question: protected double constrictionCoefficient ( double c1 , double c2 ) { double rho = c1 + c2 ; if ( rho <= 4 ) { return 1.0 ; } else { return 2.0 / ( 2.0 - rho - Math . sqrt ( Math . pow ( rho , 2.0 ) - 4.0 * rho ) ) ; } }
target:  Returns the velocity constriction coefficient.