Bernoulli’s Theorem

Bernoulli’s theorem, named after Swiss mathematician Daniel Bernoulli, is a fundamental principle in fluid dynamics. It relates the pressure, velocity, and elevation of a fluid in a streamline. Bernoulli’s theorem is based on the conservation of energy in a flowing fluid.

bernoulli's theorem

According to Bernoulli’s theorem, in an ideal fluid (one that is inviscid, incompressible, and flows steadily), the total mechanical energy per unit volume of the fluid remains constant along a streamline. This means that the sum of the pressure energy, kinetic energy, and potential energy per unit volume remains the same at any two points along a streamline, neglecting losses due to friction or external work.

The mathematical form of Bernoulli’s theorem can be expressed as:

P + ½ρv² + ρgh = constant


P is the pressure exerted by the fluid
ρ (rho) is the density of the fluid
v is the velocity of the fluid
g is the acceleration due to gravity
h is the height (elevation) of the fluid above a reference point
This equation states that the sum of the pressure, velocity head (kinetic energy per unit volume), and potential head (potential energy per unit volume) remains constant along a streamline.

Bernoulli’s theorem has various applications and is commonly used in the analysis of fluid flow, including the design of pipes, nozzles, and airfoils. It helps explain phenomena such as the lift generated by an airplane wing, the flow through a Venturi tube, and the behavior of fluids in pipes and channels. However, it is important to note that Bernoulli’s theorem is a simplified model and does not account for factors such as viscosity, compressibility, and turbulence, which may play significant roles in certain fluid flow situations.

Assumptios made in Bernoulli’s equation

Bernoulli’s equation is derived under certain assumptions about the fluid flow. The key assumptions are as follows:

Incompressible Fluid: Bernoulli’s equation assumes that the fluid is incompressible, meaning that its density remains constant throughout the flow. This assumption is suitable for many liquid flows but may not be accurate for compressible fluids, such as gases at high velocities or low pressures.

Steady Flow: The equation assumes that the fluid flow is steady, meaning that the velocity, pressure, and other flow properties do not change with time at any given point along the streamline. Transient or unsteady flows, which involve changing flow conditions over time, are not considered in Bernoulli’s equation.

Inviscid Fluid: Bernoulli’s equation assumes that the fluid is inviscid, meaning that there is no internal friction or viscosity present in the fluid. In reality, fluids do exhibit viscosity, and energy losses due to frictional effects can occur. However, these effects are neglected in the simplified Bernoulli’s equation.

Irrotational Flow: The equation assumes that the fluid flow is irrotational, meaning that the fluid particles do not rotate as they move along the streamline. This assumption implies that there are no swirls or vortices present in the flow.

No External Work: Bernoulli’s equation assumes that no external work, such as pumps or turbines, is applied to the fluid between the two points being considered. This assumption neglects the effects of external devices that may add or remove energy from the fluid.

Limitation of Bernoulli’s equation

Bernoulli’s equation, while useful in many fluid flow analyses, has several limitations that should be considered:

Viscous Effects

Bernoulli’s equation assumes that the fluid is inviscid, neglecting the effects of viscosity. In reality, fluids have viscosity, and when the flow involves high velocities, small-scale flow phenomena, or flows near solid boundaries, viscosity becomes significant. Bernoulli’s equation fails to account for these viscous effects, leading to inaccuracies in such cases.


Bernoulli’s equation assumes that the fluid is incompressible, meaning that its density remains constant. This assumption is valid for many liquid flows, but it becomes problematic when dealing with compressible fluids, such as gases at high velocities or low pressures. In these cases, the density of the fluid can vary significantly, and compressibility effects must be considered separately.

Turbulent Flow

Bernoulli’s equation is not well-suited for analyzing turbulent flows. Turbulence involves chaotic, irregular motion of fluid particles, and Bernoulli’s equation assumes smooth, steady flow conditions. Turbulence introduces additional energy losses due to eddies, vortices, and fluid mixing, which are not accounted for in the equation.

Heat Transfer and Energy Dissipation

Bernoulli’s equation does not consider heat transfer or energy dissipation effects. In real fluid flows, heat transfer may occur between the fluid and its surroundings, and energy may be lost due to various mechanisms like friction, turbulence, and shock waves. These effects can significantly impact the flow behavior but are not incorporated in the simplified Bernoulli’s equation.

Flow Geometry

Bernoulli’s equation assumes that the flow takes place along a streamline, implying that the cross-sectional area of the flow remains constant. However, in practical situations, the flow geometry may change, leading to variations in pressure and velocity distribution that cannot be captured by Bernoulli’s equation alone. Additional equations or models may be needed to account for these geometrical effects.

