Wall Flow Dynamics Research Articles

Time-resolved PIV measurement and thermal-hydraulic performance evaluation of thin film self-agitators in a rectangular channel flow

This study experimentally investigates the vortex dynamics induced by thin film self-agitators in a rectangular channel and the resultant thermal-hydraulic performance due to the disruption of the thermal boundary layer. Four channel flow features are compared: a clean channel without any agitators, a channel with double rectangular-shaped self-agitators in thicknesses of 25 µm and 125 µm, and a channel with double fishtail-shaped self-agitators in thickness of 25 µm. The effect of the self-agitator motion on the dynamic vortex shedding is characterized by employing time-resolved particle image velocimetry (TR-PIV) at the midspan of the agitator within a Reynolds number range of about 1300–6800 in a customized single channel. The instantaneous and time-averaged flow field are quantified in terms of velocity magnitude, vorticity contour, and turbulence intensity distribution. Results show that the added self-agitator designs all have positive effects on the flow mixing, especially during the flapping mode region. Near wall flow dynamics measurements quantitatively confirm the enhancement of flow boundary layer disturbance. In addition, an off-the-shelf multi-channel heatsink with an identical cross-section profile is tested under the same flow conditions to quantify the heat transfer enhancement as well as the pressure loss penalty for the four channel design features. A dimensionless synthetic thermal-hydraulic performance factor η is used to evaluate the overall benefits of the three self-agitator designs compared to the clean channel. A maximum enhancement factor of approximately 1.42 can be achieved while keeping above 1 for most of the test conditions. Therefore, there is great potential for integrating this external-energy-free vibration motion into plate fin heat exchanger applications.

Lattice Boltzmann simulations for wall-flow dynamics in porous ceramic diesel particulate filters

Flows through porous filter walls of wall-flow diesel particulate filter are investigated using the lattice Boltzmann method (LBM). The microscopic model of the realistic filter wall is represented by randomly overlapped arrays of solid spheres. The LB simulation results are first validated by comparison to those from previous hydrodynamic theories and constitutive models for flows in porous media with simple regular and random solid-wall configurations. We demonstrate that the newly designed randomly overlapped array structures of porous walls allow reliable and accurate simulations for the porous wall-flow dynamics in a wide range of solid volume fractions from 0.01 to about 0.8, which is beyond the maximum random packing limit of 0.625. The permeable performance of porous media is scrutinized by changing the solid volume fraction and particle Reynolds number using Darcy’s law and Forchheimer’s extension in the laminar flow region.

Prospectus: towards the development of high-fidelity models of wall turbulence at large Reynolds number.

Recent and on-going advances in mathematical methods and analysis techniques, coupled with the experimental and computational capacity to capture detailed flow structure at increasingly large Reynolds numbers, afford an unprecedented opportunity to develop realistic models of high Reynolds number turbulent wall-flow dynamics. A distinctive attribute of this new generation of models is their grounding in the Navier-Stokes equations. By adhering to this challenging constraint, high-fidelity models ultimately can be developed that not only predict flow properties at high Reynolds numbers, but that possess a mathematical structure that faithfully captures the underlying flow physics. These first-principles models are needed, for example, to reliably manipulate flow behaviours at extreme Reynolds numbers. This theme issue of Philosophical Transactions of the Royal Society A provides a selection of contributions from the community of researchers who are working towards the development of such models. Broadly speaking, the research topics represented herein report on dynamical structure, mechanisms and transport; scale interactions and self-similarity; model reductions that restrict nonlinear interactions; and modern asymptotic theories. In this prospectus, the challenges associated with modelling turbulent wall-flows at large Reynolds numbers are briefly outlined, and the connections between the contributing papers are highlighted.This article is part of the themed issue 'Toward the development of high-fidelity models of wall turbulence at large Reynolds number'.

Toward coherently representing turbulent wall-flow dynamics

AbstractThe complex dynamics of turbulent flow in the vicinity of a solid surface underlie numerous scientifically important processes, and pose persistently daunting challenges in many engineering applications. Since their discovery decades ago, coherent motions have presented a tantalizing prospective opportunity for constructing descriptions of wall-flow dynamics using only a relatively small number of elements. The veracity and reliability of such representations are, however, ultimately tied to their basis in the Navier–Stokes equations. In this regard, the study by Sharma & McKeon (J. Fluid Mech., vol. 728, 2013, pp. 196–238) constitutes an important contribution, as it not only provides insights regarding the mechanisms underlying wall-flow coherent motion formation and evolution, but does so within a Navier–Stokes framework.

Self-similar mean dynamics in turbulent wall flows

AbstractThis study investigates how and why dynamical self-similarities emerge with increasing Reynolds number within the canonical wall flows beyond the transitional regime. An overarching aim is to advance a mechanistically coherent description of turbulent wall-flow dynamics that is mathematically tractable and grounded in the mean dynamical equations. As revealed by the analysis of Fife, Klewicki & Wei (J. Discrete Continuous Dyn. Syst.A, vol. 24, 2009, pp. 781–807), the equations that respectively describe the mean dynamics of turbulent channel, pipe and boundary layer flows formally admit invariant forms. These expose an underlying self-similar structure. In all cases, two kinds of dynamical self-similarity are shown to exist on an internal domain that, for all Reynolds numbers, extends from$O(\nu / {u}_{\tau } )$to$O(\delta )$, where$\nu $is the kinematic viscosity,${u}_{\tau } $is the friction velocity and$\delta $is the half-channel height, pipe radius, or boundary layer thickness. The simpler of the two self-similarities is operative on a large outer portion of the relevant domain. This self-similarity leads to an explicit analytical closure of the mean momentum equation. This self-similarity also underlies the emergence of a logarithmic mean velocity profile. A more complicated kind a self-similarity emerges asymptotically over a smaller domain closer to the wall. The simpler self-similarity allows the mean dynamical equation to be written as a closed system of nonlinear ordinary differential equations that, like the similarity solution for the laminar flat-plate boundary layer, can be numerically integrated. The resulting similarity solutions are demonstrated to exhibit nearly exact agreement with direct numerical simulations over the solution domain specified by the theory. At the Reynolds numbers investigated, the outer similarity solution is shown to be operative over a domain that encompasses${\sim }40\hspace{0.167em} \% $of the overall width of the flow. Other properties predicted by the theory are also shown to be well supported by existing data.

TURBULENT FLOWS OVER ROUGH WALLS

▪ Abstract We review the experimental evidence on turbulent flows over rough walls. Two parameters are important: the roughness Reynolds number [Formula: see text], which measures the effect of the roughness on the buffer layer, and the ratio of the boundary layer thickness to the roughness height, which determines whether a logarithmic layer survives. The behavior of transitionally rough surfaces with low [Formula: see text] depends a lot on their geometry. Riblets and other drag-reducing cases belong to this regime. In flows with δ/k ≲ 50, the effect of the roughness extends across the boundary layer, and is also variable. There is little left of the original wall-flow dynamics in these flows, which can perhaps be better described as flows over obstacles. We also review the evidence for the phenomenon of d-roughness. The theoretical arguments are sound, but the experimental evidence is inconclusive. Finally, we discuss some ideas on how rough walls can be modeled without the detailed computation of the flow around the roughness elements themselves.