Effects of Electric Double Layer on Bypass Transition in Microchannel Flow
Date Issued
2004
Date
2004
Author(s)
Shiu, Huan-Ruei
DOI
en-US
Abstract
The effect of the electric double layer (EDL) on the bypass transition mechanism in the linear and nonlinear evolution stage is explored through direct numerical simulations of high resolution. Three kinds of localized disturbances are analyzed. The first one is a pair of counterrotating vortices, the second is a wall jet-like axisymmetric perturbation, while the third is a wave-packet. The time-space evolution of the perturbed field is throughly investigated at low Reynolds numbers in EDL flow by systematically comparing the results in the subcritical transition region of macro-scale Poiseuille flow. The wall normal and spanwise perturbation velocities development are both quantitatively and qualitatively similar in macro and micro flows in the linear stage. The streamwise velocity, which is initially zero for the pair of vortices and is set up by the generation of the wall normal vorticity is twice larger under the EDL effect. Both flows develop inclined strong streamwise shear layers. Overall is the close similarity of the disturbance evolution showing that the three dimensional linear mechanism in EDL flow lead to the structures that are at least as strong as in Poiseuille flow. For large amplitude perturbations, i.e. in the non linear regime the total kinetic energy associated with the EDL flow is larger compared with the Poiseuille flow because of the inherently unstable EDL inflexionnal velocity profile. The energy growth associated with axisymmetric perturbation is always larger than that associated with the counterrotating vortices. The nonlinear interactions trigger the breakdown and lead to turbulent spots, bypassing the transitional Tollmien-Schlichting instability mechanism, providing that the disturbance is strong enough and/or the Reynolds number is sufficiently high to overcome the transient growth stage. Thus the amplitude of the localized disturbance that lead to breakdown is an order of magnitude smaller in EDL flow compared to the macro flow under some circumstances.
Subjects
微流體
電雙層
旁通過渡流
微流道
microfluidic
EDL
microchannel
bypass transition
Type
thesis
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