Experiments in Fluids最新文献

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}}).

We present measurements of temperature fields and flow structures in a liquid metal Rayleigh–Bénard convection at a low Prandtl number, which were carried out for the first time using embedded fiber Bragg grating sensors (FBG) in combination with ultrasonic Doppler velocimetry (UDV). The FBG sensors enable minimally invasive, spatially resolved temperature measurements in optically opaque and electrically conductive liquids, thereby overcoming significant limitations of conventional thermocouples and optical techniques. This approach was applied in a cuboid Rayleigh–Bénard cell with an aspect ratio (Gamma = 5) filled with GaInSn. In this paper, we present measurements at two Rayleigh numbers, (textrm{Ra} = 6.8 times 10^4) and (2.1 times 10^5). At the lower Rayleigh number, a coherent three-roll structure is observed with low-frequency modulation of thermal fluctuations. At the higher Rayleigh number, a cellular convection regime emerges, featuring checkerboard-like temperature patterns in the mid-plane and periodic plume emissions. Spectral analysis reveals a dominant oscillation frequency near (f = 0.029) Hz, while autocorrelation and extremum tracking highlight strong temporal coherence near the center and more volatile plume behavior near the sidewalls. The results of the temperature measurements and the UDV velocity measurements are consistent, thus confirming the capability of FBG sensors as a robust tool for investigating the spatio-temporal dynamics in convective systems.

This review aims to provide an overview of laboratory models of nearshore morphodynamics, focusing specifically on the role of wave action. In this context, shoreline evolution is driven by the direction of sediment flux induced by wave dynamics, corresponding to erosion or accretion processes. These processes, which naturally modify the shape of coastlines, are influenced by factors such as sediment availability, wave climate, and soil strength, among others. Starting from unconsolidated sandy materials and the equilibrium-based concepts used in natural beach classification, the reanalysis of laboratory experiments shows that they can reproduce natural conditions and serve as conceptual models for understanding nearshore morphodynamics. However, the temporal evolution of the shoreline and the source of available sand are difficult to capture through equilibrium-type physical processes and therefore requires a specific focus on localized zones of the nearshore, depending on the prevailing hydrodynamics and sediment strength, i.e., consolidation. Accordingly, the local dynamics near the shoreline are closely linked to the behavior of the swash zone and to cliff erosion processes, which constitute a central focus of this review from a laboratory experiment perspective.

In urban environments, pollutant ingress from outdoor sources poses a significant challenge to indoor air quality. Cross-ventilation, while essential for passive cooling and natural airflow, can also facilitate the entry of outdoor contaminants into indoor spaces. To investigate the dynamics of outdoor-to-indoor pollutant transport, the present study employs an idealized configuration, namely, a hollow cube representing a scaled-down model building with window openings in the upstream and downstream faces, subjected to an upstream passive scalar source within an atmospheric boundary layer. The experiments are conducted in two distinct facilities: a water tunnel using Rhodamine dye as the scalar, and a wind tunnel using propane gas, all performed at a specified flow Reynolds number of ( text{Re} = U_{{{text{Ref}}}} H/nu approx 50,000 ) for a fixed boundary layer-to-cube height ratio of about 3; here, ( U_{{{text{Ref}}}} ) is the streamwise velocity at cube’s height (H) measured without the cube. The scalar, released from a ground-level upstream source, is predominantly transported by a streamwise advective flux, while relatively weaker wall-normal advective and turbulent fluxes contribute to vertical dispersion and local mixing. A fraction of the oncoming scalar enters the cube intermittently, through the upstream window. Inside, a central jet-like flow carries the scalar parcels primarily by streamwise advective flux, while also interacting with the upper and lower recirculation regions, enabling scalar exchange across these zones through wall-normal advective and turbulent fluxes. While the time-averaged concentration field inside the cube is nearly uniform, suggesting effective mixing, instantaneous concentration traces exhibit strong intermittency, with sporadic peak events, highlighting the risk of transient peak exposures. The indoor concentration exponentially decays over time once the source is turned off, with a slower decay in the upper recirculation region, implying relatively prolonged exposure near the ceiling region. Both experimental setups produce closely matching values and consistent trends in the spatio-temporal dynamics of scalar concentration, and also highlight their complementary nature, with each offering distinct advantages. The present findings will deepen our understanding of pollutant ingress and mixing in buildings in cross-ventilated flows and also offer valuable insights to future modeling of pollutant exposure in urban indoor spaces.

The relationship between in-cylinder flow structures and subsequent turbulent flame growth in a production spark-ignition engine was investigated using combined phase-locked particle image velocimetry (PIV) and high-speed flame chemiluminescence (CL) imaging via endoscopic access. Measurements were taken in a 4-cylinder production engine operating at 2000 rpm and 75 Nm torque with a stoichiometric fuel–air mixture. For two-dimensional flow field, a double-frame sCMOS camera was used to acquire 143 cycles of particle image pairs at 50°CA bTDC (15.5°CA before ignition timing). Simultaneously, time-resolved flame propagation images were captured every second combustion cycle using a high-speed CMOS camera (Vision Research Phantom v7.3) at a frame rate of 11 kHz. Broadband chemiluminescence imaging began at 50°CA bTDC and continued until 6°CA bTDC. The acquired images were analyzed in conjunction with pressure-derived heat release rates and mass fraction burned (MFB) to elucidate the relationship between engine performance and the physical characteristics of flame propagation. Analysis of instantaneous velocity fields from individual cycles unveiled substantial cyclic variations in flow structure. Combined flow–flame imaging demonstrated that the direction and magnitude of the flow near the spark region significantly influenced spark plasma orientation and early flame kernel development. Correlation map analysis indicated a strong positive correlation between local velocity magnitude and average flame speed, particularly in the vicinity of the spark plug. The high-correlation region is associated with the largest differences in the mean flow fields between slow and fast cycles.