中科大Fluent教程第四章边界条件.ppt

第四章，边界条件,概述,进口与出口边界速度速度及其分布湍流参数压力边界条件 and others.壁面,对称,周期性和轴Axis边界内部区域流体（Fluid）多孔介质（Porous Media）移动区域（Moving Cell Zones）固体（Solid）内部边界,Overview,边界条件:边界条件决定流动.数学模型求解的需要.给定进入计算区域的流率或通量.如 mass,momentum,和 energyFluid/Solid regions represented 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.,Example:Face and Cell zones associated with Pipe Flow through orifice plate,边界条件设置,每个区（流体、固体）首先在Gambit中预设，Fluent中可以修正与改动每个区域都必须有其对应的边界条件:Define Boundary Conditions.Choose the zone in Zone list.Click on selected zone type in Type listClick Set.buttonCan also select boundary zone in graphics window using right mouse button.Useful if:Setting up problem for first timeTwo or more zones of same type in problem.,流动进口与出口,Fluent中进、出计算区域的边界条件:一般流动Pressure inletPressure outlet不可压缩Velocity inletOutflow根据物理过程，选择合适的边界条件.一般准则:根据有流入与流出情况决定进口与出口的位置与形状.尽可能选择收敛性好的边界条件.在垂直边界的方向上不宜有较大的梯度.表明进口或出口位置位置选择不合理.近壁处网格的偏斜尽可能小.,可压缩流动质量进口Mass flow inlet压力远场Pressure far-field特别Inlet vent,outlet vent,intake fan,exhaust fan,速度进口：Velocity Inlets,给定速度矢量和标量进口值.进口速度知道时，给定该条件尤为方便.默认是均匀速度该边界条件针对不可压缩流动问题.总(滞止)量（温度、压力等）不定.总(滞止)量不定用以调节速度分布如果用于可压缩流动，得到的解不复合物理意义.壁面把速度进口放的太靠近障碍物.,给定曲线分布的进口,用 UDFs 定义边界条件.曲线可以是空间变化，也可以随时间变化.曲线可以:从其它 CFD 软件模拟结果中读入产生一个格式文件，具有位置及边界条件的信息。可以处理曲线分布边界条件命令:Define Profiles用 hooks把曲线加到边界条件中.,确定湍流参数,湍流经过 inlet,outlet边界,或者在远边界条件下,FLUENT 5 需要提供如下边界值湍动能 k 湍流耗散率 给定湍流参数的四种方法:直接给定 k 和。给定turbulence intensity 和 turbulence length scale给定turbulence intensity 和 turbulent viscosity ratio设定 turbulence intensity 和 hydraulic diameter湍流强度与长度尺度取决于上游来流条件，比如:透平机械出口Intensity=20%Length scale=1-10%of blade span孔板和屏风下游Intensity=10%Length scale=screen/hole size完全发展的腔道或管内流动Intensity=5%Length scale=hydraulic diameter,压力边界条件,压力边界条件要求输入表压（gauge pressure）:工作压力（Operating pressure）设置:Define Operating Conditions 适合压力边界条件设置的条件:进口流量或速度不知道(如浮力驱动的流动).外流的自由边界 或 需要确定的自由流。,压力边界条件(1),Defines total pressure,temperature,and other scalar quantities at flow inlets.Supersonic/Initial Gauge Pressure:Defines static pressure at boundaryfor 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.,压力边界条件(2),给定流动方向.流动方向不合理，得到的解可能很不符合物理现象.适合于：可压与不可压缩流动.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.压力进口的地方可能会有流出情况出现.流动方向由求解结果决定.Exhaust static pressure is defined by value specified for gauge total pressure wherever outflow occurs.,压力出口：Pressure Outlet(1),定义出口处的static(gauge)pressure.流场流入什么样的压力环境里.可以给定压力径向分布.压力出口处可能会出现回流:求解过程或者求解结果中，都可能如此.回流方向与出口边界垂直的方向.由于回流量具有“弹性”，求解收敛性能较好.回流出现时，用静压来给回流总压赋值.,压力出口边界(2),对于不可压缩流动:静压给定边界压力其它量由流场内计算外推得到.对于可压流动:静压计算不考虑当地是否是局部超音速.所有计算量从计算区域里外推计算.当进口条件设定为pressure inlet时，出口一定要用pressure outlet.,Outflow边界,Outflow边界，除了压力之外，其它量的梯度为零。FLUENT 从流场内外推边界所需的信息.特别有用的情况:求解之前，不知道速度和压力的流动问题.流动出口是，或接近是完全发展的流动.注:当求解过程中，或者求解结果具有回流时，用 Pressure Outlet 比用 Outflow出口条件更具有收敛优势.,Outflow 边界条件不能使用场合,Outflow 边界不能用于:可压缩流动.Pressure Inlet 边界条件:变密度的非定常流动.,不适合的物理问题:回流区流动方向有明显压力梯度下游影响上游流动,多出口边界条件数值模拟,应用Outflow边界条件的前提:默认设置，所有Outflow边界的质量流量为进口流量均分.默认设置流量权重(FRW)为1.如果出口的流量不同，则:制定各个出口的流量权重:mi=FRWi/FRWi.出口压力不同，用于调节各个出口流量.可以采用Pressure边界条件.,其它 Inlet/Outlet 边界条件,Mass Flow Inlet用于可压缩流动给定进口质量流量.对于不可压缩流动，无需给定.Pressure Far Field材料选择为理想气体时，才会有该选项.用于给定自由流的可压缩流动状态，给定自由流的马赫数和静压，静温等。