# Presentation: 014212

## Mitigation of Aero-Optical Effects and Subsonic Cavity Noise

- using Fluidic Actuators

- Jacob George
- Post Doc,  Caltech/OSU

- Collaborators:  Mo Samimy and Walter Lempert (Ohio State University
- 	          James Trolinger (Metrolaser, Irvine)

- Sponsor:  US AirForce

## Outline

- Research Motivation
- Experimental Apparatus and Technique
- Results (with and without flow control)
- Conclusions

## Research Motivation

- These aero-optical effects limit the performance of laser based and guided  systems.
- The goal is to understand the connection between the flow field and the optical degradation
- The basis of flow control is to either reduce the degradation as much as possible or to transform the flow into a more predictable state enabling the use of adaptive optics to remove the degradation
- To employ flow control to suppress cavity tones since they cause structural damage and fatigue
- Flow structure and wavefront distortions are analyzed with and without control

_**Research Motivation**_

- Planar Wavefront

- Aberrated Wavefront

## Experimental Apparatus and Technique 

_**Experimental Apparatus and Technique **_

- Test section area : 3” (span) by 1.5” (height)
- Cavity Dimensions: Span (3”), Depth (0.5”), 					Length variable from 2” to 3”
- Test section side-walls, ceiling and cavity bed have Fused Silica windows for easy optical access
- Ports on cavity bed for dynamic pressure measurement
- Flow diagnostics using Schlieren imaging and PIV
- Flow measurements in conjunction with wavefront interferograms using  (includes interferograms, Strehl ratio, etc.)

_**Aero-Optics Wind Tunnel**_

## Simultaneous Flow and Wavefront Measurements in Mach 0.7 Cavity Flow 

- Simultaneous measurements of flow and wavefronts are made using pressure transducers, Particle Image Velocimetry (PIV) and a wavefront sensor that utilizes the principle of phase shifting intereferometry

## Slide 6

## Flow Control implementation using Hartmann Tube Fluidic Actuator (HTFA)

- Plenum

- Plenum

- Air Inlet

- Nozzles

- Spacer

- Resonator
- tube

- Exit for the pulsating jet

- Cavity LE

- Nozzles

- Resonator Tube

- Cavity LE

- Spacer

- _Perspective View_

- _Sectional View_

- Two HTFA assemblies were used – one that delivers medium frequence forcing and other that delivers high frequency forcing.

## Definitions and Measured quantities

- Pressure Field:  Sound pressure level in dB
- Flow Field:  Streamwise velocity (U) in m/s
-                     Vorticity (Ω) in 1/s, Ω =
- Wavefront:  RMS Wavefront variance in wavelengths

- Optical Path Length (OPL):

- In practice: Optical Path Difference (OPD):

- Strehl’s ratio,        =

- Where,         is max. light irradiance for aberrated system, and
-                     is max. light irradiance without aberrations

- and

- is the RMS wavefront variance

## Results

_**Results**_

- Vorticity contour of roll-up of vortices during one cycle

## Results

_**Results**_

- Interferograms of roll-up of vortices during one cycle

## Results

_**Results**_

## Wall Pressure Spectra - Effects of Flow Control

- Strong Resonance Mode (L=1.90”): Baseline and Flow Control

-           Mode: 5.59 kHz

- Forcing leads to 18 dB
-    peak suppression

- Moderately Strong Resonance Mode (L=1.90”): Baseline and Flow Control

-           Mode: 5.69 kHz

- Forcing leads to 14 dB
-    peak suppression

## Conclusions

- The ability to simultaneously measure flow structure and the resultant wavefront distortion has been achieved which is in itself a state of the art
- Observations show a distinct correlation between the large scale structures and wavefront distortions
- The effects of different forcing cases on various cavity resonant modes in high subsonic flows yields significantly less wavefront distortions.
- The Hartmann Tube Fluidic Actuators have been shown to be successful in obtaining significant reduction in the cavity resonant tones
- Efforts are ongoing towards further quantifying the relationship between the flow structure and the wavefront distortions