Experiments in Fluids最新文献

High equilibrium wall temperatures in hypersonic flight necessitate cooling of apertures (windows) used by outward-looking electromagnetic sensors to meet material temperature limits. The use of transpiration cooling to achieve this is investigated experimentally using a flat plate model with a porous ejector in the University of Oxford’s High Density Tunnel with a Mach 6.1 freestream flow and a turbulent boundary layer. A backward-facing step either upstream or downstream of the porous ejector is introduced to represent a flight geometry after an insulating patch or shroud has been removed. Infrared thermography is used to gather spatially resolved measurements of cooling performance on and downstream of the ejector. For the conformal configuration, the cooling performance on the porous ejector itself is lower than predicted by existing correlations, and comparison with a slot cooling correlation suggests lower cooling performance for the same mass flow rate over most aperture-relevant regions considered. However, this comparison is correlation-based and geometry-specific, so should not be generalised from the present work alone. For both direct cooling on the ejector and downstream cooling, there is a significant discrepancy between the current work and previous experiments using a similar model in the same wind tunnel. With a backward-facing step upstream of the ejector, the coolant fills the separation region, producing a region with high cooling performance, moving the reattachment location downstream, and eliminating the peak in heating at the reattachment point. However, the effectiveness downstream of the reattachment is lower than for a conformal geometry due to increased mixing and lateral spread of coolant. With a backward-facing step downstream of the ejector, the cooling performance is either better than or similar to the conformal case, except for a local overshoot of around 10% near the reattachment point.

In electrostatic spray painting it is important to maximize transfer efficiency (TE), defined as the fraction of liquid sprayed that adheres to the part being painted. In this study a glycerin (33 vol%)-water mixture was atomized and carried by an air stream with velocities of 2–9 m/s towards cylinders (6.5 cm, 3.5 cm, and 1.5 cm diameters), spheres (6.5 cm diameter) and disks (10 cm diameter) and the TE determined by measuring the weight increase of the target and dividing by the weight of the liquid sprayed. Tests were done both with and without a 40 kV potential applied to the spray nozzle A stochastic model of droplet transport was developed in which flow streamlines around the targets were derived from potential flow solutions. Droplets were assumed to be charged to the Rayleigh limit and the electric field to be one-dimensional. Electrostatic and drag forces on droplets were calculated to predict their trajectories. Transfer efficiency was found to increase with airstream velocity and target size. Calculated values of TE agreed well with those measured in experiments. The inertial Stokes number, which is the ratio of the droplet relaxation time to the inertial time for the flow, characterizes the forces acting on droplets. Droplets with high St_i, which were those with large diameter and high air velocity, were more likely to land on the target surface. Applying a 40 kV potential to the spray nozzle significantly increased TE at low air velocities (2 m/s), where TE increased by approximately 40%. At high air velocities (9 m/s) the electrostatic field had little effect on TE. An electrostatic Stokes number was defined as the ratio of an electrostatic time scale to the inertial time scale. Small values of ({text{S}text{t}}_{e}) mean that electrostatic forces strongly influence droplet trajectory and are therefore most effective in improving TE by forcing small droplets transported by low velocity airflow onto the target.

Foamy-oil flow plays a critical role in enhancing heavy-oil recovery; however, the mechanisms governing bubble nucleation, growth, and coalescence in porous media remain insufficiently understood. In this study, a microfluidic porous media system was employed to experimentally investigate the statistical evolution of gas bubbles during pressure depletion processes. High-resolution images were recorded during controlled depletion tests under varying operational conditions, including pressure-decline rate, soaking time, end pressure, and cyclic injection strategies. Image-processing techniques based on threshold segmentation and connected-component labeling were applied to extract bubble characteristics, including number, size distribution, and spatial evolution at different time steps. The results reveal that bubble-size distributions can be effectively described using normal distributions, enabling quantitative tracking of mean bubble size and standard deviation over time. Analysis shows that during early stages of depletion, both mean bubble size and standard deviation increase simultaneously, indicating active nucleation and broadening of the bubble-size spectrum. Beyond a critical point ((approx 730) kPa), a divergence between these parameters is observed, suggesting a transition from nucleation-dominated behavior to growth and coalescence-dominated mechanisms. Furthermore, higher pressure depletion rates lead to increased bubble nucleation due to enhanced supersaturation, while extended soaking times promote gas diffusion up to a threshold beyond which additional soaking yields minimal impact. Cyclic experiments demonstrate that different pressure depletion strategies, particularly stepwise decreasing end-pressure patterns, significantly influence bubble generation and system performance. Overall, this study provides a comprehensive statistical framework for understanding bubble evolution in foamy-oil flow within porous media, offering insights that can be utilized to optimize production strategies in heavy-oil reservoirs.

