This study discusses a general framework to identify the unsteady features of a flow past an oscillating aerofoil in deep dynamic stall conditions. In particular, the work aims at demonstrating the advantages for the design process of the Spectral Proper Orthogonal Decomposition in accurately producing reliable reduced models of CFD systems and comparing this technique with standard snapshot-based models. Reynolds-Averaged Navier-Stokes system of equations, coupled with $k-\omega$ SST turbulence model, is used to produce the dataset, the latter consisting of a two-dimensional NACA 0012 aerofoil in the pitching motion. Modal analysis is performed on both velocity and pressure fields showing that, for vectored values, a proper tuning of the filtering process allows for better results compared to snapshot formulations and extract highly correlated coherent flow structures otherwise undetected. Wider filters, in particular, produce enhanced coherence without affecting the typical frequency response of the coupled modes. Conversely, the pressure field decomposition is drastically affected by the windowing properties. In conclusion, the low-order spectral reconstruction of the pressure field allows for an excellent prediction of aerodynamic loads. Moreover, the analysis shows that snapshot-based models better perform on the CFD values during the pitching cycle, while spectral-based methods better fit the loads’ fluctuations.

The present work provides a detailed numerical investigation of a turbopump for waterjet applications in cavitating conditions. In particular, the study focuses on the complexities of cavitation modelling, serving as a pivotal reference for future computational research, especially in off-design hydro-jet scenarios, and it aims to extend current model assessments of the existing methods, by disputing their standard formulations. Thus, a computational domain of a single rotor-stator blade passage is solved using steady-state Reynolds-Averaged Navier–Stokes equations, coupled with one-, two-, and four-equation turbulence models, and compared with available measurements, encompassing both nominal and thrust breakdown conditions. Through grid dependency analysis, a medium refinement with the Shear Stress Transport turbulence model is chosen as the optimal configuration, reducing either computational time and relative error in breakdown efficiency to 1%. This arrangement is coupled with a systematic study of the Zwart cavitation model parameters through multipliers ranging from ${10}^{-2}$ to ${10}^2$. Results reveal that properly tuning these values allows for a more accurate reconstruction of the initial phases of cavitation up to breakdown. Notably, increasing the nucleation radius reduces the difference between the estimated head rise and experimental values near breakdown, reducing the maximum error by 4%. This variation constrains vapour concentration, promoting cavitation volume extension in the passage. A similar observation occurs when modifying the condensation coefficient, whereas altering the vaporization coefficient yields opposite effects.

View Video Presentation: https://doi.org/10.2514/6.2022-0182.vid The present work investigates the three-dimensional dynamics which affects the evolution of dynamic stall over a pitching wing. The phenomenon is traced over a finite section of a spanwise periodically-extended infinite blade at Reynolds chord number of $Re_c=1.35\cdot10^5$ and a reduced pitching frequency of $k=0.1$. The geometry is obtained from NACA 0012 aerofoil unit chord extrusion. Modal analysis is conducted on the computed flow field through Spectral Proper Orthogonal Decomposition. SPOD inputs are gathered via Computational Fluid Dynamics results employing Delayed Detached Eddy Simulations with $k-\omega$ SST closure. The three-dimensional database is converted to a two-dimensional spatial distribution by exploiting the spanwise homogeneity of the system. In particular, the SPOD filter role is addressed by decomposing the velocity field. Thus, three different filter sizes are chosen to investigate the response of the modal set corresponding to the two dominant pairs. The results prove that stronger filtering actions are less effective over low-harmonic contents, already featuring high correlation levels around a single dominant frequency. Conversely, higher harmonics responses are more dependent on the filter dimension. The latter, in fact, enforces a temporal constraint that may introduce detrimental effects on the energetic optimality of the spatial modes, thus compromising the possibility to draw a direct connection with physical evolutions.

View Video Presentation: https://doi.org/10.2514/6.2023-1129.vid The present paper describes the performance of URANOS, a high-fidelity Direct and Large-Eddy Simulation Navier-Stokes solver specifically developed for wall-bounded compressible flows. The code combines cutting-edge numerical methods peculiarly developed for high-speed turbulent flow simulations and is tailored to modern high-performance computing systems due to MPI parallelization combined with multi-GPUs communication access. In particular, OpenACC directives are implemented for GPU enabling offloading computational loads onto accelerators cards, making URANOS an easily maintained solver as well as guaranteeing extreme flexibility and portability. The solver validation is detailed for a broad range of Mach numbers, from low-speed to compressible cases. In particular, velocity statistics and Reynolds stress components for canonical channel flow and turbulent boundary layer configurations obtained with URANOS well agree with high-quality DNS data. Computational performance and scaling properties are tested on several multi-GPU-equipped clusters. Thus, with URANOS, the scientific community can take advantage of a GPU-accelerated solver in dealing with fluid modeling for aerodynamics applications. The source code is available under a BSD license at the following link: \url{https://gitlab.com/fralusa/uranos_gpu}.