% Reynolds_number: The Reynold's number of a sphere in a gas.
%
% Re=Reynolds_number(R,V,rho,T,A); % neutral gas
%
% Re=Reynolds_number(R,V,rho,T,A,Z); % gas ionized
%
% ARGUMENTS
%  R	Radius of the sphere. [cm]
%  V	Velocity of sphere in gas. [cm s^-1]
%  rho	Density of gas. [g cm^-3]
%  T	Temperature of the gas. [K]
%  A	Atomic mass of the ion
%  Z	Charge per ion. [esu] (optional)
%
% RETURNS
%  The Reynold's number (dimensionless)
%
% USES
%  dynamic_viscosity
%
% SEE ALSO
%  drag_coef, drag_force, impact_parameter

% AUTHOR: Eric Tittley
%
% HISTORY
%  00 07 31 First Version
%  06 03 01 Standardized header comments.

function Re=Reynolds_number(R,V,rho,T,A,Z)

if(nargin==5) % the gas is unionized
 eta=dynamic_viscosity(T,rho,A);
elseif(nargin==6)  % The gas is ionized
 eta=dynamic_viscosity(T,rho,A,Z);
else
 error('need 5 or 6 arguments')
end
Re=R*V*rho/eta;