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1. Dominant flow features in the wake of a wind turbine at high Reynolds numbers

   https://doi.org/10.1063/5.0086746  

   Piqué, A. ; Miller, M. A. ; Hultmark, M. ( May 2022 , Journal of Renewable and Sustainable Energy)

   Dominant flow features in the near and intermediate wake of a horizontal-axis wind turbine are studied at near field-scale Reynolds numbers. Measurements of the axial velocity component were performed using a nano-scale hot-wire anemometer and analyzed using spectral methods to reveal the extent and evolution of the flow features. Experiments were conducted at a range of Reynolds numbers, of [Formula: see text], based on the rotor diameter and freestream velocity. Five different downstream locations were surveyed, between [Formula: see text], including the near wake, transition to the intermediate wake, and the intermediate wake. Three dominant wake features are identified and studied: the tip vortices, an annular shear layer in the wake core, and wake meandering. The tip vortices are shown to have a broadband influence in the flow in their vicinity, which locally alters the turbulence in that area. It is shown that shedding in the wake core and wake meandering are two distinct and independent low frequency features, and the wake meandering persists into the intermediate wake, whereas the signatures of the core shedding vanish early in the near wake. 

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2. Toward on-demand modulation and annihilation of aeroelastic limit cycle oscillations with dynamic upstream disturbance generator

   Hughes, Michael ; Gopalarathnam, Ashok ; Bryant, Matthew ( January 2021 , Proceedings of the Online Symposium on Aeroelasticity, Fluid-Structure Interaction, and Vibrations)

   Periodic upstream flow disturbances from a bluff body have recently been shown to be able to modulate and annihilate limit cycle oscillations (LCOs) in a downstream aeroelastic wing section under certain conditions. To further investigate these phenomena, we have implemented a controllable wind tunnel disturbance generator to enable quantification of the parameter ranges under which these nonlinear interactions can occur. This disturbance generator, consisting of a pitch-actuated cylinder with an attached splitter plate, can be oscillated to produce a von Karman type wake with vortex shedding frequency equal to the oscillation frequency over a range of frequencies around the natural shedding frequency of the cylinder alone. At vortex shedding frequencies away from the LCO frequency of the wing, forced oscillations were observed in the wing, but the wing did not enter self-sustaining LCOs. However, when disturbances were introduced near the LCO frequency, the initially static downstream wing entered self-sustaining oscillations in the presence of the incoming vortices, and these LCOs persisted when the disturbance generator was stopped. Annihilation of the wing LCOs was also observed disturbance vortices were introduced upstream of the wing in LCO. 

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3. Coherent motions in a turbulent wake of an axisymmetric bluff body

   https://doi.org/10.1017/jfm.2023.231  

   Zhang, Fengrui ; Peet, Yulia T. ( May 2023 , Journal of Fluid Mechanics)

   The wake flow past an axisymmetric body of revolution at a diameter-based Reynolds number$Re=u_{\infty }D/\nu =5000$is investigated via a direct numerical simulation. The study is focused on identification of coherent vortical motions and the dominant frequencies in this flow. Three dominant coherent motions are identified in the wake: the vortex shedding motion with the frequency of$St=fD/u_{\infty }=0.27$, the bubble pumping motion with$St=0.02$, and the very-low-frequency (VLF) motion originated in the very near wake of the body with the frequency$St=0.002$–$0.005$. The vortex shedding pattern is demonstrated to follow a reflectional symmetry breaking mode, whereas the vortex loops are shed alternatingly from each side of the vortex shedding plane, but are subsequently twisted and tangled, giving the resulting wake structure a helical spiraling pattern. The bubble pumping motion is confined to the recirculation region and is a result of a Görtler instability. The VLF motion is related to a stochastic destabilisation of a steady symmetric mode in the near wake and manifests itself as a slow, precessional motion of the wake barycentre. The VLF mode with$St=0.005$is also detectable in the intermediate wake and may be associated with a low-frequency radial flapping of the shear layer.

    

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4. Volumetric measurement of a Newtonian fluid flow through three-dimensional porous media using Lagrangian particle tracking (Shake-the-Box) technique

   https://doi.org/10.1063/5.0141535  

   Bagheri, Maryam ; Mirbod, Parisa ( April 2023 , Physics of Fluids)

   This work experimentally investigates the pressure-driven flow of a pure Newtonian fluid through three-dimensional (3D) porous media models. The porous media model consists of square arrays of rods that also could be interpreted as a periodic tandem rod arrangement. We employed a time-resolved three-dimensional particle tracking velocimetry (3D Shake-the-Box) technique for a range of Reynolds numbers [Formula: see text] to observe flow structures and vortex formation between the rods in porous media structures with different porosities of [Formula: see text] which corresponds to the spacing ratio of [Formula: see text], where L is the distance between the centers of the rods, and D is the diameter of the rods. For all the examined cases, we further analyzed the effect of the Reynolds number and the spacing ratio on the instantaneous and averaged patterns of velocity, vorticity, and the other flow parameters after obtaining the two-dimensional velocity fields using the bin-averaging method. We observed both symmetrical and asymmetrical patterns of structure and recirculation regions between the rods depending on the Reynolds number and spacing ratio. Increasing the Reynolds number reduced the symmetrical patterns of flow structures with respect to the centerline of the gap region, while the spacing ratio was randomly affecting the symmetry degree. Vortex shedding was considerable for the two examined high Reynolds numbers of Re = 444 and Re = 890 behind the upstream rod as the porosity increased. The backward movement of the reattachment point has been observed by increasing the Reynolds number. 

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5. Machine Learning to Classify Vortex Wakes of Energy Harvesting Oscillating Foils

   https://doi.org/10.2514/1.J062091  

   Ribeiro, Bernardo Luiz ; Franck, Jennifer A. ( March 2023 , AIAA Journal)

   A machine learning model is developed to establish wake patterns behind oscillating foils for energy harvesting. The role of the wake structure is particularly important for array deployments of oscillating foils since the unsteady wake highly influences the performance of downstream foils. This work explores 46 oscillating foil kinematics, with the goal of parameterizing the wake based on the input kinematic variables and grouping vortex wakes through image analysis of vorticity fields. A combination of a convolutional neural network with long short-term memory units is developed to classify the wakes into three classes. To fully verify the physical wake differences among foil kinematics, a convolutional autoencoder combined with [Formula: see text]-means++ clustering is used to reveal four wake patterns via an unsupervised method. Future work can use these patterns to predict the performance of foils placed in the wake and build optimal foil arrangements for tidal energy harvesting.

    

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