The Experts below are selected from a list of 270 Experts worldwide ranked by ideXlab platform

Ulrich Rist - One of the best experts on this subject based on the ideXlab platform.

  • experiments on Critical Reynolds Number and global instability in roughness induced laminar turbulent transition
    Journal of Fluid Mechanics, 2018
    Co-Authors: Dominik K Puckert, Ulrich Rist
    Abstract:

    The effects of isolated, cylindrical roughness elements on laminar–turbulent transition in a flat-plate boundary layer are investigated in a laminar water channel. Our experiments aim at providing a comparison to global linear stability theory (LST) by means of hot-film anemometry and particle image velocimetry. Although the Critical Reynolds Number from theory does not match the transition Reynolds Number observed in experiments, there are distinct experimental observations indicating a changeover from purely convective to absolute/global instability very close to the Critical Reynolds Number predicted by theory. Forcing with a vibrating wire reveals the evolution of the system dynamics from an amplifier to a wavemaker when the Critical Reynolds Number is exceeded. The mode symmetry is varicose for thick roughness elements and a changeover from varicose to sinuous modes is observed at the Critical Reynolds Number for thin roughness elements. Therefore, most predictions by global LST can be confirmed, but additional observations in the physical flow demonstrate that not all features can be captured adequately by global LST.

  • Experiments on Critical Reynolds Number and global instability in roughness-induced laminar–turbulent transition
    Journal of Fluid Mechanics, 2018
    Co-Authors: Dominik K Puckert, Ulrich Rist
    Abstract:

    The effects of isolated, cylindrical roughness elements on laminar–turbulent transition in a flat-plate boundary layer are investigated in a laminar water channel. Our experiments aim at providing a comparison to global linear stability theory (LST) by means of hot-film anemometry and particle image velocimetry. Although the Critical Reynolds Number from theory does not match the transition Reynolds Number observed in experiments, there are distinct experimental observations indicating a changeover from purely convective to absolute/global instability very close to the Critical Reynolds Number predicted by theory. Forcing with a vibrating wire reveals the evolution of the system dynamics from an amplifier to a wavemaker when the Critical Reynolds Number is exceeded. The mode symmetry is varicose for thick roughness elements and a changeover from varicose to sinuous modes is observed at the Critical Reynolds Number for thin roughness elements. Therefore, most predictions by global LST can be confirmed, but additional observations in the physical flow demonstrate that not all features can be captured adequately by global LST.

Mu Wang - One of the best experts on this subject based on the ideXlab platform.

  • the effect of expansion ratio on the Critical Reynolds Number in single fracture flow with sudden expansion
    Hydrological Processes, 2016
    Co-Authors: Jiazhong Qian, Hongbin Zhan, Xiao Wang, Mu Wang
    Abstract:

    Flow in a single fracture (SF) is an important research subject in groundwater hydrology, hydraulic engineering, radioactive nuclear waste repository and geotechnical engineering. An abruptly changing aperture is a unique type of SF. This study discusses the relation between the values of the Critical Reynolds Number (Rec) for the onset of symmetry breaking of flow and the expansion ratio (E) of SF, which is defined as the ratio between the outlet (D) and inlet (d) apertures. This study also investigates the effect of inlet aperture d on Rec for flow in an SF with abruptly changing apertures (SF-ACA) using the finite volume method. Earlier numerical and experimental results showed that flow is symmetric in respect to the central plane of the SF-ACA at small Reynolds Number (Re) but becomes asymmetric when Re is sufficiently large. Our simulations show that the value of Rec decreases with the increasing E, and the relationship between the logarithm of Rec and E can be described accurately using either a quadratic polynomial function or a logarithmic function. However, the relationship of Rec and d for a given E value is vague, and Rec becomes even less sensitive to d when E increases. This study also reveals that the hydraulic gradient (J) and flow velocity (v) follow a super-linear relationship that can be fitted almost perfectly by the Forchheimer equation. The inertial component (Ji) of J increases monotonically with Re, whereas the viscous component (Jv) of J decreases monotonically with Re. The Re value corresponding to equal inertial and viscous components of J (named as the transitional point Re) decreases when E increases, and such a transitional point Re should be closely related to the Critical Reynolds Number Rec, although a rigorous theoretical proof is not yet available. Copyright © 2015 John Wiley & Sons, Ltd.

Yasuhiro Mitani - One of the best experts on this subject based on the ideXlab platform.

  • Critical Reynolds Number for nonlinear flow through rough‐walled fractures: The role of shear processes
    Water Resources Research, 2014
    Co-Authors: M Javadi, Mostafa Sharifzadeh, Kourosh Shahriar, Yasuhiro Mitani
    Abstract:

    This paper experimentally investigates the role of shear processes on the variation of Critical Reynolds Number and nonlinear flow through rough-walled rock fractures. A quantitative criterion was developed to quantify the onset of nonlinear flow by comprehensive combination of Forchheimer's law and Reynolds Number. At each shear displacement, several high-precision water flow tests were carried out with different hydraulic gradients then the Critical Reynolds Number was determined based on the developed criterion. The results show that (i) the Forchheimer's law was fitted very well to experimental results of nonlinear fluid flow through rough-walled fractures, (ii) the coefficients of viscous and inertial pressure drops experience 4 and 7 orders of magnitude reduction during shear displacement, respectively, and (iii) the Critical Reynolds Number varies from 0.001 to 25 and experiences 4 orders of magnitude enlargement by increasing shear displacement from 0 to 20 mm. These findings may prove useful in proper understanding of fluid flow through rock fractures, or inclusions in computational studies of large-scale nonlinear flow in fractured rocks.

