Experiments in Fluids最新文献

Dispersed two-phase flows at air–water interfaces are ubiquitous in environmentally relevant flows such as in the dispersion of floating microplastics or transport processes across the air–sea interface. In the current study, we propose a method to study such flows through the study of a relatively flat turbulent free surface laden with spherical floating particles (“floaters”). The free surface is perturbed by a relatively low-mean nearly homogeneous subsurface turbulent flow that is produced in a turbulence box actuated by a 10(times)10 synthetic jet array. The free surface flow field is characterized using planar particle image velocimetry (PIV) simultaneously with Lagrangian tracking of floaters allowing insight into the floater dynamics and the surface flow coupling. This is enabled by a relatively simple setup of LED panels and a single camera. Distinction between the continuous (flow tracers) and the dispersed (floaters) phase is carried out by exploiting their size disparity and number density. The proposed method is employed to characterize the single-phase flow field and the clustering statistics of floaters for different turbulence levels, the latter achieved by varying the distance of the free surface from the jet array. Specifically, we study the effect of different turbulence levels on the floater clustering behavior. We observe that the time required for floaters to reach a clustered quasi-steady state decreases with increasing vorticity and surface divergence amplitude. In addition, the growth rate of the mean cluster size is observed to increase with increasing vorticity and surface divergence amplitude, with its temporal evolution exhibiting two distinct phases: an agglomeration phase and an equilibrium phase. In contrast, in the absence of a subsurface flow, floaters are observed to cluster at a relatively slower rate characterized by a prolonged agglomeration phase. Finally, to highlight the potential of this technique in studying floater-laden turbulent free surfaces, preliminary results of flow–floater interactions are discussed.

The axisymmetric oscillation dynamics of droplets were investigated using an integrated numerical and experimental approach. Numerically, high-resolution simulations based on the arbitrary Lagrangian–Eulerian (ALE) method were carried out under ideal conditions that neglected gravity and acoustic fields. This simplified model served as a supplementary tool to analyze the influence of initial deformation amplitude, droplet size, and viscous dissipation on frequency shifts. Experimentally, droplet oscillations were analyzed using an acoustic levitation platform integrated with a high-speed imaging system, with particular attention to three governing parameters affecting frequency shifts: droplet size, oscillation amplitude, and static aspect ratio. Comparative analysis revealed that smaller ideal droplets exhibit less negative frequency shifts during large-amplitude oscillations, whereas acoustically levitated droplets of smaller size exhibit greater negative frequency shifts as a result of acoustic radiation forces.

Improving energy efficiency in industrial cooling systems hinges on the precise control and enhancement of heat transfer, a challenge that demands innovative solutions. While traditional approaches focus on heat exchanger geometry or fluid circulation speed, modulating the fluid’s intrinsic properties presents a promising yet relatively less explored path. In this study, we use laser-induced fluorescence through a Taylor-Couette cell to experimentally identify the interplay between fluid composition and rheology, hydrodynamic regimes, and mixing enhancement ultimately leading to transfer intensification. By measuring time evolving concentration fields upon injection of dye droplets, we quantify mixing dynamics across a spectrum of complex fluids, revealing how complex physical properties induced by the presence of microparticles, nanoparticles, and polymers reshape flow patterns and/or impact mixing times. Our findings underscore the increase in advection in triggering elasto-inertial turbulence by polymer addition, a regime that dramatically accelerates mixing. While dilute particle suspensions universally enhance convective performance (to the notable exception of that nanoparticles added into wavy flows), excessive particle loading paradoxically degrades elasto-inertial mixing efficiency, thus developing non-chaotic regimes (elasto-inertial dissipation, EID). Without any visible coherent scalar structure associated to it, this recently discovered regime still outperforms laminar flow in terms of mixing. This study thus highlights the potential of complex suspensions as efficient convection enhancers and of Taylor-Couette flows as a quick way of screening the convective potential of heat transfer fluids in centrifugal and/or curved flow contexts.

