Experiments in Fluids最新文献

In this study, we introduce a novel laser imaging technique, named as scanning laser-induced fluorescence (SLIF), designed to quantify the time-averaged three-dimensional mixing behavior of inclined dense jets with and without swirl. SLIF builds upon the established planar laser-induced fluorescence (LIF) method, which is widely used for experimental studies on buoyant jets in the laboratory. Unlike traditional LIF which is typically stationary, SLIF involves towing a light sheet obliquely in a back-and-forth manner through the flow domain to generate a volumetric scan. Simultaneously, an imaging camera is also towed at an oblique angle to capture the LIF images synchronously, allowing for comprehensive three-dimensional concentration measurements within the scanned volume. Compared to previous scanning approaches, SLIF is able to provide spatial measurements without the concern of defocusing due to the fixed relative distance between the laser sheet and camera. For validations, laboratory experiments with turbulent non-buoyant jets were first conducted. The results demonstrated good agreement with existing literature data. Subsequently, SLIF was applied to quantify the mixing characteristics of 45-degree inclined dense jets with and without swirl. In particular, the experimental results confirmed that SLIF offers valuable visualizations of mixing patterns in complex swirling situations that require three-dimensional volumetric scanning. Moreover, the volumetric measurements also enable the extraction of oblique sections across the inclined dense jet, facilitating the analysis of concentration distributions at various offset planes. This capability is crucial for enhancing the understanding of three-dimensional mixing processes for inclined dense jets particularly with swirl.

We present a new model experiment to study the metal pad roll (MPR) instability, which is a limiting factor for the safe operation of aluminum reduction cells. The idea of our experiment is to replace the horizontal electrical currents in the aluminum layer, which are caused by displacements of the cryolite–aluminum interface in industrial reduction cells, with a synthetic current that is supplied through the side walls of the experimental cell. In this way, only one liquid layer of an electrically conducting fluid is required for modeling the MPR instability, allowing the experiment to operate under ambient conditions using the room-temperature liquid alloy GaInSn as the current-bearing fluid. We demonstrate that the experimental model allows self-amplifying MPR waves to be destabilized and maintained in a reproducible way. The setup is equipped with an acoustic measurement technique that facilitates precise submillimeter measurements of liquid metal surface elevations, which makes it possible to determine several key quantities such as MPR growth rates, stability onsets, saturation amplitudes, or viscous and magnetic damping rates. As the MPR destabilizing Lorentz force synthesized in the experiment can be calibrated to the Lorentz forces appearing in real two-layer cells, the proposed model experiment is intended to establish a novel framework for experimental benchmarking.

A new visualization technique using photochromism for the movement of oil film was developed and applied to an optical gasoline engine. A photochromic dye was dissolved in the oil and an arbitrary position of the oil film was illuminated by UV laser light, which makes a marker in the oil film via a photochromic reaction. The lifetime of color change by photochromism is relatively long and therefore, by tracking the movement of a marker makes it possible to visualize the movement of oil film directly. The color density was quantified based on the absorbance calculated from images taken before and after coloring in two wavelengths. Through the experimental and theoretical considerations, it was confirmed that the calculated absorbance is effective in reducing a noise originated by the color, the shape of the piston surface, the temporal variation of oil film thickness and the illuminating light intensity distribution. Furthermore, the value of the absorbance is in a very good linear relationship with the oil film thickness. This technique was applied to an optical gasoline engine and confirmed the availability of this technique. In the top land of piston surface, the oil film between the piston and cylinder liner was separated and the majority of the oil is at the piston surface and moved with the piston motion. The oil film thickness on the cylinder liner was very thin. On the contrary, at the piston skirt region, a wider region of the oil film is connected between the linear and the piston skirt. However, there are regions where the oil film between the liner and the skirt is separated by oil film rupture and the majority of the oil film is on the piston skirt. The change of the flow direction by the operating condition, i.e., the throttle condition, was able to clearly visualize, though the movement of oil film on the piston surface was very slow in normal case. The relatively fast complexed flow for opposite direction was also able to visualize by this technique.

