An improved swirling- strength criterion for identifying spanwise vortices in wall turbulence

Swirling strength, (ci), is an effective vortex indicator in wall turbulence, and existing vortex extraction methods introduce normalisation of (ci) with its root mean square as the basis for defining a universal threshold. This study presents an improved criterion by normalising the vortex indicator for prograde and retrograde vortices, respectively, with its conditional mean. Finally, a single, universal threshold is selected for extracting prograde and retrograde vortices simultaneously. The improved method was compared with the previous method based on open channel flow data measured with particle image velocimetry. Results show that while the two methods are basically equivalent in extracting prograde vortices, the improved method extracts a higher fraction of retrograde vortices. The approach of separately normalising vortex indicators associated with prograde and retrograde vortices can also be applied to other vortex extraction criteria.