Modulation of wall-attached-eddy inclination angle with the streamwise momentum induced by Very-Large-Scale Motions: A LiDAR experiment for the neutral Atmospheric Surface Layer

Coherent structures identified in turbulent boundary layers are typically characterized by a forward-leaning morphology. Experimental evidences have shown typical inclination angles of and for wall-attached eddies and Very-Large-Scale Motion (VLSMs, arising from the concatenation of wall-attached eddies), respectively, across a wide range of Reynolds numbers. However, these values are assumed as mean estimates, while the local flow dynamics that dictates the structure inclination angle are still unclear. In this work, we hypothesize that the alternance of high and low streamwise momentum induced by VLSMs can affect the local inclination angle of the underlying attached eddies, specifically reducing or increasing it, respectively. To test this hypothesis, an Atmospheric Surface Layer (ASL) flow is probed with a scanning wind light detection and ranging (LiDAR), while a simultaneous and co-located Super Large Particle Image Velocimetry (SLPIV) is utilized to probe the attached eddies closer to the ground. The positive/negative VLSM velocity fluctuations are identified to calculate conditional statistics for the attached-eddy inclination angle. The latter shows significantly different values, approximately and , in presence of either high or low streamwise momentum associated with the VLSM modulation, respectively. The combined SLPIV and LiDAR measurements corroborate that the inclination angle of wall-attached eddies is modulated by the streamwise momentum induced by VLSMs.