FT-09 · Momentum

Jet impact on deflectors

Applies the vector momentum balance for a stationary plate or a moving vane, with configurable velocity losses.

Jet, vane and change of direction

Flow rate Q · Jet diameter · Density · Deflection angle θ · °Value: 120°Velocity coefficient kVane velocity u · 0 for a stationary deflector

Resultant force

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Componente Fx

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Componente Fy

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Jet velocity

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Direction of the force

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Power transferred

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Reaction components vs. angle

Balance on a moving vane

\[\dot m=\rho A(V-u)\]

\[F_x=\rho A(V-u)^2(1-k\cos\theta)\]

\[F_y=-\rho A(V-u)^2k\sin\theta\]

The force reported acts on the deflector. For a fixed surface u=0; θ=180° represents ideal reversal of the jet.

Jet impact force calculator

The force arises because the deflector changes the velocity vector of the jet. The momentum balance must preserve both components and signs; a 180° reversal ideally maximises the axial reaction.

Reference cases

Deflection	Ideal outlet	Fixed axial force
0°	No change	0
90°	Transverse outlet	ρQV
180°	Reversal	2ρQV

Moving vanes

When the vane advances, the mass flow it intercepts depends on the relative velocity V−u. The transferred power is obtained as F_xu.

Preguntas frecuentes

Is the calculated force the one on the fluid?

The reaction on the deflector is reported; the force on the fluid acts in the opposite direction.

What does k represent?

The ratio of outlet to inlet relative velocity; it summarises the losses over the surface.

Is it valid for a real Pelton wheel?

It is a first approximation. Design requires bucket geometry, jet splitting, efficiency and peripheral speed.