Pub Date : 2021-01-01DOI: 10.1299/jfst.2021jfst0007
H. Yao, T. Nambu, Y. Mizobuchi
A combustion simulation software tool, “HINOCA”, has been developed for automotive engine analysis. HINOCA is based on fully compressible Navier-Stokes equations, which are Reynolds-averaged (RANS) or spatially-filtered (LES), and employs the Cartesian grid and immersed boundary (IB) methods to reduce the mesh generation cost. In the present paper, focusing on flow simulations using k-ε models, a robust and reliable IB method coupled with wall functions is proposed. One major aspect of the method is that different IB cell information is employed for inviscid and viscous flux evaluations at fluid-IB cell interfaces. To improve the evaluation of wall shear stress, the shear stresses on the boundaries of an IB cell are transformed into a body force acting on the adjacent fluid cell. The computational method for ε-equation and the source terms of the k-equation near IB cells are modified so that the development of the turbulent boundary layer on a flat plate is well reproduced. The effects of these modifications are validated by the 2D Zero Pressure Gradient Flat Plate problem. To improve the mass conservation property of the IB method, multiple geometric parameters are defined for IB cells; that is, different image point information is immersed on IB cell centers for evaluating the inviscid flux on each cell interface. Evaluation with the Steady State Flow Bench problem shows that the proposed method drastically improves the mass conservation property of simulations and is able with a coarse mesh to reproduce flow structures obtained by LES with a much finer mesh.
{"title":"An immersed boundary method for practical simulations of high-Reynolds number flows by k-ε RANS models","authors":"H. Yao, T. Nambu, Y. Mizobuchi","doi":"10.1299/jfst.2021jfst0007","DOIUrl":"https://doi.org/10.1299/jfst.2021jfst0007","url":null,"abstract":"A combustion simulation software tool, “HINOCA”, has been developed for automotive engine analysis. HINOCA is based on fully compressible Navier-Stokes equations, which are Reynolds-averaged (RANS) or spatially-filtered (LES), and employs the Cartesian grid and immersed boundary (IB) methods to reduce the mesh generation cost. In the present paper, focusing on flow simulations using k-ε models, a robust and reliable IB method coupled with wall functions is proposed. One major aspect of the method is that different IB cell information is employed for inviscid and viscous flux evaluations at fluid-IB cell interfaces. To improve the evaluation of wall shear stress, the shear stresses on the boundaries of an IB cell are transformed into a body force acting on the adjacent fluid cell. The computational method for ε-equation and the source terms of the k-equation near IB cells are modified so that the development of the turbulent boundary layer on a flat plate is well reproduced. The effects of these modifications are validated by the 2D Zero Pressure Gradient Flat Plate problem. To improve the mass conservation property of the IB method, multiple geometric parameters are defined for IB cells; that is, different image point information is immersed on IB cell centers for evaluating the inviscid flux on each cell interface. Evaluation with the Steady State Flow Bench problem shows that the proposed method drastically improves the mass conservation property of simulations and is able with a coarse mesh to reproduce flow structures obtained by LES with a much finer mesh.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"16 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2021-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305885","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 : 2020-03-01DOI: 10.1299/jfst.2020jfst0015
De Zhang, Y. Katayama, S. Watanabe, S. Tsuda, A. Furukawa
Compared with conventional high-specific-speed axial flow pump, better cavitation performance and compact size have been achieved in contra-rotating axial flow pump, where the rear rotor is employed additionally to the front rotor to convert the swirling flow to the pressure rise. Meanwhile, significantly deteriorated performance has also been observed at well off-design flow rates with design rotational speed. The rotational speed control (RSC) of front and rear rotors has been experimentally proved to be effective to enhance the performance. However, thorough investigations are necessary to find the optimum rotational speeds of rotors. It may be done by computational fluid dynamics (CFD) simulations, whereas it is time-consuming to cover the wide ranges of rotational speeds. Therefore, in the present paper, a fast and effective performance prediction model is established by considering radial equilibrium condition, conservation of rothalpy and mass, empirical deviation angle, bladerows interaction and empirical losses. Experimental and CFD results are employed to validate the proposed prediction model. It is found that the proposed model shows good enough accuracy in predicting performances of contra-rotating axial flow pump under RSC in broad flow rate range. Furthermore, an energy saving application of the proposed model is also illustrated for two typical system resistances. Compared with the traditional valve control under the design rotational speed operation, the RSC method can well modify the pump head to satisfy the system resistance at wide flow rate range with the significantly improved energy performance.
