Improved pressure probes have been designed to give nearly instantaneous readings of the time-varying local static pressure in flowing gases. The probes have been installed in a wind tunnel (see figure). Probes of this type could also be particularly useful as pressure-sensing instruments to help characterize the instantaneous airflow fields about aircraft; the outputs of these and other instruments could be used, for example, as inputs to advanced digital flight-control systems. These probes could also be used to measure local time-varying pressures in natural-gas pipelines.
The probe design is modular, featuring a variety of interchangeable tips with different shapes for different flow conditions; a cone-cylinder tip is optimized for supersonic flow, whereas a sphere/cylinder tip is optimized for subsonic flow. Each probe tip contains eight radial holes that intersect an internal pneumatic passage, part of which serves as a pressure-measurement plenum. The external shape and the configuration of the passages is designed so that the pressure in the plenum equals the local static pressure of the flow field in which the flow is immersed.
A dynamic-pressure transducer located in the plenum senses the rapidly varying component of static pressure; this transducer is connected to external high-speed electronic circuitry that samples the transducer output. The probe tip is mounted on a stem, which is hollow; the stem contains both (1) wires for connection to the electronic circuitry and (2) a passage that serves as a conventional pneumatic connection between the plenum and an external transducer that measures the slowly varying component of static pressure.
In tests, prototypes of these probes were found to respond to both transient and steady pressures, as intended. In supersonic-flow tests, the probes measured free-stream static pressures correctly at mach 1.6., 2.0, and 2.5. In the worst case encountered in the tests, the root-mean-square pressure measured by the tip transducer and the time-averaged pressure measured by the pneumatically coupled external transducer differed by no more than 0.02 psi (140 Pa). Transient testing of the prototype probe configuration indicates a dynamic static-pressure-measurement capability in excess of 800 measurements per second (800 Hz).
This work was done by A. Robert Porro and Michael E. Ernst of Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Mechanics category, or circle no. 122 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
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