A benchmark evaluation of event-based imaging velocimetry (EBIV) on its acquisition capability and measurement uncertainty is performed. Toward this end, a digital micro-mirror device interfaced with a pulsed laser light source is employed to generate illuminated particle images under various predefined particle diameters and concentrations, serving as the ground-truth base. For ease of comparison, a frame-based camera is used to provide the reference particle images. The measurement results indicate that the maximum frame-recovered acquisition frequency decreases as either particle image diameter or concentration increases, converging to a minimum level of 2400 Hz for the EVK4 event-based camera. Despite this lower limit of frequency, adding large-diameter and high-concentration particles may induce event overflow and then lead to incorrect velocity measurements. This deficiency can be avoided by maintaining a margin of around 5% between the maximum acquisition frequency and its lower limit, which corresponds to frequencies over 2500 Hz in this study. Furthermore, for an acquisition frequency over 2500 Hz, a diameter of 2.20 px exhibits the lowest mean velocity uncertainty, whereas, for an acquisition frequency below 2500 Hz, diameters of 2.20 and 3.06 px can both achieve the lowest uncertainty level. A linear model is also proposed to predict the maximum acquisition frequency in practical applications. This work establishes the relationship among acquisition frequency, measurement uncertainty, particle size and concentration for the EBIV system. Finally, a two-dimensional EBIV experiment on a water jet is successfully conducted at 4 kHz.

Recording onto a single-frame multiple exposures of the tracer particles has the potential to simplify the hardware needed for 3D PTV measurements, especially when dealing with high-speed flows. The analysis of such recordings, however, is challenged by the unknown time tag of each particle exposure, alongside their unknown organization into physical trajectories (trajectory tag). Using a sequence of two or more illumination pulses with a constant time separation leads to the well-known directional ambiguity problem, whereby it is not possible to distinguish the direction of motion of the tracer particles. Instead, an irregular and asymmetric sequence of time separation for the illumination pulses allows recognizing the time tag of the unique sequence of positions in the image, composing the trace. A criterion is formulated here that recognizes unambiguously the trace pattern, based upon the principle of kinematic similarity. A combinatorial algorithm is proposed whereby a signal-to-noise ratio is introduced for every candidate trace. The approach is combined with an additional criterion that favors trace regularity (minimum velocity fluctuations). The algorithm is illustrated making use of particle motion examples. Furthermore, it is assessed using 3D experimental data produced with time-resolved analysis (single-frame, single-exposure) using the Shake-the-Box method. Traces with a three-pulse sequence yield a detection rate of 85%. The latter declines with the number of pulses. Conversely, the error rate rapidly vanishes with the samples number, which confirms the reliability of trace detection criterion when more pulses are comprised in the sequence.

Graphical abstract

A novel method is presented to measure Mach number and flow angle from schlieren images of two-dimensional supersonic flows. A line detection technique is used to extract characteristic lines from schlieren images to measure the velocity direction and the Mach number at the intersection of characteristic curves. The proposed technique is independent of fluid thermodynamics and applies to dilute gas flows and non-ideal compressible flows. The velocity magnitude and fluid thermodynamics are retrieved from the fluid thermodynamic model, assuming constant total enthalpy and entropy. Mach number measurements are also obtained at solid walls by integrating the compatibility equation along the characteristic lines, using the measurements within the flowfield as initial conditions. Results are presented for two exemplary cases: an asymmetric converging–diverging nozzle and the supersonic flow around a diamond-shaped airfoil. Measured values of the Mach number and the flow angle agree with numerical predictions and indirect Mach number measurements based on pressure measurements. The reconstructed pressure and velocity magnitude values agree fairly well with available measurements and simulations in the dilute gas and in the non-ideal regimes.

Falling film flows over rectangular corrugations can exhibit intense time-oscillatory interfacial motion. This is of considerable interest for heat and mass transfer applications, where structured surfaces play a crucial role in process intensification. Our contribution relies on high-speed imaging and image processing based on an internally referenced light absorption method to obtain a full spatio-temporal characterization of the structure-induced wave evolution. After validating the customized experimental technique, particular emphasis is placed on identifying relationships between the steady and transient characteristics of aqueous falling film flows under operating conditions relevant to, e.g., falling film absorbers for (text {CO}_2) capture applications. The transient film instabilities are found to evolve from an initially steady film flow. In the investigated Reynolds number range, inertia-controlled liquid overshoot in wall-normal direction at the structure element’s upstream edges plays a crucial role in the overall flow destabilization. The developed film flow can be decomposed into a steady and a time-oscillatory flow contribution. The former is characterized by a dominant two-dimensional wave shape with a primary wavelength matching that of the bottom contour, while the latter is more isotropic in shape. Nevertheless, both flow contributions are interconnected, with high oscillation intensities being usually accompanied by a strongly sloped steady base flow. In the context of surface structure optimization, the streamwise length scale of the steady interfacial ridge induced at an isolated structure element may serve as a predictor for identifying structure spacings that exhibit particularly strong transient flow destabilization.

