GEOL 340 Sedimentology and Stratigraphy
Lecture 5
1. Fluid Properties
2. Fluid Types and Flow
Fluid Properties
Fluid = gas or liquid, but not a solid (petroleum, gas, air, water, sediment laden water)
Fluid definition = substance that changes shape easily and continuously in response to externally applied forces, does not resist shearing forces
Shear forces = parallel forces that cause sliding along parallel planes (Fig. 2.1, p. 26)
Categories of fluid forces:
a. inertial forces = ability to flow
b. viscous forces = relative thickness
c. gravitational forces = effects of gravity
Stress = externally applied forces (not only external: example
is thermal expansion)
Strain = resulting deformation
Fluid Density (r) = rho = mass/unit volume, affects:
magnitude of forces within a fluid
particle settling velocity
r increases with decreasing temperature
bipolar structure of water and it's function as a solvent
H2O = 0.998 g/ml at 20oC (700x r of air)
H2O therefore capable of carrying larger particles than air
Dynamic Flow Viscosity (m) = mu = measures ability of a fluid
to flow
m = t / (du/dy) = shear stress / velocity gradient
m a 1 / T (inversely proportional to temperature, see handout)
u = local fluid velcity; V = velocity of shearing surfaces
y = distance from shearing surface
Shear Stress (t) = shear force / unit area = dynes / cm2
acts on the fluid parallel to the surrounding surfaces
occurs at boundary of two fluids
controls erosion and entrainment
Shear Velocity (U*) = shear stress at the bed surface
= square root of shear stress / density
controls erosion and entrainment
Kinematic Viscosity (u) = nu = m / r = dynamic viscosity /
density
u a 1 / T (inversely proportional to temperature)
viscosity decreases with increasing temperature
determines extent to which flow will be turbulent
Froude Number (Fr) = ratio between inertial and gravity forces
Fr = U / = gauge of wave transmission (tranquil versus rapid)
U = mean flow velocity, g = gravitational acceleration, L =
water depth
Fr = < 1, wave velocity greater than flow velocity
waves propagate (move) upstream against current, tranquil, subcritical
Fr = > 1, waves not propagated upstream, rapid, shooting,
supercritical
different regimes create characteristic bedforms
Fluid Types and Flow (Fig. 2.2, p. 28)
Newtonian Fluids = fluids that have no strength and no change
in m
no m change with t change
example: clear water
Non-Newtonian Fluids = fluids that have no strength but have
variable m
m changes with t change
example: sediment laden water
Bingham Plastic = fluids with an intial strength to be overcome
before yielding
no m change with t change
example: concentrated sediment load - debris flow (grains in
mud matrix)
Psuedoplastic (Thixotropic) = fluids with an intial strength
until sheared, time loaded
m changes with t change
example: concentrated sediment load - liquification and turbidites