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Identifying inefficiencies in unsteady pin fin heat transfer using orthogonal decomposition / Markus Schwänen in Journal of heat transfer, Vol. 134 N° 2 (Fevrier 2012) 

Public ISBD [article]  in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 10 p. Titre : Identifying inefficiencies in unsteady pin fin heat transfer using orthogonal decomposition Type de document : texte imprimé Auteurs : Markus Schwänen, Auteur ; Andrew Duggleby, Auteur Année de publication : 2012 Article en page(s) : 10 p. Note générale : Heat transfer Langues : Anglais (eng) Mots-clés : Blades  Channel  flow  Cooling  Flow  instability  Flow  simulation  Gas  turbines  Turbulence Index. décimale : 536 Chaleur. Thermodynamique Résumé : Internal cooling of the trailing edge region in a gas turbine blade is typically achieved with an array of pin fins. In order to better understand the effectiveness of this configuration, high performance computations are performed on cylindrical pin fins with a spanwise distance to fin diameter ratio of 2 and height over fin diameter ratio of one. For validation purposes, the flow Reynolds number based on hydraulic channel diameter and bulk velocity (Re = 12,800) was set to match experiments available in the open literature. Simulations included a URANS and LES on a single row of pin fins where the URANS domain was 1 pin wide versus the LES with 3 pins. The resulting time-dependent flow field was analyzed using a variation of bi-orthogonal decomposition (BOD), where the correlation matrices were built using the internal energy in addition to the three velocity components. This enables a detailed comparison of URANS and LES to assess the URANS modeling assumptions as well as a flow decomposition with respect to the flow structure's influence on surface heat transfer. This analysis shows low order modes which do not contribute to turbulent heat flux, but instead increase the heat exchanger's global inefficiency. In the URANS study, the forth mode showed the first nonzero temperature basis function, which means that a considerable amount of energy is contained in flow structures that do not contribute to increasing endwall heat transfer. In the LES, the first non zero temperature basis function was the seventh mode. Both orthogonal basis function sets were evaluated with respect to each mode's contribution to turbulent heat exchange with the surface. This analysis showed that there exists one distinct, high energy mode that contributes to wall heat flux, whereas all others do not. Modifying this mode could potentially be used to improve the heat exchanger's efficiency with respect to pressure loss. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...] [article] Identifying inefficiencies in unsteady pin fin heat transfer using orthogonal decomposition [texte imprimé] / Markus Schwänen, Auteur ; Andrew Duggleby, Auteur . - 2012 . - 10 p. Heat transfer Langues : Anglais (eng) in Journal of heat transfer > Vol. 134 N° 2 (Fevrier 2012) . - 10 p. Mots-clés : Blades  Channel  flow  Cooling  Flow  instability  Flow  simulation  Gas  turbines  Turbulence Index. décimale : 536 Chaleur. Thermodynamique Résumé : Internal cooling of the trailing edge region in a gas turbine blade is typically achieved with an array of pin fins. In order to better understand the effectiveness of this configuration, high performance computations are performed on cylindrical pin fins with a spanwise distance to fin diameter ratio of 2 and height over fin diameter ratio of one. For validation purposes, the flow Reynolds number based on hydraulic channel diameter and bulk velocity (Re = 12,800) was set to match experiments available in the open literature. Simulations included a URANS and LES on a single row of pin fins where the URANS domain was 1 pin wide versus the LES with 3 pins. The resulting time-dependent flow field was analyzed using a variation of bi-orthogonal decomposition (BOD), where the correlation matrices were built using the internal energy in addition to the three velocity components. This enables a detailed comparison of URANS and LES to assess the URANS modeling assumptions as well as a flow decomposition with respect to the flow structure's influence on surface heat transfer. This analysis shows low order modes which do not contribute to turbulent heat flux, but instead increase the heat exchanger's global inefficiency. In the URANS study, the forth mode showed the first nonzero temperature basis function, which means that a considerable amount of energy is contained in flow structures that do not contribute to increasing endwall heat transfer. In the LES, the first non zero temperature basis function was the seventh mode. Both orthogonal basis function sets were evaluated with respect to each mode's contribution to turbulent heat exchange with the surface. This analysis showed that there exists one distinct, high energy mode that contributes to wall heat flux, whereas all others do not. Modifying this mode could potentially be used to improve the heat exchanger's efficiency with respect to pressure loss. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JHTRAO000134000002 [...]

Low pressure turbine relaminarization bubble characterization using Massively-parallel large Eddy simulations / Shriram Jagannathan in Transactions of the ASME . Journal of fluids engineering, Vol. 134 N° 2 (Fevrier 2012) 

