3 research outputs found
Stereoscopic PIV measurements using low-cost action cameras
Recently, large progress was made in the development towards low-cost PIV (Particle Image Velocimetry) for industrial and educational applications. This paper presents the use of two low-cost action cameras for stereoscopic planar PIV. A continuous wave laser or alternatively an LED was used for illumination and pulsed by a frequency generator. A slight detuning of the light pulsation and camera frame rate minimizes systematic errors by the rolling shutter effect and allows for the synchronization of both cameras by postprocessing without the need of hardware synchronization. The setup was successfully qualified on a rotating particle pattern in a planar and stereoscopic configuration as well as on the jet of an aquarium pump. Since action cameras are intended to be used at outdoor activities, they are small, very robust and work autarkic. In conjunction with the synchronization and image pre-processing scheme presented herein, those cameras enable stereoscopic PIV in harsh environments and even on moving experiments
Thermal boundary condition studies in large aspect ratio Rayleigh-B\'enard convection
We study the influence of thermal boundary conditions on large aspect ratio
Rayleigh-B\'enard convection by a joint analysis of experimental and numerical
data sets for a Prandl number and Rayleigh numbers
. The spatio-temporal experimental data are obtained
by combined Particle Image Velocimetry and Particle Image Thermometry
measurements in a cuboid cell filled with water at an aspect ratio . In addition, numerical data are generated by Direct Numerical Simulations
(DNS) in domains with and subject to different
thermal boundary conditions. Our experimental data show an increased
characteristic horizontal extension scale of the flow structures,
, for increasing Ra, which is coupled with a raise of the Biot
number Bi in particular at the cooling plate. However, we find the experimental
flow structure size to range in any case between the ones observed for the
idealized thermal conditions captured by the simulations. On the one hand, they
are larger than in the numerical case with applied uniform temperatures at the
plates, but, on the other hand, smaller than in the case of an applied constant
heat flux, the latter of which leads to a structure that grows gradually up to
the horizontal domain size. We link this observation qualitatively to
theoretical predictions for the onset of convection. Furthermore, we study the
effect of the asymmetric boundary conditions on the heat transfer. Contrasting
experimental and numerical data reveals an increased probability of far-tail
events of reversed heat transfer. The decomposition of the local Nusselt number
traces this effect back to the sign of the temperature
deviation , revealing asymmetries of the heating and cooling
plate on the thermal variance of the generated thermal plumes
Combined particle image velocimetry and thermometry of turbulent superstructures in thermal convection
Turbulent superstructures in horizontally extended three-dimensional
Rayleigh-B\'enard convection flows are investigated in controlled laboratory
experiments in water at Prandtl number . A Rayleigh-B\'enard cell with
square cross-section, aspect ratio , side length and
height is used. Three different Rayleigh numbers in the range are considered. The cell is accessible optically, such that thermochromic
liquid crystals can be seeded as tracer particles to monitor simultaneously
temperature and velocity fields in a large section of the horizontal mid-plane
for long time periods of up to 6 h, corresponding to approximately
convective free-fall time units. The joint application of stereoscopic particle
image velocimetry and thermometry opens the possibility to assess the local
convective heat flux fields in the bulk of the convection cell and thus to
analyse the characteristic large-scale transport patterns in the flow. A direct
comparison with existing direct numerical simulation data in the same parameter
range of and reveals the same superstructure patterns and
global turbulent heat transfer scaling . Slight quantitative
differences can be traced back to violations of the isothermal boundary
condition at the extended water-cooled glass plate at the top. The
characteristic scales of the patterns fall into the same size range, but are
systematically larger. It is confirmed experimentally that the superstructure
patterns are an important backbone of the heat transfer. The present
experiments enable, furthermore, the study of the gradual evolution of the
large-scale patterns in time, which is challenging in simulations of
large-aspect-ratio turbulent convection.Comment: 25 pages, 11 figure