Effets viscoélastiques dans les mesures des pressions statiques et d'arrêt

¹ Chargé de Recherches, C.N.R.S., Laboratoire d'Aérodynamique, Orsay.
² Chercheur, Laboratoire d'Aérodynamique, Orsay.
³ Maître de Conférences, E.N.S.M., Nantes.

Résumé

Total and static pressure measurements in visco-elastic fluids are subject to considerable interference. Measured dynamic pressures differ by up to 70 per cent from similar data for Newtonian fluids, due to such factors as flow velocity V0 outside probe diameter D0, viscosity μ, specific gravity Q and shear modulus of elasticity for the fluid G. It is found that there are two characteristic non-dimensional factors for the problem, one being the Reynolds number referred to D0 and the other the Deborah number relating characteristic flow to flow time. Tests have been carried out to establish the effect of the characteristic parameters, using a probe mounted on a rotating arm and moving at an exactly known rate in a circular tank and transmitting the pressure data to an external receiver. The tests were carried out in a solution of polyethylene and water over a range of concentrations. The pressure difference Dp was found to vary as the reciprocal of D, and it was seen that, depending on probe tip geometry, it either passed through a maximum or reached a step as V0 increased. The static pressure measurements were carried out in with the same equipment and the same fluids. Circular cylinders of varying outside diameter and with varying pressure tapping orifice angular position θ on a generatrix were tested at Reynolds numbers ranging from 1.52 X 101 to 6.0 X 101. The results differed from Newtonian fluid data, for which mainly D0, V0 and θ are significant. From the analysis of the results it was suspected that the difference might be due to the elastic fluid properties. The main differences were lower measured pressures in the neighbourhood of the stagnation point and a downstream shift of the separation point. The measured pressures in the intermediate zone and in the wake of the measured pressures were very close to those associated with the flow of a Newtonian fluid around a similar cylinder with delayed separation.