Laminar jet apparatuses (LJAs) are employed to study gas–liquid mass transfer and thereby provide insights into the design of technical separation and reaction equipment. The hydrodynamics of the laminar jet determines the operating window of such apparatuses and must therefore be understood for the proper evaluation of experimental results. Although substantial literature exists on free liquid jets, quantitative analyses directly tailored to LJAs and their operating conditions remain scarce. The present work addresses this gap. We investigate all hydrodynamic aspects relevant to the operation of LJAs using computational fluid dynamics (CFD) simulations: the shape of the jet surface (i.e. the gas–liquid interface), the velocity profile within the jet (including the transition from the nozzle to the free jet) and jet breakup. The study is conducted for a square-edged orifice plate nozzle under various flow conditions and analysed using dimensionless numbers. The CFD simulations account for gravity and surface tension and comprise the entire relevant domain (feed line, nozzle, liquid jet and surrounding gas phase). Complementary experiments using a LJA, in which the jet shape was measured, validate the simulation results. The findings provide quantitative guidance for rational design of LJAs and offer a foundation for replacing ad hoc assumptions in the evaluation of the experiments.
The dynamics of carboxyl-modified polystyrene colloidal particles of varying size and zeta potential were investigated in a microfluidic junction device where merging electrolyte streams generated a steady-state salt concentration gradient. The device featured a long microchannel downstream of the junction, enabling the particle dynamics to be analysed over a time scale comparable to the relaxation time of the salt gradient. In the bulk of the channel, particles migrate outwards through a diffusive-like process arising from the additive contributions of particle diffusiophoresis and wall diffusioosmosis. An analytical expression is derived for the effective diffusion coefficient, and a microfluidic approach is proposed to estimate particle diffusiophoretic mobility. Particles accumulated at the walls also displace sideways but the direction of their motion depends on the counter-acting effects of particle diffusiophoresis and wall diffusioosmosis. This motion is sub-diffusive at short distances from the junction, but either ceases or reverses further downstream. The intensity of the accumulation peaks exhibits a non-monotonic dependence on the distance from the junction and, after calibration, can serve for the microfluidic measurement of particle size or zeta potential, when one property is known a priori. The influence of electric double layer polarisation on diffusiophoretic mobility is also examined.
We experimentally investigated the turbulent flow in a water channel developing past a two-dimensional array of photovoltaic panels, with varying incidence angle and spacing. Particle image velocimetry revealed two distinct regimes, depending on the incidence angle, while the dependence on inter-panel spacing was weaker. Positive incidence angles produce high turbulence and intense transport above the panels, with strong mixing between the canopy and the external flow. Conversely, negative incidence angles yield a skimming flow with large recirculation vortices that dominate the canopy layer. Vertical profiles, ventilation rates and turbulent statistics indicate that even a modest positive incidence significantly enhances ventilation and scalar transport, whereas they are substantially lower in the case of negative incidence. A Lagrangian analysis using synthetic passive particles allowed the estimation of the residence times and turbulent dispersion coefficients. The latter were more than an order of magnitude larger for positive than for negative incidence. The results demonstrate how panel configuration changes mixing and dispersion processes, providing a fluid mechanics basis for refined microclimate models useful for guiding the design of advanced photovoltaic farms.
Abstract The characteristically low flowback recovery in shale reservoirs stems from spontaneous imbibition, a governing mechanism for fluid retention and hydrocarbon production. Despite extensive research, the fundamental processes underlying aqueous-phase transport in shales remain poorly understood. This review synthesises recent findings by characterising imbibition as a dynamic, cross-scale transport phenomenon driven by the coupling of capillary suction, chemical potential gradients and clay hydration. Unlike traditional static descriptions of this process, this review highlight how imbibition induces continuous pore-network evolution via hydration-triggered microfracture propagation and mineral-scale blockage. Geological attributes, fluid chemistry and operational parameters are systematically evaluated. We further examine the methodological transition from macroscopic monitoring to in situ visualisation, and from classical analytical solutions to multiphysics numerical frameworks. Lastly, we conclude by identifying critical knowledge gaps and outlining future perspectives in high-pressure high-temperature in situ measurements, multiscale predictive correlations and intelligent fluid systems.
