@@ -423,18 +423,25 @@ A particle cloud is a compact specification of a bed of identical circular (2D)
 | Parameter         | Type    | Description |
 | ---:              | :----:  | :---        |
 | `x[y,z]_centroid` | Real    | Centre of the cloud region in the [x,y,z]-direction. |
-| `length_x[y,z]`   | Real    | Extent of the cloud region in the [x,y,z]-direction. |
+| `length_x[y,z]`   | Real    | Extent of the cloud region in the [x,y,z]-direction for `cloud_geometry = 1`; ignored by `cloud_geometry = 2`. |
 | `num_particles`   | Integer | Number of particles to place in the region. |
 | `radius`          | Real    | Radius of every particle in the cloud. |
 | `mass`            | Real    | Mass of every particle in the cloud. |
 | `min_spacing`     | Real    | Minimum surface-to-surface gap between particles (centres are `2*radius + min_spacing` apart). |
+| `cloud_geometry`  | Integer | Shape of the cloud region. |
+| `shell_inner_radius` | Real | Inner radius for hemisphere-shell clouds (`cloud_geometry = 2`). |
+| `shell_outer_radius` | Real | Outer radius for hemisphere-shell clouds (`cloud_geometry = 2`). |
 | `moving_ibm`      | Integer | Motion flag applied to every particle (see `patch_ib(j)%%moving_ibm`). |
 | `seed`            | Integer | Random seed for reproducible placement (used by `packing_method = 1`). |
 | `packing_method`  | Integer | Algorithm used to place the particles. |
 
+- `cloud_geometry` selects the cloud region:
+  - `1` (box) uses `x[y,z]_centroid` and `length_x[y,z]` to define the region.
+  - `2` uses `x[y,z]_centroid`, `shell_inner_radius`, and `shell_outer_radius` to define a half-annulus in 2D and a hemisphere shell in 3D. Particle centres are sampled between `shell_inner_radius + radius` and `shell_outer_radius - radius`, and the flat plane is kept clear by one particle radius. The flat face is fixed at `y_centroid` in 2D and `z_centroid` in 3D; the filled region opens toward positive `y` in 2D and positive `z` in 3D. The full shell extent (`x[y,z]_centroid +/- shell_outer_radius` on the open side, and one particle radius of clearance on the flat-face side) must lie inside the computational domain; a hemisphere shell also requires at least two dimensions (`n > 0`).
 - `packing_method` selects how the `num_particles` are positioned within the cloud region:
   - `1` (rejection sampling) draws random positions and rejects any that violate `min_spacing`, producing a disordered bed. `seed` makes the placement reproducible.
   - `2` (lattice) places the particles on the optimally dense lattice for the geometry — a triangular lattice in 2D and a face-centered cubic lattice in 3D. The lattice spacing is derived from the particle density (`num_particles` over the region area/volume); if that spacing is below the required `2*radius + min_spacing`, the region is too dense and the run aborts.
+  - Hemisphere-shell clouds currently support rejection sampling only; `cloud_geometry = 2` with `packing_method = 2` is rejected during input validation.
 
 ### 5. Fluid Material's {#sec-fluid-materials}
 
 
@@ -374,9 +374,13 @@ module m_derived_types
         real(wp) :: radius  !< Particle radius
         real(wp) :: mass  !< Particle mass
         real(wp) :: min_spacing  !< Minimum surface-to-surface gap (particle centers are 2*radius + min_spacing apart)
+        real(wp) :: shell_inner_radius  !< Inner radius for shell packing
+        real(wp) :: shell_outer_radius  !< Outer radius for shell packing
         integer  :: moving_ibm  !< Motion flag: 0=static, 1=moving (forces), 2=forced path
         integer  :: seed  !< Random seed for reproducible placement
+        integer  :: cloud_geometry  !< Cloud region geometry: 1=box, 2=hemisphere shell
         integer  :: packing_method  !< Packing algorithm: 1=rejection sampling, 2=lattice
+        integer  :: periodic  !< Periodic overlap flag for box rejection packing: 0=off, 1=on
     end type particle_cloud_parameters
 
     !> Derived type annexing the physical parameters (PP) of the fluids. These include the specific heat ratio function and liquid
 
@@ -639,9 +639,13 @@ contains
             particle_cloud(i)%radius = dflt_real
             particle_cloud(i)%mass = dflt_real
             particle_cloud(i)%min_spacing = 0._wp
+            particle_cloud(i)%shell_inner_radius = dflt_real
+            particle_cloud(i)%shell_outer_radius = dflt_real
             particle_cloud(i)%moving_ibm = 0
             particle_cloud(i)%seed = 0
+            particle_cloud(i)%cloud_geometry = 1
             particle_cloud(i)%packing_method = dflt_int
+            particle_cloud(i)%periodic = 0
         end do
 
         do i = 1, num_ib_patches_max_namelist
 
@@ -200,13 +200,15 @@ contains
         ! manual: particle_cloud (runtime loop to num_particle_clouds; irregular member subset)
         do i = 1, num_particle_clouds
             #:for VAR in ['x_centroid', 'y_centroid', 'z_centroid', 'length_x', 'length_y', 'length_z', &
-                & 'radius', 'mass', 'min_spacing']
+                & 'radius', 'mass', 'min_spacing', 'shell_inner_radius', 'shell_outer_radius']
                 call MPI_BCAST(particle_cloud(i)%${VAR}$, 1, mpi_p, 0, MPI_COMM_WORLD, ierr)
             #:endfor
             call MPI_BCAST(particle_cloud(i)%num_particles, 1, MPI_INTEGER, 0, MPI_COMM_WORLD, ierr)
             call MPI_BCAST(particle_cloud(i)%moving_ibm, 1, MPI_INTEGER, 0, MPI_COMM_WORLD, ierr)
             call MPI_BCAST(particle_cloud(i)%seed, 1, MPI_INTEGER, 0, MPI_COMM_WORLD, ierr)
+            call MPI_BCAST(particle_cloud(i)%cloud_geometry, 1, MPI_INTEGER, 0, MPI_COMM_WORLD, ierr)
             call MPI_BCAST(particle_cloud(i)%packing_method, 1, MPI_INTEGER, 0, MPI_COMM_WORLD, ierr)
+            call MPI_BCAST(particle_cloud(i)%periodic, 1, MPI_INTEGER, 0, MPI_COMM_WORLD, ierr)
         end do
 
         ! manual: acoustic/probe (combined loop; complex acoustic member set)