Relationships between Large-Scale Coherent Motions and Bursting Events in a Turbulent Boundary Layer

Abstract

A turbulent boundary layer (TBL) over a flat rigid surface can be seen in many industrial and environmental flows. Therefore, for the prediction and control of turbulence and design of better industrial apparatus, it is of great importance to elucidate the spatiotemporal structure of the TBL. In TBLs, it is well known that coherent motions associated with the bursting events exist near the wall. In the outer turbulent/nonturbulent (potential) interface of the TBL, large-scale coherent motions exist, which have the size of order of the boundarylayer thickness. The latter structures are called `bulge' and `valley' and are unsteady in space and time (Robinson 1991). Therefore, it is of importance to clarify the spatiotemporal structures of these coherent motions in order to elucidate the structure of the TBL. However, it is difficult to study the entire structure of the TBL by a single-point measurement using an Ior X-type hot-wire probe. In addition, previous studies (Ichijo & Kobashi 1982, Kobashi et al. 1984, Thomas & Bull 1983) have suggested that these coherent motions are closely associated with wall static pressure fluctuations. Therefore, the simultaneous measurement of multipoint instantaneous velocities and wall static pressure fluctuations is appropriate for clarifying the spatiotemporal structure of the TBL. In this chapter, we will show the experimental results on the relationships between the bursting events occurring near the wall and the large-scale bulge and valley structures in the outer turbulent/nonturbulent interface of the TBL, and the spatiotemporal structures of the coherent motions in a zero-pressure-gradient TBL. With regard to the experiments, multipoint instantaneous streamwise velocities and instantaneous wall static pressures are simultaneously measured using a combination of a rake consisting of 23 I-type hot-wire probes, which covers the entire TBL, and a microphone pressure sensor. The velocity signals are analysed by the KL (Karhunen-Loeve) expansion (also known as the proper orthogonal decomposition (POD) method). The flow field is reconstructed by using either loweror higher-order modes to investigate the coherent motions in the TBL. The bursting events are detected by applying the VITA (Variable Interval Time Average) technique to the instantaneous velocity signals in the original flow. The large-scale bulge and valley structures are detected by using the newly proposed conditional sampling method, which is applied to both the original flow and the reconstructed flow by using the KL expansion. Further, to show the future research direction, the experimental data on the statistical