Empirical evidence of a fluctuation theorem for the wind mechanical power input into the ocean

Ocean dynamics is predominantly driven by the shear stress between the atmospheric winds and ocean currents. The mechanical power input to the ocean is fluctuating in space and time and the atmospheric wind sometimes decelerates the ocean currents. Building on 24 years of global satellite observations, the input of mechanical power to the ocean is analysed. A fluctuation theorem (FT) holds when the logarithm of the ratio between the occurrence of positive and negative events, of a certain magnitude of the power input, is a linear function of this magnitude and the averaging period. The flux of mechanical power to the ocean shows evidence of a FT for regions within the recirculation area of the subtropical gyre but not over extensions of western boundary currents. A FT puts a strong constraint on the temporal distribution of fluctuations of power input, connects variables obtained with different lengths of temporal averaging, guides the temporal down- and up-scaling and constrains the episodes of improbable events.

COMPUTATIONAL STUDY OF GEOMETRIC EFFECTS OF BOTTOM WALL MICROGROOVES ON CELL DOCKING INSIDE MICROFLUIDIC DEVICES

Cells docking inside microfluidic devices is effective in studying cell biology, cell-based biosensing, as well as drug screening. Furthermore, single cell and regularly cells docking inside the microstructure of microfluidic systems are advantageous in different analyses of single cells exposed to equal drug concentration and mechanical stimulus. In this study, we investigated bottom wall microgrooves with semicircular and rectangular geometries with different sizes which are suitable for single cell docking along the length of the microgroove in [Formula: see text]-direction and numerous cells docking regularly in one line inside the microgroove in a 3D microchannel. We used computational fluid dynamics to analyze the fluid recirculation area inside different microgrooves. The height of recirculation area in the bottom of microgroove could affect the cell’s attachment, and also materials delivery to attached cells, so the height of recirculation area may have optimum value. In addition, we analyzed the fluid drag force on cell movement toward the microgroove. This parameter was proportional to the fluid velocities in [Formula: see text] and [Formula: see text] directions in different microgrooves geometries. In different microgrooves’ geometries the fluid velocity in [Formula: see text]-direction did not change, but the fluid velocity in [Formula: see text]-direction decreased inside the microgroove. Therefore, the cell movement time inside the microgroove increased, and also the drag force in [Formula: see text]-direction could push the cells toward the bottom due to the lower drag force in [Formula: see text]-direction. The percentages of negative shear stress and average shear stress on the adhered cell surface were also calculated. The lower average shear stress, and negative shear stress around 50% on the cell surface were against cell detachment from the substrate. The results indicated that at the constant fluid inlet velocity and microchannel height, microgroove geometry and ratio of cell size to the microgroove size play pivotal roles in the cell initial adhesion to the substrate as well as the cell detachment.

Evidence of a fluctuation theorem for the input of mechanical power to the ocean at the air-sea interface from satellite data

The ocean dynamics is predominantly driven by the shear between the atmospheric winds and ocean currents. The ocean mostly receives energy, but it can also lose energy. Building on 24-years of global satellite observations, the input of mechanical power to the ocean is analysed. A Fluctuation Theorem (FT) holds when the logarithm of the ratio between the occurrence of positive and negative events, of a certain magnitude of the power input, is a linear function of this magnitude and the averaging period. The input of mechanical power into the ocean shows evidence of a FT, for regions within the recirculation area of the subtropical gyre, but not over extensions of Western Boundary Currents. A FT puts a strong constraint on the temporal distribution of fluctuations of power input, connects variables obtained with different length of temporal averaging and guides the temporal down- and up-scaling and constrains the occurrence of extreme events.

