Experimental Study on Relationships between Large-scale Coherent Motions and Bursting Events in a Turbulent Boundary Layer

Abstract

The relationships between large-scale bulge and valley structures and bursting events in a zero-pressure-gradient turbulent boundary layer (TBL) are investigated by means of laboratory experiments carried out in a wind tunnel. To investigate the spatiotemporal structures of coherent motions in a TBL, multipoint instantaneous streamwise velocities and instantaneous wall static pressures are simultaneously measured using a combination of a rake of 23 I-type hot-wire probes, which covers entire TBL, and a microphone pressure sensor. The KL (Karhunen-Loève) expansion is applied to the measured velocity signals. The flow field is reconstructed by using lower- or 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 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. The results show that at lower modes, only the large-scale motions rotating against the mean shear stress accompany the bursting events. At higher modes, however, the large-scale motions are found to be poorly related to the bursting events.

Keywords

• boundary layer
• turbulent flow
• flow measurements
• coherent motion
• wall static pressure fluctuation