Add radial grid documentation

Author: magnesj

assets/images/import/radial-grid-overview.png

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+title = " Radial Grid"
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+weight = 30
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+
+ResInsight now supports radial grids, enabling visualization and analysis of reservoir models that use radial coordinate systems. This is particularly useful for well-centric models and near-wellbore analysis.
+
+![](/images/import/radial-grid-overview.png)
+*Example of a radial grid model displayed in ResInsight showing pressure distribution around a well*
+
+## Overview
+
+Radial grids use a cylindrical coordinate system (r, θ, z) instead of the traditional Cartesian (x, y, z) system. This allows for more accurate representation of phenomena around wells and other cylindrical geometries.
+
+## Importing Radial Grid Models
+
+Radial grid models can be imported into ResInsight through the standard import workflow:
+
+1. **File → Import → Eclipse Files**
+2. Select your radial grid files (.EGRID, .INIT, .UNRST, etc.)
+3. ResInsight will automatically detect if the grid uses radial coordinates
+4. The model will be loaded and converted for visualization. Choose file reader and how to handle radial grids from **Preferences**.
+5. If the radial grid has a low number of angular cells, ResInsight will automatically create a temporary Local Grid Refinement (LGR) with higher angular refinement for improved visualization. This LGR can be seen in the project tree as "Radial LGR".
+
+## Visualization Features
+
+When working with radial grids, ResInsight provides:
+
+- **3D Visualization**: Automatic conversion from radial to Cartesian coordinates for display
+- **Property Mapping**: All standard property visualization features work with radial grids
+- **Well Trajectories**: Enhanced visualization of wells in radial coordinate systems
+- **Cross Sections**: Support for radial and angular cross-sections
+- **Hex Element Approximation**: Radial grid cells can be approximated using hexahedral elements for improved visualization. This setting can be controlled from **Preferences**
+
+## Preferences Settings
+
+Radial grid behavior can be controlled through **Edit → Preferences → EGRID Settings**:
+
+- **Radial Grid Display Mode**: Choose between:
+  - **Cylindrical**: Uses cylindrical coordinates for radial grids (default behavior). No support for local grid refinement (LGR)
+  - **Cartesian**: Forces Cartesian coordinate interpretation. This mode enstimates the cylindrical cells by hex element, and supports visualization of radial LGR.
+
+## Key Considerations
+
+- **Coordinate Conversion**: ResInsight handles the conversion between radial and Cartesian coordinates internally
+- **Property Interpolation**: Properties are correctly interpolated considering the radial grid geometry
+- **Export Functions**: Exported data maintains the original radial grid structure when appropriate
+
+## Troubleshooting
+
+If you encounter issues with radial grid imports:
+
+- Verify that your grid files contain proper radial coordinate definitions
+- Consider using a different reader for EGRID files
+- Ensure all required grid files (.EGRID, .INIT) are present
+- Review the ResInsight log for any import warnings or errors