Experiments in Fluids最新文献

This paper investigates the effects of the interaction between an under-expanded supersonic jet and a flat plate on screech generation. The plate is installed parallel to the jet axis and an experimental investigation is made for several configurations corresponding to different radial distances of the plate, H, from the nozzle axis. Acoustic measurements are carried out to evaluate the plate impact on jet aeroacoustics. The results show that, when the radial distance is less than 1.5 nozzle diameters, the plate substantially alters the staging dynamics between modes A1 and A2 and between modes B and C. Specifically, the plate induces an extinction of modes A2 and C together with an extension of modes A1 and B. The extent of flow conditions for which the extinction of modes A2 and C is observed is dependent on the jet-plate distance, the smaller the distance the wider the range of flow conditions. Furthermore, the plate induces a frequency splitting of mode B for the smallest jet-plate distance, that is, (H=0.7D). A time-frequency analysis is performed to explore in detail the staging dynamics between A1 and A2 modes. The study shows that the jet experiences a complete suppression of mode A2 for (H=0.7D) and an intermittent switch from A1 to A2 modes as the jet-plate radial distance is increased. Finally, schlieren measurements are conducted to assess whether the observed modifications of the screech behaviour might be associated with changes of the shock-cell structure. The analysis reveals that for (H=0.7D) the surface induces an attenuation of the amplitude of the secondary wavenumber peak of the shock-cell structure, thus providing a possible explanation of the A2 mode suppression observed in the noise spectra.

The separated and reattaching flow induced by an asymmetrically oscillating fence in a laminar boundary layer is investigated using time-resolved particle image velocimetry, with a focus on the evolution of shedding vortices, the characteristics of the turbulent/non-turbulent interface (TNTI), and the entrainment process. Fast-rising and slow-rising asymmetric oscillation modes are compared against sinusoidal oscillations, all at the resonant frequency. The results reveal that asymmetric oscillations effectively modulate vortex dynamics within individual cycles: The fast-rising oscillation enhances the core swirling strength, while the slow-rising mode generates larger-scale vortices and introduces a phase lag in their development. However, these asymmetric oscillations have negligible effects on the time-averaged statistical characteristics, such as the recirculation zone size, the vorticity thickness, and the turbulent kinetic energy distribution. The influence on the geometric properties of the TNTI, such as its height distribution, is also limited, primarily confined to the region near the fence. As for dynamic properties of the TNTI, the asymmetric oscillations accelerate the decay of the TNTI thickness, suggesting a premature breakdown of the primary shedding vortices. Despite this, the dimensionless TNTI thickness converges to a similar value after flow reattachment across all actuated cases. Regarding entrainment, the asymmetric oscillations alter the streamwise evolution of the engulfment flux near the reattachment point. Additionally, they moderately enhance the local entrainment velocity and flux associated with the nibbling mechanism within the recirculation zone, while reducing these parameters in the post-reattachment region. Nevertheless, the relative contributions of the nibbling and engulfment mechanisms to the total entrainment remain largely unchanged.

The background-oriented Schlieren technique has emerged as a promising method for visualizing density gradients and performing quantitative measurements. However, an inherent constraint of BOS is the compromise between spatial resolution and measurement sensitivity, as the BOS camera typically remains focused on the background pattern. To overcome the resolution-sensitivity constraint, a new BOS variant based on nominally directional rays is proposed in this paper. Instead of using diffuse, reflective background patterns, a spherically concave mirror etched with random dots has been used to create a dotted background that reflects rays in a directional manner. When combined with coaxial LED illumination, we demonstrate that the current setup can improve the spatial resolution of canonical BOS without compromising measurement sensitivity. Moreover, the proposed setup decouples the requirement for a small lens aperture to achieve a large depth of field, thereby significantly reducing the need for strong background illumination in high-speed BOS applications. To demonstrate the effectiveness of the proposed method in improving the BOS spatial resolution, both synthetic BOS image generations and experiments on low- and high-speed jets are conducted. The results show that the proposed BOS variant can be advantageous for measuring density-varying flows with a limited field of view.

