Experiments in Fluids最新文献

Images of compressible flows can be post-processed with digital imaging techniques to obtain accurate quantitative information about variables characterizing the flow. For example, the local flow Mach number can be obtained from the angle of Mach lines visualized with the schlieren method. These techniques were recently applied to supersonic flows of dense organic vapors, with the objective of obtaining accurate data to validate theory and CFD codes. Non-ideal compressible fluid dynamics (NICFD) is concerned with these flows, for which therefore the thermodynamic properties of the fluid can be modeled only with equations that are more complex than the ideal gas relations. NICFD flows are relevant, e.g., for applications in the power and chemical industry. However, currently employed image post-processing techniques used to obtain the local Mach number or shock wave angle from schlieren images, like the Hough transform, suffer from few drawbacks, namely a long computational time to obtain the relevant quantities and improvable accuracy. The investigation reported here concerns the application of known digital image processing methods to schlieren images, in this case Gabor filters and Radon transforms, to obtain the local Mach number and the shockwave angle of flows in NICFD conditions. The selected test case is the supersonic expansion of the dense vapor of hexamethyldisiloxane flowing through the nozzle test section of the ORCHID facility in operation at the Propulsion and Power laboratory of Delft University of Technology. The investigated digital image processing techniques provide values of the local Mach number with comparable uncertainty (within (5%)) as the Hough transform approach. Moreover, Mach line orientations are computed for the whole field of view, together with Mach line wavelength. It was also proven that these methods are suitable for discerning Mach line orientation even in the case of very complex flow fields, with coexisting Mach waves and shock waves.

We introduce recurrent all-pairs field transforms for stereoscopic particle image velocimetry (RAFT-StereoPIV). Our approach leverages deep optical flow learning to analyze time-resolved and double-frame particle images from on-site measurements, particularly from the ‘Ring of Fire,’ as well as from wind tunnel measurements for fast aerodynamic analysis. A multi-fidelity dataset comprising both Reynolds-averaged Navier–Stokes (RANS) and direct numerical simulation (DNS) was used to train our model. RAFT-StereoPIV outperforms all PIV state-of-the-art deep learning models on benchmark datasets, with a 68 % error reduction on the validation dataset, Problem Class 2, and a 47 % error reduction on the unseen test dataset, Problem Class 1, demonstrating its robustness and generalizability. In comparison with the most recent works in the field of deep learning for PIV, where the main focus was the methodology development and the application was limited to either 2D flow cases or simple experimental data, we extend deep learning-based PIV for industrial applications and three-component two-dimensional (3C2D) velocity estimation. We believe that this study brings the field of experimental fluid dynamics one step closer to the long-term goal of having experimental measurement systems that can be used for fast flow field estimation.

This study builds upon prior research by exploring droplet oscillation dynamics for surface tension determination using a drop-on-demand high-temperature droplet generator. Computational fluid dynamics (CFD) simulations were conducted to analyse frequency shifts over time, comparing two different materials with consistent results. The findings suggest potential for developing correction factors for oscillations with larger initial deformations. Additionally, frequency shifts relative to evolving aspect ratios of droplets starting with higher initial deformations were compared. Corrective measures can be applied, particularly beneficial for short-term measurements based on image analysis with minimal overall frequency shift. Slight asymmetry in oscillation with increasing aspect ratio could be accredited to droplet cross-sectional geometry or energy availability for returning prolate droplets to a spherical state. Experimental results indicated minimal frequency shift within a measurement duration of up to 40 ms, affirming the adequacy of using a fitted sine function without a time-dependent frequency term for overall frequency determination. A dimensionless criterion can be used to filter out unsuitable droplets. A temperature-dependent surface tension trend for AlCu10 alloy consistent with literature findings is introduced.

The detection of an interface separating two zones of low contrast and evolving in a deformable medium is very challenging. The low-contrasted images describing a partially saturated granular medium are replaced by high contrasted images generated from the correlation error maps issued from a standard digital image correlation (DIC) calculation. A robust algorithm has been developed to capture automatically the sought interface evolving in 2D space through time. Capable of detecting all morphological reliefs, this approach is completed by a sequence of steps to remove aberrant contours and track the main advancing interface. Consequently, adequate algorithms have been developed to determine physical and morphological properties that will give insights on the factors that monitor the interface propagation.

Internal waves generated by oscillating topography with a series of ridges in a stratified medium are experimentally explored. Experiments represent oscillating tidal flow in the ocean where small-scale roughness on topography cannot be fully resolved in global circulation models, but the generated internal wave field can impact global mixing and ocean dynamics. Here, the influence of topography roughness is evaluated by including different numbers of ridges, with slopes equivalent to the edge slope of the full topography, on top of the original topography. Specifically, the internal wave field generated by a wide plateau shape is compared with the same shape except with three to six Gaussian ridges overlain on the plateau. In all scenarios, a complex pattern of internal waves generated by each ridge is observed. However, the results show as the number or width of ridges increases, the waves generated by the ridges near the center of the plateau decay very quickly and in the far field the internal wave field is indistinguishable from that generated by a smooth plateau. A non-dimensional number is suggested that accounts for both the number of ridges and overall topography width while defining a limit for which plateau-like internal wave generation is expected and this form of surface roughness may be neglected.

