Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows
Abstract
:1. Introduction
2. Materials and Methods
2.1. The Hydrodynamic Forcing
2.2. Bed Morphology
2.3. Near-Bed, Mean and Maximum Shear Stress
2.4. Turbulent Fluxes and Vertical Eddy Scales
2.5. Temporal Scales of Turbulent Motions and Active Periods of Flow
3. Results
3.1. Flow Properties and the Incident Wave Field
3.2. Bed Morphology and Sediment Motion
3.3. Turbulent Flow Properties
3.3.1. Near-Bed Shear Stresses
3.3.2. Intermittent Coherent Motions
3.3.3. Vertical Turbulent Fluxes and Eddy Scales
3.3.4. Temporal Scales and Active Periods of the Flow
4. Discussion
5. Conclusions
- (1)
- Combined (2.5D) wave-current ripples were observed in all runs with comparable wave and current strengths, with intense suspensions at flow reversal that persist-longer in aligned flows.
- (2)
- Near-bed interactions outside of the theoretical combined flow boundary layer were not uniform throughout the outer flow region, with discernible difference in flow structure and momentum exchange at the two elevations.
- (3)
- The aligned flows are characterised by upward turbulent fluxes with Reynolds stresses produced throughout the outer layer, whilst opposing flows featured downward turbulent flux of eddies generated in the core flow.
- (4)
- Spatial scales of turbulence indicate similar vertical scaling of eddies with fluxes extending beyond the theoretical thickness of the combined layer, often imposed as a zero-flux boundary in numerical models. It follows that the oscillatory (combined wave-current) boundary must be thicker than predicted by commonly used combined flow models.
- (5)
- In aligned flow, active lower-frequency (wave-related) periods of flow are characterised by pairs of counter-rotating vortices implying longer-lived wall-aligned structures prevail. Opposing flows are characterised by shorter-lived intense stress-bearing structures, hinting at higher dissipation rates. Intense momentum exchanges (with vortex shedding inducing suspension clouds) and plug flow sediment transport conditions are observed when strong adverse currents correspond with the reversing wave motion within each cycle.
Author Contributions
Funding
Conflicts of Interest
References
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Test Run | (°) | (m) | (m) | (m) | (s) | ADV | (m) | (m/s) | ||
---|---|---|---|---|---|---|---|---|---|---|
Run W1 | 1.05 | 0 | -- | 0.18 * | 0.17 * | 0.18 * | 3.04 | ADV1 | 0.16 | 0.17 |
ADV2 | 0.09 | 0.14 | ||||||||
Run W2 | 1.55 | 0 | -- | 0.22 | 0.23 | 0.25 | 3.03 | ADV1 | 0.18 | 0.18 |
ADV2 | 0.12 | 0.17 | ||||||||
Run A1 | 1.05 | 0.48 | 0 | 0.17 * | 0.17 * | 0.23 * | 2.00 | ADV1 | 0.16 | 0.40 |
ADV2 | 0.07 | 0.31 | ||||||||
Run A2 | 1.05 | 0.48 | 0 | 0.17 * | 0.17 * | 0.20 * | 3.03 | ADV1 | 0.18 | 0.38 |
ADV2 | 0.07 | 0.29 | ||||||||
Run A3 | 1.55 | 0.50 | 0 | 0.20 | 0.21 | 0.26 | 3.04 | ADV1 | 0.18 | 0.28 |
ADV2 | 0.13 | 0.23 | ||||||||
Run O1 | 1.06 | 0.50 | 180 | 0.17 | 0.19 | 0.27 | 2.00 | ADV1 | 0.16 | 0.45 |
ADV2 | 0.11 | 0.38 | ||||||||
Run O2 | 1.05 | 0.38 | 180 | 0.17 | 0.17 | 0.20 | 3.03 | ADV1 | 0.17 | 0.30 |
ADV2 | 0.11 | 0.26 | ||||||||
Run O3 | 1.06 | 0.56 | 180 | 0.18 | 0.22 | 0.27 | 2.00 | ADV1 | 0.17 | 0.42 |
ADV2 | 0.11 | 0.34 | ||||||||
Run O4 | 1.55 | 0.30 | 180 | 0.17 | 0.26 | 0.32 | 3.04 | ADV1 | 0.19 | 0.27 |
ADV2 | 0.10 | 0.24 |
Test Run | (m) | (m) | Ripple Geometry | Transport Mode |