Bernoulli’s Theorem – Assumptios, Limitation, Application and More

Application of Bernoulli’s Theorem

Bernoulli’s theorem has various practical applications in fluid dynamics. Here are some common applications:


Bernoulli’s theorem is extensively used in the field of aerodynamics to explain the lift generated by airfoils, such as airplane wings. According to Bernoulli’s theorem, as air flows over the curved upper surface of an airfoil, its velocity increases, resulting in a decrease in pressure. This pressure difference between the upper and lower surfaces creates lift, allowing an airplane to fly.

Venturi Effect

The Venturi effect is a direct application of Bernoulli’s theorem. It is utilized in devices called Venturi tubes or Venturi meters, which are used to measure fluid flow rates. A Venturi tube consists of a constriction in the pipe that causes the fluid to accelerate, resulting in a decrease in pressure. By measuring the pressure difference before and after the constriction, the flow rate of the fluid can be determined.


In internal combustion engines, carburetors use Bernoulli’s principle to mix air and fuel. The Venturi effect is employed to draw fuel into the air stream as air flows through a narrowed section of the carburetor. The decrease in pressure in the narrowed section causes fuel to be drawn from the carburetor’s fuel bowl and atomized into the air, resulting in the desired fuel-air mixture for combustion.

Pitot Tubes

Pitot tubes, named after Henri Pitot, utilize Bernoulli’s theorem to measure fluid velocity. These instruments consist of a tube with one open end facing the fluid flow and another connected to a pressure-measuring device. The fluid entering the open end of the tube is brought to rest, resulting in an increase in pressure. By measuring this pressure difference, the velocity of the fluid can be determined.

Water Towers

Water towers are used to provide water pressure in distribution systems. The height of the water in the tower, combined with Bernoulli’s theorem, ensures a constant and adequate water pressure in the pipes. The elevation of the water provides potential energy, which is converted to pressure energy as it flows down to supply the distribution system.


Aerodynamics is the study of how gases, particularly air, interact with objects in motion. It plays a crucial role in understanding and designing the movement of aircraft, automobiles, rockets, and other objects through the air or other gases.

Key Principles of Aerodynamics

Bernoulli’s Principle: As mentioned earlier, Bernoulli’s principle is a fundamental concept in aerodynamics. It states that as the speed of a fluid (such as air) increases, the pressure it exerts decreases. This principle helps explain lift generation and the behavior of airflow around wings and other aerodynamic surfaces.

Lift and Drag: Lift is the upward force generated by the motion of an object through a fluid, usually air. It is essential for the flight of aircraft. Lift is primarily produced by the shape of wings, which generates lower pressure on the upper surface and higher pressure on the lower surface, creating an upward force. Drag, on the other hand, is the resistance encountered by an object moving through a fluid. Reducing drag is crucial for optimizing the efficiency and speed of vehicles.

Flow Characteristics: Understanding how air flows around objects is vital in aerodynamics. Flow characteristics, such as laminar flow and turbulent flow, affect the drag and lift of objects. Laminar flow is smooth and orderly, while turbulent flow is chaotic and characterized by eddies and vortices. The transition from laminar to turbulent flow can significantly impact the aerodynamic performance of objects.

Boundary Layer: The boundary layer is a thin layer of air adjacent to the surface of an object in motion. It is responsible for the exchange of momentum and heat between the object and the airflow. The behavior of the boundary layer affects the drag experienced by the object. In some cases, controlling or manipulating the boundary layer can help reduce drag and improve performance.

Applications of Aerodynamics

Aircraft Design: Aerodynamics plays a fundamental role in the design and performance of aircraft. It encompasses the shape and design of wings, fuselage, control surfaces, and propulsion systems. Aerodynamic principles are used to optimize lift, reduce drag, and enhance stability and control.

Automotive Design: Aerodynamics is essential in automotive engineering to reduce drag and improve fuel efficiency. Streamlining the shape of cars and optimizing airflow around them can reduce drag, increase stability, and enhance performance.

Wind Energy: The design of wind turbines relies on aerodynamics to maximize the conversion of wind energy into electricity. Understanding the flow of air over the turbine blades helps optimize their efficiency and power output.

Sports Equipment: Aerodynamics is considered in the design of sports equipment such as racing bicycles, Formula 1 cars, golf balls, and ski suits. Minimizing drag and maximizing lift or downforce can provide athletes with a competitive edge.

Space Exploration: Aerodynamics is crucial in the design and performance of spacecraft and rockets. Aerodynamic principles are applied during launch, atmospheric reentry, and controlling the trajectory of spacecraft.

Venturi Effect

The Venturi effect is a phenomenon that occurs when a fluid flows through a constricted section of a pipe or conduit. It is named after Italian physicist Giovanni Battista Venturi, who first observed and studied the effect.

