Optics of the channelling LDM (height not to scale).
Lens array and aperture plate.
Channelling LDM applied to a text block (object), showing magnified regions of interest (image on camera chip).
Ray hitting meniscus non-parallel to surface normal.
Lens array geometry.
Comparison of rms aberration of singlet and doublet lens at different magnification.
Subsection of the 30 × 30 micro-pillar field as seen by a standard microscope. The image demonstrates good quality and homogeneity of the cylindrical structures.
Wall jet generator and flow visualization system. The light sheet and camera position is shown in orientation for the PIV-measurements.
Smoke visualization of the wall jet. View from top onto the wall. The location of observed micro-pillars is indicated by a square in the bottommost image.
Ensemble average of the cross-sectional velocity field downstream of the nozzle, measured by classical PIV. The jet Reynolds number is 1290. The profiles show the development of the wall jet in the transitional region.
Image sequence showing one period of the quasi-periodic roll-up of larger vortical structures. The dashed line indicates the reference location, where the first picture of the sequence shows the same position of the roller structure as the last one. The five vertical, white bars in the uppermost image indicate the position of the micro-pillars. Each bar corresponds to one micro-pillar. The height of the bars is enlarged about four times, whereas their lateral distance scales correctly.
Calibration curve showing the normalized micro-pillar-tip deflection at different wall shear stress in the lower limit of load range. Calibration is carried out in a Couette flow apparatus generating a constant velocity gradient within the flow channel. +calibration data; ■micro-pillar side view; –fit of calibration data to equation w/[μm] = exp (τ/[Pa] + 2.6099) − 13.597.
Visualization of the flow structure in the wake of a wall-bounded cylinder with a free end at pillar Reynolds number Re = 33. The flow is from left to right. The illuminated plane is centered with the incoming flow direction and the pillar axis.
Setup for dynamic calibration. The micro-pillar field is located directly between two speakers, observed by the channelling LDM.
Maximum velocity of the speaker cone as a function of the speaker frequency.
Sensitivity of the micro-pillar field as a function of the frequency of the oscillating flow.
Setup for wall shear stress measurements in a wall jet by means of channelling LDM.
Images of the micro-pillar array at rest (top) and with arbitrary deflection at (bottom). The flow direction is from left to right.
(a) mean wall shear stress in streamwise direction , (b) mean friction coefficient C f for different Reynolds number of wall jet. The location of the micro-pillar field is at 1 < x/d < 1.2.
Probability density functions (PDF) of streamwise wall shear stress fluctuations τ u ′ for different Reynolds numbers of the wall jet. The location of the micro-pillar field is at 1 < x/d < 1.2.
PDF of at Re = 2360, fitted by a skewed Gaussian profile.
Spectrum of wall shear stress fluctuations in the transitional wall jet at 1 < x/d < 1.2. The frequency of the shear layer roll-up f = 625 Hz and its doubling are highlighted.
Mean deflection and arbitrary snapshot fluctuation of micro-pillars in the transitional wall jet at and 1 < x/d < 1.2. The direction of the flow is from left to right.
Time series of the first three POD coefficients at and 1 < x/d < 1.2.
High energetic POD modes ϕ1, ϕ2, ϕ3 of wall shear stress fluctuations obtained by measuring the fluid-imposed deflections of twenty-one micro-pillars at and 1 < x/d < 1.2. The direction of the flow is from left to right. The contribution to the total energy is given in percent above the vector field.
Lenses of the channelling LDM.
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