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http://hdl.handle.net/10985/9013
Optimal perturbations in boundary layer flows over rough surfaces
CHERUBINI, Stefania; DE TULLIO, Marco; DE PALMA, Pietro; PASCAZIO, Giuseppe
This work provides a three-dimensional energy optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of roughness elements. The immersed boundary technique has been coupled with a Lagrangian optimization in a three-dimensional framework. Four roughness elements with different heights have been studied, inducing amplification mechanisms that bypass the asymptotical growth of Tollmien-Schlichting waves. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can strongly localize the optimal disturbance. Moreover, the highest value of the energy gain is obtained for a varicose perturbation. This result demonstrates the relevance of varicose instabilities for such a flow and shows a different behavior with respect to the secondary instability theory of boundary layer streaks.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/90132013-01-01T00:00:00ZCHERUBINI, StefaniaDE TULLIO, MarcoDE PALMA, PietroPASCAZIO, GiuseppeThis work provides a three-dimensional energy optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of roughness elements. The immersed boundary technique has been coupled with a Lagrangian optimization in a three-dimensional framework. Four roughness elements with different heights have been studied, inducing amplification mechanisms that bypass the asymptotical growth of Tollmien-Schlichting waves. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can strongly localize the optimal disturbance. Moreover, the highest value of the energy gain is obtained for a varicose perturbation. This result demonstrates the relevance of varicose instabilities for such a flow and shows a different behavior with respect to the secondary instability theory of boundary layer streaks.Transient growth in the flow past a three-dimensional smooth roughness element
http://hdl.handle.net/10985/8973
Transient growth in the flow past a three-dimensional smooth roughness element
CHERUBINI, Stefania; DE TULLIO, Marco; DE PALMA, Pietro; PASCAZIO, Giuseppe
This work provides a global optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of smooth three-dimensional roughness elements. Amplification mechanisms are described which can bypass the asymptotical growth of Tollmien–Schlichting waves. Smooth axisymmetric roughness elements of different height have been studied, at different Reynolds numbers. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can localize the optimal disturbance characterizing the Blasius boundary-layer flow. Moreover, for large enough bump heights and Reynolds numbers, a strong amplification mechanism has been recovered, inducing an increase of several orders of magnitude of the energy gain with respect to the Blasius case. In particular, the highest value of the energy gain is obtained for an initial varicose perturbation, differently to what found for a streaky parallel flow. Optimal varicose perturbations grow very rapidly by transporting the strong wall-normal shear of the base flow, which is localized in the wake of the bump. Such optimal disturbances are found to lead to transition for initial energies and amplitudes considerably smaller than sinuous optimal ones, inducing hairpin vortices downstream of the roughness element.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/89732013-01-01T00:00:00ZCHERUBINI, StefaniaDE TULLIO, MarcoDE PALMA, PietroPASCAZIO, GiuseppeThis work provides a global optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of smooth three-dimensional roughness elements. Amplification mechanisms are described which can bypass the asymptotical growth of Tollmien–Schlichting waves. Smooth axisymmetric roughness elements of different height have been studied, at different Reynolds numbers. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can localize the optimal disturbance characterizing the Blasius boundary-layer flow. Moreover, for large enough bump heights and Reynolds numbers, a strong amplification mechanism has been recovered, inducing an increase of several orders of magnitude of the energy gain with respect to the Blasius case. In particular, the highest value of the energy gain is obtained for an initial varicose perturbation, differently to what found for a streaky parallel flow. Optimal varicose perturbations grow very rapidly by transporting the strong wall-normal shear of the base flow, which is localized in the wake of the bump. Such optimal disturbances are found to lead to transition for initial energies and amplitudes considerably smaller than sinuous optimal ones, inducing hairpin vortices downstream of the roughness element.Numerical investigation of hypersonic turbulent boundary layers with high-temperature effects
http://hdl.handle.net/10985/23685
Numerical investigation of hypersonic turbulent boundary layers with high-temperature effects
PASSIATORE, Donatella; SCIACOVELLI, Luca; CINNELLA, Paola; PASCAZIO, Giuseppe
A hypersonic turbulent boundary layer over a flat plate is numerically investigated. The large Mach number and temperature values in the freestream (M e = 12.48 and T e = 594.3 K, respectively) lead to a high-enthalpy regime and to the occurrence of thermochemical non-equilibrium effects. Vibrational relaxation phenomena are shown to be predominant with respect to chemical activity. In this context, high-fidelity results obtained by means of a Direct Numerical Simulation (DNS) are used as a benchmark to assess the quality of a Large-Eddy Simulation (LES) performed with a coarser wall-resolved grid. The wall-adapting local eddy viscosity approach is selected as sub-grid scale (SGS) model. The LES strategy is shown to capture the mean and fluctuating dynamic fields in the fully turbulent region quite satisfactorily, whereas transition to turbulence is slightly anticipated with respect to DNS. Both the chemical and vibrational source terms are evaluated with the filtered aerothermochemical quantities, resulting in an overestimation of the translational-vibrational energy exchange and an underestimation of dissociation chemical production rates. These results shed light on the necessity of developing more accurate closure models for the source terms, the SGS turbulence-thermochemistry interactions being important for the configuration under investigation.
