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Y. T. Chew - One of the best experts on this subject based on the ideXlab platform.

  • Near-wall hot-wire measurements Part II: Turbulence time scale, Convective Velocity and spectra in the viscous sublayer
    Experiments in Fluids, 2001
    Co-Authors: Boo Cheong Khoo, Y. T. Chew, Chiang Juay Teo
    Abstract:

    This work continues our previous studies, where experiments were performed in turbulent-channel and flat-plate boundary-layer flows using near-wall hot-wire probes. The probability density function (pdf) of the wall-shear stress and streamwise Velocity fluctuations in the viscous sublayer, buffer region and beyond were compared and analyzed. The Convective Velocity U c of the streamwise Velocity fluctuations in the very near-wall region was obtained using a two-point correlation technique. It was found that in the viscous sublayer, U c is approximately constant at 13u τ and 15u τ , respectively, for the channel and boundary-layer flows. Spectra data for the viscous sublayer are presented for the first time, and the normalized spectral plots for different flow conditions collapse at high frequencies or wavenumbers, thus indicating the possible presence of small-scale universality at different Reynolds numbers. The integral time scale corresponding to the streamwise Velocity fluctuations in the viscous sublayer is also presented

  • the dynamic response of a hot wire anemometer iv sine wave voltage perturbation testing for near wall hot wire film probes and the presence of low high frequency response characteristics
    Measurement Science and Technology, 2001
    Co-Authors: Boo Cheong Khoo, Y. T. Chew
    Abstract:

    Experiments were performed using the electronic sine-wave voltage-perturbation test to systematically study the frequency responses of near-wall hot-wire probes subjected in turn to varying magnitudes of Convective Velocity and different effects of wall influence. In addition, quartz-substrate hot-film gauges with various thicknesses of quartz coating were also investigated. Results of the high cut-off frequency obtained using the sine-wave test (fsine) were found to be in fair agreement with those obtained using the square-wave test (fS) both for hot-wire and for hot-film sensors. The sine-wave test response curve exhibited a distinct bulging effect for the hot-film gauges. For the hot-wire sensors, a much weaker bulging effect was also observed. In contrast to fS and fsine, the low frequency response characteristic corresponding to the location of the bulging effect (fbulge) compared much more favourably with the dynamic frequency response (fD) obtained by Khoo et al and Chew et al using a known near-wall fluctuating flow field. Freymuth's theory for non-cylindrical hot-film sensors incorporating the Bellhouse-Schultz model was applied to predict the responses of the hot-film wall gauges when they were subjected to electronic sine-wave testing and dynamic perturbation testing under different parametric conditions. Although it is one-dimensional in nature, the model is capable of predicting most of the trends observed in the present study and previous works by Khoo et al (1998a) and Chew et?al (1998a).

  • the dynamic response of a hot wire anemometer iii voltage perturbation versus Velocity perturbation testing for near wall hot wire film probes
    Measurement Science and Technology, 1999
    Co-Authors: Boo Cheong Khoo, Y. T. Chew
    Abstract:

    Experiments were performed for the first time using the electronic square-wave voltage-perturbation test to systematically quantify the frequency response of near-wall hot-wire probes subjected in turn to varying magnitudes of Convective Velocity, different substrate materials and changes in wall-substrate temperature. In addition, quartz-substrate hot-film gauges with various thicknesses of quartz coating were also tested. Results of were compared against the dynamic frequency response previously obtained in parts I and II using a known near-wall fluctuating flow field. Although the observed trends for and were similar, their magnitudes were vastly different, notably for the commercially available hot-film gauges, or which was up to five orders of magnitude greater than . This signifies that there are possibly inherent differences between square-wave voltage-perturbation and Velocity-perturbation tests for quantifying the frequency response of a hot-wire/hot-film system. These differences are then analysed in relation to the equation of a CTA unit put forth by Freymuth.

