All three components of the dissipation rate of the fluctuating temperature θ are measured simultaneously in the inner region of a fully developed turbulent boundary layer at a moderate Reynolds number. Measurements are made with a probe of four cold wires consisting of two closely spaced parallel vertical wires mounted a small distance upstream of two closely spaced parallel horizontal wires. In the inner region of the layer, local isotropy is not closely approximated [(∂θ/θz) 2 >(∂ θ/∂y) 2 >(∂θ/∂x) 2]. The spectral density of the sum χ[ = (∂θ/∂x) 2 + (∂θ/∂y) 2 +(∂θ/∂z) 2] is similar in shape to that of (∂θ/∂y) 2 or (∂θ/∂z) 2 , but not as rich in high frequency content as that of (∂θ/∂x) 2. The probability density of χ has a lower skewness and flatness factor and is more closely log-normal than those of the individual components. This is true regardless of whether χ and its components are unaveraged or locally averaged over a linear dimension r. When averaging is applied, departures from log-normality are diminished but do not disappear entirely. The variance σ 2 of the logarithm of the locally averaged χ is proportional to 1n r over a wide range of r (r max/r min≃30), in contrast to the individual components where this ratio may be as small as 2. The value of the Kolmogoroff constant μ θdetermined from the slope of σ 2 vs 1n r is about 0.35. This is consistent with the slope of the spectral density of χ and is also in agreement with previous best estimates of μ θ(and μ) obtained at high Reynolds numbers.
ASJC Scopus subject areas
- Condensed Matter Physics
- General Physics and Astronomy
- Mechanics of Materials
- Computational Mechanics
- Fluid Flow and Transfer Processes