Fluids and flows

Question 1

What is a fluid?

Answer 1

Water and air

Question 2

What are 3 important characteristics of fluids?

Answer 2

Density (𝜌): Determines the magnitude of forces such as shear stress that act within the fluid and on the bed.

Effective density: Density of solid particle – density of fluid.

Viscosity (μ): Ability of the fluid to flow. Defined as the ratio of shear stress (𝜏) to the rate of deformation sustained by that shear stress across the fluid. Viscosity is not constant and varies with T°

Question 3

What is fluid turbulence?

Answer 3

Random movement of parcels of fluid superimposed upon the overall flow.

Reducing the fluid velocity or increasing its density will reduce the turbulence.

A non-turbulent fluid is said to be laminar.

Question 4

What is the Froude number?

Answer 4

Ratio of inertial to gravitational forces in the flow

If Fr > 1: rapid (supercritical) flow
If Fr < 1: tranquil (subcritical) flow

Question 5

What is a hydraulic jump?

Answer 5

Sharp transition between the two stages (rapid and tranquil)

Question 6

Hvordan opfører bølger sig på lavt vs. dybt vand?

Answer 6

Lavt = ‘mærker’ bunden og reagerer på den + cirkulær svingning bliver vandret ned mod bunden

Dybt = cirkulær svingning dør ud ned mod bunden pga. indre friktion

Question 7

What are the 2 modes of particle movement?

Answer 7

In suspension = suspended in the flow

Bedload transport = rolling or saltation on floor

Question 8

What different kinds of sediment gravity flows exists?

Answer 8

Grainflow on an aeolian dune lee slope (sandbanke)

Mudflow (mud-dominated debris flow)

Turbidity flow (body + head)

Question 9

What is a pyroclastic density current?

Answer 9

Overall similar physical processes as sediment gravity flows

BUT composed of volcanic particles and hot gas, moving in the air AND on the water!