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Characteristics of recirculation zone structure behind an impulsively started circular cylinder / Chang Lin in Journal of engineering mechanics, Vol. 138 N° 2 (Fevrier 2012) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 138 N° 2 (Fevrier 2012) . - pp.184198 Titre : Characteristics of recirculation zone structure behind an impulsively started circular cylinder Type de document : texte imprimé Auteurs : Chang Lin, Auteur ; Shih-Chun Hsieh, Auteur ; Wei-Jung Lin, Auteur Année de publication : 2012 Article en page(s) : pp.184198 Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Recirculation  Velocity  Cylinders  Reynolds  number Résumé : The characteristics of the recirculation zone structure behind an impulsively started circular cylinder for Reynolds number Re ranging from 500 to 2,000 and nondimensional time T ranging from 0.5 to 5.0 are investigated experimentally using particle image velocimetry (PIV) and flow visualization techniques. On the basis of the flow visualization pictures and velocity maps, both obtained by a reference frame with a moving coordinate system, the evolution of the recirculation zone structure is studied with special emphasis on the negative velocity subzone. The negative velocity subzone is enveloped by the boundary consisting of the zero-velocity points inside the recirculation zone, and the streamwise velocity profile inside and near the negative velocity subzone is characterized by a jetlike flow moving toward the cylinder. During the evolution of the recirculation zone, the representative dimensions of the vortical structure both in the streamwise and vertical directions, including the recirculation zone length and the center-to-center distance between two primary vortex cores, are found to increase rapidly with time. Characteristic length and velocity scales are proposed first in the literature not only to demonstrate the similarity profile for streamwise velocity within and near the negative velocity subzone, but also to exhibit the similarity for geometric shape of the negative velocity subzone via the ratio of the width of the negative velocity subzone (i.e., the vertical distance between two zero-velocity points) to the center-to-center distance between two primary vortex cores. The very important observation is that the similarities of velocity profile and geometric shape represent promising self-preservation for the jetlike flow enclosed by the zero-velocity points forming the boundary of the negative velocity subzone. In addition, time variation of the nondimensional circulation of two primary vortices in the recirculation zone is studied and found to increase with an increase in the nondimensional time. Note de contenu : Article Outline 1. Introduction 2. Experimental Setup and Procedure 1. Towing Tank and Test Model 2. Flow Visualization Techniques 3. Velocity Measurement Technique 4. Movement of Carriage System and Coordinate System 3. Results and Discussions 1. Test of Two Dimensionality of Flow Field 2. Flow Visualization 3. Velocity Field 4. Characteristic Dimensions of the Recirculation Zone 5. Vorticity of Primary Vortex in Recirculation Zone 4. Conclusions ISSN : 0733-9399 En ligne : http://ascelibrary.org/emo/resource/1/jenmdt/v138/i2/p184_s1?isAuthorized=no [article] Characteristics of recirculation zone structure behind an impulsively started circular cylinder [texte imprimé] / Chang Lin, Auteur ; Shih-Chun Hsieh, Auteur ; Wei-Jung Lin, Auteur . - 2012 . - pp.184198. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 138 N° 2 (Fevrier 2012) . - pp.184198 Mots-clés : Recirculation  Velocity  Cylinders  Reynolds  number Résumé : The characteristics of the recirculation zone structure behind an impulsively started circular cylinder for Reynolds number Re ranging from 500 to 2,000 and nondimensional time T ranging from 0.5 to 5.0 are investigated experimentally using particle image velocimetry (PIV) and flow visualization techniques. On the basis of the flow visualization pictures and velocity maps, both obtained by a reference frame with a moving coordinate system, the evolution of the recirculation zone structure is studied with special emphasis on the negative velocity subzone. The negative velocity subzone is enveloped by the boundary consisting of the zero-velocity points inside the recirculation zone, and the streamwise velocity profile inside and near the negative velocity subzone is characterized by a jetlike flow moving toward the cylinder. During the evolution of the recirculation zone, the representative dimensions of the vortical structure both in the streamwise and vertical directions, including the recirculation zone length and the center-to-center distance between two primary vortex cores, are found to increase rapidly with time. Characteristic length and velocity scales are proposed first in the literature not only to demonstrate the similarity profile for streamwise velocity within and near the negative velocity subzone, but also to exhibit the similarity for geometric shape of the negative velocity subzone via the ratio of the width of the negative velocity subzone (i.e., the vertical distance between two zero-velocity points) to the center-to-center distance between two primary vortex cores. The very important