The Relationship between Flux Coefficient and Entrainment Ratio in Density Currents

Mathew Wells University of Toronto, Toronto, Ontario, Canada

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Claudia Cenedese Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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C. P. Caulfield BP Institute and Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, United Kingdom

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Abstract

The authors explore the theoretical and empirical relationship between the nonlocal quantities of the entrainment ratio E, the appropriately depth- and time-averaged flux coefficient Γ, and the bulk Froude number Fro in density currents. The main theoretical result is that E = 0.125 Γ Fro2(CU3/CL)/cosθ, where θ is the angle of the slope over which the density current flows, CL is the ratio the turbulent length scale to the depth of the density current, and CU is the ratio of the turbulent velocity scale to the mean velocity of the density current. In the case of high bulk Froude numbers Γ ∼ Fro−2 and (CU3/CL) = Cϵ ∼ 1, so E ∼ 0.1, consistent with observations of a constant entrainment ratio in unstratified jets and weakly stratified plumes. For bulk Froude numbers close to one, Γ is constant and has a value in the range of 0.1–0.3, which means that E ∼ Fro2, again in agreement with observations and previous experiments. For bulk Froude numbers less than one, Γ decreases rapidly with bulk Froude number, explaining the sudden decrease in entrainment ratios that has been observed in all field and experimental observations.

Corresponding author address: Mathew Wells, Department of Physical and Environmental Sciences, University of Toronto, 1265 Military Trail, Toronto ON M1C 1A4, Canada. Email: wells@utsc.utoronto.ca

Abstract

The authors explore the theoretical and empirical relationship between the nonlocal quantities of the entrainment ratio E, the appropriately depth- and time-averaged flux coefficient Γ, and the bulk Froude number Fro in density currents. The main theoretical result is that E = 0.125 Γ Fro2(CU3/CL)/cosθ, where θ is the angle of the slope over which the density current flows, CL is the ratio the turbulent length scale to the depth of the density current, and CU is the ratio of the turbulent velocity scale to the mean velocity of the density current. In the case of high bulk Froude numbers Γ ∼ Fro−2 and (CU3/CL) = Cϵ ∼ 1, so E ∼ 0.1, consistent with observations of a constant entrainment ratio in unstratified jets and weakly stratified plumes. For bulk Froude numbers close to one, Γ is constant and has a value in the range of 0.1–0.3, which means that E ∼ Fro2, again in agreement with observations and previous experiments. For bulk Froude numbers less than one, Γ decreases rapidly with bulk Froude number, explaining the sudden decrease in entrainment ratios that has been observed in all field and experimental observations.

Corresponding author address: Mathew Wells, Department of Physical and Environmental Sciences, University of Toronto, 1265 Military Trail, Toronto ON M1C 1A4, Canada. Email: wells@utsc.utoronto.ca

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  • Arneborg, L., V. Fiekas, L. Umlauf, and H. Burchard, 2007: Gravity current dynamics and entrainment—–A process study based on observations in the Arkona Basin. J. Phys. Oceanogr., 37 , 20942113.

    • Search Google Scholar
    • Export Citation
  • Baines, P., 2001: Mixing in flows down gentle slopes into stratified environments. J. Fluid Mech., 443 , 237270.

  • Baringer, M. O., and J. F. Price, 1997: Mixing and spreading of the Mediterranean outflow. J. Phys. Oceanogr., 27 , 16541677.

  • Billant, P., and J-M. Chomaz, 2001: Self-similarity of strongly stratified inviscid flows. Phys. Fluids, 13 , 16451651.

  • Brethouwer, G., P. Billant, E. Lindborg, and J-M. Chomaz, 2007: Scaling analysis and simulation of strongly stratified turbulent flows. J. Fluid Mech., 585 , 343368.

    • Search Google Scholar
    • Export Citation
  • Canuto, V. M., M. S. Dubovikov, and Y. Cheng, 2005: Entrainment: Local and non-local turbulence models with double diffusion. Geophys. Res. Lett., 32 , L22604. doi:10.1029/2005GL023771.

    • Search Google Scholar
    • Export Citation
  • Caulfield, C. P., W. Tang, and S. C. Plasting, 2004: Reynolds number dependence of an upper bound for the long-time-averaged buoyancy flux in plane stratified Couette flow. J. Fluid Mech., 498 , 315332.

