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APISAT 2012
Ramada Plaza Hotel, Jeju, Korea 0148 Nov. 14, 2012
Vortex Dissipation Due to Airfoil-Vortex Interaction
Toward An Improvement of The Hybrid Method of
Prescribed Wake Model/CFD
○ Keitaroh OHSHIO (Tokyo Metropolitan University)
Masahiko SUGIURA (Aviation Program Group, JAXA)
Yasutada TANABE (Aviation Program Group, JAXA)
Hideaki SUGAWARA (Ryoyu Systems, Co., Ltd.)
Masahiro KANAZAKI (Tokyo Metropolitan University)

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Presentation Outline
 Background
 Objective
 Methodologies
 Calculation results
 Change of vortex trajectory and circulation
 Sound pressure fluctuation
 Concluding remarks

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Development of BVI Noise Prediction Method
Tail rotor noise Tip vortex
Transmission noise
HSI noise Engine noise
BVI noise
BVI
BVI noise has to be reduced for
Dominant parameters
next generation helicopters.
・ Vertical distance between
blade and vortex center
(=miss-distance)
The simulation of BVI has to be
・ Vortex circulation
 high-accurate
 low-cost

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Hybrid Method of Prescribed Wake Model/CFD (1/2)
Hybrid method
CFD
Prescribed wake model
(Estimation of the induced velocity)
CFD
(Estimation of the pressure distribution on blades)
Prescribed wake model Previous study (2012, DLR)
B. G. van der Wall: Prediction of BVI Noise Radiation Variation Due to HHC
Using Advanced Prescribed Wake Methodology
Tip vortex geometry expressed by the prescribed wake model
xv = RcosΨv + μx(Ψb-Ψv ) R ：Rotor radius
yv = RsinΨv Ψv ：Vortex age
Ψb
Ψb ：Blade azimuth at vortex release
zv = -μz(Ψb-Ψv )+ ∫ (v/RΩ)dΨ V ：Freestream velocity
Ψv
αTPP ：Rotor angle of attack
μx = VcosαTPP/RΩ μz =VsinαTPP/RΩ Ω ：Rotor rotational speed
CFD：Computational Fluid Dynamics

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Hybrid Method of Prescribed Wake Model/CFD (2/2)
Overestimation
BVI Error
Error of prescribed wake model
Δ2 [dB] influences on CFD error.
Overestimation of BVISPL

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Toward An Improvement of The Hybrid Method（1/2）
Vorticity Magnitude
When using the
hybrid method ・ Difference of vortex center
between CFD and the hybrid
method affects to the accuracy
When using CFD of BVI noise prediction.
1st BVI Flow 2nd BVI ・For an improvement of the
hybrid method, it is
Rotor required to estimate the
4th BVI change of vortex center
3rd BVI
and its strength due to
the sequential BVI.

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Toward An Improvement of The Hybrid Method（2/2）
 Requirement of the modified prescribed wake model
Ω ・ It is promising approach to estimate
the change of vortex center location
and vortex circulation with 2-D AVI
simulations for development of the
modified prescribed wake model.
Wake position to modify
xv = RcosΨv+μx (Ψb-Ψv ) +Δxv (Ψb-Ψv , Γ , ・ ・ ・ )
yv = RsinΨv +Δyv (Ψb-Ψv , Γ , ・ ・ ・ )
Ψb
zv = -μz (Ψb-Ψv )+ ∫Ψv (v/RΩ)dΨ +Δzv (Ψb-Ψv , Γ , ・ ・ ・ )
AVI: Airfoil-Vortex Interaction

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Objective
・ N u m e r i c a l s i m u l a t i o n o f AV I f o r
improvement of the hybrid method
 2-D CFD simulation composed of
a single vortex and two airfoils
 Estimation of the change of vortex
c e n t e r l o c a t i o n a n d v o r t e x
c i r c u l a t i o n d u e t o s e q u e n t i a l AV I
Airfoil
Vortex
AVI

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Methodologies(1/2)
・Governing equations：2-D compressible Euler equations
・Numerical flux evaluation:
SLAU(Simple Low-dissipative Advection Upstream Splitting Method)
・Temporal scheme：
Dual-time stepping implicit method solved with LU-SGS
(Lower-Upper Symmetric-Gauss-Seidel) method
・Spatial reconstruction：
FCMT(Fourth-order Compact MUSCL TVD) method
・Grid construction：Overset grids platform Airfoil grid
Background grid

