Direct Measurements of Skin Friction in Supersonic Combustion Flow Fields

Abstract

An experimental investigation was conducted to measure skin friction along the chamber walls of supersonic combustors. A direct force measurement device was used to measure simultaneously an axial and a transverse component of the small tangential shear force passing over a nonintrusive floating element. This measurement was made possible with a sensitive piezoresistive deflection sensing unit. The floating head is mounted to a stiff cantilever beam arrangement with deflection due to the flow on the order of 0.00254 mm (0.0001 in). This allowed the instrument to be a nonnulling type. A second gage was designed with active cooling of the floating sensor head to eliminate nonuniform temperature effects between the sensor head and the surrounding wall. The key to this device is the use of a quartz tube cantilever with piezoresistive strain gages bonded directly to its surface. A symmetric fluid flow was developed inside the quartz tube to provide cooling to the backside of the floating head. Tests showed that this flow did not influence the tangential force measurement. Measurements were made in three separate combustor test facilities. Tests at NASA Langley Research center consisted of a Mach 3.0 vitiated air flow with hydrogen fuel injection at Pt = 500 psia (3466 kPa) and Tt = 3000 R (1667 K). Two separate sets of tests were conducted at the General Applied Science Laboratory (GASL) in a scramjet combustor model with hydrogen fuel injection in vitiated air at Mach = 3.3, Pt = 800 psia (5510 kPa), and Tt = 4000 R (2222 K). Skin friction coefficients between 0.001–0.005 were measured dependent on the facility and measurement location. Analysis of the measurement uncertainties indicate an accuracy to within ± 10–15 percent of the streamwise component.

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

“…Both foil and semi-conductor strain gages have their relative advantages and disadvantages for this application. 45,46,47 Also, current pressure sensor designs are not able to make accurate high-speed measurements in harsh environments, including high temperature, high Mach number flows which can cause ionization of the gas. Currently, measurements in such an environment are made through pressure taps that protect the sensor, but the process greatly reduces the frequency response.…”

“…91,92,93,94 Attached to the beam were two Luna EFPI fiber optic strain sensors, as pictured in Figure 97. The concept was to construct a skin friction sensor as close as possible to previously fabricated conventional strain gage designs that have been successful.…”

“…1 In fact, Schetz's research group has produced a series of skin friction sensors based on strain gages mounted to the cantilever beam in a non-nulling arrangement. 21,22,23 As with any sensor, new design concepts take advantage of more sensitive transducing technology as it evolves. In the current study, the sensing technology has moved to fiber optics, allowing a design principle based on deflection instead of strain, which is new in non-nulling cantilever beam designs.…”

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

“…The design was influenced by and extended from the previous successful scramjet-tested skin friction sensors of DeTurris and Chadwick 9,24,25,35 . This design was then verified by testing in the combustor of a rocket-based-combined-cycle (RBCC) engine operating in scramjet mode.…”

“…Since then, they have been used in many different facilities, including high-enthalpy scramjet wind tunnel tests 24,25 , extremely shortduration impulse wind tunnel tests 26,27 , and transonic flight tests summarizes the general methods available for measuring skin friction discussed above, both direct and indirect, and lists the relative advantages and disadvantages of each.…”