Pub Date : 2020-01-01DOI: 10.1299/jfst.2020jfst0005
Naoya Uene, H. Takeuchi, Y. Hayamizu, T. Tokumasu
We consider a Couette flow of a rarefied Ar gas with heat transfer between two wall surfaces and investigate the scattering behavior of gas molecules reflected either at a clean Pt surface or at a surface contaminated with adsorbates. Water molecules abundantly present in the atmosphere were adopted as the adsorbates. The reflection of gas molecules on the lower wall surface was simulated by Molecular Dynamics (MD) method to obtain accommodation coefficients and velocity distribution functions of gas molecules. We applied the modified reflection model of gas molecule and investigated the velocity distribution functions of the model by comparing the MD results to verify the validity. The accommodation coefficients obtained by the MD method depend on the number of adsorbed water molecules on the lower wall surface. Specifically, tangential momentum accommodation coefficient (TMAC) tended to increase and then decrease with the increase in adsorbed water molecules, but normal momentum accommodation coefficient (NMAC) tended to decrease monotonically. The velocity distribution functions of the modified reflection model approximately show the good agreement with the MD calculation but the degree of coincidence depends on the speed difference between the upper and lower wall surfaces, and the number of adsorbed water molecules on the surface.
{"title":"Study of reflection models of gas molecules on water adsorbed surfaces in high-speed flows","authors":"Naoya Uene, H. Takeuchi, Y. Hayamizu, T. Tokumasu","doi":"10.1299/jfst.2020jfst0005","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0005","url":null,"abstract":"We consider a Couette flow of a rarefied Ar gas with heat transfer between two wall surfaces and investigate the scattering behavior of gas molecules reflected either at a clean Pt surface or at a surface contaminated with adsorbates. Water molecules abundantly present in the atmosphere were adopted as the adsorbates. The reflection of gas molecules on the lower wall surface was simulated by Molecular Dynamics (MD) method to obtain accommodation coefficients and velocity distribution functions of gas molecules. We applied the modified reflection model of gas molecule and investigated the velocity distribution functions of the model by comparing the MD results to verify the validity. The accommodation coefficients obtained by the MD method depend on the number of adsorbed water molecules on the lower wall surface. Specifically, tangential momentum accommodation coefficient (TMAC) tended to increase and then decrease with the increase in adsorbed water molecules, but normal momentum accommodation coefficient (NMAC) tended to decrease monotonically. The velocity distribution functions of the modified reflection model approximately show the good agreement with the MD calculation but the degree of coincidence depends on the speed difference between the upper and lower wall surfaces, and the number of adsorbed water molecules on the surface.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304288","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jfst.2020jfst0010
Kentaro Echigo, K. Tsujimoto, T. Shakouchi, T. Ando
A single impinging jet exhibits high heat transfer performance around an impingement point on a wall. However, the heat transfer performance deteriorates as it moves away from the impingement point. Consequently, multiple impinging jets are commonly introduced to overcome the shortcomings of a single jet: inhomogeneous heat distribution on the wall and a narrow heating area. However, inhomogeneous heat transfers still occur. Therefore, a new jet control is required to improve the uniformity of heat transfer. Meanwhile, blooming jets are produced by appropriate combinations of axial and helical excitations at the nozzle exit. Using appropriately selected excitations, a jet can split into two separate jets (bifurcating jet) or spread into a shower of toroidal vortex rings. Blooming jets exhibit good performances of mixing and diffusion, suggesting possible applications in flow control. However, studies regarding the heat transfer performance of blooming jets are non-existent. In this study, we conducted direct numerical simulations of blooming jets impinging upon a wall and investigated their flow characteristics and heat transfer performances. As control parameters, the impingement distance (the distance from the nozzle to the wall) and frequency ratio (the axial excitation frequency to the helical frequency) are varied. The vortex structures and velocity magnitude reveals flow modulations due to blooming control. With the time-averaged local Nusselt number, the heat transfer performance of the blooming jets is evaluated quantitatively. Compared with uncontrolled jets, the uniformity of heat transfer of blooming jets is better, suggesting their potential application for leveling the heat transfer of impinging jets.
