arXiv subjects

Matteo Bernardini

Publications and source records attributed to Matteo Bernardini.

17 recordsLinked to original sources

arXiv2026

Effects of thermochemical modelling on a hypersonic shock-wave/turbulent boundary-layer interaction

Thermochemical non-equilibrium can alter the structure, loads, and time scales of hypersonic shock-wave/turbulent boundary-layer interactions, yet its role in fully turbulent configurations remains largely unquantified. The present work addresses this issue by performing three direct numerical simulations of an oblique shock impinging on a turbulent high-enthalpy boundary layer at edge Mach number $M_e=6.4$ and stagnation enthalpy $H_e=16.9$ MJ/kg. The simulations share identical geometry and freestream conditions, but employ a hierarchy of progressively simplified thermochemical descriptions: a finite-rate reactive case, a single-species thermally perfect gas model, and a single-species calorically perfect model. The reactive simulation shows that the shock-induced temperature rise substantially enhances chemical activity relative to the incoming boundary layer, with peak concentrations of dissociation products attained downstream of the interaction. Thus, the thermal and chemical responses are not synchronised: the composition lags the rapid thermal forcing imposed by the shock system, and turbulent Damk\"ohler numbers reach values of order unity within the recirculation region, indicating non-negligible turbulence-chemistry interaction. The comparison among the three models shows that thermally and calorically perfect descriptions yield similar predictions, whereas finite-rate chemistry produces systematic differences: a smaller separation bubble, lower post-interaction wall heat flux, lower mean and fluctuating temperatures, and a less inclined reflected shock. In the present regime, the dominant modelling distinction is therefore between frozen and chemically reacting descriptions, with caloric-model effects playing only a secondary role.

physics.flu-dyn↗

arXiv2026

On the Reynolds analogy for high-speed rough-wall flows: implications for wall modelling

We study the validity of the generalized Reynolds analogy (GRA) in compressible turbulent boundary layers over prism-shaped roughness by mining direct numerical simulation data of Mach 2 and Mach 4 compressible turbulent boundary layers with adiabatic and cooled surfaces. Although the direct influence of roughness strongly disrupts the near-wall coupling between momentum and energy, we show that this breakdown is confined to the roughness sublayer. Above this layer, the enthalpy and velocity fields recover a smooth-wall-like similarity, and the GRA becomes asymptotically valid by naturally accounting for roughness-enhanced wall shear stress and heat flux. Building on these results, we propose a GRA-based wall model for predicting heat transfer over rough surfaces, which is coupled with a drag-predictive physics-based method developed for prism-shaped roughness by means of compressibility transformations.

physics.flu-dyn↗

arXiv2025

Experimental and Numerical Analysis of the Intermittentency in a Nozzle Overexpanded-Flow

The present work reports an investigation into the statistical properties of wall-pressure fluctuations in a highly over-expanded nozzle flow, characterized by significant shock-induced flow separation. This regime is extremely hazardous to rocket nozzles, as it leads to very high off-axis loads. The database under investigation has been obtained both experimentally and numerically by means of a hybrid RANS/LES simulation of the flow issuing from a sub-scale Truncated Ideal Contour (TIC) nozzle, fed with cold air and operating at a Reynolds number on the order of 10^6. The experimental campaign was conducted in the S150 supersonic wind tunnel at the Institut PPRIME in Poitiers. The degree of over-expansion is quantified by the nozzle pressure ratio (NPR). Pressure fluctuations are extracted from several probes positioned along the nozzle wall, considering different NPR values. The intermittent behavior is investigated using conditional statistics based on the wavelet transform, which demonstrates that the aerodynamic loads of the over-expanded jet consist of intermittent bursts rather than continuous variations. The wavelet analysis reveals scale-by-scale intermittency and, in particular, shows that the wall-pressure signals exhibit a significant degree of intermittency around the frequency associated with aerodynamic side-loads. The statistics of these intermittent events, in terms of the time delay between occurrences and in terms of their amplitude, are found to be weakly sensitive to NPRs and to locations along the nozzle wall and appear to follow a universal behaviour that can be modelled by a log-normal distribution. This finding may support the development of a stochastic model of the aerodynamic side-loads.

physics.flu-dyn↗

arXiv2024

High-fidelity simulations of microramp-controlled shock wave/boundary layer interaction

