第十四章 CEL表达式语言

第十四章 14: CFX 表达式语言(CEL)CFX Expression Language (CEL) is an interpreted, declarative language that has been developed to enable CFX users to enhance their simulations without recourse to writing and linking separate external Fortran routines.You can use CEL expressions anywhere a value is required for input in ANSYS CFX.CEL can be used to:∙Define material properties that depend on other variables.∙Specify complex boundary conditions.∙Add terms to the solved equations.You can also monitor the value of an expression during the solution using monitor points. Important: There is some CEL that works elsewhere in ANSYS CFX, but not in CFD-Post. Any expression created in CFX-Pre and used as a Design Exploration output parameter could potentially cause fatal errors during the Design Exploration run, so you should create all expressions for Design Exploration output parameters in CFD-Post.14.1.1. Values and ExpressionsCEL can be used to generate both values and expressions. Values are dimensional (that is, with units) or dimensionless constants. The simplest type of definition is the dimensionless value, for example:b = 3.743You can also specify a value with units, for example:g = 9.81 [m s^-2]The dimensions of the quantities of interest for CFD calculations can be written in terms of mass, length, time, temperature and angle. The concept of units is fundamental to the behavior of values and expressions.Values can be used directly, or they can be used as part of an expression. For example, you can use an expression to add two values together:<Expr_1> = <Value_1> + <Value_2>In this example, you may want to predefine <Value_1> and <Value_2>, but this is not required. However, in order to add two quantities together, they must have the same dimension; that is, it is meaningful to add a quantity in inches to one expressed in meters, but it is not meaningful to add one expressed in kilograms to one in square feet.Expressions can also be functions of other (predefined) expressions:<Expr_2> = <Expr_1> + <Value_3>Units follow the conventions in the rest of CFX, in that a calculation has a set of solution units (by default, SI units), and that any quantity can be defined either in terms of the solution units, or any other set of units with the correct form.An expression does not have its own units string, but if it references quantities that have dimensions, these will determine the resulting units for the expression. For example, if an expression depends inversely on the square of the x coordinate, then it has implied dimensions of length to the power -2.14.1.1.1. Using Locators in ExpressionsA CFX simulation has physics areas and mesh areas; physics areas are boundaries while mesh areas are regions. These two types of area can occupy completely different spaces in a simulation; however, there is no requirement that area names be unique between physics and mesh. This can lead to ambiguities when you use these names in expressions.To avoid these ambiguities, ANSYS CFX first checks to see if "@<locator>" is a physics name; if this is not found, the name is checked in the list of mesh names. Thus if "in1" is both the name of a physics area and the name of a mesh area, "@<locator>" is taken to indicate the physics area.ANSYS CFX also has @REGION CEL syntax so that you can identify a named area as being a mesh area. Thus to identify the mesh area in1, you would use the syntax:@REGION:in1Note that if <locator> does not appear as a physics name or a mesh name, the expression fails.14.1.2. CFX Expression Language StatementsThe CFX Expression Language is declarative. You declare the name and definition of the expression using expression language statements. The statements must conform to a predefined syntax that is similar to Fortran mathematical statements and to C statements for logical expressions.The statement must consist of the following:∙ a number, optionally with associated units. This defines a constant. Constants without units are termed dimensionless.∙for mathematical expressions, one or more references to mathematical constants, system variables, or existing user variables, separatedby +(addition), - (subtraction), * (multiplication), / (division)and ^(exponentiation), with optional grouping of these by parentheses. Thesyntax rules for these expressions are the same as those for conventionalarithmetic.∙for logical expressions involving relational operators, one or more references to mathematical constants or results from mathematical expressions, separatedby <= (is less than or equal to), < (is less than), == (is equal to),!= (is not equalto), > (is greater than) and >= (is greater than or equal to) with optional groupingof these by parentheses.∙for logical expressions involving logical operators, one or more references to logical constants or results from relational operations separatedby !(negation), && (logical AND) and || (logical OR), with optional grouping byparentheses.14.1.2.1. Use of ConstantsConstants do not need to be defined prior to being used in an expression. For example, you could choose to evaluate the expression x + 5 [m]. Or, you could define a constant, b = 5 [m] and then create an expression x + b.The logical constants are false and true. Results of logical expressions are either false or true, which are evaluated as 0 and 1 (corresponding to false and true, respectively) when a numerical representation is required.The use of constants may be of benefit in generating complicated expressions or if you have several expressions that use the same constants.14.1.2.2. Expression SyntaxAll numbers are treated as real numbers.The precedence of mathematical operators is as follows (from highest to lowest):∙The power operator ^ as in x^y.∙The unary minus or negation operator - as in -x.