supplementary information

الوصول الحر: http://arxiv.org/abs/2406.01652Test

المؤلفون: Mitra, Dipanjan, Basu, Rahul, Melikizde, George I

مصطلحات موضوعية: Astrophysics - High Energy Astrophysical Phenomena

الوصف: Radio observations from normal pulsars indicate that the coherent radio emission is excited by curvature radiation from charge bunches. In this review we provide a systematic description of the various observational constraints on the radio emission mechanism. We have discussed the presence of highly polarized time samples where the polarization position angle follow two orthogonal well defined tracks across the profile, that closely match the rotating vector model in an identical manner. The observations also show the presence of circular polarization, with both the right and left handed circular polarization seen across the profile. Other constraints on the emission mechanism is provided by the detailed measurements of the spectral index variation across the profile window, where the central part of the profile, corresponding to the core component, has a steeper spectrum than the surrounding cones. Finally, the detailed measurements of the subpulse drifting behaviour can be explained by considering the presence of non-dipolar field on the stellar surface and the formation of the Partially screened Gap (PSG) above the polar cap region. The PSG gives rise to a non-stationary plasma flow, that has a multi-component nature, consisting of highly energetic primary particles, secondary pair plasma and iron ions discharged from the surface, with large fragmentation resulting is dense plasma clouds and lower density inter-cloud regions. The physical properties of the outflowing plasma and the observational constraints lead us to consider coherent curvature radiation as the most viable explanation for the emission mechanism in normal pulsars, where propagation effects due to adiabatic walking and refraction are largely inconsequential.
Comment: 31 pages, accepted for publication as review article for special issue "A New Horizon of Pulsar and Neutron Star: The 55-Year Anniversary", Journal Universe

الوصول الحر: http://arxiv.org/abs/2405.20634Test

المؤلفون: Cooke, Justin P., Jerolmack, Douglas J., Park, George I.

مصطلحات موضوعية: Physics - Atmospheric and Oceanic Physics, Physics - Fluid Dynamics, Physics - Geophysics

الوصف: The structure and intensity of turbulence in the atmospheric boundary layer (ABL) drives fluxes of sediment, contaminants, heat, moisture and CO$_2$ at the Earth's surface. Where ABL flows encounter changes in roughness -- such as cities, wind farms, forest canopies and landforms -- a new mesoscopic flow scale is introduced: the internal boundary layer (IBL), which represents a near-bed region of transient flow adjustment that develops over kilometers. This important scale lies within a gap in present observational capabilities of ABL flows, and simplified models fail to capture the sensitive dependence of turbulence on roughness geometry. Here we use large-eddy simulations, run over high-resolution topographic data and validated against field observations, to examine the structure of the ABL across a natural roughness transition: the emergent sand dunes at White Sands National Park. We observe that development of the IBL is triggered by the abrupt transition from smooth playa surface to dunes; however, continuous changes in the size and spacing of dunes over several kilometers influence the downwind patterns of boundary stress and near-bed turbulence. Coherent flow structures grow and merge over the entire $\sim$10-km distance of the dune field, and modulate the influence of large-scale atmospheric turbulence on the bed. Simulated boundary stresses in the developing IBL explain the observed downwind decrease in dune migration, demonstrating a mesoscale coupling between flow and form that governs landscape dynamics. More broadly, our findings demonstrate the importance of resolving both turbulence and realistic roughness for understanding fluid-boundary interactions in environmental flows.
Comment: 11 Pages, 4 Figures, 76 References

الوصول الحر: http://arxiv.org/abs/2311.03575Test

المؤلفون: Van Mieghem, NM, Deeb, GM, Sondergaard, L, Grube, E, Windecker, S, Gada, H, Mumtaz, M, Olsen, PS, Heiser, JC, Merhi, W, Kleiman, NS, Chetcuti, SJ, Gleason, TG, Lee, JS, Cheng, W, Makkar, RR, Crestanello, J, George, B, George, I, Kodali, S, Yakubov, SJ, Serruys, PW, Lange, R, Piazza, N, Williams, MR, Oh, JK, Adams, DH, Li, SZ, Reardon, MJ

المصدر: JAMA cardiology. 7(10):1000-1008

مصطلحات موضوعية: Medicin och hälsovetenskap

الوصول الحر: http://kipublications.ki.se/Default.aspx?queryparsed=id:150561984Test

المؤلفون: Basu, Rahul, Mitra, Dipanjan, Melikidze, George I.

