Fundamental and Experimental Aerodynamics

2D2C PIV
Two-component displacement in a plane

Figure 1: Typical experimental setup and sample recorded images

The key idea underlying PIV is to measure the velocity of tracer particles present in the flow, which are chosen small enough to be passively entrained (see the Equipment page for typical characteristics of the seedings used in our wind-tunnels).

To perform 2D2C PIV, a double-pulsed laser is supplemented with adapted optics to illuminate the seeded flow by a light sheet, at two instants separated by a very short time interval Dt. A PIV camera usually images the light sheet at a perpendicular point of view, and includes a diaphragm in order to record the diffraction images of the particles at the two instants. With this setup, one obtains sample particle images at t and t+Dt, as illustrated Figure 1 above. The frequency at which such a pair of images may be recorded defines the repetition rate of the setup. While PIV has long been restricted to low frequencies (conventional PIV, typical repetition rate 10 Hz), the recent advent of high-speed system now opens the way to time sampling of turbulent flows, with maximal repetition rates of around 20 kHz. 

Figure 2: Local cross-correlation between images at t and t+Dt

Once acquired, each image pair is processed by a dedicated software. To estimate the local motion, one of the most frequently used approaches consists in finding the local maximum of cross-correlation between the images, i.e. to match a pattern of particles in both images. This is done by dividing the images into subzones called interrogation windows. A sketch of the search for the maximum of cross-correlation is shown in Figure 2.

A large number of the existing PIV softwares solves this problem by an exhaustive computation of the correlation map, using FFT [1]. The recent software developed by the ALPIV group, FOLKI-SPIV, relies on a different algorithmic choice, leading to a highly parallel structure ideal for GPU programming, with a state-of-the-art accuracy. Read more about FOLKI-SPIV here.

Example: Shock-induced separation in the S8Ch wind-tunnel

Shock-induced separation is a frequently encountered phenomenon in external and internal transonic flows, which may cause detrimental unsteadiness and dramatically reduce the aerodynamical performances of, e.g., an airfoil or an air intake. This has motivated important research efforts at DAFE, aiming at understanding and controlling the phenomenon. Recently, a PIV campaign was conducted in the S8Ch wind-tunnel, whose lower wall is equipped with a bump in order to fix the shock location (see Figure 3).



Figure 3: Test-section of the S8Ch wind-tunnel, equipped with vortex generators

In this study, extensive 2D2C PIV measurements were performed in order to gain insight into the physics of the interaction between shock and boundary layer, as well as to characterize the action of vortex generators placed upstream of the shock, which may help decrease the spatial extent of the separation [2].


Figure 4: Mean velocity field over the bump (from Sartor et al., [2]).


Figure 5: Turbulent kinetic energy over the bump (from Sartor et al., [2]).

References

[1] Raffel M., Willert C., Wereley S., Kompenhans J., Particle image velocimetry: a practical guide, Springer-Verlag, 2007.
[2] Sartor F., Losfeld G., Bur R., Characterization by PIV of the effect of vortex generators in a transonic separated flow, submitted to the AIAA J.