Fluid Flow and sediment transport-split (1).pdf
- 1. Contents • Fluids • Types of fluids • Laminar flow • Turbulent flow • Reynolds Number • Froude Number • Particle Entrainment • Sediment Transport and Deposition
- 2. Fluids • Fluids are substances that change shape easily and continuously as external forces are applied. • They have negligible resistance to shearing forces. • Natural fluids includes, crude petroleum, natural gas , air and water. • Air, water and water containing various amounts of suspended sediments are the fluids of primary interest. • Two basic physical property: Density and Viscosity. Greatly affects the ability of fluids to erode and transport sediments.
- 3. Cont.. • Fluid density (ρ) is defined as mass per unit fluid volume. ρ= m/v • Affects magnitude of forces that act on fluid and on the bed. • Influences the movement of fluids downslope under influence of gravity. • Varies with different fluids and increases as temperature of fluid decreases. Eg. Sugar syrup • Density of water(0.998g/mL at 20ᵒc) is more than 700 times greater than that of air. This density differences influences the abilities of water and air to transport sediment
- 4. Cont.. • Fluid viscosity is a measure of the ability of fluids to flow. • Fluids with low viscosity flow readily and fluids with high viscosity flow sluggishly. • air< ice, water< honey • Viscosity of water at 20ᵒc is almost 55 times greater than air • Viscosity increases with decreasing temperature of fluids. • Viscosity has important influence on water turbulence. • Increased viscosity suppresses turbulence hence slows down the rates at which particle settle . Decreased viscosity increases the turbulence hence increases the ability to erode and transport
- 6. Cont.. • Molecular (dynamic)viscosity(µ) is the measure of resistance of a substance to change in shape taking place at finite speed during flow. It is defined as the ratio of shear stress(τ) to the rate of deformation(𝑑𝑢/𝑑𝑦) sustained across the fluid. 𝜇 = 𝜏 𝑑𝑢/𝑑𝑦 Where, τ = shear stress, du/dy = velocity gradient • Density and dynamic viscosity strongly affect the fluid behaviour. • Fluid dynamists commonly combine the two into a single parameter called kinematic viscosity(ν) which is ratio of dynamic viscosity to density 𝜈 = 𝜇/𝜌 • Kinematic viscosity is an important factor in determining the extent to which fluid flows exhibit turbulence.
- 7. Types of fluids • Depending upon the extent to which dynamic viscosity(μ) changes with shear or strain (deformation) rate, three types of fluid are defined • Newtonian fluid: fluid having no strength and does not undergo change in viscosity as shear rate increases. Eg, water. • Non- Newtonian fluids: fluid having no strength but shows variable viscosity with change in shear or strain rate. Eg, highly water saturated mud • Thixotropic : shearing destroys the strength of the fluid, the substance behaves like fluid (Non- Newtonian) until allowed to rest for a short while after which the strength is again regained. Eg, Shearing resulting from earthquake tremors, can cause liquefaction and failure of such muds.
- 8. Laminar versus turbulent flow • Fluids in motion display two modes of flow depending upon the flow velocity, fluid viscosity and roughness of bed over which flow takes place. • When the fluid viscosity is higher compared to flow velocity, the fluid flows as a straight, parallel path in form of lamina of nearly constant width. Such flow is termed as LAMINAR FLOW. • It can be visualized as parallel sheet of filaments, referred to as streamline, over the river bed. • Streamlines may curve over an object , but they never intertwine. • When the flow velocity increases and fluid viscosity decreases the stream is no longer maintained as a coherent stream but breaks up and becomes highly distorted. It moves as a series of constantly changing and deforming mass. The streamlines are intertwined in a very complex way. This type of flow is termed TURBULENT FLOW. • Turbulence is thus an irregular or random component of fluid motion. • Highly turbulent masses are termed as eddies. • Most flow of water and air under natural condition is turbulent, although flow of ice and mud flows are essentially laminar.
- 11. Reynolds Number • The fundamental differences in laminar and turbulent flow arises from the ratio of inertial forces to viscous forces. • Inertial forces which are related to velocity of fluids in motion, tend to cause fluid turbulence. • Viscous forces suppress turbulence. • The relationship of inertial to viscous forces can be shown mathematically by a dimensionless value called the Reynolds number which is expressed as • where, U is mean velocity of flow, L is length(commonly water depth) and ν is kinematic viscosity.
