arXiv subjects

Simon Demange

Publications and source records attributed to Simon Demange.

5 recordsLinked to original sources

arXiv2026

Coherent structures modeling in stenotic transitional flow via resolvent analysis

This study investigates the capability of linear modeling to characterize the transitional dynamics in an axisymmetric stenosis and attempts a low-order representation of the turbulent stresses. The transition to turbulence in stenotic flows generates wall shear stress fluctuations that strongly influence the progression of cardiovascular diseases and the risk of plaque rupture. A description of the linear mechanisms driving the forced dynamics at Reynolds number beyond transition is currently missing. Linear modeling of coherent structures is leveraged to identify the flow amplification mechanisms using the mean field from a LES at Re=4000. Global linear stability analysis reveals an unstable and sinuous stationary eigenmode that is known to destabilize the flow at lower Reynolds numbers through a weak Coanda-type wall attachment. At intermediate frequencies, resolvent analysis identifies a second amplification region within the shear-layer where the most amplified fluctuations are axisymmetric, in contrast to findings from studies at lower Reynolds numbers. The linear model is validated against SPOD. At intermediate frequencies, the optimal resolvent response mode demonstrates both high gain separation and strong alignment with the leading SPOD mode. The low-rank nature of the resolvent operator is leveraged to reconstruct the turbulent kinetic energy (TKE) and turbulent wall shear stress (tWSS) from the optimal response mode. In the immediate post-stenotic zone, axisymmetric fluctuations dominate the tWSS and exhibit low-rank dynamics. Our findings highlight that linear mechanisms effectively capture the complex post-stenotic dynamics. The successful reconstruction of turbulent quantities from mean flow data alone opens new predictive possibilities of key turbulent quantities.

physics.flu-dyn↗

arXiv2026

Uncovering Turbulent Dynamics in Stenotic Flows from 4D-flow MRI Measurements via Resolvent Analysis and Data Assimilation

This study presents a hybrid experimental and computational framework that couples in vitro 4D phase-contrast magnetic resonance imaging (4D-flow MRI) measurements with data assimilation and linear modeling to characterize the flow linear amplification mechanisms. We manufacture an idealized stenosis phantom with a cosine-shaped contraction and acquire three-dimensional (3D) mean velocity measurements at Reynolds number 3960 using 4D-flow MRI. To overcome the inherent displacement artifact, we perform data assimilation via a two-step optimization strategy using physics-informed neural network (PINN). This approach first corrects measurement artifacts before extracting the unknown mean pressure and eddy viscosity fields. The RANS-compatible mean flow then serves as the base state for global linear stability analysis (LSA) and resolvent analysis. The global LSA reveals stationary eigenmodes located in the recirculation bubble that exhibit a positive growth rate for azimuthal wavenumbers m=2 and m=3. The forced dynamics of this eigenmode dominates the low-frequency dynamics. Resolvent analysis identifies a broadband pseudo-resonance associated with the convective instability of the separated shear-layer, with maximal amplification for m=0. This methodology demonstrates how integrating sparse experimental MRI data with physics-based modeling enables the identification of mean fields and coherent structures. By leveraging the capabilities of 4D-flow MRI to non-invasively measure 3D velocity fields without requiring physical or optical access, this approach is a first step in the application of linear analysis to cardiovascular flows.

physics.flu-dyn↗

arXiv2024

Identification of structures driving trailing-edge noise. Part I -- Experimental investigation

