Gradient zero ! How to remove the fix plane in FFD setting
 

I tried to use SU2 v6.2.0 for shape optimization of axial turbine stage. The ffd is set as follows:
% Tolerance of the Free-Form Deformation point inversion
FFD_TOLERANCE= 1E-10
%
% Maximum number of iterations in the Free-Form Deformation point inversion
FFD_ITERATIONS= 1000
%
% FFD box definition: 3D case (FFD_BoxTag, X1, Y1, Z1, X2, Y2, Z2, X3, Y3, Z3, X4, Y4, Z4,
% X5, Y5, Z5, X6, Y6, Z6, X7, Y7, Z7, X8, Y8, Z8)
% 2D case (FFD_BoxTag, X1, Y1, 0.0, X2, Y2, 0.0, X3, Y3, 0.0, X4, Y4, 0.0,
% 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0)
FFD_DEFINITION= (stator, 0.140, -0.053, 0.0, 0.19, -0.053, 0.0, 0.19, 0.005, 0.0, 0.14, 0.005, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0);(rotor, 0.20, -0.03, 0.0, 0.26, -0.03, 0.0, 0.26, 0.022, 0.0, 0.2, 0.022, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0)
%
%
% FFD box degree: 3D case (x_degree, y_degree, z_degree)
% 2D case (x_degree, y_degree, 0)
FFD_DEGREE= (2 , 4 , 0 );(2 , 4 , 0 )
%
% Surface grid continuity at the intersection with the faces of the FFD boxes.
% To keep a particular level of surface continuity, SU2 automatically freezes the right
% number of control point planes (NO_DERIVATIVE, 1ST_DERIVATIVE, 2ND_DERIVATIVE, USER_INPUT)
FFD_CONTINUITY= 2ND_DERIVATIVE
%
%
% --------------------- OPTIMAL SHAPE DESIGN DEFINITION -----------------------%
%
% Optimization design variables, separated by semicolons
DEFINITION_DV= ( 15, 1.0 | wall1 | stator, 0, 0, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 1, 0, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 2, 0, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 0, 1, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 1, 1, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 2, 1, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 0, 2, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 1, 2, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 2, 2, 0.0, 1.0 ); ( 15, 1.0 | wall1 | stator, 0, 3, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 1, 3, 0.0, 1.0 ); ( 15, 1.0 | wall1 | stator, 2, 3, 0.0, 1.0 ); ( 15, 1.0 | wall1 | stator, 0, 4, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 1, 4, 0.0, 0.0 ); ( 15, 1.0 | wall1 | stator, 2, 4, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 0, 0, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 1, 0, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 2, 0, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 0, 1, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 1, 1, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 2, 1, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 0, 2, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 1, 2, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 2, 2, 0.0, 1.0 ); ( 15, 1.0 | wall2 | rotor, 0, 3, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 1, 3, 0.0, 1.0 ); ( 15, 1.0 | wall2 | rotor, 2, 3, 0.0, 1.0 ); ( 15, 1.0 | wall2 | rotor, 0, 4, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 1, 4, 0.0, 0.0 ); ( 15, 1.0 | wall2 | rotor, 2, 4, 0.0, 0.0 )

The resultant ffd box is shown in the attachment


I didn't see any intersection in FFD box and solid surface. However, after running discrete_adjoint -f xxx.cfg -z 2, it shows
Evaluate functional gradient using Algorithmic Differentiation (ZONE 1).
2 Free Form Deformation boxes.
1 Free Form Deformation nested levels.
FFD box tag: stator. FFD box level: 0. Degrees: 2, 4.
FFD Blending using Bezier Curves.
Number of parent boxes: 0. Number of child boxes: 0.
Corner points: 4. Control points: 30. Surface points: 0.
FFD box tag: rotor. FFD box level: 0. Degrees: 2, 4.
FFD Blending using Bezier Curves.
Number of parent boxes: 0. Number of child boxes: 0.
Corner points: 4. Control points: 30. Surface points: 432.

----------------- FFD technique (parametric -> cartesian) ---------------
Checking FFD box dimension.
Checking FFD box intersections with the solid surfaces.
The FFD planes j=4, intersect solid surfaces.
SU2 is fixing the planes to maintain a continuous 2nd order derivative.
Update cartesian coord | FFD box: stator. Max Diff: 0.
Checking FFD box dimension.
Checking FFD box intersections with the solid surfaces.
SU2 is fixing the planes to maintain a continuous 2nd order derivative.
Update cartesian coord | FFD box: rotor. Max Diff: 1.67111e-16.

May I ask how I can set ffd coordinate to remove the fix j=4 plane? And is there a way to set a boundary for design variable? Thanks.