Jamey Jacob

Dynamics of corotating vortex pairs in the wakes of flapped airfoils

The behaviour of a pair of corotating vortices in the wake of a flapped airfoil is experimentally studied in a water towing tank. Reynolds numbers based on total circulation of the vortices range from 1.0 × 10 4 to 6.4 × 10 4 . Planar velocity vector fields and their gradients are derived from PIV images using an adaptive Lagrangian parcel tracking algorithm. Isovorticity surfaces are extracted from time series of planar vorticity data. The behaviour of the vortices is tracked by using various moments of both the probability density distribution and the spatial distribution of their streamwise vorticity. All vortices show a Lamb-Oseen circulation distribution when they are clearly identifiable. Further, vortices from the flapless wing exhibit Lamb-Oseen velocity and vorticity distributions with slow growth. All corotating vortex pairs are observed to merge at about 0.8 orbit periods. First-order statistics of the flow field remain invariant during the merger. The higher-order moments of the vorticity distribution show strong time dependence, which implies three-dimensionality of the flow resulting from vortex stretching

Trailing Vortex Wake Growth Characteristics of a High Aspect Ratio Rectangular Airfoil

The growth characteristics of trailing vortex wakes are investigated in a towing tank. The wakes from a lifting rectangular NACA 0012 airfoil with an aspect ratio of 8 are studied up to 1400 chords downstream. The circulation Reynolds number ranged from 4 x 10 3 to 4 x 10 4 . Digital particle image velocimetry is used to measure the velocity field in the cross-stream plane from which vorticity is calculated. The vortex pair is observed to be very stable and long-lived, showing little decay in the circulation of the individual vortices. The vorticity, however, diffuses over time within the cores of the vortices.

Experimental evidence for intense vortical structures in grid turbulence

The vortical structure of homogeneous turbulence is the subject of this paper. Unsteady, three dimensional vorticity is the essential attribute of turbulence. The question we address here is the distribution of this vorticity. We know that turbulence exhibits strong intermittency. Is this intermittency intertwined with an intense vortical structure? Figure 1 shows an instantaneous cross-sectional view of the turbulent flow behind a grid. The figure summarizes our notion of what homogeneous isotropic turbulence is. There is a multitude of scales, seemingly randomly distributed. There is a multitude of structures, seemingly randomly oriented. And, when observed in sequence, the pictures are changing seemingly randomly in time. The question here is if there are basic building blocks in the flow and if so, what their structures might be. A common view is that there are coherent structures which ought to be the building blocks. As to what they are seems to depend on the flow. In particular, homogeneous isotropic turbulence, which is the most studied of turbulent flows (Batchelor [1]), presents the stiffest challenge for confirmation due to the lack of large-scale vortical structures like those in shear flows. In this regard, numerical studies are ahead of experimentation. Experimentally, the question translates first to identifying and second to quantifying the building block in figure 1.