第四章,邊界條件

1、第四章，邊界條件計算流體與傳熱傳質概述o進口與出口邊界n速度o 速度及其分布o 湍流參數(shù)n壓力邊界條件 and others.o壁面, 對稱, 周期性和軸Axis邊界o內部區(qū)域n流體（Fluid）o 多孔介質（Porous Media）o 移動區(qū)域（Moving Cell Zones）n固體（Solid）o內部邊界計算流體與傳熱傳質Overviewo邊界條件:n邊界條件決定流動. n數(shù)學模型求解的需要.o給定進入計算區(qū)域的流率或通量.n如 mass, momentum, 和 energyoFluid/Solid regions represented by cell zones.nMateri

2、al and Source terms are assigned to cell zones.oBoundaries and internal surfaces are represented by face zones.nBoundary data are assigned to face zones.Example: Face and Cell zones associated with Pipe Flow through orifice plateinletoutletwallorifice(interior)orifice_plate and orifice_plate-shadowf

3、luid計算流體與傳熱傳質邊界條件設置o每個區(qū)（流體、固體）首先在Gambit中預設，F(xiàn)luent中可以修正與改動o每個區(qū)域都必須有其對應的邊界條件:Define Boundary Conditions.nChoose the zone in Zone list.nClick on selected zone type in Type listnClick Set. buttonoCan also select boundary zone in graphics window using right mouse button.nUseful if:oSetting up problem for

4、 first timeoTwo or more zones of same type in problem.計算流體與傳熱傳質流動進口與出口oFluent中進、出計算區(qū)域的邊界條件:n一般流動一般流動oPressure inletoPressure outletn不可壓縮不可壓縮oVelocity inletoOutflowo根據物理過程，選擇合適的邊界條件.o一般準則:n根據有流入與流出情況決定進口與出口的位置與形狀.o盡可能選擇收斂性好的邊界條件.n在垂直邊界的方向上不宜有較大的梯度.o表明進口或出口位置位置選擇不合理.n近壁處網格的偏斜盡可能小.n可壓縮流動可壓縮流動o質量進口質量進口M

5、ass flow inleto壓力遠場壓力遠場Pressure far-fieldn特別特別oInlet vent, outlet vent,intake fan, exhaust fan計算流體與傳熱傳質速度進口：Velocity Inletso給定速度矢量和標量進口值.o進口速度知道時，給定該條件尤為方便.n默認是均勻速度o該邊界條件針對不可壓縮流動問題.n總(滯止)量（溫度、壓力等）不定.o總(滯止)量不定用以調節(jié)速度分布n如果用于可壓縮流動，得到的解不復合物理意義.o壁面把速度進口放的太靠近障礙物.計算流體與傳熱傳質給定曲線分布的進口o用 UDFs 定義邊界條件.n曲線可以是空間變化，

6、也可以隨時間變化.o曲線可以:n從其它 CFD 軟件模擬結果中讀入n產生一個格式文件，具有位置及邊界條件的信息。o可以處理曲線分布邊界條件命令:nDefine Profiles計算流體與傳熱傳質確定湍流參數(shù)o湍流經過 inlet, outlet邊界, 或者在遠邊界條件下, FLUENT 需要提供如下邊界值n湍動能 k 湍流耗散率 o給定湍流參數(shù)的四種方法:n直接給定 k 和 。 n給定turbulence intensity 和 turbulence length scalen給定turbulence intensity 和 turbulent viscosity ration設定 turbu

7、lence intensity 和 hydraulic diametero湍流強度與長度尺度取決于上游來流條件，比如 :n透平機械出口Intensity = 20 % Length scale = 1 - 10 % of blade spann孔板和屏風下游Intensity = 10 % Length scale = screen/hole sizen完全發(fā)展的腔道或管內流動Intensity = 5 %Length scale = hydraulic diameter計算流體與傳熱傳質壓力邊界條件o壓力邊界條件要求輸入表壓（ gauge pressure）:o工作壓力（Operating