{"title":"Performance prediction model of contra-rotating axial flow pump with separate rotational speed of front and rear rotors and its application for energy saving operation","authors":"De Zhang, Y. Katayama, S. Watanabe, S. Tsuda, A. Furukawa","doi":"10.1299/jfst.2020jfst0015","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0015","url":null,"abstract":"Compared with conventional high-specific-speed axial flow pump, better cavitation performance and compact size have been achieved in contra-rotating axial flow pump, where the rear rotor is employed additionally to the front rotor to convert the swirling flow to the pressure rise. Meanwhile, significantly deteriorated performance has also been observed at well off-design flow rates with design rotational speed. The rotational speed control (RSC) of front and rear rotors has been experimentally proved to be effective to enhance the performance. However, thorough investigations are necessary to find the optimum rotational speeds of rotors. It may be done by computational fluid dynamics (CFD) simulations, whereas it is time-consuming to cover the wide ranges of rotational speeds. Therefore, in the present paper, a fast and effective performance prediction model is established by considering radial equilibrium condition, conservation of rothalpy and mass, empirical deviation angle, bladerows interaction and empirical losses. Experimental and CFD results are employed to validate the proposed prediction model. It is found that the proposed model shows good enough accuracy in predicting performances of contra-rotating axial flow pump under RSC in broad flow rate range. Furthermore, an energy saving application of the proposed model is also illustrated for two typical system resistances. Compared with the traditional valve control under the design rotational speed operation, the RSC method can well modify the pump head to satisfy the system resistance at wide flow rate range with the significantly improved energy performance.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"15 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2020-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49523368","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.2020jfst0008
M. Ohashi, Y. Morita, Shiho Hirokawa, K. Fukagata, N. Tokugawa
Reynolds-averaged Navier–Stokes simulations (RANS) of flows around a Clark-Y airfoil with uniform blowing (UB) and uniform suction (US) are performed aiming at improvement of the airfoil performance. First, the control effect in the case with single UB or US applied on the airfoil surface is investigated at the various control locations. The magnitudeof UB/US is 0.14% of the free-stream velocity, and the control region is set at four diff erent locations on the upper and lower surfaces. The Reynolds number based on the chord length and the angle of attack are 1.5×106 and 0◦, respectively. It is found that the friction drag is decreased/increased by single UB/US control. It is also found that UB on the lower surface or US on the upper surface improves the lift-to-drag ratio, while UB on the upper surface or US on the lower surface worsens it. In the combined control of UB and US having the equal flow rate, the magnitude of blowing and suction is set at 0.14% or 0.26% of the free-stream velocity. The locations of blowing/suction and flow conditions are the same as those in the cases with either UB or US only. The simulationresult suggests that the lift-to-drag ratio is improved by the combined control of UB on the lower surface and US on the upper surface. In particular, the lift-to-drag ratio is most improved by a combination of UB on the lower rear surface and US on the upper rear surface. In contrast, a combined control of UB on the upper front surface and US on the lower rear surface is identified as the most eff ec ive case for the friction drag reduction only.
