arXiv subjects

Radouan Boukharfane

Publications and source records attributed to Radouan Boukharfane.

8 recordsLinked to original sources

arXiv2026

Discrete topology sensing in turbulence via signed pairwise graphlets of the velocity gradient tensor

A signed pairwise graphlet encoding of the velocity gradient tensor $\bm{A}$ is introduced as a minimal discrete representation of its off-diagonal structure, and its statistical relationship with the PQR topology of homogeneous isotropic turbulence is investigated using direct numerical simulation data at $Re_λ\approx 433$. Each off-diagonal component is assigned a ternary label based on a local threshold, mapping the thresholded off-diagonal sign pattern to one of $3^6=729$ discrete states. The imbalance $Δn=n_W-n_S$ between rotation-promoting and strain-promoting pairs is shown to act as a robust proxy for the sign of $Q$: $P(Q>0\midΔn)$ increases monotonically from $0\%$ at $Δn\leq-2$ to $100\%$ at $Δn=+3$, and graphlets with a single rotation-promoting pair $(Δn=+1)$ predict the stable-focus-stretching region with probability approximately $65\%$ -nearly double the global base rate of $36.5\%$- despite discarding all magnitude information. This predictive power can be largely attributed to the off-diagonal contribution of $\bm{A}$, which dominates the sign of $Q$ in a majority of flow states. The graphlet--PQR association remains statistically correlated over several Kolmogorov time scales and exhibits spatial coherence comparable to that of the $Q$ field at dissipative scales, providing a consistency check that the discrete thresholding preserves the short-range organisation inherited from the velocity-gradient field. An out-of-sample predictive log-loss test further shows that $Δn$ captures most, but not all, of the predictive content of the complete $729$-state graphlet code: the full code retains a small, reproducible gain of order $10^{-3}$ bits per observation in predicting future PQR topology beyond $Δn$ and the continuous invariants, present at finite lag and absent at zero lag.

physics.flu-dyn↗

arXiv2026

Global existence of weak solutions to incompressible anisotropic Cahn-Hilliard-Navier-Stokes system

We study the anisotropic, incompressible Cahn-Hilliard-Navier-Stokes system with variable density in a bounded smooth domain $Ω\subset \mathbb{R}^d$. This work extends previous results on the isotropic case by incorporating anisotropic surface energy, represented by $\mathfrak{F}= \int_Ω \fracε{2}\, Γ^2(\nabla ϕ) $. The thermodynamic consistency of this system, as well as its modeling background and physical motivation, has been established in \cite{anderson2000phase,taylor-cahn98, zaidni2024}. Using a Galerkin approximation scheme, we prove the existence of global weak solutions in both two- and three-dimensions $(d=2,3)$. A key ingredient in extending the local existence of approximate solutions to a global one is the application of Bihari's inequality combined with a fixed-point argument.

math.AP↗

arXiv2025

Numerical Analysis of Pure and Blended Fuel Sonic Jets in a Mach 2 Crossflow

The injection of transverse jets into supersonic compressible crossflows represents a fundamental configuration relevant to a spectrum of high-speed applications. The intricate interactions arising between the crossflow and the injected jet induce complex flow phenomena, including shock waves and vortical structures, the characteristics of which are significantly contingent upon the thermophysical properties of the injected fuel. While prior investigations have addressed the influence of various fuels, a knowledge gap persists concerning the behaviour of alternative and synthetic multicomponent fuels within this flow regime. The present work employs high-fidelity large-eddy simulations (LES) to examine the impact of ten distinct fuels-hydrogen, methane, ethylene, ammonia, syngas mixture, and a synthetic blend, alongside several NH3/H2/N2 mixtures-on the macroscopic flow structures and mixing attributes within a transverse sonic jet immersed in a Mach 2 crossflow. By maintaining a uniform momentum flux ratio across the investigated cases, the study aims to isolate the influence of the unique thermophysical properties of each fuel on the windward mixing layer, a region critically important for initial entrainment processes. The investigation quantifies the effects of molecular weight, heat capacity ratio, and density on the development and evolution of coherent structures through a detailed examination of instantaneous flow fields, vortex dynamics, scalar distributions, and turbulence statistics. The results are expected to provide pertinent insights into fuel-dependent mixing mechanisms in supersonic flows, thereby contributing to the advancement of more efficient and versatile propulsion systems.

physics.flu-dyn↗

arXiv2025

Modal analysis of oblique shock-induced flow dynamics in a supersonic reacting shear layer

