Merge pull request #180 from SciML/bgc/friction_factor_opt

Optimize `friction_factor` function

src/Hydraulic/IsothermalCompressible/utils.jl

@@ -70,6 +70,9 @@ Fluid parameter setter for isothermal compressible fluid domain.  Defaults given
     ODESystem(eqs, t, vars, pars; name, defaults = [dm => 0])
 end
 
+f_laminar(shape_factor, Re) = shape_factor * regPow(Re, -1, 1e-6) #regPow used to avoid dividing by 0, min value is 1e-6
+f_turbulent(shape_factor, Re) = (shape_factor / 64) / (0.79 * log(Re) - 1.64)^2
+
 """
     friction_factor(dm, area, d_h, density, viscosity, shape_factor)
 
@@ -95,31 +98,25 @@ Reference: Introduction to Fluid Mechanics, Fox & McDonald, 5th Edition, equatio
 """
 function friction_factor(dm, area, d_h, density, viscosity, shape_factor)
     u = abs(dm) / (density * area)
-
     Re = density * u * d_h / viscosity
-    f_laminar = shape_factor * regPow(Re, -1, 1e-6)
-
-    Re = max(Re, one(Re))
-    f_turbulent = (shape_factor / 64) * (0.79 * log(Re) - 1.64)^(-2)
-
-    f = transition(2000, 3000, f_laminar, f_turbulent, Re)
 
-    return f
+    if Re <= 2000
+        return f_laminar(shape_factor, Re)
+    elseif 2000 < Re < 3000
+        return transition(2000, 3000, f_laminar(shape_factor, Re),
+                          f_turbulent(shape_factor, Re), Re)
+    else
+        return f_turbulent(shape_factor, Re)
+    end
 end
 @register_symbolic friction_factor(dm, area, d_h, density, viscosity, shape_factor)
 Symbolics.derivative(::typeof(friction_factor), args, ::Val{1}) = 0
 Symbolics.derivative(::typeof(friction_factor), args, ::Val{4}) = 0
 
 function transition(x1, x2, y1, y2, x)
-    if x < x1
-        return y1
-    elseif x > x2
-        return y2
-    else
-        u = (x - x1) / (x2 - x1)
-        blend = 3 * u^2 - 2 * u^3
-        return (1 - blend) * y1 + blend * y2
-    end
+    u = (x - x1) / (x2 - x1)
+    blend = u^2 * (3 - 2 * u)
+    return (1 - blend) * y1 + blend * y2
 end
 
 density_ref(port) = port.ρ