Experiments in Fluids最新文献

Flows in rapidly spinning bodies, such as the iconic libration-induced flow, are key ingredients of the dynamics of stars and planetary interiors. Laboratory experiments of such flows experience a strong centrifugal acceleration, which hinders the use of classical velocimetry methods relying on particle tracking. Modal acoustic velocimetry was introduced by Triana et al. (New J Phys 16(11):113005, 2014) as a new particle-free method, inspired from helioseismology, to alleviate this problem. In this method, acoustic modes are excited in the fluid and recorded in the spinning container. Rotation and fluid flow modify the characteristics of these modes, lifting the degeneracy of non-axisymmetric modes. To date, this method has only been applied to stationary or statistically stationary flows, by measuring frequency splittings in the spectral domain. Here, we analyze time-varying libration-induced flows. We propose and test two data acquisition strategies. The first strategy operates in the frequency domain and relies on the periodicity of the flow, while the second strategy involves a high-resolution algorithm applied in the time domain. The retrieved mode frequency splittings are compared to those computed for a classical linear libration-induced flow model as reported (Greenspan The theory of rotating fluids, Cambridge University Press, Cambridge, 1968). A very good agreement is obtained, but we observe an unexpected time delay, which we attribute to the buildup time of acoustic modes. We retrieve more than 50 splitting measurements at 10 successive libration phases. Inverting these data with the SOLA method, often used in helioseismology, we derive profiles (1D inversion) and maps (2D inversion) of the azimuthally averaged fluid rotation rate. The inversions recover the main characteristics of this time-dependent flow. The 2D inversion confirms the invariance of the flow along the rotation axis. Resolution kernels show that flow can be mapped on patches that spread over approximately (5 %) of a meridian quarter-plane. Our study paves the way to the investigation of more exotic regimes of precession- or libration-induced flows.

The tailless flying wing configuration represents a typical aerodynamic layout for next-generation aircraft. Rudderless flight control technology can significantly enhance the high-stealth performance and payload capability of flying wing aircraft, making it a disruptive technology that has gained widespread attention and is being gradually applied in advanced air vehicles. The implementation of this technology holds considerable strategic value and engineering significance. This review summarizes recent advances and flight verifications in three core active flow control technologies involved in rudderless flight control of flying wing aircraft: circulation control, flow separation control, and separation induction control. The current technology status and challenges are discussed, and an outlook on future development trends is provided.

We present a newly constructed jet array with a novel driving scheme for turbulence generation in a vertical water tunnel and measurements of the turbulent flow this jet array establishes. The design of the array allows us to control the mean background flow and the turbulence intensity independently of each other. The array consists of a rectangular arrangement of 112 individually computer-controlled water jets that are aligned streamwise to the measurement section of our 8-m tall vertically recirculating water tunnel. Using solenoid valves, individual jets are activated following predefined protocols that can be tailored to obtain different turbulence statistics within the measurement section. The protocols are based on four-dimensional OpenSimplex noise, a type of gradient noise that features spatial and temporal coherence. Details of the mechanical and electrical designs are presented, together with a detailed description of the protocol generation. We show that the resulting turbulence is near homogeneous and isotropic, with a turbulence intensity of Order 1, an energy dissipation rate of order (10^{-1},mathrm {m^2/s^3}) and (textrm{Re}_{lambda }approx 1400). Additionally, we present experiments that show the effects that various system and protocol parameters have on the created flow conditions and address the streamwise development, as well as the homogeneity and isotropy of the flow.

We investigate fluid instabilities in a Hele–Shaw cell driven by the combined effects of double diffusion (DD) and Rayleigh–Taylor (RT) mechanisms, focusing on the formation of vertical fingers and overturning plumes. These patterns emerge from the interplay between sediment settling and the diffusion of a scalar (dextrose). A novel experimental design is introduced in which particle sizes are selected so that their settling velocities are slower than, comparable to, or faster than the solute diffusion rate. This systematic variation allows the relative influence of settling and diffusion to be isolated and quantified. Two primary questions are addressed. First, under what conditions do fingering instabilities arise, as opposed to pure particle settling? We find that fingering is suppressed when the particle settling velocity exceeds the DD finger tip velocity. A predictive criterion is developed based on a dimensionless gravity parameter and the ratio of characteristic settling to diffusion times. Second, when instabilities are present, which mechanism—DD or RT—dominates? As expected, DD dominates when diffusion is rapid. A transition from DD to RT is observed when the settling-to-diffusion time ratio falls below approximately 0.2, using the gap width as the characteristic length scale. This work introduces a straightforward framework for exploring competing instability mechanisms in sediment-laden, diffusive flows in an experimentally accessible, previously uncharacterized parameter space.