Efficient mixing in high-speed compressible flows, crucial for scramjet operation, can be significantly enhanced by shock wave interactions. This study employs Direct Numerical Simulations (DNS) to comprehensively examine the interaction between an oblique shock and a spatially developing turbulent mixing layer, contrasting inert and reacting (hydrogen-air combustion) cases. Utilizing streaming Dynamic Mode Decomposition (sDMD), we analyze four configurations: inert and reacting shear layers, both with and without shock impingement (at $\mathrm{Ma}_c = 0.48$). We evaluate the temporal mode growth rates, the evolution of vorticity thickness, and the spatial structures of dominant DMD modes to elucidate how shocks and heat release synergistically influence flow stability, mixing, and the underlying coherent dynamics. Results reveal that the oblique shock significantly amplifies Kelvin-Helmholtz instabilities, excites a broader spectrum of unstable temporal modes, and accelerates the growth of the vorticity thickness. Combustion-induced heat release further modifies this response, leading to a redistribution of energy among the DMD modes and indicating a complex coupled effect with shock dynamics, particularly in the enhanced excitation of high-frequency modes and the alteration of spatial structures. The modal analysis identifies distinct frequency bands associated with shock and combustion effects and characterizes the dominant spatial patterns, offering refined insights for controlling and enhancing mixing in high-speed propulsion flows.

physics.flu-dyn↗

arXiv2024

High-Precision Surrogate Modeling for Uncertainty Quantification in Complex Slurry Flows

Slurry transportation via pipelines is essential for global industries, offering efficiency and environmental benefits. Specifically, the precise calibration of physical parameters for transporting raw phosphate material to fertilizer plants is crucial to minimize energy losses and ensure secure operations. Computational fluid dynamics (CFD) is commonly employed to understand solid concentration, velocity distributions, and flow pressure along the pipeline. However, numerical solutions for slurry flows often entail uncertainties from initial and boundary conditions, emphasizing the need for quantification. This study addresses the challenge by proposing a framework that combines proper orthogonal decomposition and polynomial chaos expansions to quantify uncertainties in two-dimensional phosphate slurry flow simulations. The use of surrogate modeling methods, like polynomial chaos expansion, proves effective in reducing computational costs associated with direct stochastic simulations, especially for complex flows with high spatial variability, as observed in phosphate slurries. Numerical results demonstrate the accuracy of the non-intrusive reduction method in reproducing mean and variance distributions. Moreover, the uncertainty quantification analysis shows that the reduced-order model significantly reduces computational costs compared to the full-order model.

physics.flu-dyn↗

arXiv2023

Study of Dynamic Interaction Between Low Re Aerodynamic Load and Flexible-Biomimetic Wings with Tailorable Stiffness by FSI Modeling

In the present work, we investigate dynamic interaction and response of flexible bio-inspired morphing wing structure to a low Reynolds aerodynamic load. The aspects of inspiration are as follows. First, the segmentation of the wing into rigid and flexible segments. Considering a leading edge constitution of bone and muscle. In addition to a flexible trailing edge composed of feathers. Second, the material properties provided by experimental biology in literature are adopted such as the bending stiffness and Young's modulus. The development of numerical models allowing non uniform distribution of properties are developed and implemented into an OpenFoam finite volume solver that couples fluid dynamics to a structural solid dynamics solver through the FSI interface. In the course of this work, the validation is performed for a NACA6409 airfoil considering a rigid segment of 40\% and flexible segment 60\% chord length in order to test the aero-structure behavior for an aerodynamic load of air flow at low Reynolds number of $\boldsymbol{5\times 10^5}$ for the fluid and feather inspired material properties. The results suggest that bio-inspired techniques can be reproduced in engineering configurations.

physics.flu-dyn↗

arXiv2022

A Level-Set Immersed Boundary Method for Incompressible Flows at Subcritical Reynolds Numbers

In this work, a numerical scheme based on a level-set immersed boundary method is employed for the numerical simulation of the flow around two tandem circular cylinders in the subcritical flow regimes. Three different spacing ratios $\ell/\mathcal{D}$ (where $\ell$ is the center-to-center distance between the two cylinders with $\mathcal{D}$ being the diameter of the cylinders) from $2$ to $4$ is considered. The instantaneous flow structures, pressure distributions and hydrodynamic forces on two tandem cylinders are analyzed at a Reynolds number of $\mathcal{R}\mathfrak{e}=2.2\times 10^4$. The strategy is based on a combination of a narrow-band accurate conservative level set method and ghost-fluid framework. In this strategy, the interface is defined as the isocontour of a hyperbolic tangent function, which is advected by the fluid, and then periodically reshaped to enforce the degraded level set function being a signed distance function using a reinitialization equation based on an improved form. The latter approach takes advantage of a mapping onto a classical distance level set while much better preserving the interface shape.

physics.flu-dyn↗

arXiv2019

On the robustness and performance of entropy stable discontinuous collocation methods for the compressible Navier-Stokes equations

In computational fluid dynamics, the demand for increasingly multidisciplinary reliable simulations, for both analysis and design optimization purposes, requires transformational advances in individual components of future solvers. At the algorithmic level, hardware compatibility and efficiency are of paramount importance in determining viability at exascale and beyond. However, equally important (if not more so) is algorithmic robustness with minimal user intervention, which becomes progressively more challenging to achieve as problem size and physics complexity increase. We numerically show that low and high order entropy stable discontinuous spatial discretizations based on summation-by-part operators and simultaneous-approximation-terms technique provides an essential step toward a truly enabling technology in terms of reliability and robustness for both under-resolved turbulent flow simulations and flows with discontinuities.

math.NA↗