Experiments in Fluids最新文献

A series of experiments were performed in a tornado simulator to study tornado-like vortices, with a focus on the surface pressure fields beneath the vortices. In the experiments, the pressures on the floor beneath a single-celled vortex and a two-celled vortex were measured. The measurements were used to characterize the surface pressure fields, which revealed, among others, the spatially varying non-Gaussian distribution of the surface pressures, the existence of narrowband components in the pressure fluctuations, and the differences between the characteristics of the surface pressure fields beneath the two types of vortices. A further analysis of the time and spatial variations of the surface pressure fields enabled the identification of the phenomena that cause these major characteristics of the surface pressure fields and the specific differences in these phenomena that result in the differences in the characteristics of the surface pressure fields beneath the two types of vortices. The findings from the study of the surface pressure fields provided insights into the nature of the tornado-like vortices aloft.

This paper presents a method for measuring the wall shear stress of a flat plate in incompressible flow based on optical measurement of the viscous liquid surface deformation in a cylindrical cavity. The liquid surface deformation is measured by detecting the liquid-surface-deformation-induced displacement field of white dots randomly distributed on the dark bottom of the cavity in images taken by a camera viewing the cavity perpendicularly. Numerical simulations of the flow over a simplified surface model indicate that the liquid surface deformation is caused by the elevated dynamic pressure in the cavity. Further, an analysis based on the similarity law of velocity in a turbulent boundary layer shows there is a functional relationship between the wall shear stress and the light deflection angle charactering the liquid surface deformation. Subsequently, in experiments in a low-speed wind tunnel, the liquid surface deformation is measured using the background-oriented Schlieren (BOS) technique and local velocity profiles are obtained via hot-wire anemometry to infer the wall shear stress in a range of the incoming flow velocities. Therefore, the relationship between the wall shear stress and the light deflection angle induced by the liquid surface deformation is established. The potential error sources are discussed.

Flow characteristics of inclined jet in crossflow (JICF), especially in-hole fields, remain insufficiently explored in experimental observations of near-wall properties and turbulence statistics, due to strong optical distortions and blind spots caused by refractive indices mismatch at solid–fluid interfaces. This study represents the first application of the refractive index matching (RIM) technique in JICF research, enabling in-hole measurements through time-resolved particle image velocimetry (TR-PIV). Experiments were conducted on a round hole at four velocity ratios (VR = 0.4, 0.8, 1.2, and 1.5). Focusing on the in-hole and near-exit mean flow field, this study identified a low-speed separation zone on the downstream side near the hole inlet and a jet acceleration zone on the upstream side near the hole exit. Near the hole inlet, vortex is formed due to the high-speed shear effects on upstream sidewall. Within the low-speed zone, flow characteristics were associated with strong vorticity, high turbulent kinetic energy, and low Reynolds stress components. In contrast, turbulence in the jet acceleration zone is higher for two intermediate VRs, which depends on the momentum balance between jet and mainstream. At VR = 0.4, large-scale vortex structure was formed inside the hole. The mainstream blockage led to a counter vortex in the leading edge of hole exit, which caused strong oscillation and contributed to hairpin vortex downstream. As VR increased, more complex axial vortical structures were observed, and dominant frequencies were converted. At VR = 1.5, the high-speed jet was more stable to show more regular vortical structures inside the hole and induced shear vortices with strong K-H instabilities in external field. By clarifying in-hole flow characteristics and establishing correlations with external JICF behaviors, this study aims to enrich the data of experimental benchmark for in-hole JICF validation and provides insights for optimizing film cooling strategies.

