Experiments in Fluids最新文献

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.

In this study, the free-surface synthetic schlieren (FS-SS) method, as proposed by Moisy et al. (Exp. Fluids 46 (2009) 1021), is extended to allow a wider application of the method. The reliance on small-angle paraxial approximation is removed, which allows for shorter distances of the camera to the free surface. Also, the presence of walls and non-constant depth of the liquid layer are accounted for. The difference between the original and modified approach is evaluated using a ray tracing approach. The extension of the method is demonstrated with measurements of the water surface in a cylinder for both rotating and non-rotating cases, i.e., surface waves due to impact, a surface depression due to a spin-up vortex, and the parabolic free surface in a rotating cylinder. The measured variations in surface elevation are as small as (mathcal {O})(10 (upmu )m) with a range in paraxial angle up to ±0.56 rad (({32.3}^circ )).

Innovative short-inlet nacelle designs and disruptive technologies are essential for minimizing the drag and weight penalties of high-bypass-ratio turbofan engines. Active flow control (AFC) can mitigate inlet separation under off-design conditions by using local air injection at the inlet lip. This work investigates wind tunnel testing of continuous and pulsed blowing on a generic short-inlet UHBR nacelle at high angles of incidence (AoIs). The model is designed as a flow-through nacelle and is tested at a free stream Mach number of 0.15. The Reynolds number associated with the nacelle length is (2.4 times 10^{6}). Static pressure tabs on the model surface and a traversable total pressure rake serve to evaluate the performance of AFC. Both continuous and pulsed blowing could prevent inlet separation for AoIs of (18^{circ }) and (22^{circ }), reducing spatial total pressure non-uniformity by up to 80%. Here, the AFC performance showed strong sensitivity to the jet velocity. A comparison of required momentum revealed that pulsed blowing consumes 20% less momentum input than continuous blowing for the same improvement level. For an AoI of (26^{circ }), only continuous blowing at maximum momentum input could effectively mitigate inlet separation. This experimental study confirms the potential of AFC to improve inlet flow under off-design conditions.

Shear-thinning properties of non-Newtonian fluids such as blood significantly affect the flow in compliant vessels. In particular, under physiologically realistic pulsatile inflow conditions, these shear-thinning properties can alter the flow field and thus the fluid–structure interaction. Yet, non-Newtonian effects are frequently neglected in in vitro experiments due to increased experimental complexity. This study aims to provide a comparative analysis of the combined influence of pulsatile flow conditions and non-Newtonian fluid properties on the flow field and fluid–structure interaction in a compliant abdominal aortic bifurcation model. Therefore, a fully transparent compliant bifurcation phantom was manufactured from two-component PDMS (RTV-615) using a wax-based lost-core technique to ensure dimensional accuracy and smooth internal surfaces. The model was integrated into a pulsatile flow loop, and time-resolved three-dimensional velocity fields were acquired by three-dimensional particle-tracking velocimetry (3D PTV) using the Shake-the-Box algorithm. The flow field, static pressure, wall dilatation, and volume flux were measured for both a Newtonian fluid and a blood-analogue non-Newtonian fluid across three Reynolds numbers and two Womersley numbers, representing physiologically relevant flow regimes. The flow measurements indicate that for both fluids an increase of the Womersley number yields broader and more uniform high-velocity cores and smoother axial transitions. Results for the non-Newtonian fluid consistently exhibit wider velocity distributions and more synchronized fluid–structure interaction dynamics in all cases that were investigated. Overall, the findings suggest that the shear-thinning rheology of the non-Newtonian fluid shifts the temporal response characteristics of the coupled system toward behavior observed at higher Womersley numbers. This indicates an altered phase relationship between the flow field and wall deformation. Thus, the present measurements indicate that non-Newtonian properties significantly influence both the flow field and the fluid–structure interaction in compliant vascular models under pulsatile conditions. Combining compliant, optically transparent phantoms with time-resolved 3D PTV provides a powerful platform for quantifying these coupled effects, advancing the understanding of physiological hemodynamics, and improving the reliability of in vitro studies and simulation validation efforts.

