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Tue, 12 Nov 2024 16:52:45 GMT
20241112T16:52:45Z

Early evolution of the compressible mixing layer issued from two turbulent streams
http://hdl.handle.net/10985/10381
Early evolution of the compressible mixing layer issued from two turbulent streams
PIROZZOLI, Sergio; BERNARDINI, Matteo; MARIÉ, Simon; GRASSO, Francesco
Direct numerical simulation of the spatially developing mixing layer issuing from two turbulent streams past a splitter plate is carried out under mild compressibility conditions. The study mainly focuses on the early evolution of the mixing region, where transition occurs from a wakelike to a canonical mixinglayerlike behaviour, corresponding to the fillingup of the initial momentum deficit. The mixing layer is found to initially grow faster than linearly, and then at a sublinear rate further downstream. The Reynolds stress components are in close agreement with reference experiments and follow a continued slow decay till the end of the computational domain. These observations are suggestive of the occurrence of incomplete similarity in the developing turbulent mixing layer. Coherent eddies are found to form in the close proximity of the splitter plate trailing edge, that are mainly organized in bands, initially skewed and then parallel to the spanwise direction. Dynamic mode decomposition is used to educe the dynamically relevant features, and it is found to be capable of singling out the coherent eddies responsible for mixing layer development.
Thu, 01 Jan 2015 00:00:00 GMT
http://hdl.handle.net/10985/10381
20150101T00:00:00Z
PIROZZOLI, Sergio
BERNARDINI, Matteo
MARIÉ, Simon
GRASSO, Francesco
Direct numerical simulation of the spatially developing mixing layer issuing from two turbulent streams past a splitter plate is carried out under mild compressibility conditions. The study mainly focuses on the early evolution of the mixing region, where transition occurs from a wakelike to a canonical mixinglayerlike behaviour, corresponding to the fillingup of the initial momentum deficit. The mixing layer is found to initially grow faster than linearly, and then at a sublinear rate further downstream. The Reynolds stress components are in close agreement with reference experiments and follow a continued slow decay till the end of the computational domain. These observations are suggestive of the occurrence of incomplete similarity in the developing turbulent mixing layer. Coherent eddies are found to form in the close proximity of the splitter plate trailing edge, that are mainly organized in bands, initially skewed and then parallel to the spanwise direction. Dynamic mode decomposition is used to educe the dynamically relevant features, and it is found to be capable of singling out the coherent eddies responsible for mixing layer development.

Smallscale dynamics of dense gas compressible homogeneous isotropic turbulence
http://hdl.handle.net/10985/15598
Smallscale dynamics of dense gas compressible homogeneous isotropic turbulence
CINNELLA, Paola; GRASSO, Francesco; SCIACOVELLI, Luca
The present paper investigates the influence of dense gases governed by complex equations of state on the dynamics of homogeneous isotropic turbulence. In particular, we investigate how differences due to the complex thermodynamic behaviour and transport properties affect the smallscale structures, viscous dissipation and enstrophy generation. To this end, we carry out direct numerical simulations of the compressible Navier–Stokes equations supplemented by advanced dense gas constitutive models. The dense gas considered in the study is a heavy fluorocarbon (PP11) that is shown to exhibit an inversion zone (i.e. a region where the fundamental derivative of gas dynamics Γ is negative) in its vapour phase, for pressures and temperatures of the order of magnitude of the critical ones. Simulations are carried out at various initial turbulent Mach numbers and for two different initial thermodynamic states, one immediately outside and the other inside the inversion zone. After investigating the influence of dense gas effects on the time evolution of mean turbulence properties, we focus on the statistical properties of turbulent structures. For that purpose we carry out an analysis in the plane of the second and third invariant of the deviatoric strainrate tensor. The analysis shows a weakening of compressive structures and an enhancement of expanding ones. Strong expansion regions are found to be mostly populated by nonfocal convergence structures typical of strong compression regions, in contrast with the perfect gas that is dominated by eddylike structures. Additionally, the contribution of nonfocal expanding structures to the dilatational dissipation is comparable to that of compressed structures. This is due to the occurrence of steep expansion fronts and possibly of expansion shocklets which contribute to enstrophy generation in strong expansion regions and that counterbalance enstrophy destruction by means of the eddylike structures.