Microvortex generators (MVGs) are a promising solution to control shock wave/turbulent boundary layer interactions (SBLIs). This study examines the effects of a microramp VG on an SBLI generated by an oblique shock wave and a turbulent boundary layer using direct numerical simulations (DNSs). Two cases, with and without MVGs, are compared at free-stream Mach number equal to 2 and friction Reynolds number equal to 600. A long integration period allows assessing how MVGs affect the typical SBLI low-frequency unsteadiness. The 3D microramp wake dramatically alters the interaction, inducing spanwise modulation and topology changes of the separation. For example, tornado-like structures redistribute the recirculating flow in the spanwise and wall-normal directions. The increase in momentum close to the wall by the ramp vortices delays the onset of the separation but also leads to an increase in the intensity of the wall-pressure fluctuations. We then characterise the interaction between the arch-like vortices around the ramp wake and the SBLI. The spanwise vorticity shows that these vortices follow the edge of the separation, and their intensity is unaffected by the shocks. The shocks, instead, are deformed by the impingement of the vortices, although the spectral analysis does not reveal relevant traces of their shedding frequency at separation. These vortices, however, may be important in the separation bubble closure. A constant increase - in both value and magnitude - in the low-frequency peak is observed all along the span, suggesting that the shock oscillation remains coherent while being disturbed by the arch-like vortices.

physics.flu-dyn↗

arXiv2023

Turbulent drag reduction with streamwise travelling waves in the compressible regime

The ability of streamwise-travelling waves of spanwise velocity to reduce the turbulent skin friction drag is assessed in the compressible regime. Direct numerical simulations are carried out to compare drag reduction in subsonic, transonic and supersonic channel flows. Compressibility improves the benefits of the travelling waves, in a way that depends on the control parameters: drag reduction becomes larger than the incompressible one for small frequencies and wavenumbers. However, the improvement depends on the specific procedure employed for comparison. When the Mach number is varied and, at the same time, wall friction is changed by the control, the bulk temperature in the flow can either evolve freely in time until the aerodynamic heating balances the heat flux at the walls, or be constrained such that a fixed percentage of kinetic energy is transformed into thermal energy. Physical arguments suggest that, in the present context, the latter approach should be preferred. Not only it provides a test condition in which the wall-normal temperature profile more realistically mimics that in an external flow, but also leads to a much better scaling of the results, over both the Mach number and the control parameters. Under this comparison, drag reduction is only marginally improved by compressibility.

physics.flu-dyn↗

arXiv2023

Direct numerical simulation of supersonic boundary layers over a microramp: effect of the Reynolds number

Microvortex generators are passive control devices smaller than the boundary layer thickness that energise the boundary layer to prevent flow separation with limited induced drag. In this work, we use direct numerical simulations (DNSs) to investigate the effect of the Reynolds number in a supersonic turbulent boundary layer over a microramp vortex generator. Three friction Reynolds numbers are considered, up to $Re_{\tau}=2000$, for fixed free-stream Mach number $M_{\infty}=2$ and fixed relative height of the ramp with respect to the boundary layer thickness. The high-fidelity data set sheds light on the instantaneous and highly three-dimensional organisation of both the wake and the shock waves induced by the microramp. The full access to the flow field provided by DNS allows us to develop a qualitative model of the near wake, explaining the internal convolution of the Kelvin-Helmoltz vortices around the low-momentum region behind the ramp. The overall analysis shows that numerical results agree excellently with recent experimental measurements in similar operating conditions and confirms that microramps effectively induce a significantly fuller boundary layer even far downstream of the ramp. Moreover, results highlight significant Reynolds number effects, which in general do not scale with the ramp height. Increasing Reynolds number leads to enhanced coherence of the typical vortical structures in the field, faster and stronger development of the momentum deficit region, increased upwash between the primary vortices from the sides of the ramp - and thus increased lift-up of the wake - and faster transfer of momentum towards the wall.

physics.flu-dyn↗

arXiv2023

Assessment of heat transfer and Mach number effects on high-speed turbulent boundary layers

