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The ever-increasing computational power has shifted direct numerical simulations towards higher Reynolds numbers and large eddy simulations towards industrially-relevant flow scales. However, this increase in both temporal and spatial resolution has severely increased the computational cost of model order reduction techniques. Reducing the full data set to a smaller subset in order to perform reduced-order modeling (ROM) may be an interesting method to keep the computational effort reasonable. Moreover, non-tomographic particle image velocimetry measurements obtain a 2D data set of a 3D flow field and an interesting research question would be to quantify the difference between this 2D ROM compared to the 3D ROM of the full flow field. To provide an answer to both issues, the aim of this study was to test a new method for obtaining POD basis functions from a small subset of data initially and using them afterwards in the ROM of either the complete data set or the reduced data set. Hence, no new method of ROM is presented, but we demonstrate a procedure to significantly reduce the computational effort required for the ROM of very large data sets and a quantification of the error introduced by reducing the size of those data sets. The method applies eigenvalue decomposition on a small subset of data extracted from a full 3D simulation and the obtained temporal coefficients are projected back on the 3D velocity fields to obtain the 3D spatial modes. To test the method, an annular jet was chosen as a flow topology due to its simple geometry and the rich dynamical content of its flow field. First, a smaller data set is extracted from the 2D cross-sectional planes and ROM is performed on that data set. Secondly, the full 3D spatial structures are reconstructed by projecting the temporal coefficients back on the 3D velocity fields and the 2D spatial structures by projecting the temporal coefficients back on the 2D velocity fields. It is shown that two perpendicular lateral planes are sufficient to capture the relevant large-scale structures. As such, the total processing time can be reduced by a factor of 136 and up to 22 times less RAM is needed to complete the ROM processing.

Stereoscopic Particle Image Velocimetry (S-PIV) measurements are performed to analyse the flow field in a sudden expansion with an expansion ratio of 4.9:1. The velocity profile in the inlet tube is measured using Laser Doppler Anemometry (LDA) and the Reynolds number (Re) is varied from 3150 till 63300. The time-averaged flow field is found to be axisymmetric and has a jet-like velocity profile near the nozzle with a spreading rate comparable to free round jets. Despite the axisymmetric nature of the mean flow field, it contains unsteady coherent structures, which are analysed using both Spectral Proper Orthogonal Decomposition (SPOD) and phase averaging. It is found that the instantaneous flow field is asymmetric and it precesses around the central axis of the geometry. The structure of this precession is similar for all Reynolds numbers investigated and the Strouhal number increases with increasing Reynolds number till Re= 26000, after which it reaches a constant value of around 3 × 10−3. These precessing structures were previously only reported for swirling sudden expansion flows or flows in dedicated free jet nozzles, but this study shows they also exist in confined non swirling sudden expansion flows.

Stereoscopic Particle Image Velocimetry (S-PIV) measurements are performed to analyse the flow field in a sudden expansion with an expansion ratio of 4.9:1. The velocity profile in the inlet tube is measured using Laser Doppler Anemometry (LDA) and the Reynolds number (Re) is varied from 3150 till 63300. The time-averaged flow field is found to be axisymmetric and has a jet-like velocity profile near the nozzle with a spreading rate comparable to free round jets. Despite the axisymmetric nature of the mean flow field, it contains unsteady coherent structures, which are analysed using both Spectral Proper Orthogonal Decomposition (SPOD) and phase averaging. It is found that the instantaneous flow field is asymmetric and it precesses around the central axis of the geometry. The structure of this precession is similar for all Reynolds numbers investigated and the Strouhal number increases with increasing Reynolds number till Re =26000, after which it reaches a constant value of around 3 × 10 −3 . These precessing structures were previously only reported for swirling sudden expansion flows or flows in dedicated free jet nozzles, but this study shows they also exist in confined nonswirling sudden expansion flows.