Predicting droplet evaporation is particularly complex when the liquid phase consists of multiple components. To date, only a limited number of physical optical phenomena have been used to non-intrusively measure the composition of droplets. Laser-induced fluorescence is a promising approach, as the emission and absorption of certain fluorescent dyes are known to depend on solvent polarity, viscosity, and, more generally, the chemical environment. However, a challenge is that fluorescence signal intensity is generally sensitive to both temperature and composition. This study investigates fluorescence lifetime measurements as a robust alternative. We demonstrate that, with a well-chosen fluorescent dye, it is possible to measure the composition of bicomponent droplets using a single dye and a single detection band, with minimal constraint on detection band selection, and without ambiguity due to temperature variations. To validate the technique, it is applied to acoustically levitated droplets across several mixtures that exhibit markedly different behaviors.

The hydrodynamic influence of beveled edges on air–water flow properties in a stepped chute were studied. Air–water flow measurements were made with a double tip phase-detection conductivity probe and an ultrasonic sensor for unit discharges up to 0.565 m²/s in a beveled stepped chute for two interchangeable step heights of 0.1 m and 0.2 m. Flow regimes, the onset of aeration, and the streamwise development of air concentrations, interfacial velocities, and free-surface fluctuations were quantified. Bubble count rates, chord lengths, and their distributions were also derived from measurements with a discussion of the flow physics. A direct comparison of air–water flow properties with vertical steps revealed that bevels elongated and reduced the stability of recirculating cavities, directly influencing flow regimes and reducing the distance to the air-entrainment inception point by 20–30%. At the chute exit, beveled steps produced higher mean air concentrations, greater flow depths and reduced interfacial velocities. These results highlight the value of detailed air–water flow measurements to quantify flow properties and processes that may be used in engineering applications.

Tomographic reconstruction, a critical process for tomographic particle image velocimetry (Tomo-PIV), remains inefficient due to the required massive memories and high computational cost. In this work, a fast tomographic reconstruction technique is proposed to improve the efficiency significantly. The weighting coefficient, which represents the contribution of the voxel to the corresponding pixel intensity, is remodeled to be independent of the voxel’s positions by artificially improving the particle image resolution. Consequently, the simultaneous multiplicative algebraic reconstruction technique (SMART) is re-implemented with convolution operations. The proposed method is named Convolutional-SMART (Conv-SMART). Moreover, the numerous convolution operations are accelerated using a graphics processing unit (GPU) to further reduce the reconstruction time. A synthetic three-dimensional 3D experiment with a vortex ring is carried out to numerically evaluate the precision and efficiency of the proposed method. The results show that the speed-up ratio of Conv-SMART to the original SMART reaches about five times faster in the central processing unit (CPU) environment and 15 times faster in the GPU environment without losing accuracy when the particle density is 0.05 particles per pixel (ppp) and the resolution is 20 voxels/mm. The speed-up ratio as a function of the particle density and resolution is also provided. Conv-SMART is also applied to the left ventricular Tomo-PIV experiment. The velocity field derived from Conv-SMART is consistent with that from SMART, whereas Conv-SMART achieves 50 times faster within the GPU.

Cloud cavitation is known as a typical phenomenon of cavitating flow, in which aggregation of bubbles repeats collective growth and collapse behavior and induces shock waves. To understand the mechanism of the cloud cavitation phenomenon, it is crucial to clarify the relation between flow fields and the unsteady behavior of the cloud. In this paper, we experimentally investigated how velocity and vorticity fields appear in association with the unsteady behavior of the cloud cavitation (i.e., from its inception, growth, collapse, and finally rebound). To do this, we made the cloud by injecting a pulsed water jet into still water, and the fluorescent particle image velocimetry (PIV) method was employed to obtain two-dimensional velocity fields surrounding the cloud. The particle images were recorded by a high-speed camera with 300,000 fps, and then the velocity and vorticity fields were computed by PIV analysis. Thus, we illustrated that flows of twin vortices move along the boundary of the cloud associated with its growth and shrink behavior and collide with each other as well as disappear before the collapse. Furthermore, we showed that high-vorticity regions appear together with the twin vortices induced by the collapse. These experimental observations suggested that the growth and shrink behavior of the cloud induce the motion of the twin vortices and the collapse of the cloud creates the twin vortices with high vorticity. Finally, we made a comparison between the experimental results and the numerical simulation performed in our previous work and demonstrated the consistency of the flow structures.

Nozzle characteristics modulate the stability of liquid jets, but their role in jet robustness to external disturbances is understudied. Here we produce jets with thin elastic membranes containing a hole of approximately 500 (mu)m in undeformed diameter. Our softest membranes produce the most stable jets in the Rayleigh and first wind-induced breakup regimes. An externally applied upstream pressure pulse lasting approximately 1 ms momentarily reduces the jet breakup distance and alters morphology. The pressure pulse is generated by the strike of a coil spring against a membrane mounted to the jet relaxation chamber. Softer nozzles and higher jet velocities minimize the disruption to the otherwise steady jet. Linear temporal theory for short nozzles derived using a dilated nozzle diameter well predicts breakup length before and after the pressure pulse. We propose hypothetical states for which our pressure pulse does not affect jet stability. Pressure disturbances initiate morphological changes in the jet, introducing novel phenomena like jet thinning and exit coalescence. Our results demonstrate that nozzle compliance can play a significant role in damping undesirable disturbances.