With advances in experimental fluid mechanics, optical measurement techniques have been widely adopted owing to their non-intrusiveness and high spatiotemporal resolution. However, conventional synthetic Schlieren techniques face challenges such as strong dependence on liquid depth and limited robustness in highly noisy environments. This paper presents a single-camera synthetic Schlieren method based on double color-coded checkerboard patterns, enabling real-time, depth-independent reconstruction of free-surface topography via color-channel separation and wavenumber-domain decoupling. Experimental results demonstrate that the method can accurately capture static surface slopes in a vibration-noise environment, and successfully reconstructs dynamic surface deformations induced by water droplet impact, thereby offering a practical and reliable approach for high-precision, time-resolved measurement of free-surface 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.
Pressure-gradient-induced turbulent separation bubbles (PGI-TSBs) are investigated using time-resolved particle image velocimetry (TR-PIV) and stereoscopic PIV on multiple planes, synchronized with surface pressure measurements. Spectral analysis reveals two dominant features: a broadband low-frequency “breathing” mode and a medium-frequency peak linked to convective shear layer shedding. Spectral proper orthogonal decomposition (SPOD) identifies the low-frequency mode as a global, quasi-periodic expansion and contraction of the separated region, quantitatively capturing large fluctuations in bubble length. Furthermore, the spectral analysis modal method (SAMM) reveals that this motion is phase-locked with global pressure fluctuations and exhibits a pronounced spanwise structure correlated with sidewall vortex dynamics. This study contributes the first application of SAMM to a TSB, providing a high-resolution experimental benchmark for the low-frequency ( St_L_sep≈ 0.01 ) breathing mode, often unresolved in Direct Numerical Simulations. Crucially, the results support the interpretation that the TSB acts as a selective amplifier of upstream perturbations that excite a weakly damped global mode, manifesting physically as a three-dimensional spanwise standing wavelike structure intrinsically coupled to sidewall vortex dynamics.
Time-resolved fast-response pressure-sensitive paint (PSP) measurements on the impingement plate, synchronised with far-field microphones, are used to develop and assess a plate-resolved diagnostic framework for extracting the wall-pressure content that is most relevant to the radiated sound in dual-impinging jets. Two identical square jets impinge normally on a rigid plate at impinging distance h/D = 4 and dual-nozzle distance s/D = 3.5 (D is the equivalent diameter), covering broadband, single-tone and multi-tone regimes. Spectral proper orthogonal decomposition is applied to the PSP pressure fluctuations to obtain frequency-resolved coherent wall-loading structures. Each leading structure is analysed in wavenumber–frequency space and separated, using transport-velocity masks, into an acoustic component, a convection-dominated component, and a low-phase-speed oscillatory component. The extracted acoustic subset reproduces the dominant far-field peaks when used as input to a surface radiation model under a baffled-plate idealisation, while the total wall loading remains dominated by non-radiating content. Across regimes, the broadband state at M = 0.7 is controlled by low-phase-speed oscillation and radiates weakly, whereas single-tone lock-in at M = 0.9 and multi-tone staging at M = 1.2 correspond to a selective re-organisation of the coherent wall loading in which the convective imprint becomes more prominent and a larger fraction projects onto the radiating subset. Resonance strengthens noise primarily by redistributing the wall-loading budget towards the radiating acoustic subset, providing a basis for diagnosing and targeting the acoustically potent part of plate loading in coupled impingement flows.
This study examines scale effects in a wind tunnel study of an urban street canyon. A simplified modelling configuration was employed, comprising five consecutive canyons of height-to-width and length-to-width aspect ratios equal to 1 and 8, respectively, for a free-stream turbulence intensity of ∼ 5 ∼ 10000–20000) by a factor of ∼ 4, i.e. ranging from 23000 to 57000. Focusing on time-averaged velocity components and turbulence kinetic energy, a novel framework is introduced to facilitate the local separation of actual Reynolds number dependence from measurement uncertainty while global indices, applied to the canyon-scale measurement volume, are implemented. Results indicate that the critical values of the Reynolds number (e.g. in the range 10000–20000) that are widely adopted in the wind tunnel testing literature may be insufficient to ensure Reynolds number independence for all examined variables and for the whole volume of urban street canyon configurations, especially near wall boundaries, as Reynolds number values up to 47000 may be necessary.