    • Search Google Scholar
    • Export Citation
  • Cenedese, C., and C. Adduce, 2008: Mixing in a density-driven current flowing down a slope in a rotating fluid. J. Fluid Mech., 604 , 369388.

    • Search Google Scholar
    • Export Citation
  • Cenedese, C., and C. Adduce, 2010: A new parameterization for entrainment in overflows. J. Phys. Oceanogr., 40 , 18351850.

  • Cenedese, C., J. A. Whitehead, T. A. Ascarelli, and M. Ohiwa, 2004: A dense current flowing down a sloping bottom in a rotating fluid. J. Phys. Oceanogr., 34 , 188203.

    • Search Google Scholar
    • Export Citation
  • Christodoulou, G. C., 1986: Interfacial mixing in stratified flows. J. Hydraul. Res., 24 , 7792.

  • Crapper, P. F., and P. F. Linden, 1974: The structure of turbulent density interfaces. J. Fluid Mech., 65 , 4563.

  • Dallimore, C. J., J. Imberger, and T. Ishikawa, 2001: Entrainment and turbulence in saline underflow in Lake Ogawara. J. Hydraul. Eng., 127 , 937948.

    • Search Google Scholar
    • Export Citation
  • Ellison, T. H., and J. S. Turner, 1959: Turbulent entrainment in stratified flows. J. Fluid Mech., 6 , 423448.

  • Fer, I., G. Voet, K. S. Seim, B. Rudels, and K. Latarius, 2010: Intense mixing of the Faroe Bank Channel overflow. Geophys. Res. Lett., 37 , L02604. doi:10.1029/2009GL041924.

    • Search Google Scholar
    • Export Citation
  • Fernando, H. J. S., 1991: Turbulent mixing in stratified fluids. Annu. Rev. Fluid Mech., 23 , 455493.

  • Garcia, M., 1990: Depositing and eroding turbidity sediment driven flows: Turbidity currents. Project Rep. 306, St. Anthony Falls Hydraulic Laboratory, University of Minnesota, Minneapolis, 179 pp.

    • Search Google Scholar
    • Export Citation
  • Girton, J. B., and T. B. Sanford, 2003: Descent and modification of the overflow plume in Denmark Strait. J. Phys. Oceanogr., 33 , 13511364.

    • Search Google Scholar
    • Export Citation
  • Gregg, M. C., 1987: Diapycnal mixing in the thermocline: A review. J. Geophys. Res., 92 , 52495286.

  • Holford, J. M., and P. F. Linden, 1999: Turbulent mixing in a stratified fluid. Dyn. Atmos. Oceans, 30 , 173198.

  • Huang, H., J. Imran, and C. Pirmez, 2007: Numerical modeling of poorly sorted depositional turbidity currents. J. Geophys. Res., 112 , C01014. doi:10.1029/2006JC003778.

    • Search Google Scholar
    • Export Citation
  • Itsweire, E. C., K. N. Helland, and C. W. van Atta, 1986: The evolution of grid-generated turbulence in a stably stratified fluid. J. Fluid Mech., 162 , 299338.

    • Search Google Scholar
    • Export Citation
  • Ivey, G. N., and J. Imberger, 1991: On the nature of turbulence in a stratified fluid. Part I: The energetics of mixing. J. Phys. Oceanogr., 21 , 650659.

    • Search Google Scholar
    • Export Citation
  • Ivey, G. N., K. B. Winters, and J. R. Koseff, 2008: Density stratification, turbulence, but how much mixing? Annu. Rev. Fluid Mech., 40 , 169184.

    • Search Google Scholar
    • Export Citation
  • Jackson, L., R. Hallberg, and S. Legg, 2008: A parameterization of shear-driven turbulence for ocean climate models. J. Phys. Oceanogr., 38 , 10331053.

    • Search Google Scholar
    • Export Citation
  • Johnson, G. C., R. G. Lueck, and T. B. Sanford, 1994: Stress on the Mediterranean outflow plume. Part II: Turbulent dissipation and shear measurements. J. Phys. Oceanogr., 24 , 20842092.

