Laboratoire de Mécanique des Fluides et d'Acoustique

This work presents a complete hybrid testing methodology for flow assessment that combines computational fluid dynamics (CFD) simulations, stochastic variational Gaussian processes (SVGP), and multi-fidelity Gaussian processes (MFGP) to achieve accurate assessments with fewer measurements. Focused on minimizing energy consumption and testing times, the methodology utilizes SVGP to identify informative measurement locations based on CFD data, enabling a data-driven design of experiments (DoE) approach. This optimizes sensor placement and minimizes redundant measurements. Acquired experimental data are then fused with CFD data using a MFGP model, providing a comprehensive flow field reconstruction with associated uncertainty estimates within a Bayesian framework comparable to traditional methodologies. Two test cases demonstrate the methodology's effectiveness: the H25 axial compressor, used as a development test case due to availability of preexisting experimental data, and a state-of-the-art ECL5 ultra-high bypass ratio fan, for which the methodology was applied in a blind validation scenario. Results show a significant reduction in required measurements (around 70% for the H25 and 50% for the ECL5) without sacrificing accuracy, leading to a substantial decrease in experimental testing time. The SVGP DoE framework outperforms random and Latin hypercube sampling, effectively capturing relevant flow features with fewer measurements. The methodology also adapts to varying flow conditions, accurately predicting flow fields and quantifying uncertainties across different Reynolds numbers. This work introduces a shift in experimental campaigns, strategically optimizing measurement strategies during the planning phase, emphasizing machine learning trends in shaping new research paths toward more efficient, cost-effective, and sustainable experimental practices for fluid dynamics.

<strong class="journal-contentHeaderColor">Abstract.</strong> We present a large-scale laboratory model of the Strait of Gibraltar that reproduces realistic topography, tidal forcing, stratification, and rotation, enabling controlled investigation of key exchange processes linking the Mediterranean Sea and Atlantic Ocean. Velocity and density measurements confirm dynamic similarity with ocean observations. Analysis of the flow near Camarinal Sill shows that bottom boundary layers are the primary source of turbulent kinetic energy, exceeding contributions from shear at the interface between Atlantic and Mediterranean waters. The enhanced role of bottom-generated turbulence is linked to separation of the Mediterranean gravity current induced by an adverse pressure gradient during outflow, providing a new explanation for the well-documented detachment of the Mediterranean plume west of the sill. This detachment intensifies during spring tides, driving diluted waters farther into the Atlantic, while during neap tides bottom-generated and interfacial turbulence coincide, offering a consistent explanation for the high dissipation rates reported in field measurements. Overall, tidal forcing promotes full-depth mixing, with up to ~20 % density reduction west of CS and oscillatory ~20 % variations east, consistent with field data, and simultaneously introducing an important phase shift between velocity and density fields, with implications for parameterizing turbulent exchange and definition of the composite internal Froude number for reliable diagnose of hydraulic control. During spring tides, hydraulic control is intermittently lost during inflow and this loss propagates eastward, while additional control points arise west of the sill. Neap tides exhibit signatures of control which persist much longer during a tidal cycle as compared to spring tides, but does not propagate to the east when the tide reverses. Transport and energy budgets reveal strong longitudinal and transverse variability, highlighting the need for fully three-dimensional diagnostics. Volume transport, dominated by transverse topographic variability, exceeds salt transport by two orders of magnitude, confirming net Atlantic inflow. A phase-lagged internal bore release between the northern and southern transects is observed, consistent with field observations, and we show that it is independent of barotropic effects or Kelvin waves. These results demonstrate that high-fidelity laboratory modeling can capture the essential three-dimensional dynamics of energetic straits and provides a powerful complement to observational and numerical approaches.

ABSTRACTUrban river bathing is re‐emerging across Europe, driven by social demand and climate change impacts. The Urban Bathing Consortium, an interdisciplinary and intersectoral consortium initiated at the University of Lyon (France), is at the forefront of studying the challenges and opportunities of creating and managing healthy, safe, and accessible river bathing spaces. Through interdisciplinary collaboration among researchers and stakeholders, the consortium proposed an analytical framework, identifying seven critical dimensions for urban river bathing: the history and revival of city‐river relationships, legal and regulatory frameworks, bathing water quality, river drowning risks, river ecosystems, social perspectives, and urban planning. By examining these dimensions with state‐of‐the‐art approaches and drawing on Lyon's experiences, the study provides scientific insights and practical recommendations for future sustainable urban river bathing development. These include revitalizing historical city‐river connections, aligning local regulations with EU guidance, advancing holistic microbial water quality control, enhancing safety measures, incorporating ecological considerations, balancing competing river uses in urban planning, and addressing social needs for inclusive river governance.

