Measurements of the rate of adjustment of the secondary flow in a curved open channel with varying discharge

Measurements of the rate of adjustment of the secondary flow in a curved open channel with varying discharge

Booij, R.
Pennekamp, J.G.C.

Civil Engineering and Geosciences

Hydraulic Engineering

1984-01-01

Prediction of bottom changes in rivers and tidal channels require a thorough knowledge of the secondary flow, as this flow gives rise to bottom slopes transverse to the main flow direction. In the model used in this research, the computation of the secondary flow field is based upon an, independently computed, depth averaged main flow field. In the computation of the secondary flow it can often be assumed fully developed, i.e. adjusted to the local variables: depth, main flow velocity, etc. In case of variation of bottom topography or of main flow properties in the flow direction, the secondary flow will not be adjusted to these local variables, but will retain information about the situation considered parcels of water have met earlier during their flow. An analogous misadjustment will occur in case of time dependent main flow properties. A theoretical investigation about the degree of adjustment of secondary flow was reported by Booij and Kalkwijk (1982). They describe the rate of adjustment by a relaxation length in case of variation in the flow direction and by a relaxation time in case of variation in time. Both are defined by the reduction of the misadjustment to a factor l/e of the original misadjustment with respect to the local variables. For the value of the Chezy coefficient of 50 m^1/2/s, the relaxation length L of the secondary flow was found to be about 15 times the water depth hand the relaxation time T was found to be about the time needed by the depth averaged flow to cover a distance equal to this relaxation length.

secondary flow
curved channel

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TU Delft, Department of Hydraulic Engineering

Report 15-84

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