Experiments in Fluids最新文献

For the past twenty years, fractal grids have shown their efficiency at generating long-lasting broadband turbulence. Reference studies have so far focused on developing the most canonical self-similar turbulence far downstream of the grid; the production region was stretched out, and the maximum turbulence intensities at the peak remained limited. We explore a new region of the parameter space of fractal square grids to accelerate turbulence production, shorten the wake-interaction length, and enhance the kinetic energy in the early stages of the decay. The length-to-thickness ratio (L_0/t_0) of the first iteration is much lower than in typical grids reported in the literature, yielding peak turbulence intensities of 21–27% within (x/L_0 < 3). The experiments were conducted in a recirculating hydro-channel facility using Particle Image Velocimetry for spatially extended data and Laser Doppler Velocimetry for time-resolved centerline statistics. A sweep of progressively smaller PIV fields of view, with vector spacing down to (Delta x / eta approx 1.3), is used in the decay region to approach the dissipative range. The analysis focuses on the production region and the early-decay region within the wake-interaction length ((x < x_*)) and combines centerline statistics, characteristic length scales and the spectral signature along the flow. A scale-by-scale spectral assessment, benchmarked against forced-isotropic DNS, shows that local isotropy is recovered over the reliable resolved wavenumber range, whereas global derivative-based indicators (K_1) and (K_3) remain affected by large-scale anisotropy and finite PIV resolution. The results are consistent with the non-equilibrium scaling reported in earlier studies of fractal-grid-generated turbulence. The new grids, however, reveal that the dissipation scaling (C_epsilon sim sqrt{text {Re}_{L_0}}/text {Re}_lambda ) no longer holds when the geometry is extended to low (L_0/t_0) ratios. Using (text {Re}_{t_0}) based on the largest-bar thickness restores the collapse with the literature, indicating that the relevant global Reynolds number is set by the vortex-shedding scale at which energy is initially injected, rather than by the largest geometric scale of the grid.

The non-contact interaction of oppositely charged droplets under strong electric fields involves coupled charge transport, gas ionization, and interfacial instability, yet direct evidence for the origin of polarity-selective breakup remains limited. Here, non-contact breakup of highly conducting oppositely charged droplets was investigated in air and helium using a self-developed microscopic collision platform and high-speed imaging. By varying droplet curvature, reversing the high-voltage-electrode polarity, and comparing acidic and alkaline electrolytes, we show that breakup in air consistently initiates from the negative-polarity droplet. The polarity preference is therefore not governed by droplet geometry, electrode configuration or liquid-phase ion mobility. In helium, the breakup at the negative side becomes weaker, and in some cases, non-contact breakup occurs at both polarity sides. For a given liquid conductivity, the critical nominal electric bond number for breakup is also higher in helium than in air. We suggest that this behavior is associated with gas ionization-mediated space-charge effects in air: Slowly migrating positive ions accumulate near the negatively charged droplet, distort the local electric field and increase the surface charge density until a local sub-Rayleigh instability is reached. In helium, weak electron attachment and faster charged-particle transport reduce this field asymmetry. These results indicate that gas ionization plays an important role in regulating breakup polarity and the dynamic outcome of oppositely charged droplets, providing guidance for electrosprays, microreactors, and electric field-controlled multiphase flows.

Reduction of frictional drag is examined on a 31 cm × 15 cm flat plate covered with superhydrophobic (SHPo) longitudinal microtrenches in a high-speed water tunnel capable of imposing wall shear levels representative of operating water vessels. Experimentation capable of testing substantially larger (~ 16× larger in area than before) microtrench-array surfaces in a large, high-speed water tunnel using a standard measurement method is developed, thereby providing high-quality, rigorously controlled experimental data to support and validate turbulent SHPo drag reduction. High-performance microtrench surfaces featuring hierarchical roughness and re-entrant trench tops are microfabricated on silicon wafers, diced into 7.6 cm × 4.7 cm tiles, and assembled as a 4 × 3 array on the drag plate interfaced with a drag balance for shear-drag measurement. The water tunnel enables control of water pressure and air saturation and allows a low-grazing-angle view for detecting plastron depinning. Two microtrench geometries are tested: (i) 50 µm pitch, 90% gas fraction, 100 µm depth, and 10 mm length, designed primarily for plastron stability and (ii) 100 µm pitch, 92% gas fraction, 200 µm depth, and 20 mm length for larger slip. The former geometry achieves around 30% drag reductions at flow speeds corresponding to a wall shear stress of about 100 Pa, surpassing the previous record of 70 Pa and reaching conditions relevant to practical vessel operation.

