BOUNDARY CONDITIONS Chapter 4

Training Manual May 15, 2001 Inventory # Boundary Condition Overview Well Posed Problems Categorization of boundaries –Flow Boundary Conditions (loads) –Thermal Boundary Conditions (loads) Application –Commands –Graphical User Interface –Nodal or Solid Model

Training Manual May 15, 2001 Inventory # General Comments In CFD analyses, almost every boundary must be accounted for in some fashion. If a boundary condition is not specified for a dependent variable, the assumption is that the derivative of the variable normal to the surface is zero. Boundary conditions consist of conditions applied to any of the velocity degrees of freedom (VX,VY,VZ), or pressure (PRES), the turbulence quantities (ENKE,ENDS) or the temperature (TEMP). Heat fluxes, film coefficients, and volumetric heat sources may also be applied. Boundary conditions are set on nodes and elements with the D and SF commands. Solid model boundary conditions are applied with the DL, DA, SFL, and SFA commands… (NOT the DK command)

Training Manual May 15, 2001 Inventory # Boundary Condition Types Inlet/Outlet Symmetry Condition Stationary Wall, Moving Wall Periodic Boundaries Fixed Temperature Heat Flux Volumetric Heat Source Heat Transfer (Film) Coefficient Radiation

Training Manual May 15, 2001 Inventory # Inlets/Outlets Problems are generally velocity driven or pressure driven. Velocity into or out of the problem domain may be specified –Constant value Profile constructed with function or table Pressure may be specified across a boundary –Flow may enter or leave. –Locate pressure boundaries away from significant geometry changes. –Typically, Pressure set to 0 at outlet. Turbulence Boundary conditions –Specification optional FLOTRAN will define defaults (generally, turbulent boundary conditions need not be known in detail as the flow conditions themselves generate most of the turbulence.)

Training Manual May 15, 2001 Inventory # Symmetry Boundary Used to represent either plane of symmetry or axisymmetric centerline. Assumption is that flow patterns are symmetric. Velocity component normal to symmetry boundary is set to zero. Prevents mass or heat transfer across boundary. Symmetry boundary must be aligned with a global coordinate axis. The User must determine whether or not transient effects might result in asymmetric flow patterns. Obviously these effects will not be modeled with a symmetric geometry.

Training Manual May 15, 2001 Inventory # Stationary Wall Apply no-slip boundary condition. –All velocity components set to zero. Turbulence model boundary conditions automatically applied. –Law of the Wall, Log-Law of the Wall ( methods of treating conditions very close to the wall) –They are a part of the FLOTRAN turbulence modeling scheme. No user action is required to implement wall turbulence conditions If no thermal boundary conditions are specified, wall is treated as adiabatic boundary. No pressure specification necessary.

Training Manual May 15, 2001 Inventory # Moving Wall Moving wall - Stationary boundary. –Problem domain cannot change shape. –Examples: Flow between rotating cylinders Flow past cars: movement of the ground Wall “drags” flow or moves with it. Specify velocity normal to wall as zero. Specify velocity tangent to wall as wall speed. Tell FLOTRAN that this is not an inlet! –Flag is to set turbulent kinetic energy (ENKE) to -1. Specified as moving wall via GUI in Velocity BC Dialog Box under loads

Training Manual May 15, 2001 Inventory # Internal Flows Internal flow problems are bounded by walls, symmetry planes, and inlets/outlets. There need not be any inlets/outlets. Unspecified boundaries are permitted, but can be unstable.

Training Manual May 15, 2001 Inventory # External Flows

Training Manual May 15, 2001 Inventory # Notes on External Flows The boundary must be located far from the body. Typically, the back half of the boundary has a zero relative pressure applied. In supersonic problems, pressure and velocity may be specified in the forward section, with the back half left as an unspecified boundary. Make sure that any shock waves do not extend all the way to a specified boundary. Remove the specified condition if it does. It is sometimes desirable to specify the velocity condition all around the problem domain. (Usually requires use of initial conditions) Considering a characteristic dimension of the body as a chord length, the boundaries of the problem domain should be chord lengths from the body.