Sun, 01 Jan 2017 00:00:00 GMT
http://hdl.handle.net/10985/15598
20170101T00:00:00Z
CINNELLA, Paola
GRASSO, Francesco
SCIACOVELLI, Luca
The present paper investigates the influence of dense gases governed by complex equations of state on the dynamics of homogeneous isotropic turbulence. In particular, we investigate how differences due to the complex thermodynamic behaviour and transport properties affect the smallscale structures, viscous dissipation and enstrophy generation. To this end, we carry out direct numerical simulations of the compressible Navier–Stokes equations supplemented by advanced dense gas constitutive models. The dense gas considered in the study is a heavy fluorocarbon (PP11) that is shown to exhibit an inversion zone (i.e. a region where the fundamental derivative of gas dynamics Γ is negative) in its vapour phase, for pressures and temperatures of the order of magnitude of the critical ones. Simulations are carried out at various initial turbulent Mach numbers and for two different initial thermodynamic states, one immediately outside and the other inside the inversion zone. After investigating the influence of dense gas effects on the time evolution of mean turbulence properties, we focus on the statistical properties of turbulent structures. For that purpose we carry out an analysis in the plane of the second and third invariant of the deviatoric strainrate tensor. The analysis shows a weakening of compressive structures and an enhancement of expanding ones. Strong expansion regions are found to be mostly populated by nonfocal convergence structures typical of strong compression regions, in contrast with the perfect gas that is dominated by eddylike structures. Additionally, the contribution of nonfocal expanding structures to the dilatational dissipation is comparable to that of compressed structures. This is due to the occurrence of steep expansion fronts and possibly of expansion shocklets which contribute to enstrophy generation in strong expansion regions and that counterbalance enstrophy destruction by means of the eddylike structures.

Dense gas effects in inviscid homogeneous isotropic turbulence
http://hdl.handle.net/10985/15675
Dense gas effects in inviscid homogeneous isotropic turbulence
CINNELLA, Paola; CONTENT, C.; GRASSO, Francesco; SCIACOVELLI, Luca
A detailed numerical study of the influence of dense gas effects on the largescale dynamics of decaying homogeneous isotropic turbulence is carried out by using the van der Waals gas model. More specifically, we focus on dense gases of the Bethe–Zel’dovich–Thompson type, which may exhibit nonclassical nonlinearities in the transonic and supersonic flow regimes, under suitable thermodynamic conditions. The simulations are based on the inviscid conservation equations, solved by means of a ninthorder numerical scheme. The simulations rely on the numerical viscosity of the scheme to dissipate energy at the finest scales, while leaving the larger scales mostly unaffected. The results are systematically compared with those obtained for a perfect gas. Dense gas effects are found to have a significant influence on the time evolution of the average and root mean square (r.m.s.) of the thermodynamic properties for flows characterized by sufficiently high initial turbulent Mach numbers (above 0.5), whereas the influence on kinematic properties, such as the kinetic energy and the vorticity, are smaller. However, the flow dilatational behaviour is very different, due to the nonclassical variation of the speed of sound in flow regions where the dense gas is characterized by a value of the fundamental derivative of the gas dynamics (a measure of the variation of the speed of sound in isentropic compressions) smaller than one or even negative. The most significant differences between the perfect and the dense gas case are found for the repartition of dilatation levels in the flow field. For the perfect gas, strong compressions occupy a much larger volume fraction than expansion regions, leading to probability distributions of the velocity divergence highly skewed toward negative values. For the dense gas, the volume fractions occupied by strong expansion and compression regions are much more balanced; moreover, strong expansion regions are characterized by sheetlike structures, unlike the perfect gas which exhibits tubular structures. In strong compression regions, where compression shocklets may occur, both the dense and the perfect gas exhibit sheetlike structures. This suggests the possibility that expansion eddy shocklets may appear in the dense gas. This hypothesis is also supported by the fact that, in dense gas, vorticity is created with equal probability in strong compression and expansion regions, whereas for a perfect gas, vorticity is more likely to be created in the strong compression ones.
Fri, 01 Jan 2016 00:00:00 GMT
http://hdl.handle.net/10985/15675
20160101T00:00:00Z
CINNELLA, Paola
CONTENT, C.