∙Multiplication and division as in x*y/z.∙Addition and subtraction as in x+y-z.The precedence of logical and relational operators is as follows (from highest to lowest):∙The negation operator ! as in !x.∙The relational operators involving less than or greater than (<=, <, > and >=) as in x >= y.∙The relational operator is equal to and is not equal to (== and !=) as in x != y.∙The logical AND operator (&&) as in x && y.∙The logical OR operator (||) as in x || y.14.1.2.3. Multiple-Line ExpressionsIt is often useful, particularly with complex expressions, to use more than one line when creating your expression. CFX allows you to use multiple lines to generate an expression, provided each line is separated by an appropriate operator.For example, you may have an equation, A + B/C, that consists of three complex terms, A, B, and C. In this case, you could use three lines to simplify creating the expression:A +B/ CNote that the operator may be used at the end of a line (A +) or at the beginning of a line (/ C). You do not need to enter the operator twice.Once the expression has been created, it will appear in the Existing Definitions list box as if it were generated on a single line (A + B/C).14.2.1. CEL OperatorsCFX provides a range of mathematical, logical and operational operators as built-in functions to help you create complex expressions using the Expression details view.Table 14.1: CEL Operators14.2.2. Conditional if StatementCEL supports the conditional if statement using the following syntax:if( cond_expr, true_expr, false_expr )where:cond_expr: is the logical expression used as the conditional test∙true_expr: is the mathematical expression used to determine the result if the conditional test is true.∙false_expr : is the mathematical expression used to determine the result if the conditional test is false.Note: The expressions true_expr and false_expr are always evaluated independent of whether the evaluation of cond_expr is true or false. As a consequence, a conditional statement cannot be used to avoid division by zero as in if( x>0, 1/x, 1.0). In this case, when x=0.0, a division by zero will still occur because the expression 1/x is evaluated independent of whether x>0 is satisfied or not.14.2.3. CEL ConstantsRight-click in the Expression details view to access the following useful constants when developing expressions:Table 14.2: CEL Constants14.2.4. Using ExpressionsThe interaction with CEL consists of two phases:∙ a definition phase, and,∙ a use phase.The definition phase consists of creating a set of values and expressions of valid syntax. The purpose of the Expression details view is to help you to do this.14.2.4.1. Use of Offset TemperatureWhen using temperature values in expressions, it is generally safer to use units of [K] only. When units are used that posses an offset (for example, [C]), they are converted internally to [K]. For terms that have temperature to the power of unity, any unit conversion will include the offset between temperature scales. However, in all other cases the offset is ignored because this is usually the most appropriate behavior. You should therefore take care when specifying an expression involving non-unit powers of temperature. For example, each of the expressions below is equivalent:Temperature = 30 [C]Temperature = 303.15 [K]Temperature = 0 [C] + 30 [K]Temperature = 273.15 [K] + 30 [K]These are only equivalent because all units are to the power of unity and units other than [K] appear no more than once in each expression. The following expression will not produce the expected result:Temperature = 0 [C] + 30 [C]This is equivalent to 576.30 [K] because each value is converted to [K] and then summed. The two expression below are equivalent (as expected) because the offset in scales is ignored for non-unit powers of temperature:Specific Heat = 4200 [J kg^-1 C^-1]Specific Heat = 4200 [J kg^-1 K^-1]15.5. Quantitative Function ListThe available quantitative functions are outlined in the sections that follow.In the table that follows, <Expression> applies to CFD-Post only. CFX-Pre andCFX-Solver can only accept variables as arguments to quantitative functions. Note that for CFX-Pre and CFX-Solver, an Additional Variable can be used to pass an expression indirectly into a quantitative function.The behavior of the functions in the table below depends on the type of <Location>. Typically:∙On domains and subdomains, the functions use vertex (node) values for the operand.∙On a boundary, the functions use conservative values for the operand unless this is overridden by the Boundcon variable operator in CFX-Pre.∙On user locations in CFD-Post, the functions use values interpolated from nodal values.Table 15.4: CEL Functions in CFX-Pre/CFX-Solver and in CFD-Post15.5.1. areaThe area function is used to calculate the area of a 2D locator.area[_<Axis>[_<Coord Frame>] ]()@<Location>where:∙<Axis> is x, y, or z∙<Coord Frame> is the coordinate frame∙<Location> is any 2D region (such as a boundary or interface).An error is raised if the location specified is not a 2D object. If an axis is not specified, the total area of the location is calculated.area()@Isosurface1 calculates the total area of the location,and Isosurface1.area_y()@Isosurface1 calculates the projected areaof Isosurface1 onto a plane normal to the Y-axis.15.5.1.1. Tools > Command Editor Example>calculate area, <Location>, [<Axis>]The specification of an axis is optional. If an axis is not specified, the value held in the object will be used. To calculate the total area of the location, the axis specification should be left blank (that is, type a comma after the location specification).>calculate area, myplane calculates the area of the locator myplane projected onto a plane normal to the axis specification in the CALCULATOR object.。