An improved sensing element for skin-friction balance measurements

Allen, Jerry M.

doi:10.2514/6.1980-49

Cited by 17 publications

(24 citation statements)

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“…It has long been known that the floating head geometry is important to a successful direct measuring skin friction gage, leading to several studies of the effects of floating head geometry on measurement uncertainties 33,34 . Some of the more notable work was done by Allen 19,20 in the 1970's. He identified important floating head geometric parameters and studied their effects, including the effects of misalignment of the floating head, on the error in direct skin friction measurements.…”

Section: Floating Head Design Attributes and Concernsmentioning

confidence: 99%

“…Allen found that while a perfectly aligned floating head would have minimal error, using optimal ratios of the different geometric parameters could reduce the errors caused by misalignment 20 . Specifically, his results indicated that misalignment errors were reduced with increasing gap-to-diameter (G/D) ratio.…”

Section: Floating Head Design Attributes and Concernsmentioning

confidence: 99%

“…A schematic of his device is shown above in Figure 5. The work that followed, including that of Allen 19,20 , Voisinet 21 , and Roensch and Cadwell…”

mentioning

confidence: 99%

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Development and Ground Testing of Direct Measuring Skin Friction Gages for High Enthalpy Supersonic Flight Tests

Smith

2002

22nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference

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Aerospace Engineering (ABSTRACT)A series of direct-measuring skin friction gages were developed for a high-speed, high-temperature environment of the turbulent boundary layer in flows such as that in supersonic combustion ramjet (scramjet) engines, with a progression from free-jet ground tests to a design for an actual hypersonic scramjet-integrated flight vehicle. The designs were non-nulling, with a sensing head that was flush with the model wall and surrounded by a small gap. Thus, the shear force due to the flow along the wall deflects the head, inducing a measurable strain. Strain gages were used to detect the strain. The gages were statically calibrated using a direct force method. The designs were verified by testing in a well-documented Mach 2.4 cold flow. Results of the cold-flow tests were repeatable and within 15% of the value of C f estimated from simple theory. The first gage design incorporated a cantilever beam with semiconductor strain gages to sense the shear on the floating head. Cooling water was routed both internally and around the external housing in order to control the temperature of the strain gages. This first gage was installed and tested in a rocket-based-combined-cycle (RBCC) engine model operating in the scramjet mode. The free-jet facility provided a Mach 6.4 flow with P 0 = 1350 psia (9310 kPa) and T 0 = 2800 °R (1555 °K). Local wall temperatures were measured between 850 and 900 °R (472-500 °K). Output from the RBCC scramjet tests was reasonable and repeatable. A second skin friction gage was designed for and tested in a wind tunnel model of the Hyper-X flight vehicle scramjet engine. These unsuccessful tests revealed the need for a radically different skin friction gage design.The third and final skin friction gage was specifically developed to be installed on the Hyper-X flight vehicle. Rather than the cantilever beam and semiconductor strain gages, the third skin friction gage made use of a flexure ring and metal foil strain gages to sense iii the shear. The water-cooling and oil-fill used on the previous skin friction sensors were eliminated. It was qualified for flight through a rigorous series of environmental tests, including pressure, temperature, vibration, and heat flux tests. Finally, the third skin friction gage was tested in the Hyper-X Engine Model (HXEM), a full-scale-partial-width wind tunnel model of the flight vehicle engine. These tests were conducted at Mach 6.5 enthalpy with P 0 = 555 psia (3827 kPa) and h 0 = 900 Btu/lb m in a freejet facility. The successful testing in the wind tunnel scramjet model provided the final verification of the gage before installation in the flight vehicle engine. The development, testing, and results of all three skin friction gages are discussed.

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Section: Floating Head Design Attributes and Concernsmentioning

confidence: 99%

Section: Floating Head Design Attributes and Concernsmentioning

confidence: 99%

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Development and Ground Testing of Direct Measuring Skin Friction Gages for High Enthalpy Supersonic Flight Tests

Smith

2002

22nd AIAA Aerodynamic Measurement Technology and Ground Testing Conference

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“…These design concerns include misalignment and geometry of the floating head 68,69,70 , surface tension of the fluid in the gap introducing spurious forces 71,72 , and pressure gradient effects 72,73 . Allen 69 performed extensive studies in supersonic flows of the errors for various physical gap sizes around the head, head diameters, size and shape of the floating element lip, and protrusion or recession of the head, as depicted in Figure 49.…”

Section: Pressure Gradient Effectsmentioning

confidence: 99%

“…Allen 69 performed extensive studies in supersonic flows of the errors for various physical gap sizes around the head, head diameters, size and shape of the floating element lip, and protrusion or recession of the head, as depicted in Figure 49.…”

Section: Pressure Gradient Effectsmentioning

confidence: 99%

Development of fiber optic aerodynamic sensors for high Reynolds number supersonic flows

Pulliam¹,

Schetz²

2001

39th Aerospace Sciences Meeting and Exhibit

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Aerospace Engineering (ABSTRACT)The purpose of the project was to examine fiber optic sensors for the measurement of pressure, skin friction, temperature, and heat flux in high Reynolds number, supersonic flow. Using a standard fiber optic signal conditioning unit (specifically a broadband interferometric system using spectra), the work centered around determining under what conditions these sensors will work effectively and quantifying the total system limitations.An interferometric-based, fiber optic skin friction sensor was developed for the measurement of wall shear stress in complex, supersonic flows. This sensor type was tested successfully in laminar, incompressible flow, and supersonic flow up to Mach 1.92, Mach 2.4 and 3.0 flow, in which the sensor operated with varying success. A micromachined, fiber optic pressure sensor was also tested in these supersonic conditions, also with varying success. The accurate operation of these sensors was found to be tied to the flow conditions and the fiber optic, signal processing system.A correlation was found to exist between the energy of the flow, either through its dynamic pressure or through external disturbances such as shocks or separation, and the noise in the signals, expressed by the variance of the gap estimate, for the pressure and skin friction sensors in these flows. The energy of the flow couples with the mechanical properties of the sensor reducing the fringe contrast of the signal used by the optical signal processing system to determine a gap estimate. As the energy of the flow is increased and the sensor is excited, the fringe contrast is reduced. A practical limit of a normalized fringe contrast of 0.10 was found for producing accurate gap estimates in real flows. A consequence is that there is a limit to the dynamic pressure of the flow for the sensors to operate accurately, which is demonstrated by the varying success of the supersonic wind tunnel tests. This correlation is sensor specific, meaning that sensors can be designed to operate successfully in any flow. Also, the signal processing system, which forms the other end of the total system, could be improved to allow accurate measurements with the current sensors.iii

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