Experiments in Fluids最新文献

Background-oriented schlieren (BOS) is a non-intrusive optical method for measuring density gradients in a fluid flow based on changes of local refractive index. The density gradients can be obtained by observing the displacement between two images of a background pattern, with and without the presence of the flow. Existing methods to estimate displacements include block matching and optical flow. Image registration in computer vision seeks reasonable transformations between two images such that one matches the other and has been under-utilized in determining the displacement for BOS image processing. Deformable image registration (DIR) methods allow non-global transformations and are proposed as displacement estimation methods for processing BOS images. Numerical ray tracing simulations are performed to generate synthetic BOS images of various flows. The estimated density gradient results of block matching, optical flow, and DIR methods are compared to the ground truth (the path-averaged density gradient of the schlieren object used in ray tracing) to assess and compare their performances. The performances of these methods are also validated on experimental BOS images to determine the displacement estimation method that is most suitable for high-speed, turbulent flows.

This paper presents an image velocimetry technique that employs supercooled water droplets as seeding particles in an icing wind tunnel. The innovative work involves reconstructing the pressure distribution using the velocity field measured by the water droplets and providing the water collection efficiency at the leading edge of the airfoil using pathlines. The study compares the relative errors of the velocity fields obtained through traditional particle image velocimetry (PIV) and the supercooled water droplet image velocimetry (SWDIV) methods. Results indicate that the velocity root mean square error of SWDIV is (17.4 %) of the incoming flow velocity, and the average angle error was 5.36°. The streamlines derived from the SWDIV method align well with the water droplet trajectories calculated using the Lagrangian approach, providing a more accurate representation of the flow dynamics involving supercooled water droplets of varying sizes. Using this technique, the study quantitatively analyzes the changing characteristics of the airfoil flow field during the anti-icing and de-icing processes under plasma actuation. Key observations include identifying changes in ice configuration, evaluating water droplet collection efficiency at the leading edge, and analyzing the velocity and pressure fields. In the icing wind tunnel experiments, the incoming flow velocity was 15 (text {m/s}), resulting in a Reynolds number of (1.8 times 10^5). The liquid water content was 1.0 (text {g/m}^3), with a median volume diameter of water droplets at 20 (upmu text {m}) and an average diameter of 6.4 (upmu text {m}) The static temperature of the incoming flow was − 10 °C. The anti-icing research revealed that plasma actuation prevents icing on the leading edge while maintaining the suction peak. However, runback ice formed downstream of the actuator at low incoming flow velocities, significantly reducing local negative pressure and leading to lift loss. Moreover, the aerodynamic effects generated by plasma reduced the peak water droplet collection coefficient at the leading edge by approximately 0.05. When the airfoil’s leading edge was covered with a 5 mm thick layer of mixed ice, its geometric shape became irregular, and the pressure peak was notably diminished. Upon activation of the plasma actuator, the resulting aerodynamic and thermal coupling melted the surface ice, disrupting the adhesion between the ice and the airfoil surface. This caused the ice to detach and be carried downstream by the airflow, effectively achieving de-icing within approximately 203 s. After ice removal, the negative pressure peak on the upper surface of the airfoil’s leading edge returned to baseline levels, restoring lift to a great degree.

The effect of the wall proximity of detached ({60,mathrm{ ^{circ }}}) divergent ribs applied on one wall in a square duct on the turbulent flow field was investigated. Laser Doppler anemometry (LDA) measurements were conducted for different clearance-to-rib-height ratios in the range of 0.1–1.0 at a Reynolds number (based on the rib height and mean bulk velocity) of 5000. Mean velocities, Reynolds stresses, triple velocity correlations as well as skewness and kurtosis were determined and yield deep insights into the turbulent flow field. The results showed that a geometry-induced secondary fluid motion occurred above and below the rib. The variations in the highly three-dimensional flow field close to the ribs and the geometry-induced secondary flow motion with the clearance-to-rib-height ratio determined the development of the wall-bounded flows and separating shear layers. Large recirculation regions on the bottom duct wall were prevented by the fluid exiting the gap below the detached ribs and separated shear layers pivoted upward in lateral direction. With decreasing wall proximity, lower mean vertical flow velocities above the ribs and the increasing upward fluid flow originating from the flow in the gap attenuated an intense interaction of the separated shear layers with the wall-bounded flow within the inter-rib spacing. Since turbulent structures originated in the shear layers, distributions of high-order statistic moments depend strongly on the shear layer development. Reynolds stresses and triple velocity correlations increased in the direction of the side walls near the rib due to the lateral flow motion, and their peak regions moved away from the wall with increasing clearance-to-rib-height ratios.

