Fluent-Intro 14.5 WS08 Vortex Shedding

  • View
    62

  • Download
    22

Embed Size (px)

DESCRIPTION

Fluent-Intro 14.5 WS08 Vortex Shedding

Text of Fluent-Intro 14.5 WS08 Vortex Shedding

  • 2012 ANSYS, Inc. September 19, 2013 1 Release 14.5

    PRACE Autumn School 2013 - Industry Oriented HPC Simulations, September 21-27,

    University of Ljubljana, Faculty of Mechanical Engineering, Ljubljana, Slovenia

    Express Introductory Training in ANSYS Fluent

    Workshop 08

    Vortex Shedding

    Dimitrios Sofialidis

    Technical Manager, SimTec Ltd.

    Mechanical Engineer, PhD

  • 2012 ANSYS, Inc. September 19, 2013 2 Release 14.5

    Introduction to ANSYS Fluent

    14.5 Release

    Workshop 7b Vortex Shedding

  • 2012 ANSYS, Inc. September 19, 2013 3 Release 14.5

    Workshop Description:

    The purpose of this workshop is to introduce good techniques for transient flow modeling.

    Learning Aims:

    This workshop teaches skills for running Fluent for timedependant (transient) simulations. Topics covered include:

    Selecting a suitable timestep. Using CustomFieldFunctions (CFF).

    Autosaving results during the simulation. Generating FastFourier Transforms (FFT).

    Generating images during the simulation. Transient postprocessing in CFDPost.

    Learning Objectives:

    To show how to set up, run and postprocess a transient (timedependant) simulation, as well as additional skills in using custom field functions and FastFourierTransforms.

    I Introduction

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 4 Release 14.5

    Simulation to be Performed

    The case considered here is flow around a cylinder with a Reynolds number of 100.

    Vortex shedding will be observed. However the workshop starts with a steady state analysis assuming that the user didnt anticipate this behavior.

    This workshop demonstrates iterative and noniterative time advancement, Fast Fourier Transforms (FFT) and animations.

    The tutorial is carried out using Fluent and CFDPost in standalone mode.

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 5 Release 14.5

    Computational domain created in ANSYS DesignModeler has the following dimensions.

    Name Location Dimension

    Cylinder D1 2 m (dia.)

    Inlet Length D2 20 m = 10 D

    Outlet Length D3 30 m = 15 D

    Width D4 40 m = 20 D

    Computational Domain

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 6 Release 14.5

    Re>3.5106

    3105 < Re < 3.5106

    40 < Re < 150

    150 < Re < 3105

    515 < Re < 40

    Re < 5

    Turbulent vortex street, but

    the separation is narrower

    than the laminar case.

    Boundary layer transition to

    turbulent.

    Laminar boundary layer up to

    the separation point, turbulent

    wake.

    Laminar vortex street.

    A pair of stable vortices in the

    wake.

    Creeping flow (no separation).

    Reynolds Number Effects

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 7 Release 14.5

    Start a Fluent Project (Standalone)

    Introduction Model Setup Solving PostProcessing Summary

    Launch Fluent from the Start Menu:

    "Start Menu>ANSYS 14.5>Fluid Dynamics>Fluent".

    Select "2D" Display Mesh After Reading.

    Select the working directory you are using on your machine (may be different to that shown here).

  • 2012 ANSYS, Inc. September 19, 2013 8 Release 14.5

    Mesh [1]

    Introduction Model Setup Solving PostProcessing Summary

    Read the Fluent mesh file: "vortexsheddingcoarse.msh" ("File>Read>Mesh").

    The mesh will be read in and displayed, and the zones will be shown in the TUI window.

  • 2012 ANSYS, Inc. September 19, 2013 9 Release 14.5

    Final domain

    extent.

    Mesh [2] The mesh needs scaling.

    Select "Scale" ("Problem Setup>General>Scale"), and enter the values shown, then press "Scale". Be careful only to press "Scale" only once.

    Close the scale panel and "Check" the Mesh.

    "General>Check".

    "General>Report Quality".

    Display the grid again once scaling has been performed.

    "General>Display".

