Where does this equation come from?? Hi, In a text book I found an equation for the volumetric flow rate of air through a channel (tube). I can't find from where the equation is coming, someone an idea? The equation which I mean is: Q = - pi*d^4/(128*viscosity)* dp/dx I know that Q = A*v = pi*d^2/4 *v Also from the simplified equation of motion 'v' can be calculated as: dp/dx = viscosity * ddv/ddy (dd = second derivative) from this equation v = ?? if I fill v in, in Q = pi*d^2/4 *v will this give Q = - pi*d^4/(128*viscosity)* dp/dx ??