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*Wake Interference |
 
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*Wake Interference
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*Wake Interference |
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To specify that the present analysis should include wake interference effects, and to specify associated parameters.
Refer to Wake Interference for further information on this feature.
Note also that the old *WAKE INTERFERENCE and *UPSTREAM STRUCTURE keywords have effectively been superseded by the new wake interference keywords which are more generic and provide additional functionality. Refer to *WAKE UPSTREAM and *WAKE DOWNSTREAM for further information.
Two lines, the second of which is optional, defining general parameters, followed by one or more lines whose format(s) depends on the first of the initial two lines.
MODEL=Wake Effect Model [, OUTPUT=YES] [, OPTION=Coefficient Type]
[SET=Element Set Name]
Wake Effect Model can be HUSE, BLEVINS or USER. The format of the third and subsequent lines depends on which of these is nominated. The use of OPTION= is appropriate only for the USER model.
One line with three values:
[k1, k2, k3]
One line with three values:
[a1, a2, a3]
Data in this case consists of drag and lift coefficient data, and is specified in blocks. A first line with either TYPE=DRAG or TYPE=LIFT indicates which data is being input (they can be specified in either order). The coefficients are specified in terms of coefficient factors or absolute coefficient values, depending on the Coefficient Type selected. The first line following TYPE= lists the transverse separation values at which the coefficients are defined, normalised with respect to the drag diameter of the upstream structure. Each line after that starts with a longitudinal separation value, again normalised with respect to the drag diameter of the upstream structure, followed by coefficients corresponding to the separation values. If L represents the centre-to-centre distance between upstream and downstream structures in the longitudinal direction (direction of undisturbed flow), and T is the centre-to-centre spacing in the transverse direction (normal to undisturbed flow); then the ith normalised separations are denoted li=Li/Dd and ti =Ti/Dd . The following then summarises the above:
TYPE=DRAG
t1, t2, t3, ...... tn
l1, Cd(l1,t1), Cd(l1,t2), Cd(l1,t3) ....... Cd(l1,tn)
l2, Cd(l2,t1), Cd(l2,t2), Cd(l2,t3) ....... Cd(l2,tn)
...
...
lm, Cd(lm,t1), Cd(lm,t2), Cd(lm,t3) ....... Cd(lm,tn)
TYPE=LIFT
t1, t2, t3, ....... tn
l1, Cl(l1,t1), Cl(l1,t2), Cl(l1,t3) ....... Cl(l1,tn)
l2, Cl(l2,t1), Cl(l2,t2), Cl(l2,t3) ....... Cl(l2,tn)
...
...
lm, Cl(lm,t1), Cl(lm,t2), Cl(lm,t3) ....... Cl(lm,tn)
If lift coefficients are defined in terms of coefficient factors, it is necessary to define the actual lift coefficients themselves, and this is specified via a data block beginning with a TYPE=LIFT COEFFICIENT line. This is then followed by blocks of two lines defining lift coefficients on an element set by element set basis.
TYPE=LIFT COEFFICIENT
[SET=Element Set Name]
Wake Lift Coefficient
If the SET= line is omitted, the specified lift coefficient applies to all elements, in which case previous or subsequent blocks of lines assigning coefficients to other element sets are invalid.
*WAKE INTERFERENCE may only be invoked in a static or quasi-static analysis. Both occurrences of Element Set Name must refer to an existing element set, which must be defined in the initial static analysis of the downstream structure. The specification of drag and lift data is required if Wake Effect Model is set to USER. Drag and lift data must be specified at the same l and t values. The l and t values must be specified in ascending order. The specification of lift coefficients is required for the downstream structure if the lift data is specified in terms of coefficient factors.
The second line following *WAKE INTERFERENCE (with SET=) is required where the upstream and downstream structures are included in the same model, in which case it is used to identify the elements of the combined model which comprise the downstream structure. If the two structures are being analysed in separate models, it is optional; the default is all elements in the present (downstream) structure model. Likewise the line following TYPE=LIFT COEFFICIENT (with SET=) is optional if the two structures are being analysed in separate models; if omitted the element set again defaults to all elements (of the downstream structure).
The specification of the k1, k2 and k3 values for Huse’s formulation is optional; these default to 0.25, 1 and 0.693 (not 0.639) respectively. Likewise, the specification of the a1, a2 and a3 values for Blevins’ formulation is optional; these default to 1, 4.5 and -10.6 respectively. Coefficient Type may be either FACTORS (the default) or ABSOLUTE, and is only relevant to the User-Defined wake formulation.
The pair of lines following TYPE=LIFT COEFFICIENT are repeated as often as necessary to define wake lift coefficients for all elements of the downstream structure model.
To specify the formulation to be used in calculating wake interference effects.
Input: |
Description |
Formulation: |
The options are Huse's (the default), Blevins' and User-Defined. |
To request additional output from wake modelling calculations.