- 12. Cont.. • When viscous forces dominate, as in highly concentrated mud flows , Reynolds number are small and flow is laminar. • Very Low flow velocity or shallow depth also produces low Reynolds number and laminar flow. • As inertial force dominate and flow velocity increases, as in atmosphere and river flow, Reynolds number are large and flow is turbulent. • The transition from laminar flow to turbulent flow takes place above a critical value of Reynolds number, which commonly lies between 500 and 2000 and which depends upon the boundary conditions such as channel depth and geometry. • Thus, under a given set of boundary conditions, the Reynolds number can be used to predict whether flow will be laminar or turbulent.
- 13. Froude Number • In addition to the effects of fluid viscosity and inertial forces, gravity also plays a role in fluid flow because gravity influences the way in which a fluid transmits surface waves. • The velocity with which small gravity waves move in shallow water is given by the expression 𝑔𝐿, where, g is gravitational acceleration and L is water depth. • The ratio between inertial and gravity forces is the Froude number which is expressed as 𝐹𝑟 = 𝑈 𝑔𝐿 Where, U is the mean velocity of flow and L is the water depth. • Similar to Reynolds number Froude number is also dimensionless and hence useful in modelling studies.
- 14. Cont.. • When Fr <1 , the wave velocity is greater than flow velocity and waves can propagate upstream. Flow is termed as tranquil, streaming or subcritical. • When Fr > 1, wave velocity is lower than flow velocity and waves can’t propagate upstream. The flow is termed as rapid, shooting or supercritical. • Thus the Froude number can be used to define the critical velocity of moving water , when it changes from tranquil to rapid flow or vice versa. • The Froude number also has a relationship to flow regimes, which are defined by characteristic bedforms, such as ripples, cross beddings , plane beds etc. which develop during fluid flow over a sediment bed.
- 16. Particle entrainment • Transport of sediments by fluid flow involves two fundamental steps – Erosion and entrainment of sediments from bed – Sustained downcurrent or downwind movement of sediment along or above the bed • Term entrainment refers to process involved in lifting resting grains from the bed or simply putting them in motion. • More energy is required for entrainment . • Settling velocity is an important factor in determining the travel distance of particle before resting again on the bed.
- 17. Particle entrainment by currents • As the velocity and shear stress of a fluid moving over a sediment bed increases, a critical point is reached at which the grain begins to move downcurrent. • Smaller and lighter grains are moved first. • As shear stress increases larger grains are put in motion until finally all grains are in motion. • This critical threshold for grain movement is a direct function of several variables: shear stress, fluid viscosity and particle size ,shape and density. • The opposing forces which come into play as fluid moves across its bed include, gravity force, drag force and lift force.
- 18. Opposing forces • Gravity forces result from mass of particle and resist grain movement by frictional resistance between particles. • Drag force acts parallel to the bed and depends on boundary shear stress. • The hydraulic lift force is caused by Bernoulli effect of fluid flow over grains
- 19. Cont.. • Boundary (bed) Shear stress(τ0): stress which opposes the motion of the fluid and exists at bed surface. • To differentiate it from fluid shear stress(τ), it is defined as force per unit area parallel to the bed. • It is function of the density of fluid, slope of the bed, and water depth. • Expressed as : τ0 = γghs • Where, γ is density of fluid, g is gravitational acceleration, h is water depth and s is the slope of parallel bed and water surface(gradient). • Boundary shear stress increases directly with increasing density of moving fluid with ,increasing diameter and depth of river channel and increasing slope of streambed.
- 20. Role of settling velocity in transport • As soon the grains are entrained , they begin to fall back to bed. • The distance they travel downcurrent before resting again on bed depends upon the drag and lift forces exerted by the current, including turbulence, and the settling velocity of the particles . • A particle initially accelerates as it falls through a fluid, but acceleration gradually decreases until a steady rate of fall called terminal fall velocity is achieved. • For small particles terminal fall velocity is achieved very quickly. • The settling rate is determined by the interaction of upwardly directed forces- owing to buoyancy of fluid and downwardly directed forces arising from gravity.
