Aerodynamic measurement of high-speed base flows

crookedjourneyMechanics

Oct 24, 2013 (3 years and 5 months ago)

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Aerodynamic measurement
of high
-
speed base flows

Background

The

separated

wake

at

the

base

of

cylindrical

bodies

presents

a

number

of

challenges

to

aerodynamic

designers

dealing

with

similar

geometries,

such

as

in

missiles

or

rocket

launchers,

due

to

the

unsteady

and

three
-
dimensional

character

of

the

wake
.


Likewise,

the

measurement

of

this

flow

is

equally

challenging

and

requires

the

application

of

three
-
dimensional

measurement

techniques

capable

of

operating

at

the

high

speeds

characteristic

of

these

problems
.


This

topic

therefore

concentrates

on

two

parallel

problems
:

first,

how

to

characterize

the

unsteady

aerodynamic

phenomena

occurring

in

the

flow
.

Once

a

series

of

metrics

are

defined,

the

second

task

is

how

to

extend

current

measurement

techniques

to

enable

measurements

in

this

regime
.

Progress and
Objectives

The progress in the first two years of the PhD was an
understanding of PIV and its extension to three
-
dimensional flows: tomographic PIV. As part of this
process, the concept of fluid trajectory correlation (FTC)
evolved and became a major focus of my investigations as
a means to increase the accuracy of measurements
performed using tomographic PIV.


With the principles of FTC established, the current year has
focused on the implementation of a tomographic PIV
system suitable for high speed flows. The approach
adopted by our group involves to use of a large array of
inexpensive cameras which provides a unique ability to
acquire four images at a speed which is suitable for high
speed flows, and additionally provides enough views of the
base of the model to describe the full three
-
dimensional
motion.


The upcoming year is focusing on the application of this
system to low speed experiments and high speed
experiments to examine the underlying fluid dynamics of
the base flow problem. This will culminate in a series of
publications taking advantage of the unique measurement
capabilities available with such a camera array and with
the uniquely developed algorithms. It should be stressed
that the techniques developed in this work are also
applicable to many other high
-
speed measurement
scenarios and may find use in additional laboratories in the
future.

Publications

-

K
. Lynch, F. Scarano “Enhancing the velocity dynamic range and accuracy of time
-
resolved PIV through fluid trajectory correlation
.” 16th
Int

Symp

on Applications of Laser Techniques to
Fluid Mechanics. Lisbon, Portugal, 09
-
12 July, 2012

-

K
. Lynch, F. Scarano “A high
-
order time
-
accurate interrogation method for time
-
resolved PIV,”
Meas

Sci.
Technol

24:035305.


Base Flow Aerodynamics

As flow encounters a base geometry, it rapidly separates to
form a mixing layer separating the free stream flow from
an inner recirculation region. Proceeding downstream, the
mixing layer grows and approaches the wall.
T
he mixing
layer proceeds to impinge on the wall, leading to
reattachment.


A simplified depiction of this is provided in the above
figure; in reality, the impingement of the mixing layer on
structures in the
afterbody

is highly unsteady, leading to
high stress and the possibility of fatigue for mechanical
devices in this region. Besides the mixing layer, the
recirculation bubble also exhibits unsteady dynamics which
contribute to loading imbalances at the base, requiring
additional control authority to maintain stability. Ultimately,
the understanding of these aerodynamic phenomena can
allow control schemes to be implemented to improve
launcher performance.


High
-
Speed,

Three
-
Dimensional

Measurement

Techniques

A

primary

complication

of

the

unsteady

aerodynamic

phenomena

is

the

unsteady

loading

occurring

in

the

base

and

reattachment

region
.

This

suggests

that

linking

the

flow

phenomena

to

the

pressure

along

the

body

can

provide

insight

on

the

flow

structures

or

modes

responsible

for

undesirable

effects
.

One

method

for

measuring

the

flow

field

is

through

particle

image

velocimetry

(PIV),

which

measures

the

motion

of

particle

tracers

suspended

in

air

and

illuminated

at

multiple

times
.

The

particle

images

are

captured

in

cameras

and

analyzed

to

yield

dense,

instantaneous

information

on

the

flow

field,

as

shown

below
.

























PhD
Candidate:
Kyle Lynch

Department
:
Aerodynamics

Section: Aerodynamics

Supervisor:
F. Scarano

Promoter: F. Scarano

Start date:
07
-
07
-
2011

Funding:
FLOVIST / AFDAR


Aerospace

Engineering

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=

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D
𝐮
Dt
+
𝜇
𝛻
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Ariane

5 launcher with separated flow region
highlighted and shown in additional detail.
From Deck
et al.
Unsteadiness of an
axisymmetric separating
-
reattaching
flow: Numerical
investigation

The

connection

between

flow

field

information

and

pressure

field

information

can

be

derived

from

the

flow

governing

equations,

e
.
g
.
,




Which

relates

the

material

acceleration

to

the

pressure

gradient

field
.

The

pressure

gradient

field

can

then

be

integrated

to

yield

the

pressure
.

This

equation

requires

the

acceleration

to

be

measured,

which

is

not

typically

provided

in

a

PIV

experiment
.

Thus,

we

have

extended

traditional

PIV

by

acquiring

four

images

of

data,

and

developing

unique

algorithms

to

determine

the

acceleration

of

the

fluid

throughout

the

images
.

This

specialized

algorithm,

Fluid

Trajectory

Correlation

(FTC),

predicts

the

position

of

particles

in

time,

and

corrects

the

prediction

such

that

it

tracks

more

accurately

the

true

fluid

trajectory
.
,

as

shown

below
.

By

then

parameterizing

this

trajectory

into

polynomial

functions,

the

acceleration

can

easily

be

determined

through

differentiation
.




Example of predictor
-
corrector used in FTC algorithm.

Velocity field as determined from a PIV

experiment on
a base flow model.

12
-
camera array for acceleration measurements in high
speed flows.

Camera array performing measurements of the
azimuthal structure of the wake in a low
-
speed test.