Breuer, Michael

The authors regret the presence of a typographical error in Eq. (1) of De Nayer et al. (2022). As correctly introduced in De Nayer and Breuer (2020), the source term used to inject the wind gust should be analogous to a mass flow rate multiplied by the total flow velocity [Formula presented] and not only the gust velocity component [Formula presented]. Therefore, the source term written in the Cartesian basis [Formula presented] should read: [Formula presented] The authors would like to apologize for any inconvenience caused. CRediT authorship contribution statement G. De Nayer: Writing – original draft. M. Breuer: Resources, Supervision. K. Boulbrachene: Writing – original draft.

In this study, agglomerate breakage in homogeneous isotropic turbulence is investigated using particle-resolved direct numerical simulations. Single agglomerates composed of 500 monodisperse spherical particles are considered, and their interaction with the turbulent flow is resolved through an immersed boundary method coupled with a soft-sphere discrete element model. A range of Reynolds numbers and cohesion levels is examined to assess their influence on the breakup behavior. Detailed insights into the underlying breakage mechanisms are provided through the analysis of local flow structures and fluid stresses. Strain-dominated regions are identified as the primary contributors to the onset and propagation of particle erosion. The benefits of the particle-resolved simulation framework in capturing these physical processes in detail are demonstrated. The predicted fragment size distributions and breakup modes are analyzed leading to the outcome that erosion-driven breakage is the dominating mechanism. The time evolution of the fragment number and the main agglomerate structure is quantified. The breakage rate is evaluated and its dependence on the modified adhesion number is established, showing a power-law decay that agrees with general trends reported in the literature. In addition, the analysis of the fragment ejection direction reveals a strong alignment with the local deformation plane spanned by the most extensional and compressive strain-rate eigenvectors, indicating that breakage results from the interplay between flow stretching and compression. The results contribute to the development of physics-informed breakup kernels for use in efficient but less-detailed simulation approaches, such as point-particle Euler–Lagrange predictions with agglomerates represented by effective spheres or Euler–Euler simulations.

The recently developed wind gust generator, the adaptive nozzle (Wood and Breuer, 2025) , features a nozzle with a fully rotatable upper contour, enabling a smooth gust generation with low unwanted flow disturbances. While preserving the underlying gust-generation principle of its predecessor, the new design significantly reduces pressure losses caused by flow blockage and preserves the original horizontal trajectory of the flow along the streamwise direction. While experiments have validated the improved design, a comprehensive numerical analysis is crucial to resolve the three-dimensional flow fields across the entire computational domain. This shall also facilitate capturing the resulting transient aerodynamic loads on a wind tunnel specimen — quantities difficult to measure experimentally. To accurately capture the complex flow dynamics, high- fidelity large-eddy simulations are conducted, modeling the nozzle’s upper contour as a dynamic immersed boundary (IB). A curvilinear Eulerian grid is employed to ensure both efficient and precise spatial resolution of the problem. The moving least-squares (MLS) version of the direct forcing IB approach (Vanella and Balaras, 2009) is used to construct an IB kernel for each Lagrangian marker. Additionally, the MLS approach is also applied to construct one-sided kernel functions for Lagrangian points near the boundaries of the computational domain. Challenges related to the efficient IB simulation on curvilinear grids are addressed, and a solution is proposed within the MLS framework. The predicted results are analyzed in detail and validated against the experimental data by Wood and Breuer (2025) , providing insights into the effectiveness of the new design in generating controlled wind gusts.

Laminar–turbulent transition on the suction surface of the LM45.3p blade (20% thickness) was investigated using wall-resolved large eddy simulation (LES) at a chord Reynolds number of Re_c = 10^6 and angle of attack 4.6°. The effects of anisotropic free stream turbulence (FST) with intensities T I = 0%–7% were examined, with integral length scales scaled down from atmospheric measurements. At T I = 0%, a laminar separation bubble (LSB) forms and transition is initiated by Kelvin–Helmholtz vortices. At low FST levels (0% < T I < 2.4%), robust streak growth via the lift-up mechanism suppresses the LSB, while transition dynamics shifts from two-dimensional Tollmien–Schlichting (TS) waves (T I = 0.6%) to predominantly varicose inner and outer instabilities (T I = 1.2% and 2.4%) induced by the wall-normal shear and inflectional velocity profiles. The critical disturbance kinetic energy scales with T I ^−1.80±0.11, compared with T I ^−2.40 from Mack’s correlation. For T I > 4.5%, bypass transition dominates, driven by high-frequency boundary layer perturbations and streak breakdown via outer sinuous modes induced by the spanwise shear and inflectional velocity profiles. The scaling of streak amplitudes with T I becomes sub-linear and spanwise non-uniformity characterises the turbulent breakdown. The critical disturbance kinetic energy reduces to T I ^−0.90±0.16, marking a transition regime distinct from modal mechanisms. The onset of bypass transition (T I ≈ 2.4%−4.5%) aligns with prior studies of separated and flat-plate flows. A proposed turbulence spectrum cutoff links atmospheric measurements to wind tunnel data and Mack’s correlation, offering a framework for effective T I estimation in practical environments.

