Name: tracking fluid through space or mass
Uploaded: 2025-02-13T15:43:18.000Z
Duration: 1 min 16 s
Channel: JoVE

A control volume is a defined region in space where interactions involving mass, energy, or momentum are analyzed.
Consider water flowing through a pipe. The control volume would be a section of the pipe, and the control surface would be the boundary of this section.
As water enters and exits this control volume, conservation laws for mass, momentum, and energy are used to study the flow without tracking individual water particles.
The control volume stays fixed while fluid moves through it, reducing complexity.
In contrast, a system refers to a specific mass of fluid that is followed over time. Consider a water tank holding a fixed amount of water.
The water inside the tank represents the system. As the water drains or fills, the same system of water is tracked, focusing on its behavior and changes over time.
The system representation allows an analysis of this fixed mass of water, even as the tank&#8217;s state changes.
While the system follows the specific fluid mass, the control volume approach remains stationary and focuses on a particular region of space.

control volume and system representations

Two key frameworks are employed to analyze mass, energy, and momentum transfer: the control volume approach and the system approach. These frameworks offer different perspectives, depending on whether the focus is on a specific region in space (control volume approach) or a defined mass of fluid (system approach).
The control volume approach considers a stationary region in space through which fluid flows. This region is bounded by a control surface.&#160; For instance, in the case of water flowing through a pipe, the control volume would be a fixed section of the pipe, and the control surface would be the boundary of this section. This framework simplifies the analysis of fluid flow by applying conservation laws for mass, energy, and momentum across the control volume. The key advantage is that it focuses on the macroscopic properties of the fluid without requiring tracking of individual fluid particles. As water enters and exits the control volume, the changes in its flow characteristics are assessed at the boundaries of the control surface, making it a powerful tool for studying steady or unsteady flow conditions.
The system approach, by contrast, is concerned with a fixed mass of fluid that is tracked over time. A practical example of this is water in a tank. The water inside the tank represents a system, and as it drains or fills, the same mass of water is analyzed. The system&#39;s behavior is studied as external forces and energy transfers cause changes in the fluid&#39;s state. This approach is particularly useful when analyzing thermodynamic processes where the exact mass and its energy or momentum content are critical, such as in closed-system heat transfer or phase change scenarios.
While the control volume is fixed in space and focuses on the exchange of quantities through its surface, the system approach tracks the evolution of a specific mass of fluid. These methodologies are complementary, offering flexibility in fluid dynamics and thermodynamic analysis.

Control Volume and System Representations

A control volume is a defined region in space where interactions involving mass, energy, or momentum are analyzed.
Consider water flowing through a pipe. ...

control-volume-and-system-representations

Education

JoVE Core

Civil Engineering

Unit 1

Fluid Kinematics

KEY TERMS AND CONCEPTS

tracking fluid through space or mass

Fluid analysis often uses two linked ideas: the control volume approach and the system approach. Both help describe mass, energy, and momentum transfer, but they focus on different things. One follows a region in space. The other follows a fixed mass of fluid over time.

The control volume approach looks at a stationary area in space through which fluid moves. That area is bounded by a control surface, which is the outer edge of the region. A pipe with water flowing through it is a clear example. In that case, the control volume is a fixed section of the pipe.

This method makes it easier to apply conservation laws for mass, energy, and momentum. It does not require tracking each fluid particle. Instead, it studies how fluid enters and exits the control surface. This makes it useful for both steady flow and unsteady flow.

The system approach uses a different viewpoint. It tracks a specific mass of fluid as conditions change. Water in a tank is a good example. As the tank drains or fills, the same mass of water is treated as the system.

This approach is especially useful in thermodynamics. It helps when the exact mass and its energy or momentum content matter, such as in closed-system heat transfer or phase change. The system changes because external forces and energy transfers act on it.

Together, the control volume and system approaches give students flexible ways to study fluid dynamics and thermodynamics. One focuses on what crosses a boundary in space. The other focuses on how one mass of fluid changes over time.

Tracking Fluid Through Space or Mass

eulerian and lagrangian flow descriptions

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Tracking Fluid Motion in Space and Time

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introduction to types of flows

Flow Patterns by Direction and Time

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Streamlines, streaklines, and pathlines are three ways to describe fluid motion. Each one shows flow from a different angle. Together, they help students compare how a fluid moves in steady and unsteady flow. A streamline is a line that stays tangent to the fluid’s velocity vector at each point. That means the fluid velocity points along the streamline. Streamlines give an instant picture of the flow field, so they are useful for showing the overall direction of flow in a fluid system. In...

Fluid analysis often uses two linked ideas: the control volume approach and the system approach. Both help describe mass, energy, and momentum transfer, but they focus on different things. One follows a region in space. The other follows a fixed mass of fluid over time. The control volume approach looks at a stationary area in space through which fluid moves. That area is bounded by a control surface, which is the outer edge of the region. A pipe with water flowing through it is a clear...

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How Flow Changes Affect Particle Motion

Velocity and acceleration help describe how a fluid particle moves in steady and unsteady flow. A fluid particle follows a pathline, and its velocity depends on both position and time. Acceleration is found by differentiating velocity with respect to time, so it shows how the particle’s motion changes. In a flow field, acceleration can be written using three perpendicular components. These components show how velocity changes over time in different spatial directions. This gives a general way...

reynolds transport theorem

Reynolds Transport and Control Volumes

Reynolds transport theorem links a fluid system to a control volume. It helps explain how the time rate of change of a property can be tracked in fluid dynamics. The theorem uses extensive properties, such as mass, velocity, acceleration, temperature, and momentum. These properties depend on the size of the system. Their intensive forms are the values per unit mass. The total amount of an extensive property in a system at any instant is the sum of the amounts carried by each tiny fluid...

design example: flow through a fire extinguisher

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Fire Extinguisher Nozzle Flow and Pressure

A pressurized water fire extinguisher uses fluid dynamics to create a fast stream of water. The water is stored at a much higher pressure inside the extinguisher than the surrounding air. When the extinguisher is activated, that pressure difference forces water to flow out quickly. Bernoulli's equation helps predict the exit speed of the water. This equation links pressure and velocity. As pressure drops, velocity increases. In the extinguisher, the high internal pressure is converted into the...

key terms and concepts

Learn how the control volume and system approaches analyze mass, energy, and momentum in fluids, using examples like water in a pipe or tank.

Tracking Fluid Through Space or Mass | JoVE K12

JoVE Core Civil Engineering offers a foundational exploration of essential construction materials such as bricks, stones, steel, wood, and concrete, along with fluid mechanics topics like flow over immersed bodies, open channel flow, and dimensional modeling. It also covers key surveying concepts, including leveling, traverse adjustments, curve design, topographic mapping, and geospatial technologies (GPS & GIS). Science-in-action videos showcase laboratory research on materials, fluids, and surveying practices in real-world engineering systems.

Tracking Fluid Through Space or Mass

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