Lifting line definition
- @LIFTING_LINE_DEFINITION {
- @LIFTING_LINE_NAME {LfnLineName} {
- @IS_DEFINED_IN_FRAME {FrameName}
- @AIRSTATION_LOCATION_SCHEME {AstLocation}
- @COORDINATE_TYPE {CoordType}
- @LIFTING_LINE_PROPERTY_NAME {LfnPropName}
- @BODY_LIST {BeamName1, BeamName2, ... BeamNameN}
- @AIRSTATION_POSITION {
- @COORDINATES {x_{a1}, x_{a2}, x_{a3}}
- @CURVILINEAR_COORDINATE {s}
- @AXIAL_COORDINATE {x}
- }
- @INITIAL_POINT {x_{i1}, x_{i2}, x_{i3}}
- @CURVILINEAR_COORDINATE {s_{i}, s_{f}}
- @AXIAL_COORDINATE {x_{i}, x_{f}}
- @ORIENTATION_DISTRIBUTION_NAME {OriDistName}
- @NUMBER_OF_AIRSTATIONS {N_{ast}}
- @TIP_LOSS_FACTOR {μ}
- @MOVING_FRAME_NAME {MvgFrameName}
- @PARENT_LIFTING_LINE {ParentLfn}
- @VISUALIZATION_PARAMETERS_NAME {VizPrmName}
- @COMMENTS {CommentText}
- }
- }
NOTES
- A lifting line is a component of an aerodynamic model and is defined as a collection of N_{ast} airstations at which aerodynamic loads are computed. The motion of the lifting line is determined by that of a number of associated beams. The geometric characteristics of the lifting line, the orientation distribution and the location of the airstations, are both defined with respect to a fixed frame, FrameName. This is the frame used to copy the lifting line. Both rotors and wings may involve a collection of lifting lines.
- The lifting line properties, LfnPropName, are associated with the lifting line. This data section will define properties associated with the airstations, such as position, orientation, chord length, quarter-chord offset, and airfoil properties.
- Optionally, a tip loss factor can be defined. This tip loss factor enforces the vanishing of the lift forces near the tip of lifting line. The tip loss factor, f_{tl}, is computed as f_{tl} = tanh (1 - s/s_{f}) / (1 - μ), where s is the curvilinear coordinate of the airstation, s_{f} the curvilinear coordinate of the lifting line tip and 0 < μ < 1. Recommended values are 0.95 ≤ μ ≤ 0.99. The tip loss factor f_{tl} will be used for all airstations associated with this lifting line.
- Optionally, a moving frame, MvgFrameName can be specified. If this lifting line is associated with a rotor, this moving frame defines one of the hub frames of the rotor. If this lifting line is associated with a wing, this moving frame defines one of the wing frames of the wing.
- It is possible to attach comments to the definition of the object; these comments have no effect on its definition.
Geometry of the lifting line
Figure 1. Geometry of the lifting line.
The geometry of the lifting line is determined by the composite curve obtained by joining the curves associated with beams BeamName1, BeamName2,... BeamNameN, of the lifting line, as depicted in fig. 1. For instance, let Curve1, Curve2 and Curve3 be the curves associated with BeamName1, BeamName2 and BeamName3, respectively. The composite curve is the the union of Curve1, Curve2 and Curve3. Of course, it is assumed here that the end point of Curve1 and the starting point of Curve2 are identical and the end point of Curve2 is identical to the starting point of Curve3.
Several parameterizations are available for individual curves. However, for the composite curve defining the geometry of the lifting line, the η-coordinate is not convenient since it ranges from zero to one for each individual curve. Hence, the sole curvilinear and axial coordinates will be used here. The curvilinear coordinate measures length along the composite curve, whereas the axial coordinate measures length along axis i_{1} of triad B = (i_{1}, i_{2}, i_{3}) of fixed frame FrameName associated with the curves.
Airstation location
The location of an airstation determines both its geometric position and its connection to the structural elements of the system. It consists of two distinct elements: its geometric position and its coordinate.
- The geometric position of an airstation consists of its Cartesian coordinates, x_{a1}, x_{a2}, x_{a3}, measured in frame FrameName.
- The coordinate of an airstation consists of a single coordinate, the curvilinear or axial coordinate, along the composite curve defining the geometry of the lifting line. Within the definition of a single lifting line, all coordinates must be either curvilinear or axial. If CoordType = AXIAL_COORDINATE, all coordinates must be axial coordinates; if CoordType = CURVILINEAR_COORDINATE, all coordinates must be curvilinear coordinates.
