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Consider the energy equation for flow of a fluid in a closed conduit, which can be expressed in the form $$\frac{p_{1}}{\gamma}+\frac{V_{1}^{2}}{2 g}+z_{1}=\frac{p_{2}}{\gamma}+\frac{V_{2}^{2}}{2 g}+z_{2}+h_{\mathrm{f}}$$ where \(p_{1}, V_{1}\), and \(z_{1}\) are the pressure, average velocity, and centerline elevation at an upstream section of the conduit, \(p_{2}, V_{2},\) and \(z_{2}\) are the corresponding variables at a downstream section, \(h_{f}\) is the head loss between the two sections, \(\gamma\) is the specific weigh of the fluid, and \(g\) is the gravity constant. State the SI unit of energy and state the SI unit(s) of the terms (not the individual variables) in Equation 6.27. Explain why each of the terms in Equation 6.27 represents energy and express Equation 6.27 as a nondimensional equation.

Short Answer

Expert verified
The SI unit for energy is the Joule, and each term in the equation has an SI unit of meters. Terms represent energy due to pressure, velocity, and elevation.

Step by step solution

01

Identify the SI units for energy

The SI unit for energy is the Joule (J). In terms of mechanical energy within a fluid context, energy is often expressed in terms of head (i.e., energy per unit weight of the fluid) in meters (m). Thus, each term represents energy in the unit of length.
02

Identify terms and their SI units in the equation

Let's analyze each term in the energy equation:- \(\frac{p}{\gamma}\): \([N/m^2]/[N/m^3] = m\) (where pressure \(p\) has the SI unit of Pascal (Pa or N/m²), and \(\gamma\), specific weight, has unit N/m³. Thus, this term represents energy per unit weight in meters).- \(\frac{V^2}{2g}\): \([m^2/s^2]/[m/s^2] = m\) (velocity squared divided by twice the gravitational acceleration also yields energy per unit weight, in meters).- \(z\): \(m\) (elevation or head above a datum is inherently in meters and represents potential energy per unit weight).
03

Understand why each term represents energy

Each term in the equation represents an equivalent form of energy per unit weight:1. Pressure Energy (\(\frac{p}{\gamma}\)): Represents energy stored due to the fluid being under pressure.2. Kinetic Energy (\(\frac{V^2}{2g}\)): Energy due to fluid's motion.3. Potential Energy (\(z\)): Energy due to elevation in a gravitational field.These reasons justify why the terms represent the energy in meters.
04

Express the equation as a nondimensional form

To nondimensionalize the equation, divide through by a common term that represents these energies. Using \(z_1\), an elevation as a reference, define dimensionless terms:1. \(\tilde{p} = \frac{p}{\gamma z_1}\)2. \(\tilde{V} = \frac{V^2}{2gz_1}\)3. \(\tilde{z} = \frac{z}{z_1}\)Thus, the nondimensional equation is: \[ \tilde{p_1} + \tilde{V_1} + \tilde{z_1} = \tilde{p_2} + \tilde{V_2} + \tilde{z_2} + \frac{h_f}{z_1} \]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Fluid Dynamics
Fluid dynamics is a field of physics that focuses on the behavior of fluids, including liquids and gases, in motion. It helps us understand how fluids flow through different environments and under varying conditions. In the context of Bernoulli's Equation, fluid dynamics allows us to analyze how the movement of a fluid affects its energy distribution.
In a closed conduit, as represented in our exercise, we consider key properties like pressure, velocity, and elevation of the fluid along its flow path. These properties can change depending on the fluid's dynamics and the physical characteristics of the pipe or conduit. By understanding these changes, engineers can design systems such as pipelines, water supply networks, and even aerodynamics of vehicles.
Fluid dynamics is essential not only for calculating potential energy, kinetic energy, and pressure energy but also for predicting how these energies affect structures around us.
Energy Equation
The energy equation is central to understanding how energy is transformed and conserved in fluid systems. It is particularly evident in Bernoulli's Equation, where we equate different forms of energy as a fluid moves between two points in a conduit.
Let's break it down:
  • Pressure Energy: \[ \frac{p}{\gamma} \] signifies energy stored due to fluid pressure. It's how the fluid's pressure can do work or move to regions of lower pressure.
  • Kinetic Energy: \[ \frac{V^2}{2g} \] is the energy from the fluid's velocity, indicating how fast it's moving and related energy from that motion.
  • Potential Energy: \(z\) represents the energy due to fluid's elevation in a gravitational field. Lowering or raising a fluid affects its potential to do work.
Each term in Bernoulli's Equation is expressed concerning energy per unit weight, which, in this context, translates to meters due to its integration with fluid dynamics units like N/m² for pressure.
By maintaining an energy balance with these terms, we can better understand how fluids behave during motion and how systems are optimized for maximum efficiency.
Head Loss
Head loss is a critical concept when analyzing systems with Bernoulli’s Equation in fluid dynamics. It represents the loss of energy due to friction and other resistive forces as the fluid flows through a conduit or pipe.
In the equation, head loss \(h_f\) is added on the right-hand side to account for the energy dissipated through:
  • Friction: As the fluid moves along the surfaces of a pipe or boundary.
  • Turbulence: When the fluid's flow becomes irregular, leading to additional energy dissipation.
  • Bends and Fittings: These introduce resistance to fluid flow, causing more energy to be lost.
Head loss is predominantly energy lost in the form of heat or noise, making it vital to minimize in engineering designs. By accurately calculating and reducing head loss, systems can become more energy-efficient, leading to savings in operational costs and reducing the risk of system failures.
Nondimensional Analysis
Nondimensional analysis is a mathematical approach in fluid dynamics used to simplify complex problems by removing physical dimensions. In the context of Bernoulli's Equation, nondimensionalization helps in making comparisons between different fluid systems more straightforward and reveals key underlying similarities.
To nondimensionalize the energy equation, each term is divided by a characteristic term, such as the elevation or head \(z_1\), which acts as a reference. Here’s how it occurs:
  • Pressure: \(\tilde{p} = \frac{p}{\gamma z_1}\)
  • Velocity: \(\tilde{V} = \frac{V^2}{2gz_1}\)
  • Elevation: \(\tilde{z} = \frac{z}{z_1}\)
This transformation leads to a nondimensional form of the equation:\[ \tilde{p_1} + \tilde{V_1} + \tilde{z_1} = \tilde{p_2} + \tilde{V_2} + \tilde{z_2} + \frac{h_f}{z_1} \]By using nondimensional terms, scientists and engineers can focus on the relationships and ratios of the energies involved, facilitating easier scaling of problems across different systems or conditions. This technique is particularly useful in experiments and simulations, where direct measurement might be challenging.

