Experiments in Fluids最新文献

Interactions between vortices and normal shock waves, which are often encountered in transonic and supersonic flows, can cause the vortices to break down or burst. Experiments aimed at establishing the characteristics of these interactions are performed in the range Mach 1.3 to Mach 1.5, where little experimental data is available despite the industrial relevance of this regime. A strong compressibility effect was observed, with the non-dimensional circulation required to cause vortex breakdown reducing by about 25% from Mach 1.3 to Mach 1.5. For interactions featuring vortex breakdown, high-speed schlieren visualisation demonstrates a direct link between the bursting of the vortex and local perturbations in the wave pattern. In addition, the time-resolved analysis indicates that the Mach 1.3 interaction is characterised by intermittent breakdown events which are likely caused by feedback between vortex breakdown and adverse pressure gradient.

Five-hole probes are a well-established measurement technique for flow surveys in fluid mechanics and fluid machinery, where the probe calibration is known to become sensitive to Reynolds number below a probe-geometry-specific critical Reynolds number. Nevertheless, probes are typically calibrated at a single Reynolds number—particularly for use in incompressible flow-resulting in significant measurement errors in typical technical flows with strong velocity and Reynolds number variations. To address this limitation, this paper presents a Reynolds number-dependent calibration method based on repeated probe calibration over Reynolds numbers ranging from 2000 to 20000. A novel Reynolds number coefficient is introduced, extending conventional two-dimensional calibration maps into a three-dimensional calibration space. Two data-reduction strategies are investigated: three-dimensional interpolation and artificial neural networks. The proposed method is evaluated using the open-access Oxford Probe and compared against conventional probe calibrations at constant Reynolds number. Compared with conventional constant-Reynolds number calibrations, the proposed Reynolds number-dependent approach reduced the errors by up to ({50,mathrm{%}}) in flow angles, ({70,mathrm{%}}) in total pressure, and ({60,mathrm{%}}) in dynamic pressure. Artificial neural network-based regression provides a further reduction of approximately ({30,mathrm{%}}) relative to the interpolation-based calibration in all flow quantities. A parametric study demonstrates the effects of network architecture and size on the calibration errors.

Laser-based flow diagnostics offer significant advantages over classical probe-based intrusive instruments in regard to measurement capabilities, spatial and temporal resolution, and measurement uncertainty reduction. Due to the complex thermochemical nature of the gas exiting an arc-jet nozzle, direct experimental measurements of chemical and thermodynamic gas properties are required to accurately measure the flow enthalpy, understand test material response, and verify theoretical predictions. The presented work documents the first application of hybrid femtosecond/picosecond Coherent Anti-Stokes Raman Scattering (fs/ps-CARS) spectroscopy to non-equilibrium thermometry measurements in an arc-jet flow. Vibrational fs/ps-CARS spectroscopy in nitrogen freestream flow enabled thermodynamic measurements by fitting the spectra to a theoretical model of non-equilibrium fs/ps-CARS. Calibration measurements were used to adjust model parameters, capture the nonresonant background effects, and optimize experimental parameters for application in arc-heated Mach 4 nitrogen flow. With a strategy to limit the background radiation and address the challenges posed by the long optical paths, inherent to this class of arc-jet facilities, the measured vibrational and rotational temperatures of diatomic nitrogen were 3690 ± 87 K and 1370 ± 109 K, respectively, successfully capturing the thermochemical non-equilibrium state of the flow, and matching theoretical predictions. This demonstration lays the foundation for broadening the applicability of hybrid CARS to thermometry from other molecular flow constituents (e.g., O(_2), NO), performing simultaneous rotational and vibrational measurements, and integrating with other ultra-fast optical diagnostics for a more comprehensive flow characterization.

