PIV

Background to PIV

PIV (Particle Image Velocimetry) is a wind tunnel testing technique which is both qualitative and quantitative. Particles are injected into the tunnel, illuminated by a laser and photographed using high speed cameras. Two photos are taken in quick succession and the two frames compared to track the movement of the particles. This allows the direction and speed of the particles to be extracted and analysed. Using this information, the entire flowfield can be reconstructed and the data interpreted. There are 3 types of PIV systems:

  1. 2D: The particles are tracked in a 2D plane

  2. Stereo: The particles are tracked in a 2D plane, but out of plane motion can also be captured

  3. Tomographic: A 3D volume is reconstructed

The experiment shown here looks at flow over a hemisphere in a 2D plane. This page will only give an overview of the basic steps completed, as a full instructional is not practical given the complexity of these experimental setups.

Experimental Setup and Calibration

The first main step in setting up a PIV experiment is to ensure that the laser is aligned. Two beams exit the laser box and they must be coincident with eachother. The laser system is very sensitive and any knocks or bumps can cause the mirrors or optics to move, resulting in a misaligned beam.

Once the beam has been aligned, the cameras need to be calibrated and the field of view checked. This is done using a calibration plate, which is a grid of white dots on a black background. The calibration plate should be installed where the experiment is expected to be place. Using Scheimflug adaptors and the manual zoom, the entire plate needs to be brought into focus.

Particle Tracking

To track the flowfield, particles need to be injected into the tunnel. This is done using a vaporiser and an approved oil solution. The seeding time is largely experiment and tunnel specific and should be determined on the test day.

Once the flow is seeded, images can be taken using the PIV software. This software completes all the necessary timing and triggers to appropriately capture the flow. The user has control over settings such as the laser current and general camera settings such as apeture.

Postprocessed Results

The flowfield can be analysed using software like Tecplot, which are typically used to analyse CFD results. The image to the left shows results of the reconstructed velocity field over the hemisphere, clearly showing the large separation zone on the leeward side. A small region of recirculation is also observed upstream of the hemisphere.