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 10 Release 14.5

    Select "General" in the navigation pane and keep the "SteadyState" "PressureBased" solver.

    Keep "Laminar" setting for the "Viscous Model".

    The properties to be used for the material "air" need to be set.

    For "Density", enter "1 (kg/m3)".

    For "Viscosity", enter "0.01 (kg/ms)".

    Select "Change/Create".

    Solver & Models

    Introduction Model Setup Solving PostProcessing Summary

    Later on we will compare the Fluent results with those from a literature search. We have changed the default material properties for air to aid that comparison.

  • 2012 ANSYS, Inc. September 19, 2013 11 Release 14.5

    Boundary Conditions/Solution Methods Boundary Conditions.

    "Inlet":

    Select boundary "in".

    Set velocity to be "1 [m/s]" "normal" to boundary.

    "Outlet":

    Select boundary "out".

    Keep default of "0 [Pa]".

    "Other boundaries":

    "cylinder" is set to a "wall", no action needed.

    "sym1" and "sym2" are set to "symmetry", no action needed.

    Solution Methods.

    Select "QUICK" scheme for "Momentum" equation.

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 12 Release 14.5

    Set up residual monitors so the convergence can be monitored.

    "Monitors>Residuals>Edit".

    Make sure "Plot" is on, then "OK".

    Create points to monitor quantity. "Surface (top menu)>Point".

    Specify coordinates (2 , 1).

    "Activate" point tool to check location on the grid.

    (check out point tool before closing panel).

    "Create" then "Close".

    Surface monitor on point. "Monitors>Surface Monitors>Create".

    Select "VertexAverage" on report type and "Velocity" "Yvelocity" in field variable.

    Select "point6" (the point created above at coordinates [2,1]).

    Options: "Print to Console" & "Plot", then "OK".

    Solution Monitor

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 13 Release 14.5

    Solution Initialization Initialize the flow field based on the farfield boundary.

    Select "Standard Initialization"

    "Compute from" "in" (inlet zone).

    "Initialize".

    Save the case file.

    "File>Write Case".

    You can write case and data files with extension .gz, the files will be compressed automatically.

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 14 Release 14.5

    We have tried to solve this vortexshedding problem in a steadystate manner. Note that solution is not converging and monitor shows a regular periodic behavior.

    Run Calculation [1]

    Introduction Model Setup Solving PostProcessing Summary

    Set the number of requested iterations to "400" then "Calculate".

  • 2012 ANSYS, Inc. September 19, 2013 15 Release 14.5

    Steady state solution is asymmetric.

    Run Calculation [2] Choose Graphics and "Animations>Vectors".

    Since this is a 2D simulation, there is no need to pick a surface, just "Display".

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 16 Release 14.5

    Save Case&Data Files and Make Transient Save the Case&Data files.

    "File>Write Case&Data".

    You can write case and data files with extension .gz the files will be compressed automatically.

    To obtain a more realistic solution to this problem we will solve it again, but in a transient (time dependant) manner.

    Under "Problem Setup>General", change the time option to "Transient".

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 17 Release 14.5

    Run Calculation For the transient scheme, it is recommended to change Solver Methods. The

    default pressure velocity coupling (SIMPLE) may require more iterations to converge.

    Change to the "PISO" scheme and "2nd Order Implicit" "Transient Formulation" as shown in the image below .

    Also change the "Under Relaxation Factors" as shown in the image.

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 18 Release 14.5

    Solution Monitor Edit the Surface monitor.

    Change "Get Data Every" to "Time Step". Also set the "X Axis" to be "Time Step".

    "OK"

    Introduction Model Setup Solving PostProcessing Summary

  • 2012 ANSYS, Inc. September 19, 2013 19 Release 14.5

    sVSt

    D

    fperiod

    V

    fDSt 06.6

    .

    1

    Run Calculation [1]

    Introduction Model Setup Solving PostProcessing Summary

    Save the transient case file before starting the computation.

    We need to identify a suitable time step size for this problem.

    1) A quick way is to do a handcalculation to see how long it takes for the flow to pass through a typical grid cell. Run this, and check that convergence occurs in less that 20 iterations per timestep.

    2) Another approach is to determine the characteristic response of the system. By perfor