Drag reduction in fibre suspensions: transitional behaviour due to fibre degradation

Abstract

IT is known that the addition of small amounts of certain polymers to a fluid can produce spectacular reductions in turbulent friction^1,2. It is also possible to reduce friction by suspending macroscopic fibres in a fluid³. Superficially there are many similarities between the two effects but they are believed to involve different mechanisms because, for example, using polymers and fibres together can produce much larger reductions in drag than would be the case with either additive on its own⁴. We report here measurements of turbulent velocities in a fibre suspension, using laser–Doppler anemometry. We found that the effect of fibres on turbulent structure was not the same as that of polymers. However, when the fibre was degraded by passing it through the system several times, there was a transition to the type of structural change found in polymer solutions.