Experiments in Fluids最新文献

The wave crest (cusp) of the disturbance wave in thin liquid film flow is an important factor contributing to heat/mass transfer, e.g., fuel rods in boiling water reactors, stator/rotor blades in steam turbines, and cleaning/drying wafer processes in semiconductor manufacturing. We developed a new technique for directly detecting the thickness of wave cusps using array-based sensing with an optical waveguide film (OWF). This technique, based on geometrical optics assumptions, simultaneously obtains information on liquid films’ thickness and their interfacial shape, i.e., whether or not the local interface is convex upward. We first performed a pseudo-film flow measurement using a metal specimen to confirm the basic principle. According to the results, a meaningful signal indicating the wave-cusp passage, along with a thickness signal, was detected simultaneously. The OWF signal processing for cusp thickness detection was newly established based on this fact. We then applied this technique to a wavy liquid film flow formed on a flat plate in the entry region. A series of experiments were performed over a wide range of air speeds (j_G = 20–70 m/s). As a result, the cusp thicknesses of relatively large waves on the wavy interface were successfully extracted from the OWF output signal. Further, the major thickness variables (i.e., base film thickness, median film thickness, and cusp thickness) were compared with those of conventional thickness estimation methods, which showed reasonable agreement. This paper provides a framework for wavy thin-film flow measurements via OWF that is specialized for directly detecting local thickness profiles.

Graphical abstract

The fabrication of arterial flow phantoms for fluid dynamics studies suitable for particle image velocimetry (PIV) techniques has presented challenges. Current 3D-printed blood flow phantoms with suitable transparency for optical PIV (laserPIV) are restricted to rigid materials far from those of arterial properties. Conversely, while soft 3D-printed phantoms demonstrate promise for sufficient acoustical transparency for ultrasound PIV (echoPIV), their optical translucency presents challenges for laserPIV applicability. This dual-modality approach leverages the high spatial resolution of laserPIV for in-vitro applications and the ability of echoPIV to quantify flow in both in-vivo and in-vitro application (also inside stents), providing a more comprehensive understanding of flow dynamics. In this study, we present a series of coated thin-walled 3D-printed compliant phantoms suitable for dual-modality PIV flow imaging (i.e., laserPIV and echoPIV) methods, overcoming current 3D-printable material limitations. Stereolithographic (SLA) 3D printing was used to fabricate pipe flow phantoms from a set of commercial soft resins (flexible and elastic) as vascular tissue surrogates. To overcome low transparency and poor surface finish of soft resins, we coated the 3D-printed flow phantoms with a soft, optically transparent, photo-activated polymeric coating. The feasibility of performing dual-modality PIV was tested in an in-vitro flow setup. Our results show that the average normalized root mean square errors obtained from comparing laserPIV and echoPIV velocity profiles against the analytical solutions were 3.2% and 5.1%, and 3.3% and 5.3% for the flexible and elastic phantoms, respectively. These results indicate that dual-modality PIV flow imaging is feasible in the 3D-printed coated phantoms, promoting its future use in fabricating clinically-relevant flow phantoms.

Graphical abstract

Particle image velocimetry (PIV) is an established velocimetry technique in experimental fluid mechanics that involves determining a fluid flow velocity field from the motion of tracer particles illuminated by a laser sheet. The necessity of laser illumination poses challenges in certain applications and is a potential entry barrier due to its high cost and safety considerations. A laser-free alternative to PIV is particle shadow velocimetry (PSV), which uses images of the shadows cast by the particles on the camera sensor under back-illumination, instead of the Mie scattering signal produced by laser illumination. This study aims to compare various aspects of PSV such as depth of field, seeding density, type of illumination required, particle size, image filtering, cost-effectiveness and limitations with those of PIV. PSV and PIV measurements are taken in the wake of a flow past a cylinder and in a boundary layer developing over a flat plate. It is found that PSV is capable of achieving equivalent accuracy to PIV and is a viable alternative to PIV in certain applications where light sheet illumination creates experimental challenges.

