The Experts below are selected from a list of 123 Experts worldwide ranked by ideXlab platform

M Cuntz - One of the best experts on this subject based on the ideXlab platform.

  • generation of longitudinal flux tube waves in theoretical main sequence stars effects of model parameters
    Astronomy and Astrophysics, 2011
    Co-Authors: Diaa E Fawzy, M Cuntz
    Abstract:

    Aims. We compute the wave energy fluxes carried by longitudinal tube waves along vertically oriented thin magnetic fluxes tubes embedded in the atmospheres of theoretical main-sequence stars based on stellar parameters deduced by Kurucz and Gray. In addition, we present a fitting formula for the wave energy flux based on the governing stellar and magnetic parameters. Methods. A modified theory of turbulence generation based on the Mixing-Length Concept is combined with the magnetohydrodynamic equations to numerically account for the wave energies generated at the base of magnetic flux tubes. Results. The results indicate a stiff dependence of the generated wave energy on the stellar and magnetic parameters in principal agreement with previous studies. The wave energy flux FLTW decreases by about a factor of 1.7 between G0 V and K0 V stars, but drops by almost two orders of magnitude between K0 V and M0 V stars. In addition, the values for FLTW are significantly higher for lower in-tube magnetic field strengths. Both results are consistent with the findings from previous studies. Conclusions. Our study complements existing descriptions of magnetic energy generation in late-type main-sequence stars. Our results will be helpful for calculating theoretical atmospheric models for stars of different levels of magnetic activity.

  • generation of longitudinal flux tube waves in theoretical main sequence stars effects of model parameters
    arXiv: Solar and Stellar Astrophysics, 2010
    Co-Authors: Diaa E Fawzy, M Cuntz
    Abstract:

    Context. Continued investigation of the linkage between magneto-acoustic energy generation in stellar convective zones and the energy dissipation and radiative emission in outer stellar atmospheres in stars of different activity levels. Aims. We compute the wave energy fluxes carried by longitudinal tube waves along vertically oriented thin magnetic fluxes tubes embedded in the atmospheres of theoretical main-sequence stars based on stellar parameters deduced by R. L. Kurucz and D. F. Gray. Additionally, we present a fitting formula for the wave energy flux based on the governing stellar and magnetic parameters. Methods. A modified theory of turbulence generation based on the Mixing-Length Concept is combined with the magneto-hydrodynamic equations to numerically account for the wave energies generated at the base of magnetic flux tubes. Results. The results indicate a stiff dependence of the generated wave energy on the stellar and magnetic parameters in principal agreement with previous studies. The wave energy flux F_LTW decreases by about a factor of 1.7 between G0V and K0V stars, but drops by almost two orders of magnitude between K0V and M0V stars. In addition, the values for F_LTW are significantly higher for lower in-tube magnetic field strengths. Both results are consistent with the findings from previous studies. Conclusions. Our study will add to the description of magnetic energy generation in late-type main-sequence stars. Our results will be helpful for calculating theoretical atmospheric models for stars of different levels of magnetic activity.

Motohiko Umeyama - One of the best experts on this subject based on the ideXlab platform.

  • Time-Averaged Distributions of Velocity and Sediment-Concentration under Irregular Waves and Currents
    Coastal Engineering 2000, 2001
    Co-Authors: Motohiko Umeyama, Jan Van De Graaff
    Abstract:

    The velocity and sediment-concentration distributions in the case of irregular waves and currents are studied by using the theoretical model developed by means of a Mixing-Length Concept for sediment-laden flow, and by improving the sediment-concentration formula based on the same hypothesis. The predicted results are compared with a set of velocity and sediment-concentration data collected previously in a recirculating wave flume. The time-averaging procedure is adopted, since it eliminates the random scatter of the instantaneous components of velocities and concentration. The general characteristic of the time-averaged velocity is greatly affected by the significant-wave height and the flow direction to the generated waves, although the Mixing Length is a function of the sediment concentration. To compute the time-averaged vertical distribution of suspended sediment, two different formulae are proposed. The agreement between the simulated velocity and concentration profiles and the measured ones is satisfactory for the flow containing fine sands.

  • Vertical Distributions of Velocity and Concentration in Sediment-Laden Open-Channel Flow Based on a New Mixing-Length Concept
    Doboku Gakkai Ronbunshuu B, 1995
    Co-Authors: Motohiko Umeyama, Takao Horiguchi
    Abstract:

    A theoretical model to predict the distributions of velocity and concentration for sediment-laden flow in open channel is developed by means of a new Mixing-Length Concept. The vertical velocity distribution is obtained as a function of the vertical sediment concentration. The Karman coefficient is independent of suspended load and equal to 0.4. The solution of velocity distribution departs from the logarithmic law in the outer layer. The sediment concentration equation is derived analytically, using the vertical mass balance equation over the water depth.

  • Velocity distribution in uniform sediment-laden flow
    Journal of Hydraulic Engineering, 1992
    Co-Authors: Motohiko Umeyama, Franciscus Gerritsen
    Abstract:

    A theoretical model to predict the velocity distribution for sediment-laden flow is developed by means of a new Mixing-Length Concept. The solution is obtained as a function of the vertical sediment concentration. The von Karman coefficient is independent on suspended load and equal to 0.4. The model is applicable to an alluvial-bed condition as well as a flat-bed condition. The possibility of adapting the model to clear-water flow is also discussed. For clear-water flow, the velocity equation consists of a logarithmic term and power-series terms that explain the wake effect. The theoretical velocity distribution for sediment-laden flow departs from the logarithmic law in the outer layer. The magnitude of the departure is larger with the increase in the sediment load. The numerical calculation is carried out and its prediction is compared to five independent sets of experimental data previously obtained. The theoretical result shows good agreement in the whole flow layer with these experimental results.

