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Further Experiments on Impact-Pressure Probes in a Low-Density, Hypervelocity Flow

Report Number: AEDC TDR 62-208
Author: Bailey, A. B.
Corporate Author: ARO, Inc.
Laboratory: Arnold Engineering Development Center
Date of Publication: 1962-11
Pages: 32
Contract: AF 40(600)-1000
Project: 8950
Task: 895004
AD Number: AD0290520

An investigation of the behavior of flat-faced, impact-pressure probes with a range of orificeto-probe diameter ratios was made in heated A under conditions where free stream Mach. = 4 to 14, gas temperature = 2700 to 4300 K, and Reynolds number (based on outside probe diameter and conditions behind a normal shock)/in. = 30 to 430. At the lower Reynolds numbers the measured impact pressure was found to decrease with the pressure sensing orifice size. This result agrees with that found in heated N. As the Reynolds number increased this orifice effect became less significant, and at the higher Reynolds numbers no decrease in measured impact pressure was noted for the smallest orifice tested. This tends to confirm the assumption made in some experiments on impact-pressure probes in a low-density hypervelocity flow (AD-268 391) that this behavior is caused by a thermomolecular flow effect. Because this effect is a function solely of pressure at a particular temperature, the greater the pressure the smaller the effect. When Re2 times the square root of density in free stream over density in normal shock is less than 800 in A the measured impact pressure was less than the true impact pressure and decreased to a minimum value, approximately 93% of the true value. As the Reynolds number decreased still further, the viscous effects became dominant, and there was a sharp increase in the measured impact pressure.

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