GRASSO, Francesco
SCIACOVELLI, Luca
A detailed numerical study of the influence of dense gas effects on the largescale dynamics of decaying homogeneous isotropic turbulence is carried out by using the van der Waals gas model. More specifically, we focus on dense gases of the Bethe–Zel’dovich–Thompson type, which may exhibit nonclassical nonlinearities in the transonic and supersonic flow regimes, under suitable thermodynamic conditions. The simulations are based on the inviscid conservation equations, solved by means of a ninthorder numerical scheme. The simulations rely on the numerical viscosity of the scheme to dissipate energy at the finest scales, while leaving the larger scales mostly unaffected. The results are systematically compared with those obtained for a perfect gas. Dense gas effects are found to have a significant influence on the time evolution of the average and root mean square (r.m.s.) of the thermodynamic properties for flows characterized by sufficiently high initial turbulent Mach numbers (above 0.5), whereas the influence on kinematic properties, such as the kinetic energy and the vorticity, are smaller. However, the flow dilatational behaviour is very different, due to the nonclassical variation of the speed of sound in flow regions where the dense gas is characterized by a value of the fundamental derivative of the gas dynamics (a measure of the variation of the speed of sound in isentropic compressions) smaller than one or even negative. The most significant differences between the perfect and the dense gas case are found for the repartition of dilatation levels in the flow field. For the perfect gas, strong compressions occupy a much larger volume fraction than expansion regions, leading to probability distributions of the velocity divergence highly skewed toward negative values. For the dense gas, the volume fractions occupied by strong expansion and compression regions are much more balanced; moreover, strong expansion regions are characterized by sheetlike structures, unlike the perfect gas which exhibits tubular structures. In strong compression regions, where compression shocklets may occur, both the dense and the perfect gas exhibit sheetlike structures. This suggests the possibility that expansion eddy shocklets may appear in the dense gas. This hypothesis is also supported by the fact that, in dense gas, vorticity is created with equal probability in strong compression and expansion regions, whereas for a perfect gas, vorticity is more likely to be created in the strong compression ones.

Densegas effects on compressible boundarylayer stability
http://hdl.handle.net/10985/18556
Densegas effects on compressible boundarylayer stability
CINNELLA, Paola; GRASSO, Francesco; ROBINET, JeanChristophe; SCIACOVELLI, Luca; GLOERFELT, Xavier
A study of densegas effects on the stability of compressible boundarylayer flows is conducted. From the laminar similarity solution, the temperature variations are small due to the high specific heat of dense gases, leading to velocity profiles close to the incompressible ones. Concurrently, the complex thermodynamic properties of dense gases can lead to unconventional compressibility effects. In the subsonic regime, the Tollmien–Schlichting viscous mode is attenuated by compressibility effects and becomes preferentially skewed in line with the results based on the idealgas assumption. However, the absence of a generalized inflection point precludes the sustainability of the first mode by inviscid mechanisms. On the contrary, the viscous mode can be completely stable at supersonic speeds. At very high speeds, we have found instances of radiating supersonic instabilities with substantial amplification rates, i.e. waves that travel supersonically relative to the freestream velocity. This acoustic mode has qualitatively similar features for various thermodynamic conditions and for different working fluids. This shows that the leading parameters governing the boundarylayer behaviour for the dense gas are the constantpressure specific heat and, to a minor extent, the densitydependent viscosity. A satisfactory scaling of the mode characteristics is found to be proportional to the height of the layer near the wall that acts as a waveguide where acoustic waves may become trapped. This means that the supersonic mode has the same nature as Mack’s modes, even if its frequency for maximal amplification is greater. Direct numerical simulation accurately reproduces the development of the supersonic mode and emphasizes the radiation of the instability waves.
Wed, 01 Jan 2020 00:00:00 GMT
http://hdl.handle.net/10985/18556
20200101T00:00:00Z
CINNELLA, Paola
GRASSO, Francesco
ROBINET, JeanChristophe
SCIACOVELLI, Luca
GLOERFELT, Xavier
A study of densegas effects on the stability of compressible boundarylayer flows is conducted. From the laminar similarity solution, the temperature variations are small due to the high specific heat of dense gases, leading to velocity profiles close to the incompressible ones. Concurrently, the complex thermodynamic properties of dense gases can lead to unconventional compressibility effects. In the subsonic regime, the Tollmien–Schlichting viscous mode is attenuated by compressibility effects and becomes preferentially skewed in line with the results based on the idealgas assumption. However, the absence of a generalized inflection point precludes the sustainability of the first mode by inviscid mechanisms. On the contrary, the viscous mode can be completely stable at supersonic speeds. At very high speeds, we have found instances of radiating supersonic instabilities with substantial amplification rates, i.e. waves that travel supersonically relative to the freestream velocity. This acoustic mode has qualitatively similar features for various thermodynamic conditions and for different working fluids. This shows that the leading parameters governing the boundarylayer behaviour for the dense gas are the constantpressure specific heat and, to a minor extent, the densitydependent viscosity. A satisfactory scaling of the mode characteristics is found to be proportional to the height of the layer near the wall that acts as a waveguide where acoustic waves may become trapped. This means that the supersonic mode has the same nature as Mack’s modes, even if its frequency for maximal amplification is greater. Direct numerical simulation accurately reproduces the development of the supersonic mode and emphasizes the radiation of the instability waves.

