Rotation of the velocity measurements
To estimate [math]\displaystyle{ \varepsilon }[/math] from all the different velocity components, the measurements must be rotated into the main direction of the flow. In some instances, the instrument's frame of reference may be aligned with the direction of flow, which is ideal to account for the varying levels of anisotropy amongst components [1][2]. If this isn't the case, then the velocities' measurement frame must be rotated into that of the flow, which we refer to as the analysis frame of reference.
Methods used for rotating into the analysis frame of reference
If one intends on using only the vertical velocity component to estimate [math]\displaystyle{ \varepsilon }[/math], then the rotation of the velocity measurements into a new frame of reference may be skipped. The analysis frame of reference is thus the same as the measurement frame provided one direction is aligned with gravity. However, anisotropic velocity spectra caused when the largest turbulence scales are less than XX times the Kolmogorov length scale, may inhibit using the vertical velocity component to derive [math]\displaystyle{ \varepsilon }[/math]. In these situations, it may be possible to use the longitudinal velocity component (see Bluteau et al (2011)[2]), which requires the user to rotate the velocity in the direction of mean flow.
We will update when our final recommendation is set in stone. Also, comment about large vertical velocities on sloped bottoms... The page is too wordy
- Using time-averaged velocities in each segment
- Principal component analysis
References
- ↑ A. E. Gargett, T. R. Osborn and and P.W. Nasmyth. 1984. Local isotropy and the decay of turbulence in a stratified fluid. J. Fluid. Mech.. doi:10.1017/S0022112084001592
- ↑ 2.0 2.1 C.E. Bluteau, N.L. Jones and and G. Ivey. 2011. Estimating turbulent kinetic energy dissipation using the inertial subrange method in environmental flows. Limnol. Oceanogr.: Methods. doi:10:4319/lom.2011.9.302
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