This study presents the first systematic experimental investigation of the streamwise evolution of multi-orifice synthetic jets (MOSJs) driven by a single actuator. A centred hexagonal arrangement of seven circular orifices is examined using particle image velocimetry. Four cases are studied in which Reynolds number and Strouhal number are held constant while the orifice diameter is varied, thereby isolating the effect of the internal orifice geometry through changes in throat aspect ratio. The flow development is shown to consist of four distinct regimes: (i) a near-orifice region dominated by the balance of blowing and suction; (ii) development of individual jets; (iii) a jet-merging regime; and (iv) a merged-jet region that evolves towards self-similar scaling. Despite matched non-dimensional actuation parameters, differences in momentum flux, Reynolds number and turbulence statistics are observed. These differences scale monotonically with orifice diameter and are traced to changes in the exit velocity profile imposed by the orifice throat length. In particular, more diffuse shear layers – resulting from a longer orifice throat – lead to more efficient jet merging, higher downstream momentum and reduced dissipation. The results demonstrate that actuator-internal geometry plays a critical role in MOSJ performance and cannot be neglected in predictive or design-oriented models.
This work presents a combined numerical and experimental investigation of a rigid wing free to pitch in a transonic flow at Mach 0.8. The wing exhibits small-amplitude oscillations around an equilibrium point in deep stall, where a large separation region develops on the suction side. Fluid-structure interaction simulations suggest that the oscillation frequency originates from the unsteady motion of the juncture vortex that forms between the wind tunnel sidewall and the wing. The measured oscillation frequency is approximately 350 Hz, while the numerical prediction yields a value of 290 Hz. Additionally, a simplified criterion is proposed to estimate the juncture vortex oscillation frequency as the ratio between the mean vortex circulation and the product of the chord and the average vortex diameter.
We develop a data-driven approach to infer aerodynamic drag directly from canopy geometry by learning interpretable low-dimensional latent representations of canopy configurations, without resolving the flow field. A PixelCNN-based variational autoencoder with latent disentanglement regularisation and auxiliary observable regression is used to identify latent factors that encode the geometric features most relevant to drag. Latent traversals and mutual information analysis are employed to quantify the physical relevance of individual latent dimensions and to assess how modelling choices affect information retention. Applied to laboratory measurements of heterogeneous canopy arrays, the learned latent space organises canopy configurations according to their aerodynamic impact and enables accurate drag prediction. The results demonstrate that physically informed latent modelling provides a compact and interpretable pathway for linking complex canopy geometry to aerodynamic drag.
This paper performs a biglobal input-output analysis of the separated flow over a periodic hill to understand the flow sensitivity to disturbances. We use the volume penalty method to model the solid body of the periodic hill, which is implemented in a spectral solver to obtain the two-dimensional base flow used in the biglobal input-output analysis. We formulate the spatiotemporal frequency response operator using the tensor product, and conduct singular value decomposition of this frequency response operator to identify the dominant response mode, forcing mode and input-output amplification. Results show that disturbances with high temporal frequencies result in shear-dominant flow structures with a smaller length scale, while as the temporal frequency approaches zero, the response modes are less oscillatory in the spatial domain and display global recirculation. With an increase in the spanwise wavenumber of disturbances, flow structures grow in length scale and become less oscillatory in space, while the amplification increases and then decreases with increasing spanwise wavenumber. In general, forcing modes show a peak near the separation region downstream of the hill crest, while the response modes show a peak near the reattachment region upstream of the hill crest.