{"title":"Flow and heat transfer characteristics of blooming jets impinging upon wall using DNS","authors":"Kentaro Echigo, K. Tsujimoto, T. Shakouchi, T. Ando","doi":"10.1299/jfst.2020jfst0010","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0010","url":null,"abstract":"A single impinging jet exhibits high heat transfer performance around an impingement point on a wall. However, the heat transfer performance deteriorates as it moves away from the impingement point. Consequently, multiple impinging jets are commonly introduced to overcome the shortcomings of a single jet: inhomogeneous heat distribution on the wall and a narrow heating area. However, inhomogeneous heat transfers still occur. Therefore, a new jet control is required to improve the uniformity of heat transfer. Meanwhile, blooming jets are produced by appropriate combinations of axial and helical excitations at the nozzle exit. Using appropriately selected excitations, a jet can split into two separate jets (bifurcating jet) or spread into a shower of toroidal vortex rings. Blooming jets exhibit good performances of mixing and diffusion, suggesting possible applications in flow control. However, studies regarding the heat transfer performance of blooming jets are non-existent. In this study, we conducted direct numerical simulations of blooming jets impinging upon a wall and investigated their flow characteristics and heat transfer performances. As control parameters, the impingement distance (the distance from the nozzle to the wall) and frequency ratio (the axial excitation frequency to the helical frequency) are varied. The vortex structures and velocity magnitude reveals flow modulations due to blooming control. With the time-averaged local Nusselt number, the heat transfer performance of the blooming jets is evaluated quantitatively. Compared with uncontrolled jets, the uniformity of heat transfer of blooming jets is better, suggesting their potential application for leveling the heat transfer of impinging jets.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304690","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jfst.2020jfst0022
Syuta Sasaoka, T. Kurahashi
An algorithm to optimize texture shape under constant load conditions and a shape update equation of the design variables are proposed. The tribological properties are improved by machining grooves and holes, termed“ texture”, on frictional surfaces that are lubricated by fluid. Improvement of tribological properties, such as the friction coefficient, is likely to lead to a reduction in energy loss and extension of machine life, resulting in major economic benefits. Because tribological properties depend on the shape of the texture, the focus of this study was on the dimensional shape of the texture. Most countermeasures to this problem involve size optimization rather than shape optimization. Conventionally, when the effect of the texture shape on the friction coefficient is evaluated experimentally, the load is kept constant. However, when the texture shape is changed as part of the analysis, the pressure field changes. The load, which is the integrated value of the pressure, also changes. Therefore, it is difficult to evaluate the friction coefficient accurately. At this time, the load can be kept constant by adjusting the basic oil film thickness, which is the distance between the frictional surfaces. This occurs naturally in real-world situations. In general, when the adjoint variable method is applied to determine the texture shape, constraint conditions are included in the Lagrange function. But, in this study, the constant load condition, i.e., the constraint condition, was simply added to keep the initial load, because it is difficult to calculate the gradient of the constraint condition with respect to the design variable. Considering the above, the purpose of this study was to find an appropriate oil film thickness for a texture by shape optimization and to reduce the friction coefficient by adding an algorithm that keeps the load constant by varying the basic oil film thickness. In addition, the shape update equation for the design variable was improved, and results based on the present method were compared with those based on the steepest descent and the conjugate gradient methods. This was achieved by replacing the interpretation of the update equation using the steepest descent method with a differential equation and by applying the differential to the step length of the design variable in the Taylor expansion equation of the design variable. By improving the shape update equation, a lower performance function was obtained. Texture shape optimization was performed by the adjoint variable method using the Reynolds equation as the governing equation. The performance function is defined by the frictional force, and the friction coefficient is optimized at the same time by keeping the load constant. FreeFEM++ was used to calculate the optimal shape.