When a fluid flows through a constricted section of a pipe, its velocity increases while the pressure decreases. This is a consequence of the conservation of mass and energy, as described by Bernoulli’s theorem. According to Bernoulli’s principle, an increase in fluid velocity is accompanied by a decrease in pressure.

The Venturi effect can be explained as follows

Constriction: The flow passage narrows down, creating a constriction in the pipe. As the fluid approaches the constricted section, it is forced to accelerate due to the reduced cross-sectional area available for flow. The constriction causes the fluid velocity to increase.

Decreased Pressure: According to Bernoulli’s theorem, the increase in fluid velocity results in a decrease in pressure. This pressure decrease is often referred to as the “Venturi pressure drop” or “pressure differential.” The fluid’s kinetic energy increases at the expense of its potential and pressure energies.

Recovery: After passing through the constricted section, the fluid gradually expands back to the original pipe diameter. As the fluid expands, its velocity decreases, and the pressure returns closer to the initial value.

The Venturi effect has several practical applications

Flow Measurement: Venturi tubes or Venturi meters are commonly used to measure the flow rate of fluids. By measuring the pressure difference between the constricted and wider sections, the flow rate can be determined using calibrated relationships.

Atomization: The Venturi effect is utilized in devices such as atomizers and fuel injectors to create a fine spray or mist of a liquid. As the liquid passes through the constriction, the pressure drop causes it to break up into smaller droplets, facilitating better mixing or combustion.

Suction and Vacuum Systems: The Venturi effect is employed in suction and vacuum systems to generate suction or create a vacuum. By forcing a fluid (such as air or water) through a Venturi nozzle, a region of low pressure can be created, which allows for the suction or extraction of other fluids or gases.

Aspiration: In medical applications, the Venturi effect is used in devices called Venturi masks to deliver controlled oxygen concentrations to patients. By mixing oxygen with ambient air using a Venturi mechanism, precise oxygen concentrations can be achieved.

The Venturi effect is a fundamental principle in fluid dynamics, and its applications extend to various fields such as engineering, physics, and medicine.

Bernoulli’s Theorem – Assumptios, Limitation, Application and More


Carburetors are mechanical devices used in internal combustion engines to mix air and fuel in the proper proportions for combustion. They were widely used in older vehicles but have largely been replaced by fuel injection systems in modern cars. Nonetheless, understanding how carburetors work provides insights into the principles of engine operation.

The main components of a carburetor include

Air Intake: Carburetors have an air intake that allows atmospheric air to enter the engine. The air passes through a filter to remove contaminants before entering the carburetor.

Venturi: The venturi is a constricted section within the carburetor’s airflow passage. It creates a region of low pressure as air flows through it, based on the Venturi effect. The decrease in pressure helps draw fuel into the airflow.

Fuel Delivery System: Carburetors have a fuel delivery system consisting of a fuel bowl or reservoir, a float mechanism, and a fuel jet or nozzle. The fuel bowl stores the fuel, while the float mechanism maintains a constant fuel level by opening or closing the fuel inlet valve. The fuel jet or nozzle meters the flow of fuel into the airflow based on the engine’s requirements.

Throttle: The throttle controls the amount of airflow entering the engine. It is typically a butterfly valve that opens or closes to regulate the airflow. As the throttle opens, more air is allowed to enter, and the carburetor compensates by delivering a proportional amount of fuel.

Mixing Chamber: The mixing chamber is where the fuel and air are combined. The fuel is sprayed into the airflow via the fuel nozzle, and the airflow helps atomize and distribute the fuel evenly. The mixture of air and fuel then passes into the intake manifold for distribution to the engine cylinders.

The operation of a carburetor can be summarized as follows

Airflow: As the engine runs, air is drawn into the carburetor through the air intake.

Venturi Effect: The narrowing of the venturi in the carburetor causes the air velocity to increase, resulting in a low-pressure region. This low-pressure region helps draw fuel from the fuel bowl.

Fuel Metering: The fuel flow into the airflow is regulated by the fuel jet or nozzle, which is controlled by the fuel level in the float chamber. The level is maintained by the float mechanism, which opens or closes the fuel inlet valve as necessary.

Air-Fuel Mixture: The fuel is mixed with the incoming air in the mixing chamber. The carburetor aims to deliver the proper air-fuel ratio for efficient combustion based on engine requirements.

Throttle Control: The throttle, controlled by the accelerator pedal, regulates the airflow entering the engine. As the throttle opens or closes, the carburetor adjusts the fuel delivery to maintain the desired air-fuel ratio.

Carburetors require regular maintenance and adjustment to ensure optimal engine performance. Issues such as improper fuel mixture, clogged jets, or worn-out components can affect engine operation and fuel efficiency. While fuel injection systems have largely replaced carburetors in modern vehicles, understanding their functioning helps provide a historical perspective on engine technology.