The study focuses on the combination of two numerical approaches that are typically not used together in this manner. The first is a well-established partitioned fluid-structure interaction (FSI) simulation methodology relying on a finite-volume fluid solver for curvilinear, block-structured, body-fitted grids written in the Arbitrary Lagrangian–Eulerian (ALE) formulation, and a finite-element solver for the structural analysis. The second approach is an immersed boundary (IB) method employing a continuous and direct forcing strategy. The IB method, often applied to Cartesian grids, is also referred to as an approach to simulate fluid-structure interactions. In this study, both methods are combined to exploit their respective advantages in simulating a complex flow problem. The coupled FSI problem involves the interaction of a thin, flexible structure deforming under the dynamic load of a wind gust (task 1). The gust itself is generated by an artificial wind gust generator, which includes a paddle that partially obstructs the wind tunnel's outlet, thereby defining an FSI problem of its own (task 2). For task 1, the classical partitioned ALE approach is employed, while the IB method is more appropriate for task 2. Using available experimental measurement data for both the fluid flow and the structural deformation, the combined simulation framework is first validated for the case without gust. In a second step, the more challenging FSI problem of discrete gusts impacting the T-structure is thoroughly analyzed and the predicted data are compared with the available measurement data. For both cases without and with gusts, a very good agreement between simulation and experiment is achieved, which justifies the chosen approach.

Aerosol injectors applied in single-particle diffractive imaging experiments demonstrated their potential in efficiently delivering nanoparticles with high density. Continuous optimization of injector design is crucial for achieving high-density particle streams, minimizing background gas, enhancing x-ray interactions, and generating high-quality diffraction patterns. We present an updated simulation framework designed for the fast and effective exploration of the experimental parameter space to enhance the optimization process. The framework includes both the simulation of the carrier gas and the particle trajectories within injectors and their expansion into the experimental vacuum chamber. A hybrid molecular-continuum-simulation method [direct simulation Monte Carlo (DSMC)/computational fluid dynamics (CFD)] is utilized to accurately capture the multi-scale nature of the flow. The simulation setup, initial benchmark results of the coupled approach, and the validation of the entire methodology against experimental data are presented. The results of the enhanced methodology show a significant improvement in the prediction quality compared to previous approaches.

The paper presents a novel design of a wind gust generator based on an adaptive nozzle for wind tunnel applications and its experimental investigation. The key feature of this design is the movable upper wall of the nozzle, which adjusts the cross-section of the nozzle's outlet. For this purpose, the upper contour of the nozzle is connected to a programmable and fast-moving tooth belt axis, enabling rapid changes in the nozzle geometry to generate reproducible horizontal wind gusts that develop along a flat ground plate. The experimental setup primarily relies on particle-image velocimetry as an optical measurement technique, supported by a constant-temperature anemometer and pressure taps at specific locations. The gusts are generated using a well-defined motion pattern of the movable nozzle, following a (1-cos)-type signal. A combination of velocity and surface pressure measurements is carried out, analyzing the gust development at various positions along the ground plate in streamwise direction. Both data sets are used to quantify the adaptive nozzle's potential as an effective tool for wind gust generation, facilitating future studies on highly dynamic fluid-structure interactions under wind gust load. Additionally, the well-designed experiment is planned to serve as a valuable validation case for numerical methods.

The simultaneous measurement of multi-physical processes presents a fundamental challenge in experimental fluid mechanics. A classic example is fluid-structure interaction (FSI), where the flow around a flexible structure leads to deformations of the structure, which affect the flow field itself. Typically, the flow and structure measurements are carried out using different measurement techniques for each discipline, for example particle-image velocimetry (PIV) for the flow field measurement and digital-image correlation (DIC) for the measurements of the structural deformations. Especially time-resolved processes require a synchronization of both measurement devices to achieve temporally matching data sets. In more complex setups consisting of multiple measurement devices, the synchronization and triggering is a main challenge. Often, each device has its own specific trigger input requirements. For this reason, using a single master signal to synchronize all devices in the experimental setup is often not possible. For this purpose, customized and independent signals must be routed to each measurement tool. This paper presents a solution for accurately controlling multiple devices of arbitrary complexity. A key aspect of the approach is the utilization of a cost-effective microcontroller, which is programmed via open-source software. The microcontroller offers multiple digital and analog I/O channel options making it capable to send pre-programmed signals. In this study, the signal quality is analyzed in detail. Furthermore, the ability of the microcontroller to trigger different measurement devices is demonstrated by synchronizing a wind gust generator with a dynamic pressure transducer. Both systems are triggered by the microcontroller, which provides individual input signals for each device.