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Entropy generation in a boundary layer transitioning under the influence of freestream turbulence / Edmond J. Walsh in Transactions of the ASME . Journal of fluids engineering, Vol. 133 N° 6 (Juin 2011) 

Public ISBD [article]  in Transactions of the ASME . Journal of fluids engineering > Vol. 133 N° 6 (Juin 2011) . - 10 p. Titre : Entropy generation in a boundary layer transitioning under the influence of freestream turbulence Type de document : texte imprimé Auteurs : Edmond J. Walsh, Auteur ; Mc Eligot, Donald M., Auteur ; Brandt, Luca, Auteur Année de publication : 2011 Article en page(s) : 10 p. Note générale : Fluids engineering Langues : Anglais (eng) Mots-clés : Boundary  layer  turbulence  Convection  Entropy  Flow  simulation  Laminar  flow  Laminar  to  turbulent  transitions  Numerical  analysis Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : The objective of the present research is to develop new fundamental knowledge of the entropy generation process in laminar flow with significant fluctuations (called pre-transition) and during transition prematurely induced by strong freestream turbulence (bypass transition). Results of direct numerical simulations are employed. In the pre-transitional boundary layer, the perturbations by the streaky structures modify the mean velocity profile and induce a “quasi-turbulent” contribution to indirect dissipation. Application of classical laminar theory leads to underprediction of the entropy generated. In the transition region the pointwise entropy generation rate (S[prime][prime][prime])+ initially increases near the wall and then decreases to correspond to the distribution predicted for a fully-turbulent boundary layer as the flow progresses downstream. In contrast to a developed turbulent flow, the term for turbulent convection in the turbulence kinetic energy balance is significant and can play an important role in some regions of the transitioning boundary layer. More turbulent energy is produced than dissipated and the excess is convected downstream as the boundary layer grows. Since it is difficult to measure and predict true turbulent dissipation rates (and hence, entropy generation rates) exactly other than by expensive direct numerical simulations, a motivation for this research is to evaluate approximate methods for possible use in experiments and design. These new results demonstrate that an approximate technique, used by many investigators, overestimates the dissipation coefficient Cd by up to seventeen per cent. For better predictions and measurements, an integral approach accounting for the important turbulent energy flux is proposed and validated for the case studied. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JFEGA400013 [...] [article] Entropy generation in a boundary layer transitioning under the influence of freestream turbulence [texte imprimé] / Edmond J. Walsh, Auteur ; Mc Eligot, Donald M., Auteur ; Brandt, Luca, Auteur . - 2011 . - 10 p. Fluids engineering Langues : Anglais (eng) in Transactions of the ASME . Journal of fluids engineering > Vol. 133 N° 6 (Juin 2011) . - 10 p. Mots-clés : Boundary  layer  turbulence  Convection  Entropy  Flow  simulation  Laminar  flow  Laminar  to  turbulent  transitions  Numerical  analysis Index. décimale : 620.1 Essais des matériaux. Défauts des matériaux. Protection des matériaux Résumé : The objective of the present research is to develop new fundamental knowledge of the entropy generation process in laminar flow with significant fluctuations (called pre-transition) and during transition prematurely induced by strong freestream turbulence (bypass transition). Results of direct numerical simulations are employed. In the pre-transitional boundary layer, the perturbations by the streaky structures modify the mean velocity profile and induce a “quasi-turbulent” contribution to indirect dissipation. Application of classical laminar theory leads to underprediction of the entropy generated. In the transition region the pointwise entropy generation rate (S[prime][prime][prime])+ initially increases near the wall and then decreases to correspond to the distribution predicted for a fully-turbulent boundary layer as the flow progresses downstream. In contrast to a developed turbulent flow, the term for turbulent convection in the turbulence kinetic energy balance is significant and can play an important role in some regions of the transitioning boundary layer. More turbulent energy is produced than dissipated and the excess is convected downstream as the boundary layer grows. Since it is difficult to measure and predict true turbulent dissipation rates (and hence, entropy generation rates) exactly other than by expensive direct numerical simulations, a motivation for this research is to evaluate approximate methods for possible use in experiments and design. These new results demonstrate that an approximate technique, used by many investigators, overestimates the dissipation coefficient Cd by up to seventeen per cent. For better predictions and measurements, an integral approach accounting for the important turbulent energy flux is proposed and validated for the case studied. DEWEY : 620.1 ISSN : 0742-4795 En ligne : http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JFEGA400013 [...]

Entropy generation in the viscous parts of turbulent boundary layers / Mc Eligot, Donald M. in Transactions of the ASME . Journal of fluids engineering, Vol. 130 N° 6 (Juin 2008) 

