Unit 1: Fluids (fundamental concepts) Flashcards
What is the definition of stress?
Stress is defined as force per unit area [N/m2]
σ = F/A
What is pressure?
The stress acting perpendicular to the surface
Also called Normal Stress
P = FNormal/A
What is shear stress?
The stress acting parallell to the surface
τ= FTangent / A
What is the definition of a fluid
A fluid is a substance that deforms continuously when subject to a shear stress, no matter how small the stress is
what is the characteristics of a fluid at rest
A fluid at rest shows inability to resist shear stress
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
What is the units of density
ρ = m/V
[kg/m3]
mass contained in a unit volume
What is the units of Specific Weight
γ = ρg
[N/m3]
weight per unit volume
What is the units of Specific Gravity
SG = ρ/ρwater
[-]
the ratio of a substances weight to that of an equal volume of water
How do you define viscosity of a Newtonian fluid?
In newtonian fluids the rate of deformation is proportional to the shear stress
The viscosity of a fluid is a measure of its
“resistance to deformation”
τ = μ * du/dy = μ * dα/dt
du/dy is called the velocity gradient
What is the kinematic viscosity of a fluid?
The ratio of viscosity to density
ν = μ/ρ
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
