Natural ventilation of buildings is the flow generated by temperature differences and by the wind. The governing feature of this flow is the exchange between an interior space and the external ambient. Although the wind may often appear to be the dominant driving mechanism, in many circumstances temperature variations play a controlling feature on the ventilation since the directional buoyancy force has a large influence on the flow patterns within the space and on the nature of the exchange with the outside. Two forms of ventilation are discussed: mixing ventilation, in which the interior is at an approximately uniform temperature, and displacement ventilation, where there is strong internal stratification. The dynamics of these buoyancy-driven flows are considered, and the effects of wind on them are examined. The aim behind this work is to give designers rules and intuition on how air moves within a building; the research reveals a fascinating branch of fluid mechanics.

The fluid mechanics of natural ventilation

Experiments and simulations of the transition to turbulence in fluid flow through a quasi-2D channel reveal critical exponents consistent with directed percolation — long conjectured to be the universality class associated with the transition. Transition from laminar to turbulent flow drastically changes the mixing, transport, and drag properties of fluids, yet when and how turbulence emerges is elusive even for simple flow within pipes and rectangular channels1,2. Unlike the onset of temporal disorder, which is identified as the universal route to chaos in confined flows3,4, characterization of the onset of spatiotemporal disorder has been an outstanding challenge because turbulent domains irregularly decay or spread as they propagate downstream. Here, through extensive experimental investigation of channel flow, we identify a distinctive transition with critical behaviour. Turbulent domains continuously injected from an inlet ultimately decayed, or in contrast, spread depending on flow rates. Near a transition point, critical behaviour was observed. We investigate both spatial and temporal dynamics of turbulent clusters, measuring four critical exponents, a universal scaling function and a scaling relation, all in agreement with the (2 + 1)-dimensional directed percolation universality class.

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A universal transition to turbulence in channel flow

We examine the onset of turbulence in Waleffe flow – the planar shear flow between stress-free boundaries driven by a sinusoidal body force. By truncating the wall-normal representation to four modes, we are able to simulate system sizes an order of magnitude larger than any previously simulated, and thereby to attack the question of universality for a planar shear flow. We demonstrate that the equilibrium turbulence fraction increases continuously from zero above a critical Reynolds number and that statistics of the turbulent structures exhibit the power-law scalings of the (2+1)D directed percolation universality class.

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Universal continuous transition to turbulence in a planar shear flow

In channel flow, the transition from turbulence displays a crossover from laminar-turbulent patterns following a two-dimensional directed-percolation scenario to a regime with localized turbulent bands statistically propagating along a single direction. A simple model accounts for this bifurcation.

Bifurcations to turbulence in transitional channel flow

In the present work, zinc oxide (ZnO) thin film based Love wave acoustic device has been exploited for realization of a biosensor. These Love wave devices have been fabricated on 36°YX lithium tantalate with ZnO thin film deposited using rf sputtering technique as the guiding layer. Polydimethylsiloxane (PDMS) microchannels have been prepared and integrated on the propagation path on the fabricated device. Detection of uric acid has been demonstrated using the developed biosensing device by measuring the shift in center frequency on interaction with uric acid. The developed uric acid sensor paves way towards the realization of handheld biosensor for future wireless sensing technology.

Highly sensitive Love wave acoustic biosensor for uric acid

In this letter, we show via numerical simulations that the typical flow structures appearing in transitional channel flows at moderate Reynolds numbers are not spots but isolated turbulent bands, which have much longer lifetimes than the spots. Localized perturbations can evolve into isolated turbulent bands by continuously growing obliquely when the Reynolds number is larger than 660. However, interactions with other bands and local perturbations cause band breaking and decay. The competition between the band extension and breaking does not lead to a sustained turbulence until Re becomes larger than about 1000. Above this critical value, the bands split, providing an effective mechanism for turbulence spreading.

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Turbulent bands in plane-Poiseuille flow at moderate Reynolds numbers

Numerical simulations of plane-Poiseuille flow find a state of sparse but persistent oblique turbulent bands in a Reynolds number range between about 660 and 800, which is below the anticipated directed percolation transition.

Extended localized structures and the onset of turbulence in channel flow

The Love mode surface acoustic wave biosensor is considered as one of the most promising probing methods in biomedical research and diagnosis, which has been applied to detect the mechano-biological behaviors of cells attached to the surface of the device. Recent studies have reported the structural and functional optimization of Love wave biosensors for reducing propagation loss and improving sensitivity; however, the relevant device performance needs to be analyzed in depth in terms of device structure, electromechanical properties of piezoelectric crystal substrates, viscoelastic properties of wave guiding layers, and the effect of liquid loading. In this study, a 36° YX-LiTaO3 based Love wave sensor with a parylene-C wave guiding layer is considered as a cell-based biosensor. A theoretical model is proposed to describe the Love wave propagation in the wave guiding layer and penetration in the liquid medium. Decay length δ for the Love wave penetration in liquid is found to be in the order of ∼50 nm, w...

Theoretical analysis of a Love wave biosensor in liquid with a viscoelastic wave guiding layer

Linear stability and energy stability of plane Poiseuille flow with isotropic and anisotropic slip boundary conditions

In this letter, it is shown numerically that in plane Poiseuille flow and before the threshold of equilibrium turbulence defined by the directed-percolation universality class, a sparse turbulent state in form of localized turbulent band can sustain either by continuous increase of the turbulence fraction due to band extension when the flow domain is large enough, or by a dynamic balance between the band extension and the band breaking and decay caused by the band interaction in a finite domain. The width and tilt angle of the band keep statistically invariant during its oblique extension, a process which is not sensitive to random disturbances.

Xiangming Xiong

Papers

Turbulent bands in plane-Poiseuille flow at moderate Reynolds numbers

Extended localized structures and the onset of turbulence in channel flow

Theoretical analysis of a Love wave biosensor in liquid with a viscoelastic wave guiding layer

Linear stability and energy stability of plane Poiseuille flow with isotropic and anisotropic slip boundary conditions

Extended localized structures and the onset of turbulence in channel flow