Experiments in Fluids最新文献

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.

In this paper, we present a simple and cost-efficient single-camera measurement setup for simultaneous temperature and velocity field measurements by two-color Planar Laser Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV). The camera we used is a three-chip color camera, which utilizes an optical prism to separate the incoming light into the three primary colors: red, green, and blue, each captured by an individual monochromatic camera sensor. Compared to conventional color filter array color cameras, a three-chip camera features significantly less color cross-talk and increased spatial resolution. By illuminating PIV tracers and exciting the fluorescent dyes Rhodamine 110 and RuPhen with a blue 450 nm laser light sheet in water, the camera is capable of recording time-resolved PIV images in the blue color channel, and two-color PLIF images were recorded in the green and red color channels simultaneously. We achieve a high temperature sensitivity of up to ({5,mathrm{% ^{circ }text {C}^{-1}}}) and demonstrate the feasibility of the proposed measurement approach on a Rayleigh–Bénard convection (RBC) experiment at a Rayleigh number (text{Ra} = {1.2 times {10}^8}) and Prandtl number (text{Pr} = 6.1). The resulting simultaneous temperature and velocity data reveal detaching plumes near the heating and cooling plates and enable a direct analysis of heat transfer. The probability density function (PDF) of the dimensionless heat transfer of a short measurement shows the typical skew toward positive values.

This study investigates the influence of injector flare angle ((beta)) on the flow dynamics and dynamic stability of a counter-rotating dual-radial swirl injector. The objective is to elucidate how geometric variation modifies both hydrodynamic and thermo-acoustic instability characteristics, thereby shaping flame topology and global stability limits in swirl-stabilized combustors. A series of non-reacting and reacting experiments were conducted for flare angles (beta) = (0^{circ }), (30^{circ }), and (50^{circ }). High-speed (OH^*) chemiluminescence, stereo-PIV, and dynamic pressure measurements were acquired simultaneously to resolve unsteady flow--flame interactions. Time-resolved and spectral analyses–including spectral POD–were employed to extract coherent structures, instability modes, and coupling mechanisms between pressure and velocity oscillations. Under non-reacting conditions, increasing the flare angle enhances the interaction between primary and secondary swirl streams, leading to stronger recirculation, a larger central recirculation zone (CRZ), and intensified precessing vortex core (PVC) activity. In reacting flows, (beta)profoundly affects both static and dynamic stability. The (beta = {0}^circ) and (beta = {30}^circ) cases sustain attached V-flames dominated by longitudinal thermo-acoustic oscillations, whereas (beta = {50}^circ) exhibits a transition from bubble-type to conical vortex breakdown (BVB (rightarrow) CVB), yielding lifted flames and intermittent low-frequency oscillations. This transition weakens acoustic coupling and produces a dynamically quieter yet stable flame. The flare angle is identified as a critical geometric control parameter dictating the balance between hydrodynamic and thermo-acoustic dominance. Optimizing (beta) improves fuel–air mixing, extends the rich blow-off limit, and mitigates high-amplitude oscillations. These findings provide fundamental guidance for designing high-shear swirl injectors in next-generation low-emission gas turbine combustors with enhanced stability and reduced acoustic sensitivity.

This study presents a novel in situ experimental identification technique for wave loads on fixed structures. It utilizes measured in situ wave surface elevation data around the structure as input. By employing Green’s function and the boundary element method, a direct physical mapping between the free-surface data and the structural surface pressure is established. Furthermore, formulations for both first-order and second-order wave load identification are developed. Experiments on three different structural shapes were conducted, confirming the method’s reliability for the precise characterization of wave load amplitude and phase. Additionally, the method enables accurate prediction of the average largest wave-induced force based on the average of the largest waves around the structure. With minimal data requirements and strong adaptability to complex geometries, this approach offers a valuable tool for large-scale in situ experiments and field monitoring of marine structures.

Introduction

Refractive-index-matched (RIM) silicone phantoms play a critical role in experimental biomedical fluid mechanics, enabling detailed investigations of complex flow phenomena in anatomically accurate geometries. However, providing transparent, patient-specific and non-compliant and compliant models for detailed experimental quantitative analysis of the flow field, i.e., with high-dimensional accuracy and minimal post-processing, remains a major challenge.

Methods

This work presents a scalable manufacturing workflow based on a wax-based lost-core casting technique. High-resolution wax printing enables the three-dimensional (3D) creation of both non-compliant and compliant silicone phantoms with smooth surfaces, fine structural details, and clean core removal. The method allows for modular assembly of large geometries, and it is demonstrated on three representative models, namely a patient-specific human airway model, a generic compliant bifurcation, and a compliant patient-specific thoracic aorta.

