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The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Mon, 04 Nov 2024 07:50:11 GMT2024-11-04T07:50:11ZRoughness-induced transition by quasi-resonance of a varicose global mode
http://hdl.handle.net/10985/17804
Roughness-induced transition by quasi-resonance of a varicose global mode
BUCCI, Michele Alessandro; PUCKERT, Dominik K.; ANDRIANO, Cesare; CHERUBINI, Stefania; RIST, Ulrich; ROBINET, Jean-Christophe; LOISEAU, Jean-Christophe
The onset of unsteadiness in a boundary-layer flow past a cylindrical roughness element is investigated for three flow configurations at subcritical Reynolds numbers, both experimentally and numerically. On the one hand, a quasi-periodic shedding of hairpin vortices is observed for all configurations in the experiment. On the other hand, global stability analyses have revealed the existence of a varicose isolated mode, as well as of a sinuous one, both being linearly stable. Nonetheless, the isolated stable varicose modes are highly sensitive, as ascertained by pseudospectrum analysis. To investigate how these modes might influence the dynamics of the flow, an optimal forcing analysis is performed. The optimal response consists of a varicose perturbation closely related to the least stable varicose isolated eigenmode and induces dynamics similar to that observed experimentally. The quasi-resonance of such a global mode to external forcing might thus be responsible for the onset of unsteadiness at subcritical Reynolds numbers, hence providing a simple explanation for the experimental observations.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10985/178042017-01-01T00:00:00ZBUCCI, Michele AlessandroPUCKERT, Dominik K.ANDRIANO, CesareCHERUBINI, StefaniaRIST, UlrichROBINET, Jean-ChristopheLOISEAU, Jean-ChristopheThe onset of unsteadiness in a boundary-layer flow past a cylindrical roughness element is investigated for three flow configurations at subcritical Reynolds numbers, both experimentally and numerically. On the one hand, a quasi-periodic shedding of hairpin vortices is observed for all configurations in the experiment. On the other hand, global stability analyses have revealed the existence of a varicose isolated mode, as well as of a sinuous one, both being linearly stable. Nonetheless, the isolated stable varicose modes are highly sensitive, as ascertained by pseudospectrum analysis. To investigate how these modes might influence the dynamics of the flow, an optimal forcing analysis is performed. The optimal response consists of a varicose perturbation closely related to the least stable varicose isolated eigenmode and induces dynamics similar to that observed experimentally. The quasi-resonance of such a global mode to external forcing might thus be responsible for the onset of unsteadiness at subcritical Reynolds numbers, hence providing a simple explanation for the experimental observations.A synthetic forcing to trigger laminar-turbulent transition in parallel wall bounded flows via receptivity
http://hdl.handle.net/10985/17987
A synthetic forcing to trigger laminar-turbulent transition in parallel wall bounded flows via receptivity
PICELLA, Francesco; BUCCI, Michele Alessandro; CHERUBINI, Stefania; ROBINET, Jean-Christophe
Research on laminar-turbulent transition of wall-bounded parallel flows has usually focused on controlled scenarios where transition is triggered by perturbations having simple shapes and spectra. These disturbances strongly differ from the environmental noise usually present in experimental setups or industrial applications, where uncontrolled transition is usually observed. In this paper a new method is proposed to trigger uncontrolled transition to turbulence in wall-bounded parallel flows exploiting the receptivity of the flow to a volume forcing. Using some concepts provided by linear stability and sensitivity analysis, such as the resolvent, we propose a method for constructing a volume forcing capable of inducing stochastic velocity perturbations with a prescribed energy level, eventually leading to laminar-turbulent transition as a response of the system to external noise. The method has been tested in a channel flow configuration, using direct numerical simulations of the fully nonlinear Navier-Stokes equations in the presence of the volume forcing constructed on the basis of optimal forcing functions. Subcritical transition to turbulence induced by the prescribed forcing has been investigated and compared to other transition scenarios, where deterministic perturbations are imposed for obtaining a turbulent flow. Finally, the fully developed turbulent flows induced by the proposed method has been analysed, showing that low-order statistics and energy balance equations are practically unaffected by the continuous synthetic forcing.
