Experiments in Fluids最新文献

This paper investigates the acoustic and velocity fields due to a circular rod and an aerofoil placed in the wake of, and perpendicular to, a rod. Simultaneous measurements were conducted using a microphone array and time-resolved particle image velocimetry (TR-PIV). The interaction was characterized through acoustic spectra and the coherence between microphone signals and the three velocity components. Coherent structures were identified with spectral proper orthogonal decomposition (SPOD) using a norm based either on turbulence kinetic energy (SPOD-u) or on pressure (SPOD-p). An advantage of SPOD-p is that it identifies velocity modes associated with a large acoustic energy. Peaks of energy were observed at (St approx 0.2) and 0.4–Strouhal numbers based on rod diameter and free-stream velocity. At (St approx 0.2), the dominant feature is von Kármán vortex shedding from the rod. At (St approx 0.4), a wave-train structure in the rod wake impinging on the aerofoil leading edge is captured by the rank-1 SPOD-p mode, with coherence levels reaching 60% for the (u_2) component (upwash/downwash relative to the aerofoil). This structure also appears at (St approx 0.2), but as the rank-2 SPOD-p mode. A mode-switching occurs around (St approx 0.3): below this value, the rank-1 mode corresponds to von Kármán shedding (cylinder branch), while above it, the rank-1 mode tracks the interaction of the aerofoil with the rod wake (aerofoil branch). Both branches were also identified via beamforming using low-rank cross-spectral matrices derived from SPOD-p modes.

In this study, inverted flexible and rigid splitter plates are applied to control the vortex-induced vibration of a circular cylinder. The vortex-induced vibration (VIV) characteristics and vortex dynamics are experimentally investigated with a Reynolds number ranging from 1970 to 10,590. To examine the effect of the streamwise length, five different lengths are selected. It is indicated that the VIV of the circular cylinder is suppressed by the inverted rigid splitter plate and the suppression effect is improved with an increase in the streamwise length of the plate, however, the vibration response behaviors remain the same for all streamwise lengths. Compared to the rigid one, the inverted flexible splitter plate causes various vibration responses with the change of its streamwise length. The diverse vibration responses are related to the kinematic characteristics of the inverted flexible splitter plate and the vortex dynamics. Five vortex shedding modes, including “Kármán vortex”, “Bi-LEV + Bi-WV” (Bi-LEV: bilateral leading-edge vortex; Bi-WV: bilateral wake vortex), “2Bi-LEV + Bi-WV” (2Bi-LEV: two pairs of bilateral leading-edge vortex), “Uni-LEV + Uni-WV” (Uni-LEV: unilateral leading-edge vortex; Uni-WV: unilateral wake vortex), and “K-H instability” (Kelvin–Helmholtz instability), are found based on the number of vortices in one vibration cycle. The correlation between the VIV and the vortex shedding mode is revealed. The “Kármán vortex” and “K-H instability” modes are corresponding to a better effect of suppressing VIV. The control effect of “LEV + WV” modes is affected by the kinematics of the plate, as it is found that the vibration amplitude of the circular cylinder and the inverted flexible splitter plate is positively correlated.

To investigate the typical unsteady structures and gas–solid coupling mechanisms in particle-laden transverse jets in high-enthalpy supersonic flows, a reliable experimental approach was developed and systematically validated. The experimental system consists of a high-enthalpy supersonic flow facility and a custom-designed particle injection device. It generates a Mach 2.63 (~ 1406 m/s) flow with a total temperature of ~ 1633 K, along with continuous and stable injection of SiO₂ particles ranging from 0.1 to 150 μm in diameter. A multimodal diagnostic strategy was employed to provide insights into the flow field. High-speed focused shadowgraphy captured the aggregation and sweeping–ejection processes of large-scale vortices acting on particle clusters. Planar laser scattering (PLS) revealed three representative particle distribution patterns—fluctuation, trailing, and roller type—as well as their quasi-periodic cascading evolution. Two-dimensional, two-component particle image velocimetry (2D–2C PIV) provided quantitative measurements of the particle velocity field. The results showed peak velocities up to ~ 1300 m/s and a pronounced non-monotonic stratification linked to particle inertia effects. These results demonstrate that the developed experimental platform and diagnostic methodology can reliably capture the complex unsteady features of particle-laden transverse jets in high-enthalpy supersonic flows. This provides a robust experimental basis and essential data for advancing the understanding of particle–turbulence mixing and transport mechanisms in high-speed flows.

