arXiv subjects

P. Jordan

Publications and source records attributed to P. Jordan.

5 recordsLinked to original sources

arXiv2026

Receptivity and instability of the hypersonic flow over moderately blunt cones

With a view to identifying and understanding the linear receptivity and amplification mechanisms that underpin laminar-to-turbulent transition over blunt bodies in hypersonic flow, we use resolvent analysis to study the flow over a blunt cone with 7{\deg} half-angle at Mach number $M_{\infty} = 6$, zero angle of attack, and nose-radius-based Reynolds number $Re_{R_n} = 90000$. Optimal forcing and responses are obtained for frequencies up to 330 kHz and azimuthal wavenumbers between 0 and 200. Wall-temperature effects are accounted for by considering both isothermal ($T_w =$ 300 K) and adiabatic wall conditions. The resolvent analysis shows that stationary streak modes are the most amplified in the isothermal case, followed by entropy-layer modes between 20 and 140 kHz. In the adiabatic case, the $1^{st}$ Mack mode is the most amplified. The entropy layer, caused by the nose-tip bluntness, has a profound influence on the receptivity structures. For the optimal streak mode, the most intense receptivity structures lie deep in the entropy layer, further away from the boundary-layer edge compared to the equivalent sharp-cone streak mode. This indicates that atmospheric disturbances may excite streak-like instability without fully penetrating the boundary layer. For the entropy-layer modes, the dominant receptivity and fluctuation signatures are located within the entropy layer. An energy-budget analysis reveals that these modes are most susceptible to kinetic disturbances and they sign the most on temperature fluctuations. These modes are found to leverage a temperature mixing mechanism that exploits the baseflow's wall-normal temperature gradient in the entropy layer to grow.

physics.flu-dyn↗

arXiv2023

Acoustic resolvent analysis of turbulent jets

We perform a resolvent analysis of a compressible turbulent jet, where the optimisation domain of the response modes is located in the acoustic field, excluding the hydrodynamic region, in order to promote acoustically efficient modes. We examine the properties of the acoustic resolvent and assess its potential for jet-noise modelling, focusing on the subsonic regime. Resolvent forcing modes, consistent with previous studies, are found to contain supersonic waves associated with Mach wave radiation in the response modes. This differs from the standard resolvent in which hydrodynamic instabilities dominate. We compare resolvent modes with SPOD modes educed from LES data. Acoustic resolvent response modes generally have better alignment with acoustic SPOD modes than standard resolvent response modes. For the optimal mode, the angle of the acoustic beam is close to that found in SPOD modes for moderate frequencies. However, there is no significant separation between the singular values of the leading and sub-optimal modes. Some suboptimal modes are furthermore shown to contain irrelevant structure for jet noise. Thus, even though it contains essential acoustic features absent from the standard resolvent approach, the SVD of the acoustic resolvent alone is insufficient to educe a low-rank model for jet noise. But because it identifies the prevailing mechanisms of jet noise, it provides valuable guidelines in the search of a forcing model (Karban \textit{et al.} An empirical model of noise sources in subsonic jets. \textit{Journal of Fluid Mechanics} (2023): A18).

physics.flu-dyn↗

arXiv2021

Self-similar mechanisms in wall turbulence studied using of resolvent analysis

Self-similarity of wall-attached coherent structures in a turbulent channel at $Re_\tau=543$ is explored by means of resolvent analysis. In this modelling framework, coherent structures are understood to arise as a response of the linearised mean-flow operator to generalised, frequency-dependent Reynolds stresses, considered to act as an endogenous forcing. We assess the self-similarity of both the wall-attached flow structures and the associated forcing. The former are educed from direct numerical simulation data by finding the flow field correlated with the wall shear, whereas the latter is identified using a frequency space version of Extended Proper Orthogonal Decomposition (Bor\'{e}e, J. 2003 Extended proper orthogonal decomposition: a tool to analyse correlated events in turbulent flows. Experiments in fluids 35 (2), 188-192). The forcing structures identified are compared to those obtained using the resolvent-based estimation introduced by Towne \emph{et al}. (Towne, A., Lozano-Dur\'{a}n, A. & Yang, X. 2020 Resolvent-based estimation of space-time flow statistics. Journal of Fluid Mechanics 883, A17). The analysis reveals self-similarity of both wall-attached structures$-$in quantitative agreement with Townsend's hypothesis of self-similar attached eddies$-$and the underlying forcing, at least in certain components.

physics.flu-dyn↗

arXiv2020

Ambiguity in mean-flow-based linear analysis

Linearisation of the Navier-Stokes equations about the mean of a turbulent flow forms the foundation of popular models for energy amplification and coherent structures, including resolvent analysis. While the Navier-Stokes equations can be equivalently written using many different sets of dependent variables, we show that the properties of the linear operator obtained via linearisation about the mean depend on the variables in which the equations are written prior to linearisation. For example, we show that using primitive and conservative variables leads to differences in the singular values and modes of the resolvent operator for turbulent jets, and that the differences become more severe as variable-density effects increase. This lack of uniqueness of mean-flow-based linear analysis provides new opportunities for optimizing models by specific choice of variables while also highlighting the importance of carefully accounting for the nonlinear terms that act as a forcing on the resolvent operator.

physics.flu-dyn↗

arXiv2006

Minimal Stereoscopic Analysis for Imaging Atmospheric Cherenkov Telescope Arrays

The trajectory of a primary gamma-ray detected with an array of at least four atmospheric Cherenkov imaging telescopes can be reconstructed from the shower image centroid positions and geometrical considerations independent of the primary energy. Using only the image centroid positions some cosmic-ray discrimination is also possible. This minimal approach opens the possibility of pushing the analysis threshold to lower values, close to the hardware threshold.

astro-ph↗