Experiments in Fluids最新文献

Event-based cameras are dynamic vision sensors that acquire asynchronous, pixel-wise data when local changes in light intensity exceed a user-defined threshold. Their high temporal resolution and data efficiency are particularly advantageous for applications with rapid movement of sparse/discrete signals such as in flow velocimetry. While previous studies have demonstrated the application of event-based particle image velocimetry (PIV) and particle tracking velocimetry (PTV), the impact of key hardware limitations, specifically latency and bandwidth, on the velocity dynamic range remains underexplored. This work first characterizes event cameras using single-pulse planar illumination of a water/alumina suspension in a cuvette, where effects of laser pulse energy, particle image density/size, and contrast threshold are systematically investigated. A camera performance map showed that non-retrievable information loss occurred row-wise for event rates above (sim) 400 ev/(upmu)s as the readout interface saturated due to camera bandwidth. The observed correlation between event rate and camera latency constrained the minimum time interval between successive laser pulses, therefore limiting the velocity that can be effectively measured with event-based PIV. A subsequent set of experiments performed in an air jet with double-pulse PIV produced exploitable vector fields for a velocity dynamic range of 0.2–1.8 m/s (0.003–0.03 px/(upmu)s) with particle image densities above 0.018 ppp and uncertainties <0.4 px. Results indicate that camera latency and bandwidth introduce a complex trade-off between the time interval between laser pulses (velocity dynamic range) and particle image density (spatial resolution) to ensure robust and exploitable velocity measurements. The proposed methodology can be readily applied to evaluate other event-based cameras and serve as a practical guideline to set up and optimize event-based PIV.

Understanding the gust response of free-falling bodies such as plant seeds and debris is critical in predicting their dispersal. Furthermore, gusts can significantly affect the performance and survivability of low-inertia aerial vehicles. However, current methodologies for studying common gusts, particularly transverse gusts, which are characterised by the sudden appearance of a flow velocity component orthogonal to the flyer’s velocity, are not applicable to untethered or free-falling bodies. This article introduces a novel approach that addresses this limitation through an accelerating reference frame generating a fictitious force that temporarily scales and redirects the gravitational force. This approach is demonstrated through a first-of-its-kind vertical wind tunnel that accelerates horizontally in the direction normal to the flow with the same acceleration as the gust. A preliminary characterisation of the facility is presented. The tunnel acceleration generates the same pressure gradient as irrotational, uniform transverse gusts, without introducing the shear layer typical of Küssner’s gusts. The gust response of a free-falling dandelion diaspore to a discrete transverse gust (Wagner type) is demonstrated, but the proposed approach is suitable for arbitrary time series of transverse gusts, including Theodorsen-type periodic gusts. For the first time, this novel approach will allow investigating the dynamic response of untethered bodies to transverse gusts, including micro- and nanodrones, unpowered microrobots, plant seeds, debris and more.

Effective quantitative visualization of compressible flows across the full field of view is essential for understanding the flow topology and dynamics of supersonic jet oscillations, shock-boundary-layer interactions, and shock reflection and diffraction. The present study provides a methodology for obtaining the time-dependent density field of transient shock-dominated flows within a confined duct. A shock strong enough to induce flow separation and exhibiting unsteady behavior is introduced downstream of the throat within a divergent half duct. The incoming flow Mach number just upstream of the shock is approximately 1.47, and the Reynolds number calculated based on the height and flow properties at the throat is 1.35 (times) (10^5). We employ Mach-Zehnder interferometry with a finite-fringe setup to capture the time-resolved density field including the shock motion, utilizing a He-Ne laser as the light source and a high-speed camera as the recording device. A two-dimensional Fourier fringe analysis is employed to extract the phase information over the entire density field. Fascinating visual representations, such as the pseudo-infinite interferogram and the density field with phase information known as domain coloring, are introduced to illustrate flow topology. The oscillatory characteristics of shock motions are analyzed using both Lagrangian and Eulerian approaches, and the results are compared quantitatively. Furthermore, an uncertainty analysis is conducted to assess the accuracy of the density measurements and to reveal how shock oscillations affect density uncertainty.

