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Large-eddy simulation was used to model turbulent atmospheric surface layer (ASL) flow over canopies composed of streamwise-aligned rows of synthetic trees of height,$$h$$, and systematically arranged to quantify the response to variable streamwise spacing,$$\delta _1$$, and spanwise spacing,$$\delta _2$$, between adjacent trees. The response to spanwise and streamwise heterogeneity has, indeed, been the topic of a sustained research effort: the former resulting in formation of Reynolds-averaged counter-rotating secondary cells, the latter associated with the$$k$$- and$$d$$-type response. No study has addressed the confluence of both, and results herein show secondary flow polarity reversal across ‘critical’ values of$$\delta _1$$and$$\delta _2$$. For$$\delta _2/\delta \lesssim 1$$and$$\gtrsim 2$$, where$$\delta$$is the flow depth, the counter-rotating secondary cells are aligned such that upwelling and downwelling, respectively, occurs above the elements. The streamwise spacing$$\delta _1$$regulates this transition, with secondary cell reversal occurring first for the largest$$k$$-type cases, as elevated turbulence production within the canopy necessitates entrainment of fluid from aloft. The results are interpreted through the lens of a benchmark prognostic closure for effective aerodynamic roughness,$$z_{0,{Eff.}} = \alpha \sigma _h$$, where$$\alpha$$is a proportionality constant and$$\sigma _h$$is height root mean square. We report$$\alpha \approx 10^{-1}$$, the value reported over many decades for a broad range of rough surfaces, for$$k$$-type cases at small$$\delta _2$$, whereas the transition to$$d$$-type arrangements necessitates larger$$\delta _2$$. Though preliminary, results highlight the non-trivial response to variation of streamwise and spanwise spacing.
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Pore-resolved investigation of turbulent open channel flow over a randomly packed permeable sediment bed
Pore-resolved direct numerical simulations are performed to investigate the interactions between streamflow turbulence and groundwater flow through a randomly packed porous sediment bed for three permeability Reynolds numbers,$$Re_K=2.56$$, 5.17 and 8.94, representative of natural stream or river systems. Time–space averaging is used to quantify the Reynolds stress, form-induced stress, mean flow and shear penetration depths, and mixing length at the sediment–water interface (SWI). The mean flow and shear penetration depths increase with$$Re_K$$and are found to be nonlinear functions of non-dimensional permeability. The peaks and significant values of the Reynolds stresses, form-induced stresses, and pressure variations are shown to occur in the top layer of the bed, which is also confirmed by conducting simulations of just the top layer as roughness elements over an impermeable wall. The probability distribution functions (p.d.f.s) of normalized local bed stress are found to collapse for all Reynolds numbers, and their root-mean-square fluctuations are assumed to follow logarithmic correlations. The fluctuations in local bed stress and resultant drag and lift forces on sediment grains are mainly a result of the top layer; their p.d.f.s are symmetric with heavy tails, and can be well represented by a non-Gaussian model fit. The bed stress statistics and the pressure data at the SWI potentially can be used in providing better boundary conditions in modelling of incipient motion and reach-scale transport in the hyporheic zone.
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- Award ID(s):
- 2053248
- PAR ID:
- 10502124
- Publisher / Repository:
- Journal of Fluid Mechanics
- Date Published:
- Journal Name:
- Journal of Fluid Mechanics
- Volume:
- 971
- ISSN:
- 0022-1120
- Subject(s) / Keyword(s):
- Turbulent Flows: Turbulent boundary layers Turbulent Flows: Turbulence simulation
- 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/10.1017/jfm.2023.636
