scholarly journals On the generation of nonlinear travelling waves in confined geometries using electric fields

2014 ◽  
Vol 372 (2020) ◽  
pp. 20140066 ◽  
Author(s):  
R Cimpeanu ◽  
D. T Papageorgiou
Keyword(s):  
Numerical Simulations ◽  
Electric Fields ◽  
Travelling Waves ◽  
Travelling Wave ◽  
Direct Numerical Simulations ◽  
Small Scale ◽  
Interfacial Instabilities ◽  
Lower Electrode ◽  
Nonlinear Deformations ◽  
Moving Parts

We investigate electrostatically induced interfacial instabilities and subsequent generation of nonlinear coherent structures in immiscible, viscous, dielectric multi-layer stratified flows confined in small-scale channels. Vertical electric fields are imposed across the channel to produce interfacial instabilities that would normally be absent in such flows. In situations when the imposed vertical fields are constant, interfacial instabilities emerge due to the presence of electrostatic forces, and we follow the nonlinear dynamics via direct numerical simulations. We also propose and illustrate a novel pumping mechanism in microfluidic devices that does not use moving parts. This is achieved by first inducing interfacial instabilities using constant background electric fields to obtain fully nonlinear deformations. The second step involves the manipulation of the imposed voltage on the lower electrode (channel wall) to produce a spatio-temporally varying voltage there, in the form of a travelling wave with pre-determined properties. Such travelling wave dielectrophoresis methods are shown to generate intricate fluid–surface–structure interactions that can be of practical value since they produce net mass flux along the channel and thus are candidates for microfluidic pumps without moving parts. We show via extensive direct numerical simulations that this pumping phenomenon is a result of an externally induced nonlinear travelling wave that forms at the fluid–fluid interface and study the characteristics of the generated velocity field inside the channel.

Direct Numerical Simulations of Transitional Separation–Bubble Development in Swept–Blade Flow Conditions

2012 ◽  
Author(s):  
Joshua R. Brinkerhoff ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Numerical Simulations ◽  
Shear Layer ◽  
Pressure Field ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Direct Numerical Simulations ◽  
Small Scale ◽  
Flow Conditions ◽  
Crossflow Instability

This paper describes numerical simulations of the instability mechanisms in a separation bubble subjected to a three-dimensional freestream pressure distribution. Two direct numerical simulations are performed of a separation bubble with laminar separation and turbulent reattachment under low freestream turbulence at flow Reynolds numbers and streamwise pressure distributions that approximate the conditions encountered on the suction side of typical low-pressure gas-turbine blades with blade sweep angles of 0° and 45°. The three-dimensional pressure field in the swept configuration produces a crossflow-velocity component in the laminar boundary layer upstream of the separation point that is unstable to a crossflow instability mode. The simulation results show that crossflow instability does not play a role in the development of the boundary layer upstream of separation. An increase in the amplification rate and most amplified disturbance frequency is observed in the separated-flow region of the swept configuration, and is attributed to boundary-layer conditions at the point of separation that are modified by the spanwise pressure gradient. This results in a slight upstream movement of the location where the shear layer breaks down to small-scale turbulence and modifies the turbulent mixing of the separated shear layer to yield a downstream shift in the time-averaged reattachment location. The results demonstrate that although crossflow instability does not appear to have a noticeable effect on the development of the transitional separation bubble, the 3D pressure field does indirectly alter the separation-bubble development by modifying the flow conditions at separation.

scholarly journals Direct numerical simulations of dense suspensions: wave instabilities in liquid-fluidized beds

2007 ◽  
Vol 587 ◽  
pp. 303-336 ◽  
Author(s):  
J. J. DERKSEN ◽  
S. SUNDARESAN
Keyword(s):  
Numerical Simulations ◽  
Bulk Viscosity ◽  
Fluid Model ◽  
Travelling Waves ◽  
System Size ◽  
Direct Numerical Simulations ◽  
Fluid Particle ◽  
Spherical Particles ◽  
Particle Phase ◽  
Viscosity Term

We present results of direct numerical simulations of travelling waves in dense assemblies of monodisperse spherical particles fluidized by a liquid. The cases we study have been derived from the experimental work of others. In these simulations, the flow of interstitial fluid is solved by the lattice-Boltzmann method (LBM) and the particles move under the influence of gravity, hydrodynamic forces stemming from the LBM, subgrid-scale lubrication forces and hard-sphere collisions. We first show that the propagating inhomogeneous structures seen in the simulations are in agreement with those observed experimentally. We then use the detailed information contained in the simulation results to assess aspects of two-fluid model closures, namely, fluid–particle drag, and the various contributions to the effective stresses. We show that the rates of compaction and dilation of the particle phase in the travelling waves are comparable to the rate at which the microstructure relaxes, and that there is a pronounced effect of the rate of compaction on the average collisional normal stress. Although this effect can be expressed as an effective bulk viscosity term, this approach would require the use of a path-dependent bulk viscosity. We also find that the effective fluid–particle drag coefficient can be described well with the often-used closure motivated by the experiments of Richardson & Zaki (Trans. Inst. Chem. Engng vol. 32, 1954, p. 35). In this respect, the effect of the system size for determining the drag requires specific care. The shear viscosity of the particle phase manifests small, but clearly noticeable dependence on the rate of compaction/dilation of the particle phase. Our observations point to the need for higher-order closures that recognize the slow evolution of the microstructure in these flows and account for the effects of non-equilibrium microstructure on the stresses.

