fluent教程第四章

1、第四章，邊界條件 計(jì)算流體與傳熱傳質(zhì) 概述 ?進(jìn)口與出口邊界 ? 速度 ? 速度及其分布 ? 湍流參數(shù) ? 壓力邊界條件 and others. ?壁面, 對(duì)稱, 周期性和軸Axis邊界 ?內(nèi)部區(qū)域 ? 流體（Fluid） ? 多孔介質(zhì)（Porous Media） ? 移動(dòng)區(qū)域（Moving Cell Zones） ? 固體（Solid） ?內(nèi)部邊界 計(jì)算流體與傳熱傳質(zhì) Overview ?邊界條件: ?邊界條件決定流動(dòng). ?數(shù)學(xué)模型求解的需要. ?給定進(jìn)入計(jì)算區(qū)域的流率或通量. ?如 mass, momentum, 和 energy ?Fluid/Solid regions represen

2、ted by cell zones. ?Material and Source terms are assigned to cell zones. ?Boundaries and internal surfaces are represented by face zones. ?Boundary data are assigned to face zones. orifice (interior) orifice_plate and orifice_plate-shadow outlet wall inlet fluid Example: Face and Cell zones associa

3、ted with Pipe Flow through orifice plate 計(jì)算流體與傳熱傳質(zhì) 邊界條件設(shè)置 ?每個(gè)區(qū)（流體、固體）首先在 Gambit中預(yù)設(shè)，F(xiàn)luent中可以修正與改動(dòng) 每個(gè)區(qū)域都必須有其對(duì)應(yīng)的邊界條件 : Define ? Boundary Conditions. ?Choose the zone in Zone list. ?Click on selected zone type in Type list ?Click Set. button Can also select boundary zone in graphics window using right

4、mouse button. ?Useful if: ?Setting up problem for first time ?Two or more zones of same type in problem. ?計(jì)算流體與傳熱傳質(zhì) 流動(dòng)進(jìn)口與出口 ?Fluent中進(jìn)、出計(jì)算區(qū)域的邊界條件: ?可壓縮流動(dòng)可壓縮流動(dòng) ?一般流動(dòng)一般流動(dòng) ?質(zhì)量進(jìn)口質(zhì)量進(jìn)口Mass flow inlet ?Pressure inlet ?壓力遠(yuǎn)場(chǎng)壓力遠(yuǎn)場(chǎng)Pressure far-field ?Pressure outlet ?特別特別 ?不可壓縮不可壓縮 ?Inlet vent, outlet vent, ?Vel

5、ocity inlet intake fan, exhaust fan ?Outflow 根據(jù)物理過程，選擇合適的邊界條件. 一般準(zhǔn)則: ?根據(jù)有流入與流出情況決定進(jìn)口與出口的位置與形狀. ?盡可能選擇收斂性好的邊界條件. ?在垂直邊界的方向上不宜有較大的梯度. ?表明進(jìn)口或出口位置位置選擇不合理. ?近壁處網(wǎng)格的偏斜盡可能小. ?計(jì)算流體與傳熱傳質(zhì) 速度進(jìn)口：Velocity Inlets ?給定速度矢量和標(biāo)量進(jìn)口值 . 進(jìn)口速度知道時(shí)，給定該條件尤為方便 . ?默認(rèn)是均勻速度 該邊界條件針對(duì)不可壓縮流動(dòng)問題 . ?總(滯止)量（溫度、壓力等）不定 . ?總(滯止)量不定用以調(diào)節(jié)速度分布 ?