Aerodynamic drag accounts for about 60% of heavy truck energy expenditure at highway speeds, 20–50% for passenger vehicles and 75–90% for high-speed trains (HSTs). Reducing this drag is therefore crucially important for improving the energy efficiency of terrestrial vehicles, lowering their operating costs and mitigating greenhouse gas emissions. Over recent years, research on drag reduction (DR) has made a great progress, driven by developments in experimental diagnostics, high-fidelity numerical simulation and optimization based on machine learning (ML) or artificial intelligence (AI). This paper reviews recent advances in flow control and DR strategies for terrestrial vehicles, focusing on three key avenues, i.e., (1) understanding flow physics, (2) developing passive, active and adaptive hybrid flow control (PFC, AFC, AHC) techniques and (3) leveraging AI-based virtual engineering technology for aerodynamic shape optimization. Comparative analysis is provided for automotive vehicles (with the Ahmed body as a canonical model) and HSTs, highlighting their similarities, coherent structures and control mechanisms. Their shared flow physics points to the fact that proven DR strategies for, e.g., Ahmed bodies, could be extended for HSTs. The paper also discusses trade-offs between energy efficiency, adaptability, implementation complexity and operational reliability and outlines future challenges in scaling laboratory-proven methods to robust and real-world applications. By uniting insights from bluff- and streamlined-body aerodynamics, this review aims to provide a guide for the future research and the development of next-generation terrestrial vehicles that may achieve both performance and sustainability targets.

This study reports the first converged, non-intrusive measurement of a transverse integral scale of turbulence in an arc-jet flow, specifically a Mach 4 nitrogen freestream flow within the ONR–UTA (Office of Naval Research-University of Texas at Arlington) arc-heated plasma wind tunnel, Leste, using Femtosecond Laser Electronic Excitation Tagging (FLEET) velocimetry. A total of 4722 FLEET images capturing tagged fluid parcels were collected. To isolate the FLEET signal from the intense arc-jet background radiation, a 725 nm long-pass filter and a background subtraction algorithm were applied. The spatio-temporal displacement of the laser-induced tags was analyzed to extract axial velocity and its fluctuations along the transverse profile of the FLEET tag. Velocity determination employed a sliding signal box with 50 % overlap under a one-dimensional flow assumption, validated by comparing the FLEET tag length with the nozzle exit diameter and half-angle of the conical arc-jet nozzle. Of the dataset, 563 images were suitable for velocity measurements, and two-point correlation was used to determine the transverse integral scale. Previous work reported the first FLEET velocity measurements at the Mach 6 nozzle centerline, demonstrating the feasibility of quantitative velocimetry in arc-jet flows despite challenging background emissions. Subsequent studies extended this approach to Mach 4 flow for preliminary integral scale estimates; however, convergence was limited by the small number of usable images. The present study completes this progression by providing the first converged measurement of an integral scale of turbulence in an arc-jet flow using FLEET, which, to the authors’ knowledge, has not been previously reported in the literature. The correlation curve exhibits an almost monotonic decay to a minimum of 0.12, which, together with the low uncertainty of approximately 1.6 %, provides strong confidence in the measurement. Under the selected conditions, the mean axial velocity varied between 2330 and 2370 m/s, the turbulence intensity ranged from 3.7 to 4.5 %, and the transverse integral scale was 12.9 mm.

A novel approach is presented to simultaneously measure volume fraction (using two-tracer planar laser induced fluorescence (PLIF)) and velocity (using particle image velocimetry (PIV)) to study three-component mixing in the multilayer Rayleigh–Taylor instability, constituting of three layers in a blow-down gas tunnel. This approach enables non-intrusive, planar measurements of each of the three components in the mixture. By using acetone and anisole as PLIF tracers and a three-camera system, each tracer’s fluorescence is captured alongside scattered light from PIV particles. Key considerations for two-tracer PLIF with acetone and anisole are discussed, including spectral conflicts as well as the independence of anisole fluorescence from the presence of acetone, and vice-versa. The diagnostic is first validated and then applied to the Rayleigh–Taylor instability in a three-layer configuration, yielding previously unattainable results. This includes simultaneous volume fraction profiles, the covariances between the volume fraction fluctuations of different layers, as well as each layer’s covariance between their respective volume fraction and vertical velocity fluctuations. Such quantities are crucial for the development and validation of variable density turbulence models. The presented diagnostic is extendable to other three-component gas mixing experiments with small pressure variations, provided that the tracer volume fractions remain small.

