Early turbulence and pulsatile flows enhance diodicity of Tesla’s macrofluidic valve

Microfluidics has enabled a revolution in the manipulation of small volumes of fluids. Controlling flows at larger scales and faster rates, or macrofluidics, has broad applications but involves the unique complexities of inertial flow physics. We show how such effects are exploited in a device proposed by Nikola Tesla that acts as a diode or valve whose asymmetric internal geometry leads to direction-dependent fluidic resistance. Systematic tests for steady forcing conditions reveal that diodicity turns on abruptly at Reynolds number $${\rm{Re}}\approx 200$$ Re ≈ 200 and is accompanied by nonlinear pressure-flux scaling and flow instabilities, suggesting a laminar-to-turbulent transition that is triggered at unusually low $${\rm{Re}}$$ Re . To assess performance for unsteady forcing, we devise a circuit that functions as an AC-to-DC converter, rectifier, or pump in which diodes transform imposed oscillations into directed flow. Our results confirm Tesla’s conjecture that diodic performance is boosted for pulsatile flows. The connections between diodicity, early turbulence and pulsatility uncovered here can inform applications in fluidic mixing and pumping.

Effect of aspect ratio and inlet manifold shape on the laminar-to-turbulent transition of gas flow in rectangular microchannels

A combined experimental and numerical study on the laminar-to-turbulent transition in microchannels using gas flow is presented. The effects of two geometric parameters, namely aspect ratio (height to width) of microchannels and inlet manifold shape, are considered on the value assumed by the critical Reynolds number linked to the laminar-to-turbulent transition. To study the effect of aspect ratio, seven rectangular microchannels having an aspect ratio between 0.25 and 1.04 are micro-milled in PMMA plastic with a constant length of 100 mm. Four rectangular microchannels with different inlet shapes, namely sudden contraction, rounded entrance, V shape and bellmouth, are fabricated to analyze the effects of inlet shape. Pressure loss analyses are then performed for all 11 microchannels by evaluating both average and semi-local friction factors. The Reynolds number in correspondence of which the transition takes place is determined by observing the trend of the friction factor. In parallel, numerical simulations using an intermittency-based transitional turbulence model are also performed and results are compared with the experiments. Experimental and numerical results have demonstrated that both of the investigated geometrical characteristics (aspect ratio and inlet manifold shape) play an important role on the range of the Reynolds number between the onset of transition and the onset of fully turbulent regime for gas microflows. Experimental critical Reynolds numbers show a good agreement with the predictions of the conventional theory and are in the range of 1863–3470 for all the tested microchannels. The role of gas compressibility on the laminar-to-turbulent transition is also discussed. Graphic abstract