Conservation laws Flashcards

1
Q

system approach

A

The word system refers to a fixed quantity of mass with a boundary. However,
with time the boundary of the system may change, but the mass remains the
same (no mass crosses system boundaries). Focus on particles

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2
Q

Control volume approach

A

An arbitrary volume in space through which fluid flows.
Like a “window” for observation in the flow
Flow may cross boundaries

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3
Q

Mass flow rate

A

The amount of mass flowing through a cross section per unit time

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4
Q

Volume flow rate

A

The volume of the fluid flowing through a cross section per unit time

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5
Q

Volume flow rate for incompressible fluids

A

For incompressible fluids, ρ=constant, the volume flow rate INTO a fixed
control volume is equal to the volume flow rate OUT of the control volume

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6
Q

Mass Flow rate for steady, compressible flows

A

ρ=ρ(x,y,z) and by definition of steady flow no
fluid property varies with time. the mass flow rate INTO a fixed control volume is equal to the
mass flow flow rate OUT of the control volume

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7
Q

For uniform velocities, how can the continuity equation be simplified?

A

If the velocity can be considered uniform across the cross section, or if
we can use an average velocity in the cross section, then we can replace
the integral with the sum

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8
Q

What is a streamline?

A

A streamline is a curve that is everywhere tangent to the
instantaneous local velocity vector

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9
Q

What is the bernoulli equation

A

The Bernoulli equation is usefull approximate relation between pressure, velocity
and elevation in regions of steady, incompressible flow, where viscous forces can be
neglected.

The Bernoulli equation states that during steady, incompressible flow with negligible
friction, the various forms of mechanical energy (kinetic, potential and flow) are
converted to each other, but their sum remains constant

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10
Q

Elevation head

A

Gravitational potential
energy, z

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11
Q

Pressure head

A

Flow work, P/γ

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12
Q

Velocity head

A

Kinetic energy, V^2/2g

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13
Q

Real fluids

A

Take into account for the effects due to fluid viscosity, μ!
During fluid motion, viscosity generates shear
stresses,τ0, which tend to resist to flow.
Their work is associated with dissipation of mechanical
energy which is converted in thermal form

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14
Q

Modified Bernoulli equation for real fluids

A

H(s) = H0- deltaH

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15
Q

The momentum equation is based on newtons second law. What does it say? (oklart svar, ändra!)

A

The sum of all external forces acting on the system is equal to the time rate of
change of linear momentum of the system

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16
Q

Body forces

A

acting throughout the entire body of the control volume
(such as gravity, electric,and magnetic forces)

17
Q

Surface forces

A

acting on the control surface
(such as pressure and viscous forces)

18
Q

Momentum equation for steady flow

A

During steady flow, the amount of momentum within the control volume remains constant and
thus the time rate of change of linear momentum of the contents of the control volume is zero

19
Q

Momentum equation for uniform flow

A

Integral –> sum. If the velocity across the inlet or outlet of the CV is uniform (or can approximate), we could
simply take it outside the integral.