High-speed vehicles experience a highly challenging environment in which the free-stream Mach number and surface temperature greatly influence aerodynamic drag and heat transfer. The interplay of these two parameters strongly affects the near-wall dynamics of high-speed turbulent boundary layers in a non-trivial way, breaking similarity arguments on velocity and temperature fields, typically derived for adiabatic cases. In this work, we present direct numerical simulations of flat-plate zero-pressure-gradient turbulent boundary layers spanning three free-stream Mach numbers [2,4,6] and four wall temperature conditions (from adiabatic to very cold walls), emphasising the choice of the diabatic parameter $\mathit{\Theta}$ (Zhang, Bi, Hussain & She, J. Fluid Mech., vol. 739, pp. 392-420) to recover a similar flow organisation at different Mach numbers. We link qualitative observations on flow patterns to first- and second-order statistics to explain the strong decoupling of temperature-velocity fluctuations that occurs at reduced wall temperatures and high Mach numbers. For these cases, we find that the mean temperature gradient in the near-wall region can reach such a strong intensity that it promotes the formation of a secondary peak of thermal production in the viscous sublayer, which is in direct contrast with the monotonic behaviour of adiabatic profiles. We propose different physical mechanisms induced by wall-cooling and compressibility that result in apparently similar flow features, such as a higher peak in the streamwise velocity turbulence intensity, and distinct ones, such as the separation of turbulent scales.

physics.flu-dyn↗

arXiv2023

High-speed turbulent flows towards the exascale: STREAmS-2 porting and performance

Exascale High Performance Computing (HPC) represents a tremendous opportunity to push the boundaries of Computational Fluid Dynamics (CFD), but despite the consolidated trend towards the use of Graphics Processing Units (GPUs), programmability is still an issue. STREAmS-2 (Bernardini et al. Comput. Phys. Commun. 285 (2023) 108644) is a compressible solver for canonical wall-bounded turbulent flows capable of harvesting the potential of NVIDIA GPUs. Here we extend the already available CUDA Fortran backend with a novel HIPFort backend targeting AMD GPU architectures. The main implementation strategies are discussed along with a novel Python tool that can generate the HIPFort and CPU code versions allowing developers to focus their attention only on the CUDA Fortran backend. Single GPU performance is analysed focusing on NVIDIA A100 and AMD MI250x cards which are currently at the core of several HPC clusters. The gap between peak GPU performance and STREAmS-2 performance is found to be generally smaller for NVIDIA cards. Roofline analysis allows tracing this behavior to unexpectedly different computational intensities of the same kernel using the two cards. Parallel performance is measured on the two largest EuroHPC pre-exascale systems, LUMI (AMD GPUs) and Leonardo (NVIDIA GPUs). Strong scalability reveals more than 80% efficiency up to 16 nodes for Leonardo and up to 32 for LUMI. Weak scalability shows an impressive efficiency of over 95% up to the maximum number of nodes tested (256 for LUMI and 512 for Leonardo). This analysis shows that STREAmS-2 is the perfect candidate to fully exploit the power of current pre-exascale HPC systems in Europe, allowing users to simulate flows with over a trillion mesh points, thus reducing the gap between the Reynolds numbers achievable in high-fidelity simulations and those of real engineering applications.

physics.flu-dyn↗

arXiv2022

Unsteadiness characterisation of shock wave/turbulent boundary-layer interaction at moderate Reynolds number

A direct numerical simulation of an oblique shock wave impinging on a turbulent boundary layer at Mach number 2.28 is carried out at moderate Reynolds number, simulating flow conditions similar to those of the experiment by Dupont et al. (2006). The low-frequency shock unsteadiness, whose characteristics have been the focus of considerable research efforts, is here investigated via the Morlet wavelet transform. Owing to its compact support in both physical and Fourier spaces, the wavelet transformation makes it possible to track the time evolution of the various scales of the wall-pressure fluctuations. This property also makes it possible to define a local intermittency measure, representing a frequency-dependent flatness factor, to pinpoint the bursts of energy that characterise the shock intermittency scale by scale. As a major result, wavelet decomposition shows that the broadband shock movement is actually the result of a collection of sparse events in time, each characterised by its own temporal scale. This feature is hidden by the classical Fourier analysis, which can only show the time-averaged behaviour. Then, we propose a procedure to process any relevant time series, such as the time history of the wall-pressure or that of the separation bubble extent, in which we use a condition based on the local intermittency measure to filter out the turbulent content in the proximity of the shock foot and to isolate only the intermittent component of the signal. In addition, wavelet analysis reveals the intermittent behaviour also of the breathing motion of the recirculation bubble behind the reflected shock, and allows us to detect a direct, partial correspondence between the most significant intermittent events of the separation region and those of the wall-pressure at the foot of the shock.