Comparative Study on the Hydraulic Characteristics of Nature-Like Fishways

Due to the complex structure and the multiformity of boulder arrangements, there is currently no perfect design criterion for nature-like fishways. This paper proposes four types of nature-like fishways arranged with an impermeable partition wall (IPW), a semi-permeable partition wall (SPPW) or a fully permeable partition wall (FPPW). The hydraulic characteristics of these fishways were investigated experimentally. The results show that the discharge of the fishway arranged with a FPPW was almost twice that of an IPW fishway, and the discharge of a SPPW fishway was between the two extremes. The mean flow velocity of the FPPW fishway was larger than that of the other schemes. For the fishway arranged with an IPW, the flow information was basically consistent with that of the engineered technical fishway. In the FPPW or SPPW fishway, there was more abundant flow information and no obvious recirculation zones in the fishway pool, and these conditions are suitable for migratory fish moving up- and downstream. Notably, for the fishway arranged with two fish passages, two mainstreams were formed in the pool, which divided the flow pattern of the pool into three flow regions. A weak recirculation area was formed in the low-velocity region, which facilitates swimming for migratory fish. According to this comprehensive comparative study, the SPPW fishway with two fish passages had low discharge, abundant flow information and favorable fish migration characteristics; thus, it is the optimal fishway scheme among those studied in this paper.

Design and Bench-Scale Hydrodynamic Testing of Thin-Layer Wavy Photobioreactors

In a thin-volume photobioreactor where a concentrated suspension of microalgae is circulated throughout the established spatial irradiance gradient, microalgal cells experience a time-variable irradiance. Deploying this feature is the most convenient way of obtaining the so-called “flashing light” effect, improving biomass production in high irradiance. This work investigates the light flashing features of sloping wavy photobioreactors, a recently proposed type, by introducing and validating a computational fluid dynamics (CFD) model. Two characteristic flow zones (straight top-to-bottom stream and local recirculation stream), both effective toward light flashing, have been found and characterized: a recirculation-induced frequency of 3.7 Hz and straight flow-induced frequency of 5.6 Hz were estimated. If the channel slope is increased, the recirculation area becomes less stable while the recirculation frequency is nearly constant with flow rate. The validated CFD model is a mighty tool that could be reliably used to further increase the flashing frequency by optimizing the design, dimensions, installation, and operational parameters of the sloping wavy photobioreactor.

Hydrodynamic Light Flashing in Thin Layer Wavy Photobioreactors

In a thin-volume photobioreactor where a concentrated suspension of microalgae is circulated throughout the established spatial irradiance gradient, microalgal cells experience a time-variable irradiance. Deploying this feature is the most convenient way of obtaining the so-called “flashing light” effect, improving biomass production in high irradiance. This work investigates the light flashing features of sloping wavy photobioreactors, a recently proposed type, by introducing and validating a Computational Fluid Dynamics model. Two characteristic flow zones (straight top-bottom stream and local recirculation stream), both effective toward light flashing, have been found and characterised: a recirculation-induced frequency of 3.7 Hz and straight flow-induced frequency of 5.6 Hz were estimated. If the channel slope is increased, the recirculation area becomes less stable while the recirculation frequency is nearly constant with flow rate. The validated CFD model is a mighty tool that could be reliably used to further increase the flashing frequency by optimising the design, the dimensions, the installation and the operational parameters of the sloping wavy photobioreactor.

Bluff-Body Flames in Hot and Diluted Environments

Large eddy simulation/Flamelet progress variable approach is employed in current research to investigate how flame behaviour is influenced by bluff-body and coflow composition. We used Sydney bluff-body burner as the target burner. Computation grid in cylinder coordinates is approximately 2.7 million in total number, extended to the downstream location 80 times of jet diameter. Three coflow compositions with different oxygen ratio at the same inlet velocity are considered. Comparing to jet flames with hot and diluted coflow, instantaneous and statistical results showed that an introduction of bluff-body preheats the fuel and shortens the flame length. At lower oxygen ratio condition, a weaker reaction zone emerged, marked by lower temperature and OH concentration; the flame appeared lifted gradually, leading to a potential MILD combustion. Besides, bluff-body effect behaves differently in these flames: at lower oxygen ratio condition, a large-scale distribution of CH2O appeared as a marker of partial premixing and preignition reaction in recirculation area; on the contrary, in higher oxygen ratio case, the recirculation area brings out more reactive fuel at lower speed and higher temperature, hence ignitable in the vicinity of bluff-body.