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Methodologies(2/2)
・Investigation of the correlation between miss-distance and vortex trajectory
Scully Vortex Z (Vertical distance between airfoil and vortex)
※
M   0.626 NACA0012 (#1) NACA0012 (#2)
Zv x
c(=1.0)
10c 23c
~
~
~
~
Miss-distance Zv=0.0～1.5 AoA=0.0, 5.0[deg]
Scully vortex
ˆ
v  r2 ˆ 

Vθ
Swirl velocity 
U  2r r  r0 
2 2
Uc 
Core radius r0  0.162c 0 r0 r
ˆ
Non-dimensional circulation   0.2536（equivalent to Cl=-0.50）
※ Caradonna, F. X. et al., “An Experimental Study of Blade-Vortex Interaction Aerodynamics
and Acoustics Utilizing an Independently Generated Vortex,” NASA TM 199208790, 1999

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Presentation Outline
 Background
 Objective
 Methodologies
 Calculation results
 Change of vortex trajectory and circulation
 Sound pressure fluctuation
 Concluding remarks

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Sequential AVI
Vorticity
Zv=0.05 （AoA=0.0） Zv=0.05 （AoA=0.0）
~
~
1st AVI (#1) 2nd AVI (#2)
・ 1st airfoil divides the clockwise vortex into two vortices.
・ Counter-clockwise vortices are induced due to 1st AVI.

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Change of the vortex center location
Two cases are compared: AoA=0.0, 5.0
Original vortex
Δz
Before 2nd AVI
Δx
z
Flow
x
⊿x : In case of AoA=0.0, the
original vortex is decelerated
by counter-clockwise vortex
⊿z : In both cases, the original
vortex moves upward due to
Before 2nd AVI the induced velocity of
counter-clockwise vortex

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Change of the vortex circulation
Flow
^
Original vortex
z
Before 2nd AVI
Zv=0.05 （AoA=0.0）
x
Since the total vorticity is conserved, Original vortex
it is promising to capture the change of
z
vortex center by considering the several
vortices as a single vortex
Zv=0.25 （AoA=0.0）
x

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Presentation Outline
 Background
 Objective
 Methodologies
 Calculation results
 Change of vortex trajectory and circulation
 Sound pressure fluctuation
 Concluding remarks

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Sound pressure fluctuation(1/2)
Observation points Pressure fluctuation coef. Lavi
Δp
Lavi  2
ρ a
AoA=0.0
 Estimation of the difference between Lavi1 and Lavi2
 Since the miss-distance increases after 1st AVI,
Lavi2 is lower than Lavi1.

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Sound pressure fluctuation(2/2)
・ Investigation of the effect of miss-distance
Zv Lavi2/Lavi1
AoA=0.0 0.05 0.355
0.25 0.414
When the initial miss-distance is long (Zv=0.25),
 the intensities of both AVIs are lower than those of AoA=0.0
 larger miss-distance provides lower Lavi2

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Correlation between miss-distance and pressure fluctuation
※
AoA=0.0 According to the previous study,
the following regression between
miss-distance and Lavi is introduced
by using the calculation results
Lavi  d
Lavi0 a Zv b  c
Lavi 1
 1.2
Lavi0 2 Zv 1
Lavi0 ：Sound pressure fluctuation in case of Zv=0.0
※ Yasutada, T., Saito, S., Takasaki K., and Fujita H., “A Parametric Study on Parallel Blade-Vortex Interaction
for Helicopter Rotor,” JAXA RR-07-051E, 2008

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Identification of corrected miss-distance
AoA=0.0
Investigation of the ratio of Modification of miss-distance
Lavi1 and Lavi2 with CFD results in case of AoA=0.0
 By using regression line and initial miss-distance, corrected
miss-distance is estimated to modify the area where vortex
dissipates.
 The variation of miss-distance for modification of the hybrid
method is obtained from right-hand figure by considering that
total vorticity is conserved.

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Concluding remarks
・ N u m e r i c a l s i m u l a t i o n o f AV I f o r
improvement of the hybrid method
 After 1st AVI, several vortices are induced
and complex vortex wake flow exists.
 When considering several vortices as the single
vortex, it is required to modify the vortex center
location and vortex circulation in the hybrid method.
 Fundamental insights necessary to modify
the hybrid method are obtained.
 Parametric study for validating the modified
hybrid method is performed for future work.

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Thank you for your attention