Options available for SU2 big chunk

Author: gnorman7

_docs_v7/Gradients-Limiters-Grant.md

@@ -21,20 +21,96 @@ Grant will work on:
 Do this before diving into the comparisons (which can be a lot more work)
 Also we assume that the user will know the theory, and that they are just looking for the limiters that are available in SU2 first.
 
-We may want to link to another place in the docs where they mention that limiters can be activated after a specific number of iterations.
+
+The field `SLOPE_LIMITER_FLOW` in the `.cfg` file specifies which limiter to use, and defaults to `VENKATAKRISHNAN` (see below). Note that this option is only used if `MUSCL_FLOW = YES` (which specifies to use a second-order method).
+An example is shown here: https://su2code.github.io/tutorials/Laminar_Cylinder/. The [Turbulent Flat Plate example](https://su2code.github.io/tutorials/ Turbulent_Flat_Plate/) sets `SLOPE_LIMITER_TURB` (also defaulting to `VENKATAKRISHNAN`), which is used for the turbulence equations, rather than for the flow equations. Using a limiter similarly requires `MUSCL_TURB = YES` The settings `MUSCL_ADJFLOW`, `MUSCL_ADJTURB`, `SLOPE_LIMITER_ADJFLOW`, and `SLOPE_LIMITER_ADJTURB` set the corresponding options for the respective adjoint equations. For species transport, the options `MUSCL_SPECIES` and `SLOPE_LIMITER_SPECIES` are used.
+
+<!-- There's also: 
+% Frozen the slope limiter in the discrete adjoint formulation (NO, YES)
+FROZEN_LIMITER_DISC= NO
+ -->
+
+
+<!-- 
+CLimiterDetails.hpp:
+computeLimiters.hpp uses computeLimiters_impl.hpp and really just sets the limiter
+computeLimiters_impl.hpp passes computation of field values (cell averages), gradients, and  to the limiter
+
+also check out
+computeGradientsGreenGauss.hpp
+
+ -->
+
+The `SLOPE_LIMITER_` options above may each be changed to use different limiters, as explained below.
+
+
+All of the limiters depend on variables `proj`, `delta`, and `eps`/`eps2` (and maybe `dist` for `raisedSine` in `SHARP_EDGES`)
+
+Limiters
+
+| Type | Description | Notes |
+| --- | --- | --- |
+| `NONE`                  | No limiter                                      |  | 
+| `BARTH_JESPERSEN`       | Barth-Jespersen                                 |  | 
+| `VENKATAKRISHNAN`       | Venkatakrishnan                                 |  | 
+| `VENKATAKRISHNAN_WANG`  | Venkatakrishnan-Wang                            |  | 
+| `SHARP_EDGES`           | Venkatakrishnan with sharp edge modification    |  | 
+| `WALL_DISTANCE`         | Venkatakrishnan with wall distance modification |  |
+
+
+The `VENKAT_LIMITER_COEFF` parameter is generally a small constant, defaulting to $0.05$, but its specific definition depends on the limiter being used.
+
+For the `VENKATAKRISHNAN` option, the `VENKAT_LIMITER_COEFF` parameter refers to $K$ in $\epsilon^2=\left(K\bar{\Delta} x\right)^3$, where $\bar{\Delta}$ is an average grid size. The $K$ parameter defines a threshold, below which oscillations are not damped by the limiter, as described by [Venkatakrishnan](https://doi.org/10.1006/jcph.1995.1084). Thus, a large value will approach the case of using no limiter, while too small of a value will slow the convergence.
+
+
+[Wang](https://doi.org/10.2514/6.1996-2091)
+
+<!-- Maybe a better way to word this. -->
+After the number of iterations given by `LIMITER_ITER` (default $999999$), the value of the limiter will be frozen.
+
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+
+<!-- We can specify which limiters are applied through the fields
+constexpr size_t MAXNVAR = 32; -->
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+<!-- 
+Should I mention some possible errors
+
+SU2_MPI::Error("Too many dimensions to compute limiters.", CURRENT_FUNCTION);
+SU2_MPI::Error("Unknown limiter type.", CURRENT_FUNCTION);
+
+  constexpr size_t MAXNVAR = 32;
+
+  if (varEnd > MAXNVAR)
+    SU2_MPI::Error("Number of variables is too large, increase MAXNVAR.", CURRENT_FUNCTION);
+
+ -->
+
+
 
 ## Why Slope Limiters are used in FVM
 * TVD
 * Monotonic
 * 2nd order accuracy
 
 
-![Caption. Wow, such an elaborate caption](../../docs_files/LW_example.png){#fig:description}
-
-<!-- [image]: ../../docs_files/LW_example.png "Image Title" 
-![Alt text][image] 
-A reference to the [image](#image).s -->
-
 ### subsection example
 a subsection would go here