LAVISION
A PIV Vectormap of a vortex in air showing vector arrow array and derived vorticity
PIV velocity data plotted with pressure sensitive paint data Source: DLR Goettingen
time window allowing data to be acquired under continuous and optimal flow conditions. Also, the total time for PIV setup and data acquisition is short reducing the cost of running expensive wind tunnel facilities. The ease and speed of data gathering can make detailed comparisons between experimental and numerical models possible with even a few seconds of captured data. The origins of PIV can be traced back to the simultaneous work of several researchers in the late 1970’s developing what was termed Laser-Speckle-Velocimetry (LSV). Initial experiments were rudimentary and proved the principle of quantitative flow mapping for very low velocity flows and a small field of view. The principle is very simple. Neutrally buoyant tracers are introduced into the flow just as with LDV but typically at higher 88
densities. A laser sheet is introduced into the region of the flow that is of interest and an image of the illuminated tracer particles recorded by a camera viewing normal to the light sheet. Pulsing the laser sheet two or more times allows the flow to transport the tracers between pulses the image of the displaced particles provides a record of the flow velocity. The local point displacement of the tracer particles across these photographically recorded images was originally measured from the fringe pattern generated by passing a low power laser beam through the film. Early examples of LSV utilized a high concentration of tracer particles such that they simulated a continuous surface within the fluid. Later research showed how a reduced density of particles achieves higher reliability, spatial resolution and accuracy. Resolving individual tracer particle images was an important requirement and thus the technique came to be known as Particle Image Velocimetry (PIV). Technical development of PIV has progressed scientifically and technically to provide higher fidelity and more extensive velocity data in less time. Motivated by a scientific interest in the kind of data that PIV provides and facilitated by technologies developed in other fields, PIV has developed in its approach and
has adopted several key enabling technologies. State-of-the-art CCD imaging sensors were utilized in the 1990’s and high frame-rate CMOS sensors in the last decade. Evaluation of PIV images is now performed digitally utilizing fast Fourier transform (FFT) based cross-correlation techniques to extract particle displacements and performed in real time on multiprocessor desktop computers. Compact, high-energy, double-pulsed, solidstate lasers provide precise and efficient illumination and timing of laser pulses onto successive images thereby permitting the direction of the flow to be resolved. Photographic recording did not permit this directly. The sensitivity of modern sensors and the high energy of PIV lasers mean that wind tunnels may be seeded lightly utilizing a submicron droplet generator using non-toxic, light oil which evaporates leaving no persistent trace in the measurement volume. Integrated software suites have been developed to provide image acquisition, image processing, post processing and data visualization. International special interest committees of experts such as EuroPIV, SmartPIV and PIV Challenge have established a degree of standardization in the methodology and analysis techniques in PIV as well as providing a forum for current and future development of the technique. Almost all flows of engineering interest have significant components of velocity in three dimensions. The basic PIV technique as described measures the projection of two components of velocity. Accurate measurement of the two components of velocity parallel to the light sheet and the component normal to the light sheet can be measured by utilizing two cameras each viewing the light sheet at an oblique angle to the laser sheet. The measured two components of displacements from each of the camera perspectives are combined by triangulation to resolve three components of velocity. This extension of the PIV technique is termed Stereo PIV: as well as resolving the third component of velocity, it also results in improved accuracy of the two components of velocity in the plane of the light sheet. Stereo PIV is still restricted to velocity measurement across a plane defined by the laser sheet. Recent advances in flow mapping have led to further enhancement of the basic PIV technique to provide three-component velocity data through a volume. By expanding a standard PIV pulsed laser to fill a volume within the flow the illuminated particle cloud is recorded onto three or four cameras. As with Stereo PIV, each camera views the illuminated particles from a different perspective. However, rather than measure WIND TUNNEL INTERNATIONAL | 2009