Public ISBD [article]  in Transactions of the ASME . Journal of fluids engineering > Vol. 134 N° 2 (Fevrier 2012) . - 13 p. Titre : Low pressure turbine relaminarization bubble characterization using Massively-parallel large Eddy simulations Type de document : texte imprimé Auteurs : Shriram Jagannathan, Auteur ; Markus Schwänen, Auteur ; Andrew Duggleby, Auteur Année de publication : 2012 Article en page(s) : 13 p. Note générale : Fluids engineering Langues : Anglais (eng) Mots-clés : Low  pressure  turbine  Large-eddy  simulation Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : The separation and reattachment of suction surface boundary layer in a low pressure turbine is characterized using large-eddy simulation at Ress = 69000 based on inlet velocity and suction surface length. Favorable comparisons are drawn with experiments using a high pass filtered Smagorinsky model for sub-grid scales. The onset of time mean separation is at s/so = 0.61 and reattachment at s/so = 0.81, extending over 20% of the suction surface. The boundary layer is convectively unstable with a maximum reverse flow velocity of about 13% of freestream. The breakdown to turbulence occurs over a very short distance of suction surface and is followed by reattachment. Turbulence near the bubble is further characterized using anisotropy invariant mapping and time orthogonal decomposition diagnostics. Particularly the vortex shedding and shear layer flapping phenomena are addressed. On the suction side, dominant hairpin structures near the transitional and turbulent flow regime are observed. The hairpin vortices are carried by the freestream even downstream of the trailing edge of the blade with a possibility of reaching the next stage. Longitudinal streaks that evolve from the breakdown of hairpin vortices formed near the leading edge are observed on the pressure surface. DEWEY : 620.1 ISSN : 0098-2202 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JFEGA4000134000002 [...] [article] Low pressure turbine relaminarization bubble characterization using Massively-parallel large Eddy simulations [texte imprimé] / Shriram Jagannathan, Auteur ; Markus Schwänen, Auteur ; Andrew Duggleby, Auteur . - 2012 . - 13 p. Fluids engineering Langues : Anglais (eng) in Transactions of the ASME . Journal of fluids engineering > Vol. 134 N° 2 (Fevrier 2012) . - 13 p. Mots-clés : Low  pressure  turbine  Large-eddy  simulation Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : The separation and reattachment of suction surface boundary layer in a low pressure turbine is characterized using large-eddy simulation at Ress = 69000 based on inlet velocity and suction surface length. Favorable comparisons are drawn with experiments using a high pass filtered Smagorinsky model for sub-grid scales. The onset of time mean separation is at s/so = 0.61 and reattachment at s/so = 0.81, extending over 20% of the suction surface. The boundary layer is convectively unstable with a maximum reverse flow velocity of about 13% of freestream. The breakdown to turbulence occurs over a very short distance of suction surface and is followed by reattachment. Turbulence near the bubble is further characterized using anisotropy invariant mapping and time orthogonal decomposition diagnostics. Particularly the vortex shedding and shear layer flapping phenomena are addressed. On the suction side, dominant hairpin structures near the transitional and turbulent flow regime are observed. The hairpin vortices are carried by the freestream even downstream of the trailing edge of the blade with a possibility of reaching the next stage. Longitudinal streaks that evolve from the breakdown of hairpin vortices formed near the leading edge are observed on the pressure surface. DEWEY : 620.1 ISSN : 0098-2202 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JFEGA4000134000002 [...]

Mixing analysis in a lid-driven cavity flow at finite Reynolds numbers / Pradeep Rao in Transactions of the ASME . Journal of fluids engineering, Vol. 134 N° 4 (Avril 2012) 

Public ISBD [article]  in Transactions of the ASME . Journal of fluids engineering > Vol. 134 N° 4 (Avril 2012) . - 08 p. Titre : Mixing analysis in a lid-driven cavity flow at finite Reynolds numbers Type de document : texte imprimé Auteurs : Pradeep Rao, Auteur ; Andrew Duggleby, Auteur ; Mark A. Stremler, Auteur Année de publication : 2012 Article en page(s) : 08 p. Note générale : fluids engineering Langues : Anglais (eng) Mots-clés : inertial  effects;  chaotic  advection;  lid-driven  cavity  flow Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : The influence of inertial effects on chaotic advection and mixing is investigated for a two-dimensional, time-dependent lid-driven cavity flow. Previous work shows that this flow exhibits exponential stretching and folding of material lines due to the presence of figure-eight stirring patterns in the creeping flow regime. The high sensitivity to initial conditions and the exponential growth of errors in chaotic flows necessitate an accurate solution of the flow in order to calculate metrics based on Lagrangian particle tracking. The streamfunction-vorticity formulation of the Navier-Stokes equations is solved using a Fourier-Chebyshev spectral method, providing the necessary exponential convergence and machine-precision accuracy. Poincaré sections and mixing measures are used to analyze chaotic advection and quantify the mixing efficiency. The calculated mixing characteristics are almost identical for Re <= 1. For the time range investigated, the best mixing in this system is observed for Re = 10. Interestingly, increasing the Reynolds number to the range 10 < Re <= 100 results in an observed decrease in mixing efficacy. DEWEY : 620.1 ISSN : 0098-2202 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JFEGA4000134000004 [...] [article] Mixing analysis in a lid-driven cavity flow at finite Reynolds numbers [texte imprimé] / Pradeep Rao, Auteur ; Andrew Duggleby, Auteur ; Mark A. Stremler, Auteur . - 2012 . - 08 p. fluids engineering Langues : Anglais (eng) in Transactions of the ASME . Journal of fluids engineering > Vol. 134 N° 4 (Avril 2012) . - 08 p. Mots-clés : inertial  effects;  chaotic  advection;  lid-driven  cavity  flow Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : The influence of inertial effects on chaotic advection and mixing is investigated for a two-dimensional, time-dependent lid-driven cavity flow. Previous work shows that this flow exhibits exponential stretching and folding of material lines due to the presence of figure-eight stirring patterns in the creeping flow regime. The high sensitivity to initial conditions and the exponential growth of errors in chaotic flows necessitate an accurate solution of the flow in order to calculate metrics based on Lagrangian particle tracking. The streamfunction-vorticity formulation of the Navier-Stokes equations is solved using a Fourier-Chebyshev spectral method, providing the necessary exponential convergence and machine-precision accuracy. Poincaré sections and mixing measures are used to analyze chaotic advection and quantify the mixing efficiency. The calculated mixing characteristics are almost identical for Re <= 1. For the time range investigated, the best mixing in this system is observed for Re = 10. Interestingly, increasing the Reynolds number to the range 10 < Re <= 100 results in an observed decrease in mixing efficacy. DEWEY : 620.1 ISSN : 0098-2202 En ligne : http://asmedl.org/getabs/servlet/GetabsServlet?prog=normal&id=JFEGA4000134000004 [...]