On closures for reduced order models—A spectrum of first-principle to machine-learned avenues

Physics of Fluids, Journal Year: 2021, Volume and Issue: 33(9)

Published: Sept. 1, 2021

For over a century, reduced order models (ROMs) have been fundamental discipline of theoretical fluid mechanics. Early examples include Galerkin inspired by the Orr–Sommerfeld stability equation and numerous vortex models, which von Kármán street is one most prominent. Subsequent ROMs typically relied on first principles, like mathematical weakly nonlinear theory, two- three-dimensional models. Aubry et al. [J. Fluid Mech. 192, 115–173 (1988)] pioneered data-driven proper orthogonal decomposition (POD) modeling. In early POD modeling, available data were used to build an optimal basis, was then utilized in classical procedure construct ROM, but made profound impact beyond expansion. this paper, we take modest step illustrate modeling significant ROM area. Specifically, focus closures, are correction terms that added model effect discarded modes under-resolved simulations. Through simple examples, main principles ROMs, motivate introduce modern show how artificial intelligence, machine learning changed standard methodology last two decades. Finally, outline our vision state-of-the-art can continue reshape field

Language: Английский

---

Generation of gravitational waves from freely decaying turbulence

Journal of Cosmology and Astroparticle Physics, Journal Year: 2022, Volume and Issue: 2022(09), P. 029 - 029

Published: Sept. 1, 2022

We study the stochastic gravitational wave background (SGWB) produced by freely decaying vortical turbulence in early Universe. thoroughly investigate time correlation of velocity field, and hence anisotropic stresses producing waves. With hydrodynamical simulations, we show that unequal function (UETC) Fourier components field is Gaussian difference, as predicted "sweeping" decorrelation model. introduce a model can be extended to wavelengths around integral scale flow. Supplemented with evolution laws kinetic energy scale, this provides new UETC turbulent consistent simulations. discuss positive definite kernel, propose use Gibbs kernel for natural way ensure definiteness SGWB. The SGWB given 4-dimensional integration resulting stress Green's function. perform using Monte Carlo algorithm based on importance sampling, find result matches Furthermore, obtained from numerical simulations close agreement which source constant abruptly turns off after few eddy turnover times. Based assumption, provide an approximate analytical form spectrum its scaling initial scale. Finally, our including growth phase flow heavily influences spectral shape This highlights complete understanding generation mechanism.

Language: Английский

---

Recent Developments in Theories of Inhomogeneous and Anisotropic Turbulence

Annual Review of Fluid Mechanics, Journal Year: 2022, Volume and Issue: 55(1), P. 351 - 375

Published: Oct. 13, 2022

Understanding inhomogeneous and anisotropic fluid flows requires mathematical computational tools that are tailored to such distinct from methods used understand the canonical problem of homogeneous isotropic turbulence. We review some recent developments in theory turbulence, placing special emphasis on several kinds quasi-linear approximations their corresponding statistical formulations. Aspects have received insufficient attention literature discussed, open questions framed.

Language: Английский

---

Simulations of three-layer Richtmyer–Meshkov mixing in a shock tube

Physics of Fluids, Journal Year: 2024, Volume and Issue: 36(1)

Published: Jan. 1, 2024

The Richtmyer–Meshkov instability causes perturbations to grow after a shock traverses fluid density interface. This increases the mixing rate between from either side of We use Flash Eulerian hydrodynamic code investigate alterations when thin third layer intermediate is placed along interface, effectively creating two adjacent unstable interfaces. common occurrence in engineering applications where barrier initially separates materials. find that width similar or slightly reduced; however, total mass mixed material can actually increase. becomes more compact and efficient. However, normalized decreases, meaning finger entrainment important than simple two-layer case. effect adding central appears decrease Atwood number decreased. results are also benchmarked against experimental data tube at University Arizona.

Language: Английский

---

What is the Reynolds Number of the Solar Wind?

The Astrophysical Journal, Journal Year: 2024, Volume and Issue: 961(2), P. 182 - 182

Published: Jan. 25, 2024

Abstract The Reynolds number, Re , is an important quantity for describing a turbulent flow. It tells us about the bandwidth over which energy can cascade from large scales to smaller ones, prior onset of dissipation. However, calculating it nearly collisionless plasmas like solar wind challenging. Previous studies have used formulations “effective” expressing as function correlation scale and either Taylor or proxy dissipation scale. We find that definition number has sizable prefactor approximately 27, not been employed in previous works. Drawing 18 years data Wind spacecraft at 1 au, we calculate magnetic directly use both ion inertial length spectrum break approximations scale, yielding three distinct estimates each 12 hr interval. Average values range between 116,000 3,406,000 within general distribution past work. also considerable disagreement methods, with linear associations 0.38 0.72. Although method arguably more physically motivated, due its dependence on rate, theoretical work needed order identify most appropriate way effective numbers kinetic plasmas. As summary our observational analysis, make available product 28 au magnetospheric plasma measurements Wind.