Pitot Tubes

Pitot tubes, also known as Pitot-static tubes or Pitot probes, are devices used to measure fluid velocity, particularly in air or gases. They are commonly used in aerodynamics, aviation, and fluid mechanics to determine the speed of an object or the velocity of a fluid flow.

The Pitot tube is named after French engineer Henri Pitot, who invented it in the 18th century. It consists of two tubes: a larger, open-ended tube, known as the Pitot tube, and a smaller tube called the static tube. Both tubes are typically combined into a single device.

The working principle of a Pitot tube is based on the concept of total pressure and static pressure. Total pressure is the sum of static pressure (the pressure exerted by the fluid due to its motion) and dynamic pressure (the pressure resulting from the fluid’s velocity).

Here’s how a Pitot tube works

Total Pressure Measurement: The open-ended Pitot tube faces the oncoming fluid flow, such as air or gas. The forward-facing opening of the tube captures the total pressure of the fluid, which is a combination of static pressure and dynamic pressure. The total pressure is measured by connecting the Pitot tube to a pressure measuring device, such as a manometer or pressure transducer.

Static Pressure Measurement: The static tube is positioned perpendicular to the fluid flow, with multiple small openings facing the flow. These openings capture only the static pressure of the fluid, which is the pressure exerted by the fluid in the absence of its motion. The static pressure is also measured by connecting the static tube to a separate pressure measuring device.

Calculation of Velocity: By subtracting the measured static pressure from the total pressure, the dynamic pressure can be determined. The dynamic pressure is directly related to the velocity of the fluid flow according to Bernoulli’s equation. Using this relationship, the velocity of the fluid flow can be calculated.

Pitot tubes are commonly used in various applications

Aircraft Speed Measurement: Pitot tubes are used in aircraft to measure the airspeed or velocity of the aircraft through the surrounding air. The total pressure captured by the Pitot tube, along with the static pressure measurement from the static tube, allows for the calculation of the aircraft’s indicated airspeed (IAS) or calibrated airspeed (CAS).

Fluid Flow Measurement: Pitot tubes can be used to measure the velocity of fluid flow in pipes or channels. By positioning the Pitot tube in the flow and measuring the total and static pressures, the velocity of the fluid can be determined. This information is useful in industries such as water management, process control, and hydraulic systems.

Wind Tunnel Testing: In aerodynamic research and wind tunnel testing, Pitot tubes are used to measure the airspeed and airflow characteristics around model aircraft, vehicles, or other objects. This data helps in evaluating the aerodynamic performance and designing efficient vehicles.

Pitot tubes provide a straightforward and reliable method for measuring fluid velocity. However, it is essential to consider factors such as calibration, positioning, and potential errors to ensure accurate measurements.

Water Towers

Water towers are tall structures or elevated tanks that store water and provide water pressure in municipal water supply systems. They play a crucial role in maintaining a reliable and consistent water supply to communities.

Here are some key aspects and functions of water towers

Water Storage: Water towers serve as storage reservoirs for large quantities of water. They are typically designed to store water at an elevated height, allowing gravity to create the necessary pressure for water distribution. The height of the water tower determines the potential energy or pressure head that can be utilized.

Pressure Regulation: The elevated water in the tower creates hydrostatic pressure, which ensures adequate pressure in the water distribution system. By storing water at an elevated level, water towers provide consistent pressure throughout the distribution network, ensuring that water reaches homes and buildings on upper floors with sufficient force.

Peak Demand and Emergency Supply: Water towers help meet peak demand periods when water consumption is high. During periods of high usage, such as mornings or evenings, when water demand exceeds the rate at which it can be supplied, water is drawn from the water tower to meet the additional demand. Water towers also serve as emergency reserves during power outages or equipment failures, ensuring a continuous water supply.

System Balancing: Water towers help balance and stabilize the pressure within the water distribution system. The elevated storage provided by water towers allows for constant pressure throughout the network, compensating for variations in demand and avoiding pressure fluctuations.

Water Treatment: Water towers are often equipped with treatment facilities or receive pre-treated water to maintain water quality and prevent contamination. Water quality monitoring and treatment processes ensure that the stored water remains safe for consumption.

Fire Protection: Water towers can provide a reliable source of water for fire protection purposes. The elevated water and the associated pressure in the distribution system allow firefighters to access a significant amount of water quickly, aiding firefighting efforts and reducing the reliance on fire hydrants alone.

Bernoulli’s Theorem – Assumptios, Limitation, Application and More

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By Aditya

Hi, I’m Aditya Sharma, a professional blogger from Gurgaon, India and I launched this blog called aadityacademy on July 2021. is a mechanical Project-oriented platform run by Aditya sharma and I got the motivation to start aadityacademy blog after seeing less technical education information available on google.

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