Dual-beam skin friction interferometer

• US 4,377,343 A
• Filed: 07/10/1981
• Issued: 03/22/1983
• Est. Priority Date: 07/10/1981
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Monson, Daryl J.
• Primary Examiner(s): Corbin, John K.
• Assistant Examiner(s): Koren, Matthew W.
• Application Number:
  US06/282,192
• Time in Patent Office: 620 Days
• Field of Search: 356/351, 356/352, 356/357, 73/147
• US Class Current: 356/503
• CPC Class Codes: G01B 11/0675 using interferometry
• CPC Class Codes: G01B 9/02 Interferometers
• CPC Class Codes: G01N 13/02 Investigating surface tension of liquids

Abstract

A portable dual-laser-beam interferometer is described that nonintrusively measures skin friction by monitoring the thickness change of an oil film at two locations while said oil film is subjected to shear stress. An interferometer flat is utilized to develop the two beams. Light detectors sense the beam reflections from the oil film and the surface thereunder. The signals from the detectors are recorded so that the number of interference fringes produced over a given time span may be counted.

Claims

1. Apparatus for measuring the thickness change of an oil film on a light reflective surface while said oil film is flowing comprising:

means for fixedly directing first and second parallel coherent light beams of a known wavelength on said oil film at points A and B, respectively, for reflection by said oil film and said surface adjacent thereto, said beams being a known distance apart and said first beam being upstream from said second beam;

means for rotating the plane of polarization of said first beam 90°

with respect to the plane of polarization of said second beam;

means for separating the two beams reflected from said oil film and said surface into paths normal to each other,

light detection means interposed in each path for generating a signal representative of the beam intensity, and

means for recording each signal so that the frequency of interference fringes at said points A and B may be determined.

2. The apparatus of claim 1 wherein said means for directing said first and second coherent light beams comprises a laser and an interferometer flat.

3. The apparatus of claim 1 wherein said oil film and surface are situated within a wind tunnel and said oil film is caused to flow by the air movement within said wind tunnel.

4. Apparatus for measuring the thickness change of an oil film on a light reflective surface while said oil film is flowing comprising:

laser means for generating a coherent light beam,

an interferometer flat fixedly and obliquely interposed in said beam and producing first and second parallel beams impinging on and reflecting from said oil film and said surface adjacent thereto, said first beam being upstream of said second beam and said first and second beams impinging on said oil film at points A and B;

a half-wave retardation plate stationed between said interferometer flat and said oil and interposed in said first beam to rotate its polarization by 90°

;

a polarization beam splitter situated to receive said beams reflected from said oil film and separate them into two paths orthogonal to each other;

first and second light detectors each adapted to generate a signal representative of the light intensity received;

means for focusing the two beams emanating from said polarization beam splitter onto said light detectors, respectively;

and means for recording each light detector signal whereby the interference fringes occurring at points A and B as the oil film thickness changes may be determined.

5. Apparatus as set forth in claim 4 wherein said focusing means includes polarizers and interference filters to reduce noise at each light detector.

6. Apparatus as set forth in claim 5 wherein said oil film is located on the surface of a test object in a wind tunnel and said first and second beams and their reflections from said oil film and surface are transmitted through a window of said wind tunnel.

7. Apparatus for measuring data useful for a skin friction calculation comprising:

a wind tunnel with a window;

a test object with a light reflective surface situated within said tunnel in proximity to said window;

an oil film located on said surface and adapted to flow and change thickness when subjected to the shear stress of the air flow within said tunnel;

a laser transmitter positioned outside of said wind tunnel and capable of directing first and second light beams through said window onto said oil film at points A and B a known distance apart, said first beam being upstream of said second beam;

a laser receiver positioned outside of said wind tunnel and adapted to receive the two light beams reflected from the oil film and the test object surface thereunder, said laser receiver including means for separating the two reflected beams and focusing them on separate light detectors, said detectors generating signals representative of the interference fringes at points A and B, respectively; and

means coupled to said light detectors for recording said signals.

8. Apparatus as set forth in claim 7 wherein said laser transmitter includes a laser, an interferometer flat and a half-wave retardation plate, said interferometer flat being interposed obliquely in the light path of said laser whereby two parallel light beams are reflected from said flat, said half-wave retardation plate being interposed in the path of one of the reflected beams to rotate its polarization by 90°

.