The relationship between the geometric position and the coordinate of an airstation is based on geometric considerations: the coordinate of the airstation is determined by the location of the point along the composite curve that is at the shortest distance from the airstation geometric position.
- If CoordType = CURVILINEAR_COORDINATE, the curvilinear coordinate system along the composite curve is used to measure length along this curve. It is fully defined by specifying that s_{i} is the curvilinear coordinate associated with the initial point of coordinates x_{i1}, x_{i2}, x_{i3}, measured in frame FrameName, i.e. curvilinear coordinate s_{i} is that of the point on the composite curve that is at the shortest distance from the initial point. All airstations will have a curvilinear coordinate such that s_{i} ≤ s ≤ s_{f}, where s_{i} and s_{f} are the initial and final values of the curvilinear coordinate along the composite curve.
- If CoordType = AXIAL_COORDINATE, the axial coordinate system along the composite curve is used to measure length along axis i_{1} of the fixed frame. It is fully defined by specifying that x_{i} is the axial coordinate associated with the initial point of coordinates x_{i1}, x_{i2}, x_{i3}, measured in frame FrameName, i.e. axial coordinate x_{i} is that of the point on the composite curve that is at the shortest distance from the initial point. All airstations will have an axial coordinate such that x_{i} ≤ x ≤ x_{f}, where x_{i} and x_{f} are the initial and final values of the axial coordinate along axis i_{1} of the fixed frame.
This curvilinear or axial coordinate system will also be used to interpolate lifting line properties such as airstation orientations, chord length, and airtable definition.
Taken together, the geometric position and the curvilinear coordinate of the airstation establish the location of the airstation and its connection to the structural model. The motion of the airstation is inherited from that of the point of the beam located at the corresponding curvilinear or axial coordinate. If the airstation Cartesian coordinates are known, it is possible to determine its curvilinear or axial coordinate from the geometry of the composite curve. If the airstation curvilinear or axial coordinate is known, its Cartesian coordinates can be determined by specifying a quarter-chord offset along one axis of the airstation triad. Hence, it is not necessary to define both geometric position and coordinate of an airstation.
The AstLocation flag determines how airstation locations are defined and can take one of the following six values.
- If AstLocation = COORDINATES, the locations of the N_{ast} airstations will be specified by their coordinates, x_{a1}, x_{a2}, x_{a3}, measured in frame FrameName. This option is typically used when airloads are computed by an external aerodynamic code, as discussed in the aerodynamic interface. In this case, airstation locations are dictated by the grid used in the computational fluid dynamics code. The airstation curvilinear or axial coordinate is determined by the location of the point on the composite curve that is at the shortest distance from the airstation position, as depicted in fig. 2.
- If AstLocation = CURVILINEAR, the locations of the N_{ast} airstations will be directly specified by their curvilinear coordinates, s. This option is typically used when airloads are computed by an external aerodynamic code, as discussed in the aerodynamic interface. In this case, the locations of the airstation are dictated by the grid used in the computational fluid dynamics code. The coordinates of the airstation are obtained from the definition of the composite curve, as depicted in fig. 3.
- If AstLocation = AXIAL, the locations of the N_{ast} airstations will be directly specified by their axial coordinates, x. This option is typically used when airloads are computed by an external aerodynamic code, as discussed in the aerodynamic interface. In this case, the locations of the airstation are dictated by the grid used in the computational fluid dynamics code.
- If AstLocation = EQUALLY_SPACED, the locations of the N_{ast} airstations will be automatically specified by their curvilinear coordinates, s. This option is used when airloads are computed by the internal aerodynamic code, as discussed in the aerodynamic interface. Fig. 4 shows the configuration of the lifting line with its associated airstations. The airstations are located at equally spaced distances along the composite curve, with curvilinear coordinates s_{k} given by s_{k} = s_{i} + (s_{f} - s_{i}) / (N_{ast} + 1) k , k = 1, 2, ..., N_{ast}.
- If AstLocation = GAUSS_POINTS, the location of the N_{ast} airstations will be automatically specified by their curvilinear coordinates, s. However, instead of being equally spaced, the curvilinear coordinates of the airstations are located at the Gauss-Legendre integration points along the composite curve. This option is recommended when a three dimensional dynamic inflow model is used in conjunction with this lifting line.
