Spectral and Structure Function Estimates of Turbulence Dissipation Rates in a High-Flow Tidal Channel Using Broadband ADCPs
| Autor: | Justine M. McMillan, Alex E. Hay |
|---|---|
| Rok vydání: | 2017 |
| Předmět: |
Physics
Atmospheric Science 010504 meteorology & atmospheric sciences Meteorology Turbulence Physical constant 020209 energy Flow (psychology) Phase (waves) Ocean Engineering 02 engineering and technology Dissipation 01 natural sciences Computational physics Shear (sheet metal) Acoustic Doppler current profiler 0202 electrical engineering electronic engineering information engineering 0105 earth and related environmental sciences Communication channel |
| Zdroj: | Journal of Atmospheric and Oceanic Technology. 34:5-20 |
| ISSN: | 1520-0426 0739-0572 |
| DOI: | 10.1175/jtech-d-16-0131.1 |
| Popis: | Spectral and structure function methods are implemented to compute the dissipation rate ε from broadband, diverging-beam acoustic Doppler current profiler (ADCP) data collected at four sites in a high-flow tidal channel. This paper shows that middepth estimates of ε obtained from spectral and second-order structure function (SF2) methods are both lognormally distributed with comparable means and variances. Speed bin–averaged ε values agree to within 16%, depending on the site and tidal phase (ebb/flood). The close agreement between the two independent methods provides further support for the argument put forward by McMillan et al.: that is, that the factor-of-2 difference between shear probe and (spectral) ADCP estimates of ε was likely caused by spatial differences in turbulence levels. The agreement between the spectral and both second- and third-order structure function methods also supports the use of for the SF2 universal constant. Notably, however, the SF3 method was less robust for these data. Two additional aspects of the SF2 approach are examined in some detail: 1) the differences from upstream- and downstream-facing beams are shown to arise from the Reynolds stress and 2) the inability of the ADCP to resolve small-scale motions does not affect the estimates of ε but yields apparent Doppler noise levels that—counterintuitively—decrease with increasing flow speed and increasing dissipation rate. A modified SF2 method that accounts for the variance associated with the unresolved scales removes the flow speed dependence and yields noise level estimates that agree with the spectral values. |
| Databáze: | OpenAIRE |
| Externí odkaz: |
|