SITIS Archives - Topic Details
Program:  SBIR
Topic Num:  AF071-346 (AirForce)
Title:  Skin-Friction Measuring System for Large Wind Tunnels
Research & Technical Areas:  Air Platform, Sensors, Electronics

  Objective:  Develop a sensor system for measurements of the skin-friction distribution on wind tunnel models.
  Description:  The contribution of skin-friction to the total drag of a wind tunnel model is usually estimated from component measurements of the total drag and the integrated pressure drag. The distribution of the skin-friction force cannot be determined from such measurements, but requires measurements of the local shear-stress on the model. The measurement of the shear-stress distribution is required to evaluate the contribution of the individual components to the total skin-friction drag of a complex wind tunnel model. Accurate measurements of both pressure and skin-friction drag of the complete model is important for extrapolating model results to full-scale. The accurate measurements of the distributions are important in configuration development and CFD code validation. Pressure sensitive paints (PSP) are currently used to provide an accurate, high resolution distribution of pressure on a model surface that can be integrated to obtain the pressure component of the total drag for all or part of the model. A high productivity, low cost technique for measuring the distribution of shear-stress is required to expedite the development of future flight vehicles by identifying areas with high drag early in the test program, to determine boundary layer transition location, and to validate CFD codes that will be used to predict vehicle performance. The system must be robust enough to be operated in a large, production wind tunnel, and should require a minimal amount of time to prepare the model for testing. A coating similar to and compatible with PSP is desirable. MicroElectroMechanical Systems (MEMS) could provide a means for in situ calibration of the coating as well as a potential, interim solution that gives a coarse distribution of skin friction. The target facilities are wind tunnels which operate at Mach numbers from 0.2 to 2.0, total pressures from approximately 0.2 to 2 atmospheres, and total temperatures from ambient to 140 deg F. The acoustic levels in the tunnel are typically in the range of 145 dB, and vibration levels are high at frequencies less than 100 Hz.

  PHASE I: Demonstrate the feasibility of a skin-friction sensor system.
  
  PHASE II: Develop and demonstrate a prototype skin-friction sensor system.

  DUAL USE COMMERCIALIZATION: Military application: Potential military applications include incorporation of these systems into test facilities to meet test requirements for shear stress measurements. Commercial application: Skin-Friction Measuring System can be applied to the development of low drag automobiles, and any other industry that requires monitoring and/or improving the efficiency of fluid flow systems.

  References:  1. Rubesin, M.W., Okuno, A.F., Mateer, G.G., and Brosh, A. “Flush-Mounted Hot-Wire Gage for Skin Friction and Separation Detection Measurements.” 6th International Congress on Instrumentation in Aerospace Simulation Facilities, Ottawa, September 1975. 2. Morris, M.J., et al. “Aerodynamic Applications of Pressure-Sensitive Paint.” AIAA Paper 92-0264, 30th Aerospace Sciences Meeting, Reno, NV, January 1993. 3. Reda, D., et al. “Areal Measurements of Surface Shear Stress Vector Distributions Using Liquid Crystal Coatings.” AIAA Paper 96-0420, 34th Aerospace Sciences Meeting & Exhibit, Reno, NV, January 1996. 4. Mehregany, M., DeAnna, R.G., and Reshotko, E. “Microelectomechanical Systems for Aerodynamics Applications.” AIAA Paper 96-0421, 34th Aerospace Sciences Meeting & Exhibit, Reno, NV, January 1996.

Keywords:  Shear Stress, Skin Friction, Global Surface Measurements, Microelectromechanical Systems (MEMS)

Additional Information, Corrections, References, etc:
1. Is there a minimum time requirement we would like to see?

A minute or two per data point would not be an unreasonable goal for measuring the skin friction distribution for the full surface.

2. Are there any lighting or camera issues to be aware of? (UV, IR...)

Customers want to be able to see their model during testing; therefore, lighting that requires the model to be in total darkness in the visible spectrum is not acceptable. Also, the lights and cameras must be able to survive the pressure and temperature environment as stated in announcement.

3. What thickness is the coating limited to?

The thickness of the coating is not as important as the roughness of the finished surface. Ideally, the surface should not be rough enough to trip the boundary layer (Typically no discontinuities greater than 0.002 inches for the conditions anticipated).

4. Does the coating have to be removable?

Yes. The coating must be durable enough to survive several hours of measurements in the wind tunnel with minimal loss of material. However, it is desirable that the coating be easy to remove following the test so that there will be minimal damage to the model. Coatings that are removable with a solvent and light rubbing are more desirable than coatings that require sanding and/or scraping to remove.

5. Are oil drop/sheet methods feasible/acceptable?

While the oil drop idea is feasible and has been tested in the past, it is not the preferred method due to the lack of permanency from one test condition to another. Preferably, a coating installed similar to PSP is desired, so hours of testing can be completed with the same coating.
1. Is there a minimum time requirement we would like to see?

A minute or two per data point would not be an unreasonable goal for measuring the skin friction distribution for the full surface.

2. Are there any lighting or camera issues to be aware of? (UV, IR...)

Customers want to be able to see their model during testing; therefore, lighting that requires the model to be in total darkness in the visible spectrum is not acceptable. Also, the lights and cameras must be able to survive the pressure and temperature environment as stated in announcement.

3. What thickness is the coating limited to?

The thickness of the coating is not as important as the roughness of the finished surface. Ideally, the surface should not be rough enough to trip the boundary layer (Typically no discontinuities greater than 0.002 inches for the conditions anticipated).

4. Does the coating have to be removable?

Yes. The coating must be durable enough to survive several hours of measurements in the wind tunnel with minimal loss of material. However, it is desirable that the coating be easy to remove following the test so that there will be minimal damage to the model. Coatings that are removable with a solvent and light rubbing are more desirable than coatings that require sanding and/or scraping to remove.

5. Are oil drop/sheet methods feasible/acceptable?

While the oil drop idea is feasible and has been tested in the past, it is not the preferred method due to the lack of permanency from one test condition to another. Preferably, a coating installed similar to PSP is desired, so hours of testing can be completed with the same coating.

Questions and Answers:
Q: What are the deficiencies of the Luminescent Oil-Film method in application to the proposed testing technique?
A: The tunnels AEDC would operate the skin friction in are continuous-flow, meaning the air is recirculated. This makes the use of any fluid undesirable, as it affects the air quality, leads to additional clean up and can coat windows and equipment and enter into the compressor and other tunnel mechanical parts. While the quantity of fluid used in the Luminescent Oil-Film method is low, a cleaner way of measuring skin friction is more desirable. Secondly, with this method the model needs to be cleaned and oil reapplied if tested over a substantial time period, costing valuable test time.
Q: What are the deficiencies of the Luminescent Oil-Film method in application to the proposed testing technique?
A: The tunnels AEDC would operate the skin friction in are continuous-flow, meaning the air is recirculated. This makes the use of any fluid undesirable, as it affects the air quality, leads to additional clean up and can coat windows and equipment and enter into the compressor and other tunnel mechanical parts. While the quantity of fluid used in the Luminescent Oil-Film method is low, a cleaner way of measuring skin friction is more desirable. Secondly, with this method the model needs to be cleaned and oil reapplied if tested over a substantial time period, costing valuable test time.

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