--- title: Turbulence closure (RANS, Spalart–Allmaras) summary: Turbulent flows are computed with the Reynolds-averaged equations closed by the negative Spalart–Allmaras model. The model equation is one more row of the same discrete operator, so the Newton linearisation, the adjoint and the shape derivatives (including the wall distance) remain exact. section: theory order: 11 level: 2 keywords: [RANS, turbulence, Spalart-Allmaras, SA-neg, wall distance, eddy viscosity] --- At flight Reynolds numbers the boundary layers on an aircraft are turbulent, and resolving the turbulence itself is out of reach for design work. Gyre therefore offers the Reynolds-averaged Navier–Stokes (RANS) equations with a one-equation closure: the Spalart–Allmaras model in its *negative* form (SA-neg), the variant written for discretizations, such as high-order DG, whose solution can undershoot at the edge of a boundary layer. **[L1 Essentials] The idea in one sentence** One extra transported quantity, $\tilde\nu$, produces an eddy viscosity $\mu_t$ that is added to the molecular viscosity seen by the mean flow; everything else in the solver is unchanged. **[L2 Practice] Using it** ```yaml flow: mach: 0.2 reynolds: 5.0e6 turbulence: sa # 'none' (default) or 'sa'; needs reynolds extra: {chi_inf: 3.0, prt: 0.9} # optional: free-stream nu_tilde/nu, turbulent Prandtl number ``` With `turbulence: sa` the state has one more variable per node, $\rho\tilde\nu$, stored after the energy. No-slip walls impose $\tilde\nu = 0$ and define the wall distance; the far field and inlets supply $\tilde\nu = \chi_\infty \nu$. Outputs, gradients, error estimates and the discipline interface work as for laminar flow; `chi_inf` and `prt` are ordinary differentiable flow parameters. The model is integrated to the wall (no wall functions), so the first element must be thin enough for the polynomial inside it to resolve the viscous sublayer: see [the flat-plate study](../verification/turbulence.md) for the resolution used there. **[L2 Practice] What is and is not included** * Included: SA-neg with the 2012 modifications of Allmaras, Johnson and Spalart (the $\tilde S$ limiter that keeps the modified vorticity positive, the negative branch, the $f_{t2}$ laminar-suppression term), a constant turbulent Prandtl number, an exact curved wall distance that takes part in shape derivatives. * Not included: trip terms and transition modelling (the flow is turbulent wherever the model produces eddy viscosity), rotation/curvature and compressibility corrections, the quadratic constitutive relation, the $\tfrac23 \rho k$ term of the Boussinesq stress (neglected as is usual for this model), wall functions, other models. * Verified on a manufactured solution, a linearisation study, the zero-pressure-gradient flat plate of the NASA Turbulence Modeling Resource and adjoint-versus-finite-difference derivatives ([verification](../verification/turbulence.md)); in three dimensions a spanwise extrusion of that plate reproduces its section. Separated flows, aerofoils at flight Reynolds number and turbulent flows that are three-dimensional in themselves have not been run. ## Model equations **[L3 Mechanics] Transport equation** The working variable is $\tilde\nu$, with $\chi = \tilde\nu/\nu$. Gyre solves the conservative form for $\rho\tilde\nu$, $$ \pd{(\rho\tilde\nu)}{t} + \nabla\cdot(\rho\mathbf u\,\tilde\nu) = \rho\,(P - D) + \frac{1}{\sigma}\nabla\cdot\big(\rho(\nu + \tilde\nu f_n)\nabla\tilde\nu\big) - \frac{1}{\sigma}(\nu + \tilde\nu f_n)\,\nabla\rho\cdot\nabla\tilde\nu + \frac{c_{b2}}{\sigma}\,\rho\,\abs{\nabla\tilde\nu}^2 , \label{eq:sa} $$ with production $P$ and destruction $D$ that depend on the sign of $\tilde\nu$: | | $\tilde\nu \ge 0$ | $\tilde\nu < 0$ | |---|---|---| | $P$ | $c_{b1}(1 - f_{t2})\,\tilde S\,\tilde\nu$ | $c_{b1}(1 - c_{t3})\,S\,\tilde\nu$ | | $D$ | $\big(c_{w1} f_w - \dfrac{c_{b1}}{\kappa^2} f_{t2}\big)\dfrac{\tilde\nu^2}{d^2}$ | $-c_{w1}\dfrac{\tilde\nu^2}{d^2}$ | | $f_n$ | $1$ | $\dfrac{c_{n1} + \chi^3}{c_{n1} - \chi^3}$ | | $\mu_t$ | $\rho\tilde\nu f_{v1}$, $\; f_{v1} = \dfrac{\chi^3}{\chi^3 + c_{v1}^3}$ | $0$ | $S$ is the magnitude of the vorticity and $d$ the distance to the nearest no-slip wall. The mean flow sees the viscosity $\mu + \mu_t$ and the conductivity $c_p(\mu/\mathit{Pr} + \mu_t/\mathit{Pr}_t)$. **[L4 Theory] Closure functions and constants** For $\tilde\nu \ge 0$ the modified vorticity is $$ \tilde S = \begin{cases} S + \bar S & \bar S \ge -c_{v2} S \\[4pt] S + \dfrac{S\,(c_{v2}^2 S + c_{v3}\bar S)}{(c_{v3} - 2c_{v2})S - \bar S} & \text{otherwise} \end{cases} \qquad \bar S = \frac{\tilde\nu f_{v2}}{\kappa^2 d^2},\quad f_{v2} = 1 - \frac{\chi}{1 + \chi f_{v1}} , $$ which is positive whenever $S$ is and has a continuous first derivative at the switch. The wall function of the destruction term is $$ f_w = g\left(\frac{1 + c_{w3}^6}{g^6 + c_{w3}^6}\right)^{1/6},\qquad g = r + c_{w2}(r^6 - r),\qquad r = \min\!