The streamline upwind Petrov–Galerkin pressure-stabilizing Petrov–Galerkin formulation for incompressible Navier–Stokes equations can be used for finite element computations of high Reynolds number incompressible flow using equal order of finite element space (i.e. P k − P k {\displaystyle \mathbb {P} _{k}-\mathbb {P} _{k}} ) by introducing additional stabilization terms in the Navier–Stokes Galerkin formulation. The finite element (FE) numerical computation of incompressible Navier–Stokes equations (NS) suffers from two main sources of numerical instabilities arising from the associated Galerkin problem. Equal order finite elements for pressure and velocity, (for example, P k − P k , ∀ k ≥ 0 {\displaystyle \mathbb {P} _{k}-\mathbb {P} _{k},\;\forall k\geq 0} ), do not satisfy the inf-sup condition and leads to instability on the discrete pressure (also called spurious pressure). Moreover, the advection term in the Navier–Stokes equations can produce oscillations in the velocity field (also called spurious velocity). Such spurious velocity oscillations become more evident for advection-dominated (i.e., high Reynolds number R e {\displaystyle Re} ) flows. To control instabilities arising from inf-sup condition and convection dominated problem, pressure-stabilizing Petrov–Galerkin (PSPG) stabilization along with Streamline-Upwind Petrov-Galerkin (SUPG) stabilization can be added to the NS Galerkin formulation.
The incompressible Navier–Stokes equations for a Newtonian fluid Let Ω ⊂ R 3 {\displaystyle \Omega \subset \mathbb {R} ^{3}} be the spatial fluid domain with a smooth boundary ∂ Ω ≡ Γ {\displaystyle \partial \Omega \equiv \Gamma } , where Γ = Γ N ∪ Γ D {\displaystyle \Gamma =\Gamma _{N}\cup \Gamma _{D}} with Γ D {\displaystyle \Gamma _{D}} the subset of Γ {\displaystyle \Gamma } in which the essential (Dirichlet) boundary conditions are set, while Γ N {\displaystyle \Gamma _{N}} the portion of the boundary where natural (Neumann) boundary conditions have been considered. Moreover, Γ N = Γ ∖ Γ D {\displaystyle \Gamma _{N}=\Gamma \setminus \Gamma _{D}} , and Γ N ∩ Γ D = ∅ {\displaystyle \Gamma _{N}\cap \Gamma _{D}=\emptyset } . Introducing an unknown velocity field u ( x , t ) : Ω × [ 0 , T ] → R 3 {\displaystyle \mathbf {u} (\mathbf {x} ,t):\Omega \times [0,T]\rightarrow \mathbb {R} ^{3}} and an unknown pressure field p ( x , t ) : Ω × [ 0 , T ] → R {\displaystyle p(\mathbf {x} ,t):\Omega \times [0,T]\rightarrow \mathbb {R} } , in absence of body forces, the incompressible Navier–Stokes (NS) equations read
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