2203 Fluids

Information about 2203 Fluids

Published on February 11, 2008

Author: Massimo

Source: authorstream.com

Content

SEDIMENTS AND SEDIMENTARY ROCKS: FLUIDS:  SEDIMENTS AND SEDIMENTARY ROCKS: FLUIDS READING:  READING Prothero and Schwab Ch. 3, p. 31-38 FLUID DYNAMICS:  FLUID DYNAMICS Irrigation canals, India FLUID DYNAMICS: FLUMES:  FLUID DYNAMICS: FLUMES Ripples forming under unidirectional flow Water surface FLUID PROPERTIES:  FLUID PROPERTIES DENSITY ρ = m / v = mass / unit volume of fluid (g / cm3) air = 1.3 kg / m3 water = 1000 kg / m3 (1 g / cm3) Fluid Density affects amount and size of particles transported and the rate at which they settle out. VISCOSITY μ = τ / du/dy ratio of shear stress (τ = stress per unit area) to the rate of deformation caused by the shear stress (du/dy) (= Dynamic Viscosity) measure of substance's ability to flow or its resistance to changing its shape. TYPES OF FLUID:  TYPES OF FLUID Shear Stress (τ) Shear Strain Newtonian Fluid: High-viscosity Newtonian Fluid: Low-viscosity Non-Newtonian Fluid Yield Stress FLUID TYPES:  FLUID TYPES Newtonian Fluids Have no strength Do not change viscosity when deformed e.g., Water, air Non-Newtonian Fluids Have yield strength Change viscosity when deformed e.g., Mud flows LIFT & DRAG FORCES:  LIFT & DRAG FORCES Drag acts parallel to bed = shear stress on grain Lift Bernouilli effect of flow over projecting grains, causes pressure decrease above grain (as for plane wing) Particle motion when: Lift + Drag > Gravity When lifted into fluid, flow becomes symmetrical around grain, and lift component is eliminated Flow Lift Gravity Drag Overall Fluid Force LAMINAR vs. TURBULENT FLOW:  LAMINAR vs. TURBULENT FLOW LAMINAR FLOW TURBULENT FLOW P&S, Fig. 3.1 REYNOLDS NO. LAMINAR vs. TURBULENT FLOW:  REYNOLDS NO. LAMINAR vs. TURBULENT FLOW d = pipe diameter (or flow depth) V = velocity μ = viscosity ρ = density Re = ρ d v / μ (= turbulent / inertial forces) Turbulent Flow Re 500-2000 Laminar Flow REYNOLDS NO.:  REYNOLDS NO. Laminar Flow, Re < 0.1 Laminar Flow with some vortices, Re ~ 1-40 Laminar / Turbulent Flow Transition, Re ~ 40 - 120 Particle moving through Fluid: FROUDE NO. RAPID vs. TRANQUIL FLOW:  FROUDE NO. RAPID vs. TRANQUIL FLOW v = velocity D = depth g = accel. due to gravity Fr = v / √ gD Rapid Flow Fr ~ 1.0 = hydraulic jump Tranquil Flow TRANQUIL vs. RAPID FLOW:  TRANQUIL vs. RAPID FLOW LOWER FLOW REGIME Fr < 1.0 UPPER FLOW REGIME Fr > 1.0 BOUNDARY LAYER:  BOUNDARY LAYER Flow Sediment bed Water surface Viscous Sublayer Log Layer Most sediments deposited from fully turbulent fluids Fluid velocity in open channel: zero at base  full velocity at top Turbulent bursts High shear stress (frictional) ENTRAINMENT:  ENTRAINMENT P&S, Fig. 3.3 Bed Load Suspended Load ENTRAINMENT:  ENTRAINMENT Hjulstrom diagram Velocity (cm /s) Grain Size (mm) – log scale Range of velocity ENTRAINMENT OF GRAVEL:  ENTRAINMENT OF GRAVEL High shear stress – large gravel clasts readily entrained ENTRAINMENT OF GRAVEL:  ENTRAINMENT OF GRAVEL Velocity needed to entrain these 2 m boulders? TURKEY BROOK: GRAVEL TRANSPORT:  TURKEY BROOK: GRAVEL TRANSPORT Depth = 1.4 m Top Gravel d50 = 22 mm Bankfull Level Bottom Gravel d50 = 16 mm When brook is full, what flow velocity will move the top gravel? (use Hjulstrom Curve for an approximate answer) What Froude Number will this flow have? The creek bed is ARMOURED. Why is this important? STOKES LAW OF SETTLING:  STOKES LAW OF SETTLING d = diameter (cm) g = accel. due to gravity = 980.7 cm/sec2 ρF = fluid density (g/cm3) ρS = grain density (g/cm3) μ = water viscosity (cp) V = fall velocity (cm/sec) V = 1 (ρS – ρF)gd2 − ──────── 18 μ INERTIAL FORCE / VISCOUS FORCE CLAY IN SUSPENSION:  CLAY IN SUSPENSION Plumes of suspended sediment CLAY IN SUSPENSION:  CLAY IN SUSPENSION Australian waterhole – opaque with clay – stagnant for months STOKES LAW OF SETTLING:  STOKES LAW OF SETTLING PROBLEM: How do clay flakes reach the ocean floor? HOW DOES CLAY SETTLE?:  HOW DOES CLAY SETTLE? Flocculation Van der Waal’s forces Electrolytes (salinity) Turbulence Fecal pellets Turbidity Currents hyperpycnal flow FLOCCULATION: FUNDY:  FLOCCULATION: FUNDY SEDIMENT TRANSPORT BY THE WIND :  SEDIMENT TRANSPORT BY THE WIND Air: weak fluid but readily carries fine grains Low shear stress on bed can transport only a restricted range of grain sizes (mainly less than granules) Low buoyancy force dominance of saltation and collision, with “chain reactions” and creep effects Energetic collisions rounds grains and abrades their surfaces (“frosted” grains) Suspension of large grains is difficult due to low buoyancy. Thickness of moving air may be huge (up to 104 m). strong turbulence carries silt and clay to great heights and long distances. SEDIMENT TRANSPORT BY WIND:  SEDIMENT TRANSPORT BY WIND WIND TRANSPORT:  WIND TRANSPORT Dust storms, California P&S, p. 31 WIND TRANSPORT:  WIND TRANSPORT Sand grains in motion, Great Sand Dunes, New Mexico Sediment styles -- Wind:  Sediment styles -- Wind Small ripples to huge dune systems in deserts and coastal areas Loess and dust accession layers – 40μ grains dominant (silt) – sheets of wind-blown fines Adds dust and scattered sand to other sediments Volcanic dust – pushed to high altitudes by eruption Sediment Styles -- Wind:  Sediment Styles -- Wind Longitudinal dunes, Australia Sediment Styles -- Wind:  Sediment Styles -- Wind Gibber Plain, Australia – deflation lag Sediment Styles -- Wind:  Sediment Styles -- Wind 40 m high coastal dune, NSW

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