Fluid Mechanics 2 Chapter 6 Quiz

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Questions and Answers

What is the principal focus of the Navier-Stokes equation derivation?

  • Reduction steps from the law of thermodynamics
  • The impact of gravitational forces on fluid flow
  • Reduction steps from Cauchy's equation of motion (correct)
  • The study of isothermal and non-isothermal flows

Which equation is essential for estimating flow properties such as velocity and pressure?

  • Euler's equation
  • Cauchy’s equation of motion
  • Continuity equation (correct)
  • Bernoulli's equation

In which scenario do velocity variations occur according to the content?

  • Flow in a pipe bend (correct)
  • Static fluid at rest
  • Uniform flow through a straight pipe
  • Laminar flow in open channels

What is indicated by the operational form of Cauchy’s equation of motion?

<p>Complexity due to 2nd order tensors (B)</p> Signup and view all the answers

What does the linear momentum balance relate to in fluid dynamics?

<p>Navier-Stokes equation (B)</p> Signup and view all the answers

Which of the following best describes energy dissipation in fluid flow?

<p>Loss of energy due to friction and turbulence (C)</p> Signup and view all the answers

How can the velocity field in a pipe be characterized?

<p>It varies at different locations based on flow dynamics (D)</p> Signup and view all the answers

Which of the following is not part of the foundation for modeling a flow system?

<p>Speed of sound in fluids (A)</p> Signup and view all the answers

What characterizes a Newtonian fluid?

<p>Constant kinematic viscosity (C)</p> Signup and view all the answers

In the context of incompressible fluids, what remains constant?

<p>Density, 𝜌 (A)</p> Signup and view all the answers

For a non-Newtonian fluid, how are the viscous friction terms modified?

<p>They are modified to include the fluid's rheology. (C)</p> Signup and view all the answers

In the given velocity field, what does the variable 𝑏 represent?

<p>A constant related to the velocity in the y-direction (D)</p> Signup and view all the answers

Which assumption is NOT applicable in the provided scenario of fluid flow?

<p>The flow is three-dimensional. (B)</p> Signup and view all the answers

What is the primary purpose of the Navier-Stokes equation in fluid dynamics?

<p>To describe the motion of fluid substances (A)</p> Signup and view all the answers

What does the term 'steady flow' imply in fluid dynamics?

<p>Fluid velocity at a point remains constant over time. (C)</p> Signup and view all the answers

In the given velocity field, what does the constant 𝑐 represent?

<p>The constant velocity in the z-direction (B)</p> Signup and view all the answers

What is the primary goal of modifying Cauchy’s equation of motion into an alternative form?

<p>To eliminate the second order stress tensor (C)</p> Signup and view all the answers

In the context of Cauchy's equations, what is the significance of having zero degrees of freedom?

<p>It indicates the system has a unique solution (A)</p> Signup and view all the answers

Which component of the Cauchy’s equation corresponds to the gravitational force acting in the x-direction?

<p>𝜌𝑔𝑥 (C)</p> Signup and view all the answers

What does the symbol 𝜏 represent in the alternative form of Cauchy’s equation?

<p>Stress tensor (C)</p> Signup and view all the answers

How many unknowns are associated with pressure, P, in the context of Cauchy’s equations?

<p>10 (C)</p> Signup and view all the answers

Which of the following represents the relationship between known and unknown variables in Cauchy’s equation?

<p>3 equations with 10 unknowns (A)</p> Signup and view all the answers

In the y-direction form of Cauchy’s equation, what does the term 𝜏𝑥𝑦 represent?

<p>Stress in the x-direction due to y-direction strain (D)</p> Signup and view all the answers

Which of the following is included in the modifications made to Cauchy’s equation for the x-direction?

<p>$ ho + v_x abla v$ (C)</p> Signup and view all the answers

What is the general form of the equation representing forces in the x-direction?

<p>$ ho g_x - abla P + ( au_{xx}) + ( au_{yx}) + ( au_{zx}) = ho v_x + ho v_x v_x + ho v_y v_x + ho v_z v_x$ (D)</p> Signup and view all the answers

Which term represents the effects of fluid momentum in the x-direction?

<p>$ ho v_x + abla ho vv$ (C)</p> Signup and view all the answers

What does the symbol $ abla P$ represent in these equations?

<p>The gradient of pressure (A)</p> Signup and view all the answers

How are the directions of tensors represented in these equations?

<p>By using second order components for shear stresses (A)</p> Signup and view all the answers

In the equation for the z-direction, which components are explicitly mentioned?

