Experiments in Fluids最新文献

This study presents an experimental investigation of the aerodynamics relevant to the landing of a small Uncrewed Aerial System on a fast-moving ground vehicle. A Representative Ground Vehicle and small Uncrewed Aerial System have been constructed for experimental measurements of the wake interactions in a low-speed, recirculating wind tunnel. Quantitative flow image techniques are employed to probe how the sUAS interacts with the wake structures shed by the ground vehicle when the two are in close proximity. Tools are developed for quantitative comparisons of flow fields and turbulence spectra. Using this tool, it is possible to identify regions of the flow and relative positions of the two vehicles where wake interactions are mostly linear in nature. This near-linear wake interaction was observed to extend to the wake spectra in regions where the time-averaged flow fields were also near-linear. Finally, it is shown that these observations of a near-linear wake interaction do not hold when the sUAS interacts with highly decelerated regions of the ground vehicle wake.

Droplet impact on substrates is the cornerstone of several processes relevant to many industrial applications. Imposing substrate oscillation modifies the impact dynamics and can, therefore, be used to control the ensuing heat, mass, and energy transfer between the substrate and the impacting droplet. Previous research has shown that substrate oscillation strongly influences the spreading behavior of the droplet. In this study, we extend this understanding to examine how substrate oscillations can further modulate the retraction dynamics of the droplet, consequently affecting its long-term behavior, with a particular focus on induced jetting and subsequent breakup. We systematically examine the breakup of jets formed by the recoiling droplet through experimental investigations across a range of oscillation frequencies and amplitudes. Our findings reveal two distinct jet breakup modes: early and late, each governed by different time scales. Subsequently, we present a mechanistic description of the jetting process. Furthermore, we derive a simple scaling analysis based on energy balance to identify the critical condition required for jet breakup. Finally, we compare the experimental data with the scaling analyses to show its efficacy.

Dominant wave components within a wavefield play key hydrodynamic and morphodynamic processes. Herein, we present a method to detect and measure the parameters of these waves, such as their wavelength, propagation angle and period. Image sequences of the free surface are captured with the use of a commercial unmanned aerial system. A snapshot proper orthogonal decomposition analysis is then applied to the image sequence, and a 2D autocorrelation is performed on the resulting modes. By extracting the mode that is representative of the dominant wave signal, it is then possible to infer the wave properties of the dominant wave. The outlined procedure is applied to ocean swells, wind waves, free surface undulations along a river and propagating ship wakes. Our results demonstrate an improvement in the signal-to-noise ratio of the peak wave signal to ambient noise over the more widely used fast Fourier transform approach.

In this study, the influence of various turbulence-grid configurations is analysed on both the induced free-stream turbulence (FST) and the resulting Klebanoff modes/streaks developing in a laminar flat-plate boundary layer downstream in a laminar water channel. All results are based on hot-film and particle image velocimetry measurements as well as visualizations. The grid design and installation has been done according to common grid installation recommendations to ensure homogeneous FST causing meandering Klebanoff modes inside the boundary layer. But it was found that (i) the Klebanoff modes do not meander for all grid configurations, (ii) not all configurations cause Klebanoff modes with the expected temporal and spatial behaviour, and (iii) for some configurations, the spanwise streak spacing is strictly locked to the grid spacing (mesh width). As these observations are unreported in the literature, this study is aimed at a thorough description of the influence of key grid parameters on the FST and the resulting streaks within the boundary layer. The investigation includes the grid parameters typically reported, such as the grid-bar diameters, the associated Reynolds numbers, or the streamwise placement of the grid, but now also the grid-orientation order (horizontal/vertical or vertical/horizontal order of grid bars of the dual-plane grid), the wall-normal position of the horizontal bars relative to the leading edge of the flat plate, and the existence of palpable imperfections in the manufactured grids. The Reynolds-number range covered lies well in the lower band of wind-tunnel experiments. Thus, this study suggests that the reliability and reproducibility of future experimental studies on FST would be greatly improved if they demonstrated homogeneity in the free-stream in both spanwise and wall-normal directions, documented the ongoing meandering and wavelengths of the generated Klebanoff modes and thus (implicitly) documented the spanwise independence of the results in the temporal mean. The latter is a prerequisite for the reliable investigation of FST/isolated-roughness interactions.

Graphical abstract

The paper describes the first experimental study on the application of small contoured grooves in boat-tailed bodies characterized by vortex shedding. In particular, we experimentally investigate the flow-separation delay and drag-reducing performance of spanwise-extruded and spanwise-discontinuous grooves. For this purpose, we consider groove geometries similar to those proposed and numerically investigated by Mariotti et al. (Eur J Mech B/Fluids 74:351–362, 2019) and Pasqualetto et al. (Fluids 7:121, 2022a). The Reynolds number, based on the freestream velocity and the model crossflow dimension, is (mathrm Re=9.6cdot 10^4). In addition to serving as an experimental confirmation of previous numerical studies, an important difference is that the present experiments were conducted with a freestream turbulence intensity of 0.9%, whereas the simulations were carried out with a freestream without turbulence. This extends the applicability of this flow control device to a situation closer to real-world or industrial applications. In the experiments, we measure pressure-drag variations for different configurations and flow correlations in the spanwise direction through pressure and hot-wire measurements. The results confirm the good performance of the grooves as passive flow-control devices and the capability of grooves to delay flow separation even in a turbulent freestream. The experiments elucidate the physical mechanism leading to the enhanced performance, specifically the reduction of friction losses due to the local recirculation embedded in the groove region. However, the experiments reveal a different behavior in terms of vortex shedding correlation in the spanwise direction with the introduction of grooves of different spanwise extents. Interestingly, the spanwise-extruded grooves exhibit a weaker increase in spanwise correlation of vortex shedding in experiments compared to simulations. This difference is likely due to the presence of freestream turbulence in the wind tunnel, which is absent in simulations. As expected, the introduction of the spanwise-discontinuous groove reduces vortex shedding correlation. Consequently, in experiments the adoption of spanwise-discontinuous grooves yields fewer benefits than those previously found numerically.

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.