6、如果用于可壓縮流動(dòng)，得到的解不復(fù)合物理意義. 壁面把速度進(jìn)口放的太靠近障礙物 . 計(jì)算流體與傳熱傳質(zhì) 給定曲線分布的進(jìn)口 ?用 UDFs 定義邊界條件. ?曲線可以是空間變化，也可以隨時(shí)間變化. 曲線可以: ?從其它 CFD 軟件模擬結(jié)果中讀入 ?產(chǎn)生一個(gè)格式文件，具有位置及邊界條件的信息。 可以處理曲線分布邊界條件命令: ?Define ? Profiles 用 hooks把曲線加到邊界條件中. ?計(jì)算流體與傳熱傳質(zhì) 確定湍流參數(shù) ?湍流經(jīng)過 inlet, outlet 邊界, 或者在遠(yuǎn)邊界條件下 , FLUENT 5 需要提供如下邊界值 ?湍動(dòng)能 k ? 湍流耗散率 ? 給定湍流參數(shù)的四種

7、方法 : ?直接給定 k 和 ? 。 ?給定turbulence intensity 和 turbulence length scale ?給定turbulence intensity 和 turbulent viscosity ratio ?設(shè)定 turbulence intensity 和 hydraulic diameter 湍流強(qiáng)度與長度尺度取決于上游來流條件，比如 : ?透平機(jī)械出口 Intensity = 20 % Length scale = 1 - 10 % of blade span ?孔板和屏風(fēng)下游 Intensity = 10 % Length scale = scree

8、n/hole size ?完全發(fā)展的腔道或管內(nèi)流動(dòng) Intensity = 5 % Length scale = hydraulic diameter ?計(jì)算流體與傳熱傳質(zhì) 壓力邊界條件 ?壓力邊界條件要求輸入表壓（ gauge pressure）: pabsolute?pgauge?poperating 工作壓力（Operating pressure）設(shè)置 : ?Define ? Operating Conditions 適合壓力邊界條件設(shè)置的條件: ?進(jìn)口流量或速度不知道 (如浮力驅(qū)動(dòng)的流動(dòng)). ?外流的自由邊界 或 需要確定的自由流。 pressure level gauge press

9、ure absolute pressure operating pressure operating pressure ?vacuum 計(jì)算流體與傳熱傳質(zhì) 壓力邊界條件 (1) ?Defines total pressure, temperature, and other scalar quantities at flow inlets. 12ptotal不可壓縮流動(dòng) ?pstatic?v2 k?12 k/( k?1 )可壓縮流動(dòng) ptotal?p(1?M )static 2?Supersonic/Initial Gauge Pressure: ?Defines static pressure

10、 at boundary for locally supersonic flows. ?Used, if necessary, to initialize flow field for incompressible flows. ?Total temperature: ?must be defined for compressible flows. ?is used, if necessary, to set static temperature for incompressible flows. 計(jì)算流體與傳熱傳質(zhì) 壓力邊界條件 (2) ?給定流動(dòng)方向 . ?流動(dòng)方向不合理，得到的解可能很不

11、符合物理現(xiàn)象. 適合于：可壓與不可壓縮流動(dòng) . ?Pressure inlet boundary is treated as loss-free transition from stagnation to inlet conditions. ?Mass flux through boundary varies depending on interior solution and specified flow direction. 壓力進(jìn)口的地方可能會(huì)有流出情況出現(xiàn) . ?流動(dòng)方向由求解結(jié)果決定 . ?Exhaust static pressure is defined by value spe

12、cified for gauge total pressure wherever outflow occurs. ?計(jì)算流體與傳熱傳質(zhì) 壓力出口：Pressure Outlet (1) ?定義出口處的 static (gauge) pressure. ?流場(chǎng)流入什么樣的壓力環(huán)境里. 可以給定壓力徑向分布 . 壓力出口處可能會(huì)出現(xiàn)回流: ?求解過程或者求解結(jié)果中，都可能如此 . ?回流方向與出口邊界垂直的方向 . ?由于回流量具有“彈性”，求解收斂性能較好. ?回流出現(xiàn)時(shí)，用靜壓來給回流總壓賦值 . ?計(jì)算流體與傳熱傳質(zhì) 壓力出口邊界 (2) ?對(duì)于不可壓縮流動(dòng) : ?靜壓給定邊界壓力 ?其它量

