OBSERVATIONS OF VISCOELASTIC INSTABILITIES IN RECIRCULATION FLOWS OF BOGER FLUIDS

A purely elastic flow instability in the recirculation flow of a drive n half-cavity or blocked channel is discovered and examined both exper imentally and numerically. Video image analysis is used to determine t he critical Weissenberg number and characteristic wavenumber of the in stability while laser sheet imaging provides information about the str ucture and location of the secondary flow. To understand the basic flo w and ultimately the flow instabilities, we numerically solve the comb ined momentum conservation and constitutive equations describing the f low of. elastic fluids in the block flow geometry. Local rates of poly mer stretch are estimated with a weakly elastic expansion using the Ol droyd-B constitutive equation for the polymer stress. Numerical soluti ons for the kinematics and polymer stretch are also obtained using the Chilcott-Rallison FENE model to describe the polymeric stress. These results are discussed in the context of known purely elastic instabili ty mechanisms in order to better understand the stability of general r ecirculation flows.