Experiments in Fluids最新文献

Arctic sea ice is melting rapidly, and the Arctic is likely to experience its first ice-free summer in the next few decades unless action is taken locally. One proposed method of reducing or perhaps reversing the melting of Arctic sea ice is pumping seawater onto the surface of the sea ice where it should freeze faster and thicken the ice. This may in turn enable it to last longer or even survive the summer melting period, reflecting more sunlight and becoming stronger multi-year ice with increased resistance to future melting. Despite appearing to be a relatively simple physical problem, the technique has not been researched in depth. Here, the response of ice to water being pumped over its surface is investigated theoretically and experimentally for radial axisymmetric water flow. The dominant heat transfer mechanisms during the period shortly after placement of water onto ice are conduction through the ice away from the water–ice interface and heat transfer from the water to the interface. During this initial period of evolution, advection and radiation to the atmosphere are much smaller in magnitude and hence not included. The heat transfer from the water flow to the interface is modelled for three flows: a well-mixed uniform film flow; a uniform flow with a developing thermal boundary layer; and a laminar, viscous flow with a developing thermal boundary layer. Predictions from these models are compared with data from laboratory experiments using various initial water temperatures. The predictions of the model with a fully developed, laminar viscous flow and a developing thermal boundary layer for the evolution of the ice profile were found to be closest to the data obtained from laboratory experiments with water supplied at 0.5, 1.0 and 1.5 (^circ)C.

Within the framework of the European Union Horizon 2020 project HOMER (Holistic Optical Metrology for Aero-Elastic Research), data assimilation (DA) algorithms for dense flow field reconstructions developed by different research teams, hereafter referred to as the participants, were comparatively assessed. The assessment is performed using a synthetic database that reproduces the turbulent flow in the wake of a cylinder in ground effect, placed at the distance of one diameter from a lower wall. Downstream of the cylinder, this wall continues either in the form of a flat steady wall, or of a flexible panel undergoing periodic oscillations; these two situations correspond to two different test cases, the latter being introduced to extend the evaluation to fluid–structure interaction problems. The input data for the data assimilation algorithms were datasets containing the particle locations and their trajectories identification numbers, at increasing tracer concentrations from 0.04 to 1.4 particles/mm³ (equivalent image density values between 0.005 and 0.16 particles per pixel, ppp). The outputs of the DA algorithms considered for the assessment were the three components of the velocity, the nine components of the velocity gradient tensor and the static pressure, defined in the flow field on a Cartesian grid, as well as the static pressure on the wall surface, and its position in the deformable wall case. The results were analysed in terms of errors of the output quantities with respect to the ground-truth values and their distributions. Additionally, the performances of the different DA algorithms were compared with that of a standard linear interpolation approach. The velocity errors were found in the range between 3 and 11% of the bulk velocity; furthermore, the use of the DA algorithms enabled an increase of the measurement spatial resolution by a factor between 3 and 4. The errors of the velocity gradients were of the order of 10–15% of the peak vorticity magnitude. Accurate pressure reconstruction was achieved in the flow field, whereas the evaluation of the surface pressure revealed more challenging. As expected, lower errors were obtained for increasing seeding concentration. The difference of accuracy among the results of the different data assimilation algorithms was noticeable especially for the pressure field and the compliance with governing equations of fluid motion, and in particular mass conservation. The analysis of the flexible panel test case showed that the panel position could be reconstructed with micrometric accuracy, rather independently of the data assimilation algorithm and the seeding concentration. The accurate evaluation of the static pressure field and of the surface pressure proved to be a challenge, with typical errors between 3 and 20% of the free-stream dynamic pressure.

The frequency-modulating filtered Rayleigh scattering (FM-FRS) technique has been developed and applied for the boundary layer measurements. The FM-FRS technique effectively extracts Rayleigh scattering information from noisy low-SNR signals caused by intense wall glare. The boundary layer velocity profiles measured by FM-FRS show excellent agreement with an independent pressure probe measurement and the law-of-the-wall, including approximately 100 μm above the wall. This technique is desirable for the practical applications of the Rayleigh scattering technique for flow diagnostics to regions that were limited due to strong background and low signal-to-noise ratio.

This paper presents the application of various artificial fractal-like background patterns and a digital image correlation algorithm, to enhance the accuracy of image displacement measurement within background-oriented schlieren technology. A novel method for generating new fractal-like patterns is proposed, allowing for the combination of different pattern strengths. A more robust image displacement estimation algorithm that considers the self-affine property inherent in fractal patterns is introduced. Various synthetic flow tests, as well as real supersonic flow and combustion tests, were conducted to demonstrate the advantages of the estimation algorithm and to gain a comprehensive understanding of the applicability of different fractal-like backgrounds.

