Fluid Motion Flashcards
(26 cards)
What is steady flow?
Flow that does not change with time (velocity field stays the same)
What is plug flow?
Plug flow is a simple model where the velocity profile of the fluid is assumed to be constant across any cross-section of the pipe perpendicular to the axis of the pipe
i.e. ignoring shear stress from surface and fluid friction (no boundary layer)
What is flux?
The rate at which a substance flows through a surface
One of the key features of fluids in motion is their ability to transport substances such as volume, mass momentum, energy., heat, contaminant…
One of the key features of fluids in motion is their ability to transport substances such as volume, mass momentum, energy., heat, contaminant…
What are the assumptions made when performing integral approximations of fluid property fluxes?
Rather than considering u as another function of z as it should be, the average velocity U (Q/A) is used to simplify the integral
This approximation only works well when the velocity profile is quite uniform over A i.e. for turbulent rather than laminar flows
What is a streamline?
A streamline is defined as a line that is everywhere tangential to the instantaneous flow velocity
What is the relationship between U and streamline spacing?
U is proportional to 1/streamline spacing
Conventionally, identical discharges pass through each stream tube
Why does flow separation happen?
Moving fluids have inertia, if they move in one direction, they cannot instantaneously change direction. Thus, if an obstacle is not streamlined, the fluid will separate from the solid boundary and we will observe:
- Flow separation
- Flow reattachment
- Recirculation zone
Why do recirculation zones have higher pressure?
Pressure is the way the fluid ‘communicates’, recirculation zones hence have higher pressures to redirect fluid flow
What is a vena contracta?
The point in a fluid flow where the stream cross-section is the smallest
In which direction does a recirculation zone recirculate?
A recirculation zone recirculates in the same direction as adjacent streamlines
What is an ideal fluid?
A fluid that experiences only normal stresses
wide application e.g. wave prediction
shear forces are ignored which implies that the fluid has zero viscosity
when drawing streamlines, we assumed that the fluid was ideal as we ignored friction
What are real fluids?
Fluids that have a viscosity that generates shear forces
real fluids experience friction and create boundary layers
What is the no-slip condition?
Fluid velocity and solid surface velocity are identical at the interface between a solid surface and a fluid
This is because under real conditions, the surface is rough on a molecular level, and causes the trapping of fluid particles
What are boundary layers?
The layers of fluids that are in the immediate vicinity of a boundary surface
They experience large velocity gradients as the effects of viscosity are significant
How is the shear stress calculated?
For Newtonian fluids
tau (shear stress) = mu (dynamic viscosity) times du/dz (velocity gradient)
*(Rheology is the study of fluids not obeying this relationship - Non Newtonian fluids such as rubber, clay, suspensions, plastics, paints, blood)
What are the implication of the relationship tau = (mu)(du/dz)
any shear stress - no matter how small - causes flow as shear stress creates a velocity gradient
Why do we usually ignore fluid shear stress in this module?
since we usually assume plug flow, velocity gradient and hence shear stress = 0
What is the relationship between dynamic viscosity mu and kinematic viscosity v?
kinematic viscosity v = dynamic viscosity mu / rho
When are dynamic and kinematic viscosity used respectively?
dyn mu is used for calculating forces
kine v is used when dealing with motion
v is often used when density variations are small or can be neglected
Equation of wall shear stress?
TAUw = (mu)(du/dz at wall)
What are the two flow states?
Laminar flow - organised layered flow
e.g. ground water flow
Turbulent flow - disorganised, random flow
e.g. river flow, tidal bore, fire plume, building ventilation
What is Reynolds’ number and how is it used to characterise flow states?
Re = rhoUL/mu
(inertia/viscosity)
Re «_space;1 (viscosity dominates; laminar)
Re»_space;1 (viscosity negligible; turbulent)
Re > 4000 for turbulent pipe flow
The magnitude of Re number is what is important, most flows encountered in CivEng are turbulent
What is entrance length and how is it used to help with Re number calculations?
Entrance length is the distance a flow travels after entering a pipe before the flow becomes fully developed
In calculations, if length < entrance lenth use length
if length > entrace length use diameter