Currently, most aerodynamic optimizations for square cylinders mainly rely on corner modifications, which sacrifice usable area. The configuration of retaining sharp corners with convex side surfaces preserves space while promising aerodynamic benefits, yet it lacks systematic experimental data on how curvature regulates aerodynamic and wake behaviors. Thus, the present study focuses on the aerodynamic characteristics and wake flow features of this specific configuration, aiming to quantify the influence of convex surface curvature on its aerodynamic performance and wake dynamics. Wind tunnel experiments were conducted on a standard square cylinder and three variants with different convex surface curvatures, integrating Particle Image Velocimetry (PIV) and pressure measurement systems. Experimental results show that modifying the convex surface curvature significantly alters aerodynamic performance and wake characteristics. Increasing convex surface curvature results in a gradual decrease in the C_p,mean(C_{p,mean}) and C_p,rms(C_{p,rms}) distributions on the sides and leeward face of the cylinder, with maximum reductions of 55.89 and 94.67% compared to the standard square cylinder. This modification decreases the aerodynamic coefficient and vortex shedding intensity, enhancing overall performance. Analysis of the time-mean statistics of the wake reveals that increasing the curvature radius leads to an expansion of low-speed regions in the wake, accompanied by a decrease in the distribution and intensity of high turbulent kinetic energy. This alteration in flow patterns contributes to reduced resistance and enhanced flow stability. Additionally, Proper Orthogonal Decomposition (POD) analysis indicates that the modification of convex surfaces can concentrate the energy of the dominant modes, forming a more compact flow structure.

This study investigated the performance of hot-wire anemometry and a multi-hole Cobra pressure probe for turbulence characterization in a small-scale, 3D-printed wind tunnel operating at a Reynolds number of (2.6 times 10^{4}). Experiments were conducted under passive grid-generated turbulent conditions, resulting in freestream turbulence (FST) levels of (2%), (4%), and (8%). Despite the Cobra’s practical advantages (robust, geometry-dependent pre-calibration and simultaneous three-component velocity measurements), it showed limitations due to a restricted sampling frequency and the probe’s head geometry, leading to spectral deviation and biased turbulence estimates. Post-processing strategies were devised to mitigate these effects. A second-order polynomial spectral correction improved estimates of RMS fluctuations and the integral length scale (L). At the same time, an approach based on equilibrium dissipation laws enabled consistent estimation of dissipation-related quantities, including the Taylor microscale ((lambda)), Kolmogorov scale ((eta)), and dissipation rate ((varepsilon)), without relying on velocity gradients, which are highly dependent on the pressure probe’s limitations. Validation in a large-scale wind tunnel confirmed that corrected Cobra-based dissipation estimates could achieve relative errors below (30%) compared with hot-wire data. Overall, the results demonstrate that, when combined with the proposed corrections, the Cobra probe can provide a good estimate of turbulence features despite its intrinsic frequency limitations, offering a practical alternative to hot-wire anemometry in low-Reynolds-number turbulent flows.

This paper presents a novel adaptive control pattern (ACP) for real-time feedback control of flow separation over airfoils using sparse processing particle image velocimetry (SPPIV) and dielectric barrier discharge plasma actuators. The study addresses the challenge of suppressing separation in deep stall at low Reynolds numbers, where previous feedback-based methods often fail to maintain effective flow attachment. Unlike conventional feedback control methods such as single-step and multiple-step prediction, where control inputs are directly decided on the basis of state predictions, ACP determines the modulation frequency of the actuation. This is done according to threshold-based flow state detection, enabling the selection of effective actuation patterns for the estimated flow features. Experiments were conducted on a NACA0015 airfoil at an angle of attack of 18 degrees and a Reynolds number of (approx)66,000 using a plasma actuator positioned at the leading edge. The SPPIV system acquired data with a sampling frequency of 2,000 Hz, processed PIV at optimized limited interrogation windows, and estimated the state within 20% of the time between samples. Linear models for state estimation were generated via dynamic mode decomposition with control of the flow field representations from proper orthogonal decomposition. Results demonstrate that ACP control successfully achieves flow reattachment at 6 kV actuation voltage where other methods fail, whereas at higher voltages, ACP control combines the fast and reliable reattachment speeds of lower actuation burst frequencies of open-loop control with stable quasisteady attached conditions of higher burst frequencies. It shows that the optimal actuation frequency for driving the reattachment process is not the same as that for maintaining a reattached condition, and that guiding the actuation frequency as the reattachment process evolves can provide substantial improvements in control authority. This breakthrough brings the plasma actuator one step closer to practical control applications, with highly effective yet robust control parameters.