- If AstLocation = MATCHED, the location of the N_{ast} airstations will match those of the parent lifting line ParentLfn. This option is used when defining the lifting line associated with a wing flap, as illustrated in fig. 5. The parent lifting line ParentLfn includes the airstations associated with the wing and the airstations associated with the flap are those of the present lifting line. If airloads are computed with an external CFD codes, the airstations of the parent and present lifting lines can be chosen arbitrarily. However, if airloads are computed internally, the locations of the airstations of the present lifting line must match those of the parent lifting line. The airstations of the present lifting line will be at the shortest distance of the corresponding airstation of the parent lifting line. When using this option, the hinge location of the flap must be defined in the lifting line properties.
Figure 2. Configuration of the airstations when the airstation
location flag AstLocation = COORDINATES.
Figure 3. Configuration of the airstations when the airstation
location flag AstLocation = CURVILINEAR.
Figure 4. Configuration of the airstations when the airstation
location flag AstLocation = EQUALLY_SPACED.
Figure 5. Configuration of the airstations when the airstation
location flag AstLocation = MATCHED.
Airstation orientation
The orientation of an airstation is defined by a finite rotation that brings triad B = (i_{1}, i_{2}, i_{3}) of the lifting line fixed frame FrameName to a new triad A = (a_{1}, a_{2}, a_{3}). As depicted in fig. 6, unit vector a_{2} of this triad points toward the leading edge of the airfoil, and vector a_{3} points in the direction of positive lift. This input is required for all lifting line properties. The orientation of the airstations of the lifting line are defined by orientation distribution, OriDistName. All these orientations are defined in the lifting line fixed frame FrameName.
Figure 6. Orientation triad defined at an airstation.
The orientation triad for the airstation must be chosen carefully. If airloads are computed using a simplified, two-dimensional aerodynamic theory, plane (a_{2}, a_{3}) is the plane in which the two-dimensional aerodynamic problem will be treated. Fig. 7 illustrates the case of a curved wing with two different choices of airfoil triads. In one case, the two-dimensional aerodynamic problem is solved in the plane normal to the local axis of the wing and the corresponding airstation triad is denoted A^{n} = (a_{1}^{n}, a_{2}^{n}, a_{3}^{n}). In the second case, the two-dimensional aerodynamic problem is solved in the plane containing the far field flow velocity vector, U_{∞}, and the corresponding airstation triad is denoted A^{p} = (a_{1}^{p}, a_{2}^{p}, a_{3}^{p}). If the airfoil property tables are defined in plane (a_{2}^{n}, a_{3}^{n}), the airstation triad A^{n} would be an appropriate choice, whereas if the airtables are defined in plane (a_{2}^{p}, a_{3}^{p}), airstation triad A^{p} would be a better choice.
Figure 7. Possible choices for airstation orientation.
If airloads are computed using an external CFD code, the orientation of the airstation triad is determined by the grid configuration used in the CFD computation.
Airstation motion
The motion of the lifting line is determined by that of the beams, BeamName1, BeamName2,... BeamNameN to which it is attached. The connection between the lifting line and those beam elements is detailed in the definition of airstation location. The beam elements must be defined in sequence along the lifting line in such a way that the curves associated with the beams form a continuous, composite curve. Note that the structural characteristics (stiffness, structural twist, etc.) of the wing are inherited from the those of the beams, whereas its aerodynamic characteristics (chord-length, aerodynamic twist, etc.) are inherited from those of the lifting line.
Graphical parameters
The appearance of the lifting line during the visualization phase of the analysis can be controlled by associating visualization parameters, VizPrmName, to the element. The following representations are valid for lifting lines: RepresentationType = LINE or SYMBOL. The LINE representation depicts the lifting line as a line joining its air stations, whereas the SYMBOL representation only marks the air stations. Default value: SURFACE. The following vector fields are valid for lifting lines: VfdType = AERODYNAMIC_FORCES or AERODYNAMIC_MOMENTS.
Sensors
Sensors can be defined to extract information about lifting lines. The following SensorType and associated FrameName specifications are allowed for lifting lines: AIRSTATION_LOADS, DISPLACEMENTS, FLOW_PARAMETERS, POSITIONS, TOTAL_AIRLOADS, and VELOCITIES. (Default value: AIRSTATION_LOADS). For lifting lines, the local axis system (FrameName = LOCAL) is defined by the airstation orientation.
Sensors are located at the airstations of the lifting line as determined by the u value, which is an integer such that 1 ≤ u ≤ N_{ast} and denotes the airstation number. No v value value is accepted for lifting lines.