Use of flow visualization data to examine spatial-temporal velocity and burst-type characteristics in a turbulent boundary layer

Abstract

It is well known that turbulence production in turbulent boundary layers occurs in short, energetic bursts; however, the relationship between the results of pointwise burst detection techniques and the spatial flow structure associated with such burst-type events has not been clearly established. To address this point, a study using VITA detection of burst-type events was done which allows the direct comparison between flow visualization results and quantitative, temporal velocity profile data for a flat-plate turbulent boundary layer. Using automated image processing of hydrogen-bubble flow visualization pictures, temporal velocity profile data are established using a corrected time-of-flight technique for velocity extraction. Using the visualization-derived data, spatial-temporal velocity-derived properties (∂ u /∂ y , ∂ u /∂ t , etc.), as well as probe-type burst detection properties are established. In addition, temporal and ensemble-averaged burst-type characteristics are shown to be essentially identical to previous VITA-detected velocity probe results. The VITA approach is extended to establish the spatial extent of burst-type events and the ensemble-averaged spatial-temporal properties associated with VITA-based detection. By use of a regionalized detection procedure, the types of burst-type patterns are categorized and compared with the associated visualization sequences.