For the strong-amplitude tonal noise from a slat cove, the mechanisms of mode switching phenomenon are not well understood. In this paper, an experimental study was conducted on the slat noise of a 30P30N three-element airfoil through synchronized measurements of a far-field microphone array, wall-pressure transducers, and a hot-wire anemometry. In-house wall-pressure microphones were developed, based on MEMS microphones and flexible printed circuit board, and attached to the curved surface of the slat to measure the wall-pressure fluctuations. The time-frequency analysis through the continuous wavelet transform demonstrated that the synchronous measurements captured the temporal switching of dominant mode in slat noise and the intermittent vortex structures corresponding to the dominant mode frequency in the flow field. The dominant mode in the time-averaged spectra of far-field noise and wall-pressure fluctuations arises from the competition of strong amplitude over time between the primary modes. The time-frequency analysis based on wall-pressure microphones at different spanwise positions revealed a temporal variation of the dominant mode along the slat span. The spanwise coherence analysis indicated that the dominant mode showing stronger coherence has a longer spanwise correlation length compared to other secondary modes.

Vortices generated during underwater undulatory swimming (UUS) produced thrust and propelled swimmers. Therefore, clarifying how the vortices generated during UUS changed with different swimming velocities was crucial for improving swimming performance. This study aimed to clarify the changes in vortex structure in UUS when test flow velocities (U) were varied. A male collegiate swimmer participated in the trials, and trial swims were performed at three different U (0.8, 1.0, and 1.2 m/s). Particle image velocimetry (PIV) was used to analyze the flow fields behind the swimmer. The flow field was converted into a quasi-three-dimensional flow by performing multiple trials at the same U and averaging the results. The peak value of the vorticity component in the flow field increased with U. The structure of the vortices observed from the end of a down-kick to the beginning of an up-kick changed as U increased, and the direction of the jet flow between the vortex pairs became vertically downward. This phenomenon resembled the C-start maneuver seen in fish, where sudden acceleration prompted a more vertical downward flow direction. Such a change in vortex structure was considered to be a strategy to generate large momentum during an up-kick. This was the first study to quantify the vortex structure during UUS and reveal the variations in vortex structure with changes in swimming velocities, highlighting the importance of the down-kick to up-kick switching phase in UUS from the vortex structure in the flow field perspective.

An experimental investigation in a sector ((20^circ)) of a full-scale annular gas turbine combustor is performed. The sector combustor is optically accessible for the flow and flame visualization of the primary and exit zones of the combustor. The distinctive feature of the experimental setup is that it preserves the geometrical details of an annular combustor, which includes the casing, dome, and combustor liner. The combustor design features a series of primary and secondary dilution holes with multiple film cooling strips on the outer and inner liner. In the present study, the time-resolved particle image velocimetry (PIV) experiments are conducted on the central longitudinal plane and two azimuthal planes to gain insight into the dynamics of the sector combustor. Proper orthogonal decomposition (POD) is applied to the data to obtain the dominant dynamics of the combustor. The major coherent structures of the swirl flow field, the primary dilution jets flow field, and the dominant interaction of swirl and dilution jets are elucidated here. The azimuthal plane data provide a three-dimensional explanation of dilution jet dynamics. The dynamics of the exit zone is found to be influenced by the secondary dilution jet dynamics. The spectral properties of dynamics are illustrated from the recorded acoustic ((p^prime)) signal and the time coefficient of the POD eigenmodes. Further, the experiments are performed by blocking the dilution jets (without-DJ). These experimental data help to identify the source of the dominating frequency ((f_textrm{d})) within the combustor, which is found to be the swirl flow instabilities. Without-DJ data also showcases the role of dilution jets in convecting the swirl flow generated acoustics to the exit zone. The reconstructed flow field using POD provides physical insights into the dynamics occurring within the sector combustor.