observation is that the similarities of velocity profile and geometric shape represent promising self-preservation for the jetlike flow enclosed by the zero-velocity points forming the boundary of the negative velocity subzone. In addition, time variation of the nondimensional circulation of two primary vortices in the recirculation zone is studied and found to increase with an increase in the nondimensional time. Note de contenu : Article Outline 1. Introduction 2. Experimental Setup and Procedure 1. Towing Tank and Test Model 2. Flow Visualization Techniques 3. Velocity Measurement Technique 4. Movement of Carriage System and Coordinate System 3. Results and Discussions 1. Test of Two Dimensionality of Flow Field 2. Flow Visualization 3. Velocity Field 4. Characteristic Dimensions of the Recirculation Zone 5. Vorticity of Primary Vortex in Recirculation Zone 4. Conclusions ISSN : 0733-9399 En ligne : http://ascelibrary.org/emo/resource/1/jenmdt/v138/i2/p184_s1?isAuthorized=no

Characteristics of shear layer structure in skimming flow over a vertical drop pool / Wei-Jung Lin in Journal of engineering mechanics, Vol. 135 N° 12 (Décembre 2009) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 135 N° 12 (Décembre 2009) . - pp. 1452-1466 Titre : Characteristics of shear layer structure in skimming flow over a vertical drop pool Type de document : texte imprimé Auteurs : Wei-Jung Lin, Auteur ; Chang Lin, Auteur ; Shih-Chun Hsieh, Auteur Article en page(s) : pp. 1452-1466 Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Drop  structures  Flow  measurement  Layered  systems  Velocity Résumé : The characteristics of shear layer structure between the sliding jet and the pool for skimming flows over a vertical drop pool were investigated experimentally, using flow visualization technique and high speed particle image velocimetry. Four series of experiments having different end sill ratios (h/H=0.12, 0.43, 0.71 and 1.0, where h=end sill height and H=drop height) with various approaching flow discharges were performed to measure the detailed quantitative velocity fields of the shear layer. The mean velocities and turbulence properties were obtained by ensemble averaging the repeated measurements. From the velocity profiles, it is found that the growth of the shear layer in the downward direction as the jet slides down the pool represents the momentum exchange. Analyzing the distribution of measured velocity, the similarity profile of the mean velocity at different cross sections along the shear layer was obtained. The proposed characteristic scales provided unique similarity profiles having promising regression coefficient. The selection of these characteristic scales is also discussed. Further, the spatial variations of mean velocity profiles, turbulence intensities, in-plane turbulent kinetic energy, and Reynolds shear stress were also elucidated in detail. The imperative observation is that the Reynolds shear stress dominates the major part along the shear layer as compared to the viscous shear stress. The study also provides an insight into the flow phenomena through the velocity and turbulent characteristics. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.aip.org/vsearch/servlet/VerityServlet?KEY=JENMDT&smode=strres [...] [article] Characteristics of shear layer structure in skimming flow over a vertical drop pool [texte imprimé] / Wei-Jung Lin, Auteur ; Chang Lin, Auteur ; Shih-Chun Hsieh, Auteur . - pp. 1452-1466. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 135 N° 12 (Décembre 2009) . - pp. 1452-1466 Mots-clés : Drop  structures  Flow  measurement  Layered  systems  Velocity Résumé : The characteristics of shear layer structure between the sliding jet and the pool for skimming flows over a vertical drop pool were investigated experimentally, using flow visualization technique and high speed particle image velocimetry. Four series of experiments having different end sill ratios (h/H=0.12, 0.43, 0.71 and 1.0, where h=end sill height and H=drop height) with various approaching flow discharges were performed to measure the detailed quantitative velocity fields of the shear layer. The mean velocities and turbulence properties were obtained by ensemble averaging the repeated measurements. From the velocity profiles, it is found that the growth of the shear layer in the downward direction as the jet slides down the pool represents the momentum exchange. Analyzing the distribution of measured velocity, the similarity profile of the mean velocity at different cross sections along the shear layer was obtained. The proposed characteristic scales provided unique similarity profiles having promising regression coefficient. The selection of these characteristic scales is also discussed. Further, the spatial variations of mean velocity profiles, turbulence intensities, in-plane turbulent kinetic energy, and Reynolds shear stress were also elucidated in detail. The imperative observation is that the Reynolds shear stress dominates the major part along the shear layer as compared to the viscous shear stress. The study also provides an insight into the flow phenomena through the velocity and turbulent characteristics. DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.aip.org/vsearch/servlet/VerityServlet?KEY=JENMDT&smode=strres [...]

