The configuration and energetics of the large-scale vortex structure are presented for quasi-periodic shedding in the turbulent wake of a finite (h=d ¼ 4) square-cross-section surface-mounted cylinder protruding from a thin boundary layer (d=h ¼ 0.18). The three-dimensional large-scale structure is educed from phase averaged x-y and x-z planar data measured with particle image velocimetry (PIV). Simultaneous measurements of the surface pressure difference on either side of the obstacle were used to phase-align the PIV planar measurements. The topology of the educed structures resembles alternating half-loops interconnecting close to the base plate. The time averaging of this unsteady structure gives rise to mean streamwise vortices akin to those presented in the literature for similar geometries. This topological analysis offers a contrasting interpretation of the mean streamwise vorticity, which has, otherwise, been presumed to originate from structures generated at the leading edge of the free-end. The dynamical significance of the resolved large scale structures and unresolved fluctuating kinetic energy in the wake is presented, either part being responsible for roughly half the mean kinetic energy. A discussion of turbulence production in light of the base flow that supplies it with energy is put forward.

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We experimentally investigate the three-dimensional wake behind a finite wall-mounted square cylinder at Re = 12 000 and aspect ratio of 4. Focus is placed on the base flow and oscillatory fluctuation. Time-resolved three-dimensional velocity fields are constructed from high-frame-rate particle image velocimetry (PIV) and simultaneously recorded surface pressure measurements. All three velocity components are resolved in a rectangular near-wake region by two orthogonal dense arrays of parallel PIV planes. A key enabler is a generalized phase average incorporating a slowly varying base flow, a variable oscillation amplitude and higher harmonics. These generalizations reduce the instantaneous residual 30 % below those of a traditional phase average. Moreover, the resolved variations reveal analytical constraints of the mean flow and oscillation levels, such as the mean-field paraboloid. The proposed methodology for generalized phase averaging and for construction of three-dimensional velocity fields from two-dimensional PIV data is applicable to a large class of turbulent flows with oscillatory dynamics.

The accurate prediction of mean flow fields with high degrees of curvature, adverse pressure gradients, and three-dimensional turbulent boundary layers typically present in centrifugal compressor stages is a significant challenge when applying Reynolds averaged Navier–Stokes turbulence modeling techniques. The current study compares the steady-state mixing plane predictions using four turbulence models for a centrifugal compressor stage with a tandem impeller and a “fish-tail” style discrete passage diffuser. The models analyzed are the k-ε model (an industry standard for many years), the shear stress transport (SST) model, a proposed modification to the SST model denoted as the SST-reattachment modification (RM), and the Speziale, Sarkar, and Gatski Reynolds stress model (RSM-SSG). Comparisons with measured performance parameters—the stage total-to-static pressure and total-to-total temperature ratios—indicate more accurate performance predictions from the RSM-SSG and SST models as compared to the k-ε and SST-RM models. Details of the different predicted flow fields are presented. Estimates of blockage, aerodynamic slip factor, and impeller exit velocity profiles indicate significant physical differences in the predictions at the impeller-diffuser interface. Topological flow field differences are observed: the separated tip clearance flow is found to reattach with the SST, SST-RM, and RSM-SSG models, while it does not with the k-ε model, a larger shroud separation at the impeller exit seen with the SST and SST-RM models, and core flow differences are in the complex curved diffuser geometry. The results are discussed in terms of the production and dissipation of k predicted by the various models due to their intrinsic modeling assumptions. These comparisons will assist aerodynamic designers in choosing appropriate turbulence models, and may benefit future modeling research.

The shedding process in the near wake of a surface-mounted, square cross-section cylinder of heightto-width aspect ratio 4 at a Reynolds number of 12,000 based on free-stream velocity and the obstacle width was investigated. The boundary layer thickness was 0.18 obstacle heights based on 99% free-stream velocity. The study is performed using planar high frame-rate particle image velocimetry synchronized with pressure measurements and hot-wire anemometry. Spatial cross-correlation, instantaneous phase relationships, and phase-averaged velocity data are reported. Two dominant vortex-shedding regimes are observed. During intervals of high-amplitude pressure fluctuations on the obstacle side faces, alternate formation and shedding of vortices is observed (regime A) similar to the von Kármán process. Regime B is characterized by two co-existing vortices in the obstacle lee throughout the shedding cycle and is observed within lowamplitude pressure fluctuation intervals. Despite the coexisting vortices in the base region, opposite sign vorticity is still shed out-of-phase downstream of this vortex pair giving rise to a staggered arrangement of counterrotating vortices downstream. While the probability of occurrence of Regime B increases toward the free end, the amplitude modulation remains coherent along the obstacle height. Conditionally phase-averaged reconstructions of the flow field are consistent with the spatial distribution of the phase relationships and their probability density function. Earlier observations are reconciled showing that the symmetric shedding of vortices is a rare occurrence.

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The complex flow field in turbomachinery poses numerous challenges for turbulence modeling. Herein, results of Laser Doppler Velocimetry (LDV) measurements of a full-scale aeroengine centrifugal compresser are used to validate typical design simulation results using a mixing plane and the k-ε, SST, or RSM-SSG turbulence closure models. Generally good agreement between simulation results and LDV measurements was found. The largest discrepancies were found in the near-wall regions: the predicted boundary layers were thicker and the flow more diffusive than measured. Important differences between the simulation results using different closures are discussed.