Experiments in Fluids最新文献

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.

Piezoresistive (PR) probes can provide absolute pressure measurements, but their practical use in rotating detonation engines (RDEs) has been hindered by the harsh thermal environment of the combustion chamber. In general, the probe must be recessed to protect it from intense thermal loading of the RDE combustor. Although probe recess safeguards the hardware, measurements are subjected to thermal drift, in addition to signal attenuation, phase shift, and resonance. In this study, four different recess probe mounts are investigated experimentally in an RDE to account for the thermal drift in PR probes. A modified Wheatstone bridge is used to measure the sensor temperature during the test, and a steady-state methodology is employed for thermal compensation. Results show that all probe mount configurations provide time-varying pressure measurements with good accuracy, although pressure attenuation is greater with a smaller port diameter. The study identified that absolute and/or time-averaged pressure measurements incur large errors in a non-isothermal probe during the RDE test. Accurate absolute pressure measurements by PR pressure probes can be obtained in RDEs, but it requires a probe mount to ensure an isothermal probe during the test.

We employ advanced laser-based diagnostics to investigate the spray characteristics of a custom-designed flow-blurring (FB) atomizer. Structured Laser Illumination Planar Imaging (SLIPI)-based methods are utilized to obtain two- and three-dimensional (2D–3D) maps of the droplet Sauter Mean Diameter (SMD) and planar liquid volume fraction, while the conventional particle image velocimetry is used to measure droplet velocities within the spray in 2D. Water and ethanol sprays are investigated across a range of air-to-liquid mass ratios (ALRs = 2–5). The SLIPI-based fluorescence/scattering ratio maps of droplet relative SMD are calibrated and compared using the Phase Doppler Particle Analyzer data, demonstrating strong agreement with a low mean absolute percentage error (MAPE), especially at lower ALR. However, at higher ALR, the optically dense sprays result in increased multiple scattering effects, leading to higher MAPE values. SLIPI effectively captures the transient and polydisperse nature of FB sprays, with droplet size distributions closely following a gamma function. Additionally, a correlation for SMD is developed in terms of key non-dimensional groups, which represent the geometric parameters, physical properties of the working fluid, and spatial locations within the spray. The relevant dimensionless groups in the SMD correlation based on the geometric and physical properties are Laplace number (300-9700), liquid Reynolds number ((text{Re}_l=95) and 140), and Ohnesorge number ((text{Oh}=0.0027) and 0.0065).

A stereo camera method for tracking the position and orientation of rigid non-spherical particles is presented. The method works by comparing the images of the observed particle with that of a geometric model of the observed particle. The comparison of images is formulated as an optimization problem that determines the position and orientation of the particle at each time step. The optimization problem is solved using gradient descent by implementing the objective function in the differentiable programming library PyTorch. The method is validated using synthetic data, demonstrating robustness to both regular and irregular particle shapes. The influence of image noise, pixel density, and the resolution of the geometric model on the accuracy of the recovered position and orientation is addressed, and the results show sub-pixel accuracy to around 0.3 pixels at realistic experimental conditions. Finally, the method is applied to an experiment of non-spherical particles settling in a quiescent flow. Three regular and ten irregular particles are used, with particle Reynolds numbers ranging between 200 and 800. The results show that the instantaneous vertical velocity of the particles can vary by up to 50% due to changes in orientation. Different settling modes are identified, highlighting the importance of tracking all 6(^circ) of translational and rotational freedom to capture the dynamics of settling non-spherical particles.