description can be an imagination, but action must be real …… p m v subbarao professor...
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Description can be an Imagination, but Action must be
Real ……
P M V SubbaraoProfessor
Mechanical Engineering Department
I I T Delhi
Material Derivative using Eulerian Description
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Eulerian Description of Flow : The Concept
•Adopt the point of view that we will observe fluid properties at few selected points xi as a function of time.• The association with a given fluid particle must be broken.• Realize that as time flows different fluid particles will occupy the position xi.• Once properties are expressed as functions of (x; t) , it is said to be the Eulerian description of a fluid.
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The Lagrangian Displacement of Particle in Eulerian Description
• It is of very high importance to compare these two descriptions of a fluid and validate Euler description.
• The most basic definition is the meaning of velocity: the definition is
ttxvt
x,,
That is to say, following the particle one need to calculate the rate of change of position with respect to time.
• Given the Eulerian velocity field, the calculation of Lagrangian displacement is therefore mathematically equivalent to solving the initial value problem .• A set of ordinary differential equations for the function x(t) , with the initial condition x(0) = . • This is of very high importance to compare these two descriptions of a fluid and validate Euler description.
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Material Derivative with Imaginary Description
• Consider Eulerian quantity
t
txfttxxf
dt
txdf ppp
t
p
,,lim
,0
during any infinitesimal time t
• The time rate of change of attached to this fluid parcel is expressed as:
• Attribute this quantity to lgrangians’parcel, p.
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