<p>Pressure, shear stresses, and inertial forces (A)</p> Signup and view all the answers

What does the term $ ho v_x v_y$ signify in the context of the equations?

<p>The interaction of velocities (C)</p> Signup and view all the answers

Cauchy’s equation was introduced primarily to address what?

<p>Limitations in understanding fluid dynamics (A)</p> Signup and view all the answers

Which of the following is NOT a term found in the equations for the y-direction?

<p>$ ho v_z$ (B)</p> Signup and view all the answers

What is the primary assumption regarding the function of velocity components in the x-direction?

<p>Velocity components depend only on the x coordinate. (D)</p> Signup and view all the answers

Which differential term is specifically omitted in the equation for the y-direction?

<p>Gravity term. (A)</p> Signup and view all the answers

What is the relationship of $v_y$ as expressed in the simplified Navier-Stokes equations?

<p>$v_y = -ay + cx$ (D)</p> Signup and view all the answers

In the Navier-Stokes equations, what does the term $\mu$ represent?

<p>Viscosity of the fluid. (B)</p> Signup and view all the answers

How does the pressure gradient relate to the x-coordinate based on the Navier-Stokes equation?

<p>$\frac{\partial P}{\partial x} = \rho - a^2 x - ab$ (C)</p> Signup and view all the answers

What happens when you integrate the expression for $\partial P / \partial y$?

<p>You find the pressure $P(x, y)$ as a function of coordinates. (D)</p> Signup and view all the answers

In the Navier-Stokes equations, what is the significance of steady-state conditions?

<p>The flow velocity varies only in space but not in time. (A)</p> Signup and view all the answers

Which of the following describes how $v_x$ is affected by the x-coordinate based on the equation provided?

<p>$v_x$ is linearly dependent on $x$. (A)</p> Signup and view all the answers

What effect does the term $\rho g_y$ have in the y-direction Navier-Stokes equation?

<p>It simplifies to zero under no gravity conditions. (B)</p> Signup and view all the answers

Which of the following variables plays a key role in determining the final expression for pressure $P(x,y)$?

<p>Integration constants $C$. (B)</p> Signup and view all the answers

What does the term $a^2$ signify in the equations for velocity components?

<p>It is a parameter influencing the flow characteristics. (D)</p> Signup and view all the answers

What does the integration of the expression $\partial P / \partial y$ ultimately yield in terms of function?

<p>It produces the pressure as a function of both $x$ and $y$. (D)</p> Signup and view all the answers

What significance does the term $\partial^2 v_y / \partial y^2$ hold in the context of fluid dynamics?

<p>It reflects the rate of shear deformation in the fluid. (A)</p> Signup and view all the answers

When integrating the expression for $h_x$, what final form does it take in relation to pressure?

<p>$h_x = \rho - \frac{a^2}{2} x^2 - abx + D$ (D)</p> Signup and view all the answers

What is referred to as the rate of increase in length per unit length in linear strain?

<p>Linear strain rate (B)</p> Signup and view all the answers

What is the formula for shear strain rate between two perpendicular lines initially intersecting at a point?

<p>𝜀𝑥𝑦 = 1/2(𝜕𝑣𝑦/𝜕𝑥 + 𝜕𝑣𝑥/𝜕𝑦) (B)</p> Signup and view all the answers

Which equation represents the linear strain rate tensor for the x-component?

<p>𝜀𝑥𝑥 = 𝜕𝑣𝑥/𝜕𝑥 (C)</p> Signup and view all the answers

In shear strain rates, what expression defines 𝜀𝑦𝑥?

<p>𝜀𝑦𝑥 = 1/2(𝜕𝑣𝑦/𝜕𝑥 + 𝜕𝑣𝑥/𝜕𝑦) (C)</p> Signup and view all the answers

What does the stress tensor relate to in the context of velocity?

<p>Rate of deformation of fluid elements (D)</p> Signup and view all the answers

In the Navier-Stokes equation, which term represents viscous stress?

<p>2𝜇𝜀𝑖𝑗 (D)</p> Signup and view all the answers

How is the stress tensor for the x-direction expressed in the context of the Navier-Stokes equations?

<p>𝜏𝑥𝑥 = 2𝜇𝜕𝑣𝑥/𝜕𝑥 (B)</p> Signup and view all the answers

Which of the following terms appears in the Cauchy’s equations for fluid motion?

<p>Body force term (B)</p> Signup and view all the answers

Which of the following represents the relationship between stress and strain rate in a Newtonian fluid?