Porous trailing edges attenuate hydrodynamic pressure fluctuations that scatter as trailing-edge noise, with their effectiveness governed by their material parameters. Conventional measurements of permeability rely on steady-flow rigs, which cannot capture the dynamic response of this parameter, which is relevant for predicting balancing pressure fluctuations under grazing-flow conditions. In this study, we introduce a method for directly determining the dynamic permeability of porous trailing edges from time-resolved particle image velocimetry (PIV) data. The approach employs a lumped-system circuit analogy that links unsteady pressure gradients to through-material velocities, enabling in situ characterisation without specialised porous rigs, thereby further extending its applicability to thin trailing-edge geometries. Two materials with similar porosity but distinct internal architectures are compared against a solid baseline: a structured porous trailing edge (SPTE) and a random foam trailing edge (RFTE). The extracted permeability curves show close agreement with the analytical model of Johnson et al. (J. Fluid Mech., 1987, vol. 176, pp. 379-402), validating the method for both structured and randomised porous materials. The procedure also allows for the estimation of the equivalent viscous characteristic length and tortuosity. A detailed comparison reveals that the SPTE exhibits a lower viscous length scale and tortuosity than the RFTE, with a relatively higher dynamic permeability response at high frequencies.
Boundary layer ingestion (BLI) propulsion can improve aircraft aerodynamic efficiency, but also introduces inlet distortion that affects fan flow and stability. This study investigates the resulting unsteady flow response and loss mechanisms by performing a parallel comparison of unsteady Reynolds-averaged Navier-Stokes (URANS) and large-eddy simulation (LES) under unified geometry and boundary conditions, together with a time-sequence analysis of three representative LES instants. The results show that, compared with URANS, LES provides a more detailed depiction of the distortion pattern and internal vortical structures. LES captures the generation and mixing of fragmented vortex systems, and reveals corner separation near the stator hub and the decay of throughflow capacity, identifying major internal loss sources. The time-sequence comparison further shows that, although the distorted vortex core evolves in strength and shape, its circumferential phase remains essentially preserved, leading to a stable distorted sector at the aerodynamic interface plane. Within this sector, the rotor approaches critical incidence and triggers local separation, while the stator passages exhibit a sector-fixed, circumferentially continuous loss distribution. These findings clarify distortion-induced unsteady loss mechanisms in BLI fans and provide numerical guidance for locating loss regions and supporting distortion-tolerant design of intake-fan integrated systems.
Indoor environments are continuously subject to the effects of thermal radiation: short-wave radiation that brings in solar heat, and long-wave radiation that acts to redistribute heat. However, the effects of the latter are often overlooked. To fully understand and predict the effects of long-wave radiation indoors, spatial variations in indoor temperature must be considered. This paper develops an analytical framework to describe coupled radiative and convective heat transfers within simple representations of thermally stratified rooms, a minimal description requiring only five thermally active bodies. The model, termed 'AR5B', predicts the coupled heat fluxes from each thermal body, enabling the effects of long-wave radiation to be simply mimicked within ventilation flow models. Data from six full-scale experiments show that predictions from AR5B are of suitable accuracy. Moreover, when the AR5B results are then used to mimic the effects of radiation within a simplified ventilation flow model, the resulting thermal stratification is reasonably predicted. We show this to be in stark contrast to comparable cases but without accounting for the effects of long-wave radiation - this finding underscores that long-wave radiation plays a significant role in determining indoor environments and should be incorporated more routinely in predictive models.
We present the design, construction and initial experimental validation of the Northwestern Polytechnical University Taylor-Couette (NPU-TC) apparatus, specifically developed to explore turbulent Taylor-Couette flows under conditions relevant to ultra-high-speed rotating machinery. The apparatus features an inner cylinder capable of rotating at speed of up to 10 000 rpm, corresponding to a Taylor number $Ta = 6.4 imes 10<^>8$ , with an exceptionally narrow annular gap of 2.8 mm, yielding a radius ratio ( $\eta$ ) of 0.98. Axial-scanning particle image velocimetry is employed here for the first time in air-based TC flows at such extreme conditions, which enables detailed velocity measurements without intrusive disturbances. Our velocity measurements demonstrate the absence of large-scale coherent flow structures, indicating a transition into the ultimate turbulence regime characterised by very thin boundary layers and nearly uniform velocity distributions in the bulk region. The NPU-TC apparatus thus represents a significant advance in experimental capabilities, providing critical insights into turbulent flow behaviour in high-speed rotating machinery.
