arXiv subjects

Naofumi Ohnishi

Publications and source records attributed to Naofumi Ohnishi.

At least 19 recordsLinked to original sources

arXiv2026

Low-dimensional structure and online tracking of POD subspaces on the Grassmann manifold: application to flow around an airfoil

Representing flow states over a wide range of flow parameters and control inputs in a low-dimensional state space is a central challenge in fluid mechanics. Rather than representing the instantaneous flow field in a fixed subspace spanned by the leading proper orthogonal decomposition (POD) modes, this study regards the POD subspace itself as the flow state at each flow condition. The set of POD subspaces associated with flow conditions defines a state space on the Grassmann manifold. Diffusion maps identify the intrinsic low-dimensional structure of the family of POD subspaces, while Grassmannian rank-one update subspace estimation tracks the temporal evolution of a POD subspace online. The framework is experimentally demonstrated for flow around an airfoil. POD subspaces are extracted from wall-pressure fluctuations measured by a microphone array over a range of angles of attack and under different control inputs. The subspaces are found to lie on a one-dimensional submanifold of the Grassmann manifold. Moreover, the transition from separated to attached flow, induced by a plasma actuator, follows a reproducible trajectory along the same submanifold identified from statistically stationary flow data. The temporal evolution of the subspace is consistent with the transient evolution of the flow field observed using particle image velocimetry. These results show that POD subspaces can serve as representative flow states, enabling their temporal evolution across a wide range of flow conditions to be tracked online in a low-dimensional space based on wall-pressure fluctuations. This low-dimensional representation provides a basis for real-time flow-state estimation and feedback control.

physics.flu-dyn↗

arXiv2026

Improvement of reduced-order model for two-dimensional cylinder flow based on global proper orthogonal decomposition in terms of robustness and computational speed

Reduced-order models (ROMs) are widely used in fluid engineering to enable rapid prediction of flow fields for parametric analysis, design optimization, and control applications. Proper orthogonal decomposition (POD) is commonly employed to construct ROMs because it provides an optimal basis for representing a given flow dataset. However, POD-based ROMs often lack robustness when applied to flow conditions that differ from those included in the training data. Incorporating multiple flow conditions can improve robustness, but this generally increases the computational cost of ROM prediction, which limits practical applicability in engineering workflows. In this study, we propose a ROM framework that achieves fast and robust flow prediction even when the dataset contains a large number of flow conditions. The proposed approach employs a novel two-step order-reduction strategy based on POD. In the second reduction step, flow conditions that are most relevant to the target prediction are selectively retained, thereby reducing the computational cost without sacrificing accuracy. The performance of the proposed ROM is evaluated for a two-dimensional unsteady flow past a circular cylinder, a canonical benchmark problem in fluid engineering. The model accurately reproduces the relationship between vortex-shedding frequency and Reynolds number obtained from full numerical simulations. Furthermore, the proposed ROM reduces the computational cost by approximately 50% compared with a conventional POD-based ROM constructed using flow data at 27 different Reynolds numbers.

physics.flu-dyn↗

arXiv2025

On the conservation of physical properties in operator interpolation of parameterized hydrodynamic systems

Reduced-order models (ROMs) that capture changes in fluid systems due to variations in parameters, such as the Reynolds number or the shape of a stationary body placed in the flow, are attracting increasing attention in engineering applications. In this study, we identify linear operators that characterize the behavior of fluid systems across a wide parameter range by using flow field datasets at several representative parameter values. We then comprehensively assess the applicability of ROMs constructed through the interpolation of these operators. Specifically, we consider two intrusive operator-based ROMs: one derived from Galerkin projection and the other based on operator inference using dynamic mode decomposition (DMD). The performance of these ROMs is evaluated for flows around circular and elliptical cylinders over a range of Reynolds numbers and aspect ratios. The Galerkin-based ROM successfully predicts only the eigenvalue and eigenmode corresponding to the fundamental frequency of the Kármán vortex shedding, while other frequencies are not captured and show a decaying behavior. In contrast, the DMD-based ROM accurately predicts both the fundamental frequency and its higher harmonics. Furthermore, visualization of the linear operator matrix elements reveals that interpolation fails when the subspace includes bases contaminated by numerical errors. However, by carefully selecting the subspace dimension and reference conditions, it is possible to accurately predict eigenvalues and corresponding modes even under conditions where multiple low-frequency modes exist outside the harmonic structure of the fundamental frequency. These findings underscore the robustness of the DMD-based parametric operator ROM approach.