Training Manual May 15, 2001 Inventory # Periodic Boundaries Flow at corresponding nodes of two boundaries is unknown but identical. Boundaries must have identical node patterns. All DOF's are coupled for each periodic node pair. Apply with peri.mac macro, supplied with FLOTRAN. –Select nodes on one boundary. –Specify spatial offset between the boundaries (the selected set is the base). –Not accessed in GUI Not available as solid model load

Training Manual May 15, 2001 Inventory # Periodic Boundaries Select These Nodes Peri,0,90,0 FLOTRAN Polar Coordinates The PERI macro checks the keyopt to determine the FLOTRAN coordinate system being used and interprets the offsets accordingly.

Training Manual May 15, 2001 Inventory # Well Posed Problems Make sure enough boundary conditions are applied. Problems with two flow boundaries are straightforward: –Known pressure drop > calculate flow –Known inlet flow rate > calculate pressure drop Three Inlets/Outlets –Suppose two flows are specified…. Specify pressure at third boundary –One flow is specified.. Specify pressure at two boundaries –Try to determine from boundary conditions and geometry whether multiple solutions are physically possible...

Training Manual May 15, 2001 Inventory # More on Velocity Conditions For 2D problems, the user must determine if the nodes at the endpoints of the line are to be set. Generally, they are.. The same is true of 3D problems and areas. Note the choice of Keypoints is not allowed for FLOTRAN loadings. The best procedure for the edges of areas is to use the boundary lines option, rather than the “apply velocity on lines”. If the inlet nodes have been previously selected, click on “Pick all in the Picker and then OK”. If they haven't, use the picker to select the desired nodes before OK. Note that for walls, simply select VX,VY and if necessary VZ in the Apply V on Nodes menu. It is important to note that the priority of solid model velocity boundary conditions is correctly observed by FLOTRAN automatically. That is, the zero condition of a wall will overwrite an inlet velocity, but an inlet velocity will not overwrite a wall condition. In other words, the numbering scheme of the areas or the order in which the solid model boundary conditions are applied do not matter. ANSYS may, however, produce some warning messages regarding overwriting conditions.

Training Manual May 15, 2001 Inventory # Thermal Boundary Conditions Specified Temperature –FLOTRAN calculates the heat transfer required to maintain the wall at the specified temperature. Specified Heat Flux –FLOTRAN calculates the wall temperature associated with the specified heat flux and the flow conditions. Specified Heat Transfer (Film) Coefficient –Specify Ambient (Bulk) Temperature. FLOTRAN iterates to calculate the heat flux and the surface temperature. Volumetric Heat Sources –Can be specified in fluid or non-fluid elements. Radiation (external or surface-to-surface) –Specify surface emissivity and ambient temperature.

Training Manual May 15, 2001 Inventory # More on Thermal Boundary Conditions The heat transfer coefficient can be calculated for output at all thermal boundaries where it is not specified. It will be based on the calculated or input heat flux, the calculated or specified surface temperature, and the specified ambient temperature. Note that surface boundary conditions cannot be prescribed on internal faces. In a compressible adiabatic problem, specified inlet temperatures are ignored, as FLOTRAN uses the specified total temperature and the velocity to calculate the static temperature everywhere.

Training Manual May 15, 2001 Inventory # Summary of Boundary Conditions The negative kinetic energy specification at a moving wall is used as a flag to activate the wall turbulence calculation. It is done under VELOCITY in the GUI.

Training Manual May 15, 2001 Inventory # Boundary Condition Priority at Intersection Boundaries Note that one boundary condition can have a priority over another even if the degrees of freedom do not conflict.

Training Manual May 15, 2001 Inventory # Boundary Condition Priority at Intersecting Boundaries The solid model boundary condition transfer ensures that a zero velocity condition at a boundary is not overridden by a non-zero value. This means that the wall condition will prevail at the intersection of a wall and an inlet.

Training Manual May 15, 2001 Inventory # The GUI and Boundary Conditions Preprocessor or Solution Phase Specification of the Element type provides filtering so that only the FLOTRAN CFD conditions are presented.

Training Manual May 15, 2001 Inventory # Velocity

Training Manual May 15, 2001 Inventory # Velocity on Areas

Training Manual May 15, 2001 Inventory # Pressure DOF

Training Manual May 15, 2001 Inventory # Turbulent DOF

Training Manual May 15, 2001 Inventory # Volumetric Heat Sources

Training Manual May 15, 2001 Inventory # Heat Flux The SF or SFE command is used. Application is to nodes or element faces. Apply with a constant value. (Ensure Face Number is correct!)

