LOCAL ISOTROPY IN COMPLEX TURBULENT BOUNDARY-LAYERS AT HIGH REYNOLDS-NUMBER

To continue our (Saddoughi & Veeravalli 1994) tests of the local-isotr opy predictions of Kolmogorov's (1941) universal equilibrium theory in shear flows, we have taken hot-wire measurements of the velocity fluc tuations in complex turbulent boundary layers at several Reynolds numb ers. We have studied the plane-of-symmetry flow upstream of a 4 ft dia meter, 6 ft long circular cylinder placed with its axis vertical in th e zero-pressure-gradient turbulent boundary layer of the test-section ceiling in the 80 ft x 120 ft Full-Scale Aerodynamics Facility at NASA Ames Research Center. In the present experiments, the pressure rises strongly as the obstacle is approached and in and near the plane of sy mmetry of the flow the boundary layer is influenced by the effects of lateral divergence. In addition to the basic mean shear, partial deriv ative U/partial derivative y, the extra mean strain rates are partial derivative U/partial derivative x, partial derivative V/partial deriva tive y and partial derivative W/partial derivative z. During our exper iments a full-scale F-18 fighter aircraft, set at an angle of attack o f 50 degrees, was present in the central region of the working section . To identify the effects of the aircraft on the boundary-layer charac teristics upstream of the cylinder, we have also taken measurements wh en the wind tunnel was empty. It appears that the presence of the airc raft in the wind tunnel usefully increases the magnitude of the mean s train rates, and also significantly increases the large-scale intermit tency near the edge of the boundary layer upstream of the cylinder. Th e maximum values for the parameters that have been found to represent the effects of mean shear on turbulence are S(= Sq(2)/epsilon) approx imate to 22 and S-c(= S(nu/epsilon)(1/2)) approximate to 0.05, where for the present experiments S = 2(s(ij)s(ij)/2)(1/2). All of the prese nt results are compared with our plane turbulent boundary-layer experi ments (Saddoughi & Veeravalli 1994). In the present distorted boundary -layer cases, the maximum Reynolds numbers based on momentum thickness , R-theta, and on the Taylor (1935) microscale, R-lambda, are increase d to approximately 510 000 and 2000 respectively. These are the larges t attained in laboratory boundary-layer flows: R-theta is of the same order obtained in flight on a typical commercial aircraft or the space shuttle. In general, the current investigations confirm the conclusio ns of our earlier study. In summary, it is shown again that one decade of locally isotropic inertial subrange requires a ratio of the Kolmog orov to mean-shear timescales, S-c, of not more than approximately 0. 01. In the present non-equilibrium shear layer, this was achieved at a microscale Reynolds number of approximately 2000.