Tue, 01 Nov 2022 00:00:00 GMThttp://hdl.handle.net/10985/236852022-11-01T00:00:00ZPASSIATORE, DonatellaSCIACOVELLI, LucaCINNELLA, PaolaPASCAZIO, GiuseppeA hypersonic turbulent boundary layer over a flat plate is numerically investigated. The large Mach number and temperature values in the freestream (M e = 12.48 and T e = 594.3 K, respectively) lead to a high-enthalpy regime and to the occurrence of thermochemical non-equilibrium effects. Vibrational relaxation phenomena are shown to be predominant with respect to chemical activity. In this context, high-fidelity results obtained by means of a Direct Numerical Simulation (DNS) are used as a benchmark to assess the quality of a Large-Eddy Simulation (LES) performed with a coarser wall-resolved grid. The wall-adapting local eddy viscosity approach is selected as sub-grid scale (SGS) model. The LES strategy is shown to capture the mean and fluctuating dynamic fields in the fully turbulent region quite satisfactorily, whereas transition to turbulence is slightly anticipated with respect to DNS. Both the chemical and vibrational source terms are evaluated with the filtered aerothermochemical quantities, resulting in an overestimation of the translational-vibrational energy exchange and an underestimation of dissociation chemical production rates. These results shed light on the necessity of developing more accurate closure models for the source terms, the SGS turbulence-thermochemistry interactions being important for the configuration under investigation.Direct Numerical Simulation of a hypersonic boundary layer in chemical non-equilibrium
http://hdl.handle.net/10985/23688
Direct Numerical Simulation of a hypersonic boundary layer in chemical non-equilibrium
PASSIATORE, Donatella; SCIACOVELLI, Luca; CINNELLA, Paola; PASCAZIO, Giuseppe
The influence of high-enthalpy effects in hypersonic, spatially developing boundary layers is investigated by means of direct numerical simulations. The flow of a reacting mixture of nitrogen and oxygen over a flat plate at Mach 10, previously investigated in the literature using linear stability theory (LST), is simulated using a compu-tational domain encompassing the laminar, transitional and turbulent regimes. Transition is triggered by forcing Mack’s second mode through suction and blowing at the wall. In the laminar region, the solution matches reasonably well the locally self-similar profiles, computed under chemical non-equilibrium assumptions. Strong dissociation phenomena are observed, due to the high temperatures reached close to the (uncooled) plate surface. The transitional regime is investigated by means of modal analysis. Despite the significant chemical activity, the results confirm the classical transition scenario for high-Mach number boundary layers, for which the second-mode resonance is the main mechanism responsible for turbulent breakdown. In the turbulent region, first- and second-order statistics reveal that chemical reactions do not modify significantly dynamic quantities such as velocity and Reynolds stress profiles, but greatly affect thermal properties, due to their endothermic nature. For the configuration at hand, chemical dissociation is slower than the characteristic time-scale of the flow, and the peak of chemical activity is located in the viscous sublayer, leading to mild modifications of the turbulent field compared to a frozen-chemistry model.
Thu, 01 Apr 2021 00:00:00 GMThttp://hdl.handle.net/10985/236882021-04-01T00:00:00ZPASSIATORE, DonatellaSCIACOVELLI, LucaCINNELLA, PaolaPASCAZIO, GiuseppeThe influence of high-enthalpy effects in hypersonic, spatially developing boundary layers is investigated by means of direct numerical simulations. The flow of a reacting mixture of nitrogen and oxygen over a flat plate at Mach 10, previously investigated in the literature using linear stability theory (LST), is simulated using a compu-tational domain encompassing the laminar, transitional and turbulent regimes. Transition is triggered by forcing Mack’s second mode through suction and blowing at the wall. In the laminar region, the solution matches reasonably well the locally self-similar profiles, computed under chemical non-equilibrium assumptions. Strong dissociation phenomena are observed, due to the high temperatures reached close to the (uncooled) plate surface. The transitional regime is investigated by means of modal analysis. Despite the significant chemical activity, the results confirm the classical transition scenario for high-Mach number boundary layers, for which the second-mode resonance is the main mechanism responsible for turbulent breakdown. In the turbulent region, first- and second-order statistics reveal that chemical reactions do not modify significantly dynamic quantities such as velocity and Reynolds stress profiles, but greatly affect thermal properties, due to their endothermic nature. For the configuration at hand, chemical dissociation is slower than the characteristic time-scale of the flow, and the peak of chemical activity is located in the viscous sublayer, leading to mild modifications of the turbulent field compared to a frozen-chemistry model.Shock-wave/boundary layer interaction at high enthalpies
http://hdl.handle.net/10985/23683
Shock-wave/boundary layer interaction at high enthalpies
PASSIATORE, Donatella; SCIACOVELLI, Luca; CINNELLA, Paola; PASCAZIO, Giuseppe
The dynamics of a shock wave impinging on a freestream-perturbed high-enthalpy boundary layer is investigated by means of direct numerical simulation. The oblique shock impacts on a cooled flat-plate boundary layer with an angle of 10 degrees,
generating a reversal flow region. The combination of the freestream disturbances and the shock impingement is such that a transition to a fully turbulent regime occurs downstream of the interaction region. The analysis aims at qualifying and quantifying the role of thermochemical non-equilibrium conditions on the dynamics of the shock-wave/boundary-layer interaction.