---|---|---|---|---|
Run W1 | 0.08 | 0.04 | 2D symmetric vortex-type, wave ripples | Bedload |
Run W2 | 0.16 | 0.07 | 2D symmetric rolling grain wave ripples | Bedload |
Run A1 | 0.123 | 0.056 | 2.5D, symmetric, rolling-grain ripples | Bedload, vortex shedding |
Run A2 | 0.20 | 0.06 | 2D, symmetric, round-crested ripples | Bedload, vortex-shedding & intermittent suspension |
Run A3 | 0.23–0.27 | 0.06 | 3D current-dominated, sharp crested, asymmetric ripples, sinuous transitioning to linguoid | Bedload, vortex shedding |
Run O1 | 0.09 | 0.035 | 2D, asymmetric current-skewed ripples | Bed load, vortex shedding |
Run O2 | 0.20 | 0.06 | 2D, symmetric round-crested ripples | Bedload, vortex-shedding |
Run O3 | 0.165 | 0.078 | 2.5D, symmetric, round-crested bi-convex profile with pronounced scour on lower end of stoss | Bedload, intermittent suspension |
Run O4 | 0.028 | 0.06 | 2D, symmetric round-crested | Suspension, with avalanching along stoss at flow reversal (plug flow) |
Test Run | Estimated from Measurement | Predictions | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
ADV1 | ADV2 | (m) | Nielsen-modified Grant & Madsen (1986)—GM86 | Soulsby & Clarke (2005)—SC05 | Malarkey & Davies (2012)—MD12 | |||||||||
(Pa) | (Pa) | (Pa) | (Pa) | (Pa) | Transport-enhanced (Pa) | (m) | (Pa) | (Pa) | (m) | (Pa) | (Pa) | (m) | ||
Run W1 | 0.017 | 0.42 | 0.39 | 5.5 | 0.02 | 0.34 | 2.9 | 0.019 | 0 | 0.31 | 0.002 | 0 | 0.45 | 0.004 |
Run W2 | 0.026 | 0.50 | 0.08 | 1.7 | 0.014 | 0.29 | 2.7 | 0.019 | 0 | 0.4 | 0.0023 | 0 | 0.35 | 0.002 |
Run A1 | 0.22 | 2.36 | 0.34 | 3.78 | 0.01 | 1 | 2.6 | 0.02 | 0.46 | 1.1 | 0.0014 | 0.47 | 1.2 | 0.009 |
Run A2 | 0.12 | 2.17 | 0.16 | 2.88 | 0.011 | 1.2 | 3.7 | 0.019 | 0.44 | 0.93 | 0.002 | 0.43 | 0.8 | 0.0011 |
Run A3 | 0.062 | 1.12 | 0.14 | 2.72 | 0.015 | 1.17 | 5.18 | 0.017 | 0.43 | 0.88 | 0.0016 | 0.42 | 0.95 | 0.0019 |
Run O1 | 0.17 | 3.29 | 0.19 | 3.42 | 0.01 | 1.05 | 4.62 | 0.03 | 0.54 | 1.5 | 0.002 | 0.6 | 1.0 | 0.008 |
Run O2 | 0.061 | 1.12 | 0.1 | 1.67 | 0.014 | 0.96 | 4.29 | 0.025 | 0.29 | 0.72 | 0.0018 | 0.32 | 0.92 | 0.012 |
Run O3 | 0.16 | 2.60 | 0.19 | 3.75 | 0.01 | 1.45 | 7.4 | 0.01 | 0.66 | 1.7 | 0.0019 | 0.62 | 1.5 | 0.001 |
Run O4 | 0.06 | 1.06 | 0.25 | 4.9 | 0.016 | 1.31 | 7.22 | 0.025 | 0.19 | 0.59 | 0.0019 | 0.22 | 0.88 | 0.0023 |
Run | Wave-Only | Aligned-Flow Runs | Opposing Flow Runs | ||||||
---|---|---|---|---|---|---|---|---|---|
W1 | W2 | A1 | A2 | A3 | O1 | O2 | O3 | O4 | |
ADV1 | 0.027 to 0.03 m | 0.036 m | 0.126 m | 0.08 m | 0.06 m | 0.08 to 0.1 m | 0.06 m | 0.09 m | 0.062 m |
ADV2 | 0.03 m | 0.03; 0.06 m | 0.05 m | 0.04 m | 0.055 m | 0.1; 0.14 m | 0.06 m | 0.07 m | 0.05 m |
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Kassem, H.; Thompson, C.E.L.; Amos, C.L.; Townend, I.H.; Todd, D.; Whitehouse, R.J.S.; Chellew, E. Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows. Water 2020, 12, 3515. https://doi.org/10.3390/w12123515
Kassem H, Thompson CEL, Amos CL, Townend IH, Todd D, Whitehouse RJS, Chellew E. Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows. Water. 2020; 12(12):3515. https://doi.org/10.3390/w12123515
Chicago/Turabian StyleKassem, Hachem, Charlotte E. L. Thompson, Carl L. Amos, Ian H. Townend, David Todd, Richard J. S. Whitehouse, and Elizabeth Chellew. 2020. "Observations of Nearbed Turbulence over Mobile Bedforms in Combined, Collinear Wave-Current Flows" Water 12, no. 12: 3515. https://doi.org/10.3390/w12123515