8、pressure）設置 :nDefine Operating Conditionso 適合壓力邊界條件設置的條件:n進口流量或速度不知道 (如浮力驅動的流動).n外流的自由邊界 或 需要確定的自由流。 operatinggaugeabsolutepppgauge pressureoperating pressurepressure leveloperating pressureabsolute pressurevacuum計算流體與傳熱傳質壓力邊界條件 (1)oDefines total pressure, temperature,and other scalar quantities at

9、flow inlets.oSupersonic/Initial Gauge Pressure:nDefines static pressure at boundaryfor locally supersonic flows.nUsed, if necessary, to initialize flow field for incompressible flows.oTotal temperature:nmust be defined for compressible flows.nis used, if necessary, to set static temperature for inco

10、mpressible flows.)1/(2)211 (kkstatictotalMkpp221vppstatictotal不可壓縮流動可壓縮流動計算流體與傳熱傳質壓力邊界條件 (2)o給定流動方向.n流動方向不合理，得到的解可能很不符合物理現(xiàn)象.o適合于：可壓與不可壓縮流動.nPressure inlet boundary is treated as loss-free transition from stagnation to inlet conditions.nMass flux through boundary varies depending on interior solution

11、 and specified flow direction.o壓力進口的地方可能會有流出情況出現(xiàn).n流動方向由求解結果決定.nExhaust static pressure is defined by value specified for gauge total pressure wherever outflow occurs.計算流體與傳熱傳質壓力出口：Pressure Outlet (1)o定義出口處的static (gauge) pressure.n流場流入什么樣的壓力環(huán)境里.o可以給定壓力徑向分布.o壓力出口處可能會出現(xiàn)回流:n求解過程或者求解結果中，都可能如此.n回流方向與出口邊界

12、垂直的方向.n由于回流量具有“彈性”，求解收斂性能較好.n回流出現(xiàn)時，用靜壓來給回流總壓賦值.計算流體與傳熱傳質壓力出口邊界 (2)o對于不可壓縮流動: n靜壓給定邊界壓力n其它量由流場內計算外推得到.o對于可壓流動:n靜壓計算不考慮當?shù)厥欠袷蔷植砍羲? n所有計算量從計算區(qū)域里外推計算.o當進口條件設定為pressure inlet時，出口一定要用pressure outlet.計算流體與傳熱傳質Outflow邊界oOutflow邊界，除了壓力之外，其它量的梯度為零。oFLUENT 從流場內外推邊界所需的信息.o特別有用的情況:n求解之前，不知道速度和壓力的流動問題.n流動出口是，或接近是

13、完全發(fā)展的流動.o注: 當求解過程中，或者求解結果具有回流時，用 Pressure Outlet 比用 Outflow出口條件更具有收斂優(yōu)勢.計算流體與傳熱傳質Outflow 邊界條件不能使用場合oOutflow 邊界不能用于:n可壓縮流動.nPressure Inlet 邊界條件 : n變密度的非定常流動.o不適合的物理問題:n回流區(qū)n流動方向有明顯壓力梯度n下游影響上游流動outflow condition ill-posedoutflow condition not obeyedoutflow condition obeyedoutflow condition closely obeye

14、d計算流體與傳熱傳質XY012345600.10.20.30.40.50.60.70.8計算流體與傳熱傳質多出口邊界條件數(shù)值模擬o應用Outflow邊界條件的前提:n默認設置，所有Outflow邊界的質量流量為進口流量均分.n默認設置流量權重 (FRW)為1 .n如果出口的流量不同，則:o制定各個出口的流量權重: mi=FRWi/FRWi.o出口壓力不同，用于調節(jié)各個出口流量. o可以采用Pressure邊界條件.pressure-inlet (p0,T0)pressure-outlet (ps)2velocity-inlet (v,T0)pressure-outlet (ps)1orFRW2