Public ISBD [article]  in Transactions of the ASME . Journal of fluids engineering > Vol. 130 N° 6 (Juin 2008) . - 12 p. Titre : Entropy generation in the viscous parts of turbulent boundary layers Type de document : texte imprimé Auteurs : Mc Eligot, Donald M., Auteur ; Edmond J. Walsh, Auteur ; Eckart Laurien, Auteur Année de publication : 2009 Article en page(s) : 12 p. Note générale : Fluids engineering Langues : Anglais (eng) Mots-clés : Pointwise  entropy;  turbulent  boundary  layers Résumé : The local (pointwise) entropy generation rate per unit volume S‴ is a key to improving many energy processes and applications. Consequently, in the present study, the objectives are to examine the effects of Reynolds number and favorable streamwise pressure gradients on entropy generation rates across turbulent boundary layers on flat plates and—secondarily—to assess a popular approximate technique for their evaluation. About two-thirds or more of the entropy generation occurs in the viscous part, known as the viscous layer. Fundamental new results for entropy generation in turbulent boundary layers are provided by extending available direct numerical simulations. It was found that, with negligible pressure gradients, results presented in wall coordinates are predicted to be near “universal” in the viscous layer. This apparent universality disappears when a significant pressure gradient is applied; increasing the pressure gradient decreases the entropy generation rate. Within the viscous layer, the approximate evaluation of S‴ differs significantly from the “proper” value but its integral, the entropy generation rate per unit surface area Sap″, agrees within 5% at its edge. En ligne : http://fluidsengineering.asmedigitalcollection.asme.org/Issue.aspx?issueID=27318 [...] [article] Entropy generation in the viscous parts of turbulent boundary layers [texte imprimé] / Mc Eligot, Donald M., Auteur ; Edmond J. Walsh, Auteur ; Eckart Laurien, Auteur . - 2009 . - 12 p. Fluids engineering Langues : Anglais (eng) in Transactions of the ASME . Journal of fluids engineering > Vol. 130 N° 6 (Juin 2008) . - 12 p. Mots-clés : Pointwise  entropy;  turbulent  boundary  layers Résumé : The local (pointwise) entropy generation rate per unit volume S‴ is a key to improving many energy processes and applications. Consequently, in the present study, the objectives are to examine the effects of Reynolds number and favorable streamwise pressure gradients on entropy generation rates across turbulent boundary layers on flat plates and—secondarily—to assess a popular approximate technique for their evaluation. About two-thirds or more of the entropy generation occurs in the viscous part, known as the viscous layer. Fundamental new results for entropy generation in turbulent boundary layers are provided by extending available direct numerical simulations. It was found that, with negligible pressure gradients, results presented in wall coordinates are predicted to be near “universal” in the viscous layer. This apparent universality disappears when a significant pressure gradient is applied; increasing the pressure gradient decreases the entropy generation rate. Within the viscous layer, the approximate evaluation of S‴ differs significantly from the “proper” value but its integral, the entropy generation rate per unit surface area Sap″, agrees within 5% at its edge. En ligne : http://fluidsengineering.asmedigitalcollection.asme.org/Issue.aspx?issueID=27318 [...]

Heat transfer enhancement using laminar gas-liquid segmented plug flows / Y. S. Muzychka in Journal of heat transfer, Vol. 133 N° 4 (Avril 2011) 

Public ISBD [article]  in Journal of heat transfer > Vol. 133 N° 4 (Avril 2011) . - pp. [041902/1-9] Titre : Heat transfer enhancement using laminar gas-liquid segmented plug flows Type de document : texte imprimé Auteurs : Y. S. Muzychka, Auteur ; Edmond J. Walsh, Auteur ; P. Walsh, Auteur Année de publication : 2011 Article en page(s) : pp. [041902/1-9] Note générale : Physique Langues : Anglais (eng) Mots-clés : Heat  transfer  Segmented  flow  Two  phase  flow  Thermal  enhancement  Graetz  flow  Plug  flow  Poiseuille  flow  Taylor  flow Index. décimale : 536 Chaleur. Thermodynamique Résumé : Heat transfer enhancement using segmented nonboiling gas-liquid flow is examined. Segmentation results in a two phase flow of liquid/gas having a constant liquid fraction; i.e., no phase change occurs. In this flow configuration, enhanced heat transfer occurs as a result of a shorter effective thermal length due to internal fluid circulation in the liquid plugs. A simple theory for laminar segmented flows is developed based on scaled Graetz theory and comparisons made with existing published data from the literature and new experimental data obtained in a companion study. The proposed model is valid for an isothermal tube wall provided that the axial residence time of the flow is such that dimensionless tube length L[small star, filled]<0.1. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&smode=strresults& [...] [article] Heat transfer enhancement using laminar gas-liquid segmented plug flows [texte imprimé] / Y. S. Muzychka, Auteur ; Edmond J. Walsh, Auteur ; P. Walsh, Auteur . - 2011 . - pp. [041902/1-9]. Physique Langues : Anglais (eng) in Journal of heat transfer > Vol. 133 N° 4 (Avril 2011) . - pp. [041902/1-9] Mots-clés : Heat  transfer  Segmented  flow  Two  phase  flow  Thermal  enhancement  Graetz  flow  Plug  flow  Poiseuille  flow  Taylor  flow Index. décimale : 536 Chaleur. Thermodynamique Résumé : Heat transfer enhancement using segmented nonboiling gas-liquid flow is examined. Segmentation results in a two phase flow of liquid/gas having a constant liquid fraction; i.e., no phase change occurs. In this flow configuration, enhanced heat transfer occurs as a result of a shorter effective thermal length due to internal fluid circulation in the liquid plugs. A simple theory for laminar segmented flows is developed based on scaled Graetz theory and comparisons made with existing published data from the literature and new experimental data obtained in a companion study. The proposed model is valid for an isothermal tube wall provided that the axial residence time of the flow is such that dimensionless tube length L[small star, filled]<0.1. DEWEY : 536 ISSN : 0022-1481 En ligne : http://asmedl.aip.org/vsearch/servlet/VerityServlet?KEY=JHTRAO&smode=strresults& [...]