44,655 research outputs found
A code for direct numerical simulation of turbulent boundary layers at high Reynolds numbers in BG/P supercomputers
A new high-resolution code for the direct numerical simulation of a zero pressure gradient turbulent boundary layers over a flat plate has been developed. Its purpose is to simulate a wide range of Reynolds numbers from ReΞ = 300 to 6800 while showing a linear weak scaling up to 32,768 cores in the BG/P architecture. Special attention has been paid to the generation of proper inflow boundary conditions. The results are in good agreement with existing numerical and experimental data sets
Optimal Taylor-Couette flow: Radius ratio dependence
Taylor-Couette flow with independently rotating inner (i) and outer (o)
cylinders is explored numerically and experimentally to determine the effects
of the radius ratio {\eta} on the system response. Numerical simulations reach
Reynolds numbers of up to Re_i=9.5 x 10^3 and Re_o=5x10^3, corresponding to
Taylor numbers of up to Ta=10^8 for four different radius ratios {\eta}=r_i/r_o
between 0.5 and 0.909. The experiments, performed in the Twente Turbulent
Taylor-Couette (T^3C) setup, reach Reynolds numbers of up to Re_i=2x10^6$ and
Re_o=1.5x10^6, corresponding to Ta=5x10^{12} for {\eta}=0.714-0.909. Effective
scaling laws for the torque J^{\omega}(Ta) are found, which for sufficiently
large driving Ta are independent of the radius ratio {\eta}. As previously
reported for {\eta}=0.714, optimum transport at a non-zero Rossby number
Ro=r_i|{\omega}_i-{\omega}_o|/[2(r_o-r_i){\omega}_o] is found in both
experiments and numerics. Ro_opt is found to depend on the radius ratio and the
driving of the system. At a driving in the range between {Ta\sim3\cdot10^8} and
{Ta\sim10^{10}}, Ro_opt saturates to an asymptotic {\eta}-dependent value.
Theoretical predictions for the asymptotic value of Ro_{opt} are compared to
the experimental results, and found to differ notably. Furthermore, the local
angular velocity profiles from experiments and numerics are compared, and a
link between a flat bulk profile and optimum transport for all radius ratios is
reported.Comment: Submitted to JFM, 28 pages, 17 figure
Multidimensional hydrodynamic simulations of the hydrogen injection flash
The injection of hydrogen into the convection shell powered by helium burning
during the core helium flash is commonly encountered during the evolution of
metal-free and extremely metal-poor low-mass stars. With specifically designed
multidimensional hydrodynamic simulations, we aim to prove that an entropy
barrier is no obstacle for the growth of the helium-burning shell convection
zone in the helium core of a metal-rich Pop I star, i.e. convection can
penetrate into the hydrogen-rich layers for these stars, too. We further study
whether this is also possible in one-dimensional stellar evolutionary
calculations. Our hydrodynamical simulations show that the helium-burning shell
convection zone in the helium core moves across the entropy barrier and reaches
the hydrogen-rich layers. This leads to mixing of protons into the hotter
layers of the core and to a rapid increase of the nuclear energy production at
the upper edge of the helium-burning convection shell - the hydrogen injection
flash. As a result a second convection zone appears in the hydrogen-rich
layers. Contrary to 1D models, the entropy barrier separating the two
convective shells from each other is largely permeable to chemical transport
when allowing for multidimensional flow, and consequently, hydrogen is
continuously mixed deep into the helium core. We find it difficult to achieve
such a behavior in one-dimensional stellar evolutionary calculations.Comment: 8 pages, 8 figures - accepted for publication in Astronomy and
Astrophysics. Animations related to the manuscript can be downloaded from
http://www-astro.ulb.ac.be/~mocak/index.php/Main/AnimationsHeFlas
- âŠ