{"title":"Proposal of texture shape optimization algorithm under constant load condition and considerations on new shape update equation (Texture shape optimization for minimization of friction coefficient)","authors":"Syuta Sasaoka, T. Kurahashi","doi":"10.1299/jfst.2020jfst0022","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0022","url":null,"abstract":"An algorithm to optimize texture shape under constant load conditions and a shape update equation of the design variables are proposed. The tribological properties are improved by machining grooves and holes, termed“ texture”, on frictional surfaces that are lubricated by fluid. Improvement of tribological properties, such as the friction coefficient, is likely to lead to a reduction in energy loss and extension of machine life, resulting in major economic benefits. Because tribological properties depend on the shape of the texture, the focus of this study was on the dimensional shape of the texture. Most countermeasures to this problem involve size optimization rather than shape optimization. Conventionally, when the effect of the texture shape on the friction coefficient is evaluated experimentally, the load is kept constant. However, when the texture shape is changed as part of the analysis, the pressure field changes. The load, which is the integrated value of the pressure, also changes. Therefore, it is difficult to evaluate the friction coefficient accurately. At this time, the load can be kept constant by adjusting the basic oil film thickness, which is the distance between the frictional surfaces. This occurs naturally in real-world situations. In general, when the adjoint variable method is applied to determine the texture shape, constraint conditions are included in the Lagrange function. But, in this study, the constant load condition, i.e., the constraint condition, was simply added to keep the initial load, because it is difficult to calculate the gradient of the constraint condition with respect to the design variable. Considering the above, the purpose of this study was to find an appropriate oil film thickness for a texture by shape optimization and to reduce the friction coefficient by adding an algorithm that keeps the load constant by varying the basic oil film thickness. In addition, the shape update equation for the design variable was improved, and results based on the present method were compared with those based on the steepest descent and the conjugate gradient methods. This was achieved by replacing the interpretation of the update equation using the steepest descent method with a differential equation and by applying the differential to the step length of the design variable in the Taylor expansion equation of the design variable. By improving the shape update equation, a lower performance function was obtained. Texture shape optimization was performed by the adjoint variable method using the Reynolds equation as the governing equation. The performance function is defined by the frictional force, and the friction coefficient is optimized at the same time by keeping the load constant. FreeFEM++ was used to calculate the optimal shape.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305307","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jfst.2020jfst0003
Y. Naka, Kento Inoue, Takumi Ishizaka
The present study aims to develop an ultrasound acoustic streaming actuator for flow control. The driving force can be derived from the continuity equation and the Navier-Stokes equation for the viscous compressible flow. Commercially available transducers are used as an ultra sound source, and the acoustic and induced flow characteristics for a single and multiple transducer configurations are examined. The sound pressure distribution indicates the strong acoustic pressure fluctuation near the transducer. For the multiple transducer cases, the region of the strong pressure fluctuation is widened due to the superposition of the waves. The distributions of the induced velocity are evaluated using particle image velocimetry. It is revealed that the maximum flow velocity is about 0.04 m/s for the single transducer case, and the maximum velocity is observed slightly downstream of the high sound intensity region. Since the driving force is proportional to the square of the sound pressure intensity, the higher flow velocity can be achieved using more transducers. A transducer array having 100 transducers has been applied in a turbulent boundary layer. It is confirmed that the flow velocity near the wall increases in the case with the control, and turbulence intensity augments by approximately 17% compared with the case without the control.
{"title":"Development of an ultrasound acoustic streaming actuator for flow control","authors":"Y. Naka, Kento Inoue, Takumi Ishizaka","doi":"10.1299/jfst.2020jfst0003","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0003","url":null,"abstract":"The present study aims to develop an ultrasound acoustic streaming actuator for flow control. The driving force can be derived from the continuity equation and the Navier-Stokes equation for the viscous compressible flow. Commercially available transducers are used as an ultra sound source, and the acoustic and induced flow characteristics for a single and multiple transducer configurations are examined. The sound pressure distribution indicates the strong acoustic pressure fluctuation near the transducer. For the multiple transducer cases, the region of the strong pressure fluctuation is widened due to the superposition of the waves. The distributions of the induced velocity are evaluated using particle image velocimetry. It is revealed that the maximum flow velocity is about 0.04 m/s for the single transducer case, and the maximum velocity is observed slightly downstream of the high sound intensity region. Since the driving force is proportional to the square of the sound pressure intensity, the higher flow velocity can be achieved using more transducers. A transducer array having 100 transducers has been applied in a turbulent boundary layer. It is confirmed that the flow velocity near the wall increases in the case with the control, and turbulence intensity augments by approximately 17% compared with the case without the control.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1299/jfst.2020jfst0003","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304140","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jfst.2020jfst0004
K. Nishikawa, K. Konno, Y. Hattori
An immersed boundary method of discrete type is tested as a tool for direct numerical simulation of aeroacoustic sound. The numerical method consists of the WENO scheme, the immersed boundary method by Chaudhuri et al. (J. Comp. Phys. Vol. 230, 1731–1748 (2011)), and the perfectly matched layer together with the dyadic mesh refinement and the Runge-Kutta method. The accuracy of the method is shown to be sufficient for four basic problems: propagation of acoustic waves, aeroacoustic sound generation in a flow past a fixed circular cylinder, in a flow past an oscillating square cylinder, and from a vortex pair passing through a circular cylinder. The results confirm that the developed method can deal with moving bodies and it is accurate not only for viscous flows but also for inviscid flows.
