In this study, we examine the mixing performance of thermally induced microfluidic swirlers, which are recently developed micromixers based on mixed thermal convection. In this configuration, a swirling flow motion is induced by the combination of natural convection and a pressure-driven Poiseuille flow. An experimental investigation was carried out on a microfluidic swirler composed of a glass capillary with a square cross-section of 800 $\times$ 800 $\unicode {x03BC}$m$^2$, measuring the three-dimensional flow fields in different operating conditions using the general defocusing particle tracking technique. Furthermore, a thorough numerical analysis was performed to characterise the mixing performance for different Reynolds numbers and microchannel dimensions. Our results show that thermally induced microfluidic swirlers have an optimal range of operation for microchannel with hydraulic diameters between 400 and 1600 $\unicode {x03BC}$m and Reynolds numbers around 1, where they show an increase of mixing efficiency up to 60 % with respect to the case of pure diffusion. The swirl is activated already at moderate temperature differences of 20–30 K, making this approach compatible with most chemical and biomedical applications.

The turbulent boundary layer (TBL) development over an air cavity is experimentally studied using planar particle image velocimetry. The present flow, representative of those typically encountered in ship air lubrication, resembles the geometrical characteristics of flows over solid bumps studied in the literature. However, unlike solid bumps, the cavity has a variable geometry inherent to its dynamic nature. An identification technique based on thresholding of correlation values from particle image correlations is employed to detect the cavity. The TBL does not separate at the leeward side of the cavity owing to a high boundary layer thickness to maximum cavity thickness ratio ($\delta /t_{max}= 12$). As a consequence of the cavity geometry, the TBL is subjected to alternating streamwise pressure gradients: from an adverse pressure gradient (APG) to a favourable pressure gradient and back to an APG. The mean streamwise velocity and turbulence stresses over the cavity show that the streamwise pressure gradients and air injection are the dominant perturbations to the flow, with streamline curvature concluded to be marginal. Two-point correlations of the wall-normal velocity reveal an increased coherent extent over the cavity and a local anisotropy in regions under an APG, distinct from traditional APG TBLs, suggesting possible history effects.

Weighted and unweighted power mean methods are compared against the accuracy of the simple arithmetic average to estimate an equivalent sandgrain roughness parameter ($k_{s}$) for streamwise-heterogeneous rough surfaces. Specifically, these methods are conceptually and iteratively tested on roughness plates with surface characteristics following beta ($\beta$), uniform or Gaussian distributions. The sandgrain roughness computed using these averaging methods, $k_{s_{eq}}$, is then compared with true parameters, $k_{s_{eff}}$, estimated from the fully rough asymptote model and a modified momentum integral method that accounts for the streamwise variation of skin friction over the heterogeneous surface. The weighted power mean offers significant advantages, particularly for roughness in $\beta$ distribution, which is attributed to the distribution bias towards smaller magnitudes of $k_s$. While the advantage of using the weighted power mean over the unweighted power mean is less significant for the other surface distributions, the weighted method consistently yields the lowest discrepancy in effective drag estimates, independent of roughness configuration, and is therefore the recommended method for estimating the effective sandgrain roughness across heterogeneous surfaces.

We develop original flow-based methods to interrogate and manipulate out-of-equilibrium behaviour of ternary fluids systems at the small scale. In particular, we examine droplet and jet formation of ternary fluid systems in coaxial microchannels when an aqueous phase is injected into a solvent-rich oil phase using common fluids, such as ethanol for the aqueous phase, silicone oil for the oil phase and isopropanol for the solvent. Alcohols are often employed to impart oil and water properties with a myriad of practical uses as extractants, antiseptics, wetting agents, emulsifiers or biofuels. Here, we systematically examine the role of alcohol solvents on the hydrodynamic stability of aqueous–oil multiphase flows in square microchannels. Broad variations of flow rates and solvent concentration reveal a variety of intriguing droplet and jet flow regimes in the presence of spontaneous emulsification phenomena and significant mass transfer across the fluid interface. Typical flow patterns include dripping and jetting droplets, phase inversion and dynamic wetting and conjugate jets. Functional relationships are developed to model the evolution of multiphase flow characteristics with solvent concentration. This work provides insights into complex natural phenomena relevant to the application of microfluidic droplet systems to chemical assays as well as fluid measurement and characterisation technologies.

Accurately predicting the growth rates of stationary cross-flow instabilities is crucial for understanding transition mechanisms in swept-wing configurations, which can significantly impact aerodynamic performance. This paper introduces a model for the prediction of cross-flow instability growth rates, focusing on both accuracy and ease of implementation. The proposed model consists of straightforward expressions involving key boundary layer quantities. Validation against established methods demonstrates that the new model achieves comparable or superior accuracy in predicting growth rates. Additionally, tests conducted on a three-dimensional (3-D) prolate spheroid show strong alignment with transition lines computed by means of exact linear stability. Overall, this model provides a practical and efficient alternative for accurately predicting cross-flow transitions in complex 3-D geometries, contributing to improved aerodynamic design and analysis.

Transport in nanofluidic devices is often characterized by complex electrohydrodynamic coupling. Electro-osmotic flow (EOF), i.e. the motion of fluid due to an external electric field, is one of the most common electrohydrodynamic phenomena. However, the classical continuum description of EOF cannot be directly applied at the nanoscale, and no generic experimental techniques exist to measure EOF for nanopores just a few nanometres in size. This led to the development of approximate approaches to express EOF through experimentally accessible quantities. The most popular one, derived by Gu et al. in 2003, employs nanopore selectivity measured via reversal potential experiments and expresses EOF as the sum of water molecules dragged by each ion moving through the pore. Here, combining theoretical arguments, continuum electrohydrodynamic and molecular dynamics simulations, we discuss the limitations of these approximations. Our results indicate that, although some approximate expressions contradict basic fluid dynamics scaling arguments, they still capture the order of magnitude of EOF for very narrow biological nanopores such as MspA, CytK and CsgG. Finally, we highlight some caveats of the method, particularly when dealing with non-cylindrical biological pores and the effects of localized alterations of the pore surface charge, such as point mutations commonly employed in nanopore sensing technology.

Drop shafts play a vital role in urban drainage and tunnel sewerage systems. To gain an insight into the magnitude of transient flow fluctuations inside a drop shaft attached to a scroll vortex intake, large eddy simulations (LESs) are performed in this study. First, the LES predictions are validated against experimental data from Guo (2012), demonstrating good agreement for both the time-averaged head-discharge relationship and the minimum air-core percentage. Subsequently, the transient fluctuations of the air core inside the drop shaft are investigated, with the worst-case scenario being choking of the air core inside the drop shaft, which might lead to a grave consequence to the system response. The transient fluctuations of the air core are found to have up to 13 % variation in the non-dimensional air-core area due to dynamic contraction and expansion. Additionally, velocity characteristics at different vertical and angular locations within the drop shaft are analysed, offering new insights into vortex structures and challenging assumptions from existing analytical models. The transient simulation results also reveal a global vortex structure together with embedded small-scale vortices using the $\Omega$-criterion vortex identification method.