Graphical abstract

Normal breathing induces significant variations in Reynolds numbers throughout the human airways, resulting in distinct flow regimes. However, the onset of instabilities and the development of flow structures in this context are not yet fully understood. This study presents the application of a novel point-wise measurement technique, Correlation Velocimetry (CV), to investigate unsteady velocity variations within breathing cycles at very high temporal resolution over a strongly extended measurement duration. Our approach enabled the evaluation of velocity data from more than 1000 successive breathing cycles in a realistic airway model, providing unprecedented statistical robustness. We observed both high- and low-frequency oscillating structures with spatial and temporal coherence across all investigated breathing regimes, ranging from Reynolds numbers of 274 to 4382. The cycle-to-cycle repeatability of these structures suggests the presence of defined physical mechanisms. Contrary to previous interpretations attributing similar fluctuations to turbulence or transitional states, our analysis indicates that these oscillations likely arise from geometric features forming systems of harmonic oscillators driven by the fundamental breathing frequency. This study provides new insights into the complex, multi-scale nature of respiratory airflow dynamics, challenging existing models and offering implications for improving computational simulations and our understanding of respiratory physiology.

The influences of gas composition and temperature on supersonic retropropulsion (SRP) flowfields are experimentally explored. It is revealed that the standoff distance of the bow shock produced by SRP can be scaled to account for changes in thrust, mass flow rate, forebody size, gas composition and temperature within the high-thrust, steady flow regime. These parameters were systematically varied for Mach 2 and 3 heated jets at zero angle of attack, employing nitrogen, helium and argon within a Mach 2 nitrogen or carbon dioxide freestream. Comparisons are also made with higher freestream Mach number data from the literature for similar geometries. These datasets are used to provide new insights into multi-gas, multi-temperature SRP and are more widely transferable to flight conditions than the unheated nitrogen or air interactions that have been more widely studied. We find that the momentum ratio can successfully account for changes in gas composition and temperature. Similarly, the mass flow rate ratio can account for these variables, down to a function of Mach numbers when multiplied by functions of gas molecular weight, temperature and the ratio of specific heats that are derived from mass conservation control volume analysis. Despite the collapse of shock standoff data in this study, a small dependence on the jet Mach number is seen to remain. Some additional scatter might also be expected at low thrust conditions as the bow shock transitions between one fully dominated by the freestream and forebody geometry to one dominated by the jet exhaust. Shock radius was seen to be more variable at lower thrust levels, potentially indicative of these effects. This work greatly aids the extrapolation of SRP between experiments, simulations and flight conditions.

In this study, the JFX reflected shock tunnel freestream is characterized using pitot probes, laser absorption spectroscopy, and high-speed schlieren for shock stand-off distances over a sphere. The experiment employed two driver gases: a mixture of H₂ and CO₂, and pure He. Three lasers, operating at wavelengths around 2.0 (upmu mathrm{m}) and 1.4 (upmu mathrm{m}) with a scanning frequency of 50 kHz, were utilized to measure the properties of CO₂ and H₂O. Computational fluid dynamics simulations showed near thermo-equilibrium conditions, supporting the use of an equilibrium model to determine the temperature and partial pressure of the two species. Isentropic calculations indicate that there is no significant thermodynamic nonequilibrium in the freestream. During the effective test time, the measured and simulated results were in good agreement for both the shock stand-off distance and CO₂ partial pressure. However, the detection of H₂O indicated contamination from the driver gas, with early onset leading to an increase in the shock stand-off distance. The uncontaminated time is around 700-800 (upmu mathrm{s}) for both conditions, and the contamination onset time falls between the predicted values using different nozzle conditions, which also indicates a certain degree of contamination.

Using temperature-sensitive phosphorescent materials, particle tracing technology presents a promising avenue to simultaneously obtain temperature and velocity fields in thermal fluids. However, the application of the technique is limited by the poor particle tracking ability of inorganic phosphorescent materials, particularly in low-speed flows due to their high density. To address this problem, this study developed fluid density-matched phosphorescent microspheres. Phosphorescent microspheres with hollow structures were synthesized via emulsion polymerization, which enables them to maintain the temperature measurement functionality while exhibiting favorable fluid density-matching properties and enhanced flow field tracking capabilities. The microsphere size and average density were meticulously controlled by adjusting the stirring time and temperature. The microsphere diameters were 57–120 μm, the theoretical average densities were 0.58–3.2 g/cm³, and the operational temperatures were 0–200 °C. The result of the numerical simulation indicates that the temperature response time of the microsphere was within 1.41 ms. Based on the developed microspheres, a temperature–velocity simultaneous measurement method was developed for low-speed thermal fluids. An application demonstration simultaneously measured the temperature and velocity fields in low-speed hot–cold mixed flows. Comparison with thermocouple measurements reveals that the current method can achieve a fluid temperature measurement with an error of 1.575%. The results underscore the efficacy of fluid density-matched phosphorescent microspheres in simultaneously acquiring temperature and velocity fields in low-speed thermal flows.

Graphic abstract

A novel type of low-speed water tunnel has been designed which uses an array of submersible electric thrusters to propel water through a test section centered within a large rectangular tank. This design can operate as a suction or blowdown tunnel to alternately create uniform, low-turbulence flow or spatially and temporally varying flows such as shear layers, wakes, and gusts. The thruster array architecture also enables an order of magnitude improvement in tunnel footprint for a given test section size, eschewing the need for bulky traditional elements such as the contraction, turning, and settling chambers. Planar digital particle image velocimetry (DPIV) and single-component constant temperature anemometry (CTA) measurements of flow uniformity and turbulence intensity indicate comparable flow quality to larger traditional tunnels when operated in suction mode. Dye visualizations of low Reynolds number cylinder wake flows are also presented to demonstrate tunnel capabilities.