  • dynamic response of a hot wire anemometer part i a marginally elevated hot wire probe for near wall Velocity measurements
    Measurement Science and Technology, 1998
    Co-Authors: Boo Cheong Khoo, Y. T. Chew
    Abstract:

    Experiments were carried out to generate a known near-wall fluctuating flow field for the purpose of quantifying the dynamic response of a marginally elevated hot-wire probe. It is found that the dynamic response of the hot wire is dependent on both the wire's height (h) above the wall substrate and the Convective Velocity at the wire's location. Both larger values of and smaller values of h improve the dynamic response of a near-wall hot wire. Further experiments were also performed to investigate the effect of the thermal conductivity of the wall substrate on the response characteristics of the hot wire. A thermally more conducting material for the wall yields a better response hot-wire probe for near-wall Velocity measurements.

Yuto Bekki - One of the best experts on this subject based on the ideXlab platform.

  • consequences of high effective prandtl number on solar differential rotation and Convective Velocity
    Physics of Fluids, 2018
    Co-Authors: Bidya Binay Karak, Mark S Miesch, Yuto Bekki
    Abstract:

    Observations suggest that the large-scale Convective velocities obtained by solar convection simulations might be over-estimated (Convective conundrum). One plausible solution to this could be the small-scale dynamo which cannot be fully resolved by global simulations. The small-scale Lorentz force suppresses the Convective motions and also the turbulent mixing of entropy between upflows and downflows, leading to a large effective Prandtl number (Pr). We explore this idea in three-dimensional global rotating convection simulations at different thermal conductivity (κ), i.e., at different Pr. In agreement with previous non-rotating simulations, the Convective Velocity is reduced with the increase of Pr as long as the thermal conductive flux is negligible. A subadiabatic layer is formed near the base of the convection zone due to continuous deposition of low entropy plumes in low-κ simulations. The most interesting result of our low-κ simulations is that the Convective motions are accompanied by a change in the convection structure that is increasingly influenced by small-scale plumes. These plumes tend to transport angular momentum radially inward and thus establish an anti-solar differential rotation, in striking contrast to the solar rotation profile. If such low diffusive plumes, driven by the radiative-surface cooling, are present in the Sun, then our results cast doubt on the idea that a high effective Pr may be a viable solution to the solar Convective conundrum. Our study also emphasizes that any resolution of the conundrum that relies on the downward plumes must take into account the angular momentum transport and heat transport.Observations suggest that the large-scale Convective velocities obtained by solar convection simulations might be over-estimated (Convective conundrum). One plausible solution to this could be the small-scale dynamo which cannot be fully resolved by global simulations. The small-scale Lorentz force suppresses the Convective motions and also the turbulent mixing of entropy between upflows and downflows, leading to a large effective Prandtl number (Pr). We explore this idea in three-dimensional global rotating convection simulations at different thermal conductivity (κ), i.e., at different Pr. In agreement with previous non-rotating simulations, the Convective Velocity is reduced with the increase of Pr as long as the thermal conductive flux is negligible. A subadiabatic layer is formed near the base of the convection zone due to continuous deposition of low entropy plumes in low-κ simulations. The most interesting result of our low-κ simulations is that the Convective motions are accompanied by a change in...

  • consequences of high effective prandtl number on solar differential rotation and Convective Velocity
    Physics of Fluids, 2018
    Co-Authors: Bidya Binay Karak, Mark S Miesch, Yuto Bekki
    Abstract:

    Observations suggest that the large-scale Convective velocities obtained by solar convection simulations might be over-estimated (Convective conundrum). One plausible solution to this could be the small-scale dynamo which cannot be fully resolved by global simulations. The small-scale Lorentz force suppresses the Convective motions and also the turbulent mixing of entropy between upflows and downflows, leading to a large effective Prandtl number (Pr). We explore this idea in three-dimensional global rotating convection simulations at different thermal conductivity (κ), i.e., at different Pr. In agreement with previous non-rotating simulations, the Convective Velocity is reduced with the increase of Pr as long as the thermal conductive flux is negligible. A subadiabatic layer is formed near the base of the convection zone due to continuous deposition of low entropy plumes in low-κ simulations. The most interesting result of our low-κ simulations is that the Convective motions are accompanied by a change in the convection structure that is increasingly influenced by small-scale plumes. These plumes tend to transport angular momentum radially inward and thus establish an anti-solar differential rotation, in striking contrast to the solar rotation profile. If such low diffusive plumes, driven by the radiative-surface cooling, are present in the Sun, then our results cast doubt on the idea that a high effective Pr may be a viable solution to the solar Convective conundrum. Our study also emphasizes that any resolution of the conundrum that relies on the downward plumes must take into account the angular momentum transport and heat transport.