Tue, 01 Jan 2019 00:00:00 GMThttp://hdl.handle.net/10985/179872019-01-01T00:00:00ZPICELLA, FrancescoBUCCI, Michele AlessandroCHERUBINI, StefaniaROBINET, Jean-ChristopheResearch on laminar-turbulent transition of wall-bounded parallel flows has usually focused on controlled scenarios where transition is triggered by perturbations having simple shapes and spectra. These disturbances strongly differ from the environmental noise usually present in experimental setups or industrial applications, where uncontrolled transition is usually observed. In this paper a new method is proposed to trigger uncontrolled transition to turbulence in wall-bounded parallel flows exploiting the receptivity of the flow to a volume forcing. Using some concepts provided by linear stability and sensitivity analysis, such as the resolvent, we propose a method for constructing a volume forcing capable of inducing stochastic velocity perturbations with a prescribed energy level, eventually leading to laminar-turbulent transition as a response of the system to external noise. The method has been tested in a channel flow configuration, using direct numerical simulations of the fully nonlinear Navier-Stokes equations in the presence of the volume forcing constructed on the basis of optimal forcing functions. Subcritical transition to turbulence induced by the prescribed forcing has been investigated and compared to other transition scenarios, where deterministic perturbations are imposed for obtaining a turbulent flow. Finally, the fully developed turbulent flows induced by the proposed method has been analysed, showing that low-order statistics and energy balance equations are practically unaffected by the continuous synthetic forcing.Time-Stepping and Krylov Method for large scale instability problems
http://hdl.handle.net/10985/17840
Time-Stepping and Krylov Method for large scale instability problems
BUCCI, Michele Alessandro; CHERUBINI, Stefania; ROBINET, Jean-Christophe; LOISEAU, Jean-Christophe
With the ever increasing computational power available and the development of high-performances computing, investigating the properties of realistic very large-scale nonlinear dynamical systems has become reachable. It must be noted however that the memory capabilities of computers increase at a slower rate than their computational capabilities. Consequently, the traditional matrix-forming approaches wherein the Jacobian matrix of the system considered is explicitly assembled become rapidly intractable. Over the past two decades, so-called matrix-free approaches have emerged as an efficient alternative. The aim of this chapter is thus to provide an overview of well-grounded matrix-free methods for fixed points computations and linear stability analyses of very large-scale nonlinear dynamical systems.
Mon, 01 Jan 2018 00:00:00 GMThttp://hdl.handle.net/10985/178402018-01-01T00:00:00ZBUCCI, Michele AlessandroCHERUBINI, StefaniaROBINET, Jean-ChristopheLOISEAU, Jean-ChristopheWith the ever increasing computational power available and the development of high-performances computing, investigating the properties of realistic very large-scale nonlinear dynamical systems has become reachable. It must be noted however that the memory capabilities of computers increase at a slower rate than their computational capabilities. Consequently, the traditional matrix-forming approaches wherein the Jacobian matrix of the system considered is explicitly assembled become rapidly intractable. Over the past two decades, so-called matrix-free approaches have emerged as an efficient alternative. The aim of this chapter is thus to provide an overview of well-grounded matrix-free methods for fixed points computations and linear stability analyses of very large-scale nonlinear dynamical systems.Global stability analysis of 3D micro-combustion model
http://hdl.handle.net/10985/17884
Global stability analysis of 3D micro-combustion model
BUCCI, Michele Alessandro; CHIBBARO, Sergio; ROBINET, Jean-Christophe
We report on the linear stability of micro-combustion in pipe, where two instabilities manifest at high and low flow-rates. The combustion of a stoichiometric methane/air premixed mixture has been numerically investigated within a 3D reduced model. This model reproduces decently the flame dynamics in the range of speed between 5–100 cm/s. The flame position, the stability thresholds of the Flame with Repetitive Extinction and Ignition (FREI) and the flame shape are in accordance with the experiments. Furthermore, an analysis of the integral values of all mechanisms involved in the flame evolution has been carried out near the two stability thresholds. The phase shift between the reaction term and the radial diffusion has been identified as the source of instability in both cases. The global behavior has been then investigated with a linear stability analysis. The 2D and 3D temperature and concentration perturbations have been found by solving the eigenvalue problem obtained by linearizing the model around the basic state. Only one unstable axisymmetric mode has been found. This is in agreement with the direct numerical simulation of the model.