https://orcid.org/0000-0002-6989-5813
Abstract
The effect of flow intensity and sediment characteristics on the morphodynamic evolution of ripples was investigated numerically. The methodology is based on large-eddy simulations of turbulent oscillatory flow, coupled with sediment transport and morphodynamical evolution. The resulting ripple characteristics are in agreement with those predicted by empirical formulas. The time to equilibrium was found to depend not only on the mobility parameter, but also on the ratio of the orbital amplitude to the sediment grain diameter, with larger values leading to slower transition to equilibrium. An intense turbulence area is observed near the ripple crests. In all cases, the bedload is active mostly near the ripple crests, and its magnitude decreases significantly with increasing values of the ratio of the orbital flow amplitude to the sediment grain diameter. The suspended load is active over the entire ripple length, with its magnitude increasing significantly with increasing mobility parameter values.
Acknowledgements
The present work was supported by computational time granted from the Greek Research & Technology Network in the National HPC facility – ARIS – under project ID CoastHPC.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Notation
| ao | = | orbital amplitude (m) |
| B | = | wave skewness (–) |
| c | = | suspended sediment volumetric concentration (–) |
| c′ | = | suspended sediment volumetric concentration fluctuations (–) |
| cb | = | reference concentration (–) |
| CT | = | model constant (–) |
| co | = | maximum volumetric concentration of natural sand grains (–) |
| Dg | = | sediment grain diameter (m) |
| fc | = | IB method term for sediment (–) |
| fi | = | IB method term (–) |
| G | = | gravitational acceleration (m s-²) |
| H | = | bed elevation (m) |
| hp | = | perturbation height (m) |
| hr | = | ripple height (m) |
| K | = | turbulent kinetic energy (J m−3) |
| Lp | = | perturbation length (m) |
| Lr | = | ripple length (m) |
| Lx1 | = | domain streamwise length (m) |
| Lx2 | = | domain spanwise length (m) |
| Lx3 | = | domain height (m) |
| N | = | porosity of the bed sediment (–) |
| ne | = | number of wave cycles to equilibrium (–) |
| P | = | dynamic pressure (N m−2) |
| qbi | = | normalized bedload transport rate (–) |
| Qbi | = | bedload transport rate (m2 s−1) |
| qi | = | normalized total sediment flux (–) |
| qsi | = | normalized suspended load transport rate (–) |
| Qsi | = | suspended load transport rate (m2 s−1) |
| Re | = | Reynolds number (–) |
| S | = | sediment specific gravity (–) |
| T | = | time (s) |
| T | = | wave period (s) |
| ui | = | flow velocity components (m s−1) |
| ui′ | = | flow velocity fluctuations (m s−1) |
| U | = | external flow streamwise velocity (m s−1) |
| Uo | = | velocity amplitude (m s−1) |
| ws | = | sediment settling velocity (m s−1) |
| xi | = | spatial coordinates (m) |
| θ | = | Shields parameter (–) |
| θc | = | critical Shields parameter (–) |
| λb | = | linear concentration of suspended sediment at bed level (–) |
| Sc | = | Schmidt number (–) |
| σ | = | standard deviation (–) |
| τ | = | time-scale for the morphological evolution of ripples (s) |
| τij | = | subgrid-scale stresses (N m−2) |
| Φb | = | non-dimensional bed transport rate parameter (–) |
| ψ | = | mobility parameter (–) |
| ω | = | angular wave frequency (rad s−1) |