Input: |
Description |
Output: |
The options are No (the default) and Yes. |
(a)If you invoke this option, Flexcom presents detailed information regarding the wake modelling calculations. Specifically, for every integration point on every element of the downstream structure, the following is provided:
•Longitudinal (L) and transverse (T) centre-to-centre distance to upstream structure
•Reduced current velocity
•Drag and lift coefficients
•Lift Force
This information may be useful in some circumstances in understanding wake interference analysis results, but in general it is not required, so this option would be rarely invoked.
To specify parameters relating to Huse’s wake interference formulation.
Input: |
Description |
Set Name: |
The name of the set that contains the elements on the downstream structure that you want to include in the wake interference analysis. The default is All. See Note (a). |
k1: |
The k1 coefficient of Huse’s formulation equation. The default is 0.25. |
k2: |
The k2 coefficient of Huse’s formulation equation. The default is 1. |
k3: |
The k3 coefficient of Huse’s formulation equation. The default is 0.693. |
(a)If the two structures are in separate models, then the specification of a Set Name is optional. By default, all of the elements in the current model constitute the downstream structure. If the two structures are in the same model, then the specification of a Set Name other than All is mandatory. In this case, the downstream structure cannot consist of all of the elements in that model.
To specify parameters relating to Blevins’ wake interference formulation.
Input: |
Description |
Set Name: |
The name of the set that contains the elements on the downstream structure that you want to include in the wake interference analysis. The default is All. See Note (a). |
a1: |
The a1 coefficient of Blevins’ formulation equation. The default is 1. |
a2: |
The a2 coefficient of Blevins’ formulation equation. The default is 4.5. |
a3: |
The a3 coefficient of Blevins’ formulation equation. The default is -10.6. |
(a)If the two structures are in separate models, then the specification of a Set Name is optional. By default, all of the elements in the current model constitute the downstream structure. If the two structures are in the same model, then the specification of a Set Name other than All is mandatory. In this case, the downstream structure cannot consist of all of the elements in that model.
To specify the downstream structure to be used with the User-Defined Wake Model.
Input: |
Description |
Set Name: |
The name of the set that contains the elements on the downstream structure that you want to include in the wake interference analysis. The default is All. See Note (a). |
(a)If the two structures are in separate models, then the specification of a Set Name is optional. By default, all of the elements in the current model constitute the downstream structure. If the two structures are in the same model, then the specification of a Set Name other than All is mandatory. In this case, the downstream structure cannot consist of all of the elements in that model.
To specify drag coefficient data for the User-Defined wake model.
You use this table to enter the drag coefficient data as functions of longitudinal (L) or transverse (T) centre-to-centre structure distances. The separations are normalised with respect to the drag diameter of the upstream structure. All values default to zero unless you specify otherwise. You may also customise the ranges to meet your own specific requirements.
(a)The coefficients may be defined in terms of absolute coefficient values (as was the case in earlier versions) or as coefficient factors (this is now the default option). These factors multiply or scale the (nominal) hydrodynamic coefficients to give the coefficients to be used in the calculation of the wake-affected hydrodynamic forces.
To specify the type of wake coefficients to be used with the User-Defined wake interference model.
Input: |
Description |
Coefficients Type: |
The options are Factors (the default) and Absolute. |
(a)The coefficients may be defined in terms of absolute coefficient values (as was the case in earlier versions) or as coefficient factors (this is now the default option). These factors multiply or scale the (nominal) hydrodynamic coefficients to give the coefficients to be used in the calculation of the wake-affected hydrodynamic forces.
To specify lift coefficient data for the User-Defined wake model.
You use this table to enter the lift coefficient data as functions of longitudinal (L) or transverse (T) centre-to-centre structure distances. The separations are normalised with respect to the drag diameter of the upstream structure. All values default to 0 unless you specify otherwise. You may also customise the ranges to meet your own specific requirements.
(a)The coefficients may be defined in terms of absolute coefficient values (as was the case in earlier versions) or as coefficient factors (this is now the default option). These factors multiply or scale the (nominal) hydrodynamic coefficients to give the coefficients to be used in the calculation of the wake-affected hydrodynamic forces.
To define lift coefficients to be used with the User-Defined wake interference model.
Input: |
Description |
Set Name: |
The name of the element set. |
Wake Lift Coefficient: |
The corresponding wake lift coefficient. |
(a)One important point to note regarding the lift coefficients is that they do not necessarily represent physically meaningful quantities. Unlike drag forces, lift forces outside of the wake zone are non-existent, so while drag coefficients have a physical significance in reality, lift coefficients are somewhat notional. For this reason, the specification of lift coefficients is grouped with the specification of wake interference data, in both the user interface and in the keyword file; whereas the specification of drag coefficients is (as always) part of the specification of the standard hydrodynamic properties.