In this study, the effects of separated flow over a melting paraffin slab on atomization is considered. Experiments are conducted using an optical chamber with a paraffin wax slab exposed to high shear flows of 71–234 kg/m(^2-) s with non-vitiated heated air at 84–199 (^{circ })C. The formation of a lobe structure and leading edge instability responsible for wax atomization is imaged and quantified. The lobe height and receding angle are measured, and a simplified theory is developed to predict the steady-state shape using a sinusoidal profile. A Weber number of the leading edge is defined (We(_textrm{LE})) to non-dimensionalize and correlate the experimental data to the theory. A linear correlation is observed with an (R^2) of 0.94 between the shear correction factor and We(_textrm{LE}). The overall agreement between theory and measurements is good for the lobe height and width. Measurements of entrainment from the lobe account for (approx 20%) of the total mass loss. The size of the droplets entrained from the leading edge are recorded and normalized. Droplet size distributions for varying air fluxes and temperature are shown to collapse to the same distribution when normalized using their z-scores.

The effects of edge geometry and compressibility on the wake of a slanted afterbody model with rounded edges, relevant to cargo aircraft and high-speed train applications, are investigated through detailed experimental observations of the flowfield. The wake flow features are examined using centerline planar PIV measurements and novel Scanning-SPIV measurements to reconstruct the full mean volumetric velocity field. Planar PIV measurements at the model centerline reveal that increasing the Mach number reduces the shear layer growth rate, leading to decreased entrainment within the recirculation region. Consequently, the recirculation region increases in both length and height. Further downstream, the vortex circulation for the rounded-edge model remains nearly constant across both incompressible and compressible Mach numbers. Additional vortex properties are examined through the Reynolds-averaged vorticity transport equation applied to the volumetric flowfield measurements, revealing an increase in x-vorticity compression within the recirculation region. By Helmholtz’s vortex theorem, this increased vortex compression contributes to the growth of the recirculation region between Mach number conditions. Additionally, the dilatation term was explored, allowing for the delineation of compressibility effects on the recirculation region.

Traditional methods for directly measuring aerodynamic forces are particularly challenging at low Reynolds numbers due to the low dynamic pressures. This becomes even more challenging when rapid motions of the test articles are present, with inertial forces often larger than the aerodynamic forces. Existing methods for calculating pressure fields from experimental vector fields, such as those measured using particle image velocimetry (PIV), have constraints that make them difficult or impossible to apply to data sets that do not meet certain conditions, such as boundary condition requirements or restrictions on the grid shape of the data. This paper describes a new method of determining surface pressures and aerodynamic forces using experimentally collected velocity field data. This method leverages field erosion to constrain a point-stepping spatial integration of the pressure gradient field. A systematic method for dividing the flow field into zones based on the vorticity of the flow and the known geometry of the experiment allows for pressure in less-disturbed portions of the flow to be calculated and used as the boundary conditions for more unsteady flow regions. Surface pressures are then extracted from on or near the surface and integrated to calculate lift and drag. Two data sets are used as validation cases: a pitch-and-hold dynamic stall and static lift around an airfoil, both at low Reynolds number. The pressure-derived lift curves compare favorably with the reference data sets, demonstrating the accuracy of the new method.

We show a novel plenoptic camera architecture and demonstrate its ability to perform three-dimensional three-component velocimetry using standard multi-camera processing techniques. The field of view of the imager is approximately (10~text {mm}times 7~text {mm}times 3,text {mm}). The architecture needs only a custom lens assembly with no modification to the camera body, which allows the use of any camera with an appropriate sensor size. This plenoptic configuration directly creates multiple views of a scene side by side on the camera sensor, which are then separated and treated as if they originated from independent cameras. Standard calibration techniques are implemented to create 3D to 2D correspondence on images to determine 3D scene information. 3D velocity fields are reconstructed with the “shake-the-box” implementation of Lagrangian particle tracking. Results are validated with an axially oscillating cylinder in a refractive-index-matched experiment. The flow is the axisymmetric equivalent of Stokes second problem for which an analytical solution is known. The boundary layer is (1.24~textrm{mm}) with large accelerations and velocity gradients, which serve as a strong test case for the instrument.