{"title":"Assessment of immersed boundary method as a tool for direct numerical simulation of aeroacoustic sound","authors":"K. Nishikawa, K. Konno, Y. Hattori","doi":"10.1299/jfst.2020jfst0004","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0004","url":null,"abstract":"An immersed boundary method of discrete type is tested as a tool for direct numerical simulation of aeroacoustic sound. The numerical method consists of the WENO scheme, the immersed boundary method by Chaudhuri et al. (J. Comp. Phys. Vol. 230, 1731–1748 (2011)), and the perfectly matched layer together with the dyadic mesh refinement and the Runge-Kutta method. The accuracy of the method is shown to be sufficient for four basic problems: propagation of acoustic waves, aeroacoustic sound generation in a flow past a fixed circular cylinder, in a flow past an oscillating square cylinder, and from a vortex pair passing through a circular cylinder. The results confirm that the developed method can deal with moving bodies and it is accurate not only for viscous flows but also for inviscid flows.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1299/jfst.2020jfst0004","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304234","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jfst.2020jfst0014
Riko Uekusa, Aika Kawagoe, Yusuke Nabae, K. Fukagata
Using the resolvent analysis, we investigate how the near-wall mode primarily responsible for the friction drag is amplified or suppressed depending on the shape of the mean velocity profile of a turbulent channel flow. Following the recent finding by Kühnen et al. (2018), who modified the mean velocity profile to be flatter and attained significant drag reduction, we introduce two types of artificially flattened turbulent mean velocity profiles: one is based on the turbulent viscosity model proposed by Reynolds and Tiederman (1967), and the other is based on the mean velocity profile of laminar flow. A special care is taken so that both the bulk and friction Reynolds numbers are unchanged, whereby only the effect of change in the mean velocity profile can be studied. These mean velocity profiles are used as the base flow in the resolvent analysis, and the response of the wavenumber-frequency mode corresponding to the near-wall coherent structure is assessed via the change in the singular value (i.e., amplification rate). The flatness of the modified mean velocity profiles is quantified by three different measures. In general, the flatter mean velocity profiles are found to result in significant suppression of near-wall mode. Further, increasing the mean velocity gradient in the very vicinity of the wall is found to have a significant importance for the suppression of near-wall mode through mitigation of the critical layer.
{"title":"Resolvent analysis of turbulent channel flow with manipulated mean velocity profile","authors":"Riko Uekusa, Aika Kawagoe, Yusuke Nabae, K. Fukagata","doi":"10.1299/jfst.2020jfst0014","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0014","url":null,"abstract":"Using the resolvent analysis, we investigate how the near-wall mode primarily responsible for the friction drag is amplified or suppressed depending on the shape of the mean velocity profile of a turbulent channel flow. Following the recent finding by Kühnen et al. (2018), who modified the mean velocity profile to be flatter and attained significant drag reduction, we introduce two types of artificially flattened turbulent mean velocity profiles: one is based on the turbulent viscosity model proposed by Reynolds and Tiederman (1967), and the other is based on the mean velocity profile of laminar flow. A special care is taken so that both the bulk and friction Reynolds numbers are unchanged, whereby only the effect of change in the mean velocity profile can be studied. These mean velocity profiles are used as the base flow in the resolvent analysis, and the response of the wavenumber-frequency mode corresponding to the near-wall coherent structure is assessed via the change in the singular value (i.e., amplification rate). The flatness of the modified mean velocity profiles is quantified by three different measures. In general, the flatter mean velocity profiles are found to result in significant suppression of near-wall mode. Further, increasing the mean velocity gradient in the very vicinity of the wall is found to have a significant importance for the suppression of near-wall mode through mitigation of the critical layer.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305237","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jfst.2020jfst0018
Y. Naka, Masataka Himeda