    • Search Google Scholar
    • Export Citation
  • Kato, H., and O. M. Phillips, 1969: On the penetration of the turbulent layer into a stratified fluid. J. Fluid Mech., 37 , 643665.

  • Legg, S., and Coauthors, 2009: Improving the oceanic overflow representation in climate models: The gravity current entrainment climate process team. Bull. Amer. Meteor. Soc., 90 , 657670.

    • Search Google Scholar
    • Export Citation
  • Lienhard, J. H., and C. W. van Atta, 1990: The decay of turbulence in thermally stratified flow. J. Fluid Mech., 210 , 57112.

  • Linden, P. F., 1973: The interaction of a vortex ring with a sharp density interface: A model for turbulent entrainment. J. Fluid Mech., 60 , 467480.

    • Search Google Scholar
    • Export Citation
  • Linden, P. F., 1979: Mixing in stratified fluids. Geophys. Astrophys. Fluid Dyn., 13 , 323.

  • Linden, P. F., 1980: Mixing across a density interface produced by grid turbulence. J. Fluid Mech., 100 , 691703.

  • List, E. J., 1982: Turbulent jets and plumes. Annu. Rev. Fluid Mech., 14 , 189212.

  • Martin, J. E., and C. R. Rehmann, 2006: Layering in a flow with diffusively stable temperature and salinity stratification. J. Phys. Oceanogr., 36 , 14571470.

    • Search Google Scholar
    • Export Citation
  • Mauritzen, C., J. F. Price, T. B. Sanford, and D. Torres, 2005: Circulation and mixing in the Faroese Channels. Deep-Sea Res. I, 52 , 883913.

    • Search Google Scholar
    • Export Citation
  • Odier, P., J. Chen, M. K. Rivera, and R. E. Ecke, 2009: Fluid mixing in stratified gravity currents: The Prandtl mixing length. Phys. Rev. Lett., 102 , 134504. doi:10.1103/PhysRevLett.102.134504.

    • Search Google Scholar
    • Export Citation
  • Osborn, T. R., 1980: Estimates of the local rate of vertical diffusion from dissipation measurements. J. Phys. Oceanogr., 10 , 8389.

  • Papadakis, M. P., E. P. Chassignet, and R. W. Hallberg, 2003: Numerical simulations of the Mediterranean Sea outflow: Impact of the entrainment parameterization in an isopycnic coordinate ocean model. Ocean Modell., 5 , 325356.

    • Search Google Scholar
    • Export Citation
  • Peltier, W. R., and C. P. Caulfield, 2003: Mixing efficiency in stratified shear flows. Annu. Rev. Fluid Mech., 35 , 135167.

  • Peters, H., and W. E. Johns, 2005: Mixing and entrainment in the Red Sea outflow plume. Part II: Turbulence characteristics. J. Phys. Oceanogr., 35 , 584600.

    • Search Google Scholar
    • Export Citation
  • Prastowo, T., R. W. Griffiths, G. O. Hughes, and A. Hogg, 2008: Mixing efficiency in controlled exchange flows. J. Fluid Mech., 600 , 235244.

    • Search Google Scholar
    • Export Citation
  • Price, J. F., and M. O. Barringer, 1994: Outflows and deep water production by marginal seas. Prog. Oceanogr., 33 , 161200.

  • Princevac, M., H. J. S. Fernando, and D. C. Whiteman, 2005: Turbulent entrainment into natural gravity-driven flows. J. Fluid Mech., 33 , 259268.

    • Search Google Scholar
    • Export Citation
  • Rehmann, C. R., and J. R. Koseff, 2004: Mean potential energy change in stratified grid turbulence. Dyn. Atmos. Oceans, 37 , 271294.

  • Riley, J. J., and S. M. de Bruyn Kops, 2003: Dynamics of turbulence strongly influenced by buoyancy. Phys. Fluids, 15 , 20472059.

  • Riley, J. J., and E. Lindborg, 2008: Stratified turbulence: A possible interpretation of some geophysical turbulence measurements. J. Atmos. Sci., 65 , 24162424.

    • Search Google Scholar
    • Export Citation
  • Rohr, J. J., E. C. Itsweire, K. N. Helland, and C. W. Van Atta, 1988: Growth and decay of turbulence in a stably stratified shear flow. J. Fluid Mech., 195 , 77111.

