Density Field Measurements of a Supersonic

Impinging Jet with Microjet Control

L. Venkatakrishnan#, Alex Wiley*, Rajan Kumar*,

Farrukh Alvi*

*Advanced Aero Propulsions Laboratory (AAPL)

Florida Center for Advanced Aero Propulsion (FCAAP)

Florida A&M University and Florida State University

# National Aerospace Laboratories (NAL)

Bangalore, India

Presentation Outline

• Impinging Jet Flowfield

• Current Study

• STOVL Facility

• BOS Technique

• Experimental Results

• Conclusions

• Future Work

4/11/2012 1

Impinging Jet Flowfield

4/11/2012 2

• Resonance-Dominated Flow

• High Amplitude Unsteadiness

• Feedback Loop

• Sonic Fatigue of Aircraft

• Lift Loss

Impinging Jet Flowfield

4/11/2012 3

Shadowgraph of a supersonic impinging jet.

Nozzle Exit with Thin Shear

Layer

Reflected Acoustic Wave

Strong Acoustic Wave

Large-Scale Vortical Structure

Wall Jet

Impinging Jet Flowfield

4/11/2012 4

100

101

90

100

110

120

130

140

150

Freq (kHz)

SP

L (

dB

; re

: 20

Pa)

Representative Unsteady Spectra

Nearfield Mic• High Broadband Levels

• Strong Impinging Tone

and Harmonics

• Characteristic of an

Impinging Jet

Microjet Control

4/11/2012 5

NO

CONTROL

Microjet

Streaks

MICROJET

CONTROL

Shadowgraph of Impinging Jet with and

without Microjet Control

16 Microjets

Angled 60°

Objectives of the Current Study

4/11/2012 6

1. Unanswered questions remain.

1. Why do microjets work?

2. Their effects on the mean density field of an impinging jet.

3. Free jet effects?

Can we answer some of these questions by

learning more about the density field?

Density field to be measured using

Background Oriented Schlieren

STOVL Facility

4/11/2012 7

• Primarily used to study

the flowfield of a

supersonic impinging jet

with applications in

STOVL aircraft.

• Blowdown facility

• Ma=1.5 C-D Nozzle

• Inline Heater

• Nozzle-to-ground

distance (h) may be varied

between 2-40d (d=Nozzle

Throat Diameter).

BOS (Experimental Setup)

4/11/2012 8

Light

Source

Randomly Oriented Illuminated

Dot-Pattern Background

Optical Disturbance, i.e.

the Flow

Camera

• Experimental Setup for BOS requires

minimal hardware.

• Quantitative

• Low Cost

• Simple to Setup

BOS (Experimental Setup)

4/11/2012 9

Lift Plate and Nozzle

Dot Pattern Background

LED Cluster Light

Source (IDT)

Camera

BOS – Raw Images

4/11/2012 10

Raw Image – NO FLOW Raw Image – WITH FLOW

Cold, Mildly Underexpanded (NPR=3.8 for a M=1.5 Nozzle)

Free Jet

BOS Requires two images. One reference image recorded

without the flow and one measurement image recorded with

the flow.

BOS - Processing

4/11/2012 11x/d

y/d

-2 -1 0 1 2-4

-3

-2

-1

BOS - Processing

4/11/2012 12

Interpreting BOS Data

U

V

)(1

Gn

Dale-Gladstone Relation

x y

ZZ

ZZi

x

x

n

ni

1

Venkatakrishnan & Meier, 2004

BOS – Finding Density

4/11/2012 13

x y

),(2

2

2

2

yxSyx

• Integrate in either the x or y-

directions

• Leads to noise in the

direction of integration

• Transform to the Poisson

Equation and Solve for r, the Line-

of-Sight Integrated Projected

Density Field

),(2 yxS

orProjected Density Field

Back-Projection Tomography

4/11/2012 14

Central Plane of the Mildly

Underexpanded Cold Jet

Procedure for BOS

1. Calculate the density gradient field

using PIV style cross-correlations

between a no flow image and an image

with the flow.

2. Find the projected density field by

solution of the Poisson Equation.

3. Use Filtered Back-Projection

tomography to reconstruct the 3D

density field of the flow based on the

assumption of symmetry about the

axis.

See Venkatakrishnan and Meier, EIF 2004 for more details

x/d

y/d

-2 -1 0 1 2-4

-3

-2

-1

Free jet Results

Effects of Microjet Injection

4/11/2012 15

Significant Reductions in the magnitude of the shock cells when microjet control is applied.

No ControlMicrojet

Control

Impinging Jet Results – Baseline Flow

4/11/2012 16

Simulated Bidirectional Schlieren

• Generated using the raw density

gradient data.

• The effects of unsteadiness are

manifested as a blurring of the shear layer

near the impingement region for time-

averaged data set (250 image pairs).

x/D-2 -1 0 1 2

h/D

0

1

2

3

4

amb:0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Central Slice Density Field

•Large extent of high density in the impingement region.

X

Y

-400 -200 0 200 400

-600

-400

-200

0

Density Gradient Field

h/D=4.0

Impinging Jet Results – Microjet Control

4/11/2012 17

x/D

h/D

-2 -1 0 1 2-4

-3

-2

-1

0

Simulated Bidirectional Schlieren

with Microjet ControlDensity Gradient Field with

Microjet Control

Impinging Jet Results – Microjet Control

4/11/2012 18

x/D

h/D

-1 -0.5 0 0.5 1-2

-1.5

-1

-0.5

0

x/Dh

/D

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1-2

-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

• Microjet Injection significantly alters the density field at the nozzle exit.

• The mild shock structure is disrupted beyond recognition when microjet control is

applied.

Density Gradient Field Near the Nozzle Exit with and without Microjet Control

No Control With Control

x/D-2 -1 0 1 2

h/D

0

1

2

3

4

amb:0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

x/D-2 -1 0 1 2

h/D

0

1

2

3

4

amb:0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Baseline

Impinging Jet Results – Microjet Control

4/11/2012 19

• The extent and magnitude of high density in the impingement region is greatly reduced.

• There is a low density region behind the stand-off shock when microjet control is

applied.

Central Slice Density Near the Nozzle Exit with and without Microjet Control

Microjet

Control

MICROJET

CONTROL

Conclusions

Impinging Jet

4/11/2012 20

Magnitude of the density field is

much lower in the presence of

microjet control for the impinging jet.

x/D-2 -1 0 1 2

h/D

0

1

2

3

4

amb:0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

x/D-2 -1 0 1 2

h/D

0

1

2

3

4

amb:0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Reflected in the Unsteady Pressure

Measurements

Conclusions

4/11/2012 21

Significant changes in the free jet as

well.

The differences are NOT seen in the

acoustics.

Future Work

4/11/2012 22

• Reconstruction of the instantaneous

density field, requires multiple

simultaneous views.

• Examining the density field variation

for other impinging jet conditions: TR,

h/D, NPR, etc…

• Simultaneous density and velocity

field measurements using BOS.

Thank You

4/11/2012 23

Questions

Line-Integrations

4/11/2012 24

X-Direction Integration Y-Direction Integration