  • Critical Reynolds Number for nonlinear flow through rough walled fractures the role of shear processes
    Water Resources Research, 2014
    Co-Authors: M Javadi, Mostafa Sharifzadeh, Kourosh Shahriar, Yasuhiro Mitani
    Abstract:

    This paper experimentally investigates the role of shear processes on the variation of Critical Reynolds Number and nonlinear flow through rough-walled rock fractures. A quantitative criterion was developed to quantify the onset of nonlinear flow by comprehensive combination of Forchheimer's law and Reynolds Number. At each shear displacement, several high-precision water flow tests were carried out with different hydraulic gradients then the Critical Reynolds Number was determined based on the developed criterion. The results show that (i) the Forchheimer's law was fitted very well to experimental results of nonlinear fluid flow through rough-walled fractures, (ii) the coefficients of viscous and inertial pressure drops experience 4 and 7 orders of magnitude reduction during shear displacement, respectively, and (iii) the Critical Reynolds Number varies from 0.001 to 25 and experiences 4 orders of magnitude enlargement by increasing shear displacement from 0 to 20 mm. These findings may prove useful in proper understanding of fluid flow through rock fractures, or inclusions in computational studies of large-scale nonlinear flow in fractured rocks.

Hongbin Zhan - One of the best experts on this subject based on the ideXlab platform.

  • the effect of expansion ratio on the Critical Reynolds Number in single fracture flow with sudden expansion
    Hydrological Processes, 2016
    Co-Authors: Jiazhong Qian, Hongbin Zhan, Xiao Wang, Mu Wang
    Abstract:

    Flow in a single fracture (SF) is an important research subject in groundwater hydrology, hydraulic engineering, radioactive nuclear waste repository and geotechnical engineering. An abruptly changing aperture is a unique type of SF. This study discusses the relation between the values of the Critical Reynolds Number (Rec) for the onset of symmetry breaking of flow and the expansion ratio (E) of SF, which is defined as the ratio between the outlet (D) and inlet (d) apertures. This study also investigates the effect of inlet aperture d on Rec for flow in an SF with abruptly changing apertures (SF-ACA) using the finite volume method. Earlier numerical and experimental results showed that flow is symmetric in respect to the central plane of the SF-ACA at small Reynolds Number (Re) but becomes asymmetric when Re is sufficiently large. Our simulations show that the value of Rec decreases with the increasing E, and the relationship between the logarithm of Rec and E can be described accurately using either a quadratic polynomial function or a logarithmic function. However, the relationship of Rec and d for a given E value is vague, and Rec becomes even less sensitive to d when E increases. This study also reveals that the hydraulic gradient (J) and flow velocity (v) follow a super-linear relationship that can be fitted almost perfectly by the Forchheimer equation. The inertial component (Ji) of J increases monotonically with Re, whereas the viscous component (Jv) of J decreases monotonically with Re. The Re value corresponding to equal inertial and viscous components of J (named as the transitional point Re) decreases when E increases, and such a transitional point Re should be closely related to the Critical Reynolds Number Rec, although a rigorous theoretical proof is not yet available. Copyright © 2015 John Wiley & Sons, Ltd.

Jili Zhang - One of the best experts on this subject based on the ideXlab platform.

  • Experimental study on single-phase flow in horizontal internal helically-finned tubes: The Critical Reynolds Number for turbulent flow
    Experimental Thermal and Fluid Science, 2018
    Co-Authors: Yonghui Wang, Jili Zhang, Zhixian Ma
    Abstract:

    A correlation of the Critical Reynolds Number for turbulent flow in horizontal helically finned tubes is proposed in this study based on the analysis of experimental data from the current and six previous studies. In the experiment, the main parameters of the two tubes (Tube-1 and Tube-2), include the Number of fin (Ns), helix angle (α), and the ratio of fin height to diameter (e/Di), which are 38 and 60, 60° and 45°, and 0.0534 and 0.0222, respectively. Aqueous ethylene glycol was used as the test fluid. Pressure drop data were obtained under isothermal condition with Reynolds Number spanning from 3100 to 39500 and Prandtl Number spanning from 13.8 to 49.2. Results showed that the Critical Reynolds Numbers for turbulent flow in Tube-1 and Tube-2 were 11000 and 17000, respectively. The proposed correlation, which correlated the Critical Reynolds Number with four parameters (i.e., e, Di, α, and Ns) and a constant, predicted all the 14 groups of Critical Reynolds Number within 10% and could be applicable to internal helically finned tubes with 0.01

  • experimental study on single phase flow in horizontal internal helically finned tubes the Critical Reynolds Number for turbulent flow
    Experimental Thermal and Fluid Science, 2018
    Co-Authors: Yonghui Wang, Jili Zhang
    Abstract:

    A correlation of the Critical Reynolds Number for turbulent flow in horizontal helically finned tubes is proposed in this study based on the analysis of experimental data from the current and six previous studies. In the experiment, the main parameters of the two tubes (Tube-1 and Tube-2), include the Number of fin (Ns), helix angle (α), and the ratio of fin height to diameter (e/Di), which are 38 and 60, 60° and 45°, and 0.0534 and 0.0222, respectively. Aqueous ethylene glycol was used as the test fluid. Pressure drop data were obtained under isothermal condition with Reynolds Number spanning from 3100 to 39500 and Prandtl Number spanning from 13.8 to 49.2. Results showed that the Critical Reynolds Numbers for turbulent flow in Tube-1 and Tube-2 were 11000 and 17000, respectively. The proposed correlation, which correlated the Critical Reynolds Number with four parameters (i.e., e, Di, α, and Ns) and a constant, predicted all the 14 groups of Critical Reynolds Number within 10% and could be applicable to internal helically finned tubes with 0.01