Temperature dissipation fluctuations in a turbulent boundary layer

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.