Blandat Flashcards
(20 cards)
What is a body force
make an example of a body force
All external forces acting without physical contact on all particles of the system (and proportioal to their mass)
Gravitational and magnetic forces
What is a Surface force
make an example of a Surface force
all forces acting on a portion of the continuum system through its boundary surface
Pressure and viscous forces
Name three kinds of non-newtonian fluids.
How do they behave?
Dilatant (shear thickening): Higher rate of deformation at low shear stresses, then linear
Pseudoplastic: (shear thinning): Lower rate of deformation at low shear stresses, then linear.
Bingham plastic: behaves as a solid until a
high enough shear stress is achieved and then linear
What is the No Slip Condition
The no-slip condition shapes the velocity profile: the layer that sticks to the surface slows the velocity of the next adjacent fluid layer (because of viscous forces between the fluid layers)
The fluid has no velocity at the stationary surface
What is always true about the pressure at a point in a fluid at rest?
The pressure is isotropic:
The pressure at any point in a static fluid acts with the same magnitude in all directions
What is Stevin’s law
The pressure at any point within a fluid at rest is only proportional to the depth of that point
∂P/∂x=0, ∂P/∂y = 0, ∂P/∂z = -ρg = -γ
How does a manometer work?
A manometer uses a fluid column to measure the pressure differene
∆z = ∆P/ρg
∆P = ρgh
You can figure out the pressure by measuring the difference in elevation of the fluid.
In what conditions can the continuity equation be simplified?
Incompressible fluid: ρ is independent of time or spece > you can take it out of the integral > the densities cancel eachother
∫vdA = Q
Incompressible fluid + nondeformable CV:
∫vdA = Q = 0
Steady flow: ∂/∂t = 0 > the right side = 0 (Mass flow rate into CV = Mass flow rate out of CV)
∫ρvdA = 0
Uniform velocities at the cross section: replace the integral with a sum.
∫vdA = ΣvA = 0
When is the continuity equation used
When you are given most velocities and areas and need to calculate the last velocity or area.
Useally there are no forces in the problem
What defines steady flow?
The Mass flow rate into a fixed control volyme is equal to the Mass flow rate out of the control volume
∂/∂t = 0
What defines incompressible flow
The Volume flow rate into a fixed control volyme is equal to the Volume flow rate out of the control volume
What defines frictionless flow
When there are no viscous forces acting on the fluid
μ = 0
What are the terms of the bernoulli equation called?
Elevation head: z (gravitational potential energy)
Pressure head: P/γ (Flow work)
Velocity head: v^2/2g (Kinetic energy)
Piezometric head: z + P/γ (The sum of the first 2 terms above)
Total head: H (The sum of the first 3 terms above)
What does the components of the momentum equation represent?
FB = body forces
FS= surface forces
∂/∂t ∫ρvdV = rate of change of linear mumentum in the control volume (CV)
∫vρv*dA = rate at which linear mumentum is exiting the control surface (CS)
In what conditions can the Momentum equation be simplified?
Uniform flow:
∫vρvdA -> ΣρvAv
Steady flow:
∂/∂t ∫ρvdV = 0
When is the Momentum equation used
When you have forces, velocities and areas. Usually you have to calculate a Reaction force
How do you assess the flow regime?
You calculate the reynolds number (Re)
Laminar Re ≤ 2000
Transitional 2000 ≤ Re ≤ 4000
Turbulent Re ≥ 4000
What is relative roughness?
Roughness over the charecteristic length of the geometry
ε/D
What is pressure drop?
∆P/γ = ∆H
Happens in a horizontal pipe with steady velocity. The only thing that can change is the pressure head wich would drop in real fluids. (bernoullis equation)
What is the local losses proportional to?
the velocity squared
n V^2/2g