Language: Английский

---

Stochastic Methods and Complexity Science in Climate Research and Modeling

Frontiers in Physics, Journal Year: 2022, Volume and Issue: 10

Published: June 6, 2022

The 2021 Nobel prize for physics was awarded to two climate scientists, Syukuro Manabe and Klaus Hasselmann, the physicist Giorgio Parisi. While at first sight work of Parisi seems not be related science, this is case. developed contributed many complexity science methods which are nowadays widely used in science. Giorgi also involved development “stochastic resonance” idea explain paleoclimate variability, while Hasselmann stochastic models. Here we review discuss their from a complex systems perspective order highlight those aspects work. For instance, fractal multi-fractal analysis data now weather prediction models contain parameterizations, topics have pioneered. Furthermore, Manabe’s key understanding effects anthropogenic change by advances parameterization convection radiative forcing We how inventive research has shaped current still influencing modeling future directions.

Language: Английский

---

Three-dimensional direct numerical simulation of Rayleigh–Taylor instability triggered by acoustic excitation

Physics of Fluids, Journal Year: 2022, Volume and Issue: 34(5)

Published: April 25, 2022

Rayleigh–Taylor instability (RTI) occurs when the interface between two fluids of different densities is removed, with heavier (cold) fluid resting on top lighter (hot) in equilibrium state. This arrangement unstable due to buoyancy, absence any other forces. RTI noted across a range length scales from very small nuclear fusion supernova explosion at astrophysical scales. viewed as baroclinic if viscous actions are ignored. An accurate non-overlapping parallel algorithm used solve three-dimensional problem, employing more than 4 × 109 points and refined time step (7.69×10−8s) for direct numerical simulation. Air masses temperatures initially separated by non-conducting partition inside box (with temperature difference 200 K). The impermeable removed impulsively t = 0, ensuing triggered an acoustic mechanism involving infra ultrasonic pulses that travel either side interface. Present high precision petascale computations enable one capture disturbances unprecedented accuracy without additional interfacial disturbances. creation vorticity studied performing enstrophy budget compressible flow RTI, which shows terms dominant compared one.

Language: Английский

---

The environment to the rescue: can physics help predict predator–prey interactions?

Biological reviews/Biological reviews of the Cambridge Philosophical Society, Journal Year: 2024, Volume and Issue: unknown

Published: June 10, 2024

Understanding the factors that determine occurrence and strength of ecological interactions under specific abiotic biotic conditions is fundamental since many aspects community stability ecosystem functioning depend on patterns among species. Current approaches to mapping food webs are mostly based traits, expert knowledge, experiments, and/or statistical inference. However, they do not offer clear mechanisms explaining how trophic affected by interplay between organism characteristics physical environment, such as temperature, light intensity or viscosity. Hence, cannot yet predict accurately local will respond anthropogenic pressures, notably climate change species invasions. Herein, we propose a framework synthesises recent developments in food-web theory, integrating body size metabolism with properties ecosystems. We advocate for combination movement paradigm modular definition predation sequence, because central predator-prey interactions, generic, model needed describe all possible variation interactions. Pending sufficient empirical theoretical our help impacts well-studied factors, temperature oxygen availability, well less commonly considered variables wind, turbidity electrical conductivity. An improved predictive capability facilitate better understanding responses changing world.

Language: Английский

---

Kinetic energy spectrum and transfer within the near-dissipation range of compressible isotropic turbulence

Physics of Fluids, Journal Year: 2025, Volume and Issue: 37(1)

Published: Jan. 1, 2025

The analysis of kinetic energy spectrum and dissipation mechanisms plays a crucial role in both theoretical examination real-world implementations within compressible turbulent flows. previous works the near-dissipation range are conducted framework incompressible flow dynamics. We investigate transfer isotropic flows via highly resolved direct numerical simulations with different Mach numbers (Mt). found that flows, model is significantly influenced by number, more pronounced tail as number increases. Within range, there exist strong local spatial fluctuations flux increase grid resolution. viscous can be decomposed into solenoidal dilatational components, its component leads to stronger intermittency range. Besides, high also dissipation, which serves an alternative factor for uncovered provides foundation prediction intense events development modelings.

Language: Английский

---

Analytical developments for turbulent flows subjected to strong temperature gradients

International Communications in Heat and Mass Transfer, Journal Year: 2025, Volume and Issue: 162, P. 108550 - 108550

Published: Jan. 14, 2025

Language: Английский

---