Transcript Tri/Tet vs. Quad/Hex Meshes

‫مدل ‪k-ε‬‬
‫‪Standard k- Model‬‬
‫مزايا‪:‬‬
‫بطور گسترده ای در صنعت مورد استفاده قرار مي گيرد‪.‬‬
‫مقاالت و اسناد بسيار زيادی پيرامون رفتار اين مدل منتشر شده است‪.‬‬
‫اين مدل برای جريان توسعه يافته آشفته صادق است‪.‬‬
‫برای بسياری از مسائل مهندسي ائم از جريان سيال و انتقال حرارت‬
‫دقت قابل قبولي دارد‪.‬‬
‫معايب‬
‫برای هندسه هايي که انحنا زياد دارند‪ ،‬دقت اين روش پايين است‪.‬‬
‫در مسائل بسيار پيچيده‪ ،‬افت فسار بسيار زياد نيز اين روش دقيق‬
‫نيست‪.‬‬
‫دادن خواص سيال‬
‫شرائط کارکرد‬
‫‪-‬‬
‫تعريف فشار‬
‫تعريف شتاب جاذبه‬‫قابليت های فلوئنت‬
‫تعريف شرائط مرزي‬
‫شرط مرزي سرعت ورودی ‪Velocity‬‬
‫‪Inlets‬‬
‫• تعريف بردار سرعت در‬
‫ورود‬
‫• براي وقتي که بردار سرعت‬
‫ورودي معلوم مي باشد‪ ،‬قابل‬
‫استفاده است‪.‬‬
‫– برای پروفيل سرعت ورودی يکنواخت قابل‬
‫استفاده است‪ ،‬در غير اينصورت بايد از ‪UDF‬‬
‫استفاده کرد‪.‬‬
‫– در حالت جريان غير قابل تراکم باعث ايجاد‬
‫شائط غير فيزيکي مي شود‪.‬‬
‫• سعي کنيد که شرط مرزي‬
‫شرط مرزي سرعت ورودی‬
‫وارد کردن شرائط مرزي آشفتگي‬
‫•‬
‫چهار روش برای دادن شرط مرزي وجود‬
‫دارد‪:‬‬
‫– تعيين ‪ k‬و ‪‬‬
‫‪2‬‬
‫‪k  0.1U‬‬
‫وارد کردن شرائط مرزي آشفتگي‬
‫•‬
‫چهار روش برای دادن شرط مرزي وجود‬
‫دارد‪:‬‬
‫– تعيين مقياس طولي آشفتگی و شدت آشفتگي‬
‫‪Set turbulence intensity and‬‬
‫‪turbulence length scale‬‬
‫–‬
‫خروجي از توربين ‪Exhaust of a turbine‬‬
‫‪(= 20 %‬شدت آشفتگي) ‪Intensity‬‬
‫‪( = 1 - 10 % of blade‬مفِاس طولي)‪Length scale‬‬
‫‪span‬‬
‫– جريان کمال توسعه يافته خروجي از يک کانال‬
‫‪Fully-developed flow in a duct or pipe‬‬
‫‪( = 5 %‬شدت آشفتگي) ‪Intensity‬‬
‫‪( = hydraulic‬مفِاس طولي) ‪Length scale‬‬
‫‪diameter‬‬
‫وارد کردن شرائط مرزي آشفتگي‬
‫•‬
‫چهار روش برای دادن شرط مرزي وجود‬
‫دارد‪:‬‬
‫– تعيين شدت آشفتگی و نسبت لزجت آشفتگی‬
‫‪)Set turbulence intensity and‬‬
‫(‪turbulent viscosity ratio‬‬
‫وارد کردن شرائط مرزي آشفتگي‬
‫•‬
‫چهار روش برای دادن شرط مرزي وجود‬
‫دارد‪:‬‬
‫– تعيين شدت آشفتگي و قطر هيدروليکی( ‪Set‬‬
‫‪turbulence intensity and hydraulic‬‬
‫‪)diameter‬‬
‫ فشار‬-‫شرط مرزي ورودی‬
‫ ساير پارامترهای اسکالر‬،‫ دماي کلي‬،‫– تعريف فشار کلي‬
ptotal  pstatic 
1 2
v
2
ptotal  pstatic (1 
incompressible flows
k  1 2 k /(k 1)
M )
2
compressible flows
‫شرط مرزي ورودی‪ -‬فشار‬
‫– تعريف جهت ورودی‬
‫• عمد بر سطح‬
‫• دکر جهت‬
‫شرط مرزي خروجي‪ -‬فشار‬
‫•‬
‫تعريف مقدار فشار هيدروستاتيکي‬
‫– در حالت جريان مافوق صوت‪ ،‬مقدار فشار هيدروستاتيکي در خروج از طريق برون يابي بدست‬
‫مي آيد‬
‫شرط مرزي ورودی ‪ -‬دبي جرمي‬
‫• تعريف مقدار دبي جرمي‬
‫– در حالت جريان مافوق صوت‪ ،‬مقدار فشار‬
‫هيدروستاتيکي در خروج از طريق برون‬
‫يابي بدست مي آيد‬
‫• جهت سيال ورودی‬
‫• ساير پارامترهای مربوط به جريان‬
‫آشفته‬
‫شرط مرزی ديوار(ديوار ساکن)‬
‫ شرط عدم لغزش)‪(No Slip‬‬‫‪ -‬تعيين تنش برشي سيال بر روی ديوار‬
‫شرط مرزی ديوار(ديوار متحرک)‬
‫ انتقال‬‫ دوران‬‫‪-‬تعيين مولفه هاي سرعت‬
‫شرط مرزی حرارتي ديوار‬
‫ شار گرمايي‬‫ دما‬‫‪-‬جابجايي‬
‫شرط مرزی حرارتي ديوار‬
‫ شار گرمايي‬‫ دما‬‫‪-‬جابجايي‬
‫شرط مرزی حرارتي ديوار‬
‫ شار گرمايي‬‫ دما‬‫‪-‬جابجايي‬
‫انتخاب جنس ديوار‬
‫ پيش فرض فلوئنت برای ديوار آلومينيم مي باشد‪ .‬برای فعال کردن بقيه خواص ديوار به شکل زير‬‫عمل مي کنيم‪:‬‬
Tri/Tet vs. Quad/Hex Meshes
•
For simple geometries, quad/hex
meshes can provide high-quality
solutions with fewer cells than a
comparable tri/tet mesh.
•
For complex geometries, quad/hex
meshes show no numerical
advantage, and you can save
meshing effort by using a tri/tet
mesh.
Hybrid Mesh Example
• Valve port grid
– Specific regions can
be meshed with
different cell types.
– Both efficiency and
accuracy are
enhanced relative
to a hexahedral or
tetrahedral mesh
alone.
– Tools for hybrid
mesh generation
are available in
Gambit and TGrid.
tet mesh
hex
mesh
wedge mesh
Hybrid mesh for an
IC engine valve port
Non-Conformal Mesh Example
•
Nonconformal mesh: mesh in which grid nodes do not match up along an interface.
–
–
•
Useful for ‘parts-swapping’ for design study, etc.
Helpful for meshing complex geometries.
Example:
–
3D Film Cooling Problem
•
Coolant is injected into a duct
from a plenum
–
–
Plenum is meshed with
tetrahedral cells.
Duct is meshed with