{"title":"Parametric study toward optimization of blowing and suction locations for improving lift-to-drag ratio on a Clark-Y airfoil","authors":"M. Ohashi, Y. Morita, Shiho Hirokawa, K. Fukagata, N. Tokugawa","doi":"10.1299/jfst.2020jfst0008","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0008","url":null,"abstract":"Reynolds-averaged Navier–Stokes simulations (RANS) of flows around a Clark-Y airfoil with uniform blowing (UB) and uniform suction (US) are performed aiming at improvement of the airfoil performance. First, the control effect in the case with single UB or US applied on the airfoil surface is investigated at the various control locations. The magnitudeof UB/US is 0.14% of the free-stream velocity, and the control region is set at four diff erent locations on the upper and lower surfaces. The Reynolds number based on the chord length and the angle of attack are 1.5×106 and 0◦, respectively. It is found that the friction drag is decreased/increased by single UB/US control. It is also found that UB on the lower surface or US on the upper surface improves the lift-to-drag ratio, while UB on the upper surface or US on the lower surface worsens it. In the combined control of UB and US having the equal flow rate, the magnitude of blowing and suction is set at 0.14% or 0.26% of the free-stream velocity. The locations of blowing/suction and flow conditions are the same as those in the cases with either UB or US only. The simulationresult suggests that the lift-to-drag ratio is improved by the combined control of UB on the lower surface and US on the upper surface. In particular, the lift-to-drag ratio is most improved by a combination of UB on the lower rear surface and US on the upper rear surface. In contrast, a combined control of UB on the upper front surface and US on the lower rear surface is identified as the most eff ec ive case for the friction drag reduction only.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"15 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1299/jfst.2020jfst0008","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304788","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.2020jfst0020
Sangwon Kim, N. Oshima, Y. Murai, H. Park
To address the increasing concern regarding global warming, the International Maritime Organization (IMO) regulated the requirements regarding greenhouse gas emissions from ships. To reduce these emissions, air lubrication systems are being used as this is an energy-saving technology developed to adhere to industry requirements (ABS, 2019). Air lubrication systems can be divided into two main categories based on the size of the bubbles. One category is the microbubble method developed by McCormick and Bhattacharyya (1973), and the other is the air film method, which has been found to be practically viable in the last two decades (e.g., Fukuda et al., 1999, 2000). Meanwhile, intermediate-sized bubbles have recently attracted significant research interest, as bubble deformation plays a key role in the process of drag reduction, as studied by Moriguchi and Kato (2002) and Kitagawa et al. (2005). The intermediate-sized bubbles negatively contribute to the drag reduction performance in the downstream region of the microbubble and air film methods. However, Murai et al. (2006) investigated the drag reduction mechanism for these bubbles (10–50 mm) and discovered that a calm region is generated behind the bubble. This feature is quite different from those of the microbubble and air film methods. This previous research has proven to be a turning point in studies related to intermediate-sized bubbles and is being considered as a new technique for improving drag reduction. An additional advantage is that supplying intermediatesized bubbles is much easier than generating a high flow rate of microbubbles or stabilizing an air film over a wide area. Murai et al. (2007) and Oishi and Murai (2014) rigorously investigated and described influential characteristics such as the velocity gradient and u'v' contours related to the drag reduction of a single intermediate-sized bubble. In numerical studies, Lu et al. (2005), Sugiyama et al. (2005), Kawamura (2005), and Spandan et al. (2017) also confirmed the positive *Department of Mechanical and Space Engineering, Hokkaido university Kita-13, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan E-mail: swkim5834@eis.hokudai.ac.jp **Division of Energy and Environmental Systems, Hokkaido university Kita-13, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
