Many flows have a multi-constituent nature, where understanding the transfer of mass and momentum between different parts of the flow is key. While experiments in liquid media have long used flow tagging techniques such as dye injection to isolate parts of the flow, analogous methods that do not compromise full-field velocimetry in gas flows are highly impractical. The recent introduction of a solution of Pyrromethene 567 (P567) in di-ethyl-hexyl-sebacate (DEHS) to produce a seeding fluid capable of fluorescent emission in addition to Mie scattering promises to address this need. By locally seeding a secondary flow of interest with the modified DEHS, the fluorescent signal can be used for tagging; global seeding of main flow with standard DEHS, which only produces Mie scattering of incident laser light, is used to obtain full-field velocimetry with established PIV techniques. Performing morphological image processing and intensity-based thresholding on the fluorescent particle images yields a continuum representation of the secondary flow. This can then be combined with velocimetric information from PIV to conduct quantitative zonal analyses. This technique has been applied to the flow behind an active synthetic-jet turbulence grid and a turbulent boundary layer (TBL). With the new zonal decomposition capabilities offered, data relating to the intermittency of the flows, statistical structure of the phenomena, turbulent/non-turbulent interface (TNTI) and entrainment and detrainment can be extracted.

This paper presents the development of fluorescent tracer particles for use in gas flows as a countermeasure for undesired strong light reflections on surfaces of channel walls or obstacles and as a label for the discrimination of multi-constituent flows. The employment of fluorescent dye-doped tracer particles with a wavelength-specific optical filter enables the separation of the Stokes-shifted particle light emission from reflections on surfaces and Mie scattering from non-fluorescing particles. The fluorescent particles were made of Pyrromethene 567 (P567) and Di-Ethyl-Hexyl-Sebacate (DEHS), and the addition of P567 was not found to alter the characteristics of the particles generated. Investigations in a low-speed wind tunnel revealed that the intensity of fluorescent emission is proportional to the dye concentration at least up to 2.0 g l^-1 . The efficacy of reflection removal was investigated in a setup with a metal turbine blade placed in the flow and a laser sheet oriented to impinge the blade surface. With the installation of an appropriate optical filter, undesired light reflections were successfully removed, and reasonable vector calculations were enabled in proximity to the reflective blade surfaces. Finally, the performance of the modified DEHS was compared to conventional DEHS with the measurement of a canonical turbulent boundary layer (TBL). The flow was globally seeded with conventional DEHS and the TBL was locally seeded with fluorescing DEHS; simultaneous imaging with a notch filter confirmed that the flow is accurately tracked by the modified DEHS without additional bias. Furthermore, this indicated the possibility of using the newly developed particles to segregate portions of a flow with multiple constituents.

Lubricated textured surfaces immersed in liquid flows offer tremendous potential for reducing fluid drag, enhancing heat and mass transfer, and preventing fouling. According to current design rules, the lubricant must chemically match the surface to remain robustly trapped within the texture. However, achieving such chemical compatibility poses a significant challenge for large-scale flow systems, as it demands advanced surface treatments or severely limits the range of viable lubricants. In addition, chemically tuned surfaces often degrade over time in harsh environments. Here, we demonstrate that a lubricant-infused surface (LIS) can resist drainage in the presence of external shear flow without requiring chemical compatibility. Surfaces featuring longitudinal grooves can retain up to 50% of partially wetting lubricants in fully developed turbulent flows. The retention relies on contact-angle hysteresis, where triple-phase contact lines are pinned to substrate heterogeneities, creating capillary resistance that prevents lubricant depletion. We develop an analytical model to predict the maximum length of pinned lubricant droplets in microgrooves. This model, validated through a combination of experiments and numerical simulations, can be used to design chemistry-free LISs for applications where the external environment is continuously flowing. Our findings open up new possibilities for using functional surfaces to control transport processes in large systems.

