Experimental measurement and numerical modelling of velocity, density and turbulence profiles of a gravity current

[摘要] The velocity, density and turbulence profiles of a horizontal, saline gravity currentwere measured experimentally. Stable stratfication damped the turbulence andprevented the gravity current from becoming self-similar. The velocity and densityprfiles were measured simultaneously and non-intrusively with particle imagevelocimetry scalar (PIV-S) technology. The application of the PIV-S technologyhad to be extended in order to measure the continuously stratified gravity current.Measurement of the Reynolds fluxes and Reynolds stresses revealed the anisotropicturbulent transport of mass and momentum within the gravity current body. Thesemeasurements also allowed the interaction between turbulence and stratification tobe studied. The measured profiles were used to evaluate the accuracy of a gravitycurrent model which did not assume self-similarity. The gravity current model wasbased on a Reynolds-averaged Navier-Stokes (RANS) multispecies mixture model.The Reynolds flux and Reynolds stress profiles did not show self-similaritywith increasing downstream distance. Comparison of the vertical and horizontalReynolds fluxes showed that gravity strongly damped the vertical flux. At adownstream location, where the bulk Richardson number was supercritical, theshear production profile had a positive inner (near bed) peak and a positive outerpeak, while the buoyancy production pro le had a negative outer peak. Furtherdownstream, where the bulk Richardson number was near-critical, the outer shearand buoyancy production peaks disappeared, due to the continuous damping ofthe turbulence intensities by the stable stratification. However, near bed shearingallowed the inner shear production peak to remain. Sensitivity analyses of differentturbulence models for the gravity current model showed that the standardk -e turbulence model, as well as the Renormalization Group theory (RNG) k -eturbulence model, generally underpredicted the mean streamwise velocity profileand overpredicted the excess density pro le. The flux-gradient hypothesis, used toprovide closure for the Reynoldsuxes, modelled the vertical Reynoldsux reasonably,but not the horizontal flux. This did not compromise the results, since thehorizontal gravity current had the characteristics of a boundary-layerow, where the horizontal flux does not contribute significantly to the flow structure. It wasshown that the gravity current model, implementing the standard k -e turbulencemodel with a constant turbulent Schmidt number of ot = 1;3, produced profileswhich were within 10% - 20% of the measured profiles.

[发布机构] Stellenbosch University