There remain many uncertainties regarding the ignition promotion effects of the micro-rocket torch used for forced ignition in a scramjet engine. Therefore, a detailed investigation of the physical properties of the ejected gas is necessary as a preliminary step toward understanding the ignition promotion mechanisms. While tunable diode laser absorption spectroscopy (TDLAS) has been developed for high-precision optical measurements of combustion gases, it has become evident that accurately measuring the combustion gas temperature is challenging due to the presence of multiple variable fitting parameters. Therefore, in this study, TDLAS was employed to measure the gas temperature, and the accuracy was enhanced by integrating the background-oriented schlieren (BOS) technique to precisely determine the gas jet diameter. We successfully visualized the gas ejected from the torch nozzle using the BOS method. This visualization allowed us to fix the optical path length, which was previously treated as a variable fitting parameter. As a result, the TDLAS+BOS method improved the temperature measurement accuracy compared to TDLAS-only. Specifically, the simultaneous TDLAS + BOS measurements provided more accurate gas temperatures than TDLAS-only, with errors reduced to 7.5% for an equivalence ratio of 0.51 and 10.7% for an equivalence ratio of 1.38. These findings demonstrate that the integration of BOS with TDLAS significantly enhances the precision of temperature measurements in micro-rocket torch applications, contributing to a better understanding of the ignition mechanisms in supersonic flows.

Vortices are a fascinating flow phenomenon and an important research topic in fluid mechanics. In this study, the evolution of symmetrical vortices within a generating droplet in an altered T-shaped microchannel was investigated using microparticle image velocimetry (micro-PIV) and high-speed imaging microscopy. Transient flow patterns within the microdroplet were visualized in the focal plane, revealing a high-speed jet flow and two counter-rotating symmetric vortex pairs. These findings suggest that the internal flow field of the microdroplet exhibits intricate three-dimensional structures. The vortical flow behaviors were quantitatively characterized in detail by calculating the vorticity, rotation, shear deformation, and tensile deformation. These results are crucial for comprehending the initial flow dynamics and mixing processes occurring within generating microdroplets.

The evolution of the neutralization reaction in a stationary droplet of cylindrical shape, extracting a surfactant from the surrounding mixture in a vertical Hele-Shaw cell, was studied experimentally. In our experiment, we used a new method of the simultaneous visualization of the refractive index field of light and the distribution of acidity levels in the solutions of source reactants and reaction products within the droplet. To carry out this approach, we utilized a Fizeau interferometer and added a pH indicator to the droplet before the experiment. A digital video camera recorded the resulting interference pattern with the superimposed color distribution created by solutions with different acidity levels. The study was conducted on a system of liquids where the chemical potentials would be equal when the concentration of the extracted reactant in the droplet was much higher than in the surrounding environment. Two variants of reaction realization—with and without Marangoni convection development—were considered. We determined the structures of flows and concentration fields in the droplet and its neighborhood and traced their evolution. Also, we evaluated the characteristic times of the extraction process depending on the initial reactant concentrations and droplet sizes. It was found that the resulting Marangoni convection had an oscillatory character and continued after the reaction completion. As expected, the formation of the capillary motion intensified the progression of chemical reaction inside the droplet.

Flows over cavities are relevant to many branches of engineering and are known to be a source of instabilities, high-pressure disturbances, and large recirculating regions, leading to excessive pressure loads. In this paper, we study the dynamical behavior of a 6.44:1 length-to-depth transitional cavity flow (i.e., where the shear layer partly enters the cavity) with wall proximity and lateral apertures. Mitigation of pressure loads is investigated through steady blowing upstream of the cavity’s leading edge. Concurrent pressure and particle image velocimetry (PIV) measurements along with companion unsteady numerical simulations have been performed to identify the mechanisms underlying the flow dynamics of both baseline and controlled cases. Experiments are reproduced numerically using the Improved Delayed Detached Eddy Simulations (IDDES) approach with shear stress transport eddy viscosity model ((k-omega) SST) at a Reynolds number of (Re=2.8 times 10^5). Results underline that steady blowing changes the flow drastically upstream of the cavity by thickening the boundary layer and reducing the flow rate passing the cavity. The controlled flow transforms the dynamics of the cavity shear layer, impacting the inner cavity flow, and leads to a significant reduction of the pressure loads. This mitigation is associated with a strong reduction in turbulent momentum at the shear layers interface.

A method for analyzing liquid ligaments of a textural atomization process is presented in this article for the case of a rocket engine type-assisted atomization process under combustion. The operating point positions the atomization process in the fiber-type regime carrying an intense textural atomization process. Multiscale in nature, the method based on image analysis associates a scale distribution with a family of ligaments, this distribution being sensitive to the number, size and shape of these ligaments. The quality of scale distributions measured by image analysis depends on the spatial resolution and the precision of area measurements of surfaces with curved boundaries but described by square pixels. Part of the work consisted of improving the method for measuring scale distributions by using a sub-pixel image analysis technique and refining the surface area measurement method. Another part directed the multiscale analysis toward the estimation of the diameter distributions of the blobs that characterize the large-scale deformation of the ligaments. The analysis describes the atomization process at a level of detail never reached. For instance, assuming that the blobs are drops in formation, the estimated diameter distribution (bimodal in the case examined here) and the number of these drops are evaluated as a function of the distance from the injector. This information indicates where the process is most intense and where it stops. Furthermore, these diameter distributions receive a mathematical expression whose parameters report clear evolutions with the distance from the injector. This shows the possibility of elaborating mathematical models appropriate for textural atomization mechanisms.