Froude number is the ratio of

    1. buoyancy forces to viscous forces
    2. inertia forces to viscous forces
    3. buoyancy forces to inertia forces
    4. inertia forces to gravity forces
    Correct option is D. inertia forces to gravity forces

    The Froude number is a dimensionless quantity that compares the ratio of inertia forces to gravity forces in fluid dynamics. It is commonly used to analyze the behavior of waves, flow resistance, and the relationship between fluid velocity and channel characteristics in open channel flows.

    Definition and Formula

    The Froude number (Fr) is defined as the ratio of the square root of the inertia forces (ρV^2) to the square root of the gravity forces (ρgh), where ρ is the fluid density, V is the flow velocity, g is the gravitational acceleration, and h is the flow depth. The formula can be written as:

    Fr = V / √(gh)

    When the Froude number is less than 1, the flow is considered subcritical; when it is equal to 1, the flow is critical; and when it is greater than 1, the flow is supercritical.

    Importance of Froude Number

    The Froude number is significant because it helps engineers and scientists understand and predict the behavior of fluid flow, particularly in open channels and around objects. For example, it can be used to determine the stability of floating objects, the formation of hydraulic jumps, and the efficiency of hydraulic structures.


    The Froude number has several applications in various fields, including:

    1. Ship design: In naval architecture, the Froude number is used to determine the resistance of a ship hull to water and estimate the power required to propel it at a desired speed. The concept of hull speed, which is the maximum efficient speed for a displacement hull, is also derived from the Froude number.
    2. River engineering: The Froude number is employed in river engineering to analyze the flow characteristics and sediment transport in river channels. It helps in predicting the formation of hydraulic jumps, where a rapid change in flow velocity occurs, and designing effective sediment control measures.
    3. Hydraulic structures: The Froude number is used to evaluate the performance and efficiency of hydraulic structures like spillways, weirs, and energy dissipators. It aids in determining the flow regime and energy dissipation characteristics, which are crucial for the stability and safety of such structures.
    4. Environmental engineering: In environmental engineering, the Froude number plays a role in understanding pollutant dispersion and mixing in water bodies. It helps in predicting the dispersion patterns and the extent of dilution, which are essential for designing effective pollution control strategies.
    5. Geophysical flows: The Froude number is also applied in the study of geophysical flows, such as tsunamis, storm surges, and river floods. It helps in predicting the flow characteristics, inundation extent, and the potential impact of these events on coastal and riverine communities.


    Here are some examples illustrating the importance of the Froude number in various contexts:

    • Ship design: The design of high-speed catamarans and hydrofoil boats involves minimizing the wave-making resistance by optimizing the hull shape and configuration. The Froude number is used to predict the wave resistance and establish the relationships between hull speed, displacement, and required power.
    • Hydraulic jump: A hydraulic jump is a phenomenon that occurs when a high-velocity, supercritical flow encounters a region of lower velocity, causing a sudden rise in water surface elevation. This transition can be analyzed using the Froude number, and it is used to design energy dissipators in spillways and other hydraulic structures.
    • Tsunami modeling: Tsunami wave propagation and coastal inundation can be modeled using the shallow water wave theory, which incorporates the Froude number to predict the wave characteristics and run-up. The Froude number helps in understanding the interaction between the tsunami waves and the coastal topography, and it is used to estimate the potential inundation extent and develop hazard maps.
    • Floodplain analysis: The Froude number is used in floodplain analysis to assess the flow regime and the potential for sediment transport and erosion during flood events. By evaluating the flow characteristics in terms of the Froude number, floodplain managers can develop appropriate flood control measures and land-use planning strategies to minimize the impact of floods on communities.
    • Sediment transport: The Froude number is used in the analysis of sediment transport in rivers and estuaries. It helps in estimating the flow velocity required to initiate sediment movement and predict the sediment transport rate. This information is crucial for designing effective river and coastal management strategies, such as river training works, erosion control measures, and dredging operations.

    Limitations and Considerations

    While the Froude number is a valuable tool in fluid dynamics, it has some limitations and must be used with care. Some of the key considerations when using the Froude number are:

    1. Applicability: The Froude number is most applicable in situations where gravity and inertia forces are dominant, such as open channel flows, shallow water waves, and flows around bluff bodies. In cases where other forces, such as viscous or buoyancy forces, play a significant role, the Froude number may not provide an accurate representation of the flow behavior.
    2. Dimensionality: The Froude number is a dimensionless quantity, which means it does not have units. When comparing Froude numbers in different situations, it is essential to ensure that the quantities used in the calculation (velocity, depth, and gravitational acceleration) are consistent and dimensionally correct.
    3. Flow regime: The Froude number is sensitive to the flow regime, particularly in the transition between subcritical and supercritical flow. In these cases, the Froude number may not provide a clear indication of the flow behavior, and additional analysis or empirical data may be required.
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