A New Active Micro Mixing Strategy

2007 ◽  
Author(s):  
Sedat Tardu ◽  
Rabia Nacereddine
Keyword(s):  
Numerical Simulations ◽  
Temporal Resolution ◽  
Direct Numerical Simulations ◽  
The Other ◽  
Small Scale ◽  
Streamwise Vortices ◽  
Wall Jets ◽  
Vortical Structures ◽  
Active Structures

An active micro-mixing strategy through forcing the flow by synthetic wall jets is proposed. It is based on the interaction of induced streamwise vortices in a specific way. There is a spanwise shift between two quasi-streamwise vortices in such a way that one of them compresses the wall normal vorticity layer created by the other, leading to the generation of new wall normal vortical structures. The latter are subsequently tilted by the shear to give birth to new small-scale longitudinal active structures that are efficient in mixing. The feasibility of this strategy is shown through direct numerical simulations of high spatial and temporal resolution.

Field-induced dipolar attraction between like-charged colloids

Soft Matter ◽  
2018 ◽  
Vol 14 (22) ◽  
pp. 4520-4529 ◽  
Author(s):  
Chunyu Shih ◽  
John J. Molina ◽  
Ryoichi Yamamoto
Keyword(s):  
Numerical Simulations ◽  
Electric Fields ◽  
Double Layer ◽  
Electric Double Layer ◽  
Colloidal Particles ◽  
Direct Numerical Simulations ◽  
Ac Electric Fields ◽  
Charged Colloids ◽  
Anisotropic Interactions

The field induced anisotropic interactions between like-charged colloidal particles is studied using direct numerical simulations, where the polarization of the electric double layer is explicitly computed under external AC electric fields.

scholarly journals Frictional hysteresis and particle deposition in granular free-surface flows

2019 ◽  
Vol 875 ◽  
pp. 1058-1095 ◽  
Author(s):  
A. N. Edwards ◽  
A. S. Russell ◽  
C. G. Johnson ◽  
J. M. N. T. Gray
Keyword(s):  
Numerical Simulations ◽  
Particle Deposition ◽  
Initial Conditions ◽  
Travelling Waves ◽  
Travelling Wave ◽  
Free Surface Flows ◽  
Uniform Flow ◽  
Glass Beads ◽  
Deposit Thickness ◽  
Frictional Hysteresis

Shallow granular avalanches on slopes close to repose exhibit hysteretic behaviour. For instance, when a steady-uniform granular flow is brought to rest it leaves a deposit of thickness $h_{stop}(\unicode[STIX]{x1D701})$ on a rough slope inclined at an angle $\unicode[STIX]{x1D701}$ to the horizontal. However, this layer will not spontaneously start to flow again until it is inclined to a higher angle $\unicode[STIX]{x1D701}_{start}$, or the thickness is increased to $h_{start}(\unicode[STIX]{x1D701})>h_{stop}(\unicode[STIX]{x1D701})$. This simple phenomenology leads to a rich variety of flows with co-existing regions of solid-like and fluid-like granular behaviour that evolve in space and time. In particular, frictional hysteresis is directly responsible for the spontaneous formation of self-channelized flows with static levees, retrogressive failures as well as erosion–deposition waves that travel through the material. This paper is motivated by the experimental observation that a travelling-wave develops, when a steady uniform flow of carborundum particles on a bed of larger glass beads, runs out to leave a deposit that is approximately equal to $h_{stop}$. Numerical simulations using the friction law originally proposed by Edwards et al. (J. Fluid Mech., vol. 823, 2017, pp. 278–315) and modified here, demonstrate that there are in fact two travelling waves. One that marks the trailing edge of the steady-uniform flow and another that rapidly deposits the particles, directly connecting the point of minimum dynamic friction (at thickness $h_{\ast }$) with the deposited layer. The first wave moves slightly faster than the second wave, and so there is a slowly expanding region between them in which the flow thins and the particles slow down. An exact inviscid solution for the second travelling wave is derived and it is shown that for a steady-uniform flow of thickness $h_{\ast }$ it produces a deposit close to $h_{stop}$ for all inclination angles. Numerical simulations show that the two-wave structure deposits layers that are approximately equal to $h_{stop}$ for all initial thicknesses. This insensitivity to the initial conditions implies that $h_{stop}$ is a universal quantity, at least for carborundum particles on a bed of larger glass beads. Numerical simulations are therefore able to capture the complete experimental staircase procedure, which is commonly used to determine the $h_{stop}$ and $h_{start}$ curves by progressively increasing the inclination of the chute. In general, however, the deposit thickness may depend on the depth of the flowing layer that generated it, so the most robust way to determine $h_{stop}$ is to measure the deposit thickness from a flow that was moving at the minimum steady-uniform velocity. Finally, some of the pathologies in earlier non-monotonic friction laws are discussed and it is explicitly shown that with these models either steadily travelling deposition waves do not form or they do not leave the correct deposit depth $h_{stop}$.