This study examines the control capabilities of an array of spanwise-invariant roughness strips applied on a swept-wing boundary layer (BL) dominated by a cross-flow instability (CFI) that is forced by periodically spaced discrete roughness elements to a monochromatic wavelength. Several configurations of strip arrays are investigated, varying their height, width and chordwise periodicity. Infrared thermography is employed to track the impact on the BL transition location. Optimal configurations are identified, extending laminar flow by up to 10 % of the wing chord. Additionally, BL forced by patches of randomised surface roughness are considered, better representing realistic wing surfaces. In this scenario, the application of strip arrays with optimal geometry extends the laminar portion of the BL by almost 10 % chord and beyond when combined with a discrete roughness element array. Time-averaged particle image velocimetry (PIV) velocity fields are acquired to monitor the CFI amplitude for the various configurations. The BL spectral content in the spanwise direction is used to characterise the chordwise behaviour of individual disturbance modes, whose amplitude is found to be reduced by up to 17 % for the optimal strip configuration.
Longitudinal vortices produced by a swirl-mixing grid are experimentally explored in an upscaled model of nuclear fuel assembly. The flow is mapped using particle image velocimetry in several planes downstream of the grid. The flow, an isothermal flow geometrically similar to that in one of the standard nuclear reactors, is compared between basic grids, swirl grids and the case without fuel rods, allowing for a link to previous studies of longitudinal vortex lattices. Individual vortices are recognised using a custom-made algorithm. Analysis of vortices shows that the meandering is enhanced by the presence of fuel rods and by the presence of an upstream swirl grid. The vortex core radii do not grow in the constrained case. There is a weak anticorrelation between the vortex velocity and the actual meandering amplitude. The neighbouring vortices show a weak correlation in their circumferential velocities or energies, but they do not display any significant correlations of positions or meandering amplitudes, cutting down any hypothetical “vortex dancing”.
Wind energy is a sustainable and plentiful form of clean energy. The vertical axis wind turbine (VAWT) is one type of cost-effective, acoustically quieter and lightweight turbines. The two mainstream types of VAWTs - Darrieus (lift type) and Savonius (drag type) - have contradictory strengths and weaknesses. Darrieus turbines possesses high efficiency but suffer from poor self-starting, while Savonius turbines start easily with poor aerodynamic performance. In this study, a novel VAWT with adaptive Darrieus-Savonius hybrid blades has been designed. Wind-tunnel experiments assessed the effectiveness of the proposed design and compared it against a conventional Darrieus-type rotor with similar dimensions. The results showed that the static torque coefficient was improved by over 65% and the self-starting wind speed was reduced from 8 to 6 ms-1. The adaptive blades can remain in the Savonius configuration at low rotation speed, facilitating self-starting, and automatically transition to the Darrieus configuration at higher rotation speed, integrating and leveraging the strengths of the two types of VAWTs.
The surface pressure distribution over a circular cylinder with a small, full-span, triangular bump is examined. The geometry of the bump is an isosceles triangle, the height of which is varied from 1.33 % to 5.33 % of the diameter of the cylinder and positioned between $60^{\circ }$ and $120^{\circ }$ . The Reynolds number ( $Re = V_{\infty}D/\nu$ , where $V_\infty$ is the velocity of the freestream, $D$ is the diameter of the cylinder and $\nu$ is the kinematic viscosity) is varied between $1.1 \times 10^5$ and $1.8 \times 10^5$ . The lift and drag are estimated through the surface integral of pressure over the cylinder. The results show that the smallest bump acts as a trip for the lower Re and orientations before $70^{\circ }$ , leading to a separation farther upstream than in the case of no bump. For larger bumps, Re and orientation angles, the bump acts as a spoiler and fully separates the boundary layer at the bump. In addition, the surface pressure upstream of the bump is strongly dependent on the bump position. The lift is highest for bump position less than $90^{\circ }$ and decreases significantly with increasing bump location angle. The drag is less sensitive to the position of the bump. These findings have implications for predicting the forces on bluff bodies due to small asymmetric surface geometry features and extension to applications such as atmospheric flow over topography.