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Most popular questions from this chapter

In a particular two-dimensional flow field of an incompressible fluid in the \(x z\) plane, the \(z\) component of the momentum equation is given by $$\rho u \frac{\partial w}{\partial x}=\mu\left(\frac{\partial^{2} w}{\partial x^{2}}+\frac{\partial^{2} w}{\partial z^{2}}\right)-\rho g$$ where \(u\) and \(w\) are the \(x\) and \(z\) components of the velocity, respectively, \(\rho\) and \(\mu\) are the density and dynamic viscosity of the fluid, respectively, and \(g\) is the gravity constant. The relevant scales are the length scale, \(L,\) and the velocity scale, \(V\). Express Equation 6.30 in normalized form, using the Reynolds number, Re, defined as \(\mathrm{Re}=\rho V L / \mu,\) and the Froude number, Fr, defined as \(\mathrm{Fr}=V / \sqrt{g L},\) in the final expression. What is the asymptotic form of the governing equation as the Reynolds number becomes large?

Wind blowing at a steady velocity \(V\) toward a vertical column of diameter, \(D,\) generates velocity fluctuations behind the column that have a frequency \(\omega\). The frequency of the generated velocity fluctuations can also depend on the density and viscosity of the air. Determine a nondimensional functional relationship that relates the frequency of the velocity fluctuations to the influencing variables. Identify any named conventional dimensionless groups.

Observations on the International Space Station indicate that if a small spherical liquid droplet suspended in space is deformed slightly, then its surface will oscillate with a frequency that depends on the diameter of the droplet and the density and viscosity of the liquid. Use dimensional analysis to estimate the functional relationship between the oscillation frequency of the droplet surface and the influencing variables.

A ship of length \(L\) moves at a cruising speed of \(V\) in a calm ocean where the density and viscosity of the seawater are \(\rho\) and \(\mu\), respectively. Synoptic experimental measurements of the drag force on the ship and the influencing variables are available. Determine the dimensionless groups that should be used to organize these data to determine an empirical relation between the drag force and the influencing variables. Why is it important to include gravity as a variable?

The Froude number, Fr, at any cross section of an open channel is defined by the relation $$\mathrm{Fr}=\frac{\bar{V}}{\sqrt{g D_{\mathrm{h}}}}$$ where \(\bar{V}\) is the average velocity, \(g\) is the acceleration due to gravity, and \(D_{\mathrm{h}}\) is the hydraulic depth. The hydraulic depth is defined as \(A / T,\) where \(A\) is the flow area and \(T\) is the top width of the flow area. (a) Show that Fr is dimensionless. (b) Determine the value of Fr in a trapezoidal channel that has a bottom width of \(3 \mathrm{~m}\), side slopes \(2.5: 1(\mathrm{H}: \mathrm{V}),\) an average velocity of \(0.4 \mathrm{~m} / \mathrm{s},\) and a flow depth of \(1.5 \mathrm{~m} .\)

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