Fluid Flows, Transport Flashcards
(26 cards)
What is mass wasting?
It is a way in which sediment moves. Nothing to do with water/fluid. Just sediment moving due to gravity. Like a landslide.
What are the two types of fluid flows?
Laminar (low velocity) and turbulent (high velocity)
In terms of fluid flows, what are vectors?
forces
Is turbulent flow or laminar flow more efficient at moving particles?
Turbulent
What is Reynolds number?
Dimensionless number that expresses the ratio of the relative strength of the inertial and viscous forces in a moving fluid. The numerator approximates the inertial forces (the tendency of discrete parcels of fluid to resist changes in velocity and to continue to move uniformly in the same direction). High inertial forces disrupt laminar flow changing the parallel stream lines into turbulent eddies. Fluid inertial forces increase with higher flow velocities and/or a denser, more voluminous fluid mass.
The denominator of the equation estimates the viscous forces which are directly related to fluid viscosity. They make a fluid resistant to shearing or deformation.
R = 2rVp/u
(2radiusvelocity*density)/viscosity
viscosity and denisty are constant in the sample
A low R means laminar flow and a high E means a turbulent flow (ie better at picking up particles, faster moving).
R > 2000 turbulent
R < 500 laminar
What is the Froude number?
Ration between fluid inertial forces and fluid gravitational forces. Compares the tendency of a moving fluid (and particle borne by that fluid) to continue moving with the gravitational forces that act to stop that motion.
= flow velocity/acceleration of gravityforce of inertia1/2
= V/square root of dG
V - velocity
D - depth of flow
g - gravitational constant
Fr= 1 critical threshold
Fr <1 tranquil, steaming, subcritical flow
Fr>1 rapid shooting, supercritical flow
Important to understanding the ripples and other structures that form at the base of rapidly moving streams.
What is a hydraulic jump?
Tranquil flow gives way to rapid flow (often when the channel becomes steeper) with a smooth transition, but when a rapid flow suddenly decreases to a tranquil flow, there is an abrupt change known as a hydraulic jump — a sudden increase in depth accompanied by much turbulence.
Ex a stream moving with shallow rapid flow and appears to moving quickly and smoothly. Then suddenly erupts into a turbulent upstream breaking wave as the depth increases and flow becomes subcritical. In most such cases you are witnessing a flow that has just dropped below the threshold of Froude number 1.
What is sediment entrainment?
How particles get picked up into a flow? Two main forces: the fluid drag force exerts a horizontal force (parallel to the flow) on the particle and tends to roll it along. The torque produced by rolling will life the grain up slightly as it rolls over other particles. The fluid life force is primarly responsible for raising the particle vertically into the current. The net fluid force is the result of the horizontal drag vector and the fluid life vector producing a net movement upward and downstream
Describe Bernoilli’s principle
States that the sum of velocity and pressure on an object in a flow must be constant; if the velocity increases then the pressure must decrease and vice versa. Thus whenever a flow speeds up, it exters less pressure than slower moving parts of the flow. Think of what happens when airplane flies. The faster flow on teh top of a sed grain = less pressure which allows it to life
What is traction?
Rolling or pushing, surface creep
What is saltation?
Hopping
What is suspension?
the grain temporarily floating in a stream
Traction load and saltation load taken together constitute the ________
bedload
_________ constitutes a third mode of transport. ________ load consists of those grains that float more or less continually within the moving fluid.
Suspension, Suspended
What does Hjulstrom’s diagram tell us?
The velocity of a particle in water as it relates to its size. With bigger seds, there is more velocity needed to move them.
What is Stokes law of settling?
how the velocity with which clast settles through a fluid is calculated. As soon as a particle is lifted above the surface of a bed, it begins to sink back again. The distance it travels as it does so depends on the drag force of the current and the settling velocity of the particle. The more viscous the slower to settle. More dense, settles faster. The drag force exerted by a fluid on a falling grain is proportional to the fluid density, the diameter of the grains, the drag coefficient and the fall velocity. When density and viscosities are constant settling velocity increases with the diameter of the particle, or larger grains fall faster. Settling velocity decreases with higher viscosities and increases with denser particles.
_____ flows occur when cohesionless sediment (ex dry sand) moves downward toward the pull of gravity.
Grain
What are the four main tyoes of sediment gravity flows?
grain flow, fluidized sediment flow, debris flow, turbidity current
What is the structure left behind of a grain flow?
What is the structure left behind of a fluidized flow?
Study this sediment flows chart
What is the structure left behind of a debris flow?
What is the structure left behind of a turbidity current?
Only very slowly moving (or viscous) fluid exhibits ________ flow; most natural fluid flow is ________.
Laminar, turbulent