External forcing on a wing-tip vortex can affect its instability, and therefore an optimal perturbation can improve the aerodynamic performance of the wing. The present study examined the unsteadiness of the wing-tip vortex under periodic wing-tip vibration, and revealed its effect on the aerodynamic performance of the wing. A 3Dprinted vibrating wing-tip model was prepared, which was driven by a sheet-type piezo actuator. Phase-averaged stereo particle image velocimetry (PIV) measurements clarified that the averaged position of the vortex depends on the phase of the wing-tip vibration, and the vortex shifted further from the wing as the actuation frequency increased. The phase-averaged velocity distributions indicate that the velocity deficit inside the vortex is significantly enhanced near the end of the downstroke of the wing-tip motion. The wing-tip vortex is weakened in the mid-upstroke, and its impact depends on the actuation frequency. This is because the motion of the wing is in the same direction as the flow rolling up from the pressure side, which prevents the formation of the vortex. In the mid-upstroke phase, the turbulence quantities, e.g., the turbulent kinetic energy and the Reynolds shear stress, are significantly suppressed; these effects depend monotonically on the actuation frequency. These arguments are supported by time-resolved recordings of the flow and the wing motion. The force measurements reveal that the vibration of the wing-tip brings a positive effect on the lift-to-drag ratio.
{"title":"Impact of wing-tip vibration on the development of a wing-tip vortex","authors":"Y. Naka, Masataka Himeda","doi":"10.1299/jfst.2020jfst0018","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0018","url":null,"abstract":"External forcing on a wing-tip vortex can affect its instability, and therefore an optimal perturbation can improve the aerodynamic performance of the wing. The present study examined the unsteadiness of the wing-tip vortex under periodic wing-tip vibration, and revealed its effect on the aerodynamic performance of the wing. A 3Dprinted vibrating wing-tip model was prepared, which was driven by a sheet-type piezo actuator. Phase-averaged stereo particle image velocimetry (PIV) measurements clarified that the averaged position of the vortex depends on the phase of the wing-tip vibration, and the vortex shifted further from the wing as the actuation frequency increased. The phase-averaged velocity distributions indicate that the velocity deficit inside the vortex is significantly enhanced near the end of the downstroke of the wing-tip motion. The wing-tip vortex is weakened in the mid-upstroke, and its impact depends on the actuation frequency. This is because the motion of the wing is in the same direction as the flow rolling up from the pressure side, which prevents the formation of the vortex. In the mid-upstroke phase, the turbulence quantities, e.g., the turbulent kinetic energy and the Reynolds shear stress, are significantly suppressed; these effects depend monotonically on the actuation frequency. These arguments are supported by time-resolved recordings of the flow and the wing motion. The force measurements reveal that the vibration of the wing-tip brings a positive effect on the lift-to-drag ratio.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304923","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-01-01DOI: 10.1299/jfst.2020jfst0011
Shiho Hirokawa, Kaoruko Eto, K. Fukagata, N. Tokugawa
Friction drag reduction effect of a passive blowing on a Clark-Y airfoil is investigated. Uniform blowing, conducted in a wall-normal direction on a relatively wide surface, is generally known as an active control method for reduction of turbulent skin friction drag. In the present study, uniform blowing is passively driven by the pressure difference on a wing surface between suction and blowing regions. The suction and the blowing regions are respectively set around the leading edge and the rear part of the upper surface of the Clark-Y airfoil in order to ensure a sufficient pressure difference for passive blowing. The Reynolds number based on the chord length is 0.65×106 and 1.55×106. The angle of attack is set to 0◦ and 6◦. The mean streamwise velocity profiles on the blowing region and the downstream, measured by a traversed hot-wire anemometry, are observed to shift away from the wall by passive blowing. This behavior qualitatively suggests reduction of local skin friction on the wing surface. A quantitative assessment of the friction drag is performed using the law of the wall accounting for pressure gradients (Nickels, 2004), coupled with a modified Stevenson’s law (Vigdorovich, 2016) to account for the weak blowing. From this assessment, the local friction drag reduction effect of passive blowing is estimated to reach 4% − 23%.