In experimental studies of the turbulent/ non-turbulent interface (TNTI) of wall-bounded flows, a significant challenge is accurate detection of the interface in 2D and 3D. Detection criteria based on turbulent kinetic energy (TKE) and homogeneity of the flowfield have been applied to PIV measurements of TBLs [2, 15]. The use of local seeding (LS) as a passive tracer was also investigated to decouple TNTI detection from the reduced spatial resolution associated with velocimetric methods. This yielded the simple and robust particle image density detection method, which has good spatial resolution but lacks access to velocimetry in the potential flow [15]. With recent development of fluorescent DEHS and its application to tag the TBL, it is possible to obtain the entire flowfield in addition to segregating the flow into its turbulent and non-turbulent parts in air flows [8–10]. Finally, 3D reconstruction of the TNTI with imaging of the TBL at an oblique angle opens the door to more complex investigation and new insights of the interface [14].

Particle-image or particle-tracking velocimetry (PIV/PTV) nonintrusively provides velocity field information, and consequently, the proposition of measurements in a large plane is highly attractive from an experimental standpoint, particularly for experiments conducted in large atmospheric and flight-scale wind tunnels. Physical limits to the size of the useful field-of-view (FOV) that can be achieved with a single camera depend on striking a balance between the physical capabilities of the cameras and the physics of interest in the flow. A typical solution is to stitch together multiple smaller FOVs to achieve the large FOV of interest, presenting a number of challenges, some of which are rooted in the calibration process. For SPIV calibrations, the use of a multi-level target simplifies the calibration process. However, this is problematic for large-FOV measurements, as standardised multi-level targets are typically relatively small and expensive due to the precision-engineering required. As an alternative, LEGO bricks are extremely well-suited to the construction of large, customised multi-level targets due to their high dimensional tolerance and their stackability with high precision. In order to create a prototype two-sided multi-level SPIV target and evaluate whether the LEGO bricks can be used successfully to calibrate a large FOV, the LaVision Type 31 target was chosen as a model. In order to further augment the flexibility offered by the LEGO-based targets, the backing used to mount the baseplates was designed to be modularly reconfigurable. The resultant two-sided multi-level target has an area of approximately 380 x 1150 mm. To evaluate the target, SPIV measurements of the inflow conditions of the Atmospheric Wind Tunnel Munich (AWM) were performed; calibrations were also performed using the Type 31 target for comparison. Analysis of the datasets with both calibration targets shows good agreement in the measurement of the streamwise, out-of-plane component u. However, there is some uncertainty regarding the accuracy of the computation of in-plane components. Due to the high level of agreement in calibration parameters, out-of-plane component, and the qualitative location of flow features, this disagreement is believed to be a fixable issue. These early results indicate that with some refinement, the LEGO-based calibration target can be developed further and used for large-FOV measurements in the future.

A reinforced fiber-glass model of a NACA 4412 wing profile is designed and set-up in the Minimum-Turbulence-Level (MTL) wind-tunnel facility at KTH Royal Institute of Technology (Sweden), aiming to ...

The development and behavior of turbulent boundary layers (TBLs) under high pressure gradients as appearing on wing surfaces are still open research topics. Recent advancements in high-performance computing have allowed for the study of such TBLs through highly resolved numerical simulations. For the present study, we chose the NACA 4412 profile that has been a benchmark airfoil in the study of the development of boundary layers over wings, mainly due to its pressure-gradient distribution being quite independent of Reynolds number (Re), as well as having benign stall properties. Recently, well-resolved large-eddy simulations (LES) [1] have been performed at moderate but yet relevant Reynolds numbers over this profile, allowing to obtain accurate boundary layer data and highorder statistics. However, the experimental data is limited to the pioneering work by Wadcock and Coles [2,3] from the 1970s and 80s. Thus, we aim at providing high quality experimental data with recent advanced measurement techniques, and in guidance of the numerical work performed within the Linné FLOW Centre, new – more detailed – experiments are planned in the Minimum Turbulence Level (MTL) wind tunnel at KTH Mechanics. Corresponding preliminary experimental results are being described in a companion abstract.