  • Convective Velocity suppression via the enhancement of the subadiabatic layer role of the effective prandtl number
    The Astrophysical Journal, 2017
    Co-Authors: Yuto Bekki, H Hotta, Takaaki Yokoyama
    Abstract:

    It has recently been recognized that the Convective velocities achieved in current solar convection simulations might be overestimated. The newly revealed effects of the prevailing small-scale magnetic field within the convection zone may offer possible solutions to this problem. The small-scale magnetic fields can reduce the Convective amplitude of small-scale motions through the Lorentz-force feedback, which concurrently inhibits the turbulent mixing of entropy between upflows and downflows. As a result, the effective Prandtl number may exceed unity inside the solar convection zone. In this paper, we propose and numerically confirm a possible suppression mechanism of Convective Velocity in the effectively high-Prandtl number regime. If the effective horizontal thermal diffusivity decreases (the Prandtl number accordingly increases), the subadiabatic layer which is formed near the base of the convection zone by continuous depositions of low entropy transported by adiabatically downflowing plumes is enhanced and extended. The global Convective amplitude in the high-Prandtl thermal convection is thus reduced, especially in the lower part of the convection zone via the change in the mean entropy profile, which becomes more subadiabatic near the base and less superadiabatic in the bulk.

Bidya Binay Karak - One of the best experts on this subject based on the ideXlab platform.

  • consequences of high effective prandtl number on solar differential rotation and Convective Velocity
    Physics of Fluids, 2018
    Co-Authors: Bidya Binay Karak, Mark S Miesch, Yuto Bekki
    Abstract:

    Observations suggest that the large-scale Convective velocities obtained by solar convection simulations might be over-estimated (Convective conundrum). One plausible solution to this could be the small-scale dynamo which cannot be fully resolved by global simulations. The small-scale Lorentz force suppresses the Convective motions and also the turbulent mixing of entropy between upflows and downflows, leading to a large effective Prandtl number (Pr). We explore this idea in three-dimensional global rotating convection simulations at different thermal conductivity (κ), i.e., at different Pr. In agreement with previous non-rotating simulations, the Convective Velocity is reduced with the increase of Pr as long as the thermal conductive flux is negligible. A subadiabatic layer is formed near the base of the convection zone due to continuous deposition of low entropy plumes in low-κ simulations. The most interesting result of our low-κ simulations is that the Convective motions are accompanied by a change in the convection structure that is increasingly influenced by small-scale plumes. These plumes tend to transport angular momentum radially inward and thus establish an anti-solar differential rotation, in striking contrast to the solar rotation profile. If such low diffusive plumes, driven by the radiative-surface cooling, are present in the Sun, then our results cast doubt on the idea that a high effective Pr may be a viable solution to the solar Convective conundrum. Our study also emphasizes that any resolution of the conundrum that relies on the downward plumes must take into account the angular momentum transport and heat transport.Observations suggest that the large-scale Convective velocities obtained by solar convection simulations might be over-estimated (Convective conundrum). One plausible solution to this could be the small-scale dynamo which cannot be fully resolved by global simulations. The small-scale Lorentz force suppresses the Convective motions and also the turbulent mixing of entropy between upflows and downflows, leading to a large effective Prandtl number (Pr). We explore this idea in three-dimensional global rotating convection simulations at different thermal conductivity (κ), i.e., at different Pr. In agreement with previous non-rotating simulations, the Convective Velocity is reduced with the increase of Pr as long as the thermal conductive flux is negligible. A subadiabatic layer is formed near the base of the convection zone due to continuous deposition of low entropy plumes in low-κ simulations. The most interesting result of our low-κ simulations is that the Convective motions are accompanied by a change in...

