Observations of elastic instabilities in lid-driven cavity flow

An experimental investigation of the lid-driven cavity flow of a Boger flui d is completed in order to verify the scaling theory for the aspect ratio d ependence of the critical conditions for elastic instability as proposed by Grillet, Yang, Khomami and Shaqfeh [A.M. Grillet, B. Yang, B. Khomami, E.S .G. Shaqfeh, Modeling of viscoelastic lid driven cavity flow using finite e lement simulations, J. Non-Newtonian fluid Mech. 88 (1999) 99-131]. Measure ments of the critical conditions in shallow cavities show that the critical Weissenberg number and the wavelength of the instability are independent o f the cavity aspect ratio, Lambda, for aspect ratios less than unity. Our e xperiments also demonstrate the importance of polymer concentration on the stability of cavity flows. The effect of concentration is included in new s caling arguments based on those developed for Taylor-Couette and Dean flow elastic instabilities; thus, we successfully describe experimental results at different polymer concentrations. Further experiments are described usin g flow-induced birefringence to measure the polymer stresses prior to the o nset of instability in order to investigate the role of the downstream stre ss boundary layer in the stability of the flow. Measurements taken in three aspect ratios provide evidence that, for shallow cavities, the polymer str ess emanating from the downstream corner can reach the region of curved str eamlines at the bottom of the cavity, and possibly result in an elastic ins tability. This is consistent with the scaling theory proposed by Grillet et al. [A.M. Grillet, B. Yang, B. Khomami, E.S.G. Shaqfeh, Modeling of viscoe lastic lid driven cavity flow using finite element simulations, J. Non-Newt oniam Fluid Mech. 88 (1999) 99-131]. (C) 2000 Elsevier Science B.V. All rig hts reserved.