University of Minnesota
Aerospace Engineering and Mechanics
Fall 1997 Seminar Series



Genesis, dynamics and control of near-wall coherent structures in a fully turbulent boundary layer


Professor Fazle Hussain
Department of Mechanical Engineering
University of Houston


Abstract


It is now well established that the enhanced drag and heat transfer of turbulent boundary layers is dominated by slender longitudinal vortices immediately near the wall. The geometry and dynamics of near-wall vortices are not well understood and often controversial. We educe coherent structures (CS) near the wall from a numerically simulated turbulent channel flow using a conditional scheme which extracts the entire extent of locally dominant vortical structures. Such structures are detected from the instantaneous flow field using our newly developed vortex definition - a region of negative lambda2 , the second largest eigenvalue of the tensor Sik Skj + omegaik omegajk - which accurately captures the structure details, unlike velocity, vorticity or pressure-based eduction schemes. Extensive testing shows that lambda2 correctly captures vortical structures, even in the presence of strong shear occurring near the wall of a boundary layer. Our conceptual model of the CS array reproduces experimentally observed important events reported in the literature, such as VITA, gradient and counter-gradient Reynolds stress distributions (Q1, Q2, Q3 and Q4 and their relative contributions), wall pressure variation, elongated low-speed streaks, spanwise shear, etc. Notably, the often heralded hairpin vortices, not to be confused with hairpin-shaped vortex line bundles, are absent both in the instantaneous and ensemble-averaged fields.

Further, we have discovered a new primary instability (of low-speed "streaks"), responsible for near-wall CS formation in fully turbulent boundary layers. A two-dimensional analytical base flow, containing no x-dependence or streamwise vorticity - modeling the instantaneous flow during quiescent phase- is considered. This flow is stable to varicose instability but unstable to sinuous instability which is inviscid in nature and resembles oblique mode instability in a mixing layer. The vortical structure resulting from the nonlinear phase of this instability agrees with the CS educed from DNS of fully turbulent flow.

Based on this discovery, we have developed effective new control approaches for turbulent boundary layers, via large-scale streak manipulation, which exploit the crucial role of streaks in vortex generation and hence drag. Using control flows with no x variation, a spanwise wavelength of 400 wall units, and an amplitude of only 5% of the channel centerline velocity, we find a significant sustained drag reduction : 20% for imposed counterrotating streamwise swirls and 50% for colliding spanwise wall jet-like forcing. These results suggest promising new drag reduction strategies, e.g. passive vortex generators and spanwise jets from x-aligned slots, involving large-scale (hence more durable) actuation and requiring no wall sensors or feedback logic.


Friday, October 10, 1997
209 Akerman Hall
2:30-3:30 p.m.


Refreshments served after the seminar in 227 Akerman Hall.
Disability accomodations provided upon request.
Contact Audrey Stark-Evers, Senior Secretary, 625-8000.