CFD Analysis and Simulation Techniques

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Questions and Answers

What is one of the key advantages of using CFD analysis compared to physical testing?

CFD analysis does not require any simulation of dangerous scenarios.

CFD analysis is more accurate than physical tests.

CFD analysis eliminates the need for redesign.

CFD analysis is less expensive and faster than experiments. (correct)

What stage in the Simulation Driven Design does CFD specifically contribute to?

Conceptual development (correct)

Final prototype testing

Physical prototype creation

Design validation

Which aspect of fluid flow does CFD analysis provide?

A hypothetical overview without specifications.

Physical properties of the materials used in construction.

A complete description of the fluid flow within the region of interest. (correct)

Solely the variables involved in a simplified model.

In the context of CFD, what does the term 'post-processing' refer to?

The evaluation and interpretation of simulation results. (D) Signup and view all the answers

How does CFD support the redesign process in product development?

By simulating modifications to systems before implementation. (A) Signup and view all the answers

What type of simulation is being conducted in the mixing elbow setup?

Conjugate Heat Transfer (CHT) (C) Signup and view all the answers

What is the temperature of the cold water entering the large inlet?

295 K (D) Signup and view all the answers

What is the flow velocity of the water entering from the small inlet?

1.2 m/s (A) Signup and view all the answers

Which tool is used to verify the overall dimensions in the simulation workflow?

Measure Tool (B) Signup and view all the answers

In what order does the simulation workflow start?

Import CAD, Meshing, Setup Solution (C) Signup and view all the answers

What is the main purpose of the simulation described?

To predict temperature changes in the water (B) Signup and view all the answers

What role does the large inlet play in the mixing elbow setup?

It serves for cold water entry (A) Signup and view all the answers

What is the significance of verifying essential dimensions in this simulation process?

To validate the accuracy of the heat transfer model (C) Signup and view all the answers

What is the first step to enable heat transfer in the Flow_Steady set?

Select any inner face as wetted face (A) Signup and view all the answers

What should you do after completing each important section in the Flow_Steady setup?

Save the file with a new name (B) Signup and view all the answers

When setting a cylindrical face during the Region Mesh Control, what is the specified radius?

25si (D) Signup and view all the answers

Which process is indicated as the next step after selecting a circular face in Region Mesh Control?

Select a vertex (D) Signup and view all the answers

What option is specified for the average element size in the mesh settings?

2mm (B) Signup and view all the answers

What should follow the creation of a flow solution in the Flow_Steady set?

Enabling heat transfer (D) Signup and view all the answers

In the process of combining bodies, what is one of the preliminary actions to take?

Select all bodies (D) Signup and view all the answers

Which of the following is NOT a step mentioned in the context of the Flow_Steady setup?

Adjust temperature settings (B) Signup and view all the answers

What is the first step to visualize vectors in the cutting plane?

Set plot to vector. (D) Signup and view all the answers

What can be adjusted to improve the visualization of streamlines?

Color of the fluid components. (C) Signup and view all the answers

How many seeds should be set when creating a rake for the large inlet face?

100 seeds. (C) Signup and view all the answers

What is the purpose of creating streamlines in this context?

To analyze velocity results. (C) Signup and view all the answers

Which action should be performed to create streamlines for both rakes?

Create rakes separately for each inlet face. (B) Signup and view all the answers

What should you do after selecting a small inlet face for streamlines?

Create a new rake. (A) Signup and view all the answers

Why would you want to hide the Elbow in the visualization process?

To better visualize the streamlines. (B) Signup and view all the answers

What is the primary role of setting the number of seeds in rake creation?

It establishes the density of streamlines. (A) Signup and view all the answers

What does the instantaneous velocity equation represent in Reynolds decomposition?

The sum of time-averaged and fluctuating components (A) Signup and view all the answers

In Reynolds decomposition, what is the time average of the fluctuating velocity equal to?

Zero (D) Signup and view all the answers

What do Reynolds stresses represent in the context of turbulent flow?

Unknowns introduced by Reynolds-averaging (B) Signup and view all the answers

Which equation represents the Reynolds-Averaged Navier-Stokes (RANS) equation?

$\rho \frac{\partial u}{\partial t} + \nabla \cdot U \cdot U = -\nabla P + \rho g + \mu \nabla^2 U$ (A) Signup and view all the answers

Which of the following components is NOT part of the Reynolds-Averaged Navier-Stokes equation?

Turbulence intensity (C) Signup and view all the answers

What does the Reynolds stress tensor indicate?

It expresses fluctuations in velocity and their impact (D) Signup and view all the answers

Which term is used to designate the variance of the velocity fluctuating component?

Mean square velocity fluctuation (A) Signup and view all the answers

How is the time-averaged velocity calculated in Reynolds decomposition?

$\bar{u}i = \frac{1}{t + \Delta t} \int{t}^{t + \Delta t} u_i dt$ (A) Signup and view all the answers

What is a key characteristic of the analytical approach to fluid flow?