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10985/178842016-01-01T00:00:00ZBUCCI, Michele AlessandroCHIBBARO, SergioROBINET, Jean-ChristopheWe report on the linear stability of micro-combustion in pipe, where two instabilities manifest at high and low flow-rates. The combustion of a stoichiometric methane/air premixed mixture has been numerically investigated within a 3D reduced model. This model reproduces decently the flame dynamics in the range of speed between 5–100 cm/s. The flame position, the stability thresholds of the Flame with Repetitive Extinction and Ignition (FREI) and the flame shape are in accordance with the experiments. Furthermore, an analysis of the integral values of all mechanisms involved in the flame evolution has been carried out near the two stability thresholds. The phase shift between the reaction term and the radial diffusion has been identified as the source of instability in both cases. The global behavior has been then investigated with a linear stability analysis. The 2D and 3D temperature and concentration perturbations have been found by solving the eigenvalue problem obtained by linearizing the model around the basic state. Only one unstable axisymmetric mode has been found. This is in agreement with the direct numerical simulation of the model.Influence of freestream turbulence on the flow over a wall roughness
http://hdl.handle.net/10985/20464
Influence of freestream turbulence on the flow over a wall roughness
BUCCI, Michele Alessandro; CHERUBINI, Stefania; ROBINET, Jean-Christophe; LOISEAU, Jean-Christophe
The effect of freestream turbulence on the dynamics of an incompressible flow past a cylindrical roughness element in subcritical conditions (i.e., for Reynolds numbers below the onset of linear instability) has been investigated by the joint application of direct numerical simulations, linear modal and nonmodal stability analyses, and dynamic mode decomposition. At first, the influence of the Reynolds number and the ratio of the boundary layer’s thickness to roughness height on the three-dimensional spatiotemporal (global) stability of the flow has been investigated. Depending on the operating conditions, the leading instability can either be varicose (symmetric) or sinuous (antisymmetric). In both cases, when the flow is excited by broadband frequency forcing, dynamic mode decomposition extracts only varicose coherent structures even though optimal response analysis predicts a strong amplification of sinuous disturbances having frequency close to that of the marginally stable sinuous eigenmode. This apparent discrepancy is attributed to the fact that the sinuous instability is sensitive to a very limited range of frequencies barely excited by freestream turbulence while varicose disturbances are associated with high amplification in a much wider frequency range. Hence, in this case the flow behaves as an amplifier of varicose perturbations rather than a resonator. Consequences on the subsequent transition to turbulence have been studied, highlighting that varicose perturbations extract energy from the near-wake region, get continuously amplified due to the excitation provided by freestream turbulence, and eventually give rise to a shedding of hairpin vortices.
Fri, 01 Jan 2021 00:00:00 GMThttp://hdl.handle.net/10985/204642021-01-01T00:00:00ZBUCCI, Michele AlessandroCHERUBINI, StefaniaROBINET, Jean-ChristopheLOISEAU, Jean-ChristopheThe effect of freestream turbulence on the dynamics of an incompressible flow past a cylindrical roughness element in subcritical conditions (i.e., for Reynolds numbers below the onset of linear instability) has been investigated by the joint application of direct numerical simulations, linear modal and nonmodal stability analyses, and dynamic mode decomposition. At first, the influence of the Reynolds number and the ratio of the boundary layer’s thickness to roughness height on the three-dimensional spatiotemporal (global) stability of the flow has been investigated. Depending on the operating conditions, the leading instability can either be varicose (symmetric) or sinuous (antisymmetric). In both cases, when the flow is excited by broadband frequency forcing, dynamic mode decomposition extracts only varicose coherent structures even though optimal response analysis predicts a strong amplification of sinuous disturbances having frequency close to that of the marginally stable sinuous eigenmode. This apparent discrepancy is attributed to the fact that the sinuous instability is sensitive to a very limited range of frequencies barely excited by freestream turbulence while varicose disturbances are associated with high amplification in a much wider frequency range. Hence, in this case the flow behaves as an amplifier of varicose perturbations rather than a resonator. Consequences on the subsequent transition to turbulence have been studied, highlighting that varicose perturbations extract energy from the near-wake region, get continuously amplified due to the excitation provided by freestream turbulence, and eventually give rise to a shedding of hairpin vortices.