Trailing-edge (TE) noise is the main contributor to the acoustic signature of flows over airfoils. It originates from the interaction of turbulent structures in the airfoil boundary layer with the TE. This study experimentally identifies the flow structures responsible for TE noise by decomposing the data into spanwise modes and examining the impact of spanwise coherent structures on sound emission. We analyse a NACA0012 airfoil at moderate Reynolds numbers, ensuring broadband TE noise, and use synchronous measurements of surface and far-field acoustic pressure fluctuations with custom spanwise microphone arrays. Our results demonstrate the key role of coherent structures with large spanwise wavelengths in generating broadband TE noise. Spanwise modal decomposition of the acoustic field shows that only waves with spanwise wavenumbers below the acoustic wavenumber contribute to the radiated acoustic spectrum, consistent with theoretical scattering conditions. Moreover, a strong correlation is found between spanwise-coherent (zero wavenumber) flow structures and radiated acoustics. At frequencies corresponding to peak TE noise emission, the turbulent structures responsible for radiation exhibit strikingly large spanwise wavelengths, exceeding $60\%$ of the airfoil chord length. These findings have implications for numerical and experimental TE noise analysis and flow control. The correlation between spectrally decomposed turbulent fluctuations and TE noise paves the way for future aeroacoustic modelling through linearized mean field analysis. A companion paper further explores the nature of the spanwise-coherent structures using high-resolution numerical simulations of the same setup.

physics.flu-dyn↗

arXiv2024

Identification of structures driving trailing-edge noise. Part II -- Numerical investigation

The aim of the present work is to investigate the mechanisms of broadband trailing-edge noise generation to improve prediction tools and control strategies. We focus on a NACA 0012 airfoil at 3 degrees angle of attack and chord Reynolds number Re = 200,000. A high-fidelity wall-resolved compressible implicit large eddy simulation (LES) is performed to collect data for our analysis. The simulation is designed in close alignment with the experiment described in detail in the companion paper (Demange et al. 2024b). Zig-zag geometrical tripping elements, added to generate a turbulent boundary layer, are meshed to closely follow the experimental setup. A large spanwise domain is used in the simulation to include propagative acoustic waves with low wavenumbers. An in-depth comparison with experiments is conducted showing good agreement in terms of mean flow statistics, acoustic and hydrodynamic spectra, and coherence lengths. Furthermore, a strong correlation is found between the radiated acoustics and spanwise-coherent structures. To investigate the correlation for higher wavenumbers, spectral proper orthogonal decomposition (SPOD) is applied to the spanwise Fourier-transformed LES dataset. The analysis of all SPOD modes for the leading spanwise wavenumbers reveals streamwise-travelling wavepackets as the source of the radiated acoustics. This finding, confirming observations from experiments in the companion paper, leads to a new understanding of the turbulent structures driving the trailing-edge noise. By performing extended SPOD based on the acoustic region, we confirm the low rank nature of the acoustics, and a reduced-order model based on acoustic extended SPOD is proposed for the far-field acoustic reconstruction.

physics.flu-dyn↗

arXiv2024

Linear stability and spectral modal decomposition of three-dimensional turbulent wake flow of a generic high-speed train

This work investigates the spatio-temporal evolution of coherentstructures in the wake of a high-speed train. SPOD is used to extract energy spectra and empirical modes for both symmetric and antisymmetric components of the fluctuating flow field. The spectrum of the symmetric component shows overall higher energy and more pronounced low-rank behavior compared to the antisymmetric one. The most dominant symmetric mode features periodic vortex shedding in the near wake, and wave-like structures in the far wake. The mode bispectrum further reveals the dominant role of self-interaction of the symmetric component, leading to first harmonic and subharmonic triads of the fundamental frequency, with remarkable deformation of the mean field. Then the stability of the three-dimensional wake flow is analyzed based on two-dimensional local linear stability analysis combined with a non-parallelism approximation approach. Temporal stability analysis is first performed, showing a more unstable condition in the near wake. The absolute frequency of the near-wake eigenmode is determined based on spatio-temporal analysis, then tracked along the streamwise direction to find out the global mode growth rate and frequency, which indicate a marginally stable global mode oscillating at a frequency close to the most dominant SPOD mode. The global mode wavemaker is then located, and the structural sensitivity is calculated based on the direct and adjoint modes derived from a local analysis, with the maximum value localized within the recirculation region close to the train tail. Finally, the global mode is computed by tracking the most spatially unstable eigenmode in the far wake, and the alignment with the SPOD mode is computed as a function of streamwise location. By combining data-driven and theoretical approaches, the mechanisms of coherentstructures in complex wake flows are well identified and isolated.

physics.flu-dyn↗