physics.flu-dyn↗

arXiv2025

Energy transfer and budget analysis for transient process with phase-averaged reduced-order model

We derive a phase-averaged representation of transient flows based on the eigenmodes of a data-driven linear operator that approximates the Navier-Stokes dynamics. In performing phase averaging, it is assumed that, at each instant during the transient evolution, the eigenmode amplitude remains invariant, while only the complex phase angle differs among distinct realizations of the transient process. From this modal-phase perspective, the linear operator is defined as the best-fit operator that represents phase-different transient evolutions. By introducing a time-varying dynamic mode decomposition with a phase-control strategy formulated from this modal-phase perspective, time-varying eigenmodes are extracted from numerical simulations. In this formulation, the transient process is decomposed into time-varying eigenmodes, phase-shift angles, and amplitude coefficients. Furthermore, by averaging the Navier-Stokes equations over the phase-shift angle, a frequency-domain form of the equations can be derived at any given instant, assuming that the phase-shift angle is time-independent. This frequency-domain representation reveals the instantaneous energy budget and the presence of energy transfer through triadic interactions. The proposed analysis is demonstrated using a canonical example of two-dimensional flow around a circular cylinder transitioning from a steady to an unsteady state. The time-varying dynamic mode decomposition with phase control is shown to capture the transient evolution of the frequency components accurately. In addition, the temporal evolution of the energy budget and transfer distribution reveals that transient growth processes exhibit different time-dependent characteristics of energy transfer, even in cylinder flows at Reynolds numbers that eventually lead to a periodic state.

physics.flu-dyn↗

arXiv2025

Uncovering triadic interaction relationships latent in Mode A behind a circular cylinder

Coherence between multiple low-frequency components latent in the flow fields characterizes the nonlinear aspects of fluid dynamics. This study reveals the existance of the distinct frequency components and their interaction relation of the classical Mode A of the cylinder wake. Primaries are one-third of the Karman vortex shedding frequency (third-subharmonic) and bubble pumping, known as the previous study. However, when the spanwise domain size in numerical simulations is sufficiently large, their interaction is obscured by the presence of numerous frequency components. To address this, we introduce a process in which distinct frequency components gradually emerge by starting with a small spanwise domain size and then gradually increasing it from 3.3D to 4.7D, where D represents the diameter of the cylinder. From 3.3D to 3.5D, only the vortex shedding frequency harmonics are present. Third-subharmonic frequency appeared ranging from 3.5D to 3.7D. Bispectral mode decomposition reveals that the harmonics of the third-subharmonic frequency govern the flow in this domain size. The bubble pumping is emergence in the flow fields between 3.7D and 3.8D. The frequency component after this emergence is not only the harmonics of bubble pumping and periodic nature is disrupt. Nonlinear interactions between bubble pumping, the Karman vortex, and the third-subharmonic component complicate the temporal behavior of the flow field. Utilizing the constraint of the spanwise domain size, our approach effectively reveals the interaction relationship between frequency components inherent a flow field with a significant number of frequency components.