Hybrid pulse-burst Doppler global velocimetry/pulse-burst, cross-correlation Doppler global velocimetry was demonstrated for the first time in the NASA Langley 4-foot Supersonic Unitary Plan Wind Tunnel, allowing simultaneous acquisition of mean and instantaneous high-repetition-rate velocity fields. The technique was used to make planar velocity measurements across an oblique shockwave generated by a large splitter plate set at a − 2° angle of attack in a Mach 2.4 flow. Sequences of more than 100 consecutive instantaneous velocity fields were obtained at a rate of 100 kHz. Velocity fields from both the mean and instantaneous measurements from the hybrid technique indicated errors less than 1 percent of anticipated velocities. Likewise, assessment of measurement precision yielded velocity measurements of approximately 1.6 percent of the local velocities. Additional demonstrations in different flowfields including a subsonic axisymmetric jet and a supersonic Mars reentry vehicle model further illustrate the utility of the new hybrid method. Evaluation of these other test cases indicated the potential for retroactive extraction of unsteady velocity information from previously acquired mean data.

Graphic abstract

Volumetric three-component particle tracking velocimetry flow measurements were taken to characterize the vortical structures past isolated and sheltered low-aspect-ratio finite wall-mounted circular cylinders (FWMCCs) immersed in a turbulent boundary layer. In addition to an isolated FWMCC case, sheltering effects are investigated for tandem arrangements at streamwise spacings of 2d and 6d, where d is the FWMCCs’ diameter. For each spacing, two height ratios (h_1/h_2 = 1) and 0.5 are considered, where (h_1) and (h_2) are the heights of the upstream and downstream FWMCCs, respectively. Measurements were taken for FWMCCs occupying 20% or less of the incoming boundary layer thickness. For the isolated FWMCC, the wake is dominated by two types of coherent vortical structures: an arch vortex with an inverted U shape and a streamwise counter-rotating vortex pair. The wake of sheltered FWMCCs is also dominated by a coherent U-shaped arch vortex and streamwise vortices; however, sheltering weakens these structures and affects their characteristics. For fully sheltered downstream FWMCCs ((h_1/h_2 = 1)), the wake includes a single streamwise counter-rotating vortex pair originating at the upstream FWMCC. When the free end of the downstream FWMCC is unsheltered ((h_1/h_2 = 0.5)), a streamwise counter-rotating vortex pair is induced at the downstream free end; this vortex pair merges, farther downstream, with the streamwise vortex pair originating at the upstream FWMCC. A third streamwise counter-rotating vortex pair is observed past the downstream FWMCC when it is sheltered by a FWMCC with (h_1/h_2 = 0.5) positioned 2d upstream, further highlighting the dependence of the vortical structure organization on both the height ratio and streamwise spacing.

The plasma around a reentry spacecraft causes the charged particles to interact with the electromagnetic waves emitted by the on-board antennas, and the vehicle experiences radio communication difficulties. A proposed way to mitigate the radio blackout is the magnetic field alleviation technique that consists of superimposing a magnetic field onto the flow, converting the plasma into an anisotropic medium, and changing its refractive index. The applied magnetic field leads to the creation of an extraordinary wave that can propagate for plasma frequencies higher than the radio signal frequency. In this work, a probe containing a cryogenically-cooled high-temperature superconducting magnet is used to study the effect of an applied magnetic field on the plasma flow and on the radio signal propagation, in the VKI Plasmatron facility. The magnetized plasma is characterized by optical emission spectroscopy, stagnation heat flux, and dynamic pressure measurements. The experimental radio signal measurements are conducted using conical horn antennas, operating at frequencies in the K(_text{a})-band. An antenna is placed inside of the magnetic probe, transmitting toward a stagnant air plasma flow. The applied magnetic field causes an increase of the flow temperature, leading to an augmentation of the plasma frequency and stagnation heat flux, due to the Hall effect. No significant effects are observed in the signal transmission and attenuation, while the signal reflection trend is consistent with the variation of magnetic field strength, and plasma and collision frequencies. The dependency of the Faraday rotation with the magnetic field and its direction is observed. While a clear demonstration of the magnetic window is not conclusively observed in the transmission parameters, the behavior of the reflection coefficient shows that the radio blackout mitigation is feasible at optimal combinations of flow ionization.