{"title":"Experimental investigation on friction drag reduction on an airfoil by passive blowing","authors":"Shiho Hirokawa, Kaoruko Eto, K. Fukagata, N. Tokugawa","doi":"10.1299/jfst.2020jfst0011","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0011","url":null,"abstract":"Friction drag reduction effect of a passive blowing on a Clark-Y airfoil is investigated. Uniform blowing, conducted in a wall-normal direction on a relatively wide surface, is generally known as an active control method for reduction of turbulent skin friction drag. In the present study, uniform blowing is passively driven by the pressure difference on a wing surface between suction and blowing regions. The suction and the blowing regions are respectively set around the leading edge and the rear part of the upper surface of the Clark-Y airfoil in order to ensure a sufficient pressure difference for passive blowing. The Reynolds number based on the chord length is 0.65×106 and 1.55×106. The angle of attack is set to 0◦ and 6◦. The mean streamwise velocity profiles on the blowing region and the downstream, measured by a traversed hot-wire anemometry, are observed to shift away from the wall by passive blowing. This behavior qualitatively suggests reduction of local skin friction on the wing surface. A quantitative assessment of the friction drag is performed using the law of the wall accounting for pressure gradients (Nickels, 2004), coupled with a modified Stevenson’s law (Vigdorovich, 2016) to account for the weak blowing. From this assessment, the local friction drag reduction effect of passive blowing is estimated to reach 4% − 23%.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1299/jfst.2020jfst0011","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305218","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2019-07-28DOI: 10.1115/ajkfluids2019-5255
H. Teramoto, T. Kiwata, Kako Yajima
An experimental study is conducted to investigate the flow characteristics of multiple elliptic jets issuing from a 6 × 6 nozzle array at a relatively low-Reynolds number (Re = 4.3 × 103). Two aspect ratios of the multiple elliptic nozzles (equivalent diameter, de, of a nozzle was 6 mm), namely a/b = 2.25 and 6.25, where a and b are the radii of the major and minor axes of an elliptic nozzle, respectively, and two nozzle azimuthal orientations, namely the same and alternate azimuthal orientation arrangements, were used. The mean and fluctuating velocities were measured using a constant-temperature hot-wire anemometer. The multiple jets located at the side of the ambient fluid were stretched due to interactions between the self-induced flow of an elliptic vortex ring and the secondary flow caused by the entrainment of the ambient fluid. For a/b = 2.25, axis switching occurred only once in the range of 1 < x/de ≤ 3 for both nozzle azimuthal orientations. For a/b = 6.25 and the same azimuthal orientation arrangement, axis switching occurred only once at 3 < x/de ≤ 5; axis switching did not occur for the alternate azimuthal orientation arrangement. Thus, the flow characteristics of multiple elliptic jets are influenced by the azimuthal orientation of adjoining nozzles.
{"title":"Influence of nozzle aspect ratio and orientation on flow characteristics of multiple elliptic jets","authors":"H. Teramoto, T. Kiwata, Kako Yajima","doi":"10.1115/ajkfluids2019-5255","DOIUrl":"https://doi.org/10.1115/ajkfluids2019-5255","url":null,"abstract":"\u0000 An experimental study is conducted to investigate the flow characteristics of multiple elliptic jets issuing from a 6 × 6 nozzle array at a relatively low-Reynolds number (Re = 4.3 × 103). Two aspect ratios of the multiple elliptic nozzles (equivalent diameter, de, of a nozzle was 6 mm), namely a/b = 2.25 and 6.25, where a and b are the radii of the major and minor axes of an elliptic nozzle, respectively, and two nozzle azimuthal orientations, namely the same and alternate azimuthal orientation arrangements, were used. The mean and fluctuating velocities were measured using a constant-temperature hot-wire anemometer. The multiple jets located at the side of the ambient fluid were stretched due to interactions between the self-induced flow of an elliptic vortex ring and the secondary flow caused by the entrainment of the ambient fluid. For a/b = 2.25, axis switching occurred only once in the range of 1 < x/de ≤ 3 for both nozzle azimuthal orientations. For a/b = 6.25 and the same azimuthal orientation arrangement, axis switching occurred only once at 3 < x/de ≤ 5; axis switching did not occur for the alternate azimuthal orientation arrangement. Thus, the flow characteristics of multiple elliptic jets are influenced by the azimuthal orientation of adjoining nozzles.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2019-07-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43238132","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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