Mega riverbed-patterns: linear and weakly nonlinear perspectives

In this paper, we explore the mega riverbed-patterns, whose longitudinal and vertical length dimensions scale with a few channel widths and the flow depth, respectively. We perform the stability analyses from both linear and weakly nonlinear perspectives by considering a steady-uniform flow in an erodible straight channel comprising a uniform sediment size. The mathematical framework stands on the dynamic coupling between the depth-averaged flow model and the particle transport model including both bedload and suspended load via the Exner equation, which drives the pattern formation. From the linear perspective, we employ the standard linearization technique by superimposing the periodic perturbations on the undisturbed system to find the dispersion relationship. From the weakly nonlinear perspective, we apply the centre–manifold-projection technique, where the fast dynamics of stable modes is projected on the slow dynamics of weakly unstable modes to obtain the Stuart–Landau equation for the amplitude dynamics. We examine the marginal stability, growth rate and amplitude of patterns for a given quintet formed by the channel aspect ratio, wavenumber of patterns, shear Reynolds number, Shields number and relative roughness number. This study highlights the sensitivity of pattern formation to the key parameters and shows how the classical results can be reconstructed on the parameter space.

Drag coefficients of large instream wood – mystery or science?

<p>Drag coefficients convert flow velocity into the force exerted on a body and hydraulic head loss. They spatially integrate all properties of a given configuration into one single parameter. Therefore, drag coefficients are widely used in engineering, including environmental flow applications such as large wood log. However, the scatter in drag coefficients reported from previous studies clearly indicate that universality is no longer given and predictions underlie large uncertainties. Deeper analyses are mostly restricted due to insufficient hydraulic data – which in many studies is simply the discharge and the derived cross-sectionally averaged velocity.</p><p>It is obvious, that the ‘ideal’ drag coefficients from infinite and low-turbulence wind tunnel studies with the iconic c<sub>D</sub> = 1.1 for the subcritical regime (10<sup>4</sup> < Re < 10<sup>5</sup>) do not apply anymore. Instead, disturbances play a major role. For large wood, these are typically (i) blockage of the finite river cross-sectional area, (ii) the proximities of the bed below and the free-surface above, (iii) interaction with the free-stream turbulence, (iv) wake interference and (v) 3D-effects such as free ends of the cylindrical or channel aspect ratio.</p><p>To search for a more robust predictive scheme, we started flume experiments with horizontal cylinder configurations and measured the flow field in vertical profiles as well as the overall drag. First results shifted our emphasis from drag to velocity coefficients like the ones used in energy and momentum equations to account for non-uniformity. This seems reasonable, both because of the the squared represention of velocity in the drag term and because of the significant non-uniformities in large wood constellations.</p>

The effect of channel aspect ratio on air entrapment during imbibition in soil-on-a-chip micromodels with 2D and 2.5D pore structures

“Soil-on-a-chip” micromodels designed with X-ray CT images were fabricated by tabletop CNC machining and dry adhesive bonding. The competition between film flow and piston displacement causes the air entrapment differences in 2D and 2.5D micromodels.

Parameters of liquid cooling thermal management system effect on the Li-ion battery temperature distribution

In order to investigated the influence on the liquid cooling system cooling effect by changing the structural parameters, single Li-ion battery heat generation model is conducted, and used in following simulation. Subsequently, sixteen models are designed by orthogonal array, and the results are obtained by extremum difference analysis, which can quantify the influence degree, identify major and minor factors, and find the relatively optimum combination. Finally, different channel entrance layout is adopted to investigated. With a series of work, the effective of single battery heat generation model is proved by the discharge experiment. The coolant velocity has most evident influence on the Li-ion battery temperature rise, rectangular channel aspect ratio is second one, and the heat conducting plate thickness has the smallest influence. Similarly, for Li-ion battery temperature difference, the effect of heat conducting plate thickness and rectangular channel aspect ratio as the same, both are secondary factor, and coolant velocity is main factor. With different channel entrance layout, both the maximum temperatures denote a same upward trend, and better balance temperature distribution is obtained by adopt Case C system which with alternating arrange channel entrance layout.

Numerical study of the effect of channel aspect ratio on particle focusing in acoustophoretic devices

Acoustophoretic microfluidic devices are promising non-contact and high-throughput tools for particle manipulation. Although the effectiveness of this technique has been widely demonstrated for applications based on micrometer-sized particles, the manipulation and focusing of sub-micrometer ones is challenging due to the presence of acoustic streaming. In this article, our study has the aim to investigate and understand which geometrical parameters could be changed to limit the acoustic streaming effect. We numerically study the well-known rectangular cross section of a microfluidic channel and perform a parametric study of the aspect ratio for several particle sizes. The efficiency of the focusing, is explored for different sized particles in order to identify a trend for which the acoustic streaming does not drastically influence the focusing motion of the particles. The possibility to efficiently separate different solid components in liquid suspensions, i.e. the whole blood, is crucial for all applications that require a purified medium such as plasmapheresis or an increase of the concentration of specific subpopulation as the outcome, such as proteomics, cancer biomarker detections and extracellular vesicles separation.

Effect of Aspect Ratio on Overall Thermal Performance of Forced Convective Heat Transfer Utilizing Turbulent Nanofluid Flow

This work is concerned with studying the performance of SiO2–water nanofluid flow through a three-dimensional straight mini-channel with different values of aspect ratio (AR) of (0.5, 1.0, and 1.6) and a fixed hydraulic diameter under a uniform heat flux. The governing equations are developed and solved numerically using the finite volume method for a single-phase flow with standard Kappa-epsilon (κ–ε) turbulence model via a user-defined function (UDF) over Reynolds number (Re) range of (10,000-35,000). Numerical results indicated that the average Nusselt number ratio increases as Reynolds number and volume concentration of the nanoparticles increase for all values of the channel aspect ratio. The results indicated that the maximum enhancement of the heat transfer coefficient (benefit) achieved is 94.69% at AR=0.5, along with the lowest increase of pressure drop (penalty) of 13.1%. The highest performance evaluation criterion (PEC) of 1.64 is found at AR=0.5, Re=35,000, and 5% concentration.