<p>𝜏𝑖𝑗 = 𝜇𝜀𝑖𝑗 (C)</p> Signup and view all the answers

What is considered an assumption in the reduction to the Navier-Stokes equations?

<p>Incompressible flow (C)</p> Signup and view all the answers

In the context of pressure, which term in the Navier-Stokes equations denotes pressure gradient force?

<p>𝜕𝑃/𝜕𝑥 (D)</p> Signup and view all the answers

Which set of terms contributes to the nonlinear forces in the Navier-Stokes equations?

<p>Inertial terms (C)</p> Signup and view all the answers

What does the symbol 𝜇 represent in fluid mechanics?

<p>Dynamic viscosity (D)</p> Signup and view all the answers

What does modifying the Navier-Stokes equations aim to achieve?

<p>Simplify the computation process (A)</p> Signup and view all the answers

Flashcards

Cauchy's Equation of Motion

Cauchy's equation of motion is a fundamental equation in fluid dynamics that describes the motion of a fluid element. It expresses the balance of forces acting on a fluid element, including inertial forces, pressure forces, and viscous forces.

Navier-Stokes Equation Derivation

The Navier-Stokes equation is a set of partial differential equations that describe the motion of viscous, incompressible fluids. It is derived from Cauchy's equation of motion by introducing viscous forces.

Navier-Stokes Equation Applications

The Navier-Stokes equation is a powerful tool for understanding and predicting fluid flow behavior in various engineering applications. It is used to model and analyze fluid flow in pipes, around objects, and in many other scenarios.

Continuity Equation

The continuity equation expresses the conservation of mass. In fluid dynamics, it states that the rate of change of mass within a control volume is equal to the net mass flow rate into the volume.

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Flow in a Pipe

Fluid flow in a pipe is a common example of how the Navier-Stokes equation is used to analyze fluid behavior. The equation helps predict the velocity profile and pressure distribution along the pipe.

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Flow Around Objects

The Navier-Stokes equation can be used to model and predict fluid flow around different shapes and objects. It is essential for understanding aerodynamics and other fluid-related phenomena.

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Flow in a Bend

Fluid flow in a bend is a complex problem that can be analyzed using the Navier-Stokes equation. The equation helps predict the velocity and pressure changes as the fluid navigates the bend.

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Isothermal Flow

Isothermal flow is a type of fluid flow where the temperature remains constant throughout the system. This type of flow can be analyzed using the Navier-Stokes equation by considering the heat transfer involved.

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Cauchy's Equation

A fundamental equation in fluid dynamics that describes the conservation of linear momentum in a fluid. It's essentially a force balance equation, where the forces acting on a fluid element (pressure, viscous forces, and gravity) are equated to the rate of change of momentum of that element.

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Pressure (P)

The force per unit area exerted by a fluid on a surface, acting perpendicular to the surface.

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Viscous stress (𝝉)

The force per unit area acting parallel to the surface, arising due to the internal friction between fluid layers in motion.

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Gravitational acceleration (g)

The acceleration due to gravity, approximately 9.81 m/s^2, acting on a fluid element.

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Density (ρ)

The density of the fluid, representing its mass per unit volume.

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Fluid velocity (v)

The velocity vector of the fluid element, representing its speed and direction of motion.

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Fluid acceleration (a)

The rate of change of the fluid velocity vector with respect to time. It indicates how the speed and direction of the fluid element are changing over time.

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Velocity gradient (∇v)

The partial derivatives of the velocity vector with respect to the spatial coordinates (x, y, z). It represents the spatial variability of the velocity field, describing how velocity changes across the fluid.

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Alternative form of Cauchy's equation

The alternative form of Cauchy's equation of motion expresses the balance of forces acting on a fluid element. It involves pressure, stress tensor, gravity, and acceleration.

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Degree of Freedom in Cauchy's Equations

The degree of freedom in solving Cauchy's equations depends on the number of unknowns and equations. It represents the difference between the number of unknowns and the number of independent equations.

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Pressure (P) in Cauchy's Equation

Pressure (P) is a scalar quantity representing the force per unit area acting perpendicular to a surface. It is an important term in Cauchy's equation, affecting the fluid's motion.

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Stress Tensor (𝝉) in Cauchy's Equation

The stress tensor (𝝉) is a tensor quantity representing the forces acting on a fluid element. It captures how these forces are distributed in different directions.