This study has firstly provided an instantaneous refractive index compensation on velocity measurement in a reacting field where the 3D refractive index and the velocity distribution were measured by Background-oriented Schlieren tomography (BOST) and planar particle image velocimetry (PIV). A one-to-nine endoscope system was integrated with a camera to provide nine views of a turbulent non-piloted Bunsen flame. The 3D refractive index field was reconstructed from a neural network. A low-speed PIV system was applied to capture 2D velocity distribution across the central plane simultaneously. To synchronise the BOST system with PIV, two digital delay/pulse generators were synchronised to generate two groups of signals with different frequencies for two systems with a fixed phase delay. The pixel shifting on the PIV plane was resolved by estimating the gradient of the thermal-induced refractive index between the PIV camera and the imaging plane. The magnitude of the instantaneous velocity error caused by the light deflection was estimated ((pm ,2%)) for a small non-pilot flame. By inversely considering the velocity error, the error effect caused by the instantaneous refractive index displacement was firstly removed. Such a technique provides a well-defined method that can resolve the same velocity error in PIV measurement in larger flames, significantly improving the accuracy of PIV in reacting flows.

This study presents an experimental investigation into the vortex dynamics within lined slit–cavities subjected to coupled grazing flow and acoustic excitation. Central to this investigation is the role of velocity ratio ({U}^{*}) (defined as the ratio of mainstream velocity to acoustic particle velocity) within the range of 0 to 56.6 in modulating flow-acoustic characteristics. An experimental setup integrating microphones, pressure transducer arrays, and particle image velocimetry (PIV) systems was developed to synchronously capture acoustic responses, pressure fluctuations, and unsteady flow behaviors. Crucially, the PIV system incorporated a field-programmable gate array, leveraging its real-time computation capability to ensure precise synchronization of acoustic-fluidic interactions during phase-locked measurements. Transmission loss analysis reveals a critical threshold at ({U}^{*}=) 14.9 that bifurcates the acoustic response into two distinct regimes: weak influence regime (0 (le {U}^{*}<) 14.9) and strong influence regime (14.9 (<{U}^{*}le) 56.6). Subsequently, the time-averaged flow fields and spatiotemporal vortex evolution characteristics were comparatively investigated in two distinct regimes. In contrast to the symmetric vortex evolution observed in the absence of grazing flow, two distinct evolution patterns are identified: a separated vortex evolution under weak flow-convection effects and a merged vortex evolution under strong flow-convection effects. The systematic analysis of acoustic-vortex conversion efficiency was conducted sequentially through pressure fluctuations, velocity fluctuations, and dominant modes. The results reveal that high-speed grazing flow significantly suppresses coherent structures within the slit.

An experimental study was conducted to measure spray transfer efficiency, defined as the mass fraction of sprayed droplets that adhere to a target surface, and the rate of surface coverage by impacting droplets. The objective was to determine how transfer efficiency and surface coverage rates vary with droplet size distribution and air velocity, which is important in selecting spray parameters in painting and coating applications. The study was conducted using a wind tunnel consisting of a 6.5-cm-diameter tube connected to a tubular fan, producing controlled airflow velocities from 2 to 9 m/s. Sprays of canola oil or a 33 vol% glycerin–water mixture were introduced into the airstream and directed toward a 10-cm-diameter target disk, where the mass of deposited droplets was measured to evaluate transfer efficiency. Transfer efficiency was calculated by dividing the total target weight change by the weight of liquid sprayed. Droplet diameter distributions were measured using a direct imaging method. A high-speed camera was used to photograph droplets landing on the substrate and the rate of area coverage by impacting droplets measured. Transfer efficiency and surface coverage rates increase with airstream velocity. Larger droplets, whose motion is dominated by inertia, have a higher probability of reaching the target and a higher transfer efficiency. Below a critical Stokes number (St < 0.25), droplets fail to reach the target, irrespective of velocity. Droplet trajectories were modeled using an analytical solution to the inviscid stagnation flow problem to determine air velocities and calculate drag forces on droplets. A stochastic model of droplet transport and deposition accurately predicts transfer efficiencies and rates of surface coverage, except at higher (> 5 m/s) velocities where turbulence in the flow increases.