Drag Linearization in ANSYS Aqwa
Richard May – May – Team Lead, Aqwa development
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© 2012 ANSYS, Inc.
November 26, 2012
Drag Linearization in Aqwa‐Line
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© 2012 ANSYS, Inc.
November 26, 2012
New input Workbench
ASCII input file JOB TD42
LINE
TITLE
ALTD42: BARGE + 8 TUBEs (Cd=Ca=2)
OPTIONS PPEL REST GOON LDRG SFBM END RESTART
3
4
5
09
NONE
10
NONE
11
NONE
12
NONE
13
SPEC
ALTD41
13SPDN
0.0
END13PSMZ
0.3
14
NONE
15
NONE
16
NONE
17
NONE
18
NONE
© 2012 ANSYS, Inc.
November 26, 2012
1.8
8.0
11.0
Drag Linearization in Aqwa‐Line
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1.
Drag linearization has to be done for a specific spectrum, so the results are strictly only valid for that spectrum.
2.
Reads spectrum defined in Deck 13 and uses that spectrum for EVERY WAVE DIRECTION in Aqwa‐Line; i.e. the spectral direction in Deck 13 is irrelevant.
3.
Produces new RAOs that are output in the .LIS and .PLT files.
4.
The new RAOs are not written into the database so not used in any subsequent analysis. Nor are the QTFs re‐calculated.
5.
We are currently working on adding drag linearization to Aqwa‐Fer.
© 2012 ANSYS, Inc.
November 26, 2012
New ASCII output
New output in the LIS file headed * * L I N E A R I S E D
D R A G
P A R A M E T E R S
F O R
S T R U C T U R E
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
% Critical damping Damping matrix Linearized drag forces RAOs with linearized drag
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© 2012 ANSYS, Inc.
November 26, 2012
1 * *
New graphical output
The PLT file contains new results as shown below
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© 2012 ANSYS, Inc.
November 26, 2012
Calculations (1) •
•
In the calculation of drag force u.|u| is a non‐linear term in which the |u| can be replaced by a factor multiplied by the r.m.s. velocity in order to create an equivalent linear term. In the literature (Borgman 1967) this factor is given as √(8/π). We thus get Full Drag Force
= ½.ρ.A.Cd .u.|u|
Linearized Drag Force = ½.ρ.A.Cd .α.urms .u where u = relative fluid velocity and α = √(8/π) •
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urms is calculated using the spectrum input in Deck 13. As u is relative velocity the calculation is iterative.
© 2012 ANSYS, Inc.
November 26, 2012
Calculations (2) Modified RAO equation
[‐ω2 {M(ω)+MS} – i ω {C(ω) + CDL(θ) }+ K ] X(ω,θ) = FD(ω,θ) + FK(ω,θ) + FDL(ω,θ) Calculate: •
RAOs
•
relative velocity for each component of the spectrum
•
•
•
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rms velocity drag coefficients new RAOS – check for convergence
© 2012 ANSYS, Inc.
November 26, 2012
Calculations (3) When current is included, we assume that the linearization factor is in the form β = √ [ (α*urms)2 + γ uc2 ] Where γ = 2 [ 2 – e‐r ] r = uc/(α urms) Found by numerical investigation of the error in dissipation of drag energy
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© 2012 ANSYS, Inc.
November 26, 2012
Calculations (4) Calculations use Gaussian quadrature
TUBE and STUB – 3 Gauss points
DISC – 9 Gauss points
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© 2012 ANSYS, Inc.
November 26, 2012
Shear Force and Bending Moment for Spar
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© 2012 ANSYS, Inc.
November 26, 2012
Mass distribution file All mass must be in the .MSD file. TUBE mass is ignored in the same way as PMAS elements ALSP2 - FULL SPAR NEUTRAL- Z, 0, 0 Sect i on Mas s STEP 01, 6485157. 3, STEP 02, 6562790. 3, STEP 03, 6640423. 3, STEP 04, 6718056. 3, STEP 05, 6795689. 4, STEP 06, 6873322. 4, STEP 07, 6950955. 4, STEP 08, 7028588. 4, STEP 09, 7106221. 4, STEP 10, 7183854. 4, STEP 11, 7261487. 5,
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© 2012 ANSYS, Inc.
X 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
November 26, 2012
Y 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
Z - 217. - 212. - 207. - 202. - 197. - 192. - 187. - 182. - 177. - 172. - 167.
5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5,
Zmi n - 220, - 215, - 210, - 205, - 200, - 195, - 190, - 185, - 180, - 175, - 170,
Zmax - 215, - 210, - 205, - 200, - 195, - 190, - 185, - 180, - 175, - 170, - 165,
Kzz 15 15 15 15 15 15 15 15 15 15 15
Shear Force and Bending Moment for Spar
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© 2012 ANSYS, Inc.
November 26, 2012
