Experiments in Fluids最新文献

Obtaining reliable restart of hypersonic inlets in the event of accidental unstart remains a key performance metric that constrains the operational boundary of powered hypersonic vehicles. The present work investigates the restart dynamics in a self-starting axisymmetric hypersonic Buseman inlet, which have a strong practical relevance in future platforms. The restart process was triggered by rapidly decreasing the aerodynamic blockage from a high-bandwidth counter injected jet, which allows the examination of the restart dynamics without being masked by the back pressure transience. Two-dimensional time-resolved pressure fields over inlet and isolator surfaces and time-resolved external shock field during the restart process are obtained at 10 kHz repetition rate. Both these measurements revealed a complex unstart shock motions during restart that include small amplitude relatively broadband oscillations prior to restart, a large amplitude periodic oscillation at the early restart phase, and subsequently followed by a dominant downstream shock motion. A complex flow structure within the external contraction portion of the inlet was discernable in the schlieren imagery with shear layer eddies being spilled out of the inlet. The pressure fields further evidenced a strong variation in the streamwise and azimuthal direction reinforcing the occurrence of flow spillage along both directions. Overall, the restart duration was determined to be approximately 11 ms, which is substantially greater than the unstart time scale of 8 ms obtained in the same inlet using the same back pressuring system. The critical process that extends the restart time is the periodic shock oscillations, which is termed as intermediate buzz in prior investigations, that pervade the first half of the restart process. The power spectral density of the unstart shock motions pointed to a shift in the mechanisms that drive the unstart shock motions during restart.

Delta wing aircraft generates prominent leading-edge vortex systems, which create a low-pressure region on the leeward side of the wing. The size and position of this low-pressure region significantly influence the aircraft’s aerodynamic performance. Using a 65° delta wing configuration, this paper develops a physics-driven approach based on point-vortex theory: employing a sparse pressure sensor array to identify leading-edge vortex structures and evaluate vortex influence in the wing’s leeward flow. Pressure measurements across the leeward surface of a delta wing track the trajectory and strength of the leading-edge vortex in a measured cross-flow plane. And the gradient of the pressure standard deviation curve (which changes with angle of attack) acts as a reliable indicator for identifying vortex breakdown within that section. Crucially, the dimensionless parameter K, derived from a simplified point-vortex model, quantifies interaction strength as a function of peak suction. Significantly, measurements obtained well ahead of the trailing edge correlate well with the leading-edge vortex’s influence. This holds true even under unsteady free-stream conditions or when vortex breakdown occurs leeward, provided it does not reach the instrumented planes. This approach can advance the intelligent and unmanned development of future aircraft and, furthermore, provide support for early warnings related to high angle of attack (AOA) maneuvers in next-generation fighter aircraft.

We propose a method to perform accurate temperature measurements using laser-induced fluorescence (LIF). We use a sCMOS color camera and a two-dye solution consisting of RuPhen and fluorescein, excited at 450 nm. By varying the relative concentration of the dyes, we can tune the temperature sensitivity of the color channels. This enables a robust laser power correction, reducing the effects of experimental noise compared to the convectional ratiometric approach. Furthermore, the overall temperature sensitivity is only slightly lower compared to that of the temperature-sensitive dye, which is not the case if a ratiometric analysis is performed. We demonstrate the capabilities of our method using a benchtop setup with precisely controlled temperatures. Our error analysis shows that an accuracy better than 0.5 (^circ C) can be achieved. The correction method can be applied to other fluorescence measurement techniques, including pressure-sensitive paints (PSPs).

Surface tension is a key parameter for understanding interfacial behavior in liquids and plays a crucial role in both fundamental scientific research and practical engineering applications. This paper presents a novel method for measuring surface tension, which is based on quantifying the capillary force between particles of equal diameter. The method employs a micro-thrust measurement system from aerospace engineering to directly measure the capillary forces and utilizes an approximate solution to the Young–Laplace equation for liquid bridges formed between spherical particles, establishing a quantitative relationship between capillary force and surface tension. Additionally, CCD industrial cameras were used to capture morphological changes during the stretching of the liquid bridge and analyze gravitational effects via the Bond number to validate the approximate calculation. Surface tension values were measured for dimethyl silicone oil, dodecane, and tetralin, with experimental results demonstrating good agreement with commercial instruments, exhibiting a maximum error below 5%. The proposed method requires minimal liquid volume and is independent of liquid density, offering a reliable and practical approach for surface tension characterization.

This paper proposes a pyramid-based coarse-to-fine background-oriented schlieren (BOS) tomography method to improve three-dimensional reconstruction accuracy in fields with large density gradients. First, a pyramidal BOS digital system with multiple resolutions is constructed to enable synchronized upsampling and downsampling of both flow fields and background images. Subsequently, at each pyramid level, image warping and forward linear/nonlinear tracking are used to progressively correct the projection data and projection matrices, enabling tomography reconstruction at different spatial scales and high-resolution refinement. The performance of the proposed method was demonstrated in both synthetic and real BOS cases.

