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Understanding the interactions between turbulent and non-turbulent motions has been a persistent challenge faced by the community studying stably stratified turbulent flows. For flows with high Reynolds number, high Rossby number, and stable stratifications, non-turbulent motions share a common characteristic to involve physical mechanisms acting against instability development. Because turbulence is generated through energy cascade via instability development, the presence of non-turbulent motions is expected to modify the energy distribution across scales compared to that of solely turbulent motions. The objective of this work is to identify statistical signals of non-turbulent motions caused by stable stratification. The need to resolve energy-containing motions in both space and time requires high-frequency time series of velocity fluctuations collected using arrays of sonic anemometers. The analysis is performed using data from the Canopy Horizontal Array Turbulence Study (CHATS), during which a total of 31 sonic anemometers were deployed on a horizontal array and on a 30-m tower. Compared to other field campaigns which were also equipped with arrays of sonic anemometers, CHATS took an important advantage of already published nighttime canopy-scale waves derived from aerosol backscatter lidar images. After precluding complexities caused by nonstationarity and horizontal heterogeneity, signals of non-turbulent motions caused by stable stratification are identified from spatial autocorrelations of time-block-averaged velocity fluctuations. These signals agree with existing understanding of turbulent canopy flows and two-dimensional Kelvin-Helmholtz instability development, which predicts a critical wavelength at which motions shift from free instability growth to internal gravity waves. The estimates of critical wavelengths and buoyancy periods agree well with the overall properties of nighttime canopy-scale waves derived from lidar images.
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Homogeneous turbulence in a random-jet-stirred tank
We report an investigation into random-jet-stirred homogeneous turbulence generated in a vertical octagonal prism-shaped tank where there are jet arrays on four of the eight vertical faces. We show that the turbulence is homogeneous at all scales in the central region of the tank that span multiple integral scales in all directions. The jet forcing from four sides in the horizontal direction guarantees isotropy in horizontal planes but leads to more energy in the horizontal fluctuations com- pared with the vertical fluctuations. This anisotropy between the horizontal and vertical fluctuations decreases at smaller scales, so that the inertial and dissipation range statistics show isotropic behavior. Using four jet arrays allows us to achieve higher turbulence intensity and Reynolds number with a shorter jet merging distance compared to previous facilities with two-facing arrays. By changing the array-to-array distance, the parameters of the algorithm that drives random-jet stirring, and attachments to the exits of each jet, we show that we are able to vary the turbulence scales and Reynolds number. We provide scaling relations for the turbulent fluctuation velocity, integral scale, and dissipation rate, and we show how these scales of motion are primarily determined by the properties of individual jets and the diffusion of their momentum with distance from the nozzles. Finally, we examine the signatures of individual jets in the turbulent velocity spectra and report the conditions under which individual jet flows, not fully mixed with the background turbulence, produce a spectral peak and the corresponding frequency associated with the jet forcing timescale.
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- Award ID(s):
- 2211704
- PAR ID:
- 10497876
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Experiments in Fluids
- Volume:
- 64
- ISSN:
- 0723-4864
- Page Range / eLocation ID:
- 185
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1007/s00348-023-03721-9
