Investigation of Pressure and Friction Force

Distributions on a Model Tail Wing Using

Pressure Sensitive Paint and Surface StressSensitive Film

S.D. Fonov, J.W. Crafton, L.P. Goss and G.E. JonesInnovative Scientific Solutions, Inc. 2766 Indian Ripple Road, Dayton, OH 45440

B.R.F. SchulzeEADS - Military Air Systems, 81663 Munich, Germany

Abstract This paper describes measurement methodologyand comparative results obtained using both thePressure Sensitive Paint (PSP) and novel Surface StressSensitive Film (S3F) techniques. A wind tunnel test wasperformed by EADS together with ISSI to investigatePSP at low speeds in order to compare absolute PSPpressures and S3F relative pressure data on a typicalhigh speed aircraft model. Furthermore the ability of theS3F film to sense wall shear stresses was demonstrated.

Measurements were conducted in the flow range of10-50 m/s, angles of attack of 100 and 300 and with twomodel configurations (with and without an air brake).

I. INTRODUCTION

The Pressure-Sensitive Paint (PSP) technique has beenapplied extensively by numerous research and industrialengineers for the study of high speed flows [1]. While thePSP technique has proven to be an effective tool in highspeed flows, its deployment to low speed wind tunnels hasbeen hindered by its inherent limited pressure sensitivity,usually not better then 10/0KPa, and its high temperaturesensitivity 0.1- lKPa/0C. One technique that has shownpotential as a low speed pressure sensor is Shear and StressSensitive Films (S3F) [2,3] The active element of the S3Ftechnique is a thin film made of an elastomer which deformsunder the application of a surface load. The pressure andskin friction measurement is accomplished by monitoringthe normal and tangential displacement (deformation) of theelastomer film surface and converting these displacementsinto surface loads.

The S3F shear modulus ,u and thickness h determine theresponse of the film to the pressure, Sp, and friction, Sf,, thespatial measurement resolution, 6, and the temporalresolution, I. It is possible to create films having a shearmodulus as low as 2OPa with thicknesses ranging from 1-micron to several millimeters. As described in [3], S3F is

mainly sensitive to pressure gradients where the film'spressure response can be roughly estimated as

Sp- 0.1gradPhhl u (1)The sensitivity of the film to the skin friction componentcan be estimated as

Sf -F*h/,. (2)The spatial resolution of the film is approximately the filmthickness h, while the response time can be estimated as thefirst shear resonance frequency for an elastic layer withdensity p given by:

f1 X tph (3)

One of the mayor advantages of this technique is thatthe sensitivity of the film to pressure and friction force canbe controlled in very large range 0.1.. 105 unlike thepressure paints employed in the PSP technique.

II. MODEL AND EXPERIMENTAL SETUP

The PSP (a two-channel temperature compensated FIBbased paint) and the S3F film were applied on the upperright and left surfaces of a model tail wing (see Fig. 1). Forincorporating the S3F the port tail plane was modified bymilling a flat cavity into its surface. In addition the tail planeconfiguration was covered directly with the film withoutany hardware modification on the port side while thestarboard side was painted with PSP. In this case S3Fcreated on black adhesive film was wrapped around leadingedge and covered about 7500 of wing surface. S3F thickness(including adhesive substrate was about 1mm. The S3Fsensitivity which is determined by the shear modulus andfilm thickness was adjusted for the dynamic pressure rangeof 100 - 1000 Pa of the flow which allowed quantitativemeasurements at speeds as low as 10 m/s.

1-4244-1600-0/07/$25.00 ©2007 IEEE.

The complete model was mounted on a rear stingsuspension which was installed in the open test section ofIm x 1 m Low Speed Wind Tunnel B of the TechnicalUniversity in Munich / Germany, which provided test timeand technical support to perform this investigation. Testruns were performed at speeds between 10 m/s and 50 m/sand AOA between 50 and 300

PSP and S3F data were acquired simultaneously usingtwo CCD cameras (PCO-1600) and LED array light sources.Filter switch was installed before "PSP" camera to acquirepressure sensitive and reference images.

The image acquisition and processing is similar to thatemployed in the standard PSP measurements. The outputpower of the LED arrays (2-Watt) was sufficient to achieve20-50% of the CCD dynamic range with an exposure timeof 30 to 100 ms. Imaging of the PSP required 256 framesaverages while imaging of the S3F required only a singleframe to provide good SNR.

Figure 1 Tail plane Model Configuration (Left - S3F; Right - PSP)

The PSP/S3F data acquisition and processing includesthe following steps:

* Image acquisition at "wind-off' and "wind-on"conditions for S3F plus additional acquisition ofreference images for PSP

* Alignment of wind-on to wind-off images using a

combination of cross-correlation and optical flowtechniques which provide information about sheardisplacement components for S3F images.

