On the structure of acceleration in turbulence

Acceleration and spatial velocity gradients are obtained simultaneously in an isotropic turbulent flow via three dimensional particle tracking velocimetry. We observe two distinct populations of intense acceleration events: one in flow regions of strong strain and another in regions of strong vorticity. Geometrical alignments with respect to vorticity vector and to the strain eigenvectors, curvature of Lagrangian trajectories and of streamlines for total acceleration, a=Du/Dt and for its convective part, ac=(u⋅∇)u, are studied in detail. We discriminate the alignment features of total and convective acceleration statistics, which are genuine features of turbulent nature from those of kinematic nature. We find pronounced alignment of acceleration with vorticity. Similarly, a and especially ac are predominantly aligned at 45°with the most stretching and compressing eigenvectors of the rate of the strain tensor, λ1, and λ3, respectively. Via autocorrelation functions of acceleration, conditioned on preferential directions, the vorticity vector field is found to play an important role as an ordering reference axis for acceleration orientation. Associating a velocity–acceleration structure function with an energy flux gives a clear indication that a strong energy flux occurs via compression in strain dominated events and via stretching in vorticity dominated events.

► Acceleration structure is revealed in respect to the local spatial velocity gradients.
► Genuine turbulence properties are distinguished from the kinematics of velocity gradients.
► High acceleration events are probable due to strong vorticity and due to strong rate of strain events.
► “Vortex trapping” and “jet colliding” are comparable in creating strong acceleration events.
► 3D-PTV is substantially improved to reach the high fidelity data on spatial/temporal velocity gradients.