Results

Mechanical and geometric tests confirm that the compliant phantoms replicate physiologically relevant vessel properties, with a measured Young’s modulus of 1.71 MPa and wall thickness variations below 1%. The phantoms are integrated into flow circuits, and the velocity distribution in the phantoms is measured using volumetric 3D particle-tracking velocimetry (PTV) using the Shake-the-Box (STB) algorithm. Time-resolved measurements under steady and pulsatile inflow conditions reveal detailed flow structures and fluid–structure interactions in both non-compliant and compliant models.

Conclusions

The presented workflow enables reproducible, high-fidelity RIM phantoms for experimental studies of biomedical flows. Combined with advanced flow diagnostics, it provides a powerful platform for exploring pathophysiological mechanisms, validating simulations, and evaluating the performance of medical devices in realistic geometries.

In the existing particle image velocimetry (PIV) data processing approach, mask technology is usually employed to obscure regions that are not required for calculations, ensuring that information within these regions does not affect velocity calculation results. A stable and universally applicable dynamic mask method can greatly reduce the efforts by researchers. This study proposes a novel PIV dynamic masking technique—the dynamic mask based on image registration (DMIR). This method works by identifying the mapping relationships of object changes across different images, enabling the transformation of existing mask information into a new mask. The dynamic masking capability of this method is not limited by the medium characteristics or motion forms of the object being masked. The proposed method is evaluated using synthetic images and two test cases, which demonstrates its great potential for PIV data processing under real multiphase flow scenarios.

This experimental study examines the flow control efficacy of symmetric synthetic jets (SSJs) on a circular cylinder wake at (Re=2000), employing particle image velocimetry (PIV) to quantify wake characteristic, vortex dynamics and turbulent statistics. The SSJs, strategically positioned at the time-averaged flow separation points ((theta =pm 90^{circ })) with momentum coefficient (C_mu =1.34), which corresponds to the SSJs Reynolds number (Re_{overline{u_0}}=95), are frequency-modulated across three dimensionless actuation frequencies ((f_{text {sj}}^*=f_{text {sj}}/f_0=) 1, 2 and 3). Results demonstrate that frequency-tuned momentum injection alters the wake structure through three frequency-dependent effects: The shedding pattern changes from 2 S to 2P; the near-wake recirculation and stagnation zones shrink; and vorticity is more confined near the cylinder, reducing downstream fluctuations. Lagrangian coherent structure (LCS) analysis shows that repelling LCSs move closer to the cylinder, while attracting LCSs converge toward (y=0), consistent with a narrower wake. Reduction of the near-wake stagnation zone and recirculation region sizes indicates a progressive suppression as the SJs actuation frequency increases. Vorticity confinement in the immediate cylinder wake suppresses large-scale vortex development while promoting earlier roll-up near the cylinder and lowering velocity fluctuations downstream. The study establishes frequency-modulated SSJs as an analysis tool for multiscale wake control, offering two practical effects: stronger local mixing and weaker large-scale unsteadiness. These findings provide new insights into bluff body flow control strategies and their implications for regulating aerodynamic forces.

This study investigates the interaction of a vortex ring with perforated plates of different thicknesses, with a focus on the near-wall events in the downstream region. Experimental techniques like planar laser-induced fluorescence imaging and particle image velocimetry were employed to understand the flow physics. Two different configurations of interaction, based on the alignment of the vortex ring relative to the center hole of the perforation, were investigated. Three different plate thicknesses (3, 5, and 8 mm) were considered to elucidate its role on the downstream physics. The study was conducted for a vortex ring with a circulation-based Reynolds number ((Re_Gamma )) of 9000. The arrival of a vortex ring near the perforated plate resulted in the formation of mushroom-like structures in the downstream near the wall, influencing the jets formed below. The kinetic energy of the fluid due to these structures was found to be as high as (sim) 20–30% of the total kinetic energy achieved in the downstream region. The downstream (Gamma) peak was found to be nearly independent of the plate thickness, whereas the peak kinetic energy and enstrophy decreased with higher thickness. The shear layer interaction dynamics between downstream jets are explained through instantaneous and time-averaged velocity and vorticity fields. The cumulative slug model is used to estimate the (Gamma) growth in the downstream using the centerline peak velocity of each jet. Finally, a scaling is proposed that yields an inverse relation between the time-averaged axial velocity peak in the downstream and the aspect ratio (AR = ratio of plate thickness to hole diameter).