Blog Entry #1

| 2 Comments | 0 TrackBacks

Laser Doppler Velocimetry (LDV) vs. Particle Image Velocimetry (PIV)

 

Laser Doppler Velocimetry (LDV) is a technique that fluid mechanic researchers use to make instantaneous velocity measurements of fluid flow. The technique is non-intrusive, and can deliver independent measurements of ambient conditions, it also measures three directional components, and can virtually access any flow region with the aid of fiber optics, and has a dynamic range from natural convection to supersonic velocities. LDV makes use of the coherent wave nature of laser light. The crossing of two laser beams of the same wavelength produces areas of constructive and destructive interference patterns. The interference pattern, known as a 'fringe' pattern is composed of planar layers of high and low intensity light. Velocity measurements are made when particles 'seeded' in the flow pass through the fringe pattern created by the intersection of a pair of laser beams. These particles scatter light in all directions when going through the beam crossing. This scattered light is then collected by a stationary detector which receives optics connected to a photomultiplier. The frequency of the scattered light is Doppler shifted and referred to as the Doppler frequency of the flow. This Doppler frequency is proportional to a component of the particles velocity which is perpendicular to the planar fringe pattern produced by the beam crossing. In order to obtain three components of velocity, three sets of fringe patterns need to be produced at the same region in space.

ldv_pdi_NIVHMq.jpg

Particle Image Velocimetry is a planar laser light sheet technique in which the light sheet is pulsed twice, and images of fine particles lying in the light sheet are recorded on a video camera or a photograph. The displacement of the particle images is measured in the plane of the image and used to determine the displacement of the particles in the flow. The most common way of measuring displacement is to divide the image plane into small interrogation spots and cross correlate the images from the two time exposures. The spatial displacement that produces the maximum cross-correlation statistically approximates the average displacement of the particles in the interrogation cell. Velocity associated with each interrogation spot is just the displacement divided by the time between the laser pulses. In PIV, the velocity vectors are derived from sub-sections of the target area of the particle-seeded flow by measuring the movement of particles between two light pulses. The flow is illuminated in the target area with a light sheet. The camera lens images the target area onto the sensor array of a digital camera. The camera is able to capture each light pulse in separate image frames. Once a sequence of two light pulses is recorded, the images are divided into small subsections called interrogation areas (IA). The interrogation areas from each image frame are cross-correlated with each other, pixel by pixel. The correlation produces a signal peak, identifying the common particle displacement, DX. An accurate measure of the displacement - and thus also the velocity - is achieved with sub-pixel interpolation.

 

PIV_img1_xl.jpg

PIV and LDV measurement results agree with each other well in general for both mean velocity and fluctuations of the velocity components. The average disagreement level of the mean velocity between PIV and LDV measurement results was found to be within 3% of the target velocity for the PIV system parameter selection. Bigger disagreements between the PIV and LDV measurement results were found to concentrate at high shear regions. The spatial resolution and temporal resolution differences of the PIV and LDV measurements and the limited frames of the PIV instantaneous results were suggested to be the main reasons for the disagreement.

Source Cited:

PIV/LDV MEASUREMENTS OF MEAN VELOCITY AND TURBULENCE IN A COMPLEX OPEN CHANNEL FLOW, B-S. Hyun, R. Balachandar, K. Yu, and V.C. Patel.

<http://www.oplanchina.com/data/upload/ldv_pdi_NIVHMq.jpg>

<http://www.dlr.de/as/en/Portaldata/5/Resources/images/abteilungen/abt_ev/artikel/PIV_img1_xl.jpg>

 VIDEO   Particle Image Velocimetry System in a Low Speed Wind Tunnel

 

LINKS

http://www.onepetro.org/mslib/servlet/onepetropreview?id=00027474&soc=SPE

http://www.tsi.com/en-1033/segments/fluid_mechanics.aspx

No TrackBacks

TrackBack URL: https://blogs.psu.edu/mt4/mt-tb.cgi/164368

2 Comments

I wish i was smarter so that i could understand exactly what this means. But i really like this post!

Leave a comment

Search This Blog

Full Text  Tag

Recent Entries

Blog Entry #4
AIRCRAFT COMPONENTS This description is intended for freshman and sophomore who have a basic understanding of an airplane. It uses…
Blog Entry #3
IDEAL WEBSITES TO FIND INFORMATIONS IN THE AEROSPACE FIELD! AIAA ONLINE PUBLICATIONS Link: www.aiaa.org The American Institute of Aeronautics…
Blog Entry#2
How to Setup a Mazak Multiplex Machine (For First Time Operators) The description of this paper is intended for first…