A Study of Direct Measuring Skin Friction Gages for High Enthalpy Flow Applications

This study concerns the design, analysis, and initial testing of a novel skin friction gage for applications in three-dimensional, high-speed, high-enthalpy flows. Design conditions required favorable gage performance in the Arc-Heated Facilities at Arnold Engineering Development Center. Flow conditions are expected to be at Mach 3.4, with convective heat properties of h= 1,500 W/(m°·K) (264 Btu/(hr·ft°·°R)) and T_aw= 3,900 K (7,000 °R). The wall shear stress is expected to be as high as τ_w= 2,750 Pa (0.40 psi) with a correlating coefficient of skin friction value around C_f= 0.0035. Through finite element model and analytical analyses, a generic gage design is predicted to remain fully functional and within reasonable factors of safety for short duration tests. The deflection of the sensing head does not exceed 0.025 mm (0.0001 in). Surfaces exposed to the flow reach a maximum temperatures of 960 K (1,720 °R) and the region near the sensitive electronic components experience a negligible rise in temperature after a one second test run.

The gage is a direct-measuring, non-nulling design in a cantilever beam arrangement. The sensing head is flush with the surrounding surface of the wall and is separated by a small gap, approximately 0.127 mm (0.005 in). A dual-axis, semi-conductor strain gage unit measures the strain in the beam resulting from the shear stress experienced by the head due to the flow. The gage design incorporates a unique bellows system as a shroud to contain the oil filling and protect the strain gages. Oil filling provides dynamic and thermal damping while eliminating uniform pressure loading. An active water-cooling system is routed externally around the housing in order to control the temperature of the gage system and electronic components. Each gage is wired in a full-bridge Wheatstone configuration and is calibrated for temperature compensation to minimize temperature effects.

Design verification was conducted in the Virginia Tech Hypersonic Tunnel. The gage was tested in well-documented Mach 3.0, cold and hot flow environments. The tunnel provided stagnation temperatures and pressures of up to T₀= 655 K (1,180 °R) and P₀= 1,020 kPa (148 psi) respectively. The local wall temperatures ranged from T_w= 292 to 320 K (525 to 576 °R). The skin friction coefficient measurements were between 0.00118 and 0.00134 with an uncertainty of less than 5%. Results were shown to be repeatable and in good concurrence with analytical predictions.

The design concept of the gage proved to be very sound in heated, supersonic flow. When it worked, it did so very effectively. Unfortunately, the implementation of the concept is still not robust enough for routine use. The strain gage units in general were often unstable and proved to be insufficiently reliable. The detailed gage design as built was subject to many potential sources of assembly misalignment and machining tolerances, and was susceptible to pre-loading. Further recommendations are provided for a better implementation of this design concept to make a fully functional gage test ready for Arnold Engineering Development Center.