Validation of the LES approach in Kiva-3V on a square duct geometry

A numerical investigation of chaotic laminar flow and heat transfer in isothermal-wall square-channel configurations is presented. The computations, based on a finite-volume method with the SIMPLEC algorithm, are conducted in terms of Péclet numbers ranging from 7 to 7×105. The geometries, based on the split-and-recombine (SAR) principle, are first proposed for micromixing purposes, and are then optimized and scaled up to three-dimensional minichannels with 3-mm sides that are capable of handling industrial fluid manipulation processes. The aim is to assess the feasibility of this mass- and heat-transfer technique for out-of-laboratory commercial applications and to compare different configurations from a process intensification point of view. The effects of the geometry on heat transfer and flow characteristics are examined. Results show that the flux recombination phenomenon mimicking the baker’s transform in the SAR-1 and SAR-2 configurations produces chaotic structures and promotes mass transfer. This phenomenon also accounts for higher convective heat transfer exemplified by increased values of the Nusselt number compared to the chaotic continuous-flow configuration and the baseline plain square-duct geometry. Energy expenditures are explored and the overall heat transfer enhancement factor for equal pumping power is calculated. The SAR-2 configuration reveals superior heat-transfer characteristics, enhancing the global gain by up to 17-fold over the plain duct heat exchanger.

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Pressure-driven flow of a vesicle through a square microchannel

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.887 ◽

2018 ◽

Vol 861 ◽

pp. 447-483 ◽

Cited By ~ 4

Author(s):

Joseph M. Barakat ◽

Shamim M. Ahmmed ◽

Siva A. Vanapalli ◽

Eric S. G. Shaqfeh

Keyword(s):

Boundary Element ◽

Rotational Symmetry ◽

Stress Singularity ◽

Lubrication Theory ◽

Low Flow ◽

Square Duct ◽

Square Channel ◽

Distinct Property ◽

Duct Geometry ◽

Square Microchannel

The relative velocity and extra pressure drop of a single vesicle flowing through a square microchannel are quantified via boundary element simulations, lubrication theory and microfluidic experiments. The vesicle is modelled as a fluid sac enclosed by an inextensible, fluidic membrane with a negligible bending stiffness. All results are parametrized in terms of the vesicle sphericity (i.e. the reduced volume) and flow confinement (i.e. the ratio of the vesicle radius to the channel hydraulic radius). Direct comparison is made to previous studies of vesicle flow through circular tubes, revealing several distinct features of the square-channel geometry. Firstly, fluid in the suspending medium bypasses the vesicle through the corners of the channel, which in turn reduces the dissipation created by the vesicle. Secondly, the absence of rotational symmetry about the channel axis permits surface circulation in the membrane (tank treading), which in turn reduces the vesicle’s speed. At very high confinement, both theory and experiment indicate that the vesicle’s speed can be reduced below the mean speed of the suspending fluid through this mechanism. Finally, the contact area for lubrication is greatly reduced in the square-duct geometry, which in turn weakens the stress singularity predicted by lubrication theory. This fact directly leads to a breakdown of the lubrication approximation at low flow confinement, as verified by comparison to boundary element simulations. Since the only distinct property assumed of the membrane is its ability to preserve surface area locally, it is expected that the results of this study are applicable to other types of soft particles with immobilized surfaces (e.g. Pickering droplets, gel beads and biological cells).

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Comparing Different Turbulence Modelling Approaches for Square Duct Simulations Using the Kiva Code

Computational Technologies for Fluid/Thermal/Structural/Chemical Systems With Industrial Applications, Volume 2 ◽

10.1115/pvp2004-3133 ◽

2004 ◽

Author(s):

V. Huijnen ◽

L. M. T. Somers ◽

R. S. G. Baert ◽

L. P. H. de Goey

Keyword(s):

Internal Combustion Engines ◽

Turbulence Modelling ◽

Time Dependent ◽

Combustion Engines ◽

Strong Anisotropy ◽

Square Duct ◽

Algebraic Stress Model ◽

Duct Geometry ◽

Computational Resources ◽

Les Model

In internal-combustion engines (ICE) the fluid dynamics is characterized by strong anisotropy. The standard two equation k–ε model is well known to be not appropriate in this case. A detailed study of the numerical modelling properties of the well known Kiva-3V code has been performed for two different approaches to anisotropic turbulence modelling. In the first approach a Smagorinsky-type LES model is evaluated. In the second approach a Time-Dependent RANS model has been adopted, using the Explicit Algebraic Stress Model of Gatski and Speziale [1]. For validation of both approaches numerical simulations of a turbulent flow in a square duct geometry are compared to DNS data. It is concluded from this work that the applied RANS approach is the best available practise to model the anisotropic properties of the fluid flow for ICE simulations as long as the computational resources to perform real LES simulations remain limited.

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Inertial regimes in a curved electromagnetically forced flow

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.876 ◽

2017 ◽

Vol 813 ◽

pp. 860-881 ◽

Cited By ~ 2

Author(s):

J. Boisson ◽

R. Monchaux ◽

S. Aumaître

Keyword(s):

Magnetic Field ◽

Secondary Flow ◽

High Frequency ◽

Characteristic Length ◽

Low Frequency ◽

Frequency Mode ◽

Square Duct ◽

High Frequency Mode ◽

Duct Geometry ◽

Low Frequency Mode

We investigated experimentally the flow driven by a Lorentz force induced by an axial magnetic field $\boldsymbol{B}$ and a radial electric current $I$ applied between two fixed concentric copper cylinders. The gap geometry corresponds to a rectangular section with an aspect ratio of $\unicode[STIX]{x1D702}=4$ and we probe the azimuthal and axial velocity profiles of the flow along the vertical axis by using ultrasonic Doppler velocimetry. We have performed several runs at moderate magnetic field strengths, corresponding to moderate Hartmann numbers $M\leqslant 300$. At these forcing parameters and because of the geometry of our experimental device, we show that the inertial terms are not negligible and an azimuthal velocity that depends on both $I$ and $B$ is induced. From measurements of the vertical velocity we focus on the characteristics of the secondary flow: the time-averaged velocity profiles are compatible with a secondary flow presenting two pairs of stable vortices, as pointed out by previous numerical studies. The flow exhibited a transition between two dynamical modes, a high- and a low-frequency one. The high-frequency mode, which emerges at low magnetic field forcing, corresponds to the propagation in the radial $r$-direction of tilted vortices. This mode is consistent with our previous experiments and with the instability described in Zhao et al. (Phys. Fluids, vol. 23 (8), 2011, 084103) taking place in an elongated duct geometry. The low-frequency mode, observed for high magnetic field forcing, consists of large excursions of the vortices. The dynamics of these modes matches the first axisymmetric instability described in Zhao & Zikanov (J. Fluid Mech., vol. 692, 2012, pp. 288–316) taking place in an square duct geometry. We demonstrated that this transition is controlled by the inertial magnetic thickness $H^{\prime }$ which is the characteristic length we introduce as a balance between the advection and the Lorentz force. The key point here is that when the inertial magnetic thickness $H^{\prime }$ is comparable to one geometric characteristic length ($H/2$ in the vertical or $\unicode[STIX]{x0394}r$ in the radial direction) the corresponding mode is favoured. Therefore, when $H^{\prime }/(H/2)\approx 1$ we observe the high-frequency mode taking place in an elongated duct geometry, and when $H^{\prime }/\unicode[STIX]{x0394}r\approx 1$ we observe the low-frequency mode taking place in square duct geometry and high magnetic field.

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