This research provides experimental insights into the interactions between vortices in lined double-slit cavities, under simultaneous tangential flow and acoustic excitation. Notably, at high sound pressure levels, noise reduction is governed by the conversion of acoustic energy into vortical kinetic energy and its subsequent dissipation; this work therefore focuses on how the intensity of the tangential flow modulates the acoustic response and vortex–vortex interactions. To achieve synchronous measurement of acoustic responses, pressure fluctuations, and unsteady flow behavior, an integrated setup incorporating microphone arrays, pressure transducers, and particle image velocimetry (PIV) was implemented. The PIV system was synchronized via a field-programmable gate array (FPGA), ensuring precise phase-locked measurements through its real-time computing capability. Analysis of the transmission loss demonstrates that the double-slit configuration yielded greater acoustic attenuation compared to a single-slit geometry. This performance enhancement was found to be more pronounced under conditions of strong flow-convection effects. Subsequently, a comparative analysis was performed to examine the vortex dynamics under both weak and strong flow-convection effects. Under weak flow-convection effects, the separated vortex was primarily transported downstream, while under strong effects, the vortex system merged with the shear layer. Furthermore, the mechanism of acoustic-to-flow energy conversion was investigated through an analysis of pressure pulsations and identified coherent structures. The results revealed that the tangential flow significantly modulates the vortex–vortex interactions, as evidenced by a clear shift in the dominant mode from the interaction of separated vortices to the interaction within the internal cavity vortex system.

This study experimentally investigates the impact of streamwise finlet height on the three-dimensional turbulent structures and wall-pressure fluctuations over a NACA0018 airfoil. The presence of finlets of three heights all induces two pairs of counter-rotating streamwise vortices, leading to a pronounced lifting effect behind them, followed by a recovery process downstream. The intensity of this lifting effect strongly depends on the finlet height. Short finlets convey the original streak structures upward, maintaining a spatial distance from the wall. Differently, the medium-height and tall finlets disrupt the streamwise velocity streaks in the turbulent boundary layer and generate new hairpin-like vortices in the detached shear layer. The stronger lifting effect displaces turbulent structures away from the wall, leading to a significant reduction in Reynolds shear stress and pressure fluctuations in the mid- and high-frequency bands. The best pressure reduction performance is obtained for median-height finlets. Excessively tall finlets introduce additional turbulent structures, thereby elevating low-frequency wall-pressure fluctuations and compromising high-frequency suppression.

Accurate surface temperature and heat flux measurements are essential for evaluating aerothermal loads on high-speed vehicles, including atmospheric entry capsules and space exploration vehicles. Temperature-sensitive paint provides a non-intrusive approach for global heat flux measurement in impulse ground-testing facilities, but its application is often limited by low signal levels caused by short test durations, high-frame-rate imaging, and the associated short exposure times. In this study, a particle-enhanced temperature-sensitive paint was developed and optimized by incorporating calcium silicate microparticles to improve luminescence output and measurement precision. Characterization tests showed that the optimized formulation increased emission intensity by 21% and temperature sensitivity by 11% relative to the reference temperature-sensitive paint, while retaining nearly twice the normalized emission intensity at 500 kPa. Shock tube validation demonstrated that the enhanced signal level reduced the standard deviation of reconstructed heat flux measurements by up to 33% under short-duration transient conditions, indicating the potential applicability of the particle-enhanced formulation for high-speed aerothermodynamic diagnostics.

Helium-filled soap bubbles have emerged as state-of-the-art fluid flow tracers for large-scale volumetric measurements over the past 15 years. Owing to their relatively large size, ranging from a few hundred microns to a few millimeters, they scatter significantly more incident light than conventional micron-sized fog particles. Although this enables orders-of-magnitude larger measurement volumes and reduced costs associated with illumination, they are also impacted by challenges associated with reliable and consistent generation, dispersion in particle response times, short bubble lifetimes and wind tunnel seeding methods. In this review, we discuss the parameters that govern the behavior of helium-filled soap bubbles in fluid flows and summarize the efforts to-date to quantify and validate their behavior. We highlight specific aspects that will play an important role in establishing their use in future experiments as well as avenues to tackle these challenges.

The ascent of a toroidal air bubble in water was experimentally investigated using shadowgraphy and PIV. The bubble was generated by injecting a pulsed air jet vertically upwards into water. The bubble volume and its formation conditions were varied. The dependencies of the effective radius and ascent velocity on the elevation height were determined, and the trajectory of the bubble’s core center was constructed. Based on the analysis of the velocity field and geometric parameters, the circulation and the bubble expansion coefficient were calculated. The Froude number, based on circulation and characteristic bubble size, was found to scatter randomly about a mean value. It was established that the trajectory and the expansion coefficient of the toroidal bubble do not depend on the volume but are related to the value of the Froude number. An empirical dependence of the expansion coefficient on the Froude number was obtained, which is approximately inversely proportional to the Froude number, in qualitative agreement with the theory of self-similar buoyant vortices.