A comprehensive framework to enhance numerical simulations in the spectral-element code Nek5000

Massaro, D.; Peplinski, A.; Stanly, R.; Mirzareza, S.; Lupi, V.; Mukha, T.; Schlatter, P.

doi:10.1016/j.cpc.2024.109249

Computer Physics Communications

2024

DOI: 10.1016/j.cpc.2024.109249

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A comprehensive framework to enhance numerical simulations in the spectral-element code Nek5000

D. Massaro,

A. Peplinski,

R. Stanly

et al.

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2024

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Cited by 7 publications

References 38 publications

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High-fidelity simulations of the flow around a NACA 4412 wing section at high angles of attack

Mallor,

Vinuesa,

Örlü

et al. 2024

International Journal of Heat and Fluid Flow

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High-fidelity simulations of the flow around a NACA 4412 wing section at high angles of attack

Mallor,

Vinuesa,

Örlü

et al. 2024

International Journal of Heat and Fluid Flow

View full text Add to dashboard Cite

Generating synthetic turbulence with vector autoregression of proper orthogonal decomposition time coefficients

Stanly,

Du,

Xavier

et al. 2024

J. Fluid Mech.

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This study introduces vector autoregression (VAR) as a linear procedure that can be used for synthesizing turbulence time series over an entire plane, allowing them to be imposed as an efficient turbulent inflow condition in simulations requiring stationary and cross-correlated turbulence time series. VAR is a statistical tool for modelling and prediction of multivariate time series through capturing linear correlations between multiple time series. A Fourier-based proper orthogonal decomposition (POD) is performed on the two-dimensional (2-D) velocity slices from a precursor simulation of a turbulent boundary layer at a momentum thickness-based Reynolds number, $Re_{\theta }=790$ . A subset of the most energetic structures in space are then extracted, followed by applying a VAR model to their complex time coefficients. It is observed that VAR models constructed using time coefficients of 5 and 30 most energetic POD modes per wavenumber (corresponding to $66\,\%$ and $97\,\%$ of turbulent kinetic energy, respectively) are able to make accurate predictions of the evolution of the velocity field at $Re_{\theta }=790$ for infinite time. Moreover, the 2-D velocity fields from the POD–VAR when used as a turbulent inflow condition, gave a short development distance when compared with other common inflow methods. Since the VAR model can produce an infinite number of velocity planes in time, this enables reaching statistical stationarity without having to run an extremely long precursor simulation or applying ad hoc methods such as periodic time series.

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Global stability of the flow past a stepped cylinder

Massaro,

Schlatter

2024

J. Fluid Mech.

Self Cite

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We investigate the global instability mechanism of the flow past a three-dimensional stepped cylinder. A comprehensive study is performed for different diameter ratios of the two joined cylinders ( $r=D/d$ ) ranging from $r=1.1$ to $r=4$ . Independently of $r$ , the spectrum of the linearised Navier–Stokes operator reveals a pair of complex conjugate eigenvalues, with Strouhal number $St \approx 0.11$ . The initial transition is triggered by a two-dimensional mechanism of the larger cylinder only, not affected by the presence of the junction and the smaller cylinder ( $Re_{D,cr}\approx 47$ ). The structural sensitivity analysis is used to identify where the instability mechanism acts. The onset of transition is solely localised in the large cylinder wake (L cell), where the wavemaker has two symmetric lobes across the separation bubble. When the Reynolds number increases, a second and a third unstable pair of complex conjugate eigenvalues appears. They are localised in the small cylinder (S) wake and modulation (N) region. For any $r$ , the appearance of unstable eigenmodes resembling the three cells S–N–L in the wake is observed. The nonlinear simulation results support this finding, in contrast with the previous classification of the laminar vortex shedding in direct (L–S) and indirect (L–N–S) modes interaction Lewis & Gharib (Phys. Fluids, vol. 4, 1992, pp. 104–117). This result indicates that each cell undergoes a supercritical Hopf bifurcation for any $r$ . As $r$ approaches $1$ , the modal linear stability results also show an unstable eigenmode in the wake of the small cylinder resembling a new modulation cell, named N2, similar to the N cell but mirrored with respect to the junction plane.

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A comprehensive framework to enhance numerical simulations in the spectral-element code Nek5000

Cited by 7 publications

References 38 publications

High-fidelity simulations of the flow around a NACA 4412 wing section at high angles of attack

High-fidelity simulations of the flow around a NACA 4412 wing section at high angles of attack

Generating synthetic turbulence with vector autoregression of proper orthogonal decomposition time coefficients

Global stability of the flow past a stepped cylinder

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