Graphic Abstract

Focusing shocks are created underwater by exploding 10-(mu)m-thick copper foils with circular and polygonal geometries. Their symmetry and trajectory are characterized to assess this technique’s potential contributions to fundamental and applied investigations of nonlinear wave propagation and high-energy-density phenomena. The foils are exploded using a pulsed power generator which delivers kiloamp currents in microseconds. Current and voltage time traces of the explosions are recorded concurrently with high-speed shadowgraph images of the shocks. The electric waveforms of the explosions of different foil geometries resemble each other, showing peak resistive voltages, currents, and powers around 10 kV, 300 kA, and 2.5 GW, respectively. By extracting the shocks’ trajectories through statistical analysis of the shadowgraph images, it is found that circular foils, whether free standing or attached to the inside of a plastic shell, create shocks which accelerate up to Mach 1.7. Comparable Mach numbers are achieved by exploding a circular wire array of 32 100-(mu)m-diameter copper wires, indicating that foil designs perform similarly to this traditional design. In contrast, free-standing polygonal foils create shocks which travel at a constant near-sonic speed, seemingly behaving as non-interacting weak planar shocks. This contradicts the theoretically predicted reshaping and acceleration of such shocks; manufacturing imperfections are suspected to cause this unexpected behavior. Alternate designs in which foils are attached to polygonal plastic shells are tested and found to create shocks which do reshape and accelerate.

Mechanism of spray breakup in a swirl coaxial aerated injector (SCAI) that couples internal effervescent mixing with a swirl stream is reported. Shadowgraph experiments span liquid flow rate, injection pressure, gas-to-liquid ratio (GLR) and swirl momentum ratio (MR) to map how parametric variations influence the near-field atomization pathway. The images reveal a consistent sequence of mechanisms—intact core, interfacial waves, ligament formation and fine fragment clouds—whose location and intensity shift systematically with different operating points. Increasing liquid injection pressure tightens the core and advances breakup toward the lip; increasing GLR promotes early stripping, broad liquid-film shedding and a wider droplet-size spectrum; increasing liquid flow rate (at fixed aeration) lengthens the intact region and delays fragmentation; adding coaxial swirl shortens the intact length, widens the cone and redistributes fragments into an annular shell. To quantify these changes, we apply snapshot proper orthogonal decomposition (POD) to large image datasets captured at each operating point in order to isolate the coherent structures. Four spectral modes were sufficient for every test case. These findings provide a reduced-order description that links injector settings to breakup physics and serve as guidance for injector spray design.

The primary breakup process of an atomizing spray is an intriguing area of research, for which the fluid mechanics governing primary breakup at liquid–gas interfaces (LGIs) remain largely under-resolved. This work evaluates the performance of wavelet-based optical flow velocimetry (wOFV) for measuring velocities at the LGI during the primary breakup process of an atomizing liquid jet. Sequential 2D scalar images, rendered from a direct numerical simulation (DNS) of a temporally evolving atomizing liquid jet, are used as ground-truth data to quantify wOFV accuracy at different stages of breakup. The sharp interfacial intensity gradients inherent to spray images enable wOFV to achieve good accuracy with an error down to 6.5% for the images analyzed. Two regularization terms are assessed (first-order Horn & Schunck and second-order Laplacian) and yield comparable error magnitudes, likely due to the flow exhibiting a strong principal flow direction. Accuracy improves markedly as breakup progresses due to increasing interfacial corrugation and reduced local regions of motion ambiguity. In this work, wOFV is shown to provide 20–40% higher accuracy than cross-correlation-based methods with superior vector resolution. This application of wOFV shows better accuracy compared to previous optical flow investigations involving diffuse scalar fields. The reduced motion ambiguity from sharp interfacial intensity features of the LGI results in accuracy levels closer to those achieved for discrete particle images. A simple warping-based procedure is introduced to provide an estimate of wOFV accuracy in the absence of ground-truth data. This procedure is tested on both synthetic images as well as experimental images of a turbulent jet undergoing primary breakup. Findings show good agreement with error metrics from ground-truth data and can provide valuable guidance in (lambda) selection and in estimating the uncertainty of wOFV. Findings demonstrate the proficiency of wOFV to resolve velocity estimates along interfaces with high-resolution and acceptable accuracy, providing promising opportunities to study the fluid mechanics of primary breakup, which will be presented in future studies.

Graphical abstract

This paper investigates the interaction between shock waves and boundary layers in half-nozzles under varying nozzle pressure ratios (NPR) and inlet air humidity. Three nozzle geometries (N1, N2, N3) with distinct expansion rates were tested experimentally and numerically under varying nozzle pressure ratios (NPR = 1.4, 1.6, 1.8) and relative humidity levels (30%, 50%, 70%). Experimental methods included high-speed Schlieren imaging (6000 fps) and high-frequency pressure transducers (> 10 kHz), while numerical simulations employed the Unsteady Reynolds-Averaged Navier–Stokes (URANS) method with the k-ω SST turbulence model. Results show that increasing NPR elevates oscillation frequencies, while higher humidity amplifies these frequencies. The URANS method predicted primary frequencies accurately but required hybrid URANS/LES approaches for broader spectral resolution.

In this paper, we report on the dynamics of micro-jet impact and spreading on a solid substrate. The jets are the product of electrical discharge in a liquid confined to a capillary tube; the resulting spark creates a pressure impulse which rapidly deforms the concave meniscus into a fine jet ((d_{jet} sim O(100),upmu)m) and then a heat-induced vapor bubble which expands and ejects more liquid from the end of capillary tube, as previously described in Rohilla and Marston (Exp Fluids 64:90, 2023) and Lawal et al. (Int J Pharm 674:125400, 2025). Here, we provide insight into the impact and spreading of these micro-jets across a range of fluid properties. We found jet speeds up to 81 m/s encompassing a wide range of Weber and Reynolds numbers (We (sim O(10^{1} {-} 10^{4})) and Re (sim O(10^{1} {-} 10^{4}))), with different modes such as simple deposition, fine/early splash, and violent splashing. In accordance with previous reports on splashing in drop impact, we found that a modified Weber number based on the ejecta sheet, We (= rho delta u_{ej}^{2}/sigma), provides a concise way to delineate phenomena such as deposition vs. splashing, while maximum spreading is best described using, (beta _{max } sim sqrt{text{We}}).