Micro Scale Flow

1. Micro Scale Flow Measure at the Right Magnitude!

2. Boundary Flow • When water flows over a solid (?) surface (boundary) frictional drag slows the velocity • Water flows most slowly near the boundary and the velocity gradually increases with increasing distance above the boundary to some “free stream velocity”

3. Boundary Layers Air-sea interface 1 ? 3 Water mass interface 2 Benthic boundary layer

4. Temperature (°C) Upper water mass = Surface waters Warmer Less saline Less dense Depth (m) Colder More saline Denser Lower water mass = Bottom waters Salinity (‰) or 0 0 ‘Thin layers’ Cowles 1998; Osborn 1998 Pycnocline Pycnocline Pycnocline

5. t = 0 s (a) t = 30 s (b) t = 1 minute (c) t = 5 minutes (d) Within the Pycnocline t = 1 minute t = 1 minute

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7. Type of Flow • Laminar Flow – slow flow over a flat surface, with all water particles following the exact same path. • Molecular Viscosity – component that contributes to frictional drag in liquids, “stickiness” • Turbulent Flow – flow becomes faster, water particles start to move in random eddies, net movement remains in the direction of flow • Eddy Viscosity – produced by turbulent flow. Can be several orders of magnitude greater than molecular viscosity.

8. Water flows over a relatively smooth, flat surface Viscous Sublayer: Conditions approach laminar flow up to several millimeters, molecular viscosity dominates Log Layer: rate of increase in velocity decreases logarithmically with increasing distance from the boundary (can be cm to M thick) Fully turbulent layer Velocity ProfileSmooth Turbulent Flow

9. Microscale Flow and Sediment Transport • The thickness of the viscous sublayer considerably affects the movement of sediment • The degree of turbulent mixing within the boundary layer determines the level of exchange between particulate and dissolved materials on the seafloor and in the water column. • Shear Velocity – measure of magnitude of turbulent fluctuations in velocity within the boundary layer near the seabed • Cannot be directly measured – calculated from velocity gradient

10. Shear Velocity (u*) Calculation u* = uz k log (z/z0) Uz = mean current speed at some height (z) above the seafloor k = Von Karman’s constant (5.75) Z0 = “hydraulic roughness” – can be estimated as the y-intercept of a plot of log height (z) above the seafloor (y-axis) and the mean current speed (u) (x-axis)

11. Shear Stress (T) • Force applied per unit area to the seabed • Important in determining if a particle of sand or mud can be lifted away from the bed • Causes decrease in magnitude in flow near the boundary T = ρu*2 ρ = density of seawater (~1020 kg m-3)

12. Sediment Movement • Movement begins when shear stress increases to the point where frictional and gravitational forces holding grains to the bed are overcome • Varies with grain size • Critical Shear velocity

13. Assumptions for our Calculations • “Free Stream” velocity remains constant • Smooth turbulent flow is assumed • Mixture of sand and water is denser than water alone – tends to dampen out turbulence

14. Marsh-McBirney Current Meter • Water moving in a magnetic field produces a voltage (Faraday’s Law) • Voltage is linearly proportional to the water velocity – basis for Marsh-McBirney • Sensor contains an electromagnet and two external electrodes • Flow around the probe intersects magnetic flux lines, generating voltages • Voltages are displayed in ft/sec or m/sec

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