physics.flu-dyn↗

arXiv2018

Quadratic conservative scheme for relativistic Vlasov--Maxwell system

For more than half a century, most of the plasma scientists have encountered a violation of the conservation laws of charge, momentum, and energy whenever they have numerically solve the first-principle equations of kinetic plasmas, such as the relativistic Vlasov--Maxwell system. This fatal problem is brought by the fact that both the Vlasov and Maxwell equations are indirectly associated with the conservation laws by means of some mathematical manipulations. Here we propose a quadratic conservative scheme, which can strictly maintain the conservation laws by discretizing the relativistic Vlasov--Maxwell system. A discrete product rule and summation-by-parts are the key players in the construction of the quadratic conservative scheme. Numerical experiments of the relativistic two-stream instability and relativistic Weibel instability prove the validity of our computational theory, and the proposed strategy will open the doors to the first-principle studies of mesoscopic and macroscopic plasma physics.

physics.plasm-ph↗

arXiv2017

Validation of Radiative Transfer Computation with Monte Carlo Method for Ultra-Relativistic Background Flow

We developed a three-dimensional radiative transfer code for an ultra-relativistic background flow-field by using the Monte Carlo (MC) method in the context of gamma-ray burst (GRB) emission. For obtaining reliable simulation results in the coupled computation of MC radiation transport with relativistic hydrodynamics which can reproduce GRB emission, we validated radiative transfer computation in the ultra-relativistic regime and assessed the appropriate simulation conditions. The radiative transfer code was validated through two test calculations: (1) computing in different inertial frames and (2) computing in flow-fields with discontinuous and smeared shock fronts. The simulation results of the angular distribution and spectrum were compared among three different inertial frames and in good agreement with each other. If the time duration for updating the flow-field was sufficiently small to resolve a mean free path of a photon into ten steps, the results were thoroughly converged. The spectrum computed in the flow-field with a discontinuous shock front obeyed a power-law in frequency whose index was positive in the range from 1 to 10 MeV. The number of photons in the high-energy side decreased with the smeared shock front because the photons were less scattered immediately behind the shock wave due to the small electron number density. The large optical depth near the shock front was needed for obtaining high-energy photons through bulk Compton scattering. Even one-dimensional structure of the shock wave could affect the results of radiation transport computation.

astro-ph.HE↗

arXiv2017

Structure-preserving operators for thermal-nonequilibrium hydrodynamics

Radiation hydrodynamics simulations based on the one-fluid two-temperature model may violate the law of energy conservation because the governing equations are expressed in a nonconservative formulation. Here, we maintain the important physical requirements by employing a strategy based on the key concept that the mathematical structures associated with the conservative and nonconservative equations are preserved, even at the discrete level. To this end, we discretize the conservation laws and transform them via exact algebraic operations. The proposed scheme maintains the global conservation errors within the round-off level. In addition, a numerical experiment concerning the shock tube problem suggests that the proposed scheme well agrees with the jump conditions at the discontinuities regulated by the Rankine-Hugoniot relationship. The generalized derivation allows us to employ arbitrary central difference, artificial dissipation, and Runge-Kutta methods.

physics.plasm-ph↗

arXiv2011

Explosive nucleosynthesis in the neutrino-driven aspherical supernova explosion of a non-rotating 15$M_{\odot}$ star with solar metallicity

We investigate explosive nucleosynthesis in a non-rotating 15$M_\odot$ star with solar metallicity that explodes by a neutrino-heating supernova (SN) mechanism aided by both standing accretion shock instability (SASI) and convection. To trigger explosions in our two-dimensional hydrodynamic simulations, we approximate the neutrino transport with a simple light-bulb scheme and systematically change the neutrino fluxes emitted from the protoneutron star. By a post-processing calculation, we evaluate abundances and masses of the SN ejecta for nuclei with the mass number $\le 70$ employing a large nuclear reaction network. Aspherical abundance distributions, which are observed in nearby core-collapse SN remnants, are obtained for the non-rotating spherically-symmetric progenitor, due to the growth of low-mode SASI. Abundance pattern of the supernova ejecta is similar to that of the solar system for models whose masses ranges $(0.4-0.5) \Ms$ of the ejecta from the inner region ($\le 10,000\km$) of the precollapse core. For the models, the explosion energies and the \nuc{Ni}{56} masses are $ \simeq 10^{51} \rm erg$ and $(0.05-0.06) \Ms$, respectively; their estimated baryonic masses of the neutron star are comparable to the ones observed in neutron-star binaries. These findings may have little uncertainty because most of the ejecta is composed by matter that is heated via the shock wave and has relatively definite abundances. The abundance ratios for Ne, Mg, Si and Fe observed in Cygnus loop are well reproduced with the SN ejecta from an inner region of the $15\Ms$ progenitor.