为了应对日益增长的全球变暖问题，国际海事组织(IMO)对船舶温室气体排放的要求进行了规范。为了减少这些排放，正在使用空气润滑系统，因为这是一种符合行业要求的节能技术(ABS, 2019)。根据气泡的大小，空气润滑系统可分为两大类。一类是McCormick和Bhattacharyya(1973)开发的微泡法，另一类是空气膜法，在过去二十年中被发现实际上是可行的(例如，Fukuda et al.， 1999, 2000)。与此同时，中等大小的气泡最近引起了很大的研究兴趣，因为正如Moriguchi和Kato(2002)以及Kitagawa等人(2005)所研究的那样，气泡变形在减阻过程中起着关键作用。在微气泡和气膜方法的下游区域，中等大小的气泡对减阻性能有负贡献。然而，Murai et al.(2006)研究了这些气泡(10-50 mm)的减阻机制，发现气泡后面产生了一个平静区域。这一特点与微泡法和气膜法有很大的不同。这项先前的研究已被证明是中型气泡研究的一个转折点，并被认为是一种改善减阻的新技术。另一个优点是，提供中等大小的气泡比产生高流速的微气泡或在大范围内稳定空气膜要容易得多。Murai等人(2007)和Oishi和Murai(2014)严格研究并描述了与单个中等大小气泡减阻相关的速度梯度和u'v'等高线等影响特性。陆在数值研究中,et al。(2005),Sugiyama et al。(2005),河村建夫(2005),和Spandan et al。(2017)也证实了积极*机械,航天工程,北海道大学Kita-13 Nishi-8, Kita-ku,札幌北海道060 - 8628,日本电子邮件:swkim5834@eis.hokudai.ac.jp * *部门的能源和环境系统中,北海道大学Kita-13 Nishi-8, Kita-ku,札幌北海道060 - 8628,日本
{"title":"Numerical investigation of a single intermediate-sized bubble in horizontal turbulent channel flow","authors":"Sangwon Kim, N. Oshima, Y. Murai, H. Park","doi":"10.1299/jfst.2020jfst0020","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0020","url":null,"abstract":"To address the increasing concern regarding global warming, the International Maritime Organization (IMO) regulated the requirements regarding greenhouse gas emissions from ships. To reduce these emissions, air lubrication systems are being used as this is an energy-saving technology developed to adhere to industry requirements (ABS, 2019). Air lubrication systems can be divided into two main categories based on the size of the bubbles. One category is the microbubble method developed by McCormick and Bhattacharyya (1973), and the other is the air film method, which has been found to be practically viable in the last two decades (e.g., Fukuda et al., 1999, 2000). Meanwhile, intermediate-sized bubbles have recently attracted significant research interest, as bubble deformation plays a key role in the process of drag reduction, as studied by Moriguchi and Kato (2002) and Kitagawa et al. (2005). The intermediate-sized bubbles negatively contribute to the drag reduction performance in the downstream region of the microbubble and air film methods. However, Murai et al. (2006) investigated the drag reduction mechanism for these bubbles (10–50 mm) and discovered that a calm region is generated behind the bubble. This feature is quite different from those of the microbubble and air film methods. This previous research has proven to be a turning point in studies related to intermediate-sized bubbles and is being considered as a new technique for improving drag reduction. An additional advantage is that supplying intermediatesized bubbles is much easier than generating a high flow rate of microbubbles or stabilizing an air film over a wide area. Murai et al. (2007) and Oishi and Murai (2014) rigorously investigated and described influential characteristics such as the velocity gradient and u'v' contours related to the drag reduction of a single intermediate-sized bubble. In numerical studies, Lu et al. (2005), Sugiyama et al. (2005), Kawamura (2005), and Spandan et al. (2017) also confirmed the positive *Department of Mechanical and Space Engineering, Hokkaido university Kita-13, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan E-mail: swkim5834@eis.hokudai.ac.jp **Division of Energy and Environmental Systems, Hokkaido university Kita-13, Nishi-8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"15 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305041","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.2020jfst0001
Kota Tomiuka, Y. Nishio, S. Izawa, Y. Fukunishi
In this study, the receptivity of a flat-plate boundary layer was studied by introducing a thin sheet-type disturbance. An airfoil-shaped device was used to generate a thin disturbance without velocity deficit in which a steady jet was ejected from its trailing edge to the downstream. Despite the absence of strong disturbances in the freestream outside the boundary layer, streaky structures similar to an ordinary bypass transition were generated. They meandered slowly in the spanwise direction where their downstream parts were oscillating in a delayed fashion. Turbulent spots were formed in the further downstream region. Consequently, the energy growth of the low frequency band in the velocity fluctuation spectrum was found to originate from this meandering motion of the streaks, whereas the growth of the middle- and high-frequency bands was attributed to the appearances of the turbulent spots.