Extended particle streak velocimetry (E-PSV) is a novel approach for comprehensive 2D flow measurement. It extends the measuring range of particle streak velocimetry (PSV) via particle tracking velocimetry (PTV). By using long camera exposure when recording moving tracer particles, streaks are created in areas of high flow velocities (PSV). In areas of low velocity, in contrast, particles are imaged point-shaped (PTV). E-PSV hereby offers the advantage of continuous measurement with PSV-typical setups, particularly when areas close to the wall and vortices require to be recorded simultaneously with areas of high velocity. For precise extraction of the flow information, a new model for the description of particle images is presented. It is based on the assumption that the intensity of a tracer can be modeled by a 2D Gaussian function. The temporal integral of the moving Gaussian is approximated by combining analytical calculation with values from a lookup table. We show that by this method even curved streaks can be reconstructed with subpixel accuracy under noise and quantization effects. The technique is demonstrated using a film flow in vicinity of a microstructure.

Flow field visualization techniques, e.g., schlieren and shadowgraphy, are indispensable in fluid mechanics research and application. In this paper, we present a novel technique, named holographic focusing schlieren imaging (HFSI), which takes only a single-camera and single-shot configuration to achieve three-dimensional (3D) flow visualization. The essence of this technique is that a coherent reference wave is introduced to interfere with the wavefront yielded by the traditional focusing schlieren (FS) method, forming a hologram. The reconstruction of the hologram directly yields the FS results along the test volume slice by slice with adjustable intervals, achieving 3D visualization. To demonstrate the capability of HFSI, a proof-of-concept setup was established, and experiments were performed using a compressed air jet, with a comparison to the FS method. The result shows that HFSI image reconstruction remarkably refocuses the out-of-focus jet flow, yielding similar schlieren effect observed in the FS images. The proposed HFSI holds significant practical value in some scenarios, such as in a wind tunnel, as it requires only one pair of parallel windows to achieve 3D flow visualization.

The effects of polymer additives on turbulent fluid flows have attracted massive attention since the discovery of significant drag reduction by polymers in wall-bounded flows. Here we present an experimental study on the polymer–turbulence interaction at the center of the turbulent von Kármán swirling (VKS) flow system where the flow is far away from the boundary (bulk turbulence) with tomographic particle image velocimetry (Tomo-PIV). We used water–glycerol mixture to tune the viscosity of the working fluids, which facilitates us to resolve the dissipative scales and thus obtain all the nine components of the velocity gradient tensor directly. Our experiments demonstrate that at the center of the VKS flow, anisotropic properties extend from large scale to small scale, but gradually weaken with increasing Reynolds number. In polymeric turbulence, it is found that with increasing polymer concentration both the spatial averaged root mean square velocity and the average energy dissipation rate first decrease and then tend to stay at a constant value when concentration exceeds a critical value, implying that the effect of polymers saturates at high polymer concentration. We also find that the small scales become more anisotropic with the increasing concentration. The axisymmetry of small scales, however, is always retained, which can be employed to estimate the average energy dissipation rate from the planar PIV data. Moreover, we reveal that the number of the tube-like structures, the elementary structure in Newtonian turbulence, is strongly inhibited by the polymer additives, whereas the size of the tube-like structures is greatly enlarged.

Interferometric particle imaging (IPI) is used to measure both the size distribution and concentration of microbubbles (with a diameter less than 100 micron) in water. Using a new method for calibration makes it possible to obtain quantitative results for the concentration of microbubbles. The results are validated using imaging with a long-range microscope shadowgraph (LMS). Estimates of the size distribution and concentration from both IPI and LMS agree within uncertainty limits. The relative uncertainty in the IPI concentration estimation is about 10% and is mostly due to the finite number of detected bubbles. It is shown that the performance of the bubble-image detection algorithm needs to be quantified to obtain a reliable estimate of the concentration obtained with IPI.

The role of front wheels in wheel-vehicle aerodynamic interactions is investigated for a simplified square-back vehicle by changing the bluntness of the front wheels. The interactions between the rear wheels and the vehicle wake are varied by modifying the distance between the rear wheels and the vehicle base. This results in a significant increase of the base drag for decreasing rear wheel/vehicle base distance, and we show that this increase is very sensitive to the bluntness of the front wheels: the smaller the bluntness, the larger base drag increases. We show that the base pressure decreases, and therefore, the drag increment varies linearly with the upstream velocity seen by the rear wheels, of course, significantly influenced by the bluntness of the front wheels. The main mechanism responsible for these pressure drag changes is a mean mass transfer from the wake of the main body to the wakes of the rear wheels. Based on this understanding, a physical model is proposed to explain the scaling law observed experimentally.