hexahedral cells.
Plenum part can be replaced with new
geometry with reduced meshing
effort.
Set Up the Numerical Model
• For a given problem, you will need to:
– Select appropriate physical models.
• Turbulence, combustion, multiphase, etc.
– Define material properties.
• Fluid
• Solid
• Mixture
–
–
–
–
–
Prescribe operating conditions.
Prescribe boundary conditions at all boundary zones.
Provide an initial solution.
Set up solver controls.
Set up convergence monitors.
Compute the Solution
• The discretized conservation equations are solved iteratively.
– A number of iterations are usually required to reach a
converged solution.
• Convergence is reached when:
– Changes in solution variables from one iteration to the next
are negligible.
• Residuals provide a mechanism to help monitor this trend.
– Overall property conservation is achieved.
• The accuracy of a converged solution is dependent upon:
– Appropriateness and accuracy of the physical models.
– Grid resolution and independence
– Problem setup
• A converged and grid-independent solution on a well-posed
problem will provide useful engineering results!
Examine the Results
• Examine the results to review solution and to extract useful
engineering data.
• Visualization can be used to answer such questions as:
–
–
–
–
–
–
What is the overall flow pattern?
Is there separation?
Where do shocks, shear layers, etc. form?
Are key flow features being resolved?
Are physical models and boundary conditions appropriate?
Are there local convergence problems?
• Numerical reporting tools can be used to calculate quantitative
results:
– Lift and drag
– Average heat transfer coefficients
– Surface-averaged quantities
Tools to Examine the Results
• Graphical tools
– Grid, contour, and vector plots
– Pathline and particle trajectory plots
– XY plots
– Animations
• Numerical reporting tools
– Flux balances
– Surface and volume integrals and averages
– Forces and moments
Consider Revisions to the Model
• Are physical models appropriate?
–
–
–
–
Is flow turbulent?
Is flow unsteady?
Are there compressibility effects?
Are there 3D effects?
• Are boundary conditions correct?
– Is the computational domain large enough?
– Are boundary conditions appropriate?
– Are boundary values reasonable?
• Is grid adequate?
– Can grid be adapted to improve results?
– Does solution change significantly with adaption, or is the
solution grid independent?
– Does boundary resolution need to be improved?
Review for Demo
• Problem Identification and Pre-Processing
1. Define your modeling goals.
2. Identify the domain you will model.
3. Design and create the grid.
• Solver Execution
4. Set up the numerical model.
5. Compute and monitor the solution.
• Post-Processing
6. Examine the results.
7. Consider revisions to the model.
FLUENT DEMO

Startup Gambit




load database
define boundary zones
export mesh
Startup Fluent




GUI
Problem Setup
Solve
Post-Processing
Operating System Basics: Unix
• Basic Unix commands:
– pwd - prints the name current working directory
• Your home directory is home/fluent/.
– ls - lists the files in the current directory
– cd - change working directories (cd .. to go up one
directory).
• The environment variable $TRAINPATH contains a shortcut to
the directory where training files are stored. For example:
cp $TRAINPATH/fluent5.x/tut/elbow/elbow.msh
.
will copy the mesh file for the first example problem into your
current working directory.
• To start Fluent 5:
% fluent 2d &
• To start Fluent 4.5: % fluent -r4.5 &
Operating System Basics: Windows NT
• PC users will find tutorials under c:\Fluent.Inc\fluent5.x\
tut\. This directory is write-protected.
• Save files to your home directory, c:\users\fluent\.
• Fluent can be started from the command prompt or from the start
menu:
– Command Prompt
• fluent 2d
– Start Menu
• Start  Programs  Fluent Inc  Fluent 5.x
• !Note: It is recommended that you restart Fluent for each tutorial for
both Unix and NT systems to avoid mixing solver settings from
different tutorials.