--- a/src/Grids/Grids.jl	Wed Jun 26 12:41:03 2024 +0200
+++ b/src/Grids/Grids.jl	Fri Jun 28 17:02:47 2024 +0200
@@ -19,6 +19,7 @@
 export eval_on
 export componentview
 export ArrayComponentView
+export normal
 
 export BoundaryIdentifier
 export TensorGridBoundary

--- a/src/Grids/mapped_grid.jl	Wed Jun 26 12:41:03 2024 +0200
+++ b/src/Grids/mapped_grid.jl	Fri Jun 28 17:02:47 2024 +0200
@@ -80,3 +80,27 @@
     end
 end
 
+"""
+    normal(g::MappedGrid, boundary)
+
+The outward pointing normal as a grid function on the boundary
+"""
+function normal(g::MappedGrid, boundary)
+    b_indices = boundary_indices(g, boundary)
+    σ =_boundary_sign(component_type(g), boundary)
+    return map(jacobian(g)[b_indices...]) do ∂x∂ξ
+        ∂ξ∂x = inv(∂x∂ξ)
+        k = grid_id(boundary)
+        σ*∂ξ∂x[k,:]/norm(∂ξ∂x[k,:])
+    end
+end
+
+function _boundary_sign(T, boundary)
+    if boundary_id(boundary) == Upper()
+        return one(T)
+    elseif boundary_id(boundary) == Lower()
+        return -one(T)
+    else
+        throw(ArgumentError("The boundary identifier must be either `Lower()` or `Upper()`"))
+    end
+end

--- a/test/Grids/mapped_grid_test.jl	Wed Jun 26 12:41:03 2024 +0200
+++ b/test/Grids/mapped_grid_test.jl	Fri Jun 28 17:02:47 2024 +0200
@@ -183,4 +183,15 @@
     lg = equidistant_grid((0,0), (1,1), 10, 11)
     @test logicalgrid(mg) == lg
     @test collect(mg) == map(x̄, lg)
+
+
+    @testset "normal" begin
+        @test normal(mg, CartesianBoundary{1,Lower}()) == fill(@SVector[-1,0], 11)
+        @test normal(mg, CartesianBoundary{1,Upper}()) == fill(@SVector[1,0], 11)
+        @test normal(mg, CartesianBoundary{2,Lower}()) == fill(@SVector[0,-1], 10)
+        @test normal(mg, CartesianBoundary{2,Upper}()) ≈ map(boundary_grid(mg,CartesianBoundary{2,Upper}())|>logicalgrid) do ξ̄
+            α = 1-2ξ̄[1]
+            @SVector[α,1]/√(α^2 + 1)
+        end
+    end
 end