Flow over a sphere at elevated pressures: An analysis of the near-wake using spectral proper orthogonal decomposition

This study advances the understanding of temporal dynamics in flow over a sphere in the near-wake by applying spectral proper orthogonal decomposition (SPOD) to time resolved particle image velocimetry experimental data collected at elevated pressures (3–6 MPa) over a wide range of Reynolds numbers (19 200–49 500). From the acquired velocity vector fields, the statistical values of mean and fluctuating velocities were computed along with their associated uncertainties. The work uniquely characterizes flow structures using SPOD spectra, showing that as Reynolds numbers increase, energetic peaks shift to lower frequencies, highlighting larger, slower-evolving structures. At higher Reynolds numbers, spectral peaks converge, suggesting previously distinct peaks merge into a broader one. Visualizing SPOD modes revealed large eddies at low frequencies and smaller structures in the wake at high frequencies. The investigation covers various aspects, including point-wise spectral analysis of the Reynolds decomposed streamwise and spanwise velocity components inside and outside the wake region using power spectral density, flow reconstruction using SPOD modes, and fills gaps in the literature regarding the relationship of the flow separation angle at these Reynolds numbers. The findings support Kolmogorov's theory of turbulence and offer insights into the oscillatory nature and energy dynamics of flow around a sphere. This characterization of flow dynamics at elevated pressures fills a gap in the literature, offering insights that enhance understanding of turbulence in engineering applications like pebble bed gas-cooled reactors, providing the nuclear industry with valuable data at prototypical conditions through advanced measurement and analysis techniques.