What is jet impact on stationary plate?

Short Answer:

Jet impact on a stationary plate refers to the phenomenon that occurs when a high-speed jet of fluid strikes a solid surface that is not moving. The jet exerts a force on the plate due to the change in momentum of the fluid particles as they are deflected or stopped upon impact. This force is called the impact force or reaction force, and it depends on the velocity, mass flow rate, and angle of impact of the fluid jet.

When a jet of water hits a stationary plate normally or at an angle, the direction of flow changes, creating a reaction force on the plate. This principle is widely used in hydraulic turbines, nozzles, and fluid testing experiments to study momentum transfer and energy conversion in fluids.

Detailed Explanation :

Jet Impact on Stationary Plate

The jet impact on a stationary plate is an important topic in fluid mechanics that explains how a fluid jet exerts a force when it strikes a fixed or immovable surface. The impact produces a change in the velocity of the fluid particles, which leads to a change in momentum. According to Newton’s second law of motion, the rate of change of momentum of a fluid is directly proportional to the force exerted on it. Hence, the impact of a jet is a practical application of the momentum principle.

When a fluid jet of high velocity emerges from a nozzle and strikes a plate, the plate experiences a force equal to the change in momentum of the fluid in the direction of impact. The magnitude of this force depends on factors such as velocity of jet, density of fluid, area of jet, and the angle between the jet and the plate surface.

Types of Jet Impact on Stationary Plate

The impact of a jet on a stationary plate can occur in two main ways:

  1. Impact of Jet on Flat Plate (Normal or Inclined)
  2. Impact of Jet on Curved Plate

Each case is explained below.

  1. Impact of Jet on Flat Plate

When a jet of water strikes a flat plate, the direction of flow is either stopped completely (if the plate is normal to the jet) or diverted (if the plate is inclined).

(a) Jet Striking Normally on a Flat Plate
When the plate is held perpendicular (90°) to the jet direction, the entire velocity of the fluid in the direction of impact is brought to rest.
Let,

  •  = Velocity of jet (m/s)
  •  = Density of fluid (kg/m³)
  •  = Area of jet (m²)
  •  = Mass flow rate (kg/s)

Then, the force exerted on the plate (impact force) is given by:

This force acts in the direction of jet and is equal to the rate of change of momentum.

(b) Jet Striking an Inclined Flat Plate
If the plate is inclined at an angle  to the jet, only the normal component of the velocity contributes to the change in momentum. The fluid slides along the plate after impact, losing only a portion of its velocity.
The effective force is given by:

This shows that the force decreases as the angle of inclination increases.

  1. Impact of Jet on Curved Plate

Curved plates are commonly used in hydraulic machines like Pelton turbines because they allow smooth flow deflection and efficient momentum transfer.

When a jet strikes a curved plate, the direction of flow changes significantly, often up to 180°, but the velocity magnitude remains nearly constant if there are no losses.

For a stationary curved plate deflecting the jet through an angle :

If the jet is deflected by 180°, the total change in momentum is doubled, giving the maximum force:

This concept is the basis of impulse turbines where water jets strike curved vanes to produce rotational motion.

Applications of Jet Impact on Stationary Plate

  1. Hydraulic Turbines:
    The working of impulse turbines like the Pelton wheel is based on the impact of water jet on stationary curved surfaces, which converts kinetic energy into mechanical energy.
  2. Fluid Mechanics Experiments:
    Jet impact setups are used in laboratories to measure impact forces and verify momentum equations.
  3. Jet Propulsion Systems:
    The reverse principle of jet impact (reaction) is applied in rockets and jet engines to produce thrust.
  4. Industrial Cleaning and Cutting:
    High-pressure water jets used in surface cleaning and cutting processes rely on the same principle of impact force generated by the jet.
  5. Hydraulic Testing Equipment:
    Devices used to test resistance and strength of materials under fluid impact are based on jet impact analysis.

Factors Affecting Jet Impact on Stationary Plate

  1. Velocity of Jet:
    Higher velocity increases momentum change and hence, the impact force.
  2. Mass Flow Rate:
    The greater the flow rate, the greater is the force exerted on the plate.
  3. Angle of Impact:
    The direction of the jet relative to the plate affects how much of the velocity component is utilized for force generation.
  4. Shape of the Plate:
    Curved plates allow smoother flow direction changes and generate higher forces compared to flat plates.
  5. Fluid Density:
    Denser fluids exert greater impact forces at the same velocity.

Mathematical Example (Conceptual Explanation)

Suppose a jet of water with velocity 20 m/s and area 0.01 m² strikes a stationary flat plate normally. The density of water is 1000 kg/m³.

Then,

Hence, the jet exerts a force of 4000 N on the plate. This example shows how momentum change results in a significant impact force even for small jets.

Conclusion:

The jet impact on a stationary plate is an important concept that demonstrates how a moving fluid exerts force through momentum change. It helps engineers design efficient hydraulic systems like turbines, nozzles, and impact devices. The direction, shape of plate, and velocity of jet all influence the magnitude of the force. Understanding this concept is essential for converting fluid energy into mechanical work efficiently.