It involves resolving mathematical equations governing fluid motion. (C) Signup and view all the answers

Which of the following techniques is used in the experimental approach to fluid flow?

Hot wire anemometry (C) Signup and view all the answers

What does the numerical approach to fluid flow rely on for its predictions?

Digital computers and conservation laws. (A) Signup and view all the answers

Which fluid flow cases are suitable for the analytical approach?

Simple flow cases with certain approximations. (C) Signup and view all the answers

In the context of experimental methods, which device is used to measure fluid velocity?

Pitot tube (A) Signup and view all the answers

What limitation is associated with analytical solutions in fluid dynamics?

They can only work with mathematical approximations. (A) Signup and view all the answers

What is the primary goal of using hot wire anemometry in the experimental approach?

To measure rapidly varying velocities at a point. (D) Signup and view all the answers

What is the main purpose of dividing the fluid region into finite regions in the numerical approach?

To create algebraic equations for computations. (C) Signup and view all the answers

Study Notes

Introduction to CFD

CFD is a branch of physics studying fluids (liquids, gases, plasmas) and forces acting on them.
Fluid mechanics is the branch of physics that studies fluids (liquids, gases, plasmas) and the forces acting on them.
"Fluid" is a substance continuously deforming under shearing stress.
"Mechanics" deals with both stationary and moving bodies under force influence.

Flow Solution Approaches

Fluid flow can be studied analytically, experimentally, or numerically.
Analytical approach uses mathematical equations to model the flow for cases like Couette flow, Hagen-Poiseuille flow, and Convergent-Divergent Nozzle.
Experimental approach involves studying a prototype in a test facility. Measurement of fluid properties (temperature, pressure) is done using equipment like hot-wire anemometry, pitot tubes, and mass flow meters. Flow visualization techniques like colored oil and particle tracers (smoke, lasers) are also used.
Numerical approach uses digital computers to model the flow using conservation laws (mass, momentum, energy) and dividing the fluid region into finite regions. CFD (Computational Fluid Dynamics) is a numerical approach.

Analytical Approach

Analytical solution involves solving governing equations for the physics of the phenomenon under study.
A general solution for all physical problems is complex.
The approach is effective for simple flow cases with simplified geometry, dimensionality and compressibility.
Examples of such flows include Couette flow, Hagen-Poiseuille flow, and Convergent-Divergent Nozzle.

Experimental Approach

An experimental solution utilizes a prototype device within a test facility.
Fluid properties (temperature, pressure) are measured using equipment.
Hot wire anemometry measures rapidly varying velocities at a point. Other equipment includes pitot tubes and mass flow meters.
Flow visualization techniques are used for understanding fluid behavior in a physical model (colored oil, smoke, laser light).

Numerical Approach

Numerical solutions use computer simulations to model fluid flow via the conservation laws for mass, momentum and energy of fluid in motion.
The fluid region of interest is discretized into finite regions.
Governing equations are discretized into algebraic equations, which are calculated over time and space by computers.
The result is a comprehensive description of the fluid's movement.

CFD Advantages

Performance evaluation of systems before installation
Comprehensive data on a system.
Variety of post processing options for deeper insights into the system.
Low cost compared to experimental testing in terms of time and resources. Simulation of dangerous or expensive experiments is also enabled.

CFD Generic Process

Model Preparation: Convert geometries to watertight volumes suitable for CFD analysis. Select physics models, material properties, forces, heat sources. Assign boundary conditions.
Mesh Setup and Meshing: Define mesh settings for surface and volume meshing. Run meshing.
Solver Run, Monitoring and Post-Processing: Run CFD simulation. Monitor while running. Post-process results after the simulation is complete.

Physics - Governing Equations

Governing equations model incompressible, viscous, heat-conducting fluids. Equations describe transport of mass, momentum, and energy.
Fluid behavior is described in Eulerian perspective (at a fixed point) or in Lagrangian perspective (with the fluid particle).

Turbulence - Physics of Turbulence

Turbulence is an essential feature of fluid flows with significant and irregular velocity field variations in time and position.
Examples of turbulent flows include: rivers, ocean currents, cyclones, bush fires, and flows over bodies (cars, aircraft).
Characteristics of turbulence: irregularity, non-repetition, three dimensional unsteady eddies, and property of flow, not fluid property.

Turbulence - Reynolds Number

Reynolds number (Re) represents the ratio of inertial force to viscous force in a fluid flow. Calculated as:
Re = ρuL/μ ,
where: ρ is fluid density, u is flow velocity, L is characteristic length, and μ is dynamic viscosity.
Re provides a method to characterize the flow patterns (laminar or turbulent) and determine dynamic similarity between flow cases.

Turbulence - Critical Reynolds Number

The critical Reynolds number (Recr) signals the transition from laminar to turbulent flow. Values depend on specific flow conditions. Parameters to consider are pipe diameter (for pipe flow), distance from the leading edge (for flow over a flat plate), and obstacle diameter (for an obstacle in cross flow).

