Turbulent mixing in a meandering non-buoyant chemical plume is far less understood than in a straight plume – partially due to difficulty separating the plume meander from the turbulent fluctuations. This study presents simultaneous particle image/tracking velocimetry (PIV/PTV) and laser-induced fluorescence (LIF) measurements of a phase-locked meandering chemical plume, the motion of which is forced by the periodic oscillation of a diverting plate. The plume evolves in a turbulent boundary layer in a moderate-Reynolds-number open channel flow.
Don Webster,
Georgia Institute of Technology
For the meandering plume, the centerline phase-averaged concentration decreases more rapidly with downstream distance and the plume width increases more rapidly with downstream distance (as ) compared to the straight plume (as ). Furthermore, the concentration fields and transverse profiles are asymmetric about the plume centerline in the meandering plume. Nevertheless, the transverse profiles can be modeled by a Gaussian shape in a segmented manner. The velocity fields indicate that the large-scale alternating-sign vortices induced by the diverting plate are the dominant feature of the flow. The vortices induce the plume to meander and govern the spatial distribution of the phase-averaged concentration.
Further, a phenomenological model of chemical filament transport by the vortical motion explains local peaks in the phase-averaged concentration along the plume centerline. Analysis of the covariance of the turbulent fluctuations of velocity and concentration, i.e. the turbulent flux, reveals that the spatial distribution of the turbulent quantities is governed by the large-scale alternating-sign vortices that induce the plume meander. The spatial variation of turbulent flux agrees well with the spatial variation of the phase-averaged concentration gradient. As a result, the eddy diffusivity framework effectively models the turbulent flux. As expected from turbulent mixing theory, the eddy diffusivity coefficient plateaus at a constant value once the plume width reaches the size of the largest eddies. However, when the plume width is less than the size of the largest eddies, the eddy-diffusivity coefficient scales with the plume width to the = 1 power.
Donald Webster, Ph.D., P.E. is the Karen & John Huff School Chair and Professor in the School of Civil & Environmental Engineering (CEE) at the Georgia Institute of Technology in Atlanta, Georgia. Dr. Webster earned a B.S. from the University of California, Davis (1989), and M.S. (1991) and Ph.D. (1994) degrees from the University of California, Berkeley. He joined the Georgia Tech faculty in September 1997 after completing a postdoctoral research appointment at Stanford University and holding a non-tenure-track faculty position at the University of Minnesota.
Dr. Webster’s research expertise lies in environmental fluid mechanics focused on the influence of fluid motion and turbulence on biological systems. His contributions have been in three arenas: 1) illuminating the fluid mechanics related to sensory biology and biomechanics; 2) developing advanced experimental techniques and facilities; and 3) translating research results into bio-inspired design. In recognition of these contributions, Dr. Webster is a Sustaining Fellow of the Association for the Sciences of Limnology and Oceanography (ASLO) and Fellow of the American Society of Civil Engineers (ASCE). He has won numerous awards including the Felton Jenkins, Jr. Hall of Fame Faculty Award, Class of 1934 Outstanding Innovative Use of Education Technology Award, the Eichholz Faculty Teaching Award, and the British Petroleum Junior Faculty Teaching Excellence Award.