Abstract
We demonstrate the viability of using four low-cost smartphone cameras to perform Tomographic PIV. We use colored shadows to imprint two or three different time-steps on the same image. The back-lighting is accomplished with three sets of differently-colored pulsed LEDs. Each set of Red, Green & Blue LEDs is shone on a diffuser screen facing each of the cameras. We thereby record the RGB-colored shadows of opaque suspended particles, rather than the conventionally used scattered light. We subsequently separate the RGB color channels, to represent the separate times, with preprocessing to minimize noise and cross-talk. We use commercially available Tomo-PIV software for the calibration, 3-D particle reconstruction and particle-field correlations, to obtain all three velocity components in a volume. Acceleration estimations can be done thanks to the triple pulse illumination. Our test flow is a vortex ring produced by forcing flow through a circular orifice, using a flexible membrane, which is driven by a pressurized air pulse. Our system is compared to a commercial stereoscopic PIV system for error estimations. We believe this proof of concept experiment will make this technique available for education, industry and scientists for a fraction of the hardware cost needed for traditional Tomo-PIV.
Highlights
Flow visualization and quantitative velocity measurements are the foundations of experimental fluid mechanics
In this report we show that low-cost smartphone cameras and LED illumination can be used to perform Tomo-Particle Image Velocimetry (PIV) based on colored shadows (Tomo-Particle Shadow Velocimetry (PSV))
The single channel concentration in ppp is relatively low compared to traditional tomographic PIV, but is necessary to separate the particles for each color channel without devising too much overlap that may affect the color separation and low quality reconstruction
Summary
Flow visualization and quantitative velocity measurements are the foundations of experimental fluid mechanics. Particle Image Velocimetry (PIV) is the most powerful modern technique to measure extended velocity fields[1,2,3,4] This relies on seeding the flow-volume with small tracer particles which are illuminated with pulsed light and digital cameras capture images of their displacement with time. Including a high-precision calibration, the frames from all the cameras are used in an iterative algorithm using Multiplicative Algebraic Reconstruction Technique (MART) based on a Multiple Line of Sight (MLOS) initialization[7] This allows 3-D reconstruction of the particle locations in the volume to obtain three-component velocity fields (3D-3C), based on multi-step 3-D cross-correlation[6]. Watamura et al.[21] have used space color-coded light to extract the 3-D location of particles within a volumetric illumination produced with an LCD projector, using a single camera and thereby reducing costs significantly
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