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PIV systems rely on a combination of seed particles, laser light intensity, and camera positioning to collected velocity data. Careful attention is needed when selecting the ap-propriate setup configuration for PIV systems. This paper focuses on several areas for improvement in PIV data collection for a compressible turbulent boundary layer study o...
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... Investigations on tracer particles ability to follow the flow are carried out on an exemplary RoI (at the trachea): The hydraulic diameter d h is 25 mm in this exemplary measurement region, and the average velocity magnitude of the fluid is set to v ≈ 0.08 m/s. The estimations are made for the largest particles with a diameter of d p = 50 µm since smaller particles will lower the relaxation time [34,35]. The Stokes law is valid for a creeping flow around a single particle and small fluid's Reynolds numbers Re f . ...
The human nose serves as the primary gateway for air to enter the respiratory system and plays a vital role in respiratory homeostasis. Nasal breathing difficulties are a significant health concern, leading to substantial healthcare costs for patients. Understanding nasal airflow dynamics is crucial for comprehending respiratory mechanisms. This article presents a detailed study using tomo-PIV to investigate nasal airflow dynamics while addressing its accuracy. The study includes the manufacturing of a transparent silicone model based on a clinical CT scan, refractive index matching to minimize optical distortions, and precise flow rate adjustments based on physiological breathing cycles. This method allows for spatial high-resolution investigations in different regions of interest within the nasopharynx during various phases of the breathing cycle. The results demonstrate the accuracy of the investigations, enabling detailed analysis of flow structures and gradients. This spatial high-resolution tomo-PIV approach provides valuable insights into the complex flow phenomena occurring during the physiological breathing cycle in the nasopharynx.
The study’s findings contribute to advancements in non-free-of-sight experimental flow investigation of complex cavities under nearly realistic conditions. Furthermore, reliable and accurate experimental data is crucial for properly validating numerical approaches that compute this patient-specific flow for clinical purposes.
... After machining, both models were nickel chrome plated with a thickness of less than 10 µm, and the airfoil further polished with a buffing wheel, all to achieve a near mirror finish. Since very near-wall measurements in the boundary layer are of interest, the mitigation of laser flare was a priority; past work has shown high effectiveness of mirrored surfaces (e.g., Konrath et al. 2008;Pierce and Lu 2012). Due to machining considerations, the airfoil trailing edge has a finite thickness of 0.21 mm. ...
Wind turbines with thick blade profiles experience turbulent, periodic approach flow, leading to unsteady blade loading and large torque fluctuations on the turbine drive shaft. Presented here is an experimental study of a surrogate problem representing some key aspects of the wind turbine unsteady fluid mechanics. This experiment has been designed through joint consideration by experiment and computation, with the ultimate goal of numerical model development for aerodynamics in unsteady and turbulent flows. A cylinder at diameter Reynolds number of 65,000 and Strouhal number of 0.184 is placed 10.67 diameters upstream of a NACA 63215b airfoil with chord Reynolds number of 170,000 and chord-reduced frequency of \(k=2\pi f\frac{c}{2}/V=1.5\). Extensive flow field measurements using particle image velocimetry provide a number of insights about this flow, as well as data for model validation and development. Velocity contours on the airfoil suction side in the presence of the upstream cylinder indicate a redistribution of turbulent normal stresses from transverse to streamwise, consistent with rapid distortion theory predictions. A study of the boundary layer over the suction side of the airfoil reveals very low Reynolds number turbulent mean streamwise velocity profiles. The dominance of the high amplitude large eddy passages results in a phase lag in streamwise velocity as a function of distance from the wall. The results and accompanying description provide a new test case incorporating moderate-reduced frequency inflow for computational model validation and development.
Masking particle image velocimetry images can be a time consuming process in experiments involving moving bodies. Aircraft store separation and pitching airfoil investigations are examples of studies where particle image velocimetry data has to be acquired for many different body positions to properly resolve the attributes of the unsteady flow field. Laser reflections, perspective projection of the body, and non-uniform illumination in raw images can cause intensity-based algorithmic masking routines to fail in determining the correct mask for each individual image. A novel approach for automatic masking in moving body experiments is introduced in this paper. A pitching airfoil experiment is used to demonstrate the capability of the methodology. Photogrammetry is used to locate the airfoil model in the images and map the 3D body to the image space and define a mask. After performing photogrammetry for several known angles of attack and calibrating the photogrammetry parameters for each angle, a model was developed to estimate the parameters at other angles of attack where calibration data are not available. Results indicate that obtaining the necessary parameters required for masking a few reference images allows for robust automated masking of the remaining images leading to significant time reduction in the masking process. Masks generated using the automated masking technique show agreement with those generated by direct photogrammetry; the error in the predicted location remains less than 0.3% of the chord length. The automatically-generated masks prove to be effective when processing PIV images with the airfoil in different locations.
Microvortex generators (MVGs) have been proposed as devices for alleviating the adverse effects of shock/boundary-layer interactions [1]. MVGs in supersonic flow are generally skewed tetrahedral protuberanceswhose height is less than the boundary layer thickness (Fig. 1). Anderson et al. [1] providedMVG design guidelines, such as the standoff distance from an MVG array to the shock impingement location.
Innovations in conventional surface and planar laser scattering visualizations revealed complex structures in the Mach 2.5 flow past a sharp-edged, sub-boundary-layer ramp with swept sides that is one type of micro vortex generator (MVG). The incoming flow separated over the leading edge despite the ramp angle being below the threshold for incipient separation. The separation produced a weak trailing horseshoe vortex system. The flow over the top of the MVG separated off the slant edges to produce a large primary vortex pair. Extra details were revealed at the trailing edge with at least two pairs of singularities. Vortex filaments spring from these singularities. Symmetry breaking from the confluence of the two primary vortices was observed as an unsteady wake to result in a train of possibly ring or hairpin vortices trailing downstream.
Graphical Abstract
Surface reflections of high-intensity laser light are a common concern when conducting particle image velocimetry (PIV) measurements. Consequences range from a poor signal-to-noise ratio (overexposure) in near-surface areas up to camera sensor damage. The severity depends on the interplay between three factors: surface properties, laser light intensity and relative camera position. In stereoscopic or tomographic PIV setups, material selection is often the only factor which can be adapted. We present a systematic comparative study, involving different materials and surface treatments. Their potential to mitigate surface reflections is quantified against the reference case of a flat black painted wooden surface. The largest reduction of surface reflection intensity is obtained by applying fluorescent paint on wood or by employing electropolished steel. The more widely used flat black painted wood shows poor behavior.