In this article, we study experimentally the dispersion of colloids in a two-dimensional, time independent, Rayleigh-B\'enard flow in the presence of salt gradients. Due to the additional scalar, the colloids do not follow exactly the Eulerian flow-field, but have a (small) extra-velocity $\mathbf{v}_{\mathrm{dp}} = D_{\mathrm{dp}} \nabla \log C_s$, where $D_\mathrm{dp}$ is the phoretic constant and $C_s$ the salt concentration. Such a configuration is motivated by the theoretical work by Volk \textit{et al.} (JFM, 2022), which predicted enhanced transport or blockage in a stationary cellular flow depending on the value of a blockage coefficient. By means of High Dynamical Range Light Induced Fluorescence, we study the evolution of the colloids concentration field at large P\'eclet number. We find a good agreement with the theoretical work although a number of hypotheses are not satisfied, as the experiment is non homogeneous in space and intrinsically transient. In particular we observe enhanced transport when salt and colloids are injected at both ends of the Rayleigh-B\'enard chamber, and blockage when colloids and salt are injected together and phoretic effects are strong enough.

The increasing diversity of UAV applications requires the ability to detect and track them accurately. We address the interest of a 3D LiDAR network to better cover urban canyons. Detection and tracking of objects using a low-cost 3D LiDAR have primarily been developed for on-vehicle sensors in the context of autonomous vehicles and ADAS with processing choices particularly suited for ground objects detection. We compare trackingby- detection strategies based on object classification through supervised learning and tracking-before-detection strategies that perform tracking without object classification. We discuss the need to favour the latter approach in a defence context for identifying protean threats. We conduct flight tests with a drone to gather real data from a low-cost 3D LiDAR, as well as ground-truth drone position data, to create a test database for UAV detection and tracking methods. The test scenario includes variations in the drone’s altitude, distance to the sensor, and flight speed, in order to evaluate the tracking methods under realistic conditions and to test the limits of the low-cost 3D LiDAR sensor. Based on these data, we implement two processing methods for multi-target UAV tracking. We evaluate the performance of these approaches in terms of estimation quality. In a broader sense, we assess the applicability of classical approaches developed for autonomous vehicles and ground object detection and tracking using on-vehicle 3D LiDAR sensors, in an anti-drone defence context in urban areas. This assessment focuses on detecting and tracking protean UAV based on degraded 3D LiDAR measurements. We identify innovation avenues to improve or surpass these approaches for this purpose.

An analytical theory is developed that describes acoustic microstreaming produced by the interaction of an oscillating gas bubble with a viscoelastic particle. The bubble is assumed to undergo axisymmetric oscillation modes, which can include radial oscillation, translation and shape modes. The oscillations of the particle are excited by the oscillations of the bubble. No restrictions are imposed on the ratio of the bubble and the particle radii to the viscous penetration depth and the separation distance, as well as on the ratio of the viscous penetration depth to the separation distance. Capabilities of the developed theory are illustrated by computational examples. The shear stress produced by the acoustic microstreaming on the particle’s surface is calculated. It is shown that this stress is much higher than the stress predicted by Nyborg’s formula (1958 J. Acoust. Soc. Am.30, 329–339), which is commonly used to evaluate the time-averaged shear stress produced by a bubble on a rigid wall.

