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Research Papers: Flows in Complex Systems

J. Fluids Eng. 2018;140(12):121101-121101-10. doi:10.1115/1.4040225.

This work presents an application of the partially averaged Navier–Stokes (PANS) equations for an external vehicle flow. In particular, the flow around a generic truck cabin is simulated. The PANS method is first validated against experiments and resolved large eddy simulation (LES) on two static cases. As a consequence, PANS is used to study the effect of an active flow control (AFC) on a dynamic oscillating configuration. The oscillation of the model represents a more realistic ground vehicle flow, where gusts (of different natures) define the unsteadiness of the incoming flow. In the numerical study, the model is forced to oscillate with a yaw angle 10 deg > β > –10 deg and a nondimensional frequency St = fW/Uinf = 0.1. The effect of the periodic motion of the model is compared with the quasi-static flow condition. At a later stage, the dynamic configuration is actuated by means of a synthetic jet boundary condition. Overall, the effect of the actuation is beneficial. The actuation of the AFC decreases drag, stabilizes the flow, and reduces the size of the side recirculation bubble.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(12):121102-121102-15. doi:10.1115/1.4040228.

The performance of the transition models on three-dimensional (3D) flow of wings with aspect ratios (AR) of 1 and 3 at low Reynolds number was assessed in this study. For experimental work; force measurements, surface oil and smoke-wire flow visualizations were performed over the wings with NACA4412 section at Reynolds numbers of 2.5 × 104, 5 × 104, and 7.5 × 104 and the angles of attack of 8 deg, 12 deg, and 20 deg. Results showed that the AR had significant effects on the 3D flow structure over the wing. According to the experimental and numerical results, the flow over the wing having lower ARs can be defined with wingtip vortices, axial flow, and secondary flow including spiral vortex inside the separated flow. When the angle of attack and Reynolds number was increased, wing-tip vortices were enlarged and interacted with the axial flow. At higher AR, flow separation was dominant, whereas wing-tip vortices suppressed the flow separation over the wing with lower AR. In the numerical results, while there were some inconsistencies in the prediction of lift coefficients, the predictions of drag coefficients for two transition models were noticeably better. The performance of the transition models judged from surface patterns was good, but the k–kLω was preferable. Secondary flow including spiral vortices near the surface was predicted accurately by the k–kLω. Consequently, in comparison with experiments, the predictions of the k–kLω were better than those of the shear stress transport (SST) transition.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2018;140(12):121103-121103-15. doi:10.1115/1.4040232.

This paper presents a hydrodynamic study of a propeller turbine runaway based on flow simulations and measurements results. Runaways are considered one of the most structurally damaging conditions a hydraulic turbine may encounter. This study focuses specifically on the flow dynamics in the runner and draft tube of a model propeller turbine installed on the test stand of the Hydraulic Machines Laboratory of Laval University, Quebec, Canada. The controlled runaway event reproduced on the test stand was part of a larger study into transient flow conditions. Besides global performance parameters, the measurements also featured 31 pressure transducers mounted on two runner blades. Using those measurements' results, both as boundary conditions and for validation purposes, unsteady Reynolds-averaged Navier-Stokes simulations of the entire turbine were performed. Those simulations featured transient boundary conditions to reproduce discharge and runner speed variations. Using wavelet transforms analysis, the evolution of the dominant pressure fluctuations is tracked in both, the measurements and the simulations. The wavelet analysis revealed the presence of pressure fluctuations with frequencies at a fraction of the runner rotation speed. Numerical results revealed that a vortex structure in the draft tube, similar to a part-load vortex rope, is the cause of those high-pressure fluctuations in the runner. A slight flow separation is observable on the pressure side of the blades but does not alter the flow in the inter-blade channels. Comparisons between experimental and numerical data also outline the limits of the methodology related, among others, with the imposition of strict boundary conditions.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2018;140(12):121201-121201-16. doi:10.1115/1.4040301.

In the present study, the deformation of a droplet is numerically modeled by considering the dynamic model for electric charge migration at the drop interface under the effect of a uniform electric field. The drop and its ambient are both considered behaving as leaky dielectric fluids. Solving the charge conservation equation at the interface, which is the most important part of this study, the effect of conduction and convection of charges on different deformation modes will be explored. In this work, the interface is followed by the level set method and the ghost fluid method (GFM) is used to model the jumps at the interface. Physical properties are also chosen in a way that solving the charge conservation equation becomes prominent. The small drop deformation is investigated qualitatively by changing various effective parameters. In cases, different patterns of charges and flows are observed indicating the importance of electric charges at the interface. It is also shown that the transient behavior of deformation parameter can be either a monotonic or a nonmonotonic approach toward the steady-state. Moreover, large drop deformations are studied in different ranges of capillary numbers. It will be shown that for the selected range of physical parameters, considering the dynamic model of electric charges strongly affects the oblate deformation. Nevertheless, for the prolate deformation, the results are approximately similar to those obtained from the static model.

Commentary by Dr. Valentin Fuster

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