## Calculation of losses |

An important part of the program is loss calculation.

The algorithm for flow calculation is based on the equations of Euler, Bernoulli and Colebrook.

Friction losses depending of local geometry, velocity, roughness and Reynolds number

is the base for global loss mechanisms. This means that the loss mechanisms are valid

for all types and sizes of turbines. This goes also for tunnels and tubes. Wetted perimeter

is calculated from the section geometry, roughness, actual flow velocity and Reynolds number

as input in Colebrook's equation gives losses. Observe that the friction losses are not exactly

proportional to the square of flow. Friction losses are the dominating losses around the

turbine design point.

The design point is in the program defined as the operating point where all attack losses is zero

and the flow rotation in the draft tube is minimum. This point is for normal Francis turbines

close to best point, but for reversible pump turbines and Kaplan turbines the difference

between design point and best point may be considerable.

The scale effect on efficiency related to efficiency differences for model and prototype

is automatically taken care of due to the generic properties of loss mechanisms.

The Reynolds number effect on attack- and trailing losses or generally whirl losses

is not well known. But it is not demanded the same exactness for loss calculations

far from normal operating point. During runaway are whirl losses the essential losses.

Calculation of runaway speed may therefore suffer due to that.

Whirl losses is in the program based on empirical data which is "globalised",

meaning that the same loss mechanism is used for all turbines.