Publication Date:
1997-10-10
Description:
To continue our (Saddoughi & Veeravalli 1994) tests of the local-isotropy predictions of Kolmogorov's (1941) universal equilibrium theory in shear flows, we have taken hot-wire measurements of the velocity fluctuations in complex turbulent boundary layers at several Reynolds numbers. We have studied the plane-of-symmetry flow upstream of a 4 ft diameter, 6 ft long circular cylinder placed with its axis vertical in the zero-pressure-gradient turbulent boundary layer of the test-section ceiling in the 80 ft × 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 symmetry of the flow the boundary layer is influenced by the effects of lateral divergence. In addition to the basic mean shear, ∂U/∂y, the extra mean strain rates are ∂U/∂x, ∂V/∂y and ∂W/∂z. During our experiments a full-scale F-18 fighter aircraft, set at an angle of attack of 50°, was present in the central region of the working section. To identify the effects of the aircraft on the boundary-layer characteristics upstream of the cylinder, we have also taken measurements when the wind tunnel was empty. It appears that the presence of the aircraft in the wind tunnel usefully increases the magnitude of the mean strain rates, and also significantly increases the large-scale intermittency near the edge of the boundary layer upstream of the cylinder. The maximum values for the parameters that have been found to represent the effects of mean shear on turbulence are S*(≡ Sq2/ε) ≈ 22 and S*c(≡ S(ν/ε)1/2) ≈ 0.05, where for the present experiments S ≡ 2(SijSij/2)1/2. All of the present results are compared with our plane turbulent boundary-layer experiments (Saddoughi & Veeravalli 1994). In the present distorted boundary-layer cases, the maximum Reynolds numbers based on momentum thickness, Rθ, and on the Taylor (1935) microscale, Rλ, are increased to approximately 510000 and 2000 respectively. These are the largest attained in laboratory boundary-layer flows: Rθ is of the same order obtained in flight on a typical commercial aircraft or the space shuttle. In general, the current investigations confirm the conclusions of our earlier study. In summary, it is shown again that one decade of locally isotropic inertial subrange requires a ratio of the Kolmogorov 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.
Print ISSN:
0022-1120
Electronic ISSN:
1469-7645
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
,
Physics
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