* Alignment of wind-on and wind-off referenceimages to wind-off sensitive image for PSP

* Subtraction of the bulk model displacements usingreference markers and cross-correlation/optical-flow technique for S3F images

* Image ratioing to provide information about thenormal displacement field (S3F) and pressure field(PSP)

* Spatial filtering to suppress noise due to the surfacemarker structure (S3F) and to increase SNR (PSP)

* Pressure field (PSP) and surface loadsreconstruction for S3F images

Shear displacement vectors were calculated in 32 by 32pixels interrogation windows periodically shifted by 16pixels in both directions which produced an array of 100 by80 data points. The combination of cross-correlation andoptical-flow techniques provides for a resolution in sheardisplacement measurement of 0.01 pixel with a standarddeviation of 0.02 to 0.03 pixels. These estimations are basedon measurements using test images with knowndisplacements. All image processing operations are realizedin "ProImage" package and "FreeFem++" software packageis used for surface load reconstruction.

III. EXPERIMENTAL RESULTS

All Measurements were conducted with two modelconfigurations: with and without an air brake (Fig.2)

Figure 2 Air brake installed near by tail wing having milled cavityfor S3F

Figure 3 displays PSP and S3F measurement results fora flow velocity range of 20 - 50 m/s. While the S3Ftechnique yields both pressure and skin frictionmeasurements, it requires a more complex data processingscheme than PSP. As mentioned above S3F imageprocessing provides the 3D displacement components of theelastic film on the model surface. To obtain pressure andfriction force distributions from the 3D deformations, it isnecessary to solve the 3D inverse problem. A simplified 2Dapproach was used in the current investigations. A strongvortex near the leading edge creates a pressure field with adominant pressure gradient in the direction perpendicular tomodel's leading edge. The 2D data processing approach canbe applied along this direction. A comparison of S3F andPSP measurement results is presented in Figs.4 and 5.Surface displacement data was taken along section A-A andconverted into surface loads. A comparison of the calculatedvariation in the Cp coefficients for S3F (blue line) and PSP(red line) is presented in Fig. 5 showing very goodagreement (excluding the region near by the leading edge).

1.5

100 200X, pixel

20m/s 40m/s 50m/sFigure 3 PSP (upper row - pressure) and S3F (middle row - normalcomponent of displacement field, lower row - magnitude of sheardisplacement field) data for flow velocity range 20-50m/s, angle ofattack 300.

a) b)Figure 4 PSP pressure field (fig. a, flipped and zoomed) and normalcomponent of surface displacement field (fig.b) for flow velocity

40m/s, AOA=300

PSP Pressure fields presented at Fig. 6 demonstratebrake influence for flow velocity 50m/s and angle of attack50. Brake creates local upstream pressure compressionregion and global modification of flow circulation aroundtail wing that is visualized as less decompression near byleading edge.

300 400

c)Figure 5. Comparison of S3F and PSP data in section A-A

.....~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~..... ....w::~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ...... ......Figure 6. PSP Pressure fields for "no-brake" (left) and "brake" (right)configurations, V=50m/s, angle of attack 50

This effect is more evident at higher angle of attack asit is shown at Figures 7 and 8. Decompression magnitudeunder leading edge vortex is about 4000Pa for "no-brake"configuration and 1 500Pa for "brake" configurations.

~~~~~~~~~~~~~~~.................

..............~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~............. .....

Figure 7. PSP Pressure fields for "no-brake" (left) and "brake" (right)configurations, V=30m/s, angle of attack 300

S3h provides quantitative data at flow velocity ItOmcs asit is shown at Fig 9. Presented there normal deformationfields are result of combined action of pressure gradient andfriction force under leading edge vortex.

1

2. S. Fonov, G. Jones, J. Crafton, V. Fonov, L. Goss, "TheDevelopment of Optical Techniques for the Measurement ofPressure and Skin Friction", Measurement Science andTechnology, 2006, v17, N. 6 pp. 1261-1268

3. S. D. Fonov, E.G. Jones, J. W. Crafton, and L. P. Goss. "UsingSurface Stress Sensitive Films for Pressure and FrictionMeasurements in Mini- and Micro-Channels" ICIASF2007Proceedings.

Figure 8. PSP Pressure fields for "no-brake" (left) and "brake" (right)configurations, V=50m/s, angle of attack 300

Figure 9. S3F normal deformation fields for "no-brake" (left) and "brake"(right) configurations, V=1Om/s, angle of attack 300

Further increment in flow velocity (Fig. 10) emphasizesbrake influence on global load distribution.

Figure 10. S3F normal deformation fields for "no-brake" (left) and "brake"

(right) configurations, V=30m/s, angle of attack 300

IV. CONCLUSIONS

it was demonstrated that SF provides possibility toextend quantitative flow visualization at low velocitieswhich are not reachable by currently available PSPformulations. More complex data processing scheme isprice for this challenge. Side by side PSP and 53Fmeasurements in overlapping flow parameters regionsdemonstrated good agreement even for simplified planardeformation model. One of the major advantages of the 53Ftechnique is possibility to tune sensitivity to the processesunder study especially those in which there are largepressure and shear force gradients.

REFERENCES

1. H. J. Bell, E. T. Schairer, L. A. Hand, R.D.Mehta, "SurfacePressure Measurements Using Luminescent Coating", Annu.Rev. Fluid Mech. 2001, v33, pp. 155-206