Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Rotorcraft
Aeroacoustic
Prediction
using Momentum Source Model in an
Overset Computational Fluid
Dynamics Framework
Mohanamuraly Pavanakumar
MS student
Aerospace Engineering
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Outline
Background/Motivation
Introduction/Goals
Acoustics Theory
Preliminary Study
Approach
Overview
Finite Volume Formulation
Momentum source term
Overset grid
Preliminary Results
Conclusion and Work in progress
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Background

Introduction
Complex, unsteady flow
field of Heavy Lift
configurations lead to
high noise levels
Aerodynamic
interaction (BVI,
fuselage, wings, etc) are
difficult to foresee
during design
In military application
noise level determines
the operable distance
from enemy territory
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Background

Introduction
Noise calculation should be integrated into design
to address problems early
Need for fast acoustic prediction method with
ability to simulate a wide range of fluid physics
Two potential codes already in use/development for
design level calculations of rotorcrafts
Rot3dc
©
(
Rajagopalan
et al, ISU)
Stretched Cartesian structured grid framework
Incompressible finite volume formulation
Rotor represented by time averaged Momentum disk
Disadvantage of
losing individual blade information
IBSEN
(
under development by Christopher
Hennes
PhD student)
O
verset grid framework (equally spaced grids)
C
ompressible Euler flow solver
Immersed boundary condition for wall treatment
Requires
more computation
time due to higher fidelity
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Background

Goals
Noise prediction of full Heavy

Lift rotorcraft
configuration for design and validation
purpose
Use existing CFD code base and add
modifications whenever necessary
Ability to predict Aerodynamic interaction
of appendages with blades, an important
source of noise in heavy lift configurations
Typical run time for a full configuration
(including setup time) to be a few days
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Thickness
displacement of fluid
generates sound
Loading
accelerating force distribution
generates sound
(includes BVI noise)
Quadrupole
All volume sources,
non

linear effects
nonuniform sound speed
M > 1
Background

Acoustics Theory
PSU

WOPWOP solves the
Ffowcs

Williams
Hawkins (FWH) equation using Formulation
1A of
Frassat
f
= 0
describes the
integration surface
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Background

Acoustics Theory
Chord

wise integrated blade load or compact load
are used for the loading noise calculation
CFD is used to predict the blade loads
Time rate of change of the loads generate noise
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Background

Preliminary study
Rot3dc code was used to make acoustic
prediction of rotorcraft configurations by
coupling with PSU

WOPWOP
Rotor configurations simulated
Isolated Rotor in Hover [1]
CH47 tandem rotor configuration
Discuss results from simulations and assess
the use of current version of Rot3dc for
Aeroacoustic
computation
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Background

Preliminary study
Isolated Rotor in Hover
Rotor Properties
[1]
Blades
2
Collective (experiment)
11 deg
Tip Speed
122
m/s
C
T
value (trim)
0.00708
(a)
(
b
)
(
c
)
10D
10D
10D
Mean
Normalised
Load (MNL)
MNL = 1
–
Load at (
r,Ψ
)
Average Load
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Background

Preliminary study
CH

47 tandem rotor
Thrust Distribution Baseline Grid
(25 points across rotor)
Thrust Distribution Fine Grid
(45 points across rotor)
Rotor Properties
Blades
3
Collective
10.8 deg
Tip
speed
215
m/s
Rotor
Radius
9.14
m
Rotor 1
Rotor 2
Thrust of Individual Isolated Rotors
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Background

Preliminary study
CH

47 tandem rotor (Acoustics)
Rotor 1
Rotor 2
(A)
(
B
)
(
C
)
10D
1)
Noise generated by the numerical
error is comparable to that of the
rotor wake interference
2)
The frequency of this noise lies in the
range of the BPF and first few higher
harmonics and cannot be filtered
from the spectrum
3)
There is a large uncertainty
associated with grid density near the
rotor and noise, especially, if rotor is
not aligned with the grid
4)
The grid density requirement, as
indicated by the Isolated rotor cases,
are too large to be simulated as serial
computation for full configuration
cases
10D
10D
Top view
Side view
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Immersed Boundary Solver for Environment Noise
Grid points do not lie on body surface
Surface boundary condition must be applied to points
closest to surface location
Overset grids are used to control changes in grid spacing
Automatic grid splitting and load balancing
Background

Preliminary study
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Background

Preliminary study
Current simulation time ~7 days/rev on 24
processors against our goals of 1 or 2 days
Rot3dc can produce results in the given time but
suffers from numerical issues that affect noise
calculations
Also, time averaging removes important
transient information in the blade loads
In addition, Rot3dc is a proprietary code and not
amenable to modifications
The ideal way is to combine the advantage of
both methods and build a new layer in IBSEN to
model rotor as momentum sources similar to
Rot3dc
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach

Overview
Observer
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach

Overview
Implemented as a
separate solver
module in IBSEN
Line source model
for rotor blades
Compressible Euler
Solver but in Finite
Volume framework
Complete overset +
parallel support
Finite Volume Solver
Inflow
(Actuator line)
Trim Code
2D sectional load
(per unit span of
blade)
Blade Element Code
Source Term
V=V(r,θ,z)
Flapping
Pitching
LeadLag
Momentum Source Unstructured Finite Volume Solver
Tip Correction
IBSEN
Cartesian Grid Solver
Overset Boundary
data exchange
Regularization
&
Volume Integration
f
S
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach

Overview
Explicit Finite Difference and structured finite
volume schemes have numerical stability issues
with
centerline singularity
of cylindrical grids
Unstructured Finite volume methods have good
stability characteristics but poor dispersion and
dissipation characteristics
A tradeoff is made here as the blade are mostly
modeled and the acoustic prediction is done using
load rather than flow field
Unstructured grids generated from a structured
cylindrical grid are used to enclose the rotor
sources
Polyhedral cells
are used at centerline to remove
possible numerical instability due to highly
skewed finite volumes
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Finite Volume Formulation
Governing Equations
Rotor Momentum Source term
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Finite Volume Formulation
Finite Volume Formulation
The interfacial fluxes are evaluated using
the left (L) and right (R) cell state
(Roe’s approximate Riemann solver)
Roe average
Jacobian
matrix
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Momentum Source Term
Rotor Source Term
Blades represented by volumetric body force
term in the governing equation and dependent
on the force generated by the blade (F)
Line sources are continuous along the span but
impulse function along chord and thickness
direction similar to compact load
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Momentum Source Term
Line sources generates induced velocity
when introduced into the fluid
Additional work done term should be
added to satisfy energy equation
Final form of source term
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Momentum Source Term
Gaussian functions with prescribed
spreading distance are used to smooth the
source along the thickness and chord
direction to remove singularity
Smoothed/Regularized source term
Standard deviation of the Gaussian function
Location of the source in the Chord/Thickness
direction
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Momentum Source Term
The load per unit blade span is obtained
using the Blade Element Theory
–
given
blade properties, angular, forward flight
and induced velocity
The induced velocity is obtained from the
CFD flow field, with zero induced velocity as
initial condition
A trim loop can be added to correct the
blade parameters for a particular flight
condition

currently blade parameter are
fixed
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Momentum Source Term
Rotor Properties
[1]
Blades
2
Collective
11 deg
Omega
52
rad/s
Domain
Radius
1.3 rotor
radius
Domain
Thickness
2 blade chord
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Overset Grid
The flow field information of the rotor (source)
grid communicated via the overset boundary
Existing Tri

Linear interpolation in IBSEN was
designed and tested for Cartesian grids, which
are isotropic
Cylindrical grids are fine near the center and
coarser away from the center

anisotropic
The interpolation was tested before
implementation in the Unstructured Finite
Volume Solver
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Overset Grid
Interpolation Test function
Test Geometry Setup
Grid and Test Function
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Approach
–
Overset Grid
Overall error reduction from 2.3% to 0.5%
Trilinear Interpolation
Weighted Least Squares
Interpolation
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Preliminary Results
Isolated Rotor in Hover (overset grid setup)
Two inner most grid in the overset grid setup
Side View
Top View
Grid Information
Extent of
Grid along X
10D
Extent of
Grid along Y
10D
Extent of
Grid along Z
10D
Spanwise
resolution
0.1m
Azimuthal
resolution
5
o
FV Grid
Thickness
0.5c
Total number of FV
Faces
87711
Total number of Nodes
1.2
e6
Scheme
order
1
Z
Y
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Work Underway
Interface the finite volume solver with
IBSEN parallel overset grid framework
Couple CFD output to PSU

WOPWOP to
calculate noise
Blade Element Calculation for forward flight
Minimal Rotor Trim Code
Polyhedral cell data output for visualization
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Work Underway
Cases
Planned
Isolated
Rotor
case
from
NACA
report
of
Rabott
et
al,
NACA

TN

3688
,
1956
ROBIN
Fuselage/Rotor
interaction
–
Laser
Velocimetry
Measurements
of
Elliot,
Althoff
and
Sailey
,
NASA

TM

100542
Vol
.
1
,
1988
Higher
harmonic
control
Acoustics
Rotor
Test
–
II
(HART

II),
Baseline
forward
flight
case
(comparison
of
inflow
from
PIV
and
acoustics),
2002
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
Summary
Mohanamuraly Pavanakumar,
Dept. of Aerospace Engineering, Penn State
References
Enter the password to open this PDF file:
File name:

File size:

Title:

Author:

Subject:

Keywords:

Creation Date:

Modification Date:

Creator:

PDF Producer:

PDF Version:

Page Count:

Preparing document for printing…
0%
Comments 0
Log in to post a comment