    • Search Google Scholar
    • Export Citation
  • Sherwin, T. J., 2010: Observations of the velocity profile of a fast and deep oceanic density current constrained in a gully. J. Geophys. Res., 115 , C03013. doi:10.1029/2009JC005557.

    • Search Google Scholar
    • Export Citation
  • Shih, L. H., J. R. Koseff, G. N. Ivey, and J. H. Ferziger, 2005: Parameterization of turbulent fluxes and scales using homogeneous sheared stably stratified turbulence simulations. J. Fluid Mech., 525 , 193214.

    • Search Google Scholar
    • Export Citation
  • Smyth, W. D., and J. N. Moum, 2000a: Length scales of turbulence in stably stratified mixing layers. Phys. Fluids, 12 , 13271342.

  • Smyth, W. D., and J. N. Moum, 2000b: Anisotropy of turbulence in stably stratified mixing layers. Phys. Fluids, 12 , 13431362.

  • Smyth, W. D., J. N. Moum, and D. R. Caldwell, 2001: The efficiency of mixing in turbulent patches: Inferences from direct simulations and microstructure observations. J. Phys. Oceanogr., 31 , 19691992.

    • Search Google Scholar
    • Export Citation
  • Stillinger, D. C., K. N. Helland, and C. W. Van Atta, 1983: Experiments on the transition of homogeneous turbulence to internal waves in a stratified fluid. J. Fluid Mech., 131 , 91122.

    • Search Google Scholar
    • Export Citation
  • Strang, E. J., and H. J. S. Fernando, 2001a: Entrainment and mixing in stratified shear flows. J. Fluid Mech., 428 , 349386.

  • Strang, E. J., and H. J. S. Fernando, 2001b: Vertical mixing and transports through a stratified shear layer. J. Phys. Oceanogr., 31 , 20262048.

    • Search Google Scholar
    • Export Citation
  • Taylor, G. I., 1945: Dynamics of a mass of hot gas rising in air. U.S. Atomic Energy Commission, MDDC-919, LA Rep. 236, 19 pp. [Available online at http://www.fas.org/sgp/othergov/doe/lanl/docs1/00407667.pdf].

    • Search Google Scholar
    • Export Citation
  • Turner, J. S., 1973: Buoyancy Effects in Fluids. Cambridge University Press, 367 pp.

  • Turner, J. S., 1986: Turbulent entrainment: The development of the entrainment assumption, and its application to geophysical flows. J. Fluid Mech., 173 , 431471.

    • Search Google Scholar
    • Export Citation
  • Umlauf, L., and L. Arneborg, 2009: Dynamics of rotating shallow gravity currents passing through a channel. Part I: Observation of transverse structure. J. Phys. Oceanogr., 39 , 23852401.

    • Search Google Scholar
    • Export Citation
  • Wells, M. G., 2007: Influence of Coriolis forces on turbidity currents and sediment deposition. Particle-Laden Flow: From Geophysical to Kolmogorov Scales, B. J. Geurts, H. Clercx, and W. Uijttewaal, Eds., ERCOFTAC Series, Vol. 11, Springer, 331–343.

    • Search Google Scholar
    • Export Citation
  • Wells, M. G., and J. S. Wettlaufer, 2007: The long-term circulation driven by density currents in a two-layer stratified basin. J. Fluid Mech., 572 , 3758.

    • Search Google Scholar
    • Export Citation
  • Wells, M. G., and P. Nadarajah, 2009: The intrusion depth of density currents flowing into stratified water bodies. J. Phys. Oceanogr., 39 , 19351947.

    • Search Google Scholar
    • Export Citation
  • Xu, J. P., M. A. Noble, and L. K. Rosenfeld, 2004: In-situ measurements of velocity structure within turbidity currents. Geophys. Res. Lett., 31 , L09311. doi:10.1029/2004GL019718.

    • Search Google Scholar
    • Export Citation
  • Zellouf, Y., P. Dupont, and H. Peerhossaini, 2005: Heat and mass fluxes across density interfaces in a grid-generated turbulence. Int. J. Heat Mass Transfer, 48 , 37223735.

    • Search Google Scholar
    • Export Citation
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