Schlieren imaging in conjunction with a high-speed camera was used to observe the behavior of metered helium plumes as they transition from laminar to turbulent flow in an air environment. The plumes were visualized at twelve jet Reynolds numbers ranging from 200 to 2980. The fractal dimension of the flows was obtained by applying a box counting algorithm to the recorded schlieren images. The results were analyzed to determine the correlation between the Reynolds number of the flow and the fractal dimension of the observed turbulence. A trend of increasing fractal dimension with increasing Reynolds number was observed for several different types of schlieren cutoffs including horizontal cutoff, vertical cutoff, circular cutoff, focused shadowgraphy and de-focused shadowgraphy. The vertical cutoff and focused shadowgraphy imaging methods showed the most consistent results for the fractal dimension characterization during the laminar to turbulent transition. For transitional plumes, it was observed that fractal dimension increased with distance from the jet outlet.

Biomimetic microstructured surfaces hold significant potential to alter the characteristics of flow fields and are actively being investigated for their role in noise reduction in the propellers of unmanned underwater vehicles (UUVs). However, accurately measuring the noise generated by UUVs remains a challenge, which complicates the validation of noise reduction strategies and leaves the underlying mechanisms insufficiently understood. In this study, we propose a general noise measurement method for UUVs that involves identifying the main noise sources, designing noise reduction strategies, and validating their effectiveness. Experimental results indicate that the propeller is the main noise source, prompting the design of biomimetic propellers incorporating serrated edges and surface microstructures inspired by humpback whales. These biomimetic propellers are subsequently installed on a UUV for testing, and the results demonstrate a significant noise reduction of up to 6.67 dB. To further validate the noise reduction effects, additional experiments are conducted to measure the motor power consumption and assess the hydrodynamic performance of the different propellers. The motor power consumption is slightly higher for the optimized propellers compared to the original design, indicating that the noise reduction effect can indeed be attributed to the changes in the propeller design, rather than fluctuations in motor performance. Additionally, the characteristic curves of the propellers revealed that while the biomimetic propellers produce lower thrust compared to the original propeller, they maintain stable performance across varying operating conditions. By combining experimental measurements with numerical simulations, we elucidate the underlying noise reduction mechanisms of biomimetic propellers, specifically the breakup of large-scale vortices into smaller, lower-energy vortices. This process reduces the energy of the large-scale vortices and redistributes it over a broader frequency spectrum. These findings provide a robust theoretical and experimental foundation for developing efficient, low-noise underwater propulsion systems, demonstrating profound academic and practical implications.

A novel photogrammetry procedure is proposed for projecting 3D surface oil flow visualizations from 2D images over an arbitrary computer-aided design (CAD) model of a 3D surface. The technique yields a 3D view of the oil flow pattern on the model surface from different view angles and allows rendering it along with off-surface particle image velocimetry (PIV) flow field measurements. The technique is based on a recently developed simplified photogrammetry procedure proposed by Gluzman (Exp Fluids 63:118, 2022), which is modified to account for the full, large flow field over the 3D surface. The procedure requires only two reference markers on the given surface geometry CAD model, a small checkerboard placed on a flat surface near the 3D geometry, and a single camera that should capture at least three images after the experiment from any view angle in order to calibrate the projection. The proposed procedure allows removing the elaborate steps typically required in photogrammetry while retaining the same measurement accuracy. In particular, we demonstrate the application of the technique to study turbulent smooth-body flow separation over a Gaussian bump geometry in Gray (Part I-Exp Investig, 2023). Our photogrammetric procedure allows us to obtain a 3D view from different angles of the surface oil flow pattern warped over the CAD model geometry and coupled with stereo PIV plane flow field cuts, which provides important insights into the character of the flow separation.