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Gravitational Acceleration (g) in Cauchy's Equation

The gravitational acceleration (g) is a vector quantity that points downwards and influences the motion of the fluid due to gravity.

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Velocity (v) in Cauchy's Equation

The velocity (v) of the fluid is a vector quantity that represents the speed and direction of the fluid's movement.

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Density (ρ) in Cauchy's Equation

The density (ρ) of the fluid is a scalar quantity representing the mass per unit volume of the fluid. It affects the fluid's response to forces in Cauchy's equation.

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Newtonian fluid

A type of fluid where the viscosity remains constant, regardless of the applied shear stress.

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Incompressible fluid

A fluid whose density does not change with pressure, meaning its volume remains constant.

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Navier-Stokes Equation

The mathematical expression that describes the motion of viscous fluids by relating forces, pressure, and viscosity.

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Non-Newtonian Fluid

A fluid whose viscosity changes with the applied shear stress, meaning it doesn't flow consistently.

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Steady flow

A steady flow means the velocity at any point in the fluid doesn't change over time.

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Two-dimensional flow

A flow where the velocity varies only in two dimensions, meaning the motion can be described within a plane.

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Gravity force

The force exerted by the fluid's weight per unit volume. It acts downwards due to gravity.

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Linear Strain Rate

A measure of how much a material stretches or compresses in a given direction due to an applied force.

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Shear Strain Rate

A measure of how much a material changes shape due to an applied force.

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Strain Rate Tensor

A mathematical tool used to represent the rate of deformation of a material in all directions.

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Strain Rate Tensor as a function of velocity

A relationship that links the components of the strain rate tensor to the velocity components of the fluid.

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Stress Tensors as a function of velocity

Relating the stress tensors to the velocity components helps understand the forces acting within a deforming fluid.

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Relationship between Strain Rate and Stress Tensor

The strain rate tensor is a measure of how the material is deforming, while the stress tensor represents the forces acting within the material.

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Simplified Navier-Stokes Equation

A simplified form of the Navier-Stokes equation, obtained by assuming a constant viscosity and neglecting certain terms.

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Derivation of Navier-Stokes Equation from Cauchy's Equation

The Navier-Stokes equation is derived by substituting the stress tensors in terms of velocity into Cauchy's equation.

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Navier-Stokes Equation in Different Coordinate Systems

The Navier-Stokes equation can be expressed in different coordinate systems, such as Cartesian coordinates, cylindrical coordinates, and spherical coordinates.

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Navier-Stokes Equation for the x-direction

The Navier-Stokes equation for the x-direction describes how the velocity of the fluid in the x-direction changes with time and space due to various factors.

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Navier-Stokes Equation for y and z directions

The Navier-Stokes equation can be written for the y-direction and z-direction to describe the complete motion of the fluid in three dimensions.

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Applications of Navier-Stokes Equation

The Navier-Stokes equation is used to simulate and predict various fluid phenomena, like weather patterns, ocean currents, and blood flow.

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Simplifications of the Navier-Stokes Equation

Simplifying the Navier-Stokes equation can make it easier to solve in specific scenarios. The specific terms to neglect or simplify will depend on the particular problem and the underlying assumptions.

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What is the Navier-Stokes equation?

The Navier-Stokes equation is a set of equations that describe the motion of viscous fluids. It is derived from Newton's second law and accounts for forces like pressure, viscosity, and gravity.

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How are Navier-Stokes equations simplified?

The equation is simplified based on the assumptions made about the flow. For example, if the flow is steady, the partial derivative of velocity with respect to time is zero.

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What is the significance of velocity component relationships?

The relationship between velocity components indicates how the fluid moves in different directions. For example, if the flow is one-dimensional, only one velocity component depends on its corresponding coordinate.

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How are simplified Navier-Stokes equations solved?

The simplified Navier-Stokes equation can be solved to obtain the velocity profile and pressure distribution of the fluid. The solution involves finding the velocity components and pressure terms.

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How is pressure gradient related to velocity components?

The pressure gradient in the x-direction depends on the velocity components based on the simplified Navier-Stokes equation. It reflects the change of pressure along the x-axis.

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How is the pressure gradient in the y-direction determined?

The pressure gradient in the y-direction is also determined by the simplified Navier-Stokes equation. It represents the change of pressure in the y-direction.

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How is the pressure distribution obtained from the pressure gradient equations?

Integration of the pressure gradient equations with respect to x or y yields the pressure distribution in two dimensions. This represents the pressure variation across the entire flow region.

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What is the process of obtaining the pressure distribution from the pressure gradient equations?