Bubble departure from walls in cross-flow conditions has been studied. This process plays a fundamental role in numerous applications, from gas-liquid contacting, heterogeneous nucleation in cavitation, to flow boiling. Predicting the size of the detaching bubble is essential for controlling the liquid–gas interfacial area in gas/liquid systems encountered in these applications. However, existing data on bubble departure in cross-flow are often restricted to low wall shear rates where bubble deformation is minor and thus negligible. Moreover, the effect of surface wettability on bubble departure is usually not reported in detail. Therefore, in this study, we analyse bubble deformation and departure from artificial nucleation sites for elevated wall shear rates using high-speed imaging. In addition, the surface wettability was modified and quantities related to dynamic wetting phenomena were examined. Our findings clearly show that wettability of the surface and the dynamic pressure imposed on the bubble by the flow jointly dictate how a bubble deforms prior to departure and how large its size is on departure. In addition, different mechanisms of departure are identified and reported in a regime map based on the critical deformation and the wall shear rate.

An experimental study of the stability of a centrifuged liquid–liquid interface at the non-uniform rotation is hereby presented. Two immiscible liquids, different in density and having high viscosity contrast, fill a horizontal, circular Hele-Shaw (H-S) cell rotating about the vertical axis. At the continuous rotation, the liquids form an axisymmetric core-annular configuration. Then, the rotation rate is modulated as a sum of constant and periodically varying values, and the oscillatory Kelvin–Helmholtz (K–H) instability is sought for. The interface dynamics is assessed by taking photographs and numerically processing the images recorded. With the increase in the amplitude of rotation rate modulation, the interface loses stability giving rise to an azimuthally periodic, quasi-stationary wavy pattern—the “frozen wave” driven by the oscillatory K–H instability. The evolution of its properties in the supercritical regime of oscillations is studied depending on the values of the rotation rate and the frequency and amplitude of librations. The experimental results are analyzed in comparison to the case of a vertical H-S cell at non-uniform rotation. The role of orientation of the gravity field is discussed, and its impact on the threshold of instability onset is found. A conclusion is made that the gravity-induced asymmetry in combination with the choice of the dimensionless frequency may reduce the interface stability by engaging an additional vibrational mechanism.

The effect of free rotation of the cylindrical container wall on the vortex flow of a liquid in a model of a centrifugal reactor is studied. A method for intensifying the vortex flow in a cylindrical reactor by adaptive free rotation of the side wall under the action of viscous fluid drag is proposed. The medium in the cylindrical container is agitated by the upper disk rotation. Other parts of the container can freely rotate relative to their axes with a rotational speed determined by the level of friction in the bearing. Thus, the thermal friction energy of the working fluid with a fixed wall is redistributed to the kinetic energy of rotation of the entire reactor system. The kinematic characteristics and flow structure are examined using experimental Particle Image Velocimetry (PIV) measurements and numerical simulations in Star-CCM + . Results show that even minimal free rotation significantly suppresses vortex breakdown, which is typically observed when the cylindrical container is fixed. Furthermore, the maximum values of the tangential velocity with a rotating wall are 2–3 times higher than in a stationary cylinder, and the flow swirl is an order of magnitude higher, which indicates a significant decrease in hydraulic resistance. These findings are relevant for the development of energy-efficient vortex flow control technologies and the improvement of reactor systems, in particular, in the development of new models of compact centrifugal reactors for biological, chemical, and energy industries.