PIPE-FLOW OF A THIXOTROPIC LIQUID

Detailed measurements of mean velocity and velocity fluctuation levels (axial, tangential and radial) have been carried out using a laser Do ppler anemometer for fully developed pipe flow of an aqueous solution of Laponite, a synthetic clay. The equilibrium rheological structure o f this thixotropic liquid is well characterised by the Herschel-Bulkle y model. Velocity profiles calculated for a Herschel-Bulkley fluid pro ve to be a very accurate representation of the measurements for lamina r flow at Reynolds numbers below about 1500. The measured profiles dev elop an unexplained asymmetry for higher Reynolds numbers until the fl ow undergoes transition to turbulence. The fluid is drag reducing unde r turbulent flow conditions with relative levels of tangential and rad ial turbulence intensity suppressed in comparison with water whilst th e axial turbulence intensity is little different. Under all flow condi tions it is evident that the fluid rheology is far from structural equ ilibrium, with values for the apparent yield stress and effective visc osity determined from near-wall velocity measurements considerably bel ow those obtained from a rheometer.