Optimal transient growth in compressible turbulent boundary layers
http://hdl.handle.net/10985/18612
Optimal transient growth in compressible turbulent boundary layers
ALIZARD, Frédéric; PIROZZOLI, Sergio; BERNARDINI, Matteo; GRASSO, Francesco
The structure of zeropressuregradient compressible turbulent boundary layers is analysed using the tools of optimal transient growth theory. The approach relies on the extension to compressible flows of the theoretical framework originally developed by Reynolds & Hussain (J. Fluid Mech., vol. 52, 1972, pp. 263–288) for incompressible flows. The model is based on a densityweighted triple decomposition of the instantaneous field into the contributions of the mean flow, the organized (coherent) motions and the disorganized background turbulent fluctuations. The mean field and the eddy viscosity characterizing the incoherent fluctuations are here obtained from a direct numerical simulation database. Most temporally amplified modes (optimal modes) are found to be consistent with scaling laws of turbulent boundary layers for both inner and outer layers, as well as in the logarithmic region, where they exhibit a selfsimilar spreading. Four freestream Mach numbers are considered: $\mathit{Ma}_{\infty }=0.2$, 2, 3 and 4. Weak effects of compressibility on the characteristics length and the orientation angles are observed for both the inner and the outerlayer modes. Furthermore, taking into account the effects of mean density variations, a universal behaviour is suggested for the optimal modes that populate the log layer, regardless of the Mach number. The relevance of the optimal modes in describing the nearwall layer dynamics and the eddies that populate the outer region is discussed.
Thu, 01 Jan 2015 00:00:00 GMT
http://hdl.handle.net/10985/18612
20150101T00:00:00Z
ALIZARD, Frédéric
PIROZZOLI, Sergio
BERNARDINI, Matteo
GRASSO, Francesco
The structure of zeropressuregradient compressible turbulent boundary layers is analysed using the tools of optimal transient growth theory. The approach relies on the extension to compressible flows of the theoretical framework originally developed by Reynolds & Hussain (J. Fluid Mech., vol. 52, 1972, pp. 263–288) for incompressible flows. The model is based on a densityweighted triple decomposition of the instantaneous field into the contributions of the mean flow, the organized (coherent) motions and the disorganized background turbulent fluctuations. The mean field and the eddy viscosity characterizing the incoherent fluctuations are here obtained from a direct numerical simulation database. Most temporally amplified modes (optimal modes) are found to be consistent with scaling laws of turbulent boundary layers for both inner and outer layers, as well as in the logarithmic region, where they exhibit a selfsimilar spreading. Four freestream Mach numbers are considered: $\mathit{Ma}_{\infty }=0.2$, 2, 3 and 4. Weak effects of compressibility on the characteristics length and the orientation angles are observed for both the inner and the outerlayer modes. Furthermore, taking into account the effects of mean density variations, a universal behaviour is suggested for the optimal modes that populate the log layer, regardless of the Mach number. The relevance of the optimal modes in describing the nearwall layer dynamics and the eddies that populate the outer region is discussed.

Lattice Boltzmann method for miscible gases: A forcingterm approach
http://hdl.handle.net/10985/19695
Lattice Boltzmann method for miscible gases: A forcingterm approach
VIENNE, Lucien; MARIÉ, Simon; GRASSO, Francesco
A lattice Boltzmann method for miscible gases is presented. In this model, the standard lattice Boltzmann method is employed for each species composing the mixture. Diffusion interaction among species is taken into account by means of a force derived from kinetic theory of gases. Transport coefficients expressions are recovered from the kinetic theory. Species with dissimilar molar masses are simulated by also introducing a force. Finally, mixing dynamics is recovered as shown in different applications: an equimolar counterdiffusion case, Loschmidt's tube experiment, and an opposed jets flow simulation. Since collision is not altered, the present method can easily be introduced in any other lattice Boltzmann algorithms.