astro-ph.HE↗

arXiv2011

Effects of Rotation on Stochasticity of Gravitational Waves in Nonlinear Phase of Core-Collapse Supernovae

By performing three-dimensional (3D) simulations that demonstrate the neutrino-driven core-collapse supernovae aided by the standing accretion shock instability (SASI), we study how the spiral modes of the SASI can have impacts on the properties of the gravitational-wave (GW) emission. To see the effects of rotation in the non-linear postbounce phase, we give a uniform rotation on the flow advecting from the outer boundary of the iron core, whose specific angular momentum is assumed to agree with recent stellar evolution models. We compute fifteen 3D models in which the initial angular momentum as well as the input neutrino luminosities from the protoneutron star are changed in a systematic manner. By performing a ray-tracing analysis, we accurately estimate the GW amplitudes generated by anisotropic neutrino emission. Our results show that the gravitational waveforms from neutrinos in models that include rotation exhibit a common feature otherwise they vary much more stochastically in the absence of rotation. The breaking of the stochasticity stems from the excess of the neutrino emission parallel to the spin axis. This is because the compression of matter is more enhanced in the vicinity of the equatorial plane due to the growth of the spiral SASI modes, leading to the formation of spiral flows circulating around the spin axis with higher temperatures. We point out that a recently proposed future space interferometers like Fabry-Perot type DECIGO would permit detection of these signals for a Galactic supernova.

astro-ph.HE↗

arXiv2009

Ray-Tracing Analysis of Anisotropic Neutrino Radiation for Estimating Gravitational Waves in Core-Collapse Supernovae

We propose a ray-tracing method to estimate gravitational waves (GWs) generated by anisotropic neutrino emission in supernova cores. To calculate the gravitational waveforms, we derive analytic formulae in a useful form, which are applicable also for three-dimensional computations. Pushed by evidence of slow rotation prior to core-collapse, we focus on asphericities in neutrino emission and matter motions outside the protoneutron star. Based on the two-dimensional (2D) models, which mimic SASI-aided neutrino heating explosions, we compute the neutrino anisotropies via the ray-tracing method in a post-processing manner and calculate the resulting waveforms. With these computations, it is found that the waveforms exhibit more variety in contrast to the ones previously estimated by the ray-by-ray analysis (e.g., Kotake et al. (2007)). In addition to a positively growing feature, which was predicted to determine the total wave amplitudes predominantly, the waveforms are shown to exhibit large negative growth for some epochs during the growth of SASI. These features are found to stem from the excess of neutrino emission in lateral directions, which can be precisely captured by the ray-tracing calculation. Due to the negative contributions and the neutrino absorptions appropriately taken into account by the ray-tracing method, the wave amplitudes become more than one-order-of magnitude smaller than the previous estimation, thus making their detections very hard for a galactic source.On the other hand, it is pointed out that the GW spectrum from matter motions have its peak near $\sim 100$ Hz, which could be characteristic for the SASI-induced supernova explosions.(abridged)

astro-ph.HE↗

arXiv2009

Stochastic Nature of Gravitational Waves from Supernova Explosions with Standing Accretion Shock Instability