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.
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 ± 0.015 at 18.1 pixel/mm and 500 frame/s to ± 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 3.6×10^5 and 7.2×10^5 and angles of attack from -8^∘ to 8^∘ . 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.
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 𝒪 (10 m) with a range in paraxial angle up to ±0.56 rad ( 32.3^∘ ).
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.
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
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
High-spatial resolution particle image velocimetry (PIV) and particle tracking velocimetry (PTV) analyses were conducted to investigate the zero-pressure-gradient compressible turbulent boundary layers at Mach 4.2 with Re_θ = 2323 - 3220 . The PTV analysis achieves a spatial resolution of O(0.01) mm in the near-wall region to resolve the flow in the viscous sublayer and buffer layer. Several estimation methods have been used to determine the skin friction coefficient based on the streamwise velocity distribution, e.g., the linear relation in the viscous sublayer, the Spalding equation, the logarithmic law, and the wake law. The van Driest II transformed skin friction coefficients agree well with the Blasius (1913) relation with a deviation less than 5 y^+ = 13 and y^* = 18 in inner and semi-local scalings. Furthermore, the convergence of the two-point spatial correlation is improved using an ensemble-averaged method. The inclination angles of the R_uu contours are approximately 8 ^∘ in the logarithmic region and 13 ^∘ in the wake region, which are different from those observed in incompressible turbulent boundary layers (TBLs). When the wall-normal integral scale is normalized by the characteristic structural length defined by Pirozzoli and Bernardini (2011), a plateau is observed. This indicates that self-similarity of the wall-normal length scale exists for the near-wall structures in CTBLs. Moreover, the uniform velocity zones (UVZs) in CTBLs are also examined. The mean number of UVZs ranges from approximately 1.5to 2 in the friction Reynolds number range of Re_τ≈ 180 - 230 , in line with the logarithmic increase reported by de Silva et al. (2016). From a statistical perspective, the turbulent/non-turbulent interface (TNTI) appears to primarily modulate the thicknesses of the UVZs, rather than their numbers.
Understanding liquid behavior on the surface of rapidly rotating turbine blades is essential for clarifying drain formation in wet-steam turbines. However, direct experimental visualization of droplet impact and subsequent liquid transport under the combined conditions of ultra-high peripheral velocity and low ambient pressure has not been reported. In this study, we present high-speed visualization of droplet impact and liquid transport on an actual turbine bucket at peripheral velocities of 30–430 m/s under ambient pressures of 1–5 kPa. The results show that prompt splashing occurs even in this low-pressure regime, contrary to splash-suppression trends reported for lower-impact velocity and non-rotating conditions. Impinging droplets form thin films, ligaments, and droplet trains on the surface; despite their different morphologies, these liquid phases exhibit similar transport characteristics governed by the balance between centrifugal and Coriolis forces. At the bucket tip, accumulated liquid is stretched by centrifugal force and undergoes Rayleigh breakup, with the resulting droplet diameter decreasing systematically with increasing rotational speed. A minimal numerical model incorporating centrifugal, Coriolis, and effective interfacial resistance qualitatively reproduces the observed liquid transport trajectories on the actual bucket geometry. These combined experimental visualizations and numerical modeling provide a consistent physical description on rapidly rotating surfaces under low-pressure, ultra-high-speed conditions, and offer a basis for understanding drain-related loss mechanisms in wet-steam turbines.
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 % in flow angles, 70 % in total pressure, and 60 % in dynamic pressure. Artificial neural network-based regression provides a further reduction of approximately 30 % 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.
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
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.