This study presents prediction and experimental measurement of non-uniform surface temperature distributions on a flat plate test article for a flow control method. The test article features strips of materials (copper and MACOR) with dissimilar thermal properties interacting with a developing boundary layer to establish a passively controlled, non-uniform temperature profile. Experiments with a pre-heated model are conducted within the Imperial College supersonic wind tunnel at Mach 2.75. Infrared thermography is used for surface temperature measurements. Results from the experiments confirm the passive surface temperature control, demonstrating temperature variations between the strips. Furthermore, the physics-informed thermal model, with boundary conditions derived from actual wind tunnel conditions, predicts non-uniform surface temperature distributions that quantitatively align with the experimental results. Higher temperature differences between the strips of dissimilar materials can be achieved by increasing the difference between the recovery temperature and the initial temperature. These results validate the proposed actuation method’s ability to generate significant spanwise surface temperature variations. This validation of the actuation method serves as a prerequisite for future experimental campaigns, which will focus on measuring the induced velocity streaks and assessing their efficacy for boundary layer transition delay in hypersonic environments.

We present the results of experimental studies and two-dimensional hydrodynamic simulations of underwater electrical explosions of semi-cylindrical wire arrays. In the experiments, a pulse generator delivers a current pulse of ~ 250 kA amplitude rising in ~ 1 µs to a 5-mm radius semi-cylindrical array of copper or aluminum wires. Experiment and simulation results display the generation of a radially symmetric and converging strong shock wave (SSW) and the generation of a blade-shaped supersonic jet (SJ) with a velocity reaching 1.6 km/s. Simulations predict that in the vicinity of the implosion axis, the density, pressure, and temperature can reach 1.5 g/cm³, 6 × 10⁹ Pa and 500 K, respectively. Thus, this approach can be used for studies of properties of materials placed at the convergence axis or by interaction with the blade-shaped SJ with easy access of diagnostics. Additionally, the simulations show that the internal structure of the jet is not uniform, consisting of bubbles and voids that result in significantly smaller density than the normal density of water. Finally, explosions of arrays of different diameter wires result in the generation of converging SSW with satisfactorily uniform azimuthal distribution. This indicates a resistance positive feedback mechanism that stabilizes the explosion process across wires of different diameters.

This paper presents free-oscillation experiments of a blunt body conducted in a high-speed wind tunnel, with the model motion measured using photogrammetry. A faceted blunt model, mounted on a spherical air bearing, is free to rotate in roll, pitch, and yaw in response to the freestream flow (M = 2). Four synchronised high-speed cameras capture the model from multiple angles, and the unique coded targets printed on the model’s surface are reconstructed as points in 3D space, achieving accuracy within (text {1}^{circ }) for both static and dynamic measurements. The Kabsch algorithm is used to find the optimal rotation between two point clouds, hence allowing reconstruction of the angular motion over the entire run. The method shows promise for free-oscillation tests in high-speed ground facilities, offering advantages over ballistic range and free-flight tests such as a constant freestream velocity and hundreds of oscillation cycles. This capability enables the observation of dynamic instabilities that develop over extended timescales, thus revealing a precessional instability previously reported only for slender bodies at hypersonic Mach numbers.

The efficacy of reconstructing three-dimensional (3D) volumes of time-evolving three-component velocity from planar experimental measurements is explored within strong shear-distorting turbulent flows using Taylor’s frozen turbulence hypothesis. In flows with a strong mean shear-rate the instantaneous turbulence structure is spuriously distorted with the classical Taylor’s hypothesis method, where the local ensemble average velocity is used as the convective velocity to reconstruct 3D volumes. In the current study, two additional models for the convective velocity that extend the classical Taylor’s hypothesis approach are explored with varied levels of mean shear-rate and turbulence intensity in order to reconstruct 4D velocity fields for time-resolved analysis of turbulence structure. The classical Taylor’s frozen turbulence hypothesis is compared with local time average and instantaneous convective velocities, with and without a Poisson solver step to maintain continuity. Direct numerical simulation data of a turbulent channel flow from the Johns Hopkins Turbulence Database are used to assess the accuracy of the three methods while varying mean shear-rate and turbulence intensity independently. The three methods are also applied to time-resolved sPIV measurements on transverse planes within the near-wall surface layer of a canonical flat-plate turbulent boundary layer to assess the statistical means of the reconstructions. The instantaneous convective velocity method is generally found to be most accurate at reconstructing instantaneous velocity fields, although systematic biases are observed in mean statistics. In flows with lower turbulence intensities, the local time average convective velocity is comparable for significantly lower computation and implementation costs.