Open-channel bifurcations exhibit complex three-dimensional flow structures, such as separation zones and recirculating motions. While traditional representations of the recirculation flow in bifurcation are two-dimensional and closed, recent studies suggest the presence of open three-dimensional (3-D) helical recirculation flows. The present work addresses this question by employing particle tracking velocimetry to measure the time-averaged 3-D velocity field in a simplified bifurcation geometry. The measurements reveal that all examined flow configurations exhibit an open separation zone along the lateral branch’s inner wall, with no evidence of closed, two-dimensional recirculation. Instead, two distinct helical recirculation patterns emerge, governed by the width-to-depth aspect ratio upper B divided by h B/h $B/h$ . In shallow flows ( upper B divided by h greater than B/h> $B/h \gt $ 4), a vertical-axis helix conveys the recirculating flow from the bed to the free surface. In deeper flows ( upper B divided by h less than B/h< $B/h \lt $ 4), the recirculation axis transitions from vertical upstream to streamwise downstream, generating a more complex motion. A key finding is the presence of an ‘entrance gate’ in both patterns: a spatially confined near-bed region through which flow enters the separation zone near the intersection, before escaping further downstream near the surface or at mid-depth. This study challenges conventional closed-zone representations, instead revealing two types of open, three-dimensional helical recirculation patterns with finite but prolonged residence times.
A symmetric two-panel turbulence facility based on loudspeaker-driven synthetic jets is developed to generate negligible-mean-flow homogeneous turbulence in air. The system consists of an array of independently controlled actuators operated using an extended sunbathing algorithm that enables three independent actuation strategies: random frequency switching, random power modulation and spatiotemporal on/off switching. These modes, used individually or in combination, allow systematic generation and fine-tuning of a broad range of turbulence states. Time-resolved velocity fields are measured using particle image velocimetry (PIV) to characterise the resulting flow in terms of turbulent kinetic energy, mean flow, homogeneity and isotropy. Beyond conventional turbulence statistics, facility-induced coherent structures and their energetic contributions are quantified using proper orthogonal decomposition (POD). The results demonstrate that increasing forcing randomness progressively suppresses coherent organisation and reduces coherent energy content while maintaining high turbulence intensity and low mean flow. In addition to negligible-mean-flow operation, the facility can generate controlled mean-flow rotation in laboratory turbulence by imposing a power gradient across the actuator array, without a rotating platform. Overall, the facility offers a flexible and highly configurable experimental platform for generating homogeneous turbulence, tailored turbulence states and turbulence with controlled mean-flow rotation within a confined environment.
The three-dimensional unsteady flow structures around an aerofoil with leading-edge tubercles at angles of attack up to the post-stall regime are experimentally characterised by high-speed three-dimensional particle tracking, physics-informed neural network data assimilation and proper orthogonal decomposition. At mild incidence ( less than or equivalent to 5 Superscript ring ≲5∘ ${\lesssim}5^{\circ }$ ), tubercles generate a spanwise-periodic system of counter-rotating vortices, strongly altering the boundary layer from its inception. The vortices adhere to the aerofoil profile, modulating the shear layer by alternating downwash and upwash motions. In the stall regime, the flow features large-scale separation cells spanning several tubercles with no apparent periodicity. Proper orthogonal decomposition reveals a dominant vertical flapping motion of the shear layer with a Strouhal number of upper S t almost equals 0.1 St≈0.1 $St \approx 0.1$ (based on chord length and free-stream velocity). The second most energetic mode ( upper S t almost equals 0.3 St≈0.3 $St \approx 0.3$ ) captures part of a streamwise travelling wave, indicating a shedding behaviour. In the deep-stall regime of greater than or equivalent to 25 Superscript ring ≳25∘ ${\gtrsim}25^{\circ }$ , the unsteadiness is ascribed to the interaction among neighbouring separated patches. The kinematic description of the flow is further utilised to infer the aerodynamic loads through pressure estimation. These observations help understand the gradual attenuation of lift, ascribed to an extended envelope of cellular separation, which contrasts with the abrupt stall observed for straight-edge aerofoils.
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.