Tue, 01 Jan 2019 00:00:00 GMT
http://hdl.handle.net/10985/19695
20190101T00:00:00Z
VIENNE, Lucien
MARIÉ, Simon
GRASSO, Francesco
A lattice Boltzmann method for miscible gases is presented. In this model, the standard lattice Boltzmann method is employed for each species composing the mixture. Diffusion interaction among species is taken into account by means of a force derived from kinetic theory of gases. Transport coefficients expressions are recovered from the kinetic theory. Species with dissimilar molar masses are simulated by also introducing a force. Finally, mixing dynamics is recovered as shown in different applications: an equimolar counterdiffusion case, Loschmidt's tube experiment, and an opposed jets flow simulation. Since collision is not altered, the present method can easily be introduced in any other lattice Boltzmann algorithms.

Numerical Investigation of High‑Speed Turbulent Boundary Layers of Dense Gases
http://hdl.handle.net/10985/21896
Numerical Investigation of High‑Speed Turbulent Boundary Layers of Dense Gases
PASSIATORE, Donatella; CINNELLA, Paola; GRASSO, Francesco; SCIACOVELLI, Luca; GLOERFELT, Xavier
Highspeed turbulent boundary layers of a dense gas (PP11) and a perfect gas (air) over flat plates are investigated by means of direct numerical simulations and large eddy simulations. The thermodynamic conditions of the incoming flow are chosen to highlight dense gas effects, and laminartoturbulent transition is triggered by suction and blowing. In the paper, the behavior of the fully developed turbulent flow region is investigated. Due to the low characteristic Eckert number of dense gas flows ( Ec = U2
∞∕cp,∞T∞ ), the mean velocity profiles are largely insensitive to the Mach number and very close to the incompressible
case even at high speeds. Secondorder velocity statistics are also weakly affected by the flow Mach number and the velocity spectra are characterized by a secondary peak in the outer region of the boundary layer because of the higher local friction Reynolds number. Despite the incompressiblelike velocity and Reynoldsstress profiles, the strongly nonideal
thermodynamic and transportproperty behavior of the dense gas results in unconventional distributions of the fluctuating thermophysical quantities. Specifically, density and viscosity fluctuations reach a peak close to the wall, instead of vanishing as in perfect
gas flows. Additionally, dense gas boundary layers exhibit higher values of the fluctuating Mach number and velocity divergence and a larger dilatationaltosolenoidal dissipation ratio in the nearwall region, which represents a major deviation from highMachnumber perfect gas boundary layers. Other significant deviations are represented by the more symmetric probability distributions of fluctuating quantities such as the density and velocity divergence, due to the more balanced occurrence of strong expansion and compression events.
Sun, 01 Mar 2020 00:00:00 GMT
http://hdl.handle.net/10985/21896
20200301T00:00:00Z
PASSIATORE, Donatella
CINNELLA, Paola
GRASSO, Francesco
SCIACOVELLI, Luca
GLOERFELT, Xavier
Highspeed turbulent boundary layers of a dense gas (PP11) and a perfect gas (air) over flat plates are investigated by means of direct numerical simulations and large eddy simulations. The thermodynamic conditions of the incoming flow are chosen to highlight dense gas effects, and laminartoturbulent transition is triggered by suction and blowing. In the paper, the behavior of the fully developed turbulent flow region is investigated. Due to the low characteristic Eckert number of dense gas flows ( Ec = U2
∞∕cp,∞T∞ ), the mean velocity profiles are largely insensitive to the Mach number and very close to the incompressible
case even at high speeds. Secondorder velocity statistics are also weakly affected by the flow Mach number and the velocity spectra are characterized by a secondary peak in the outer region of the boundary layer because of the higher local friction Reynolds number. Despite the incompressiblelike velocity and Reynoldsstress profiles, the strongly nonideal
thermodynamic and transportproperty behavior of the dense gas results in unconventional distributions of the fluctuating thermophysical quantities. Specifically, density and viscosity fluctuations reach a peak close to the wall, instead of vanishing as in perfect
gas flows. Additionally, dense gas boundary layers exhibit higher values of the fluctuating Mach number and velocity divergence and a larger dilatationaltosolenoidal dissipation ratio in the nearwall region, which represents a major deviation from highMachnumber perfect gas boundary layers. Other significant deviations are represented by the more symmetric probability distributions of fluctuating quantities such as the density and velocity divergence, due to the more balanced occurrence of strong expansion and compression events.

