36 research outputs found
Optimal subpixel interpolation in particle image velocimetry
It is shown that among the different techniques for particle image velocimetry subpixel interpolation, only the "sinc"-kernel creates an optimal result in that it completely suppresses spurious spectral sidelobes. An efficient method is introduced for the computation of the subpixel-accurate correlation peak position without any systematic errors. A connection is made with the kernel-dependent observation of the peak-locking phenomeno
Flow measurements near a Reynolds ridge
The Reynolds ridge is a well-known phenomenon first observed
in 1854 by Henry David Thoreau. It was then rediscovered
by Langton in 1872, but Reynolds was the first to recognize
that the surface tension difference was the physical mechanism
behind its formation and saw the equality between the case of
a spreading film and that of a stagnant film met by oncoming
flow. However, it wasn't until McCutchen in 1970 that the
prediction of a boundary layer forming beneath the film was
introduced as the cause of the surface deformation rise ahead
of the film due to the retardation of the flow. The first quantitative
theory of the ridge was formed by Harper and Dixon
(1974), who stated that the surface tension gradient balances
the viscous shear stress generated in the boundary layer. Experimental
studies of the ridge so far include Schlieren visualizations
by Sellin (1968) as well as by Scott (1982) who measured
the surface slope across the ridge and found good comparisons
between the theoretical results of Harper and Dixon. Finally, it
was Scott who recognized that even at very low levels of surface
contamination the Reynolds ridge is found to exist
Seeding of high temperature air flow
Seeding of high temperature air flows is problematic because common seeding materials can ignite and may pose a threat for human health in case of inhalation. Sodium chloride is proposed as a seeding material to avoid these problems. The generation of crystals using an ultrasonic atomizer is described and the measured particle size distribution is presented. The described method has the additional advantage that the particle size can easily be adjusted. This allows the operator to comply with the experimental requirements in terms of resolvable frequencies. The described method has been successfully applied to LDA measurements in a 500°C air flo
Amplitude measurements of Faraday waves
A light reflection technique is used to measure quantitatively the surface
elevation of Faraday waves. The performed measurements cover a wide parameter
range of driving frequencies and sample viscosities. In the capillary wave
regime the bifurcation diagrams exhibit a frequency independent scaling
proportional to the wavelength. We also provide numerical simulations of the
full Navier-Stokes equations, which are in quantitative agreement up to
supercritical drive amplitudes of 20%. The validity of an existing perturbation
analysis is found to be limited to 2.5% overcriticaly.Comment: 7 figure
Fluid surface imaging using microlens arrays
The paper describes a new technique for measuring surface slopes at the air–water interface. A microlens array is employed to optically sample the fluid surface at a large number of measurement points in parallel. The focal plane image of the lenslet array is recorded by a standard video camera, generating a data set of typically 4000 candidate samples for each video frame. The displacement of the focal spots is analyzed using a cross-correlation technique augmented by a topological identification algorithm which makes use of the hexagonal structure of the lenslet array. Surface height profiles are computed by integrating the surface slope field