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As the fluid flows outward, the area of flow increases. As a result, to satisfy continuity equation, the velocity decreases and the streamlines spread out. The velocity at all points at a given distance from the source is the same. Fig 2 - Streamlines and potential lines for source flow. The velocity of fluid flow can be given as -
The velocity satisfies the continuity equation for incompressible flow: ∇ ⋅ u = 0. {\displaystyle \quad \nabla \cdot \mathbf {u} =0.} Although in principle the stream function doesn't require the use of a particular coordinate system, for convenience the description presented here uses a right-handed Cartesian coordinate system with ...
This analogy is usually invoked when discussing the continuity equation, the divergence of the field and the divergence theorem. The analogy sometimes includes swirls and saddles for points that are neither of the two. In the case of electric fields the idea of flow is replaced by field lines and the sources and sinks are electric charges.
Other equations in physics, such as Gauss's law of the electric field and Gauss's law for gravity, have a similar mathematical form to the continuity equation, but are not usually referred to by the term "continuity equation", because j in those cases does not represent the flow of a real physical quantity.
In the analysis of a flow, it is often desirable to reduce the number of equations and/or the number of variables. The incompressible Navier–Stokes equation with mass continuity (four equations in four unknowns) can be reduced to a single equation with a single dependent variable in 2D, or one vector equation in 3D.
Also once we find the stream function for a particular flow we are assured that the continuity equation is satisfied. This is most easily accomplished in 2D, steady, incompressible flow, where the continuity equation has only two terms. [1] It is also possible to define stream function for 2D, steady, compressible flow [2] as follows:
2D Contour Plot of Taylor Green Vortex. In fluid dynamics, the Taylor–Green vortex is an unsteady flow of a decaying vortex, which has an exact closed form solution of the incompressible Navier–Stokes equations in Cartesian coordinates.
The flow is axisymmetric ( ∂... / ∂θ = 0). The flow is fully developed ( ∂u x / ∂x = 0). Here however, this can be proved via mass conservation, and the above assumptions. Then the angular equation in the momentum equations and the continuity equation are identically satisfied.