G D Johnston objects to Bryan Leyland's characterisation of tidal current energy in terms of static head--but this is the basis of

Bernoulli's equation and is conventional (Letters, PE March).

The unsteady

Bernoulli's equation is used for time historical evaluation of the velocity potential and is given as:

INVESTIGATION OF THE EFFECTS OF AN EXIT PIPE ON THE CALCULATION OF DRAINAGE TIME OF WATER FROM A TANK USING

BERNOULLI'S EQUATION.

Based on the

Bernoulli's equation in cross sections 1-1 and 2-2 (Fig.

Bernoulli's equation is the integral of the Euler's equation and applies to general initial and final velocities and pressure.

Each section's resistance was then modeled by applying

Bernoulli's equation or Forchheimer-extended Darcy's Law.

Bernoulli's equation tells us that if we have a fluid flowing in a pipe, and a sudden enlargement exists in the pipe, there will be a corresponding drop in pressure.

Subsonic airflow satisfies conservation of mass as well as

Bernoulli's equation so the ratio of test area to nozzle area can be calculated given the pressure developed by the fan and the desired test velocity.

This principle defines the conservation of momentum, which leads to

Bernoulli's equation when viscous forces are neglected, steady flow and constant density are assumed.

Solutions were obtained using (1) a three-dimensional solution of the Navier-Stokes equation using computational fluid dynamics and (2)

Bernoulli's equation and a momentum balance in one dimension.