k-εمدل Standard k- Model

مزايا:مي قرار استفاده مورد صنعت در ای گسترده بطور

گيرد.مدل اين رفتار پيرامون زيادی بسيار اسناد و مقاالت

. است شده منتشر. است صادق آشفته يافته توسعه جريان برای مدل اين

و سيال جريان از ائم مهندسي مسائل از بسياری برای. دارد قبولي قابل دقت حرارت انتقال

معايبروش اين دقت دارند، زياد انحنا که هايي هندسه برای

. است پاييناين نيز زياد بسيار فسار افت پيچيده، بسيار مسائل در

. نيست دقيق روش

سيال خواص دادن

کارکرد شرائط

تعريففشار - جاذبه- تعريفشتاب

فلوئنت های قابليت

مرزي شرائط تعريف

ورودی سرعت مرزي شرطVelocity Inlets

ورود • در سرعت بردار تعريف

سرعت • بردار که وقتي برايقابل باشد، مي معلوم ورودي

. است استفادهقابل – يکنواخت ورودی سرعت پروفيل برای

از بايد اينصورت غير در است، UDFاستفاده. کرد استفاده

ايجاد – باعث تراکم قابل غير جريان حالت در. شود مي فيزيکي غير شائط

مرزي • شرط که کنيد سعيمانع نزديکی در سرعت

. نباشد ورودی

ورودی سرعت مرزي شرط

آشفتگي مرزي شرائط کردن واردمرزي • شرط دادن برای روش چهار

: دارد وجود و kتعيين –

2U1.0k

آشفتگي مرزي شرائط کردن وارد

مرزي • شرط دادن برای روش چهار: دارد وجود

شدت – و آشفتگی طولي مقياس تعيين Set turbulence intensity andآشفتگي

turbulence length scaleتوربين – از Exhaust of a turbineخروجي

Intensity ( آشفتگي % 20 =(شدتLength scale( طولي 10 - 1 = (مفOاس % of blade

span

يک – از خروجي يافته توسعه کمال جريان Fully-developed flow in a duct orکانال

pipeIntensity ( آشفتگي % 5 = (شدتLength scale ( طولي hydraulic = (مفOاس

diameter

آشفتگي مرزي شرائط کردن واردمرزي • شرط دادن برای روش چهار

: دارد وجودنسبت – و آشفتگی شدت تعيين

آشفتگی لزجت)Set turbulence intensity and

turbulent viscosity ratio(

آشفتگي مرزي شرائط کردن واردمرزي • شرط دادن برای روش چهار

: دارد وجودقطر – و آشفتگي شدت تعيين

Set turbulenceهيدروليکی)intensity and hydraulic diameter)

- فشار ورودی مرزي شرط

اسکالر – پارامترهای ساير کلي، دماي کلي، فشار تعريف

(1/)2 (2

11) kk

statictotal Mk

pp

2

2

1vpp statictotal incompressible flows

compressible flows

- فشار ورودی مرزي شرطتعريفجهتورودی –

سطح • بر عمدجهت • دکر

- فشار خروجي مرزي شرطهيدروستاتيکي • فشار تعريفمقدار

در – هيدروستاتيکي فشار مقدار مافوقصوت، حالتجريان درآيد بدستمي يابي برون طريق از خروج

جرمي - دبي ورودی مرزي شرطدبيجرمي • تعريفمقدار

مافوقصوت، – حالتجريان درهيدروستاتيکي فشار مقدار

يابي برون طريق از خروج درآيد بدستمي

ورودی • جهتسيالجريان • به مربوط پارامترهای ساير

آشفته

) ( ساکن ديوار ديوار مرزی شرطلغزش - عدم (No Slip )شرط

ديوار- روی بر سيال برشي تنش تعيين

) ( متحرک ديوار ديوار مرزی شرطانتقال -

دوران- سرعت- هاي مولفه تعيين

مرزی ديوار شرط حرارتيگرمايي - شاردما- جابجايي-

مرزی ديوار شرط حرارتيگرمايي - شار

دما- جابجايي-

مرزی ديوار شرط حرارتيگرمايي - شار

دما- جابجايي-

ديوار جنس انتخاب . خواص - بقيه کردن فعال برای باشد مي آلومينيم ديوار برای فلوئنت فرض پيش

: کنيم مي عمل زير شکل به ديوار

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.

Hybrid mesh for an IC engine valve port

tet mesh

hex mesh

wedge mesh

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 ductfrom a plenum

– Plenum is meshed withtetrahedral cells.

– Duct is meshed withhexahedral 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-Processing1. Define your modeling goals.2. Identify the domain you will model.3. Design and create the grid.

• Solver Execution4. Set up the numerical model.5. Compute and monitor the solution.

• Post-Processing6. 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.