  • consequences of high effective prandtl number on solar differential rotation and Convective Velocity
    Physics of Fluids, 2018
    Co-Authors: Bidya Binay Karak, Mark S Miesch, Yuto Bekki
    Abstract:

    Observations suggest that the large-scale Convective velocities obtained by solar convection simulations might be over-estimated (Convective conundrum). One plausible solution to this could be the small-scale dynamo which cannot be fully resolved by global simulations. The small-scale Lorentz force suppresses the Convective motions and also the turbulent mixing of entropy between upflows and downflows, leading to a large effective Prandtl number (Pr). We explore this idea in three-dimensional global rotating convection simulations at different thermal conductivity (κ), i.e., at different Pr. In agreement with previous non-rotating simulations, the Convective Velocity is reduced with the increase of Pr as long as the thermal conductive flux is negligible. A subadiabatic layer is formed near the base of the convection zone due to continuous deposition of low entropy plumes in low-κ simulations. The most interesting result of our low-κ simulations is that the Convective motions are accompanied by a change in the convection structure that is increasingly influenced by small-scale plumes. These plumes tend to transport angular momentum radially inward and thus establish an anti-solar differential rotation, in striking contrast to the solar rotation profile. If such low diffusive plumes, driven by the radiative-surface cooling, are present in the Sun, then our results cast doubt on the idea that a high effective Pr may be a viable solution to the solar Convective conundrum. Our study also emphasizes that any resolution of the conundrum that relies on the downward plumes must take into account the angular momentum transport and heat transport.

Boo Cheong Khoo - One of the best experts on this subject based on the ideXlab platform.

  • Near-wall hot-wire measurements Part II: Turbulence time scale, Convective Velocity and spectra in the viscous sublayer
    Experiments in Fluids, 2001
    Co-Authors: Boo Cheong Khoo, Y. T. Chew, Chiang Juay Teo
    Abstract:

    This work continues our previous studies, where experiments were performed in turbulent-channel and flat-plate boundary-layer flows using near-wall hot-wire probes. The probability density function (pdf) of the wall-shear stress and streamwise Velocity fluctuations in the viscous sublayer, buffer region and beyond were compared and analyzed. The Convective Velocity U c of the streamwise Velocity fluctuations in the very near-wall region was obtained using a two-point correlation technique. It was found that in the viscous sublayer, U c is approximately constant at 13u τ and 15u τ , respectively, for the channel and boundary-layer flows. Spectra data for the viscous sublayer are presented for the first time, and the normalized spectral plots for different flow conditions collapse at high frequencies or wavenumbers, thus indicating the possible presence of small-scale universality at different Reynolds numbers. The integral time scale corresponding to the streamwise Velocity fluctuations in the viscous sublayer is also presented

  • the dynamic response of a hot wire anemometer iv sine wave voltage perturbation testing for near wall hot wire film probes and the presence of low high frequency response characteristics
    Measurement Science and Technology, 2001
    Co-Authors: Boo Cheong Khoo, Y. T. Chew
    Abstract:

    Experiments were performed using the electronic sine-wave voltage-perturbation test to systematically study the frequency responses of near-wall hot-wire probes subjected in turn to varying magnitudes of Convective Velocity and different effects of wall influence. In addition, quartz-substrate hot-film gauges with various thicknesses of quartz coating were also investigated. Results of the high cut-off frequency obtained using the sine-wave test (fsine) were found to be in fair agreement with those obtained using the square-wave test (fS) both for hot-wire and for hot-film sensors. The sine-wave test response curve exhibited a distinct bulging effect for the hot-film gauges. For the hot-wire sensors, a much weaker bulging effect was also observed. In contrast to fS and fsine, the low frequency response characteristic corresponding to the location of the bulging effect (fbulge) compared much more favourably with the dynamic frequency response (fD) obtained by Khoo et al and Chew et al using a known near-wall fluctuating flow field. Freymuth's theory for non-cylindrical hot-film sensors incorporating the Bellhouse-Schultz model was applied to predict the responses of the hot-film wall gauges when they were subjected to electronic sine-wave testing and dynamic perturbation testing under different parametric conditions. Although it is one-dimensional in nature, the model is capable of predicting most of the trends observed in the present study and previous works by Khoo et al (1998a) and Chew et?al (1998a).