13、由流場(chǎng)內(nèi)計(jì)算外推得到 . 對(duì)于可壓流動(dòng) : ?靜壓計(jì)算不考慮當(dāng)?shù)厥欠袷蔷植砍羲?. ?所有計(jì)算量從計(jì)算區(qū)域里外推計(jì)算 . 當(dāng)進(jìn)口條件設(shè)定為 pressure inlet 時(shí)，出口一定要用pressure outlet. ?計(jì)算流體與傳熱傳質(zhì) Outflow邊界 ?Outflow邊界，除了壓力之外，其它量的梯度為零。 ?FLUENT 從流場(chǎng)內(nèi)外推邊界所需的信息. ?特別有用的情況: ?求解之前，不知道速度和壓力的流動(dòng)問題. ?流動(dòng)出口是，或接近是完全發(fā)展的流動(dòng). ?注: 當(dāng)求解過程中，或者求解結(jié)果具有回流時(shí)，用 Pressure Outlet 比用 Outflow出口條件更具有收斂優(yōu)勢(shì). 計(jì)算

14、流體與傳熱傳質(zhì) Outflow 邊界條件不能使用場(chǎng)合 ?Outflow 邊界不能用于: ?可壓縮流動(dòng). ?Pressure Inlet 邊界條件 : ?變密度的非定常流動(dòng). ?不適合的物理問題 : ?回流區(qū) ?流動(dòng)方向有明顯壓力梯度 ?下游影響上游流動(dòng) outflow condition ill-posed outflow condition not obeyed outflow condition obeyed outflow condition closely obeyed 計(jì)算流體與傳熱傳質(zhì) 0.80.70.60.5Y0.40.30.20.10012X3456計(jì)算流體與傳熱傳質(zhì) 多出口邊

15、界條件數(shù)值模擬 ?應(yīng)用Outflow邊界條件的前提: ?默認(rèn)設(shè)置，所有Outflow邊界的質(zhì)量流量為進(jìn)口流量均分. ?默認(rèn)設(shè)置流量權(quán)重 (FRW)為1 . ?如果出口的流量不同，則: ?制定各個(gè)出口的流量權(quán)重: mi=FRWi/?FRWi. ?出口壓力不同，用于調(diào)節(jié)各個(gè)出口流量. 可以采用Pressure邊界條件. velocity-inlet (v,T0) or pressure-inlet (p0,T0) FRW1 velocity inlet ?FRW2 pressure-outlet (ps)1 pressure-outlet (ps)2 計(jì)算流體與傳熱傳質(zhì) 其它 Inlet/Outl

16、et 邊界條件 ?Mass Flow Inlet ?用于可壓縮流動(dòng)給定進(jìn)口質(zhì)量流量. ?對(duì)于不可壓縮流動(dòng)，無需給定. Pressure Far Field ?材料選擇為理想氣體時(shí)，才會(huì)有該選項(xiàng). ?用于給定自由流的可壓縮流動(dòng)狀態(tài)，給定自由流的馬赫數(shù)和靜壓，靜溫等。 Exhaust Fan/Outlet Vent ?如果出口有個(gè)壓力抬升或損失，可以采用exhaust fan/outlet vent給定出口壓力抬升或損失系數(shù)，以及環(huán)境壓力與溫度。 Inlet Vent/Intake Fan ?inlet vent/intake fan用于進(jìn)口給定壓力的損失系數(shù)或壓力抬升，需要給定流動(dòng)方向，環(huán)境（進(jìn)

