Takacs double-exponential sedimentation velocity function.
function vSfun input Real X; input Real Xf; output Real vS; parameter Real v0slash = 250.0 "max. settling velocity in m/d"; parameter Real v0 = 474.0 "max. Vesilind settl. veloc. in m/d"; parameter Real rh = 0.000576 "hindered zone settl. param. in m3/(g SS)"; parameter Real rp = 0.00286 "flocculant zone settl. param. in m3/(g SS)"; parameter Real fns = 0.00228 "non-settleable fraction in -"; end vSfun;