Graphical abstract

Building on prior work demonstrating that underwater microsecond timescale electrical explosions of semicylindrical wire arrays produce radially converging strong shock waves and blade-shaped supersonic jets reaching 1.6 km/s [Experiments in Fluids 67, 25 (2026)], this study presents experimental investigations and two-dimensional hydrodynamic simulations aimed at optimizing jets generation from tamped, V-shaped electrically exploded foils. Aluminum and copper foils with varying apex angles were exploded using μs and sub-μs pulse generators with peak currents up to 300 kA, in two tamping media: water and 20% gelatin/water solution. Shock formation and jet dynamics were characterized using multi-frame shadowgraphy and X-ray imaging. Results show that the highest tip velocities, up to 4.5 km/s, are achieved with a 60° apex angle, sub-μs drivers and gelatin tamping. These high-velocity jets are capable of accelerating gram-scale targets to ~ 0.3 km/s and driving shocks in fluid and solid media at pressures up to ~ 7 GPa, offering a promising platform for dynamic material characterization under extreme loading conditions.

We demonstrate a new capability to study scalar transport, combining coherent anti-Stokes Raman scattering (CARS) and femtosecond laser electronic excitation tagging (FLEET) for simultaneous measurements of temperature, pressure, and velocity along a one-dimensional (1D) measurement volume. Multi-parameter measurements were performed along a ~ 2-mm line in an underexpanded sonic jet on a single-laser-shot basis at 20 Hz. The femtosecond–picosecond (fs-ps) CARS instrument with an early and late probe pulse captured collision-free and collision-altered pure-rotational CARS spectra for measurements of temperature and pressure, respectively. The femtosecond pump/Stokes pulse from the CARS measurement was re-used as the FLEET excitation source and aligned with the one-dimensional CARS measurement volume at a time delay of several nanoseconds to eliminate interference with the CARS measurement. Radial profiles of temperature, pressure, and axial velocity were recorded at axial locations of 6.5, 15, and 19 mm, corresponding to measurements in the cold isentropically expanding gases before the Mach disk, the edge of the slip line downstream of the Mach disk, and through the inner shear layer developed from the slip line, respectively. From measurements performed in the nearly steady flow upstream of the Mach disk, estimates of measurement precision for temperature, pressure, and velocity were found to be 2%, 7%, and 3% about the mean, respectively. Flow downstream of the Mach disk was seen to have higher fluctuations, particularly near the inner shear layer. Measured flow properties match previous experimental and theoretical results in underexpanded jets. This study demonstrates the feasibility of the combined CARS-FLEET instrument for scalar transport measurements and provides a baseline for future applications in high-speed wind tunnel facilities.

We report on results from wind tunnel experiments examining the flight of European starlings (Sturnus vulgaris) in both steady flows and in the presence of a structured vortex wake generated by a pitching airfoil. The investigation employs synchronized high-speed kinematic tracking alongside time-resolved particle image velocimetry (PIV) to assess the birds’ flight mechanics and interactions with unsteady flow. When flying in the presence of a vortex wake, the starlings exhibited distinct kinematic adjustments relative to their trajectories in clean steady flow. The wake footprints observed through PIV corroborated the kinematic measurements, revealing a phase alignment between the birds’ wingbeats and the periodic wake background. The combined circulation, resulting from the merging of the bird’s flapping wake and the mechanically generated wake, closely resembles an in-phase superposition of the two components in the non-interactive cases.

Twyman–Green interferometry (TGI) has been used far less frequently than Mach–Zehnder interferometry for quantitative measurements of shock-dominated flows, despite its compact optical configuration. In this study, the underlying reason for this limited adoption is theoretically investigated. Optical analysis reveals a fundamental inconsistency in the conventional TGI image-superposition concept: The forward- and return-pass images are formed at different image planes and therefore cannot be simultaneously focused on a single camera sensor. This limitation can introduce systematic errors in quantitative measurements. To overcome this issue, offset Twyman–Green interferometry (Offset-TGI) is proposed. By introducing a spatial offset between the forward and return optical paths, the method utilizes a single properly focused image for interferogram formation, providing a physically consistent implementation of TGI for quantitative density measurements. The proposed method is validated through density-field measurements of a highly underexpanded round jet containing a Mach disk. The reconstructed density distributions and the density ratio across the Mach disk show good agreement with semi-empirical relations, theoretical predictions, and established optical diagnostics. These results demonstrate that Offset-TGI provides reliable quantitative measurements in shock-dominated flows while preserving the compact, single-beam-splitter architecture that makes TGI attractive for space-constrained experiments.

