A Stereo-PIV system installed inside a High Reynolds Number Test Model for measuring wall shear stresses from the velocity gradients in the viscous sublayer

Owing to the technical limitations, very little experimental data is available for the wall shear stresses generated by the high Reynolds number flows around bodies with varying surface pressure distributions. Cases involving 3D flow separation are particularly challenging due to the large variation in the magnitude and direction of wall stresses. This presentation introduces a high-resolution stereo PIV system that is installed inside a model to measure the wall shear stresses from the velocity gradients in the viscous sublayer. This system is designed to be positioned inside a 3 m long, inclined, 6:1 prolate spheroid that will be tested at Reynolds numbers varying between 1.5 x 10⁶ to 5 x 10⁷. The sample area is 10 x 8 mm. The flow is seeded locally by slowly injecting 2 μm particles from a series of 100 μm diameter micro-injectors located upstream of the sample area. The light source is an external laser whose beam is transmitted to the sample area via an optical fiber, that illuminates the near wall sample plane through a small window installed on the surface. Images are recorded by a pair of 4000 x 3000 pixels CMOS cameras whose data are multiplexed optically and transmitted to an external data acquisition system. These cameras view the sample area via a curved window matched with the body geometry using a transparent polymer that has the same refractive index as water. Data analysis involves ensembled correlation using an interrogation window size of 10 x 10 μm. Sample data and associated uncertainties will be discussed.