Air relative permeability is a key parameter controlling gas transport in unsaturated porous media, yet its dependence on saturation history and pore structure remains insufficiently constrained by experimental data. In this study, air relative permeability was measured during controlled drainage and imbibition processes in homogeneous sands with contrasting grain-size distributions. Steady-state air-flow experiments were conducted in a laboratory column, and air permeability was determined from imposed gas-flow rates and measured pressure gradients. Water saturation was quantified independently from expelled-water volumes, allowing reproducible characterization of permeability–saturation relationships along defined hydraulic paths. The results reveal pronounced hysteresis in air relative permeability between drainage and imbibition, with strongly dependent behaviour on grain-size distribution. In the fine sand, air relative permeability decreases progressively with increasing water saturation, whereas in the coarse sand, it remains close to its maximum value over a wide saturation range during imbibition before collapsing abruptly near full saturation. Comparisons with permeability predictions derived from independently fitted water retention curves show significant discrepancies, particularly during imbibition, where saturation-based models fail to reproduce the observed permeability plateau and sharp decline. These findings demonstrate that air relative permeability in granular media cannot be described solely as a function of water saturation and that saturation history and pore structure exert a first-order control on gas transport. The experimental dataset provides a benchmark for evaluating and improving constitutive models for gas flow in unsaturated porous media.

In this work, we present planar time-resolved particle image velocimetry measurements focusing on the vortex ring merging from twin parallel pulsed jets, generated by the nozzle spacing (S/D₀) from 1.49 to 3.20 and stroke ratio (L/D₀) between 2 and 4. The results show a critical spacing ratio, of approximately 3, below which the pulsed jets interact and merge after a certain formation time (for 1.49 < S/D₀ < 3.20) or right at the exit of the tubes (for S/D₀ < 1.49). Using the vortex core identification method based on the swirling strength criterion, the streamwise merge position is identified for different conditions and appears to be highly dependent on nozzle spacing, while not being influenced by L/D₀. High nozzle spacing delays, following a quartic trend, the streamwise merge position until reaching a threshold level beyond which no merge exists anymore. Time–frequency analysis and the oscillatory motions of the vortex ring tilt angle were also examined to identify characteristic signatures and elucidate the flow dynamics during vortex merging. For S/D₀ = 2.45, a cooperative tilting instability driven by mutual induction precedes vortex ring merging. In contrast, for S/D₀ = 3.20, where vortex rings are initially well separated, the weakening of the mutual energy and induction with increasing separation, together with energy dissipation associated with their wave-like oscillatory motions, appears to inhibit the coherent alignment required for reconnection and subsequent merging. This experimental study improves our understanding of twin pulsed jets and vortex ring interactions with implications that span from cardiovascular flows to underwater jet propulsion systems, and highlights the need for further theoretical investigation based on energy considerations to better understand the underlying mechanism.

Graphical abstract

Biologically inspired undulated cylinder geometries significantly influence wake structure and vortex-induced vibrations in turbulent flows, yet the underlying mechanisms are not fully understood. It is further unknown how these influences extend to multi-cylinder arrays, where interacting wakes accumulate and mix. This study examines the wake characteristics of an undulated cylinder and a smooth elliptic cylinder in single- and multi-cylinder array configurations, using particle image velocimetry and proper orthogonal decomposition (POD). Relative to the smooth elliptic cylinder, the undulated geometry induces pronounced spanwise-axial variability, characterized by enhanced near-wake mean momentum and weak far-wake signatures, consistent with reduced turbulent kinetic energy. While mean momentum signatures are more comparable across array configurations, POD modal energy reveals a less coherent, more homogeneous distribution for the array of undulated cylinders, indicating increased three-dimensionality and flow complexity. Despite this complexity, a low-order description of the Reynolds shear stress is achieved to within 15% RMS reconstruction error using only 5% of the total POD modes, highlighting distinctions not apparent from mean-flow quantities alone. These characteristics offer broad potential for low-order modeling, flow control, and sensing applications.