Lagrangian acceleration statistics in a fully developed turbulent channel
flow at $Re_\tau = 1440$ are investigated, based on tracer particle tracking in
experiments and direct numerical simulations. The evolution with wall distance
of the Lagrangian velocity and acceleration time scales is analyzed. Dependency
between acceleration components in the near-wall region is described using
cross-correlations and joint probability density functions. The strong
streamwise coherent vortices typical of wall-bounded turbulent flows are shown
to have a significant impact on the dynamics. This results in a strong
anisotropy at small scales in the near-wall region that remains present in most
of the channel. Such statistical properties may be used as constraints in
building advanced Lagrangian stochastic models to predict the dispersion and
mixing of chemical components for combustion or environmental studies.Comment: accepted for publication in Physical Review Fluid
The present work focuses on possible heat transfer enhancement from a heating plate towards tap water in forced convection by means of 2MHz ultrasound. The thermal approach allows to observe the increase of local convective heat transfer coefficients in the presence of ultrasound and to deduce a correlation between ultrasound power and Nusselt number. Heat transfer coefficient under ultrasound remains constant while heat transfer coefficient under silent conditions increases with Reynolds number from 900 up to 5000. Therefore, heat transfer enhancement factor ranges from 25% up to 90% for the same energy conditions (supplied ultrasonic power=110W and supplied thermal power=450W). In the same time cavitational activity due to 2MHz ultrasound emission was characterized from mechanical and chemical viewpoints without significant results. At least, Particle Image Velocimetry (PIV) measurements have been performed in order to investigate hydrodynamic modifications due to the presence of 2MHz ultrasound. It was therefore possible to propose a better understanding of heat transfer enhancement mechanism with high frequency ultrasound.
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