Simulation of Viscous Fingering Instability by the Lattice Boltzmann Method
http://hdl.handle.net/10985/19696
Simulation of Viscous Fingering Instability by the Lattice Boltzmann Method
VIENNE, Lucien; MARIE, Simon; GRASSO, Francesco
The viscous fingering instability is successfully simulated within a lattice Boltzmann framework. Each species of the mixture is governed by its own kinetic equation and a force takes into account the diffusion between species. The influence of the porous medium is mimicked by using the gray lattice Boltzmann model or the Brinkman force model. In this study, both representations of the porous medium yield equivalent results. Then a physical analysis of the instability is performed and two different dynamical behaviour are stated and discussed. Finally, it is observed that a high Péclet number intensify the instability and the viscous dissipation stemming from the DarcyBrinkman equations delay the development of the fingers in the case of large effective viscosity.
Tue, 01 Jan 2019 00:00:00 GMT
http://hdl.handle.net/10985/19696
20190101T00:00:00Z
VIENNE, Lucien
MARIE, Simon
GRASSO, Francesco
The viscous fingering instability is successfully simulated within a lattice Boltzmann framework. Each species of the mixture is governed by its own kinetic equation and a force takes into account the diffusion between species. The influence of the porous medium is mimicked by using the gray lattice Boltzmann model or the Brinkman force model. In this study, both representations of the porous medium yield equivalent results. Then a physical analysis of the instability is performed and two different dynamical behaviour are stated and discussed. Finally, it is observed that a high Péclet number intensify the instability and the viscous dissipation stemming from the DarcyBrinkman equations delay the development of the fingers in the case of large effective viscosity.

CFDdriven symbolic identification of algebraic Reynoldsstress models
http://hdl.handle.net/10985/23745
CFDdriven symbolic identification of algebraic Reynoldsstress models
BEN HASSAN SAIDI, Ismaïl; SCHMELZER, Martin; CINNELLA, Paola; GRASSO, Francesco
Reynoldsstress models (EARSM) from highfidelity data is developed building on the frozentraining SpaRTA algorithm of [1]. Corrections for the Reynolds stress tensor and the production of transported turbulent quantities of a baseline linear eddy viscosity model (LEVM) are expressed as functions of tensor polynomials selected from a library of candidate functions. The CFDdriven training consists in solving a blackbox optimization problem in which the fitness of candidate EARSM models is evaluated by running RANS simulations. The procedure enables training models against any target quantity of interest, computable as an output of the CFD model. Unlike the frozentraining approach, the proposed methodology is not restricted to data sets for which full fields of highfidelity data, including second flow order statistics, are available. However, the solution of a highdimensional expensive blackbox function optimization problem is required. Several steps are then undertaken to reduce the associated computational burden. First, a sensitivity analysis is used to identify the most influential terms and to reduce the dimensionality of the search space. Afterwards, the Constrained Optimization using Response Surface (CORS) algorithm, which approximates the blackbox cost function using a response surface constructed from a limited number of CFD solves, is used to find the optimal model parameters. Model discovery and crossvalidation is performed for three configurations of 2D turbulent separated flows in channels of variable section using different sets of training data to show the flexibility of the method. The discovered models are then applied to the prediction of an unseen 2D separated flow with higher Reynolds number and different geometry. The predictions of the discovered models for the new case are shown to be not only more accurate than the baseline LEVM, but also of a multipurpose EARSM model derived from purely physical arguments. The proposed deterministic symbolic identification approach constitutes a promising candidate for building accurate and robust RANS models customized for a given class of flows at moderate computational cost.
©2022 Elsevier Inc. All rights reserved.