The wave crest (cusp) of the disturbance wave in thin liquid film flow is an important factor contributing to heat/mass transfer, e.g., fuel rods in boiling water reactors, stator/rotor blades in steam turbines, and cleaning/drying wafer processes in semiconductor manufacturing. We developed a new technique for directly detecting the thickness of wave cusps using array-based sensing with an optical waveguide film (OWF). This technique, based on geometrical optics assumptions, simultaneously obtains information on liquid films’ thickness and their interfacial shape, i.e., whether or not the local interface is convex upward. We first performed a pseudo-film flow measurement using a metal specimen to confirm the basic principle. According to the results, a meaningful signal indicating the wave-cusp passage, along with a thickness signal, was detected simultaneously. The OWF signal processing for cusp thickness detection was newly established based on this fact. We then applied this technique to a wavy liquid film flow formed on a flat plate in the entry region. A series of experiments were performed over a wide range of air speeds (j_G = 20–70 m/s). As a result, the cusp thicknesses of relatively large waves on the wavy interface were successfully extracted from the OWF output signal. Further, the major thickness variables (i.e., base film thickness, median film thickness, and cusp thickness) were compared with those of conventional thickness estimation methods, which showed reasonable agreement. This paper provides a framework for wavy thin-film flow measurements via OWF that is specialized for directly detecting local thickness profiles.

Graphical abstract

The fabrication of arterial flow phantoms for fluid dynamics studies suitable for particle image velocimetry (PIV) techniques has presented challenges. Current 3D-printed blood flow phantoms with suitable transparency for optical PIV (laserPIV) are restricted to rigid materials far from those of arterial properties. Conversely, while soft 3D-printed phantoms demonstrate promise for sufficient acoustical transparency for ultrasound PIV (echoPIV), their optical translucency presents challenges for laserPIV applicability. This dual-modality approach leverages the high spatial resolution of laserPIV for in-vitro applications and the ability of echoPIV to quantify flow in both in-vivo and in-vitro application (also inside stents), providing a more comprehensive understanding of flow dynamics. In this study, we present a series of coated thin-walled 3D-printed compliant phantoms suitable for dual-modality PIV flow imaging (i.e., laserPIV and echoPIV) methods, overcoming current 3D-printable material limitations. Stereolithographic (SLA) 3D printing was used to fabricate pipe flow phantoms from a set of commercial soft resins (flexible and elastic) as vascular tissue surrogates. To overcome low transparency and poor surface finish of soft resins, we coated the 3D-printed flow phantoms with a soft, optically transparent, photo-activated polymeric coating. The feasibility of performing dual-modality PIV was tested in an in-vitro flow setup. Our results show that the average normalized root mean square errors obtained from comparing laserPIV and echoPIV velocity profiles against the analytical solutions were 3.2% and 5.1%, and 3.3% and 5.3% for the flexible and elastic phantoms, respectively. These results indicate that dual-modality PIV flow imaging is feasible in the 3D-printed coated phantoms, promoting its future use in fabricating clinically-relevant flow phantoms.

Graphical abstract

Particle image velocimetry (PIV) is an established velocimetry technique in experimental fluid mechanics that involves determining a fluid flow velocity field from the motion of tracer particles illuminated by a laser sheet. The necessity of laser illumination poses challenges in certain applications and is a potential entry barrier due to its high cost and safety considerations. A laser-free alternative to PIV is particle shadow velocimetry (PSV), which uses images of the shadows cast by the particles on the camera sensor under back-illumination, instead of the Mie scattering signal produced by laser illumination. This study aims to compare various aspects of PSV such as depth of field, seeding density, type of illumination required, particle size, image filtering, cost-effectiveness and limitations with those of PIV. PSV and PIV measurements are taken in the wake of a flow past a cylinder and in a boundary layer developing over a flat plate. It is found that PSV is capable of achieving equivalent accuracy to PIV and is a viable alternative to PIV in certain applications where light sheet illumination creates experimental challenges.

Extended particle streak velocimetry (E-PSV) is a novel approach for comprehensive 2D flow measurement. It extends the measuring range of particle streak velocimetry (PSV) via particle tracking velocimetry (PTV). By using long camera exposure when recording moving tracer particles, streaks are created in areas of high flow velocities (PSV). In areas of low velocity, in contrast, particles are imaged point-shaped (PTV). E-PSV hereby offers the advantage of continuous measurement with PSV-typical setups, particularly when areas close to the wall and vortices require to be recorded simultaneously with areas of high velocity. For precise extraction of the flow information, a new model for the description of particle images is presented. It is based on the assumption that the intensity of a tracer can be modeled by a 2D Gaussian function. The temporal integral of the moving Gaussian is approximated by combining analytical calculation with values from a lookup table. We show that by this method even curved streaks can be reconstructed with subpixel accuracy under noise and quantization effects. The technique is demonstrated using a film flow in vicinity of a microstructure.