With advances in experimental fluid mechanics, optical measurement techniques have been widely adopted owing to their non-intrusiveness and high spatiotemporal resolution. However, conventional synthetic Schlieren techniques face challenges such as strong dependence on liquid depth and limited robustness in highly noisy environments. This paper presents a single-camera synthetic Schlieren method based on double color-coded checkerboard patterns, enabling real-time, depth-independent reconstruction of free-surface topography via color-channel separation and wavenumber-domain decoupling. Experimental results demonstrate that the method can accurately capture static surface slopes in a vibration-noise environment, and successfully reconstructs dynamic surface deformations induced by water droplet impact, thereby offering a practical and reliable approach for high-precision, time-resolved measurement of free-surface flows.

Time-resolved fast-response pressure-sensitive paint (PSP) measurements on the impingement plate, synchronised with far-field microphones, are used to develop and assess a plate-resolved diagnostic framework for extracting the wall-pressure content that is most relevant to the radiated sound in dual-impinging jets. Two identical square jets impinge normally on a rigid plate at impinging distance h/D = 4 and dual-nozzle distance s/D = 3.5 (D is the equivalent diameter), covering broadband, single-tone and multi-tone regimes. Spectral proper orthogonal decomposition is applied to the PSP pressure fluctuations to obtain frequency-resolved coherent wall-loading structures. Each leading structure is analysed in wavenumber–frequency space and separated, using transport-velocity masks, into an acoustic component, a convection-dominated component, and a low-phase-speed oscillatory component. The extracted acoustic subset reproduces the dominant far-field peaks when used as input to a surface radiation model under a baffled-plate idealisation, while the total wall loading remains dominated by non-radiating content. Across regimes, the broadband state at M = 0.7 is controlled by low-phase-speed oscillation and radiates weakly, whereas single-tone lock-in at M = 0.9 and multi-tone staging at M = 1.2 correspond to a selective re-organisation of the coherent wall loading in which the convective imprint becomes more prominent and a larger fraction projects onto the radiating subset. Resonance strengthens noise primarily by redistributing the wall-loading budget towards the radiating acoustic subset, providing a basis for diagnosing and targeting the acoustically potent part of plate loading in coupled impingement flows.

This study investigates natural convection in a differentially heated cylindrical annulus using Wollaston shearing interferometry. Temperature-induced refractive index variations in silicone oil B5 are recorded as interferograms and processed with a windowed Fourier transform to reconstruct the two-dimensional line-of-sight mean temperature field, providing the general objective of this study, a quantitative description of the mean temperature distribution. To validate the approach, complementary three-dimensional numerical simulations are performed. The resulting temperature fields are used to generate synthetic interferograms. A qualitative and quantitative comparison shows moderate agreement in fringe structure, fringe density and flow features, including the characteristic crescent-shaped convective pattern, with reconstructed mean temperature fields consistent with numerical results. Remaining discrepancies are attributed to optical distortions, non-ideal thermal boundary conditions and light deflection in regions of strong gradients. Overall, the results demonstrate a robust framework and non-intrusive method for quantitative temperature reconstruction in convective flows.

This review summarizes state-of-the-art knowledge on phosphor thermometry (PT), which has evolved into a powerful tool for studying complex thermal fluid problems and testing mechanical and aerospace systems. As the thermographic phosphors covering room temperature to 2000 K are now well established, full-field temperature measurements have been achieved for both on-surface and in-flow scenarios. Recent studies have significantly advanced every aspect of this technology, including phosphor development, coating/particle fabrication, measurement strategy and system integration. Sources of measurement uncertainty have been comprehensively studied regarding the key components of measurement system, as well as the interfering factors commonly faced in harsh environment. The fundamental research and technological advances of PT push its limit toward more challenging applications including hypersonic aerothermodynamic experiment, turbine blade testing, engine combustion diagnostics and others. Interdisciplinary research has driven the maturing of PT technology and will propel future breakthroughs in methodology and application.

This paper examines the occurrence of the Coandã effect in the context of automotive windshield defrosting. A generic, parameterized pseudo-2D test rig representing the main windshield defrosting components of a passenger vehicle is introduced. By externally controlling key geometric features such as the defrost duct angle and dashboard position, attachment of the defrost jet to the dashboard due to the Coandã effect is induced, which is incompatible with the functional requirements of passenger vehicle HVAC systems. By exploiting the tendency of Coandã flows to exhibit hysteresis, steady mono- and bistable states are identified in which the turbulent defrost jet remains attached either to the windshield or the dashboard over extended time periods, as well as unsteady configurations characterized by approximately memoryless switching between states. Two-dimensional two-component particle image velocimetry (2D2C PIV) is used to analyze the mean flow fields of the identified jet states in terms of velocity and turbulent kinetic energy contours, maximum velocity decay, and half-width growth rates. In addition, pressure coefficients measured along the windshield centerline demonstrate how proximity to the dashboard alters the pressure distribution to resemble that of a shallower or steeper duct angle through deflection of the defrost jet. Proper orthogonal decomposition is applied to the PIV data to extract the switching modes and their associated temporal coefficients in unsteady states, with subsequent statistical analysis indicating that the observed state switching exhibits Poissonian characteristics. Beyond the present study, the experimental data are intended to serve as a benchmark for the validation and assessment of CFD results.