Contaminants such as microplastics, ash, and pollen are constantly carried between the ocean and atmosphere via raindrops, splash droplets, and breaking waves. These contaminants have an impact on marine life via bioaccumulation and alter the chemistry of the ocean, consequently impacting our weather prediction and radar systems. However, there are few studies involving deep pool drop impacts and even fewer where the impacting drop contains particulates, which would simulate the phenomena that naturally occur in our environment. For our study, we created a simplified system to mimic pollutant-laden raindrops impacting the ocean. Utilizing high-speed imaging, we characterized the effects of particle size, particle density, and seeding density on the splash phenomena and final particulate distribution. It was found that, while the particle properties (size, density, seeding density) did not alter the overall splash regimes, they did influence the dimensions of various characteristic splash morphologies, albeit to a much lesser extent than the impact energy of the droplet. We also identified relationships between the particle properties and the particulate distribution. Particle size and density have opposite effects on the percent of particulates in the bulk of the pool and on the surface. Seeding density has a significant influence only when there are no splash droplets. The percentage of particulates re-entering the atmosphere via splash droplets, on the other hand, are significantly affected by the particle properties; however, the exact effect is not yet clear. Understanding the particulate-ocean–atmosphere interactions allows us to improve our predictive system and ocean-simulating models.

A high-frequency dynamic calibration system for pressure-sensitive paint (PSP) was developed using cavity-induced jets. The dominant frequency for dynamic calibration was adjusted to the range of 9–16 kHz by scaling the cavity. The dominant flapping frequencies corresponding to different cavity lengths were identified. The distributions of pressure fluctuations across cavities were measured using fast PSP technology. An anodized-aluminum PSP (AA-PSP) with a free-based porphyrin luminophore (H(_2)TCPP) was utilized for pressure measurement. Instantaneous pressure fields were measured by combining a high-frequency dual-pulse laser and a high-speed camera. The denoising technique combining filtering and singular value decomposition (SVD) was applied, and the dominant pressure fluctuation components were extracted. The spatially averaged pressure amplitude and the uniformity of the pressure fluctuation fields were evaluated based on SVD. Potentially optimal calibration regions, exhibiting both high amplitude and uniformity, were identified on the Pareto front. Among them, the region exhibiting the highest pressure amplitude was selected as the optimal calibration region, which is located around the downstream slot. A point-measurement system for dynamic calibration of PSPs was constructed. The dynamic calibration of fast PSPs was performed. The gain attenuation and phase delay in the range of 9–16 kHz were evaluated. The calibration results exhibit good continuity with those obtained below 9 kHz using an acoustic resonance tube system. The cut-off frequency of AA-H(_2)TCPP and PC-Ru(dpp)(_3) at -3 dB are approximately 19.9 kHz and 18.9 kHz, respectively.

The catastrophic failure of the Ariane 5 ECA heavy launcher in December 2002 underscored significant gaps in understanding the dynamic loads acting on the vehicle’s base region. Investigations revealed that aerodynamic interactions between the nozzle exhaust plume and the launcher wake, particularly the phenomenon of “buffeting,” led to intense oscillatory mechanical loads. This study addresses the challenges in accurately replicating these effects in both experimental and numerical models, given the wide range of time and spatial scales and the complexities of high-temperature gas flows. Traditional experimental approaches have relied on cold nozzle flows, which, while practical, fail to capture key high-temperature effects influencing aerodynamic excitations. To overcome these limitations, a novel experimental setup was developed using a wind tunnel model integrated with a solid propellant combustion chamber, producing hot exhaust gases under realistic conditions. Advanced diagnostics, including high-speed particle image velocimetry (PIV) at (8500,textrm{Hz}), high-speed schlieren imaging, and Kulite pressure sensors at (17000,textrm{Hz}), enabled detailed characterization of the unsteady flow dynamics in the base region. Key findings reveal two dominant aerodynamic modes in the base region: flapping, characterized by asymmetric flow motion, and swinging, associated with symmetric oscillations. These modes are the primary drivers of the unsteady loads acting on the nozzle and adjacent structures. At a Mach number of 0.8, the flapping mode is found to resonate with the jet noise mechanism known as screeching, resulting in strong, alternating oscillatory loads on the wake region. This resonance amplifies the mechanical stresses experienced by the structure and, from the authors’ perspective, represents the most plausible explanation for the excessive loads that led to the catastrophic failure of Ariane 5 Flight 157. The study successfully replicates realistic flight conditions for investigating buffeting phenomena, bridging the gap between cold-flow experiments and actual high-temperature exhaust flows. This achievement enhances predictive modeling capabilities by providing a more accurate understanding of the dynamic interactions in the base region. To support this, a schematic model of the feedback cycle was developed to visualize and capture the interacting effects, thereby facilitating a deeper physical understanding. Consequently, the research contributes to the development of safer and more efficient space launch systems by enabling improved design strategies to mitigate such damaging aerodynamic loads. The consistency of these results with prior cold-flow studies further validates the robustness of the experimental approach and the universality of the identified aerodynamic mechanisms across varying flow conditions.