From 97c45fc9edb6f0437a8f7545a8f7583e3f14d665 Mon Sep 17 00:00:00 2001
From: Runda Ji <runda@flexcompute.com>
Date: Wed, 12 Nov 2025 12:02:29 -0500
Subject: [PATCH] add relative size to translator

---
 .../translator/volume_meshing_translator.py   |  3 ++-
 ...to_json_legacy_farfield_relative_size.json | 14 +++++++++++
 .../test_volume_meshing_translator.py         | 24 +++++++++++++++++++
 3 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 tests/simulation/translator/ref/volume_meshing/ref_param_to_json_legacy_farfield_relative_size.json

diff --git a/flow360/component/simulation/translator/volume_meshing_translator.py b/flow360/component/simulation/translator/volume_meshing_translator.py
index 0f0a663bd..b757fe7c9 100644
--- a/flow360/component/simulation/translator/volume_meshing_translator.py
+++ b/flow360/component/simulation/translator/volume_meshing_translator.py
@@ -278,7 +278,8 @@ def get_volume_meshing_json(input_params: SimulationParams, mesh_units):
 
         if isinstance(zone, AutomatedFarfield):
             translated["farfield"] = {
-                "planarFaceTolerance": meshing_params.defaults.planar_face_tolerance
+                "planarFaceTolerance": meshing_params.defaults.planar_face_tolerance,
+                "relativeSize": zone.relative_size,
             }
             if zone.method == "quasi-3d-periodic":
                 translated["farfield"]["type"] = "quasi-3d"
diff --git a/tests/simulation/translator/ref/volume_meshing/ref_param_to_json_legacy_farfield_relative_size.json b/tests/simulation/translator/ref/volume_meshing/ref_param_to_json_legacy_farfield_relative_size.json
new file mode 100644
index 000000000..4aae336b9
--- /dev/null
+++ b/tests/simulation/translator/ref/volume_meshing/ref_param_to_json_legacy_farfield_relative_size.json
@@ -0,0 +1,14 @@
+{
+  "faces": {},
+  "farfield": {
+    "planarFaceTolerance": 1e-06,
+    "relativeSize": 100.0,
+    "type": "quasi-3d"
+  },
+  "refinementFactor": 1.0,
+  "volume": {
+    "firstLayerThickness": 1e-03,
+    "gapTreatmentStrength": 0.0,
+    "growthRate": 1.25
+  }
+}
\ No newline at end of file
diff --git a/tests/simulation/translator/test_volume_meshing_translator.py b/tests/simulation/translator/test_volume_meshing_translator.py
index 2b8079b96..6501c9625 100644
--- a/tests/simulation/translator/test_volume_meshing_translator.py
+++ b/tests/simulation/translator/test_volume_meshing_translator.py
@@ -5,6 +5,7 @@
 
 import flow360.component.simulation.units as u
 from flow360.component.simulation.framework.param_utils import AssetCache
+from flow360.component.simulation.framework.updater_utils import compare_values
 from flow360.component.simulation.meshing_param.face_params import (
     BoundaryLayer,
     PassiveSpacing,
@@ -15,6 +16,7 @@
     MeshingParams,
 )
 from flow360.component.simulation.meshing_param.volume_params import (
+    AutomatedFarfield,
     AxisymmetricRefinement,
     CustomZones,
     RotationVolume,
@@ -556,3 +558,25 @@ def test_surface_mesh_user_defined_farfield_disallow_any_ghost():
                 )
             with pytest.raises(pd.ValidationError):
                 PassiveSpacing(entities=[GhostCircularPlane(name="symmetric")], type="projected")
+
+
+def test_farfield_relative_size():
+    with SI_unit_system:
+        param = SimulationParams(
+            meshing=MeshingParams(
+                defaults=MeshingDefaults(
+                    boundary_layer_first_layer_thickness=1e-3, boundary_layer_growth_rate=1.25
+                ),
+                volume_zones=[AutomatedFarfield(method="quasi-3d", relative_size=100.0)],
+            )
+        )
+    translated = get_volume_meshing_json(param, u.m)
+    ref_path = os.path.join(
+        os.path.dirname(os.path.abspath(__file__)),
+        "ref",
+        "volume_meshing",
+        "ref_param_to_json_legacy_farfield_relative_size.json",
+    )
+    with open(ref_path, "r") as fh:
+        ref_dict = json.load(fh)
+    assert compare_values(translated, ref_dict)