This study introduces a single-camera synthetic Schlieren technique for measuring free-surface topography in fluid layers over a smoothly varying solid bed. By modeling light refraction through weakly deformed air–liquid and weakly varying liquid–solid interfaces, we establish a linear relationship between free-surface gradients and pattern displacements that yields an explicit bed-independent formulation for upward-looking configurations. The proposed framework incorporates coordinate mapping to correct refraction-induced parallax distortion influenced by the bed shape. Validation experiments featuring sinusoidal bed topography and wedge-shaped sloped bed topography achieve accurate spatiotemporal reconstruction of both static capillary meniscus profiles and dynamic water drop ripple evolution. The present method advances experimental capabilities for quantifying interfacial hydrodynamics in multi-layer fluid systems.

Surface roughness modifies the flow dynamics over static surfaces and can significantly affect the instantaneous generation of lift and drag. This study presents force and flow measurements on NACA0012 foils covered with simple, commercially available spherical-cap roughness elements. We varied the roughness area coverage relative to the propulsive area from 0% (smooth) to 35% (mid-rough) and 70% (full-rough). Our experiments survey an angle of attack and a Reynolds number range of (-2^circ le alpha le 20^circ) and 10,000 (lessapprox Re lessapprox) 55,000, respectively. Within this parameter space, surface roughness leads to small alterations in time-averaged statistics of lift and drag. In contrast, it leads substantial changes in unsteady force and flow behavior. Specifically, surface roughness reduces lift fluctuations, up to (sim 60%), due to decreased pressure fluctuations on the foil surface. This reduction is accompanied by a modest decrease in time-averaged lift coefficient and an increase in time-averaged drag coefficient. Drag fluctuations increase by up to (sim 30%), except near stall, where both lift and drag fluctuations decrease. Roughness also mitigates flow separation, as indicated by reduced velocity fluctuations and a delayed stall onset in the (C_L(alpha )) curves. These results show that surface roughness influences not only time-averaged statistics but also the instantaneous response of lift, drag, and flow fields. Our findings offer insights into the hydrodynamic function of shark-skin-inspired surfaces and demonstrate how simple, distributed roughness can provide passive control of boundary layer behavior and flow separation.

This letter presents an adaptation of a PIV-based pressure reconstruction for non-Newtonian fluid flows. This adapted method is applied to the free-surface flow of a viscoplastic fluid (featuring a yield stress) on a slope and interacting with an obstacle positioned over the channel bed and submerged in respect of the flow. For such a yield stress fluid, the viscosity is not constant but varies locally, which requires appropriate consideration in the momentum conservation equation. In addition, flow is characterized by regions where the fluid behaves like a solid, i.e., where the shear stress does not surpass the fluid yield stress. Then, the flow is characterized by the presence of non-sheared regions which require specific treatment in the pressure reconstruction process. This letter presents the principles of the adapted pressure reconstruction method, validates its implementation using two-dimensional numerical data, and demonstrates its feasibility by applying it to experimental 2D-PIV data from the literature.

The global drive toward carbon–neutral marine propulsion has positioned ammonia as a compelling alternative fuel due to its zero-carbon emissions. However, ammonia flash-boils easily, and there is no consensus in the literature on which optical techniques to use. Hence, this study aims to identify suitable optical methods to characterize flashing liquid ammonia sprays. Ammonia sprays were investigated in a constant volume chamber under ambient pressures (0.2–20 bar), corresponding to pressure ratio (R_P) values from 42.85 to 0.43 at a fuel temperature of 20 °C. Four optical diagnostics: Diffused Back Illumination (DBI), Mie scattering, shadowgraphy, and schlieren, were applied to compare their suitability. A single-hole injector and high-speed imaging were used to measure spray tip penetration (STP) and spray width (SW). All methods produced comparable STP values; however, differences were observed under flash boiling. DBI captured the liquid core, while shadowgraphy and schlieren detected a wider spray, indicating that they detected ammonia vapor and dispersed fine droplets; however, schlieren's additional detection of shock waves at high pressures made data processing more difficult. Mie scattering lost accuracy in wide plumes due to scattering-induced signal loss. It was concluded that a combination of DBI and shadowgraphy is most suitable to detect the dense liquid core, the vapor, and any dispersed atomized droplets.