We study properties of gravitational waves based on the three-dimensional simulations, which demonstrate the neutrino-driven explosions aided by the standing accretion shock instability (SASI). Pushed by evidence supporting slow rotation prior to core-collapse, we focus on the asphericities in neutrino emissions and matter motions outside the protoneutron star. By performing a ray-tracing calculation in 3D, we estimate accurately the gravitational waveforms from anisotropic neutrino emissions. In contrast to the previous work assuming axisymmetry, we find that the gravitational waveforms vary much more stochastically because the explosion anisotropies depend sensitively on the growth of the SASI which develops chaotically in all directions. Our results show that the gravitational-wave spectrum has its peak near $\sim 100$ Hz, reflecting the SASI-induced matter overturns of $\sim O(10)$ ms. We point out that the detection of such signals, possibly visible to the LIGO-class detectors for a Galactic supernova, could be an important probe into the long-veiled explosion mechanism.

astro-ph.HE↗

arXiv2008

Effects of Rotation on Standing Accretion Shock Instability in Nonlinear Phase for Core-Collapse Supernovae

We studied the effects of rotation on standing accretion shock instability (SASI) by performing three-dimensional hydrodynamics simulations. Taking into account a realistic equation of state and neutrino heating/cooling, we prepared a spherically symmmetric and steady accretion flow through a standing shock wave onto a proto-neutron star (PNS). When the SASI entered the nonlinear phase, we imposed uniform rotation on the flow advecting from the outer boundary of the iron core, whose specific angular momentum was assumed to agree with recent stellar evolution models. Using spherical harmonics in space and Fourier decompositions in time, we performed mode analysis of the nonspherical deformed shock wave to observe rotational effects on the SASI in the nonlinear phase. We found that rotation imposed on the axisymmetric SASI did not make any spiral modes and hardly affected sloshing modes, except for steady l=2, m=0 modes. In contrast, rotation imposed on the non-axisymmetric flow increased the amplitude of spiral modes so that some spiral flows accreting on the PNS were more clearly formed inside the shock wave than without rotation. The amplitudes of spiral modes increased significantly with rotation in the progressive direction.

astro-ph↗

arXiv2007

Three-Dimensional Simulations of Standing Accretion Shock Instability in Core-Collapse Supernovae

We have studied non-axisymmetric standing accretion shock instability, or SASI, by 3D hydrodynamical simulations. This is an extention of our previous study on axisymmetric SASI. We have prepared a spherically symmetric and steady accretion flow through a standing shock wave onto a proto-neutron star, taking into account a realistic equation of state and neutrino heating and cooling. This unperturbed model is supposed to represent approximately the typical post-bounce phase of core-collapse supernovae. We then have added a small perturbation (~1%) to the radial velocity and computed the ensuing evolutions. Not only axisymmetric but non-axisymmetric perturbations have been also imposed. We have applied mode analysis to the non-spherical deformation of the shock surface, using the spherical harmonics. We have found that (1) the growth rates of SASI are degenerate with respect to the azimuthal index m of the spherical harmonics Y_l^m, just as expected for a spherically symmetric background, (2) nonlinear mode couplings produce only m=0 modes for the axisymmetric perturbations, whereas m=!0 modes are also generated in the non-axisymmetric cases according to the selection rule for the quadratic couplings, (3) the nonlinear saturation level of each mode is lower in general for 3D than for 2D because a larger number of modes are contributing to turbulence in 3D, (4) low l modes are dominant in the nonlinear phase, (5) the equi-partition is nearly established among different m modes in the nonlinear phase, (6) the spectra with respect to l obey power laws with a slope slightly steeper for 3D, and (7) although these features are common to the models with and without a shock revival at the end of simulation, the dominance of low l modes is more remarkable in the models with a shock revival.

astro-ph↗

arXiv2007

The g-mode Excitation in the Proto Neutron Star by the Standing Accretion Shock Instability