Training Manual May 15, 2001 Inventory # Convection The SF or SFE command is used. The bulk temperature specification is used to evaluate heat transfer at the surface. If the tapered option is used, the settings on all but the first node will be ignored. Note that the bulk temperature may vary with table input

Training Manual May 15, 2001 Inventory # Radiation : Ambient Make sure Stefan-Boltzmann constant is in the correct Units! ANSYS default value is in (BTU/hr/in 2 /R 4 ) 5.67x10 -8 (Watts/m 2 /K 4 )

Training Manual May 15, 2001 Inventory # Radiation : Surface-to-Surface Up to 10 enclosures may be defined. All surfaces in an enclosure look at each other. The effect on the fluid temperature is indirect, being manifested by changes in surface temperatures….

Training Manual May 15, 2001 Inventory # Non-Constant Boundary Conditions Tables - “This Vs. That” (see Basic Procedures, Loading) –Detailed example in Thermal chapter. –Independent variables as a function of the load applied are shown on the following page. –Tables may be created at any time

Training Manual May 15, 2001 Inventory # FLOTRAN Analyses ConditionIndependent Variable Nodal DOFTIME, X, Y, Z, TEMP, VELOCITY, PRESSURE Nodal DOF for ALE formulationTIME, X, Y, Z, TEMP, VELOCITY, PRES, Xr, Yr, Zr Heat Flux/Film CoefficientTIME, X, Y, Z, TEMP, VELOCITY, PRESSURE Element Heat GenerationTIME, X, Y, Z, TEMP, VELOCITY, PRESSURE Nodal Heat GenerationTIME, X, Y, Z, TEMP, VELOCITY, PRESSURE Nodal Body ForceTIME, X, Y, Z, TEMP, VELOCITY, PRESSURE RadiationTIME, X, Y, Z, TEMP, VELOCITY, PRESSURE

Training Manual May 15, 2001 Inventory # Displaying Boundary Condition Symbols When boundary conditions are applied, the symbol for plotting them is automatically turned on. For large models, plotting the nodes with the boundary condition symbols turned on can be time consuming. –Turn off Symbols after initial verification of model –Utility Menu: Plot Ctrls > Symbols

Training Manual May 15, 2001 Inventory # Symbols...

Training Manual May 15, 2001 Inventory # Initial Conditions Initial conditions are applied at the beginning of a transient analysis. External flow problems benefit from using the free stream velocity as an initial condition for a steady state analysis. Loads > Apply… Set a value...

Training Manual May 15, 2001 Inventory # Creation of An “Infinitely Thin” Wall The goal is to avoid the solid model inconvenience of modeling the actual thickness of a separating wall. The procedure involves merging nodes rather than solid model entities such as keypoints. (Why not just set velocities at a line to zero? Because then pressure would be continuous across the wall…) Example Problem –Re = 100 –Model an obstruction (wall) in the flow path of a duct

Training Manual May 15, 2001 Inventory # Create Geometry Create the four areas with obstruction at the end of the first set of areas… –Total height of flow path: 1.0 –Distance to obstruction: 3.0 –Distance behind obstruction: 7.0 –Height of obstruction: 0.5 Do Not Merge Keypoints !!

Training Manual May 15, 2001 Inventory # Establish line Divisions - flow direction The lines in the main flow direction of area 1 must be the same as 2, and 3 must match 4. Bias the mesh towards the inlet and walls….. Note that the lines on top of each other at the boundaries must each be done. Flip the bias of the outlet area lines if necessary...

Training Manual May 15, 2001 Inventory # The Transverse Direction The two “upper areas” must have the same number of divisions (e.g. 9) The two “lower areas” must each have different numbers of elements (e.g. 9 for the inlet, 10 for the outlet) Bias the mesh towards the walls and the tip of the obstruction. Take care, as there are lines on top of each other …. (The picture has the Y distortion factor set to 5)

Training Manual May 15, 2001 Inventory # Mesh The Areas No merging of any entities is done before the meshing! Unselect the nodes at the bottom of the wall on both sides!! Then merge the nodes..

Training Manual May 15, 2001 Inventory # Merge the nodes and you will get the following message Take care that a line is preserved on each side of the wall –Boundary conditions will subsequently be applied to both sides of the wall. The Merging of Nodes

Training Manual May 15, 2001 Inventory # Checking the Model Select the external nodes and plot them Scale factor has been removed.

Training Manual May 15, 2001 Inventory # Results: Velocity Vectors Pressure Contours