The pressure distribution is found by integrating the pressure gradient equations with respect to x, y, or both. The integration involves finding the integral of the pressure gradient terms.

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What factors influence the pressure distribution?

The pressure distribution is influenced by the velocity components and the integration constants. The integration constants are determined by boundary conditions of the flow.

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What kind of function is the pressure distribution?

The pressure distribution is a function of both x and y coordinates. It represents the pressure variation at every point in the flow domain.

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What are the pressure gradients in the x and y directions?

The pressure gradients in the x and y directions are given by 𝜕𝑃/𝜕𝑥 and 𝜕𝑃/𝜕𝑦, respectively. These represent the rate of change of pressure along the corresponding directions.

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How are the integration constants in the pressure distribution equation determined?

The integration constants in the pressure distribution equation are unknown values that can be determined by applying suitable boundary conditions, such as specifying the pressure at certain locations in the flow domain.

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What is the importance of pressure distribution in fluid flow analysis?

The pressure distribution is a crucial parameter in fluid flow analysis. It provides information about the pressure forces acting on the fluid and helps predict the flow behavior.

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What is the significance of the simplified Navier-Stokes equation in studying pressure distribution?

The simplified Navier-Stokes equation provides a framework for understanding the pressure distribution in viscous flow. It helps analyze fluid behavior and predict the pressure field in various applications.

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Study Notes

Fluid Mechanics 2 - KIL 3002

  • Course code: KIL 3002
  • Course title: Fluid Mechanics 2
  • Department: Chemical Engineering
  • University: Universiti Malaya

Chapter 6: Derivation of Navier-Stokes Equations

  • Derivation steps: Reduction steps from Cauchy's equation of motion
  • Cauchy's equation of motion: An equation of motion which includes stress tensor terms
  • Navier-Stokes equation: Results from the reduction of Cauchy's equation of motion

Derivation of Navier-Stokes Equations

  • Reduction of equation from Cauchy's equation of motion, with the simplification of viscosity terms as a function of velocity
  • Navier-Stokes equation's components can be expressed as a function of velocity components
  • Constitutive equation relates viscous stress tensors with strain rates
  • Formula: τij = 2µεij

Determination of Flow Properties

  • Modeling a flow system estimates flow velocity and pressure at different locations
  • Methods are derived from the law of physics
  • Conservation of mass (Continuity equation)
  • Linear momentum balance (Navier-Stokes equation)
  • Flow patterns and energy dissipations are factors to consider for modeling

Cauchy's Equation of Motion

  • Original form: complex, uses 2nd-order tensors
  • Alternative form: introduced to overcome complexity and limitations of the original form
  • Variables: Pressure (P), Velocities (vx, vy, vz), Stress tensors (Txx, Tyx, Tzx, Tyy, Tzy, Tzz)
  • Degree of freedom relates to the number of equations and unknowns.

Reduction to Navier-Stokes Equations

  • Simplifying the stress tensor components in terms of velocity components
  • Constitutive equation relates viscous stress tensors with strain rates, valid for incompressible and Newtonian fluids.
  • Formula: τij = 2µεij
  • Strain rate tensor relation to velocity components using different relations.

Reduction to Navier-Stokes Eqn for X-Direction

  • Substituting stress tensors in Cauchy's equations yields the Navier-Stokes equation for the x-direction.
  • Includes density, pressure, viscosity, and velocity components.
  • Relevant for Newtonian fluids.

Reduction to Navier-Stokes Eqn - Simplifications

  • Simplifying the Navier-Stokes equations for incompressible fluids, setting ∇•v = 0.
  • Resultant equations have reduced complexity, making calculations more manageable for incompressible fluids.
  • Compact notation for easier handling of variables.
  • Combining the x, y, and z-components of the Navier-Stokes equation to form a general form for any direction.
  • Simplified notation to represent the three equations of motion using compact notation.
  • General form of the Navier-Stokes equation allows for a general solution for any direction (x,y,z).
  • Assumptions made—Newtonian fluid, constant kinematic viscosity (µ), constant density (ρ), and incompressible fluids, flow is steady and 2-dimensional, gravity is negligible.

Application of Navier-Stokes Equation – Example

  • Example of a steady, two-dimensional, incompressible velocity field.
  • Determining the pressure field as a function of x and y
  • Simplification of the Navier-Stokes to solve for x and y components of the equations
  • Pressure field functions are determined by using integration techniques and by substituting equations to solve for the differential variables for x and y components.

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