Rotorcraft Aeroacoustic Prediction

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22 Φεβ 2014 (πριν από 3 χρόνια και 8 μήνες)

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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
Lead-Lag
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