17、口）壓力及溫度等參數(shù)。 ?計(jì)算流體與傳熱傳質(zhì) 21.751.51.25Y10.750.50.25000.51X1.52計(jì)算流體與傳熱傳質(zhì) 壁面邊界條件 ?用于分界流體與固體區(qū)域 . 對(duì)于粘性流體流動(dòng) , 不考慮壁面滑移 : ?壁面切向上的流體速度與壁面移動(dòng)速度相同 . ?壁面法向上的流體速度為零。 熱邊界條件 : ?有幾種選項(xiàng)供選擇 . 湍流計(jì)算可以考慮壁面粗糙度的影響 . ?壁面切應(yīng)力和換熱取決于當(dāng)?shù)亓鲃?dòng)場(chǎng)的計(jì)算結(jié)果。 可以給定壁面的平移速度或旋轉(zhuǎn)速度 . ?也可以給定壁面切應(yīng)力 . ?計(jì)算流體與傳熱傳質(zhì) 對(duì)稱（Symmetry）邊界條件 ?可以減少計(jì)算區(qū)域，用于減少計(jì)算量 . 流場(chǎng)和計(jì)算區(qū)

18、域必須符合對(duì)稱條件 : ?對(duì)稱面上的法向速度為零 ?對(duì)稱面上所有量在其法向上的梯度為零。 無需給定任何值 . ?但必須給定軸對(duì)稱的正確位置 . Also used to model slip walls in viscous flow ?對(duì)稱平面 計(jì)算流體與傳熱傳質(zhì) Periodic Boundaries ?Used when physical geometry of interest and expected pattern of flow/thermal solution have periodically repeating nature. ?Reduces computational

19、effort in problem. Two types available in FLUENT 5 . ?p = 0 across periodic planes. ?Rotationally or translationally periodic. ?Rotationally periodic boundaries require axis of rotation be defined in fluid zone. ?p is finite across periodic planes. ?Translationally periodic only. ?Models fully devel

20、oped conditions. ?Specify either mean ?p per period or net mass flow rate. ?By default, periodic boundaries defined in Gambit are assumed to be translational in FLUENT 5 . 計(jì)算流體與傳熱傳質(zhì) Periodic Boundaries: Examples ? ? ?p = 0: 4 tangential inlets ? ? ?p 0: computational domain Rotationally periodic bou

21、ndaries flow direction Streamlines in a 2D tube heat exchanger Translationally periodic boundaries 計(jì)算流體與傳熱傳質(zhì) Axis Boundaries ?Used: ?At centerline (y=0) of an axisymmetric grid ?Where multiple grid lines meet at a point in a 3D O-type grid Specify: ?No inputs required ?AXIS boundary 計(jì)算流體與傳熱傳質(zhì) Cell Z

22、ones: Fluid ?Fluid zone = group of cells for which all active equations are solved. Fluid material input required. ?Single species, phase. Optional inputs allow setting of source terms: ?mass, momentum, energy, etc. Define fluid zone as laminar flow region if modeling transitional flow. Can define z

23、one as porous media. Define axis of rotation for rotationally periodic flows. Can define motion for fluid zone. 計(jì)算流體與傳熱傳質(zhì) Porous Media Conditions ?Porous zone modeled as special type of fluid zone. ?Enable Porous Zone option in Fluid panel. ?Pressure loss in flow determined via user inputs of resist

24、ance coefficients to lumped parameter model. Used to model flow through porous media and other “ distributed ” resistances, e.g., ?Packed beds ?Filter papers ?Perforated plates ?Flow distributors ?Tube banks ?計(jì)算流體與傳熱傳質(zhì) Moving Zones ?Single Zone Problems: ?Rotating Reference Frame Model ?define zone

25、as Moving Reference Frame ?limited applicability Multiple Zone Problems: ?Each zone defined as moving reference frame: ?Multiple Reference Frame Model ?least accurate, least demanding on CPU ?Mixing Plane Model ?field data are averaged at the outlet of one zone and used as inlet boundary data to adj

26、acent zone. ?Each zone defined as Moving Mesh: ?Sliding Mesh Model ?must also define interface. ?Mesh positions are calculated; time-accurate simulations ?relative motion must be tangential (no normal translation) ?計(jì)算流體與傳熱傳質(zhì) Cell Zones: Solid ?“Solid” zone = group of cells for which only heat conduction problem solved. ?No flow equations solved Material being treated as solid may actually be fluid, but it is assumed that no convection takes place. Only required input is material type ?So appropriate material properties used. Optional inputs allow you to set volumet