{"title":"Bypass transition in a boundary layer subject to sheet-type freestream disturbance","authors":"Kota Tomiuka, Y. Nishio, S. Izawa, Y. Fukunishi","doi":"10.1299/jfst.2020jfst0001","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0001","url":null,"abstract":"In this study, the receptivity of a flat-plate boundary layer was studied by introducing a thin sheet-type disturbance. An airfoil-shaped device was used to generate a thin disturbance without velocity deficit in which a steady jet was ejected from its trailing edge to the downstream. Despite the absence of strong disturbances in the freestream outside the boundary layer, streaky structures similar to an ordinary bypass transition were generated. They meandered slowly in the spanwise direction where their downstream parts were oscillating in a delayed fashion. Turbulent spots were formed in the further downstream region. Consequently, the energy growth of the low frequency band in the velocity fluctuation spectrum was found to originate from this meandering motion of the streaks, whereas the growth of the middle- and high-frequency bands was attributed to the appearances of the turbulent spots.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"1 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1299/jfst.2020jfst0001","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304374","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.2020jfst0002
K. Miyaji, Takumi Inoue
Stochastic flow simulation methods based on the polynomial chaos expansion (PCE) are developed and verified to quantify the propagation of a geometric uncertainty of a quasi-one dimensional flow in a supersonic wind tunnel. The effect of uncertainty in the area of diffuser throat, i.e. second throat, on the wind tunnel starting problem is focused on, where a slight change in the area can cause a large jump of the shock wave resulting in a breakdown of the supersonic test conditions. Two major numerical techniques in our intrusive PCE are the multi-wavelet (MW) basis and the multi-element (ME) PCE, in order to properly deal with discontinuous responses of output variables, which are caused by the shock wave and its jump at started/unstarted mode change. Single-element spectral PCE using Legendre basis and the Haar-wavelet are also included as special cases of the MW, and the methods are all compared with Monte-Carlo Simulations (MCS) executed by the deterministic code. Response surfaces of the pressure by the employed PCEs qualitatively agree with the result of MCS except the spectral PCE. Furthermore, from quantitative evaluations by the probability density function (PDF) of the output on a rather complicated response surface with several discontinuities, the ME-PCE best agrees with the MCS at much lower computation costs.
{"title":"Evaluation of discontinuity treatment in intrusive polynomial chaos for uncertainty quantification of a nozzle flow in CFD","authors":"K. Miyaji, Takumi Inoue","doi":"10.1299/jfst.2020jfst0002","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0002","url":null,"abstract":"Stochastic flow simulation methods based on the polynomial chaos expansion (PCE) are developed and verified to quantify the propagation of a geometric uncertainty of a quasi-one dimensional flow in a supersonic wind tunnel. The effect of uncertainty in the area of diffuser throat, i.e. second throat, on the wind tunnel starting problem is focused on, where a slight change in the area can cause a large jump of the shock wave resulting in a breakdown of the supersonic test conditions. Two major numerical techniques in our intrusive PCE are the multi-wavelet (MW) basis and the multi-element (ME) PCE, in order to properly deal with discontinuous responses of output variables, which are caused by the shock wave and its jump at started/unstarted mode change. Single-element spectral PCE using Legendre basis and the Haar-wavelet are also included as special cases of the MW, and the methods are all compared with Monte-Carlo Simulations (MCS) executed by the deterministic code. Response surfaces of the pressure by the employed PCEs qualitatively agree with the result of MCS except the spectral PCE. Furthermore, from quantitative evaluations by the probability density function (PDF) of the output on a rather complicated response surface with several discontinuities, the ME-PCE best agrees with the MCS at much lower computation costs.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"1 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1299/jfst.2020jfst0002","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304395","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.2020jfst0006
T. Kamiya, M. Asahara, T. Miyasaka
not been studied thoroughly. Additionally, this study examines the effect of the wake on the liquid column deformation. Although the drag coefficient of a solid sphere in gas flow is relatively simple and has been widely studied for a long time, it cannot be directly applied as the drag coefficient of a liquid drop in gas flow because of the deformation and disintegration of liquid drops and the internal flow effect. Hence, the drag coefficients of deforming liquid drops in gas flow have been measured. Hsiang and Faeth (1995) observed liquid drop deformation and breakup for shock wave-initiated disturbances in air. They obtained the drag coefficient from the trajectory of the center of mass for a liquid drop at a Reynolds number of 1000–2000. For this range, the increasing effect of the drag coefficient derived from deformation Abstract The objective of this study is to clarify the effect of a wake on liquid column drag and deformation for a high-Weber-number ( We ) flow. A simulation was performed for a liquid column with a high We behind a shock wave, using a ghost fluid method as a two-phase flow solver. The simulated We were 500, 1000, 2000, 3000, and 4000. Relatively large oscillation of the drag coefficients was observed for We = 500 and 1000. This feature of the drag coefficients was possibly caused by varying pressure on the downstream interface. The pressure variation is derived from the wake. In addition, it was suggested that such varying pressures could contribute to the flattening of the liquid column.