Tue, 01 Feb 2022 00:00:00 GMT
http://hdl.handle.net/10985/23745
20220201T00:00:00Z
BEN HASSAN SAIDI, Ismaïl
SCHMELZER, Martin
CINNELLA, Paola
GRASSO, Francesco
Reynoldsstress models (EARSM) from highfidelity data is developed building on the frozentraining SpaRTA algorithm of [1]. Corrections for the Reynolds stress tensor and the production of transported turbulent quantities of a baseline linear eddy viscosity model (LEVM) are expressed as functions of tensor polynomials selected from a library of candidate functions. The CFDdriven training consists in solving a blackbox optimization problem in which the fitness of candidate EARSM models is evaluated by running RANS simulations. The procedure enables training models against any target quantity of interest, computable as an output of the CFD model. Unlike the frozentraining approach, the proposed methodology is not restricted to data sets for which full fields of highfidelity data, including second flow order statistics, are available. However, the solution of a highdimensional expensive blackbox function optimization problem is required. Several steps are then undertaken to reduce the associated computational burden. First, a sensitivity analysis is used to identify the most influential terms and to reduce the dimensionality of the search space. Afterwards, the Constrained Optimization using Response Surface (CORS) algorithm, which approximates the blackbox cost function using a response surface constructed from a limited number of CFD solves, is used to find the optimal model parameters. Model discovery and crossvalidation is performed for three configurations of 2D turbulent separated flows in channels of variable section using different sets of training data to show the flexibility of the method. The discovered models are then applied to the prediction of an unseen 2D separated flow with higher Reynolds number and different geometry. The predictions of the discovered models for the new case are shown to be not only more accurate than the baseline LEVM, but also of a multipurpose EARSM model derived from purely physical arguments. The proposed deterministic symbolic identification approach constitutes a promising candidate for building accurate and robust RANS models customized for a given class of flows at moderate computational cost.
©2022 Elsevier Inc. All rights reserved.

DNS of turbulent flows of dense gases
http://hdl.handle.net/10985/23742
DNS of turbulent flows of dense gases
SCIACOVELLI, Luca; CINNELLA, Paola; GLOERFELT, Xavier; GRASSO, Francesco
The influence of dense gas effects on compressible turbulence is investigated by means of numerical simulations of the decay of compressible homogeneous isotropic turbulence (CHIT) and of supersonic turbulent flows through a plane channel (TCF). For both configurations, a parametric study on the Mach and Reynolds numbers is carried out. The dense gas considered in these parametric studies is PP11, a heavy fluorocarbon. The results are systematically compared to those obtained for a diatomic perfect gas (air). In our computations, the thermodynamic behaviour of the dense gases is modelled by means of the MartinHou equation of state. For CHIT cases, initial turbulent Mach numbers up to 1 are analyzed using mesh resolutions up to 5123. For TCF, bulk Mach numbers up to 3 and bulk Reynolds numbers up to 12000 are investigated. Average profiles of the thermodynamic quantities exhibit significant differences with respect to perfectgas solutions for both of the configurations. For highMach CHIT, compressible structures are modified with respect to air, with weaker eddy shocklets and stronger expansions. In TCF, the velocity profiles of dense gas flows are much less sensitive to the Mach number and collapse reasonably well in the logarithmic region without any special need for compressible scalings, unlike the case of air, and the overall flow behaviour is midway between that of a variableproperty liquid and that of a gas.
Sat, 01 Apr 2017 00:00:00 GMT
http://hdl.handle.net/10985/23742
20170401T00:00:00Z
SCIACOVELLI, Luca
CINNELLA, Paola
GLOERFELT, Xavier
GRASSO, Francesco
The influence of dense gas effects on compressible turbulence is investigated by means of numerical simulations of the decay of compressible homogeneous isotropic turbulence (CHIT) and of supersonic turbulent flows through a plane channel (TCF). For both configurations, a parametric study on the Mach and Reynolds numbers is carried out. The dense gas considered in these parametric studies is PP11, a heavy fluorocarbon. The results are systematically compared to those obtained for a diatomic perfect gas (air). In our computations, the thermodynamic behaviour of the dense gases is modelled by means of the MartinHou equation of state. For CHIT cases, initial turbulent Mach numbers up to 1 are analyzed using mesh resolutions up to 5123. For TCF, bulk Mach numbers up to 3 and bulk Reynolds numbers up to 12000 are investigated. Average profiles of the thermodynamic quantities exhibit significant differences with respect to perfectgas solutions for both of the configurations. For highMach CHIT, compressible structures are modified with respect to air, with weaker eddy shocklets and stronger expansions. In TCF, the velocity profiles of dense gas flows are much less sensitive to the Mach number and collapse reasonably well in the logarithmic region without any special need for compressible scalings, unlike the case of air, and the overall flow behaviour is midway between that of a variableproperty liquid and that of a gas.