Define the viscosity beta as a constant instead of hard-coded

src/const.h

@@ -21,7 +21,10 @@
 #define SWIFT_CONST_H
 
 /* SPH Viscosity constants. */
-#define const_viscosity_alpha 0.8f
+/* Cosmology defaults: a=0.8, b=1.5a. Planetary defaults: a=1.5, b=2.0a */ 
+#define const_viscosity_alpha 1.5f
+#define const_viscosity_beta_over_alpha 2.0f
+
 #define const_viscosity_alpha_min \
   0.1f /* Values taken from (Price,2004), not used in legacy gadget mode */
 #define const_viscosity_alpha_max \

src/hydro/Planetary/hydro_iact.h

@@ -189,7 +189,7 @@ __attribute__((always_inline)) INLINE static void runner_iact_force(
   /* Compute sound speeds and signal velocity */
   const float ci = pi->force.soundspeed;
   const float cj = pj->force.soundspeed;
-  const float v_sig = ci + cj - 3.f * mu_ij;
+  const float v_sig = ci + cj - 2.f * const_viscosity_beta_over_alpha * mu_ij;
 
   /* Now construct the full viscosity term */
   const float rho_ij = 0.5f * (rhoi + rhoj);
@@ -315,7 +315,7 @@ __attribute__((always_inline)) INLINE static void runner_iact_nonsym_force(
   const float cj = pj->force.soundspeed;
 
   /* Signal velocity */
-  const float v_sig = ci + cj - 3.f * mu_ij;
+  const float v_sig = ci + cj - 2.f * const_viscosity_beta_over_alpha * mu_ij;
 
   /* Construct the full viscosity term */
   const float rho_ij = 0.5f * (rhoi + rhoj);