This study quantifies the viscous interaction between propeller tip vortices and a turbulent boundary layer developing over a semi-elliptic leading-edge plate, located downstream. The experimental wind-tunnel set-up is designed to be representative of the tractor–propeller–wing configuration. Using stereoscopic particle image velocimetry and static wall-pressure measurements, the near-wall flow topology is resolved over the plate, semi-immersed in the propeller slipstream. The results show that the interaction exhibits high spatio-temporal coherence and is dominated by a coupling between primary and secondary vortical structures. Two distinct interaction regions are identified relative to the tip-vortex core: on the inboard side, towards the slipstream interior, the boundary-layer flow experiences strong velocity gradient transitions and amplified near-wall vorticity. The flow on the outboard side, moving out of the slipstream, exhibits wall-parallel velocity deficits and vorticity lift-up consistent with unsteady vortex-induced separation mechanisms. Spanwise velocity induced by the wall-normal component of the primary vortex connects these two regions, with the secondary vortex structure identified as enhancing boundary-layer lift-up on the outboard side. Although no local flow reversal occurs under the tested conditions, localised shear amplification and vorticity roll-up indicative of separation-like behaviour were observed. These findings advance the understanding of viscous slipstream–boundary-layer interaction and its implications for tractor–propeller–wing integration.
Winds in the atmospheric boundary layer (ABL) display a wide range of velocity profiles and turbulence properties that affect wind turbine wake dynamics. However, standard concurrent-precursor large eddy simulations (LES) often neglect phenomena such as mesoscale patterns, limiting the range and controllability of inflow parameters that can be studied. Here, we propose a synthetic inflow LES method with high inflow controllability to allow parameters such as shear, turbulence, and Coriolis effects to be varied independently, facilitating the efficient exploration of wake dynamics across the full range of conditions observed in the field. The synthetic inflow method faithfully reconstructs wake dynamics when compared with standard concurrent-precursor LES. We then run a suite of over 600 LES cases to investigate the ABL processes that most affect wake dynamics. We find that wake recovery strongly depends on inflow wind veer, especially at low turbulence intensities, due to the elongation of the skewed wake. Furthermore, we identify a novel scaling relation that collapses wake deflections and dynamics onto the combination of shear and veer. The suite of LES cases elucidates ABL regimes and wake dynamics where current and future wind turbines may operate, building toward improved wake modeling for wind farm design and control.
Chirped coherent Rayleigh-Brillouin scattering (CRBS) is a flow diagnostic technique that offers high signal-to-noise ratios and nanosecond temporal resolution. To extract information of dilute gas flow, experimental spectra must be compared with theoretical predictions derived from the Boltzmann equation. In this work, we develop a MATLAB code that deterministically solves the Boltzmann equation (with a modelled collision kernel for the inverse power-law potential) to compute CRBS spectra, enabling each line shape to be obtained in approximately one minute. We find that the CRBS spectrum is highly sensitive to the intermolecular potential and that rapid chirping generates fine ripples around the Rayleigh peak along with spectral asymmetries.
The urban canopy affects wind in complex ways, making it challenging to predict wind-driven natural ventilation and cooling in buildings. Using large eddy simulations of coupled outdoor and indoor airflow, we study how the surrounding urban canopy and wind angle influence ventilation rates through four ventilation configurations: cross, corner, dual-room and single-sided. Flow visualisations demonstrate how both large-scale flow patterns and local interference effects can influence ventilation rates by 50 %–85 %. In general, lower density canopies give higher ventilation rates, and wind angles that align with a direct path between two openings also lead to higher ventilation rates. However, interference effects from surrounding buildings can significantly change the local wind speed and direction, thus also changing ventilation rates. The magnitude of these interference effects depends on both the wind angle and surrounding building geometries. The effect of wind angle is less pronounced in a higher density canopy, where the urban canopy geometry more strongly guides the flow. The results demonstrate that the canopy’s effect on ventilation rates is much more complex than those suggested by existing natural ventilation parametrisations.
