# In-Vitro Experimental Modeling of Oscillatory Respiratory Flow in a CT-Scanned OSAHS Tract
## 3 PIV Techniques for the ETA Model with OSAHS  
The other goal of this research was to apply the above experimental rig to measure the oscillatory respiratory flow velocity in an ETA model with OSAHS. The LaVision 2D-2C (two-dimensional, two-component) PIV system was used to measure the unsteady flow velocity at the two representative sections, i.e., section A-A and section B-B, as shown in Figure 2a,b.  
A double pulse yttrium-aluminum-garnet (YAG) laser and a high-speed charge-coupled device (CCD) camera were employed to obtain digital images. A laser (Evergreen Corp.) with power up to 600 mJ per pulse was used to provide double laser sheets with frequency of 15 Hz, wavelength of 532 nm and thickness of approximately 1 mm to induce fluorescence of tracers. The CCD camera (Imager SX 4M) was used to store digital images. The camera features an interline transfer chip with progressive-scan low-noise readout, high spatial resolution of 2360 \(\times\) 1776 pixels (4 Mpixel, 5.5 \(\times\) 5.5 \(\upmu\)m\({}^{2}\) per pixel size) in 12-bit mode, and fast frame rates up to 31 Hz. Here, 15 Hz frame rates were used to match the frequency of the laser pulse.  
Determining the distinctive time (\(dt\)) delay of two subsequent particle images (double shutter) in the PIV measurements of oscillatory flow is an important task since the \(dt\) setting must be adapted to the maximum flow speed as well as the flow reversal condition [32]. Here, \(dt\) was determined according to the CCD spatial resolution, field of view of the camera, estimated average velocity, and target pixel shift of tracing particles in double shutter, which were 2360 \(\times\) 1776 pixels, 15 \(\times\) 11.3 cm\({}^{2}\), 0.7 m/s, and 7 pixels, respectively. The \(dt\) and the duration of exposure time of particle images were determined to be 500 \(\upmu\)s and 200 \(\upmu\)s, respectively. The length of camera image (11.3 cm) was smaller than that in ETA model (approximately 20 cm); consequently, one PIV measurement of A-A section had to be divided into two sub-sections. The two digital images obtained from the two sub-sections measurements at the same phase were then stitched to form a whole PIV measurement image. Within the present modeling cycle (8.5 s), 128 groups of double shutter images were continuously acquired with the PIV system and controlled with a synchronizer. All steps of control, acquisition and post processing were carried out in a DaVis 10.0.5 software environment [36].  
The reflected light is very harmful to both the camera CCD itself and the CCD acquisition quality of near-wall flows. To avoid light reflection from walls, good optical index-matching, suitable tracer fluorescence and a long-pass glass filter were employed [16]. The working fluid with good optical index-matching not only eliminates optical distortions, but also reduces wall light reflection. Fluorescent tracers were used to scatter 590 nm wavelength light under 532 nm wavelength laser irradiation. Using the long-pass glass filter before the camera, the reflected light with wavelength below 550 nm will be filtered out, thus greatly alleviating wall reflection.  
The long-pass glass filter and fluorescent tracers used in PIV measurement will not distort measured results. The long-pass glass filter had more than 99% transmittance of the scattered light with wavelength above 550 nm. The mean diameter \(d_{\mathrm{p}}\) and density \(\rho_{\mathrm{p}}\) of fluorescent tracers were 4 \(\upmu\)m and 3.4 g/cm\({}^{3}\), respectively. The response time (\(\tau_{\mathrm{p}}\)) of tracing particles defined as \(d_{\mathrm{p}}\)\({}^{2}\rho_{\mathrm{p}}\)/(18\(\mu\)) was calculated as \(2.93\times 10^{-7}\) s [30]. The dynamic viscosity \(\mu\)of the water-glycerol mixture was \(1.03\times 10^{-2}\) Pa-s.