scholarly journals Direct Numerical Simulations of a Smoke Cloud–Top Mixing Layer as a Model for Stratocumuli

2013 ◽  
Vol 70 (8) ◽  
pp. 2356-2375 ◽  
Author(s):  
Alberto de Lozar ◽  
Juan Pedro Mellado
Keyword(s):  
Numerical Simulations ◽  
Turbulent Flux ◽  
Inversion Layer ◽  
Mixing Layer ◽  
Direct Numerical Simulations ◽  
Small Scale ◽  
Inversion Point ◽  
Molecular Flux ◽  
Definition Of ◽  
Averaged Model

Abstract A radiatively driven cloud-top mixing layer is investigated using direct numerical simulations. This configuration mimics the mixing process across the inversion that bounds the stratocumulus-topped boundary layer. The main focus of this paper is on small-scale turbulence. The finest resolution (7.4 cm) is about two orders of magnitude finer than that in cloud large-eddy simulations (LES). A one-dimensional horizontally averaged model is employed for the radiation. The results show that the definition of the inversion point with the mean buoyancy of 〈b〉(zi) = 0 leads to convective turbulent scalings in the cloud bulk consistent with the Deardorff theory. Three mechanisms contribute to the entrainment by cooling the inversion layer: a molecular flux, a turbulent flux, and the direct radiative cooling by the smoke inside the inversion layer. In the simulations the molecular flux is negligible, but the direct cooling reaches values comparable to the turbulent flux as the inversion layer thickens. The results suggest that the direct cooling might be overestimated in less-resolved models like LES, resulting in an excessive entrainment. The scaled turbulent flux is independent of the stratification for the range of Richardson numbers studied here. As suggested by earlier studies, the turbulent entrainment only occurs at the small scales and eddies larger than approximately four optical lengths (60 m in a typical stratocumulus cloud) perform little or no entrainment. Based on those results, a parameterization is proposed that accounts for a large part (50%–100%) of the entrainment velocities measured in the Second Dynamics and Chemistry of the Marine Stratocumulus (DYCOMS II) campaign.

scholarly journals On peculiar property of the velocity fluctuations in wall-bounded flows

2005 ◽  
Vol 9 (1) ◽  
pp. 3-12 ◽  
Author(s):  
Jovan Jovanovic ◽  
Rafaela Hillerbrand
Keyword(s):  
Statistical Analysis ◽  
Numerical Simulations ◽  
Direct Numerical Simulations ◽  
Small Scale ◽  
Wall Region ◽  
Major Conclusion ◽  
Velocity Fluctuations ◽  
Small Scales ◽  
Wall Bounded Flows ◽  
Near Wall

Statistical analysis of the velocity fluctuations is performed for the near-wall region of wall-bounded flows. By demanding that the small-scale part of the fluctuations satisfies constraints imposed by local ax symmetry it was found that the small scales must be entirely suppressed in the near-wall region. This major conclusion is well supported by all available data from direct numerical simulations.

scholarly journals A Comparison of Statistical Dynamical and Ensemble Prediction Methods during Blocking

2008 ◽  
Vol 65 (2) ◽  
pp. 426-447 ◽  
Author(s):  
Terence J. O’Kane ◽  
Jorgen S. Frederiksen
Keyword(s):  
Kinetic Energy ◽  
Numerical Simulations ◽  
Direct Interaction ◽  
Gulf Of Alaska ◽  
Direct Numerical Simulations ◽  
Ensemble Prediction ◽  
Small Scale ◽  
Error Growth ◽  
Closure Model ◽  
Non Gaussian

Abstract In this paper error growth is examined using a family of inhomogeneous statistical closure models based on the quasi-diagonal direct interaction approximation (QDIA), and the results are compared with those based on ensembles of direct numerical simulations using bred perturbations. The closure model herein includes contributions from non-Gaussian terms, is realizable, and conserves kinetic energy and enstrophy. Further, unlike previous approximations, such as those based on cumulant-discard (CD) and quasi-normal (QN) hypotheses (Epstein and Fleming), the QDIA closure is stable for long integration times and is valid for both strongly non-Gaussian and strongly inhomogeneous flows. The performance of a number of variants of the closure model, incorporating different approximations to the higher-order cumulants, is examined. The roles of non-Gaussian initial perturbations and small-scale noise in determining error growth are examined. The importance of the cumulative contribution of non-Gaussian terms to the evolved error tendency is demonstrated, as well as the role of the off-diagonal covariances in the growth of errors. Cumulative and instantaneous errors are quantified using kinetic energy spectra and a small-scale palinstrophy production measure, respectively. As a severe test of the methodology herein, synoptic situations during a rapid regime transition associated with the formation of a block over the Gulf of Alaska are considered. In general, the full QDIA closure results compare well with the statistics of direct numerical simulations.

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