An experimental investigation of the relative diffusion of particle pairs in three-dimensional turbulent flow

The particle tracking (PT) technique is used to study turbulent diffusion of particle pairs in a three-dimensional turbulent flow generated by two oscillating grids. The experimental data show a range where the Richardson-Obukhov law [r(2)] = C epsilont(3) is satisfied, and the Richardson-Obukhov constant is found to be C = 0.5. A number of models predict much larger values. Furthermore, the distance-neighbour function is studied in detail in order to determine its general shape. The results are compared with the predictions of three models: Richardson (1926), Batchelor (1952) and Kraichnan (1966a). These three models predict different behaviours of the distance-neighbour function, and of the three, only Richardson's model is found to be consistent with the measurements. We have corrected a minor error in Kraichnan's (1996a) Lagrangian history direct interaction calculations with the result that we had to increase his theoretical value from C = 2.42 to C = 5.5.