- 21. Sediment transport • Once sediment has been entrained , the transport path that it takes downcurrent is function of settling velocity. Current velocity and turbulence. • The sediment load may consist entirely of very coarse particles, entirely of very fine particles, or mixtures of coarse and fine particles. • Coarse sediments such as sand and gravel moves on or very close to the bed during transport and is considered to constitute the bed load. • Finer materials carried higher up in main flow above the bed makes up the suspended load. • If the shear velocity is greater than the settling velocity materials remain in suspension.
- 22. Cont.. • Bed Load Transport: Particles larger than sand size are commonly transported as bed load. The running water in 4 different ways transport sediment which are rolling, sliding, creep, and saltation. The first two are confined to stream bed and contribute to traction transport. Involves rolling of large or elongated grains and sliding of grains over past each other. Creep results from grains being pushed at short distance along the bed in downcurrent direction due to impact of other moving grains. Tangential forces operating on particle may occasionally force it to leap or move forward into a state of temporary suspension until the force of gravity brings it down to the bed. This whole process is then repeated and particles jumps, constituting saltation movement. The saltating grains can raise upto 1mts .
- 23. Cont.. • Suspended Load Transport: as the flow strength of water current increase, turbulence intensity increases near the bed. Particle trajectories become longer, more irregular and higher up from bed than trajectories of saltating grains. The upward component of fluid motion due to turbulence increases to such point that it balances the gravitational force allowing the particles to remain suspended. This behaviour is called intermittent suspension. The smaller particles because of low settling velocity remain in continuous suspension and are carried along at almost the same velocity as the fluid flow. • Wash Load Transport: much of the sediment load undergoing continuous suspension transport is composed of fine, clay-size particles with very low settling velocities. In rivers, this sediment is derived either from upstream sources areas or by erosion of the banks, rather than from stream bed, and is called the wash load.
- 25. Transport by Wind • Wind can be considered a very low density, low viscosity fluid that is capable of flowing and bringing about sediment transport. • Entrainment of grains by wind action is strongly affected by impact of moving grains hitting the bed. This lower threshold for grain movement is called impact threshold. • Fine sand and particles size below it are commonly transported. Sand –size particles move by traction (surface creep) and saltation and dust size particles by suspension. • The transport takes place at relatively high wind velocities and the flow is commonly turbulent, characterized by eddies of various size moving with different speeds and directions. Suspended load carried by the wind are called dustloads.
- 26. Transport by Glacial Ice The high viscosity of glacial ice causes it to flow very slowly. Glacier are capable of entraining huge volumes of sediment by scraping and plucking the underlying bedrock and adjacent valley walls. since viscosity is high therefore flow is laminar, sediments of all size are carried along with moving ice in contact with the bed and suspended at various heights above the base of the glacier. When the front of a glacier melts , the sediment load is dumped as unsorted, poorly layered glacier moraine.
- 27. Deposits of Fluid Flows • Sediments entrained and transport by moving water and wind stops, and deposition occurs when the local hydrologic and wind conditions change and the shear stress of bed is reduced. • Decrease in bed slope, increase in bed roughness, loss of water volume, decreased wind velocity , change in topography and weather conditions are the principal reasons. • Sediment deposition may be temporary or permanent. • Sediments deposited in river channels and point bars, beach environment etc. can be reentrained and subjected to continued transport depending on seasonal changes. • Some river and lake sediment, wind transported sediment may become deposited in continental settings and be preserved for longer periods. • The great bulk of sediment undergoing transportation ultimately finds its way into ocean basin.
- 28. Cont.. • Sediments deposited by fluid flow of water or wind are commonly characterized by layers or beds of various thickness depending upon depositional conditions with variety of sedimentary structures. • Sediments deposited from traction currents commonly preserve sedimentary structures such as cross- beds, ripple marks and pebble imbrication that display directional features from which ancient fluid flow direction can be determined. • Sediments deposited by suspension lack this structures and instead show fine laminations. • Wind is competent to transport and deposit particles in size range of sand to clay but water currents deposits may range in size from clay to cobbles to boulders. These variations reflect wide range of energy variations of wind and water. • Sediments deposited by glaciers are characteristically poorly layered and extremely poorly sorted with particles ranging from meter size boulder to clay size grains.