add option to specify initial baseflow density in Kelvin-Helmholtz

src/ConservationLaws/euler_navierstokes.jl

@@ -307,19 +307,21 @@ Domain should be [0,2]²
 """
 struct KelvinHelmholtzInstability <: AbstractGridFunction{2}
     γ::Float64
+    ρ_0::Float64
     N_c::Int
-    function KelvinHelmholtzInstability(conservation_law::EulerEquations{2})
-        return new(conservation_law.γ,4)
+    function KelvinHelmholtzInstability(conservation_law::EulerEquations{2},
+        ρ_0::Float64=0.5)
+        return new(conservation_law.γ,ρ_0,4)
     end
 end
 
 @inline function evaluate(f::KelvinHelmholtzInstability, 
     x::NTuple{2,Float64}, t::Float64=0.0)
     x_rel = (x[1]-1, x[2]-1)
     B = tanh(15*x_rel[2] + 7.5) - tanh(15*x_rel[2] - 7.5)
-    ρ = 0.5 + 0.75*B
+    ρ = f.ρ_0 + 0.75*B
     p = 1.0
-    u = 0.5*(B-1) 
+    u = 0.5*(B-1)
     v = 0.1*sin(2π*x_rel[1])
     return SVector{4}(ρ,ρ*u,ρ*v,p/(f.γ-1) + 0.5*ρ*(u^2 + v^2))
 end