Collaborative Research: Physics of lutoclines and laminarization extracted from turbulence-resolved numerical investigations on sediment transport in wave-current bottom boundary layer (NSF, OCE-1130217).
A Numerical Modeling Framework for Cohesive Sediment Transport Driven by Waves and Tidal Currents (ONR, N00014-14-1–0586 ).
Several prior field observations on the continental shelves reveal a variety of seabed states due to wave-current driven sediment transport. Their occurrences have several critical implications. For example, the formation of lutocline indicates trapping of fine sediment near the bed and the resulting large density anomaly may yield significant offshore sediment transport on the continental shelf through wave-supported gravity flows. When surface waves propagate over muddy seabed, high wave dissipation rate is often observed during the waning stage of a storm as the fluid mud layer becomes significantly suppressed. Recent microstratigraphy study of mud deposits suggests a three-part sedimentary microfabrics that can be associated with the processes occur during wave-supported gravity flow events. The main challenges of modeling wave-induced fluid mud transport are the coupling between sediment and turbulence, the transitional nature of turbulent flow, rheology and the polydispersed nature of transport. Our proposed research addresses these challenges and the resulting research efforts are valuable in further interpreting critical processes observed in the mud-dominant coastal environment.
Our recent numerical investigations reveal the existence of several distinct regimes of wave-induced fine sediment transport ranging from well-mixed transport, to the formation of lutocline, and eventually a complete flow laminarization due to a range of sediment availability and settling velocity. The numerical model is based on an Eulerian-Eulerian two-phase formulation simplified for fine sediment (small particle response time) while resolving all the scales of turbulence-sediment interactions, . In this study, we propose to further investigate several critical science issues via numerical simulations. Firstly, we plan to complete a phase diagram of flow regimes as a function of wave Reynolds number, bulk Richardson number (sediment availability) and nondimensional settling velocity with a series of well-planned simulations. The results will allow us to study the hypothesized major differences between the tidal and wave boundary layers in response to sediments. Secondly, with a better understanding on the onset of laminarization, we plan to enhance the numerical model with capability of non-Newtonian rheology in order to study more realistic wave-current driven fluid mud transport and to further quantify hydrodynamic dissipation associated with each flow regime.
 Traykovski, P., Geyer, W. R., Irish, J. D. & Lynch, J. F. (2000) The role of wave-induced fluid mud flows for cross-shelf transport on the Eel River continental shelf. Cont. Shelf Res. 20,2113–2140.
 Macquaker, J. H. S., Bentley, S/ J., and Bohacs, K. M., (2010) Wave-enhanced sediment-gravity flows and mud dispersal across continental shelves: Reappraising sediment transport processes operating in ancient mudstone successions, Geology, v. 38, no. 10, p947-950, doi:10.1130/G31093.1.
 Ozdemir, C. E., Hsu, T.-J. Balachandar, S., (2010) A Direct numerical simulation study on bottom boundary layer turbulence under a single solitary wave: instability mechanisms, flow transience and characteristics of flow turbulence, J. Fluid Mech., 731, 545-578.
 Cortese, T. & Balachandar, S., 1995 High performance spectral simulation of turbulent flows in massively parallel machines with distributed memory. International Journal of Supercomputer Applications 9 (3), 187–204.
 Balachandar, S. & Eaton, J. K. (2010) Turbulent dispersed multiphase flow. Annu. Rev. Fluid Mech. 42, 111–133.
- Cheng, Z., X. Yu, T.-J. Hsu, C. E. Ozdemir, and S. Balachandar (2015a), On the transport modes of fine sediment in the wave boundary layer due to resuspension/deposition: A turbulence-resolving numerical investigation, J. Geophys. Res. Oceans, 120, 1918–1936, doi:10.1002/2014JC010623.
- Cheng, Z., X. Yu, T.-J. Hsu, S. Balachandar (2015b) A numerical investigation of fine sediment resuspension in the wave boundary layer – uncertainties in particle inertia and hindered settling, Computers and Geosciences, 83, 176-192
- Ozdemir, C. E., Hsu, T.-J., Balachandar, S., Direct numerical simulations of transition and turbulence in Stokes boundary layer, Physics of Fluids, 26, 045108, ; doi: 10.1063/1.4871020.
- Yu, X., C. E. Ozdemir, Hsu, T.-J., Balachandar, S., Turbulence Modulations due to Sediment-induced Density Stratification and Rheology during Fine Sediment Transport in an Oscillatory Channel – a Numerical Investigation, Journal of Waterway, Port, Coastal, and Ocean Engineering, 140(2), 160-172.
- Yu, X., Hsu, T.-J., Balachandar, S., (2013) A spectral-like turbulence-resolving scheme for fine sediment transport in the bottom boundary layer, Computers and Geosciences, 61, 11-22.
- Yu, X., T.-J. Hsu, and S. Balachandar (2013), Convective instability in sedimentation: Linear stability analysis, J. Geophys. Res. Oceans. 118, 256–272, doi:10.1029/2012JC008255.
- Ozdemir, C. E., T.‐J. Hsu, and S. Balachandar (2011), A numerical investigation of lutocline dynamics and saturation of fine sediment in the oscillatory boundary layer, J. Geophys. Res., 116, C09012, doi:10.1029/2011JC007185.
- FineSed3D Version 1.0, open-source code available through Community Surface Dynamics Modeling System (CSDMS) code repository (download).