Experiments in Fluids最新文献

Experiments were carried out to investigate the jet characteristics in various spray modes of electrohydrodynamic spray. Through high-speed imaging, the jet morphology of different spray modes was observed and three distinct regimes were proposed: single cone-jet, meniscus multi-jet, and edge multi-jet. The operating domains of various jet regimes of electrohydrodynamic spray within a relatively wide range of flow rates and applied potentials were identified. In the cone-jet regime, both the jet breakup length and the cone semi-angle increase with the rise of the electro-Bond number (EBo) and decrease with the increase of the electro-Weber number (EWe). This behavior shares similarities with classical hydrodynamic sprays where the balance of inertial and capillary forces governs the jet characteristics. In the meniscus multi-jet regime, the liquid meniscus height grows with the increase of (EBo) but is nearly independent of (EWe). In the edge multi-jet regime, the jet breakup length decreases with the increase of (EBo) and is hardly affected by (EWe). Meanwhile, the jet deviation angle is almost independent of both (EBo) and (EWe). The jet breakup length varies linearly with (EBo), with fitting coefficients of 1.8 and -0.35 in the cone jet regime and the edge multi-jet regime respectively. The jet diameter increases with the increase of (EWe) but is almost independent of (EBo). The results indicate that in the cone jet regime, the jet diameter increases with (EWe) in a power law with a coefficient of 0.6, while in the edge multi-jet regime, the jet diameter increases linearly with (EWe) with a coefficient of 0.018. These findings confirm that the stable edge multi-jet structure is robust.

The receptivity of the natural bi-modal flow asymmetry along a finite circular cylinder at high incidence in a uniform cross-flow to perturbations is investigated in wind tunnel experiments. Azimuthal perturbations along its forebody are effected by spatially and temporally varying distributed bleed actuation driven by pressure differences across an array of surface ports. While the base flow can evolve into a random one-sided asymmetric mode that is characterized by a pair of counter-rotating streamwise vortices of unequal circulations and consequently unequal aerodynamic loads, it is shown that azimuthally segmented bleed actuation can lead to either prescribed two-sided flow asymmetry or prescribed flow symmetry. Local interactions of the bleed flow along the forebody can either disrupt the formation of the one of the CW or CCW vortices in a prescribed manner by enhancing its circulation relative to its counterpart and displacing it away from the surface or can form a symmetric pair of counter-rotating vortices of nominally equal circulations. Consequently, control of the flow symmetry can lead to either side forces of prescribed sign (up to C_s ≈ −3.5 or 3.5) or nearly balanced side forces. Temporal azimuthal regulation of the bleed about the point of flow symmetry leads to rapid corresponding variations in the aerodynamic side loads by azimuthal excursions of bleed orientations of only ±15°. These findings indicate that the characteristic random onset of flow asymmetry and side loads over cylindrical bodies at high incidence can be overcome by forebody azimuthal aerodynamic bleed to yield either balanced or prescribed aerodynamic loads. Furthermore, asymmetry can also be effected at lower angles of incidence when the flow is naturally symmetric.

We describe a newly commissioned Ludwieg-tube facility at the University of Maryland, capable of producing high-speed multi-phase flows. The wind tunnel is operational at Mach 3.75–4 for unit Reynolds numbers spanning (sim)2.5(times)10(^{6}) m(^{-1}) to (sim)24(times)10(^{6}) m⁻¹, with steady test times between 30 and 40 ms, and an average startup time of 15 ms. A particle-size characterization study is performed for droplets produced from a 5% volume concentration of Di-Ethyl-Hexyl-Sebacate (DEHS) in methanol, resulting in known conditions for monodisperse DEHS droplet sizes ranging from 4.1 to 15.6 μm. Fluorescence spectroscopy is used to measure the particle concentration in the test section. The results for the particle concentration distribution are presented along with data showing the repeatability of tests. Numerical simulations for particle transport through the nozzle are used to inform tunnel design and corroborate results from the spectroscopy experiments.

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.