Exhaust Fan/Outlet Vent如果出口有个压力抬升或损失，可以采用exhaust fan/outlet vent给定出口压力抬升或损失系数，以及环境压力与温度。Inlet Vent/Intake Faninlet vent/intake fan用于进口给定压力的损失系数或压力抬升，需要给定流动方向，环境（进口）压力及温度等参数。,壁面边界条件,用于分界流体与固体区域.对于粘性流体流动,不考虑壁面滑移:壁面切向上的流体速度与壁面移动速度相同.壁面法向上的流体速度为零。热边界条件:有几种选项供选择.湍流计算可以考虑壁面粗糙度的影响.壁面切应力和换热取决于当地流动场的计算结果。可以给定壁面的平移速度或旋转速度.也可以给定壁面切应力.,对称（Symmetry）边界条件,可以减少计算区域，用于减少计算量.流场和计算区域必须符合对称条件:对称面上的法向速度为零对称面上所有量在其法向上的梯度为零。无需给定任何值.但必须给定轴对称的正确位置.Also used to model slip walls in viscous flow,Periodic Boundaries,Used when physical geometry of interest and expected pattern of flow/thermal solution have periodically repeating nature.Reduces computational 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 developed 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.,Periodic Boundaries:Examples,p=0:,p 0:,Axis Boundaries,Used:At centerline(y=0)of an axisymmetric gridWhere multiple grid lines meet at a point in a 3D O-type gridSpecify:No inputs required,AXIS boundary,Cell Zones: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 zone as porous media.Define axis of rotation for rotationally periodic flows.Can define motion for fluid zone.,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 inputsof resistance coefficients to lumped parametermodel.Used to model flow through porous mediaand other“distributed”resistances,e.g.,Packed bedsFilter papersPerforated platesFlow distributorsTube banks,Moving Zones,Single Zone Problems:Rotating Reference Frame Modeldefine zone as Moving Reference Framelimited applicabilityMultiple Zone Problems:Each zone defined as moving reference frame:Multiple Reference Frame Modelleast accurate,least demanding on CPUMixing Plane Modelfield data are averaged at the outlet of one zoneand used as inlet boundary data to adjacent zone.Each zone defined as Moving Mesh:Sliding Mesh Modelmust also define interface.Mesh positions are calculated;time-accurate simulationsrelative motion must be tangential(no normal translation),Cell Zones:Solid,“Solid”zone=group of cells for which only heat conduction problem solved.No flow equations solvedMaterial being treated as solid may actually be fluid,but it is assumed that no convection takes place.Only required input is material typeSo appropriate material properties used.Optional inputs allow you to set volumetric heat generation rate(heat source).Need to specify rotation axis if rotationally periodic boundaries adjacent to solid zone.Can define motion for solid zone,Internal Face Boundaries,Defined on cell facesDo not have finite thicknessProvide means of introducing step change in flow properties.Used to implement physical models representing:FansRadiatorsPorous jumpPreferable over porous media-exhibits better convergence behavior.Interior wall,Summary,Zones are used to assign boundary conditions.Wide range of boundary conditions permit flow to enter and exit solution domain.Wall boundary conditions used to bound fluid and solid regions.Repeating boundaries used to reduce computational effort.Internal cell zones used to specify fluid,solid,and porous regions.Internal face boundaries provide way to introduce step change in flow properties.,