Flow characteristics around a circular cylinder placed horizontally above a plane boundary / Wei-Jung Lin in Journal of engineering mechanics, Vol. 135 N° 7 (Juillet 2009) 

Public ISBD [article]  in Journal of engineering mechanics > Vol. 135 N° 7 (Juillet 2009) . - pp. 697-716 Titre : Flow characteristics around a circular cylinder placed horizontally above a plane boundary Type de document : texte imprimé Auteurs : Wei-Jung Lin, Auteur ; Chang Lin, Auteur ; Shih-Chun Hsieh, Auteur Article en page(s) : pp. 697-716 Note générale : Mécanique appliquée Langues : Anglais (eng) Mots-clés : Vortex  shedding  Eddies  Cylinders  Boundary  layers  Reynolds  number. Résumé : Flow characteristics around a circular cylinder positioned near a plane boundary (on which laminar boundary layer flow develops in the absence of circular cylinder), are investigated for Reynolds numbers R ranging from 7.8×102 to 1.15×104. Particle image velocimetry and fiber laser Doppler velocimetry were used to measure the velocity fields and velocity time histories, respectively. Flow structures are particularly revealed using flow visualization technique at R=7.8×102 for gap ratios G/D (where G is the net gap between the surface of circular cylinder and the plane boundary), varying from 0 to 4. Based on the experimental results, the variation of Strouhal number of shedding vortex (or eddy) with G/D, the mechanism of vortex shedding suppression, and the streamwise velocity profiles of the upper shear layers and gap flows for small G/D are all discussed. Although the regular, alternate vortex shedding is suppressed for G/D<0.5, the periodicity could be detected due to the vortex (or eddy) shedding from the upper shear layer of the circular cylinder. Gap flow switching randomly is found and first put forward to be the main reason of multipeak or broadband spectral characteristics of the shedding event at a certain small gap ratio. It is also found that the streamwise velocity profiles of the upper shear layer, where periodic shedding eddies originate, exhibit well-behaved similarity. In addition, a unique similarity of mean streamwise velocity profiles of the gap flows is demonstrated for G/D<=0.3. For R<4×103, the S increases as G/D decreases to its maximum around G/D~=0.5 and then decreases as G/D decreases. For R>=4×103, although most of the previous studies indicate that the S is insensitive to G/D, the present study shows that S still increases as G/D decreases but the variations of S are in a small range (i.e., 0.18<=S<=0.22). DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JENMDT000 [...] [article] Flow characteristics around a circular cylinder placed horizontally above a plane boundary [texte imprimé] / Wei-Jung Lin, Auteur ; Chang Lin, Auteur ; Shih-Chun Hsieh, Auteur . - pp. 697-716. Mécanique appliquée Langues : Anglais (eng) in Journal of engineering mechanics > Vol. 135 N° 7 (Juillet 2009) . - pp. 697-716 Mots-clés : Vortex  shedding  Eddies  Cylinders  Boundary  layers  Reynolds  number. Résumé : Flow characteristics around a circular cylinder positioned near a plane boundary (on which laminar boundary layer flow develops in the absence of circular cylinder), are investigated for Reynolds numbers R ranging from 7.8×102 to 1.15×104. Particle image velocimetry and fiber laser Doppler velocimetry were used to measure the velocity fields and velocity time histories, respectively. Flow structures are particularly revealed using flow visualization technique at R=7.8×102 for gap ratios G/D (where G is the net gap between the surface of circular cylinder and the plane boundary), varying from 0 to 4. Based on the experimental results, the variation of Strouhal number of shedding vortex (or eddy) with G/D, the mechanism of vortex shedding suppression, and the streamwise velocity profiles of the upper shear layers and gap flows for small G/D are all discussed. Although the regular, alternate vortex shedding is suppressed for G/D<0.5, the periodicity could be detected due to the vortex (or eddy) shedding from the upper shear layer of the circular cylinder. Gap flow switching randomly is found and first put forward to be the main reason of multipeak or broadband spectral characteristics of the shedding event at a certain small gap ratio. It is also found that the streamwise velocity profiles of the upper shear layer, where periodic shedding eddies originate, exhibit well-behaved similarity. In addition, a unique similarity of mean streamwise velocity profiles of the gap flows is demonstrated for G/D<=0.3. For R<4×103, the S increases as G/D decreases to its maximum around G/D~=0.5 and then decreases as G/D decreases. For R>=4×103, although most of the previous studies indicate that the S is insensitive to G/D, the present study shows that S still increases as G/D decreases but the variations of S are in a small range (i.e., 0.18<=S<=0.22). DEWEY : 620.1 ISSN : 0733-9399 En ligne : http://ascelibrary.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JENMDT000 [...]