In continuum mechanics, the material derivative describes the time rate of change of some physical quantity (like heat or momentum) of a material element that is subjected to a space-and-time-dependent macroscopic velocity field. The material derivative can serve as a link between Eulerian and Lagrangian descriptions of continuum deformation.
For example, in fluid dynamics, the velocity field is the flow velocity, and the quantity of interest might be the temperature of the fluid. In which case, the material derivative then describes the temperature change of a certain fluid parcel with time, as it flows along its pathline (trajectory).
There are many other names for the material derivative, including:
advective derivative
convective derivative
derivative following the motion
hydrodynamic derivative
Lagrangian derivative
particle derivative
substantial derivative
substantive derivative
Stokes derivative
total derivative, although the material derivative is actually a special case of the total derivative
The material derivative is defined for any tensor field y that is macroscopic, with the sense that it depends only on position and time coordinates, y = y(x, t):
where ∇y is the covariant derivative of the tensor, and u(x, t) is the flow velocity. Generally the convective derivative of the field u·∇y, the one that contains the covariant derivative of the field, can be interpreted both as involving the streamline tensor derivative of the field u·(∇y), or as involving the streamline directional derivative of the field (u·∇) y, leading to the same result.
Only this spatial term containing the flow velocity describes the transport of the field in the flow, while the other describes the intrinsic variation of the field, independent of the presence of any flow. Confusingly, sometimes the name "convective derivative" is used for the whole material derivative D/Dt, instead for only the spatial term u·∇. The effect of the time-independent terms in the definitions are for the scalar and tensor case respectively known as advection and convection.
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In continuum mechanics the flow velocity in fluid dynamics, also macroscopic velocity in statistical mechanics, or drift velocity in electromagnetism, is a vector field used to mathematically describe the motion of a continuum. The length of the flow velocity vector is the flow speed and is a scalar. It is also called velocity field; when evaluated along a line, it is called a velocity profile (as in, e.g., law of the wall).
Une particule fluide, en mécanique des fluides, est un volume élémentaire de fluide d'échelle mésoscopique. L’échelle mésoscopique est typiquement de l'ordre du micromètre. C'est une échelle d'une part suffisamment petite pour que la grandeur étudiée puisse être considérée comme ponctuelle, et d'autre part suffisamment grande pour pouvoir considérer le milieu comme continu, c'est-à-dire ne pas avoir à faire une étude discrète de toutes les molécules.
Un fluide incompressible est un fluide dont le volume est considéré comme constant quelle que soit la pression qu'il subit, tout fluide étant en réalité sensible à la pression. Par nature, tous les fluides sont compressibles, certains plus que d'autres, et en phase gazeuse considérablement plus qu'en phase liquide. La compressibilité d'un fluide mesure la variation de volume d'une certaine quantité de ce fluide lorsqu'il est soumis à une pression extérieure.
The goal of the course is to introduce relativistic quantum field theory as the conceptual and mathematical framework describing fundamental interactions such as Quantum Electrodynamics.
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