Turbulent Flow vs. Laminar Flow

Laminar flow exhibits a velocity field with layered velocity vectors at low Reynolds numbers. This is characterized by inertial force being smaller than viscous force.
Turbulent flow has irregular flow patterns and mixing due to chaotic eddies at high Reynolds numbers. The inertial force is greater than the viscous force.

Challenges in Simulating Turbulent Flows

Three-dimensional time-dependent flow
Irregular and chaotic nature
Non-repetitive
Vast range of length & time scales
Length scale reduction with increasing Reynolds Number
Sensitivity to boundary & initial conditions

Modeling of Turbulence - Methods Comparison

Various models of turbulence exist, including RANS, LES, and DNS, with differing ranges of modeled physics and computational cost. The most computationally expensive models (DNS) capture the smallest scales of turbulence, while the computationally least expensive (RANS) models often employ approximations.

RANS - Reynolds Decomposition

Instantaneous velocity components can be decomposed into time-averaged component and fluctuating components (u = ū + u').
The time average of velocity fluctuations is zero (ū = 0). The time average of variance uu' is not zero (u u'≠ 0).
Reynolds decomposition provides a framework for deriving Reynolds-Averaged Navier-Stokes equation.

RANS - Reynolds-Averaged Navier-Stokes (RANS) Equation

The Reynolds stresses (Tij) in RANS equations are unknowns, requiring turbulence models.

RANS - General Form of Turbulence Models

The general form of turbulence models shows how different parts of the equation influence variables in relation to time and space, including convective term, diffusion term, production term, dissipation term, and unsteady term. Turbulent variables are denoted by φ.

RANS - Turbulence Models

Spalart-Allmaras, k-ε, Realizable k-ε, and k-ω (including SST k-ω) are different turbulence models with varying characteristics and suitability for specific applications.
Features like near-wall effects (or near wall damping term) are important aspects of some models.

Near-wall Turbulence - Law of the Wall

The law of the wall describes velocity near a wall in turbulent wall-bounded flow. It's a semi-empirical expression relating velocity to the distance from the wall. This behavior is independent of conditions further from the wall.

Near-wall Turbulence - Y+

Y⁺ is a nondimensionalized distance from the wall, representing the ratio of turbulent and laminar effects within a computational cell.
It is useful in numerical models for selecting density of mesh near boundaries.

Near-wall Turbulence - Boundary Layer Regions

Viscous/Laminar sublayer: Turbulent motion stops near the wall due to the no-slip condition. Viscous shear stresses dominate.
Buffer Layer: A transition zone between laminar and logarithmic sublayers wherein velocity gradients are high but flow is largely turbulent.
Logarithmic Layer: A region where the velocity profile follows a logarithmic law.

Near-wall Turbulence - Boundary Layer Resolving Approaches

Models need computational meshes appropriate for near-wall region phenomena resolution.
Full boundary layer resolution using many cells to capture steep wall gradients.
Wall functions use empirical non-linear relationships to model regions further from the wall.

Numerical Methods

FEM (Finite Element Method) and FVM (Finite Volume Method) are different approaches to numerical modeling. FEM excels at near-wall aspects and has rich expression powers across interfaces, unlike the FVM.

Workshop 1: Manifold Flow Simulation

This workshop explores CFD analysis workflows using SimLab for a manifold pipe model, analogous to an inlet manifold in an engine.
The model aims for near-equal flow distribution among passages.
The workflow involves importing CAD, meshing, setting up the solution, and visualization of results.

Workshop 2: Mixing Elbow Steady State CHT

This workshop demonstrates a typical industrial mixing pipe scenario.
Hot and cold water mix within the elbow, necessitating a Conjugate Heat Transfer (CHT) steady state simulation. The simulation is set up by importing CAD, creating the fluid domain, meshing, defining the solution, and visualising the results.

Workshop 3: Mixing Elbow Transient CHT

This transient workshop considers variable inlet water temperatures.
The solution utilizes initial conditions from the steady state simulation.
The solver is disabled for flow, focusing only on heat transfer.

Workshop 4: Thermal FSI Mixing Elbow

This workshop models a one-way coupled simulation of thermal fluid-structure interaction (FSI).
Pressure and temperature loads from the CFD simulation of Workshop 2 are mapped onto the structural mesh.

More About CFD Volume Meshing

CFD mesh tools create boundary layers (BL) and tetrahedral elements for the fluid interior. Input includes surface mesh bodies. The type of fluid bodies (e.g. inlets/outlets) affect how boundary layers are generated. Solid bodies might not need boundary layers.
Various methods for controlling thickness of the first layer are discussed, along with total layer counts (numbers).

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Description

Test your knowledge on Computational Fluid Dynamics (CFD) and its role in simulation-driven design. This quiz covers the advantages of CFD, its contribution to fluid flow analysis, and the post-processing techniques involved. Explore how CFD supports product development and the importance of dimension verification in simulations.