Analysis of the Fractal Dimension of the Edge Boundary of a Helium Plume in Air During Laminar to Turbulent Transition

Schlieren imaging was used to observe the behavior of a metered helium plume as it transitioned from laminar to turbulent flow in an air environment. A high-speed camera was used to record the plume's behavior, edge position, and shape during transition. Jet Reynolds numbers were calculated for the plume as it transitioned from laminar to turbulent flow. Trends in the fractal behavior of the density fluctuations of the plume and edge characteristics were studied during transition as a function of Reynolds number. The spatially resolved fractal dimension of the density contours visualized in schlieren was estimated using pointwise mass methods, while the whole flow fractal dimension was estimated using box counting. The results gathered from the experimentation were used to validate estimates of turbulent kinetic energy (TKE) from correlated image velocimetry (CIV) based velocity extraction. CIV results for TKE in jet flow agree with jet behaviors in literature from CFD. Uncertainty in the fractal measurement due to limited image resolution prevents drawing strong conclusions on the spatial variation.