Abstract The operational capabilities of turbofan engines encounter limitations due to instabilities arising from tightly coupled interactions among aerodynamics, acoustics, and structural mechanics. Modern fans and compressors exhibit non-synchronous vibration (NSV), leading to safety-critical blade oscillations. In contrast to self-excited phenomena such as flutter, NSV stems from the convection of aerodynamic disturbances that synchronize with blade eigenmodes. Understanding this phenomenon is challenging, as its intricate interaction patterns and the occurrence of flow separations constrain the predictive capabilities of current state-of-the-art methods. To establish a comprehensive benchmark dataset on the aeroelastic behavior of modern ultra-high bypass ratio (UHBR) architectures, the European CleanSky-2 project CATANA aimed to examine a carbon composite fan stage, ECL5, utilizing multiphysical instrumentation. Recently, experiments on a structurally tuned reference configuration were conducted, revealing high-amplitude NSV at multiple subsonic speedlines. The observed interaction modes and instability onset differed significantly from numerical predictions using both linearized Reynolds-averaged Navier–Stokes and unsteady Reynolds-averaged Navier–Stokes with prescribed harmonic blade motion. In an effort to enhance the dataset, two additional fan configurations with identical blade profile geometries were investigated: one featured a structurally mistuned rotor with approximately doubled frequency variation of all eigenmodes compared to the reference, and the other involved a case with locally increased tip clearance on individual blades. This paper presents the experimental results of a sensitivity study and explores the influence of structural mistuning and tip clearance variation associated with manufacturing tolerances. Contrary to the intended outcome, it will be demonstrated that the mistuned case exhibited higher blade vibration amplitudes than the reference case during NSV. Detailed instrumentation reveals that the mistuning pattern was effectively transferred to the rotating system, but aerodynamic mistuning, particularly concerning tip clearance, emerged as a dominant factor. The non-synchronous forced-response nature of NSV during highly throttled operation ultimately dictates the observed response levels under different conditions, necessitating a thorough analysis to evaluate the robustness of a specific configuration. These results contribute valuable insights to the open dataset for the ECL5 configuration, benefiting the research community. Particularly noteworthy is the detailed capture of blade-to-blade variations in this research, which will prove instrumental in validating numerical methods.

Prediction of the acoustic performance of 3D printed materials is investigated at normal and grazing incidence. A direct numerical (microscopic) simulation that solves the full set of Navier–Stokes equations is used as a reference. It is compared with a macroscopic approach in which the material is represented by an equivalent fluid. The materials have a periodic microstructure, consisting either of a single network of spherical or cubic cavities connected by cylindrical channels or of a double-nested network. The samples are printed using the stereolithography technique and are tested using an impedance tube and a duct test bench. For single network geometries, the results of sound absorption at normal and grazing incidence predicted using the equivalent fluid approach are in good agreement with those obtained by the microscopic approach. Comparisons with impedance tube measurements confirm that both approaches can accurately predict the absorption coefficient of the samples. For the in-duct liner configuration, the transmission loss measurements and predictions show similar evolution with frequency change, despite the discrepancy in amplitude. For the double network geometry, the equivalent fluid approach cannot exactly reproduce the results obtained with the direct numerical simulation. Finally, while the predictions with the microscopic approach provide a good match with the impedance tube measurements, only a poor agreement is obtained using the duct testing bench.

Inserting vegetation within the urban environment mitigates the urban heat island effect, the flooding risk, and improves air quality. However, its aerodynamic effect has remarkable impact on the pollutant transport and, consequently, on human health comfort. Indeed, the presence of vegetation within an urban canyon leads to non-trivial patterns of pollutant concentration and mass fluxes, as a consequence of complex mean and turbulent velocity fields. In addition to the vegetation density, the flow structure within canyons is influenced by their geometry and by the wind direction.&amp;#160;&amp;#160; The aim of the present study is to experimentally investigate the velocity field within a canyon, varying the vegetation density and the wind direction. We measured flow velocity statistics within an indefinitely long street canyon, with unit height-to-width ratio, subject to a neutrally stratified boundary layer modeled in the wind tunnel of &amp;#201;cole Centrale de Lyon. The aerodynamic impact of vegetation was reproduced by inserting plastic miniatures of trees along the two long sides of the canyon. We considered an empty canyon and a vegetated canyon, whose longitudinal axes are oriented with angles of 0&amp;#176;, 30&amp;#176;, and 60&amp;#176; with respect to the external wind flow.&amp;#160; Results reveal that when the canyon is inclined with respect to the external wind direction the mean flow follows a complex helicoidal structure. The presence of trees decreases significantly the mean longitudinal velocity and weakens the transversal circulation in the inclined canyon. The dampening effect of the mean longitudinal flow is more marked increasing the inclination angle of the canyon. Turbulent fluctuations are enhanced above the tree crowns, mostly when the wind blows parallel to the canyon axis. On the contrary, turbulent fluctuations decreases at tree trunk and crown levels, in particular when the canyon is inclined of 60&amp;#176; with respect to the external wind direction. Spectra of the velocity signal show that the presence of trees induces an evident shift of the energy peak towards high frequencies.&amp;#160; The collected data constitute a step forward to understand and modeling the urban microclimate.