Wed, 29 Mar 2023 00:00:00 GMThttp://hdl.handle.net/10985/236832023-03-29T00:00:00ZPASSIATORE, DonatellaSCIACOVELLI, LucaCINNELLA, PaolaPASCAZIO, GiuseppeThe dynamics of a shock wave impinging on a freestream-perturbed high-enthalpy boundary layer is investigated by means of direct numerical simulation. The oblique shock impacts on a cooled flat-plate boundary layer with an angle of 10 degrees,
generating a reversal flow region. The combination of the freestream disturbances and the shock impingement is such that a transition to a fully turbulent regime occurs downstream of the interaction region. The analysis aims at qualifying and quantifying the role of thermochemical non-equilibrium conditions on the dynamics of the shock-wave/boundary-layer interaction.Direct Numerical Simulation of hypersonic boundary layers in chemical non-equilibrium
http://hdl.handle.net/10985/23687
Direct Numerical Simulation of hypersonic boundary layers in chemical non-equilibrium
PASSIATORE, Donatella; SCIACOVELLI, Luca; PASCAZIO, Giuseppe; CINNELLA, Paola
The influence of high-temperature effects on compressible wall-bounded turbulence is investigated by means of a direct numerical simulation of a hypersonic, chemically out-of-equilibrium, turbulent boundary layer. The analysis aims at assessing the effects of chemical reactions on turbulence, also by comparing the results with those of a frozen flow. We will present a detailed analysis of the turbulent statistics and near-wall dynamics; the validity of some classical scalings and Reynolds analogy will also be discussed.
Sun, 01 Aug 2021 00:00:00 GMThttp://hdl.handle.net/10985/236872021-08-01T00:00:00ZPASSIATORE, DonatellaSCIACOVELLI, LucaPASCAZIO, GiuseppeCINNELLA, PaolaThe influence of high-temperature effects on compressible wall-bounded turbulence is investigated by means of a direct numerical simulation of a hypersonic, chemically out-of-equilibrium, turbulent boundary layer. The analysis aims at assessing the effects of chemical reactions on turbulence, also by comparing the results with those of a frozen flow. We will present a detailed analysis of the turbulent statistics and near-wall dynamics; the validity of some classical scalings and Reynolds analogy will also be discussed.A high-order scheme for the numerical simulation of high-enthalpy hypersonic flows
http://hdl.handle.net/10985/23686
A high-order scheme for the numerical simulation of high-enthalpy hypersonic flows
PASSIATORE, Donatella; SCIACOVELLI, Luca; CINNELLA, Paola; PASCAZIO, Giuseppe
A high-order shock-capturing finite-difference scheme for scale-resolving numerical simulations of hypersonic high-enthalpy flows, involving thermal non-equilibrium effects, is presented. The suitability of the numerical strategy for such challenging configurations is assessed in terms of accuracy and robustness, with special focus on shock-capturing capabilities. The approach is demonstrated for a variety of thermochemical non-equilibrium configurations.
Fri, 01 Jul 2022 00:00:00 GMThttp://hdl.handle.net/10985/236862022-07-01T00:00:00ZPASSIATORE, DonatellaSCIACOVELLI, LucaCINNELLA, PaolaPASCAZIO, GiuseppeA high-order shock-capturing finite-difference scheme for scale-resolving numerical simulations of hypersonic high-enthalpy flows, involving thermal non-equilibrium effects, is presented. The suitability of the numerical strategy for such challenging configurations is assessed in terms of accuracy and robustness, with special focus on shock-capturing capabilities. The approach is demonstrated for a variety of thermochemical non-equilibrium configurations.