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Metrology – Understanding the Turbulence that Causes Jet Noise

Velocity measurements using high-repetition-rate stereoscopic PIV make it possible to observe the large scale dynamics of jet turbulence in a wind tunne, opening the way to progress in the understanding of jet noise generation.

For the first time, in the ONERA R4Ch wind-tunnel at Meudon, a series of PIV (Particle Image Velocimetry) data acquisitions was carried out at a high rate (2500 images per second), in the R4 Wind Tunnel at Meudon. With the stereoscopic installation it has been possible to measure the three components of the velocity in a plane transversal to the flow, thus allowing experimental access to the large scale dynamics of turbulence.

Large-scale axial velocity fluctuations in the jet core are represented on the cylinder. The yellow and blue structures are eddy structures of the smaller scale turbulence in th eshear layer. [Enlarge image ]

The jet flow studied in this wind tunnel has a high Reynolds number (200,000), which means that the turbulence studied is fully developed. The measurement of velocity and charactérization of turbulence through PIV under these conditions constitutes a real achievement since these measurements are usually performed at much lower Reynolds numbers.

This work, carried out within Samuel Davoust’s PhD thesis, provides a better understanding of the dynamic interactions between the various turbulence scales. These interactions are key elements in the generation of jet noise in aeronautic engines.