Experiments in Fluids最新文献

Turbulent boundary layers (TBL) over various area coverages of rough surfaces were studied in a wind tunnel using hot-wire anemometry. The rough surfaces are built by randomly distributed sand grains with an average of 0.7 and 0.9 mm heights, covering 12%, 25%, 38%, 55%, and 75% of the flat-plate surface area. This paper aims to correlate the average roughness height (k) and roughness area coverage (RAC) with the Hama roughness function (Delta U^+) and to explore the influence of these roughness characteristics towards the TBL. The correlation between these roughness variables suggests that as the RAC increases, the (Delta U^+) increases and reaches a peak value at (38%) RAC. However, beyond the (38%) of RAC, the (Delta U^+) values decrease and then saturate. The mutual sheltering effect at a certain pitch-to-roughness height ratio is suspected to be the underlying mechanism for this pattern. Another finding in this study is that for surfaces with RAC < 100%, the Hama roughness function does not scale with the roughness effective slope. This finding suggests that the RAC is an important factor for any roughness TBL study, especially for surfaces with RAC < 100%. Finally, this study also correlates the two roughness characteristics (k and RAC) with the fully rough asymptote of Nikuradse (1933) to estimate the associated equivalent sand grain roughness height, (k_textrm{s}). The analysis indicates that the majority of the investigated cases are reaching the fully rough condition, and some are in the transitionally rough regime.

The deflagration to detonation transition (DDT) in acetylene–oxygen mixtures is experimentally investigated in a 1.5 m long, 80 mm inner diameter tube to quantify the influence of initial pressure, equivalence ratio, and geometric modification on the transition length. Tests are conducted over initial pressures of 0.3–1.5 bar and equivalence ratios ϕ = 0.4–1.6. Acetylene–oxygen is selected owing to its high detonation sensitivity, which enables DDT studies at relatively low initial pressures (0.3–1.0 bar) and lean equivalence ratios (ϕ = 0.4–0.6), where conventional hydrocarbon fuels typically fail to transition. Under lean and low pressure conditions, the deflagration phase is markedly prolonged, with run-up distances extending to about 1.5 m for P₀ = 0.5 bar and ϕ = 0.5 in the clean tube configuration. To mitigate this, a small diameter cavity resonator tube (CRT) is introduced within the deflagration region downstream of the ignition source. The CRT modifies the local combustion dynamics by enhancing flame acceleration and triggering a localized cavity induced detonation mechanism, thereby significantly reducing the DDT run-up distance to 0.5 m. This mechanism is active primarily during the transition phase and has limited influence once a fully developed detonation is established. The results demonstrate that targeted geometric modifications, combined with controlled initial conditions, provide a passive strategy to manipulate flame acceleration and DDT dynamics in large diameter confined reactive systems, with direct relevance to detonation based propulsion devices.

Particle-laden liquid flows, such as sand-water flows encountered in turbidity currents, are difficult to measure due to optical-opacity. Radiation-based methods, such as X-ray densitometry, are a useful tool to measure the local volume fraction regardless of opacity; however, applying the method to flows with comparable density of phases is challenging due to poor signal-to-noise ratios (SNR). In this study, time-resolved X-ray densitometry using a medical-style X-ray source and a hybrid photon-counting (HPC) detector is applied to perform line-of-sight-integrated volume fraction measurements in a turbid flow of simulated sand (glass particles) and water. Measurements were performed over a range of simulated particle volume fractions using known thicknesses of glass matching the composition of the particles. As each pixel counts incident photons in a given exposure time, spatial and temporal summation is indistinguishable from collecting data with larger pixels or longer exposures. Leveraging this, an a posteriori approach to significantly improve measurement SNR is presented. Using this method with expected values between 0 and 0.113, the maximum measured volume fraction error is improved from (pm 0.015) at 18.1 pixel/mm and 500  frame/s to (pm 0.005) at 2.6 pixel/mm and 100  frame/s. The methodology is demonstrated on a lock-exchange turbidity current in a laboratory setting, showing the viability of photon-counting densitometry to the study of sand-water flows. This approach can enable detailed study of the internal dynamics particle-laden liquid flows, thus providing crucial data.

This study presents an experimental investigation of the flow around a three-dimensional flexible hydrofoil at Reynolds numbers between ({3.6}times {10}^{5}) and ({7.2}times {10}^{5}) and angles of attack from ({-8}^{circ }) to ({8}^{circ }). Time-resolved particle image velocimetry, force-balance measurements, deflection, and hydrophone data are used to analyze the hydrodynamic forces and wake dynamics in both the time and frequency domains. The results are compared with those of a rigid reference hydrofoil. Under specific flow conditions, a strong coupling is observed between the fluid motion, hydrodynamic loading, and the hydrofoil’s natural vibration modes. This lock-in behavior shows a pronounced dependence on the angle of attack, resulting in a significant redistribution of turbulent kinetic energy in the wake and enhanced coherence in the trailing-edge vorticity.