مصطلحات موضوعية: Astrophysics - High Energy Astrophysical Phenomena

الوصف: We have studied the single pulse emission from two pulsars, PSR B0844-35 and PSR B1758-29, over a wide frequency range of 300-750 MHz using the uGMRT. The two pulsars have relatively wide profiles with multiple components, that are a result of the line of sight traversing near center of the emission beam. In both pulsars the single pulse sequences show the presence of two distinct emission states, where the profiles become much brighter at the center with prominent core components during one of the modes, while in the other mode the single pulses show odd-even subpulse drifting with periodicity around 2$P$, $P$ being the rotation period of the pulsar. The centrally bright mode was seen for 10 percent of the observing duration in PSR B0844-35, which usually lasted for short durations of around 10 pulses, but had two longer sequences of around 100 pulses. On the contrary the centrally bright mode was dominant in PSR B1758-29 and was seen for around 60 percent of the observing duration. PSR B1758-29 also showed period amplitude modulations of 60-70$P$ in both modes. The mode changing in these two pulsars facilitates investigation of the sparking process in the inner acceleration region, dominated by non-dipolar magnetic fields. The change in the surface magnetic field configurations likely results in the emission mode change.
Comment: 24 pages, 14 figures, accepted for publication in ApJ

الوصول الحر: http://arxiv.org/abs/2310.19725Test

المؤلفون: Bin, Yuanwei, Park, George I., Lv, Yu, Yang, Xiang I. A.

مصطلحات موضوعية: Physics - Fluid Dynamics

الوصف: We examine and benchmark the emerging idea of applying the large-eddy simulation (LES) formalism to unconventionally coarse grids where RANS would be considered more appropriate at first glance. We distinguish this idea from very-large-eddy-simulation (VLES) and detached-eddy-simulation (DES), which require switching between RANS and LES formalism. LES on RANS grid is appealing because first, it requires minimal changes to a production code; second, it is more cost-effective than LES; third, it converges to LES; and most importantly, it accurately predicts flows with separation. This work quantifies the benefit of LES on RANS-like grids as compared to RANS on the same grids. Three canonical cases are considered: periodic hill, backward-facing step, and jet in cross flow. We conduct direct numerical simulation (DNS), proper LES on LES grids, LES on RANS-quality grids, and RANS. We show that while the LES solutions on the RANS-quality grids are not grid converged, they are twice as accurate as the RANS on the same grids.

الوصول الحر: http://arxiv.org/abs/2310.09367Test

المؤلفون: Chen, Peng E S, Bin, Yuanwei, Yang, Xiang I A, Shi, Yipeng, Abkar, Mahdi, Park, George I.

مصطلحات موضوعية: Physics - Fluid Dynamics

الوصف: Assessing the compliance of a white-box turbulence model with known turbulent knowledge is straightforward. It enables users to screen conventional turbulence models and identify apparent inadequacies, thereby allowing for a more focused and fruitful validation and verification. However, comparing a black-box machine-learning model to known empirical scalings is not straightforward. Unless one implements and tests the model, it would not be clear if a machine-learning model, trained at finite Reynolds numbers preserves the known high Reynolds number limit. This is inconvenient, particularly because model implementation involves retraining and re-interfacing. This work attempts to address this issue, allowing fast a priori screening of machine-learning models that are based on feed-forward neural networks (FNN). The method leverages the mathematical theorems we present in the paper. These theorems offer estimates of a network's limits even when the exact weights and biases are unknown. For demonstration purposes, we screen existing machine-learning wall models and RANS models for their compliance with the log layer physics and the viscous layer physics in a priori manner. In addition, the theorems serve as essential guidelines for future machine-learning models.

الوصول الحر: http://arxiv.org/abs/2310.09366Test

المؤلفون: Shahim, B, Cohen, DJ, Asch, FM, Bax, J, George, I, Rück, A, Ben-Yehuda, O, Kar, S, Lim, DS, Saxon, JT, Zhou, Z, Lindenfeld, J, Abraham, WT, Mack, MJ, Stone, GW

المصدر: The American journal of cardiology. 223:7-14

مصطلحات موضوعية: Medicin och hälsovetenskap

الوصول الحر: http://kipublications.ki.se/Default.aspx?queryparsed=id:238788821Test

المؤلفون: Cooke, Justin P, Campbell, Matthew F, Steager, Edward B, Bargatin, Igor, Yim, Mark H, Park, George I

مصطلحات موضوعية: Physics - Fluid Dynamics

الوصف: Micro aerial vehicles are making a large impact in applications such as search-and-rescue, package delivery, and recreation. Unfortunately, these diminutive drones are currently constrained to carrying small payloads, in large part because they use propellers optimized for larger aircraft and inviscid flow regimes. Fully realizing the potential of emerging microflyers requires next-generation propellers that are specifically designed for low-Reynolds number conditions and that include new features advantageous in highly viscous flows. One aspect that has received limited attention in the literature is the addition of roughness to propeller blades as a method of reducing drag and increasing thrust. To investigate this possibility, we used large eddy simulation to conduct a numerical investigation of smooth and rough propellers. Our results indicate that roughness produces a 2% increase in thrust and a 5% decrease in power relative to a baseline smooth propeller operating at the same Reynolds number of Rec = 6500, held constant by rotational speed. We corroborated our numerical findings using thrust-stand-based experiments of 3D-printed propellers identical to those of the numerical simulations. Our study confirms that surface roughness is an additional parameter within the design space for micro-propellers that will lead to unprecedented drone efficiencies and payloads.
Comment: 23 Pages, 9 Figures

الوصول الحر: http://arxiv.org/abs/2306.17087Test