Accuracy

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Intrusiveness:

One of the advantages of PSP is that it is essentially a non-intrusive measurement. Unfortunately, the application of PSP to a model surface can alter the aerodynamic characteristics of the model. These alterations can have both viscid and inviscid effects. In the viscous regime, roughness and unevenness of the PSP coating can easily effect transition of the flow over the surface. In the inviscid regime, the thickness of the PSP coating may alter the model's geometry which would result in less accurate results.

Miscellaneous Considerations:

No experimental environment is perfectly clean. The dust and dirt that is present in any tunnel will eventually accumulate on the model surface, which can inhibit both the excitation and emission illumination (this is more of a problem around the leading edges). This dirt may also erode the PSP coating, which can also be easily contaminated chemically by other substances in the flow or simply by skin oils deposited on the surface during handling.

NASA Technician inspecting a wind tunnel model coated with PSP
NASA Technician inspecting a wind tunnel model coated with PSP

Uncertainty:

While it is extremely difficult to isolate the uncertainty of PSP measurements, a general idea of the uncertainty can be obtained by comparing the pressures obtained from PSP to those obtained using conventional taps.

For a test performed on a generic transport that will be described later, the overall rms pressure difference between PSP and taps was 1526 Pa, or 0.22 psi. More recent tests, which were performed at moderate supersonic Mach numbers, had an overall rms pressure difference of 344 Pa or 0.05 psi.





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