We present a high-accuracy Background Oriented Schlieren system utilizing a commercial LCD monitor as a programmable background. This self-luminous source provides uniform illumination and a reconfigurable random-dot pattern, overcoming the limitations of printed targets. Displacement fields from RAW 14-bit images of propane–butane gas jets (cold and combusting) are retrieved using Farnebäck optical flow applied independently to each RGB channel and then averaged. This multi-channel approach, combined with the display’s uniformity and RAW acquisition, achieves a low noise floor of 0.018 pixels ((120,text {nm}) displacement in the image plane) for the flame and 0.03 pixels ((200,text {nm}) displacement) for the cold jet. To extract quantitative data, we solve the inverse problem by fitting a parametric refractive index model to the experimental displacement profiles using forward ray tracing (4th-order Runge–Kutta) and Adam optimization. The retrieved parameters accurately capture the flow physics, showing an increase of the refractive index in the cold jet and a decrease in the flame, with root-mean-square errors of (0.3 , upmu text {m}) and (0.4 , upmu text {m}), respectively. A direct comparison with printed background under identical conditions shows that the LCD display reduces the displacement noise floor by factors of 2–4. This work demonstrates that combining a programmable display with advanced optical flow and model-based inversion provides an effective tool for quantitative flow diagnostics, achieving sub-micron displacement sensitivity from a single camera view.

The stability and performance of confined supersonic jets, such as those encountered in vacuum ejectors, are strongly influenced by unsteady flow phenomena and hysteresis. In this work, a detailed time-resolved study is conducted using unsteady pressure measurements and schlieren visualization to examine how key geometric and operating parameters affect these behaviors. Parameters, such as nozzle configuration, design Mach number, nozzle lip thickness, and duct geometry, including length-to-height and area ratios, are systematically investigated. The results indicate that hysteresis is primarily governed by overall pressure recovery, which is controlled by the geometric parameters. While convergent–divergent nozzles improve evacuation performance compared to convergent nozzles, they also lead to a wider hysteresis loop. Also, increasing the duct length and appropriately selecting the area ratio reduce the width of hysteresis and enhance startup stability. However, unsteadiness of pressure in these configurations is found to be dominated by local aeroacoustic mechanisms. A reduction in nozzle lip thickness significantly attenuates screech amplitudes, whereas an increase in Mach number shifts screech to lower frequencies while increasing its amplitude. Additionally, longer ducts provide effective acoustic damping of screech and the associated duct resonance modes. These findings reveal that global stability (hysteresis) and local aeroacoustic phenomena (unsteadiness) can be independently modified with appropriate geometric modifications. This offers practical design guidance for improving ejector performance while limiting adverse effects due to unsteady pressure oscillations.

Heterodyne background-oriented schlieren is examined here as a diagnostic that separates phase and amplitude effects in reactive flows, and it is applied to a lean premixed swirl-stabilised methane flame. In this approach, a periodic fringe background and Fourier demodulation yield line-of-sight displacement fields associated with refractive-index gradients, while the fringe amplitude modulation isolates intensity redistribution in a manner that is mathematically decoupled from phase. Building on this separation, ray-tracing analysis clarifies how the optical system and the refractive-index field jointly shape the recorded image, thereby revealing the physical origin of the fringe amplitude modulation signal. In particular, the fringe amplitude modulation field is shown to behave similarly to a focussed shadowgraph that preferentially amplifies high-gradient, small-scale features linked to combustion and suppresses low-curvature convection features. This selectivity is governed by the acquisition aperture through depth-of-focus and aperture-related ray blockage. Finally, controlled experiments with extensive image acquisition corroborate these interpretations and provide practical guidance on carrier-filtering and acquisition settings to avoid spectral overlap and maintain accuracy.

Graphical abstract

As the application of pressure-sensitive paint (PSP) extends from fundamental experiment to industrial testing, its measurement accuracy is increasingly limited by systematic errors induced by corner reflections in geometrically complex configurations. To overcome the limitations of conventional reflection correction methods in spatial resolution and accuracy, we propose a novel reflection elimination method based on a differentiable luminescence ray-tracing model. This method directly operates on 2D images and solves the inverse problem to recover the native reflection-free luminescence via gradient-driven optimization. It does not require precomputation of global view factor matrices and coordinate mapping as in conventional methods and therefore preserves full spatial resolution and reduces computational cost. In a numerical benchmark test on a 90° corner model, the proposed method reduces the mean relative deviation due to corner reflection from 25.2% to 3.8%, eliminating 84.9% of reflection-induced luminescent signal. In the wind-tunnel experiments of blade-endwall film cooling effectiveness, over 75% of the reflection-induced errors in PSP measurement are removed, demonstrating that the proposed method provides a robust and efficient solution for improving the accuracy of PSP measurements on geometrically complex models.

The third iteration of a magnetic resonance velocimetry (MRV) challenge comparison activity consisting of 3D flow measurements for a turbulent flow in a water channel moving past centrally positioned cubic obstacles is reported. In this challenge iteration, MRV measurements are coupled with either temperature or concentration measurements to extend the diagnostic utility. Five research teams from around the world conducted the measurements on a single shared apparatus. The water channel included partial elements along the channel sidewalls that varied in height and precluded easy optical accessibility. In addition to a turbulent mainstream flow, a secondary flow entered the channel between the second and third cubic elements from a square injector hole at the channel bottom wall. The injector jet interacts with the mainstream flow and mixes turbulently as it advects downstream. For the selected flow regime and water solutions used as working fluids, the mixing of the higher temperature or concentration through the secondary flow with the mainstream flow satisfies the same dynamics so that both temperature and concentration fields behave as passive scalars and can be directly compared. The measurements are explained in detail by each participating team, and the results are interpolated onto a common grid and compared using line profiles, contour plots, and isosurfaces.