A new urban canopy parameterization scheme for wind environment simulations

This paper concerns urban canopies populated with tall slender buildings. To clarify the controlling factors of urban canopies, we simulated a series of single high-rise buildings under fully developed turbulence at a realistic scale by large-eddy simulation. We then analysed correlations between the drag force and recirculation area, frontal area, surface area, floor area, porosity and inlet velocity. Our results show that the recirculation length and recirculation area were proportional to the width, height and wind speed, but were inversely proportional to the length of a building. New equations for the recirculation length and area are presented. The maximum error of the recirculation length equation was 6.66%, and the maximum error of the recirculation area equation was 7.49%. The drag source characteristic length was found to be proportional to the recirculation area, frontal area and surface area and inversely proportional to the porosity and height, but was not closely related to floor area. A new local scale drag source model was developed and applied to a complex urban canopy of Xi’an. The model was applied to 7 × 7 buildings and show good agreement with the solid wall simulation results.

A Correlative CFD Study Between Recirculation Area and FPM in VHC Design

A major part of asthma treatment is made by the use of preventive inhalation drugs. The Pressurized Metered-Dose Inhalator (pMDI) has been the backbone device for this treatment, due to its simplicity, portability and widely acceptance. But no device comes without its limitations, and pMDI is hard to handle properly by elders and children < 5 years old, resulting in reduced amount of drug to the patient lungs. Add-on devices (e.g. spacers) were developed to mitigate the need for coordination and reduce the oral/throat deposition, namely the Valved Holding Chambers (VHC). These devices are incorporated with a one-way valve and a chamber that allows the spray droplets to rapidly reduce their size upon release on a stagnated and confined flow. The VHC main ability, in terms of efficiency, is to reduce the coarse fraction (i.e. particles with diameter > 4.7μm) of the plume by impaction and allow the fine fraction to be inhaled by the patient. The VHC geometry will play a very importance role in the entrapment of small drug particles (i.e. fine fraction). The hypothesis proposed by this study is that a small particle has more probability to be trapped in geometries with higher recirculation areas (and stagnation zones). These macro vortices will cause a particle with small Stokes number to be entrapped; to assess this hypothesis a numerical study was modelled. The numerical study was carried out on an idealized geometry of a VHC device, using a 2D axisymmetric approach. Different coordinates for a “corner” point, were tested. FLUENT® was used to obtain the unsteady numerical solution, meshes were generated using Meshing® software from ANSYS®. Once the flow field is stabilized (around 0.6s), a pMDI spray was injected into the domain during 0.1 seconds and the simulation continued until perform 4 seconds. The simulation takes into account the vaporization of the HFA-134a propellant present in the droplets of spray and a User Defined Function (UDF) for modeling the particle -wall interaction. The post-processing of the results included the calculation of the recirculation area and the Fine Particle Mass (FPM) that exits the domain. Results show that the percentage of recirculation area decreases linearly with the increase of axial position of the corner point, and rapidly increases with the radial displacement. FPM results are not so linear; nevertheless they show opposite behavior to the recirculation area. Additionally, results show that high recirculation area reduces the amount of FPM emitted. Data can be correlated through a power function (FPM = 101.805*Area−0.244; R2 = 0.460). Results are more strongly correlated for lower values of radial displacement. The results seem to corroborate the hypothesis that smaller particles tend to be entrapped by recirculation areas.

Manipulating flow separation: sensitivity of stagnation points, separatrix angles and recirculation area to steady actuation

A variational technique is used to derive analytical expressions for the sensitivity of several geometric indicators of flow separation to steady actuation. Considering the boundary layer flow above a wall-mounted bump, the six following representative quantities are considered: the locations of the separation point and reattachment point connected by the separatrix, the separation angles at these stagnation points, the backflow area and the recirculation area. For each geometric quantity, linear sensitivity analysis allows us to identify regions which are the most sensitive to volume forcing and wall blowing/suction. Validations against full nonlinear Navier−Stokes calculations show excellent agreement for small-amplitude control for all considered indicators. With very resemblant sensitivity maps, the reattachment point, the backflow and recirculation areas are seen to be easily manipulated. By contrast, the upstream separation point and the separatrix angles are seen to remain extremely robust with respect to external steady actuation.