{"title":"Effect of a wake on drag and deformation of liquid column at high Weber numbers","authors":"T. Kamiya, M. Asahara, T. Miyasaka","doi":"10.1299/jfst.2020jfst0006","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0006","url":null,"abstract":"not been studied thoroughly. Additionally, this study examines the effect of the wake on the liquid column deformation. Although the drag coefficient of a solid sphere in gas flow is relatively simple and has been widely studied for a long time, it cannot be directly applied as the drag coefficient of a liquid drop in gas flow because of the deformation and disintegration of liquid drops and the internal flow effect. Hence, the drag coefficients of deforming liquid drops in gas flow have been measured. Hsiang and Faeth (1995) observed liquid drop deformation and breakup for shock wave-initiated disturbances in air. They obtained the drag coefficient from the trajectory of the center of mass for a liquid drop at a Reynolds number of 1000–2000. For this range, the increasing effect of the drag coefficient derived from deformation Abstract The objective of this study is to clarify the effect of a wake on liquid column drag and deformation for a high-Weber-number ( We ) flow. A simulation was performed for a liquid column with a high We behind a shock wave, using a ghost fluid method as a two-phase flow solver. The simulated We were 500, 1000, 2000, 3000, and 4000. Relatively large oscillation of the drag coefficients was observed for We = 500 and 1000. This feature of the drag coefficients was possibly caused by varying pressure on the downstream interface. The pressure variation is derived from the wake. In addition, it was suggested that such varying pressures could contribute to the flattening of the liquid column.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"1 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1299/jfst.2020jfst0006","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304547","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.2020jfst0019
K. Takamure, T. Degawa, R. Kano, T. Uchiyama
The characteristics of a fine-bubble plume passing through two tandem cylinders are investigated. Fine-bubbles with a mean diameter of 0.055 mm are released by water electrolysis from electrodes placed at the bottom of a tank. They induce an upward water flow around them with rising due to buoyancy. Orthogonally to the axis of such fine–bubble plume, two cylinders with a diameter D of 30 mm are arranged in tandem. The distance between the cylinders, L, ranges between 1.5D and 3D. This study visualizes the bubbles and water flow around the cylinders. It also measures the bubble velocity distribution. The experiments reveal the water and bubble shear layers originating at the sides of the lower cylinder and make clear their behavior around the upper cylinder. This study elucidates the effects of L on the characteristics of the fine-bubble plume, such as the stagnant bubbly flow and the bubbly wake around the cylinders.
{"title":"Effect of cylinders on the characteristics of a fine-bubble plume","authors":"K. Takamure, T. Degawa, R. Kano, T. Uchiyama","doi":"10.1299/jfst.2020jfst0019","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0019","url":null,"abstract":"The characteristics of a fine-bubble plume passing through two tandem cylinders are investigated. Fine-bubbles with a mean diameter of 0.055 mm are released by water electrolysis from electrodes placed at the bottom of a tank. They induce an upward water flow around them with rising due to buoyancy. Orthogonally to the axis of such fine–bubble plume, two cylinders with a diameter D of 30 mm are arranged in tandem. The distance between the cylinders, L, ranges between 1.5D and 3D. This study visualizes the bubbles and water flow around the cylinders. It also measures the bubble velocity distribution. The experiments reveal the water and bubble shear layers originating at the sides of the lower cylinder and make clear their behavior around the upper cylinder. This study elucidates the effects of L on the characteristics of the fine-bubble plume, such as the stagnant bubbly flow and the bubbly wake around the cylinders.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"15 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66304995","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.2020jfst0021
Yoshiki Mizuno, S. Koshizuka
This paper presents the development of a statistical emulator to estimate free-surface heights with less computational time than a particle method. Particle methods can simulate free-surface flow problems by solving NavierStokes and continuity equations, but they require more computational time as the number of particles becomes greater in computational domains. Accordingly, it is not pragmatic to conduct statistical analysis of free-surface problems with respect to a variety of initial conditions by particle methods. In the place of the simulation methods, statistical emulators can estimate predictive values in these problems with less computational time. In this study, we apply a Gaussian process for designing a statistical emulator of the Explicit Moving Particle Simulation (EMPS) method and predict free-surface heights in dam break problems. Once it is developed based on a dataset made from only one simulation run of a dam break problem, the Gaussian process emulator is able to approximate these heights in other dam break problems. By measuring the coefficient of determination, root mean squared error, and mean absolute error, we evaluate the accuracy of emulated free-surface heights in dam break problems where the shapes of water columns are distinct from the original shape at the initial condition. We alter the initial lengths in the x-direction and the initial heights in the z-direction remaining the same initial width in the y-direction. Consequently, in terms of the computational speed and the accuracy, it is demonstrated that we can adopt the Gaussian process emulator as a replacement of the EMPS simulator especially when free-surface flow analysis is repeatedly conducted with different initial conditions.
