Biomedical Engineering Reference
In-Depth Information
Figure 2.17
Comparisonoflowratesinamicroluidicnetwork.Thesamevaluesofthecontour
levelsofthevelocityshowthedifferenceinthelowrateinthesmallbranchofabifurcationbetween
aNewtonian(NWT)andnon-Newtonian(NNWT)luid.(a)Afteronebifurcation,thechangeinlow
ratecanreach5-10%foraninletvelocityof2-3mm/s,and(b)aftertwobifurcations,thechange
inlowratecanreach20-30%.
presents fluctuating random vertices, even at very small scales. It is intuitive to think
that microflows will predominantly be laminar since there is a strong limitation
for vertices development and randomness due to the proximity of the solid walls.
One important point here is that flow recirculation (vortex) does not mean that
the flow is turbulent. In Figure 2.18, turbulence only starts at the transition Reyn-
olds number of 100. Stable recirculation vortices at a Reynolds number of 50 are
still laminar. Turbulence is associated with time fluctuations even with steady state
boundary conditions.
A nondimensional number (the Reynolds number) determines the ratio of iner-
tia (convective forces) and viscous forces
UD
ν
Re
=
(2.35)
Figure 2.18
Differentpatternsoflaminarandtransitionallowsbehindacylinder.