15、velocity inletFRW1計算流體與傳熱傳質其它 Inlet/Outlet 邊界條件oMass Flow Inletn用于可壓縮流動給定進口質量流量.n對于不可壓縮流動，無需給定.oPressure Far Fieldn材料選擇為理想氣體時，才會有該選項.n用于給定自由流的可壓縮流動狀態(tài)，給定自由流的馬赫數(shù)和靜壓，靜溫等。oExhaust Fan/Outlet Ventn如果出口有個壓力抬升或損失，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。oInlet Vent/Intake Fanninlet vent/intake fa

16、n用于進口給定壓力的損失系數(shù)或壓力抬升，需要給定流動方向，環(huán)境（進口）壓力及溫度等參數(shù)。計算流體與傳熱傳質XY00.511.5200.250.50.7511.251.51.752計算流體與傳熱傳質壁面邊界條件o用于分界流體與固體區(qū)域.o對于粘性流體流動, 不考慮壁面滑移:n壁面切向上的流體速度與壁面移動速度相同.n壁面法向上的流體速度為零。o熱邊界條件:n有幾種選項供選擇.o湍流計算可以考慮壁面粗糙度的影響.n壁面切應力和換熱取決于當?shù)亓鲃訄龅挠嬎憬Y果。o可以給定壁面的平移速度或旋轉速度.n也可以給定壁面切應力.計算流體與傳熱傳質對稱（Symmetry）邊界條件o可以減少計算區(qū)域，用于減少計算

17、量.o流場和計算區(qū)域必須符合對稱條件:o對稱面上的法向速度為零o對稱面上所有量在其法向上的梯度為零。o無需給定任何值.n但必須給定軸對稱的正確位置.oAlso used to model slip walls in viscous flow對稱平面計算流體與傳熱傳質Axis BoundariesoUsed:nAt centerline (y=0) of an axisymmetric gridnWhere multiple grid lines meet at a point in a 3D O-type gridoSpecify:nNo inputs requiredAXIS boundar

18、y計算流體與傳熱傳質Cell Zones: FluidoFluid zone = group of cells for which all active equations are solved.oFluid material input required.nSingle species, phase.oOptional inputs allow setting of source terms:nmass, momentum, energy, etc.oDefine fluid zone as laminar flow region if modeling transitional flow.

19、oCan define zone as porous media.oDefine axis of rotation for rotationally periodic flows.oCan define motion for fluid zone.計算流體與傳熱傳質Porous Media ConditionsoPorous zone modeled as special type of fluid zone.nEnable Porous Zone option in Fluid panel.nPressure loss in flow determined via user inputsof

20、 resistance coefficients to lumped parametermodel.oUsed to model flow through porous mediaand other “distributed” resistances, e.g.,nPacked bedsnFilter papersnPerforated platesnFlow distributorsnTube banks計算流體與傳熱傳質Moving ZonesoSingle Zone Problems:nRotating Reference Frame Modelodefine zone as Movin

21、g Reference Frameolimited applicabilityoMultiple Zone Problems:nEach zone defined as moving reference frame:oMultiple Reference Frame Modelnleast accurate, least demanding on CPUoMixing Plane Modelnfield data are averaged at the outlet of one zoneand used as inlet boundary data to adjacent zone.nEac

22、h zone defined as Moving Mesh:oSliding Mesh Modelnmust also define interface.nMesh positions are calculated; time-accurate simulationsnrelative motion must be tangential (no normal translation)計算流體與傳熱傳質Cell Zones: Solido“Solid” zone = group of cells for which only heat conduction problem solved.nNo

23、flow equations solvedoMaterial being treated as solid may actually be fluid, but it is assumed that no convection takes place.oOnly required input is material typenSo appropriate material properties used.oOptional inputs allow you to set volumetric heat generation rate (heat source).oNeed to specify rotation axis if rotationally periodic boundaries adja