{"title":"Gaussian process emulation of particle method for estimating free-surface heights","authors":"Yoshiki Mizuno, S. Koshizuka","doi":"10.1299/jfst.2020jfst0021","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0021","url":null,"abstract":"This paper presents the development of a statistical emulator to estimate free-surface heights with less computational time than a particle method. Particle methods can simulate free-surface flow problems by solving NavierStokes and continuity equations, but they require more computational time as the number of particles becomes greater in computational domains. Accordingly, it is not pragmatic to conduct statistical analysis of free-surface problems with respect to a variety of initial conditions by particle methods. In the place of the simulation methods, statistical emulators can estimate predictive values in these problems with less computational time. In this study, we apply a Gaussian process for designing a statistical emulator of the Explicit Moving Particle Simulation (EMPS) method and predict free-surface heights in dam break problems. Once it is developed based on a dataset made from only one simulation run of a dam break problem, the Gaussian process emulator is able to approximate these heights in other dam break problems. By measuring the coefficient of determination, root mean squared error, and mean absolute error, we evaluate the accuracy of emulated free-surface heights in dam break problems where the shapes of water columns are distinct from the original shape at the initial condition. We alter the initial lengths in the x-direction and the initial heights in the z-direction remaining the same initial width in the y-direction. Consequently, in terms of the computational speed and the accuracy, it is demonstrated that we can adopt the Gaussian process emulator as a replacement of the EMPS simulator especially when free-surface flow analysis is repeatedly conducted with different initial conditions.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"15 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305141","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.2020jfst0016
Y. Naka, Akira Kagami
The aerodynamic performance of the multi-rotor drone under the wall proximity has been investigated by experiment and numerical simulation. The propeller airflow along with the wall deflects toward the wall due to the Coanda effect, and it yields a negative impact on the aerodynamic performance. The present study aims to reveal the link between the propeller thrust and the propeller airflow under different wall proximity conditions. The deflection of the flow is confirmed by the flow visualization, and the wall pressure exhibits the signature of flow attachment both in the profiles of the mean and the fluctuation. The force measurement indicates that the degradation of the thrust is significant enough to affect the stability of the drone body. A possible reason of the decrease in thrust is found in the streamwise velocity distribution. The velocity distributions obtained by the numerical simulation indicate that the swirling motion is significantly suppressed due to the wall proximity effect. Moreover, the pressure distribution on the propeller surface explains the decrease of the thrust. The magnitude of the pressure difference becomes smaller when the propeller blade approaches very close to the wall.
{"title":"Coanda effect of a propeller airflow and its aerodynamic impact on the thrust","authors":"Y. Naka, Akira Kagami","doi":"10.1299/jfst.2020jfst0016","DOIUrl":"https://doi.org/10.1299/jfst.2020jfst0016","url":null,"abstract":"The aerodynamic performance of the multi-rotor drone under the wall proximity has been investigated by experiment and numerical simulation. The propeller airflow along with the wall deflects toward the wall due to the Coanda effect, and it yields a negative impact on the aerodynamic performance. The present study aims to reveal the link between the propeller thrust and the propeller airflow under different wall proximity conditions. The deflection of the flow is confirmed by the flow visualization, and the wall pressure exhibits the signature of flow attachment both in the profiles of the mean and the fluctuation. The force measurement indicates that the degradation of the thrust is significant enough to affect the stability of the drone body. A possible reason of the decrease in thrust is found in the streamwise velocity distribution. The velocity distributions obtained by the numerical simulation indicate that the swirling motion is significantly suppressed due to the wall proximity effect. Moreover, the pressure distribution on the propeller surface explains the decrease of the thrust. The magnitude of the pressure difference becomes smaller when the propeller blade approaches very close to the wall.","PeriodicalId":44704,"journal":{"name":"Journal of Fluid Science and Technology","volume":"15 1","pages":""},"PeriodicalIF":0.8,"publicationDate":"2020-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"66305304","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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