\Big(\frac{\tilde\nu}{\tilde S\kappa^2 d^2},\, r_\text{lim}\Big), $$ and $f_{t2} = c_{t3}\exp(-c_{t4}\chi^2)$. Constants: $c_{b1} = 0.1355$, $c_{b2} = 0.622$, $\sigma = 2/3$, $\kappa = 0.41$, $c_{w1} = c_{b1}/\kappa^2 + (1 + c_{b2})/\sigma$, $c_{w2} = 0.3$, $c_{w3} = 2$, $c_{v1} = 7.1$, $c_{v2} = 0.7$, $c_{v3} = 0.9$, $c_{t3} = 1.2$, $c_{t4} = 0.5$, $c_{n1} = 16$, $r_\text{lim} = 10$; $\mathit{Pr}_t = 0.9$ and $\chi_\infty = 3$ by default (`prt`, `chi_inf`). **[L3 Mechanics] Why the negative branch** A polynomial that represents the sharp edge of a boundary layer undershoots, so $\tilde\nu$ becomes slightly negative there. The original model then loses its footing: the eddy viscosity turns negative and the diffusion coefficient can change sign. SA-neg continues the equation to $\tilde\nu < 0$ so that it stays well posed: no eddy viscosity, a diffusion coefficient that remains positive through $f_n$, and source terms that drive $\tilde\nu$ back towards zero. The positive and the negative branch join with continuous first derivatives, which is what Newton's method and the adjoint need. ## Discretization **[L3 Mechanics] One more row of the same operator** Equation \eqref{eq:sa} goes through the machinery of the mean-flow equations ([DGSEM](dgsem.md), [BR2](viscous-br2.md)): * **Convection.** In the volume, the two-point mass flux of the mean flow times the arithmetic mean of $\tilde\nu$; on faces, upwinding of $\tilde\nu$ with the interface mass flux $f_\rho^*$ of the Riemann solver, $f^*_{\rho\tilde\nu} = \tfrac12 f_\rho^*(\tilde\nu_L + \tilde\nu_R) - \tfrac12\sqrt{(f_\rho^*)^2 + \delta^2}\,(\tilde\nu_R - \tilde\nu_L)$. * **Diffusion.** The conservative diffusion term is one more row of the viscous flux and takes the BR2 path unchanged. $\tilde\nu$ and $\rho$ join the velocity and the temperature as gradient variables. * **Sources.** Production, destruction and the two non-conservative gradient terms are evaluated with the *BR2 gradient* $q = G + \sum_f r_f$, the same lifted gradient the viscous volume flux uses, so that they see the jumps between elements. **[L3 Mechanics] Sources are integrated at Gauss points** The Gauss–Lobatto solution nodes include points on the wall, where $d = 0$ and $\tilde\nu/d$ is undefined for a discrete $\tilde\nu$ that satisfies its wall condition only weakly. The source is therefore integrated with Gauss quadrature: state and BR2 gradient are interpolated to the $(p+1)^d$ Gauss points of each element, where $d > 0$, the source is evaluated there, and the result is tested against the nodal basis, $$ (J R)_{i} \mathrel{-}= \frac{1}{w_i}\sum_g \ell_i(\xi_g)\, w_g\, J_g\, s_t(U_g, q_g, d_g). $$ **[L4 Theory] Lifting the gradient into the source: dual consistency** A source that depends on the gradient of the solution is adjoint consistent only if it is evaluated with a gradient that includes the interface contributions (Oliver and Darmofal). Using the element-local gradient $G$ alone would leave the primal scheme consistent but make the discrete adjoint inconsistent with the continuous one, and functionals would lose their superconvergence. The BR2 gradient $q$ already contains the face liftings, so the same array serves the viscous flux and the source. **[L4 Theory] Smoothness: what was changed in the model** Newton's method and the adjoint need a residual with continuous first derivatives. The model's own switches (the $\tilde S$ limiter and the sign branch) have them by construction. The remaining kinks are smoothed: | Quantity | Model | Gyre | Size of the change | |---|---|---|---| | vorticity magnitude | $\abs{\boldsymbol\omega}$ | $\sqrt{\abs{\boldsymbol\omega}^2 + \epsilon^2} - \epsilon$ | at most $\epsilon = 10^{-6}$ (units $u_\infty/L_\text{ref}$); exactly zero in irrotational flow | | limiter of $r$ | $\min(r, r_\text{lim})$ | $C^1$ blend on $r_\text{lim} \pm 1$ | none for $r \le 9$ | | $f_{v1}$ and $\mu_t$ at $\tilde\nu = 0$ | $\chi^3$ | $\max(\chi, 0)^3$ | none ($C^2$ already) | | interface upwinding | $\abs{f_\rho^*}$ | $\sqrt{(f_\rho^*)^2 + \delta^2}$ | a dissipation floor $\delta = 10^{-3}\bar\rho$ | **[L3 Mechanics] Boundary conditions** | Condition | $\tilde\nu$ | |---|---| | no-slip walls | $\tilde\nu = 0$, in the convective flux and in the BR2 lifting (Dirichlet) | | `farfield` | exterior state $\chi_\infty\mu_\infty$; the mass-flux upwinding takes it on inflow and the interior value on outflow | | `total_inlet` | $\chi_\infty\,\mu(T_b)$ at the boundary state | | slip walls, symmetry, `pressure_outlet` | the interior value; no diffusive flux | ## Wall distance **[L3 Mechanics] Exact distance to the curved wall** The wall distance is computed once per discretization for every Gauss point: a k-d tree over samples of the wall faces proposes candidate faces, and a projected Gauss–Newton iteration on each candidate's polynomial (of the geometry degree) finds the closest point. The result is stored as interpolation weights of the foot point on the geometry nodes of its wall face, not as a number. **[L3 Mechanics] The wall distance is part of the shape derivative** Because the foot point is stored as weights on mesh nodes, the distance is re-evaluated inside the differentiable geometry, $$ d(X) = \abs{x_g(X) - x_\text{foot}(X)} , $$ whenever the node coordinates $X$ change. By the envelope theorem the derivative of a minimum with respect to a parameter is the partial derivative at the minimiser, so holding the foot-point parameters fixed gives the exact first derivative of the distance with respect to $X$, as long as the closest face does not change. Shape derivatives of turbulent cases therefore include the wall-distance sensitivity; the [adjoint study](../verification/turbulence.md) checks them against finite differences that recompute the distance. ## Solver **[L3 Mechanics] Row weights** In the units of the solver the mean-flow equations are of order one and the turbulence equation of order $\tilde\nu \sim \chi/\mathit{Re}$, six or seven orders smaller at flight Reynolds number. Unweighted, an inexact Krylov solve and the convergence monitor would not see it. The rows of the turbulence equation are therefore weighted by $\mathit{Re}/\chi_\text{ref}$ with $\chi_\text{ref} = 100$: the Newton residual norm is $\norm{s\,R_w}$, the forward Krylov solver works on $(SA)x = Sb$ and the adjoint one on $(S^{-1}A\T)x = S^{-1}b$. The solutions are those of the unscaled systems; only the norm the tolerances refer to changes. The [implicit solver](implicit-solver.md), the block-Jacobi preconditioner and p-multigrid are otherwise the same as for laminar flow, with element blocks one variable larger. **[L3 Mechanics] Limits and open questions** * The eddy viscosity enters the stable time step and the block preconditioner, but there is no positivity enforcement for $\rho$, $p$ or $\mu + \mu_t$ beyond what the model provides. * Start-up from a uniform free stream relies on pseudo-transient continuation; there is no dedicated turbulence start-up strategy and no measured robustness on separated flows. * Wall-resolved meshes have element aspect ratios of the order of a thousand at the wall. Element-block Jacobi does not see that anisotropy: used alone it leaves linear solves on such meshes slow or short of their tolerance. Two remedies exist. The piecewise-constant coarse level of the p-multigrid hierarchy (`solver.levels` at its default, which includes $p = 0$) makes them converge with block Jacobi (solver statistics of the [adjoint study](../verification/turbulence.md#adjoint-derivatives)). The line smoother (`solver.smoother: line`, [how it works](implicit-solver.md#preconditioning-without-assembling-a-matrix)) inverts the coupling along the wall-normal lines of elements exactly and needs a fraction of the Krylov iterations ([measured](../verification/solvers.md#line-smoothing-on-a-wall-resolved-mesh)). There is no incomplete-factorisation preconditioner, and nothing has been measured on a turbulent mesh larger than the flat plate of the preset. * The entropy analysis of the [two-point fluxes](two-point-fluxes.md) covers the mean-flow convective terms only; nothing is proven for the model equation. References: Allmaras, Johnson and Spalart (2012); Oliver and Darmofal (2009); see the [bibliography](../reference/bibliography.md).