DNS investigation of the dynamical behaviour of trailing vortices in unbaffled stirred vessels at transitional Reynolds numbers

Flow in an unbaffled stirred vessel agitated by a four-bladed radial impeller is investigated by using direct numerical simulations at Re = 320 and 1600. We observe fluctuations in the power consumption with a peak frequency at ca. three times the impeller rotational speed for both Reynolds numbers. It is discovered that these fluctuations are associated with a periodic event in the wake of the blades, which involves alternating growth and decay of the upper and lower cores of the trailing vortex pair as well as up-and-down swinging motion of the radial jet. Moreover, the phase relation between the wakes of the different blades is examined in detail. Further studies using fractal-shaped blades show that the exact blade shape does not have a strong influence on this phenomenon. However, the wake interaction between the blades, hence the number of blades, has a direct influence on the unsteadiness of trailing vortices.