Full-field surface pressure reconstruction using deflectometry and the virtual fields method

Abstract

This thesis presents a technique for the reconstruction of full-field surface pressure distributions with low differential amplitudes. The method is demonstrated in two different setups with air jets impinging on flat plates. Surface deformations were obtained using deflectometry, a highly sensitive technique for slope measurement. The surface slopes in combination with the plate material constitutive mechanical parameters were used as input for pressure reconstructions with the Virtual Fields Method (VFM), which is an application of the principle of virtual work. Both static and dynamic pressure distributions were reconstructed in full-field with this technique. Results were compared with pressure transducer measurements at discrete points. The observed pressure amplitudes were between O(1) Pa – O(100) Pa. The spatial extent of the investigated flow structures was O(1) mm – O(10) mm. Dynamic pressure information was extracted from time-resolved deflectometry data using temporal band-pass filters or phaselocked measurements. Dynamic Mode Decomposition (DMD) was used on time-resolved data to identify relevant spatial information that correspond to specific frequencies. Despite the low differential amplitudes, dynamically important spatio-temporal events could be observed. Full-field measurements of such small-scale, low-amplitude pressure events are not possible with established pressure measurement techniques. A finite element model was employed in combination with artificial grid deformation to assess the uncertainty of the pressure reconstructions. Finally, challenges and limitations in achieving the metrological performance for resolving the observed surface slopes of O(0.1) mm km^-1 – O(10) mm km^-1 as well as in the pressure reconstruction approach are described and strategies for future applications are discussed.

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Identifiers

ORCID for Rene Kaufmann: orcid.org/0000-0001-6248-7027

ORCID for Bharathram Ganapathisubramani: orcid.org/0000-0001-9817-0486

ORCID for Fabrice Pierron: orcid.org/0000-0003-2813-4994

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