Boulbrachene, Khaled

To generate horizontal wind gusts in a classical wind tunnel, Wood, Breuer, and Neumann [A novel approach for artificially generating horizontal wind gusts based on a movable plate: The paddle,” J. Wind Eng. Ind. Aerodyn. 230, 105170 (2022)] developed a new wind gust generator denoted the “paddle.” The working principle relies on the partial blocking of the outlet of the wind tunnel nozzle by a plate that vertically moves into the free-stream. Based on laser-Doppler anemometer measurements of the velocity at only a few locations, the basic functionality of the device was proven. The objective of the present contribution is to numerically mimic the gust generator and the flow field induced by the paddle in the test section. Contrary to the single-point measurements, the three-dimensional time-resolved simulation delivers the entire flow field and thus allows to investigate all details of the generated gust. To describe the paddle motion, the immersed boundary method with a continuous and direct forcing approach is implemented into a finite-volume flow solver for large-eddy simulations. A uniform and a non-uniform distribution of the Lagrangian markers are investigated where the latter ensures that an excessive increase in the computational resources required can be avoided. The predictions allow to characterize the resulting flow features induced by the paddle in great detail. Furthermore, a comparison of the numerical and experimental results is carried out based on the time histories of the streamwise and vertical velocity components at certain positions showing a close agreement. Finally, the forces acting on the fluid by the moving paddle are evaluated.

The paper is concerned with numerical investigations on the effect of vertical wind gusts on airfoils in a parameter range relevant for Micro-Air Vehicles. Using a simplifiedsubstitute model instead of an elastic wing, a rigid but elastically mounted airfoil with two degrees of freedom (heave and pitch) is considered. The coupled problem is tackled by a partitioned fluid–structure interaction coupling scheme based on the large-eddy simulation (LES) technique and a rigid-body solver. In order to describe the effect of deterministic 1-cosine gusts of different gust lengths and gust strengths, the split velocity method (SVM) is incorporated into the simulation framework relying on the Arbitrary Lagrangian–Eulerian (ALE) formulation on temporally varying control volumes. First the flow fields and the corresponding aerodynamic forces during the direct airfoil–gust interaction are compared for a fixed and an elastically mounted airfoil. The intrinsic study on the elastic case includes nine different gust scenarios in the transitional Reynolds number regime in order to investigate the resulting flow fields and motion patterns and to answer the question whether limit-cycle oscillations (LCO) or even flutter can be induced. The results show that in seven of the studied cases, the airfoil–gust interaction leads to sustained heave and pitch oscillations of bounded amplitudes (i.e., LCO). Further investigations clarify that this can be physically attributed to the laminar separation taking place on the upper and lower surfaces of the airfoil. The two strongest gust cases, however, excite the airfoil to levels above its critical angle of attack and triggered a pitch-induced diverging flutter. An energy analysis of both characteristic scenarios (i.e., LCO and flutter) further elucidates the differences between both cases. The former case is driven by the heave motion, whereas the pitch DOF acts as an energy sink. Contrarily, in the case of flutter the pitching motion is powering the coupled system, whereas the heave motion dissipates energy.

The paper addresses the simulation of turbulent wind gusts hitting rigid and flexible structures. The purpose is to show that such kind of complex fluid–structure interaction (FSI) problems can be simulated by high-fidelity numerical techniques with reasonable computational effort. The main ingredients required for this objective are an efficient method to inject wind gusts within the computational domain by the application of a recently developed source-term formulation, an equally effective method to prescribe the incoming turbulent flow and last but not least a reliable FSI simulation methodology to predict coupled problems based on a partitioned solution approach combining an LES fluid solver with a FEM/IGA solver for the structure. The present application is concerned with a rigid and a membranous hemisphere installed in a turbulent boundary layer and impacted by wind gusts of different strength. The methodology suggested allows to inject the gusts in close vicinity of the object of interest, which is typically well resolved. Therefore, the launch and transport of the wind gust can be realized without visible numerical dissipation and without large computational effort. The effect of the gusts on the flow field, the resulting forces on the structure and the corresponding deformations in case of the flexible structure are analyzed in detail. A comparison between the rigid and the flexible case makes it possible to work out the direct reaction of the deformations on the force histories during the impact. Furthermore, in case of the flexible structure the temporal relationships between local or global force developments and the local deformations are evaluated. Such predictions pinpoint the areas of high stresses and strains, where the material is susceptible to failure.

The objective of the present paper is to revisit two well-known wind gust injection methods in a consistent manner and to assess their performance based on different application cases. These are the field velocity method (FVM) and the split velocity method (SVM). For this purpose, both methods are consistently derived pointing out the link to the Arbitrary Lagrangian Eulerian formulation and the geometric conservation law. Furthermore, the differences between FVM and SVM are worked out and the advantages and disadvantages are compared. Based on a well-known test case considering a vertical gust hitting a plate and a newly developed case taking additionally a horizontal gust into account, the methods are evaluated and the deviations resulting from the disregard of the feedback effect in FVM are assessed. The results show that the deviations between the predictions by FVM and SVM are more pronounced for the horizontal gust justifying the introduction of this new test case. The main reason is that the additional source term in SVM responsible for the feedback effect of the surrounding flow on the gust itself nearly vanishes for the vertical gust, whereas it has a significant impact on the flow field and the resulting drag and lift coefficients for the horizontal gust. Furthermore, the correct formulation of the viscous stress tensor relying on the total velocity as done in case of SVM plays an important role, but is found to be negligible for the chosen Reynolds number of the present test cases. The study reveals that SVM is capable of delivering physical results in contradiction to FVM. It paves the way for investigating further complex gust configurations (e.g., inclined gusts) and practical applications towards coupled fluid–structure interaction simulations of engineering structures impacted by wind gusts.