physics.flu-dyn↗

arXiv2020

Passive flow-separation control in a dual-bell rocket nozzle

A delayed detached eddy simulation of a sub-scale cold-gas dual-bell nozzle flow at high Reynolds number and in sea-level mode is carried out at nozzle pressure ratio NPR=45.7. In this regime the over-expanded flow exhibits a symmetric and controlled flow separation at the inflection point, that is the junction between the two bells, leading to the generation of a low content of aerodynamic side loads with respect to conventional bell nozzles. The nozzle wall-pressure signature is analysed in the frequency domain and compared with the experimental data available in the literature for the same geometry and flow conditions. The Fourier spectra in time and space (azimuthal wavenumber) show the presence of a persistent tone associated to the symmetric shock movement. Asymmetric modes are only slightly excited by the shock and the turbulent structures. The low mean value of the side-loads magnitude is in good agreement with the experiments and confirms that the inflection point dampens the aero-acoustic interaction between the separation-shock and the detached shear layer.

physics.flu-dyn↗

arXiv2020

STREAmS: a high-fidelity accelerated solver for direct numerical simulation of compressible turbulent flow

We present STREAmS, an in-house high-fidelity solver for large-scale, massively parallel direct numerical simulations (DNS) of compressible turbulent flows on graphical processing units (GPUs). STREAmS is written in the Fortran 90 language and it is tailored to carry out DNS of canonical compressible wall-bounded flows, namely turbulent plane channel, zero-pressure gradient turbulent boundary layer and supersonic oblique shock-wave/boundary layer interactions. The solver incorporates state-of-the-art numerical algorithms, specifically designed to cope with the challenging problems associated with the solution of high-speed turbulent flows and can be used across a wide range of Mach numbers, extending from the low subsonic up to the hypersonic regime. The use of cuf automatic kernels allowed an easy and efficient porting on the GPU architecture minimizing the changes to the original CPU code, which is also maintained. We discuss a memory allocation strategy based on duplicated arrays for host and device which carefully minimizes the memory usage making the solver suitable for large scale computations on the latest GPU cards. Comparison between different CPUs and GPUs architectures strongly favor the latter, and executing the solver on a single NVIDIA Tesla P100 corresponds to using approximately 330 Intel Knights Landing CPU cores. STREAmS shows very good strong scalability and essentially ideal weak scalability up to 2048 GPUs, paving the way to simulations in the genuine high-Reynolds number regime, possibly at friction Reynolds number $Re_τ > 10^4$. The solver is released open source under GPLv3 license and is available at https://github.com/matteobernardini/STREAmS.

physics.comp-ph↗

arXiv2019

Blockchains Meet Distributed Hash Tables: Decoupling Validation from State Storage

The first obstacle that regular users encounter when setting up a node for a public blockchain is the time taken for downloading all the data needed for the node to start operating correctly. In fact, this may last from hours to weeks for the major networks. Our contribution is twofold. Firstly, we show a design that enables mining and validation of new blocks keeping only a very small state. Secondly, we show that it is possible to store the state of the blockchain in a distributed hash table obtaining a wide spectrum of trade-offs between storage committed by the nodes and replication factor. Our proposal is independent from the consensus algorithm adopted, and copes well with transactions that involve smart contracts.

cs.DC↗

arXiv2017

Numerical investigation of supersonic shock-wave/boundary-layer interaction in transitional and turbulent regime

We perform direct numerical simulations of shock-wave/boundary-layer interactions (SBLI) at Mach number M = 1.7 to investigate the influence of the state of the incoming boundary layer on the interaction properties. We reproduce and extend the flow conditions of the experiments performed by Giepman et al., in which a spatially evolving laminar boundary layer over a flat plate is initially tripped by an array of distributed roughness elements and impinged further downstream by an oblique shock wave. Four SBLI cases are considered, based on two different shock impingement locations along the streamwise direction, corresponding to transitional and turbulent interactions, and two different shock strengths, corresponding to flow deflection angles 3 degreees and 6 degrees. We find that, for all flow cases, shock induced separation is not observed, the boundary layer remains attached for the 3 degrees case and close to incipient separation for the 6 degrees case, independent of the state of the incoming boundary layer. The findings of this work suggest that a transitional interaction might be the optimal solution for practical SBLI applications, as it removes the large separation bubble typical of laminar interactions and reduces the extent of the high-friction region associated with an incoming turbulent boundary layer.

