In PIV, a flow field is measured through the use of optical imaging techniques.
The first component to PIV is a flow field of a liquid whose properties are known, such as a jet of air. Once the flow field is generated, "seeder" particles - small droplets of a fluid whose properties are similar to those of the main fluid – are evenly mixed into the flow. These particles act as trackers, similar to how a rubber ducky in a bathtub can show the current within the tub. A laser is shone through a prism, refracted into a plane, and aimed through the area of interest within the flow field. The laser light refracts off the seeder particles within the flow field, which is then picked up by a high-speed camera positioned outside the plane of the laser, pointing inwards. The laser and camera are synchronized in short and rapid pulses to reduce blurriness, and these pulses often come in pairs in rapid succession (like a heartbeat). The data from the camera is sent to a computer, which performs various computations to create a velocity field for every pair of images.
The first thing the computer does when it is passed the data images is divide them into the pairs that correspond to each pair of pulses. Then, each image is divided into sections, called "windows," each of which will later on be assigned a velocity vector. The computer compares each window in the first image of each pair with the corresponding window in the second image in order to find this vector.
To do this, the computer finds the correspondence between the first window and each potential translation the particles in the window could've followed to reach the second window - called "cross-correlation." The correspondence is a numerical value that can be plotted against the x and y translation values, so that each potential movement in both dimensions has a correspondence value. The highest correspondence value will correlate to the movement that will cause the particles in the first image to line up the best with those in the second image after being translated by that movement. This movement is then combined with the known amount of time that passed between the frames in the pair, creating a velocity vector that can be assigned to that window in that frame.
This process is then repeated for each pair of windows within the pair of frames, and then again for each pair of frames within the data set. This creates a velocity field for each pair of frames.
Advantages and disadvantages of PIV
Unlike other methods, which use a measurement device placed within the flow to measure the local velocity, PIV measures the flow nonintrusively. This prevents the flow from being disturbed by measurement devices and increases the accuracy of the data.
Similarly, other methods which use a measurement device placed within the flow can only measure the local velocity, meaning they need multiple devices in order to create a flow field. This can result in a significantly disturbed flow. PIV can measure the velocity field of a two-dimensional area without additional disruption.
While seeder particles with similar properties to the main fluid should follow the fluid accurately, some divergence is expected. The closer the properties to the main fluid, the higher the accuracy.
Standard PIV is unable to measure flow movement in the direction perpendicular to the plane of the laser. This can be addressed by using two cameras, a technique called Spectroscopic PIV.
Example of PIV data
Raw PIV data. Seeder particles are visible as white specks, and the red on the right is the jet tube.
Data with velocity vectors added. Green vectors are those selected automatically by the computer's cross-correlation algorithm; orange vectors are the computer's backup choices after the incorrect cross-correlation peak was rejected manually.
Data with velocity vectors with an overlay. The overlay displays the velocity vector magnitude, with blue being the minimum and yellow being the maximum. Various other properties can also be displayed, such as the horizontal and vertical components of velocity, the velocity direction, and the vorticity.
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