Breuer, Michael

Simultaneously measuring the fluid flow around a flexible structure and the resulting deformations during short-term yet highly dynamic flow events is the focus of this fluid-structure interaction (FSI) study. These scenarios occur when a wind gust impacts a flexible structure, leading to extreme loads and significant deflections. To mimic such gusts, a specifically designed wind gust generator is used within a wind tunnel featuring an open test section. A high-speed particle-image velocimetry system records the flow field, while the digital-image correlation technique captures the structural deformations. That allows to perform synchronized coupled fluid-structure measurements for a \mbox{T-structure} under wind gust load. The time-resolved measurements are repeated up to 104 times, allowing for phase-averaging of both the flow and the structural data, and to examine the convergence of the statistics. A comprehensive analysis of the instantaneous and phase-averaged data reveals that the flow field in the vicinity of the structure undergoes noticeable changes during the gust impact. The recirculation region behind the T-structures perceptibly increases when the gust hits the structure. A maximum deformation of about 10% of its height is observed during the highly dynamic gust event. Given (1) the availability of synchronously recorded data for both the fluid flow and the structure deformation, (2) the simplicity of the structure's geometry, and (3) the moderate Reynolds number of about 4 x 10^4, this case also serves as a well-suited benchmark test case for evaluating simulation methodologies for strongly coupled, highly dynamic FSI problems.

The Direct Simulation Monte Carlo (DSMC) method was widely used to simulate low density gas flows with large Knudsen numbers. However, DSMC encounters limitations in the regime of lower Knudsen numbers (Kn<0.05). In such cases, approaches from classical computational fluid dynamics (CFD) relying on the continuum assumption are preferred, offering accurate solutions at acceptable computational costs. In experiments aimed at imaging aerosolized nanoparticles in vacuo a wide range of Knudsen numbers occur, which motivated the present study on the analysis of the advantages and drawbacks of DSMC and CFD simulations of rarefied flows in terms of accuracy and computational effort. Furthermore, the potential of hybrid methods is evaluated. For this purpose, DSMC and CFD simulations of the flow inside a convergent–divergent nozzle (internal expanding flow) and the flow around a conical body (external shock generating flow) were carried out. CFD simulations utilize the software OpenFOAM and the DSMC solution is obtained using the software SPARTA. The results of these simulation techniques are evaluated by comparing them with experimental data (1), evaluating the time-to-solution (2) and the energy consumption (3), and assessing the feasibility of hybrid CFD-DSMC approaches (4).

In this study, a recently developed data-driven model for the collision-induced agglomerate breakup (CHERD 195, 2023) is evaluated. It is especially intended for Euler–Lagrange simulations of flows with high mass loadings, where coupled CFD–DEM predictions are too expensive. Therefore, a surrogate model relying on the hard-sphere approach in which agglomerates are represented by effective spheres was developed. Based on a variety of DEM simulations, artificial neural networks were trained to predict the post-collision number of arising fragments, their size distribution and their velocities. In the present contribution, the agglomerate collision model is assessed using the particle-laden flow through a T-junction. Since two fluid streams with agglomerates are injected at both opposite ends, the setup is particularly suitable for investigating breakage caused by collisions. Two flow configurations (laminar flow at Re = 130 and turbulent flow at Re = 8000) and two different powders (primary particle diameter of 0.97 and 5.08 micrometers) are taken into account. The latter allows to study the influence of the strength of the agglomerates on the collision-induced breakage. The laminar case offers the possibility to evaluate the effect of the collision angle in detail. The collision-induced breakage proves to be the most dominant deagglomeration mechanism in both the laminar and turbulent flow scenario. Nevertheless, the role of the fluid stresses and especially the drag stress becomes more prominent in the turbulent case, while in the laminar flow their effects are negligible.

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.

In a former study carried out by Wood et al. (2022) and Wood and Breuer (2023) a novel approach for the generation of artificial wind gusts in a wind tunnel setup was presented denoted “the paddle”. The device generates wind gusts by dynamically blocking the nozzle outlet area with a rigid plate inducing a sudden rise and drop of the velocity signal. This procedure leads to highly reproducible wind gusts within a certain region of the test section depending on the kinematic settings of the paddle. However, outside of this restricted region the flow is dominated by a highly fluctuating flow regime. This effect limits the space of the test section, which can be used for experimental investigations. In order to reduce the negative effects of the paddle, a modified design of the gust generator is presented in this contribution. For this purpose, a second wind tunnel is designed using the same automation equipment as the paddle. However, the new setup comprises a nozzle with a fully movable upper contour in order to generate smoother wind gusts. The working principle and a comarison between the gusts generated by the paddle and the new device are presented based on a closely matched motion pattern of the servo driver unit and an identical blocking ratio. In summary, the adaptive nozzle reduces the drawbacks of the paddle such as the large flow separation and the velocity undershoot on the falling flank of the gust. Additionally, previously not attainable gust shapes can be generated leading to a greater variety of flow conditions for experimental studies on fluid-structure interaction (FSI).

The interaction of several instabilities and the influence of free-stream turbulence on laminar-turbulent transition on a 20% thick wind-turbine blade section with a laminar separation bubble (LSB) are investigated with wall-resolved large-eddy simulations (LES). Turbulence intensities (TI) of 0%, 2.2%, 4.5%, 8.6%, and 15.6% at chord Reynolds number 105 are considered. Linear receptivity occurs for the most energetic disturbances; high-frequency perturbations are excited via non-linear mechanisms for TI >= 8:6%. Unstable Tollmien–Schlichting (TS) waves appear in the inflectional flow region for TI <= 4:5%, shifting to inviscid Kelvin–Helmholtz (KH) modes upon separation and forming spanwise rolls. Sub-harmonic secondary instability occurs for TI = 0%, with rolls intertwining before transition. Streaks spanwise modulate the rolls and increase their growth rates with TI for TI <= 4:5%, reducing separation and shifting transition upstream. The TI = 4:5% case presents the highest perturbations, leading to the smallest LSB and most upstream transition. Earlier inception of TS/KH modes occurs on low-speed streaks, inducing premature transition. However, for TI = 8:6%, the effect of the streaks is to stabilize the attached mean flow and front part of the LSB. This occurs due to the near-wall momentum deficit alleviation, leading to the transition delay and larger LSB than TI = 4:5%. This also suppresses separation and completely stabilizes TS/KH modes for TI = 15:6%. Linear stability theory predicts well the modal evolution for TI <= 8:6%. Optimal perturbation analysis accurately computes the streak development upstream of the inflectional flow region but indicates higher amplification than LES downstream due to the capture of low-frequency, oblique modal instabilities from the LSB. Only low-amplitude [O(1%)] streaks displayed exponential growth in the LES since non-linearity precludes the appearance of these modes.

The aerodynamic properties of a high-slenderness finned projectile with and without a threadlike roughness element on its shaft are evaluated by force and moment balance measurements in a supersonic blow-down tunnel. The test conditions cover a Mach number range from 2.5 to 4.1 at diameter-based Reynolds numbers in the order of 8 × 10^5. The forebody axial force is increased by approximately 15 % by the roughness element at zero angle of attack. Further measurements at non-zero angles of attack show that this axial force increase growths over-proportionately with the angle of attack. Whereas a minor influence by the roughness element is observed for the normal force, the pitching moment is either decreased by about 7 % or increased by about 13 % depending on the flow condition and angle of attack. Flow visualization by a high-resolution schlieren setup gives insight into the flow physics responsible for these phenomena. The schlieren setup also enables the detection of the laminar-turbulent boundary layer transition.