  • the dynamic response of a hot wire anemometer iii voltage perturbation versus Velocity perturbation testing for near wall hot wire film probes
    Measurement Science and Technology, 1999
    Co-Authors: Boo Cheong Khoo, Y. T. Chew
    Abstract:

    Experiments were performed for the first time using the electronic square-wave voltage-perturbation test to systematically quantify the frequency response of near-wall hot-wire probes subjected in turn to varying magnitudes of Convective Velocity, different substrate materials and changes in wall-substrate temperature. In addition, quartz-substrate hot-film gauges with various thicknesses of quartz coating were also tested. Results of were compared against the dynamic frequency response previously obtained in parts I and II using a known near-wall fluctuating flow field. Although the observed trends for and were similar, their magnitudes were vastly different, notably for the commercially available hot-film gauges, or which was up to five orders of magnitude greater than . This signifies that there are possibly inherent differences between square-wave voltage-perturbation and Velocity-perturbation tests for quantifying the frequency response of a hot-wire/hot-film system. These differences are then analysed in relation to the equation of a CTA unit put forth by Freymuth.

  • dynamic response of a hot wire anemometer part i a marginally elevated hot wire probe for near wall Velocity measurements
    Measurement Science and Technology, 1998
    Co-Authors: Boo Cheong Khoo, Y. T. Chew
    Abstract:

    Experiments were carried out to generate a known near-wall fluctuating flow field for the purpose of quantifying the dynamic response of a marginally elevated hot-wire probe. It is found that the dynamic response of the hot wire is dependent on both the wire's height (h) above the wall substrate and the Convective Velocity at the wire's location. Both larger values of and smaller values of h improve the dynamic response of a near-wall hot wire. Further experiments were also performed to investigate the effect of the thermal conductivity of the wall substrate on the response characteristics of the hot wire. A thermally more conducting material for the wall yields a better response hot-wire probe for near-wall Velocity measurements.

Barbara Wohlmuth - One of the best experts on this subject based on the ideXlab platform.

  • on the unsteady darcy forchheimer brinkman equation in local and nonlocal tumor growth models
    Mathematical Models and Methods in Applied Sciences, 2019
    Co-Authors: Marvin Fritz, Ernesto A B F Lima, Tinsley J Oden, Barbara Wohlmuth
    Abstract:

    A mathematical analysis of local and nonlocal phase-field models of tumor growth is presented that includes time-dependent Darcy–Forchheimer–Brinkman models of Convective Velocity fields and models...

  • on the unsteady darcy forchheimer brinkman equation in local and nonlocal tumor growth models
    arXiv: Analysis of PDEs, 2018
    Co-Authors: Marvin Fritz, Ernesto A B F Lima, Tinsley J Oden, Barbara Wohlmuth
    Abstract:

    A mathematical analysis of local and nonlocal phase-field models of tumor growth is presented that includes time-dependent Darcy-Forchheimer-Brinkman models of Convective Velocity fields and models of long-range cell interactions. A complete existence analysis is provided. In addition, a parameter-sensitivity analysis is described that quantifies the sensitivity of key quantities of interest to changes in parameter values. Two sensitivity analyses are examined; one employing statistical variances of model outputs and another employing the notion of active subspaces based on existing observational data. Remarkably, the two approaches yield very similar conclusions on sensitivity for certain quantities of interest. The work concludes with the presentation of numerical approximations of solutions of the governing equations and results of numerical experiments on tumor growth produced using finite element discretizations of the full tumor model for representative cases.