Improving mixing efficiency is critical for the development of energy-efficient intensified processes. Mixing complex and shear-sensitive fluids remains challenging: turbulence is needed to enhance mixing, but must be generated with minimal local shear to prevent fluid degradation. Oscillating grids offer a promising solution in this context. In this work, we investigate passive scalar mixing in a tank stirred by fractal oscillating grids. Injection experiments combined with pH-sensitive planar laser-induced fluorescence are used to quantify the effects of grid oscillation parameters (frequency, amplitude) and grid geometry on mixing time and local turbulent scalar dynamics. Results show that increasing the grid oscillation Reynolds number linearly increases mixing efficiency, suggesting optimal operation of oscillating grids at moderate amplitudes and frequencies. Fractal grids outperform conventional Cartesian grids, with the I-Fractal geometry providing the best performance. The large-scale advection induced by grid oscillations—typically stronger for fractal grids—plays a key role in the mixing process by accelerating scalar patch transport toward the high-turbulence, grid-swept region. The effective fractal dimension of the grid pattern or of the scalar field does not determine mixing efficiency. While mixing through turbulent cascade prevails all along the process for grids with weak large-scale advection, fractal grids allow it to be sustained on a larger range of scales. These findings pave the way for the design of efficient fractal oscillating grid mixers with potential applications in the scale-up of high-value bio-processes.

The flow characteristics of a permeable forward-facing step (PFFS) are investigated with particle image velocimetry, and the Reynolds number is (text {Re}_h=3000), 6000 and 9000, and the porosity of the step is (varphi =0), 0.36, 0.56 and 0.75. The influences of the porosity on the flow characteristics are examined. The recirculation region area before the step is decreased as (varphi ) increases from 0.36 to 0.75, which is attributed to the pressure-relief effect of the permeable step. As (varphi ) increases from 0.36 to 0.75, the reattachment length and the recirculation area above the step are increased for (text {Re}_h=3000), while they exhibit a decreasing trend for (text {Re}_h=6000) and 9000. Fluid exchange at upper porous interface is dominated by the blowing of fluid, the higher porosity enhances the fluid exchange intensity, and the fluid exchange intensity is increased as (text {Re}_h) increases. Moreover, for the three (text {Re}_h), the Reynolds stresses and turbulent kinetic energy before the step are decreased as (varphi ) increases from 0.36 to 0.75. For (text {Re}_h=3000), at (varphi =0.36), the corner vortex evolves to the upstream region of the step, and the spanwise vortex is generated from the separated shear layer above the step, and it moves downstream with concentrated vorticity. As (varphi ) increases to 0.75, the corner vortex is mainly distributed before the step, and the spanwise vortex is easily dissipated during its evolution due to the intensified fluid exchange at the interface. In addition, the blowing of fluid induces the vortical structures with positive vorticity at the pores of the porous interface. This study is helpful for understanding the modulation effects of the porous media on the forward-facing step flow, and it may be utilized for flow control of the wall-mounted structures in engineering applications.

Free-surface synthetic Schlieren (FS-SS) is a high-resolution, refraction-based optical technique for measuring the instantaneous elevation of a liquid interface. Under the assumptions of small amplitude, small slope, and small paraxial angle, the method yields a linear relationship between the gradient of the surface elevation and the apparent displacement field of a refracted pattern imaged through the surface. Here, we propose three new, nonlinear extensions of the FS-SS method that are specifically dedicated to telecentric imaging. Paraxial distortions are eliminated with a telecentric lens, thereby simplifying the optical model. This allows us to derive nonlinear surface reconstruction models that reach beyond the usual limits of small slope and small wave magnitudes. We implement these nonlinear surface reconstruction algorithms and compare them to the original, linear reconstruction algorithm in three different experiments, using a solid glass lens, spreading oil drops, and nonlinear Faraday waves. At the price of a few iterations, we can realize nonlinear surface reconstructions that are more precise, in particular when we reach high slopes or high-amplitude regimes. We share a library that encodes these nonlinear surface reconstruction algorithms.