Requirements for BVI Noise Study
Sponsored by WHL
Romuald Morvant
Computational Fluid Dynamics group
University of Glasgow
Helicopter Study Weekend,
the Burn, April 2002
Summary
1

Previous progress
2

Vortex generation

Requirements
3

Review of CAA methods

Requirements
4

Conclusions
5

Further work
Review of the possible high

order schemes
Previous Work
Implementation of the UNFACtored method
Debugging of the 3D CHIMERA mode
=>RESULTS
7A Model Rotor
Results on a Coarse Grid & CHIMERA
Pressure and Normal force coefficients
CHIMERA Grids
HOVER test case:
Incorrect solutions on CHIMERA grid
Load distribution, Normal force coefficient

Reason? => vortex located in the overlapping
area (lower order of accuracy)

Solutions: Froude conditions?
Boundary conditions?
Numerical scheme?
Results
TEST Cases
SOLUTIONS
GAIN
LANN
Wing
0.25
ONERA
FORWARD nonlifting
0.95
2bladed
7A
Transonic HOVER flight
4bladed
Single grid
0.23
Chimera grid

Current Work
Aeroacoustic objective => BVI study
Generation of a vortex in EROS and PMB
Use of the ICEM package to generate the grid
Transformation of the grid
EROS

Vortex Generation
470
200
2 C

H grid, 8 chords length
EROS

Vortex

Airfoil Interaction
EROS

Decay in Vortex Strength
PMB

Vortex Generation
UNIFORM grid
Fine H

grid:
1000 x 135 x 2

> test for
the vortex convection
PMB

Vortex Convection
PMB

Decay in Vortex Strength
PMB

Pressure Evolution at the Vortex Core
CYCLE

>
Preserving Vortex Characteristics
HIGH

ORDER ACCURATE SCHEME
Convection of the vortex (less points/wavelength)
PB of dispersion for any scheme
FINE & SMOOTH GRID:
Capture the small scales
Memory requirements

> Necessity of parallelisation

> Use of PMB and test of the chosen implemented
scheme on PMB (cf: Unfactored method)
CFD Methods for CAA

Complicated Flow Field, strong sound directivity

Thickness and load noise are well predicted

Blade vortex interaction noise less well predicted
Near

Field Methods
CAA is currently moving towards CFD especially
for near

field predictions

> Computation of the fluid

flow mean motion

> Computation of the source of noise

> Capture of the flow non

linearities
Far

Field Methods
1

Kirchhoff method
Choice of the Kirchhoff surface
Calculation of the overall noise level
2

FW

H
Computation of the quadruple noise
Noise mechanisms are decoupled
Dispersion Relation
for
centered
schemes
V=d
w
/dk, dispersion velocity
V=d
w
/dk, dispersion velocity
Dispersion Relation
for upwind biased schemes
Discontinuous case
High Order Schemes & Discontinuities
Smooth case
One

Dimensional Case
CAA Requirements
High resolution between scales of the flow

> Low dissipation
•
High

order numerical scheme
with the appropriate grid's size
•
Representative flow

field data from CFD
Use of the Kirchhoff formulation
Faster calculations for the FW

H method
Summary of Current Efforts
Simulation of a vortex

airfoil interaction
Large dissipation of the vortex (larger near the airfoil)
Incorrect convection of the vortex
High

order accurate in space scheme
Grid refinement
Test: aeroacoustic calculations
Further Work
1) Implementation of an aeroacoustic module:
Surface extractor in PMB
Use of the Kirchhoff method
Test for simple test cases (ball & Gaussian source)

> Being tested
2) Choice of the high

order scheme and implementation
Further Work
Check of the capabilities of the solver
for the vortex generation
Use of the CFD data to get the frequency at which
the BVI occurs for a documented test case
Check the quality of the generated vortex
Check the capabilities of the schemes
Refine the domain so as to get good results
HIGH

ORDER SCHEMES
How to get higher

order of accuracy
over large period of time?
TIME

Explicit
Stability: Dx~=Dt

> non physical

Implicit
Smaller dispersion error, more effective when
coupled with high

order accurate in space

> Implicit 4
th
order exists (midpoint rule)
PB: the linearisation of the convective terms (2
nd
order)
How to get higher

order of accuracy
over large period of time?
SPACE : highly accurate spatial derivative of the flow (few
points/wave length)

> high

accuracy finite difference method:

Explicit scheme (large stencil)

Compact scheme (smaller stencil, use of the
derivative of neighbouring points)
High

order accurate scheme
1) Fourier series analysis
Spectral method
Dispersion Relation Preserving scheme (DRP)
Good quality wave resolution
Improper capture of wave discontinuities (central)
Not suitable when Mach increases

> Use smoothing techniques
High

order accurate scheme
2) Truncated Taylor Series Method
ENO, WENO, WCNS

> Explicit

> Under development (FV)
2

4 MacCormack

> Explicit & Implicit

> Central (unpublished papers)
Compact

>Explicit & Implicit

> More mature
damping scalar: solution less physical
filtering: high

order filter
Compact Schemes
CENTRAL:
they can be spectral

like
spurious oscillations (smoothing terms)
UPWIND:
dissipative behaviour appropriate at/near Bs
dissipative error (highest wave numbers)
Difficulties:

it requires smooth meshes

it must be adapted to the Bs

very sensitive to BCs on coarse grid
Compact Scheme
AEROACOUSTIC
Resolution in the wave number domain:

> Advantage of the filtering

> Computational cost: not much, truncation error
decreases and the number of required points near
the Bs diminish
Proposed Schemes
TIME:

implicit 2nd order accurate
SPACE:

3rd order upwind

biased in space

4th central

differences scheme
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