Diaa E Fawzy - One of the best experts on this subject based on the ideXlab platform.

  • generation of longitudinal flux tube waves in theoretical main sequence stars effects of model parameters
    Astronomy and Astrophysics, 2011
    Co-Authors: Diaa E Fawzy, M Cuntz
    Abstract:

    Aims. We compute the wave energy fluxes carried by longitudinal tube waves along vertically oriented thin magnetic fluxes tubes embedded in the atmospheres of theoretical main-sequence stars based on stellar parameters deduced by Kurucz and Gray. In addition, we present a fitting formula for the wave energy flux based on the governing stellar and magnetic parameters. Methods. A modified theory of turbulence generation based on the Mixing-Length Concept is combined with the magnetohydrodynamic equations to numerically account for the wave energies generated at the base of magnetic flux tubes. Results. The results indicate a stiff dependence of the generated wave energy on the stellar and magnetic parameters in principal agreement with previous studies. The wave energy flux FLTW decreases by about a factor of 1.7 between G0 V and K0 V stars, but drops by almost two orders of magnitude between K0 V and M0 V stars. In addition, the values for FLTW are significantly higher for lower in-tube magnetic field strengths. Both results are consistent with the findings from previous studies. Conclusions. Our study complements existing descriptions of magnetic energy generation in late-type main-sequence stars. Our results will be helpful for calculating theoretical atmospheric models for stars of different levels of magnetic activity.

  • generation of longitudinal flux tube waves in theoretical main sequence stars effects of model parameters
    arXiv: Solar and Stellar Astrophysics, 2010
    Co-Authors: Diaa E Fawzy, M Cuntz
    Abstract:

    Context. Continued investigation of the linkage between magneto-acoustic energy generation in stellar convective zones and the energy dissipation and radiative emission in outer stellar atmospheres in stars of different activity levels. Aims. We compute the wave energy fluxes carried by longitudinal tube waves along vertically oriented thin magnetic fluxes tubes embedded in the atmospheres of theoretical main-sequence stars based on stellar parameters deduced by R. L. Kurucz and D. F. Gray. Additionally, we present a fitting formula for the wave energy flux based on the governing stellar and magnetic parameters. Methods. A modified theory of turbulence generation based on the Mixing-Length Concept is combined with the magneto-hydrodynamic equations to numerically account for the wave energies generated at the base of magnetic flux tubes. Results. The results indicate a stiff dependence of the generated wave energy on the stellar and magnetic parameters in principal agreement with previous studies. The wave energy flux F_LTW decreases by about a factor of 1.7 between G0V and K0V stars, but drops by almost two orders of magnitude between K0V and M0V stars. In addition, the values for F_LTW are significantly higher for lower in-tube magnetic field strengths. Both results are consistent with the findings from previous studies. Conclusions. Our study will add to the description of magnetic energy generation in late-type main-sequence stars. Our results will be helpful for calculating theoretical atmospheric models for stars of different levels of magnetic activity.

G. Brusasca - One of the best experts on this subject based on the ideXlab platform.

  • An assessment of Mixing-Length closure schemes for models of turbulent boundary layers over complex terrain
    Boundary-Layer Meteorology, 1995
    Co-Authors: S. Finardi, F. Trombetti, F. Tampieri, G. Brusasca
    Abstract:

    The effectiveness of closure assumptions implemented in turbulent boundary-layer models is rather uncertain over complex terrain. Different closure schemes for Reynolds shear stress based on the Mixing-Length Concept are compared with data from wind tunnel experiments over complex terrain and the results are analysed on the basis of second-order moment equations. A good estimation of the vertical momentum flux velocity scale turns out to be given by the standard deviation of the vertical velocity while the turbulent kinetic energy scaling gives less satisfactory results in regions where turbulence anisotropy is large. Fairly good results are given by closure models implementing a shear-limited Mixing-Length already proposed for non-logarithmic wind profiles, while large errors characterize traditional Mixing-Length formulations.

Lars Johanning - One of the best experts on this subject based on the ideXlab platform.

  • Quantifying turbulence from field measurements at a mixed low tidal energy site
    Renewable Energy, 2016
    Co-Authors: Abdessalem Bouferrouk, Jonathan Hardwick, Antonella M. Colucci, Lars Johanning
    Abstract:

    Abstract This study explores typical characteristics of the mean and turbulent profiles at a mixed low tidal energy site (40 m mean water depth) where the waves have limited effects on the currents. The turbulence profiles were derived from secondary current data using a 5-beam ADCP which was optimised for wave measurements. The tidal currents have peak flows of ∼1 m/s during spring tide. The turbulence intensity is no less than 10% at peak flows and compares well with values at other tidal channels (at ∼5 m from seabed). The Reynolds stresses show symmetry at the neap tide but less so for the spring tide. Although the qualitative profiles of TKE are similar between the neap and spring tides, the values of TKE for flood flow are the largest throughout the deployment. The integral Length scales are in good agreement with theory, and with estimates based on the Mixing Length Concept. The measured turbulence parameters are sensitive to flow inhomogeneity, Doppler noise, and ADCP tilt. The findings demonstrate the practical benefits of exploiting secondary current data at a mixed low tidal energy site for estimating typical turbulence characteristics; such information can be used to define design standards and protocols for marine energy devices.