Experiments in Fluids最新文献

We describe the implementation of a 3d Lagrangian particle tracking (LPT) system based on event-based vision (EBV) and demonstrate its application for the near-wall characterization of a turbulent boundary layer (TBL) in air. The viscous sublayer of the TBL is illuminated by a thin light sheet that grazes the surface of a thin glass window inserted into the wind tunnel wall. The data simultaneously captured by three synchronized event cameras are used to reconstruct the 3d particle tracks within (400,upmu text{m}) of the wall on a field of view of (12.0,text{mm} times 7.5,text{mm}). The velocity and position of particles within the viscous sublayer permit the estimation of the local vector of the unsteady wall shear stress (WSS) under the assumption of linearity between particle velocity and WSS. Thereby, time-evolving maps of the unsteady WSS and higher-order statistics are obtained that are in agreement with DNS data at matching Reynolds number. Near-wall particle acceleration provides the rate of change of the WSS which exhibits fully symmetric log-normal superstatistics. Two-point correlations of the randomly spaced WSS data are obtained by a bin-averaging approach and reveal information on the spacing of near-wall streaks. The employed compact EBV hardware coupled with suited LPT tracking algorithms provides data quality on par with currently used, considerably more expensive, high-speed framing cameras.

In a microgravity environment, where gravity is almost absent, surface tension plays a dominant role in the behavior of fluid motion. The motion of fluids within storage tanks can lead to significant oscillations of the tank itself or even the entire vehicle, posing substantial safety risks. Solid foam particle anti-sloshing technology represents a novel research topic in addressing this issue. The anti-sloshing technology using solid foam particles is a novel research topic. In this experiment, ping-pong balls were used to simulate individual foam particles, and a transparent liquid tank was designed to observe their motion. Two different hydrophobic materials were applied to the surface of the balls to alter their surface tension in water. A drop tower was used to create a microgravity environment, and the motion of the balls in water under microgravity was recorded with a camera. Both static and dynamic analyses were conducted on the balls under normal gravity and microgravity conditions, considering water resistance, surface tension, and added mass forces. The control equations for the position and velocity of the ball’s center of mass were derived. The experimental results showed that under microgravity, hydrophilic balls tend to submerge into the water, while hydrophobic balls move upwards, away from the water surface. The equilibrium adsorption position of the hydrophilic balls differed significantly between microgravity and normal gravity conditions, with noticeable oscillatory movement in the vertical direction. The experimental results showed good agreement with the dynamic model.

In the present study, the propagation velocity and lateral spreading angle of turbulent spots on a wedge in transient hypersonic boundary layer flows were investigated and characterized by measuring the heat flux distribution using a fast-response temperature-sensitive paint (TSP) in a shock tunnel facility. The shock tunnel was operated under the over-tailored condition and provided with the low- to high-unit Reynolds number during a test duration, which was realized by the contact surface arrival in the shock tube that changed the temperature and density of the reservoir gas. Power spectral density estimated from the pressure recordings may indicate that the boundary layer flow was transitional. In TSP measurements, the global heat flux distribution on the wedge was accurately obtained qualitatively. Surprisingly, turbulent spots were visualized using TSP for each unit Reynolds number condition. The propagation velocities at the head, peak heat flux point, and tail of turbulent spots for low- and high-unit Reynolds number conditions were approximated from the TSP images to be 87 ~ 96%, 69 ~ 73%, and 52 ~ 57% of the boundary layer edge velocities at each condition, respectively. These results were in good agreement with the available data in previous investigations. The lateral spreading angle of turbulent spots was also measured to be 5° ~ 10° from the TSP images. This study showed that the fast-response TSP had the ability to visualize the temporally resolved turbulent spots by measuring the heat flux on the wedge.

Graphical abstract

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.