The so-called "acoustic revival mechanism" of core-collapse supernova proposed recently by the Arizona group is an interesting new possibility. Aiming to understand the elementary processes involved in the mechanism, we have calculated the eigen frequencies and eigen functions for the g-mode oscillations of a non-rotating proto neutron star. The possible excitation of these modes by the standing accretion shock instability, or SASI, is discussed based on these eigen functions. We have formulated the forced oscillations of $g$-modes by the external pressure perturbations exerted on the proto neutron star surface. The driving pressure fluctuations have been adopted from our previous computations of the axisymmetric SASI in the non-linear regime. We have paid particular attention to low l modes, since these are the modes that are dominant in SASI and that the Arizona group claimed played an important role in their acoustic revival scenario. Here l is the index of the spherical harmonic functions, $Y_l^m$. Although the frequency spectrum of the non-linear SASI is broadened substantially by non-linear couplings, the typical frequency is still much smaller than those of g-modes, the fact leading to a severe impedance mismatch. As a result, the excitations of various $g$-modes are rather inefficient and the energy of the saturated g-modes is $\sim 10^{50}$erg or smaller, with the g_2-mode being the largest in our model. Here the g_2-mode has two radial nodes and is confined to the interior of the convection region. The energy transfer rate from the g-modes to out-going sound waves is estimated from the growth of the g-modes and found to be $\sim 10^{51}$erg/s in the model studied in this paper.

astro-ph↗

arXiv2006

Gravitational Radiation from Standing Accretion Shock Instability in Core-Collapse Supernovae

We present the results of numerical experiments, in which we study how the asphericities induced by the growth of the standing accretion shock instability (SASI) produce the gravitational waveforms in the postbounce phase of core-collapse supernovae. To obtain the neutrino-driven explosions, we parameterize the neutrino fluxes emitted from the central protoneutron star and approximate the neutrino transfer by a light-bulb scheme. We find that the waveforms due to the anisotropic neutrino emissions show the monotonic increase with time, whose amplitudes are up to two order-of-magnitudes larger than the ones from the convective matter motions outside the protoneutron stars. We point out that the amplitudes begin to become larger when the growth of the SASI enters the nonlinear phase, in which the deformation of the shocks and the neutrino anisotropy become large. From the spectrum analysis of the waveforms, we find that the amplitudes from the neutrinos are dominant over the ones from the matter motions at the frequency below $\sim 100$ Hz, which are suggested to be within the detection limits of the detectors in the next generation such as LCGT and the advanced LIGO for a supernova at 10 kpc. As a contribution to the gravitational wave background, we show that the amplitudes from this source could be larger at the frequency above $\sim$ 1 Hz than the primordial gravitational wave backgrounds, but unfortunately, invisible to the proposed space-based detectors.

astro-ph↗

arXiv2006

Inelastic Neutrino-Helium Scatterings and Standing Accretion Shock Instability in Core-Collapse Supernovae

We present the results of numerical experiments, in which we have investigated the influence of the inelastic neutrino-helium interactions on the standing accretion shock instability supposed to occur in the post-bounce supernova core. The axisymmetric hydrodynamical simulations of accretion flows through the standing accretion shock wave onto the protoneutron star show that the interactions are relatively minor and the linear growth of the shock instability is hardly affected. The extra heating given by the inelastic reactions becomes important for the shock revival after the instability enters the non-linear regime, but only when the neutrino luminosity is very close to the critical value, at which the shock would be revived without the interactions. We have also studied the dependence of the results on the initial amplitudes of perturbation and the temperatures of mu and tau neutrinos.

astro-ph↗

arXiv2005

Toward Radiation-Magnetohydrodynamic Simulations in Core-Collapse Supernovae

We report a current status of our radiation-magnetohydrodynamic code for the study of core-collapse supernovae. In this contribution, we discuss the accuracy of our newly developed numerical code by presenting the test problem in a static background model. We also present the application to the spherically symmetric core-collapse simulations. Since close comparison with the previously published models is made, we are now applying it for the study of magnetorotational core-collapse supernovae.

astro-ph↗