Experiments in Fluids最新文献

This study investigates natural convection in a differentially heated cylindrical annulus using Wollaston shearing interferometry. Temperature-induced refractive index variations in silicone oil B5 are recorded as interferograms and processed with a windowed Fourier transform to reconstruct the two-dimensional line-of-sight mean temperature field, providing the general objective of this study, a quantitative description of the mean temperature distribution. To validate the approach, complementary three-dimensional numerical simulations are performed. The resulting temperature fields are used to generate synthetic interferograms. A qualitative and quantitative comparison shows moderate agreement in fringe structure, fringe density and flow features, including the characteristic crescent-shaped convective pattern, with reconstructed mean temperature fields consistent with numerical results. Remaining discrepancies are attributed to optical distortions, non-ideal thermal boundary conditions and light deflection in regions of strong gradients. Overall, the results demonstrate a robust framework and non-intrusive method for quantitative temperature reconstruction in convective flows.

This review summarizes state-of-the-art knowledge on phosphor thermometry (PT), which has evolved into a powerful tool for studying complex thermal fluid problems and testing mechanical and aerospace systems. As the thermographic phosphors covering room temperature to 2000 K are now well established, full-field temperature measurements have been achieved for both on-surface and in-flow scenarios. Recent studies have significantly advanced every aspect of this technology, including phosphor development, coating/particle fabrication, measurement strategy and system integration. Sources of measurement uncertainty have been comprehensively studied regarding the key components of measurement system, as well as the interfering factors commonly faced in harsh environment. The fundamental research and technological advances of PT push its limit toward more challenging applications including hypersonic aerothermodynamic experiment, turbine blade testing, engine combustion diagnostics and others. Interdisciplinary research has driven the maturing of PT technology and will propel future breakthroughs in methodology and application.

This paper examines the occurrence of the Coandã effect in the context of automotive windshield defrosting. A generic, parameterized pseudo-2D test rig representing the main windshield defrosting components of a passenger vehicle is introduced. By externally controlling key geometric features such as the defrost duct angle and dashboard position, attachment of the defrost jet to the dashboard due to the Coandã effect is induced, which is incompatible with the functional requirements of passenger vehicle HVAC systems. By exploiting the tendency of Coandã flows to exhibit hysteresis, steady mono- and bistable states are identified in which the turbulent defrost jet remains attached either to the windshield or the dashboard over extended time periods, as well as unsteady configurations characterized by approximately memoryless switching between states. Two-dimensional two-component particle image velocimetry (2D2C PIV) is used to analyze the mean flow fields of the identified jet states in terms of velocity and turbulent kinetic energy contours, maximum velocity decay, and half-width growth rates. In addition, pressure coefficients measured along the windshield centerline demonstrate how proximity to the dashboard alters the pressure distribution to resemble that of a shallower or steeper duct angle through deflection of the defrost jet. Proper orthogonal decomposition is applied to the PIV data to extract the switching modes and their associated temporal coefficients in unsteady states, with subsequent statistical analysis indicating that the observed state switching exhibits Poissonian characteristics. Beyond the present study, the experimental data are intended to serve as a benchmark for the validation and assessment of CFD results.

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.