Gradient Richardson number measurements in a stratified shear layer

This paper presents instantaneous local gradient Richardson number Ri(g)(t) measurements in a stratified shear layer using a novel laser-Doppler anemometer and conductivity probe assembly with a resolution of Delta z = 0.27 cm, The aim was to study the dependence of Ri(g)(t) on the bulk Richardson number Ri(o). The shear layer was established between two co-flowing streams of different densities and velocities, and the motion field within the shear layer allowed the development of Kelvin-Helmholtz (K-H) instabilities, internal waves and turbulence. Ri(g)(t) was also measured at lesser resolutions (Delta z > 1.8 cm) using conventional measurement techniques. Although the mean background flow was quasi-steady, Ri(g)(t) was highly time dependent due to the variable internal strain field induced by the combined effect of instabilities, waves and turbulence. When K-H instabilities were present, the time-averaged gradient Richardson number <(Ri(g))over bar> (Delta z = 0.27 cm) was approximately a constant 0.06 +/- 0.02, irrespective of Ri(o). When K-H instabilities were absent, <(Ri(g))over bar> (Delta z = 0.27 cm) assumed larger values that are dependent on Ri(o). <(Ri(g))over bar> (Delta z > 1.8 cm) was always found to be dependent on Delta z and <(Ri(o))over bar>. It is argued that <(Ri(g))over bar> should be measured with a resolution better than the scale of density overturns to properly account for vertical small-scale processes of the stratified shear layer. The measurements are consistent with the notion that when Ri(o) < 10 or so the energy supplied to a sheer layer at large scales can be dissipated at smaller scales by the turbulence associated with the breakdown of K-H instabilities.