The effects of suspended sediment concentration on turbulence in an annular flume

Abstract

The effect of suspended sediment concentrations (SSC) on fluid turbulence in an annular flume was investigated. Flow speed was held constant at 0.57 m s−1, and the resulting turbulent conditions were recorded using a 3-D Acoustic Doppler Velocimeter (ADV) at height (z) of 8.5 cm above the bed. The suspended material was composed of a natural glacial clay made up of particles smaller than 6 μm. The SSC in the flume were increased from clear water to 4800 mg l−1 in nine discrete increments; temporal variations of SSC were monitored using three optical backscatter sensors (OBS) mounted in the flume wall at heights of 0.03, 0.10 and 0.20 m above the flume base. The results showed that turbulent intensity ( $$\overline { {q}^{2}}/ \overline {U} ^{2}$$ ) and energy dissipation rate (ɛ) did not change significantly between clear water and 200 mg l−1, but decreased by nearly 30% in the SSC range between 200 and 2400 mg l−1. Above 2400 mg l−1, no further decrease was observed. Analyses of the velocity variances over narrow frequency bands (0.2 Hz wide) from 0 to 12.5 Hz showed that most of the flow turbulent energy (~70–80%) was contained within the lower frequencies i.e. larger eddies, and that these eddies experienced the greatest decrease in energy due to turbidity. It is proposed that these patterns are the consequence of the increase in suspended sediment concentrations and of the vertical stratification of sediments for SSC >200 mg l−1.