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  • Simulating wave boundary layers

    Simulation of monochromatic wave boundary layers using CFX
  • Simulation of monochromatic wave boundary layers using CFX
  • Simulation of monochromatic wave boundary layers using CFX
  • Simulation of monochromatic wave boundary layers using CFX
  • Simulation of monochromatic wave boundary layers using CFX

Simulation of monochromatic wave boundary layers using CFX

The ability to accurately define the seabed boundary layer is important as it defines the bed friction and velocity distribution near the bed. The most controlled way of evaluating the predictive capability of CFX for boundary layer flow is to compare with laboratory measurements.

This preliminary study was carried out to ensure confidence in the performance of CFX in simulating wave, current and combined wave and current boundary layers. This was an essential step taken as part of a longer-term plan to investigate scouring around marine structures including the sediment aspects of the scouring process.

A review and initial selection of laboratory boundary layer test data for use in the CFX simulations was carried out. The literature research identified that the physical modelling studies described in Jensen (1989) and Jonsson and Carlson (1976) contained the most suitable comparison data for the initial wave alone test cases. Both these sets of tests used oscillating water tunnels to represent prototype scale conditions.

The CFX model has been run and the results compared with the oscillating water tunnel experiments of Jensen (1989) with a uniform rough bed and also the classic experiment of Jonsson and Carlsen (1976) with an artificial triangular strip roughness.

The output from a 1DV k-ε boundary layer model has also been used as a benchmark for comparison with CFX.

Conclusions

From the velocity simulation the CFX and k-ε models show similar levels of agreement with the experimental data of Jensen (1989) and the agreement is reasonable. The results from the simulation of Jonsson and Carlsen’s test 1 show that within the CFX model set up there is some distortion of the velocity profile along the tunnel section and that the boundary layer thickness is lower than that being measured and also being predicted by the 1DV k-ε model.

At this stage it has not been possible to identify the cause of this distortion, but this has possible implications for other studies where similar set up conditions are involved. Further research is required to resolve this issue.

Overall, the model simulations show that CFX is capable of providing a reasonable representation of the wave boundary layer under, rough, flat bed conditions. Further work is required to resolve the poor results obtained when artificial bedforms are present.

Authors

John Harris; Richard Whitehouse; Dave Robinson

Keywords

Oscillatory boundary layers; numerical; oscillating water tunnel CFX; turbulence modelling

Completed

2009

References

Jensen, BL (1989). Experimental investigation of turbulent oscillatory boundary layers. Series Paper 45, Inst. Hydrodyn. and Hydr., Tech. University of Denmark, Lyngby.

Jensen, BL, Sumer, BM and Fredsøe, J (1989). Turbulent oscillatory boundary layers at high Reynolds numbers. J. Fluid Mech., Vol. 206, pp. 265 - 297.

Jonsson, IG and Carlsen, NA (1976). Experimental and theoretical investigations in an oscillatory turbulent boundary layer. J. Hydr. Res., Vol. 14, No. 1, pp. 45 - 60.

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"The ability to accurately define the seabed boundary layer is important as it defines the bed friction and velocity distribution near the bed."

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