physics.flu-dyn↗

arXiv2016

Mixed convection in turbulent channels with unstable stratification

We study turbulent flows in planar channels with unstable thermal stratification, using direct numerical simulations in a wide range of Reynolds and Rayleigh numbers and reaching flow conditions which are representative of asymptotic developed turbulence. The combined effect of forced and free convection produces a peculiar pattern of quasi--streamwise rollers occupying the full channel thickness with aspect--ratio considerably higher than unity; it has been observed that they have an important redistributing effect on temperature and momentum. The mean values and the variances of the flow variables do not appear to follow Prandtl's scaling in the flow regime near free convection, except for the temperature and vertical velocity fluctuations, which are more affected by turbulent plumes. Nevertheless, we find that the Monin--Obukhov theory still yields a useful representation of the main flow features. In particular, the widely used Businger--Dyer relationships provide a convenient way of accounting for the bulk effects of shear and buoyancy, although individual profiles may vary widely from the alleged trends. Significant deviations are found in DNS with respect to the commonly used parametrization of the mean velocity in the light-wind regime, which may have important practical impact in models of atmospheric dynamics. Finally, for modelling purposes, we devise a set of empirical predictive formulas for the heat flux and friction coefficients which can be used with about $10\%$ maximum error in a wide range of flow parameters.

physics.flu-dyn↗

arXiv2016

Detached eddy simulation of shock unsteadiness in an over-expanded planar nozzle

This work investigates the self-excited shock wave oscillations in a three-dimensional planar over-expanded nozzle turbulent flow by means of Detached Eddy Simulations. Time resolved wall pressure measurements are used as primary diagnostics. The statistical analysis reveals that the shock unsteadiness has common features in terms of the root mean square of the pressure fluctuations with other classical shock wave/boundary layer interactions, like compression ramps and incident shocks on a flat plate. The Fourier transform and the continuous wavelet transform are used to conduct the spectral analysis. The results of the former indicate that the pressure in the shock region is characterized by a broad low-frequency content, without any resonant tone. The wavelet analysis, which is well suited to study non stationary process, reveals that the pressure signal is characterized by an amplitude and a frequency modulation in time.

physics.flu-dyn↗

arXiv2016

Heat transfer and wall temperature effects in shock wave turbulent boundary layer interactions

Direct numerical simulations are carried out to investigate the effect of the wall temperature on the behavior of oblique shock-wave/turbulent boundary layer interactions at freestream Mach number $2.28$ and shock angle of the wedge generator $φ= 8^{\circ}$. Five values of the wall-to-recovery-temperature ratio ($T_w/T_r$) are considered, corresponding to cold, adiabatic and hot wall thermal conditions. We show that the main effect of cooling is to decrease the characteristic scales of the interaction in terms of upstream influence and extent of the separation bubble. The opposite behavior is observed in the case of heating, that produces a marked dilatation of the interaction region. The distribution of the Stanton number shows that a strong amplification of the heat transfer occurs across the interaction, and the maximum values of thermal and dynamic loads are found in the case of cold wall. The analysis reveals that the fluctuating heat flux exhibits a strong intermittent behavior, characterized by scattered spots with extremely high values compared to the mean. Furthermore, the analogy between momentum and heat transfer, typical of compressible, wall-bounded, equilibrium turbulent flows does not apply for most part of the interaction domain. The pre-multiplied spectra of the wall heat flux do not show any evidence of the influence of the low-frequency shock motion, and the primary mechanism for the generation of peak heating is found to be linked with the turbulence amplification in the interaction region.

physics.flu-dyn↗

arXiv2015

DNS of laminar-turbulent boundary layer transition induced by solid obstacles

Results of numerical simulations obtained by a staggered finite difference scheme together with an efficient immersed boundary method are presented to understand the effects of the shape of three-dimensional obstacles on the transition of a boundary layer from a laminar to a turbulent regime. Fully resolved Direct Numerical Simulations (DNS), highlight that the closer to the obstacle the symmetry is disrupted the smaller is the transitional Reynolds number. It has been also found that the transition can not be related to the critical roughness Reynolds number used in the past. The simulations highlight the differences between wake and inflectional instabilities, proving that two-dimensional tripping devices are more efficient in promoting the transition. Simulations at high Reynolds number demonstrate that the reproduction of a real experiment with a solid obstacle at the inlet is an efficient tool to generate numerical data bases for understanding the physics of boundary layers. The quality of the numerical method to fully resolve the small scales, that is one ingredient for a DNS was shown by a comparison of the exponential range of the velocity spectra, in Kolmogorov units, with those for isotropic turbulence. The good comparison reinforces the idea of local isotropy at the smallest scales.

physics.flu-dyn↗