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Journal ArticleDOI

A nonintrusive laser interferometer method for measurement of skin friction

01 Jan 1983-Experiments in Fluids (Springer-Verlag)-Vol. 1, Iss: 1, pp 15-22
TL;DR: In this article, a method is described for monitoring the changing thickness of a thin oil film subject to an aerodynamic shear stress using two focused laser beams, which is then simply analyzed in terms of the surface skin friction of the flow, including the effects of arbitrarily large pressure and skinfriction gradients, gravity, and time-varying oil temperature.
Abstract: A method is described for monitoring the changing thickness of a thin oil film subject to an aerodynamic shear stress using two focused laser beams. The measurement is then simply analyzed in terms of the surface skin friction of the flow. The analysis includes the effects of arbitrarily large pressure and skinfriction gradients, gravity, and time-varying oil temperature. It may also be applied to three-dimensional flows with unknown direction. Applications are presented for a variety of flows including two-dimensional flows, three-dimensional swirling flows, separated flows, supersonic high-Reynolds-number flows, and delta-wing vortical flows.

Summary (1 min read)

2.1 Principle

  • A llne of oll is applied ahead of the front beam, the flow is started, and the oil flows downstream past the two beams.
  • The laser beams measure the time rate of change of the oil film's slope by monitoring the tlme-dependent optical interference as discussed in Section 2.2.
  • This information, in turn, is used to compute the average skin friction during the measurementperiod using the oil-flow theory and data-reductionequationsdiscussedin Section 2.3.

2.2 Instrument

  • The two-beam _nstrument cannot be used if wind-tunnel geometries require angles in the range between 30°and 70°..
  • There, the angles are too near the oil Brewster angle of 54°, where the oil reflects the p-polarization poorly.
  • One method of avoiding this problem is to increase the incidence angle to a value beyond 70°.

3.1 AxisymmetricTwo-DimensionalBoundary-LayerFlow

  • The initial verificationexperimentsfor the two-beam laser interferometer method were performedby Monson and Higuchi (1981) and later repeated by Monson (1983) in a simple two-dimensionalboundary-layerflow with no gradients.
  • The tunnel has a cylinder mounted along its centerlineon which the skin friction was measured.
  • (A section of the cylinder can be rotated to produce a swirling boundary layer, but the cylinderwas stationaryfor these tests.).

I I

  • Large plexlglass side windows allow laser beam access in and out of the tunnel.
  • The error bars on the mean laser interferometerdata representconfidencelimits of 95%.
  • Excellent agreement between the two methods and with theory is observed for the axial components.
  • Monson (1983) finds that shallow flow angles result both in long oil-flow path lengthswhich cause a persistenceof oil surfacewaves, and large errors in measured skin friction caused by small errors in applied oil llne direction.
  • As a result, this angle is probably close to the lower limit for which the present method can accurately measure the transverse skin-friction component in threedimensional flows.

4. Conclusions

  • Limitations to the method occur in flows possessing high dust levels, at very high skin-frlctlon levels, or when measuring transverse skin-frlction components in three-dlmensional flows nearly perpendicular to the local flow direction.
  • In spite of these limitations, this method has been used to successfully measure skin-friction levels between O.1 and 120 N/m 2, and transverse components in three-dimensional flows within 3@ of perpendicular to the local flow direction.

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NASA Technical Memorandum 84300
NASA-TM-84300 19830004123
A NonintrusiveLaserInterferometer
Methodfor Measurementof
SkinFriction
Daryl J. Monson
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October 1982
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NationalAeronauticsand
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NASA Technical Memorandum 84300
A NonintrusiveLaserInterferometer
Methodfor Measurementof
SkinFriction
Daryl J. Monson, Ames ResearchCenter, Moffett Field, California
NASA
NationalAeronautics and
SpaceAdministration
Ames ResearchCenter
MoffettField,California94035
_/?j---/,_,.37_#-


!
Abstract
A method is described for monitoring the changing thickness of a thin oil film
subject to an aerodynamic shear stress using two focused laser beams. The measure-
ment is then simply analyzed in terms of the surface skin friction of the flow.
The analysis includes the effects of arbitrarily large pressure and skin-friction
gradients, gravity, and time-varying oil temperature. It may also be applied to
three-dimensional flows with unknown direction. Applications are presented for a
variety of flows including two-dlmensional flows, three-dimensional swirling flows,
separated flows, supersonic high Reynolds number flows, and delta-wing vortical
flows.

Citations
More filters
01 Jun 1981
TL;DR: In this paper, a portable dual-laser-beam interferometer that nonintrusively measures skin friction by monitoring the thickness change of an oil film subject to shear stress is described.
Abstract: A portable dual-laser-beam interferometer that nonintrusively measures skin friction by monitoring the thickness change of an oil film subject to shear stress is described. The method is an advance over past versions in that the troublesome and error-introducing need to measure the distance to the oil leading edge and the starting time for the oil flow has been eliminated. The validity of the method was verified by measuring oil viscosity in the laboratory, and then using those results to measure skin friction beneath the turbulent boundary layer in a low speed wind tunnel. The dual-laser-beam skin friction measurements are compared with Preston tube measurements, with mean velocity profile data in a "law-of-the-well" coordinate system, and with computations based on turbulent boundary-layer theory. Excellent agreement is found in all cases. (This validation and the aforementioned improvements appear to make the present form of the instrument usable to measure skin friction reliably and nonintrusively in a wide range of flow situations in which previous methods are not practical.)

1 citations

Journal ArticleDOI
TL;DR: In this article, a laser reflection method is employed to determine the shear stress due to impingement of a low density supersonic free jet issuing out from a convergent divergent nozzle on a flat plate.
Abstract: The details of laser reflection method (LRM) for the determination of shear stress in low density transitional flows are presented. The method is employed to determine the shear stress due to impingement of a low density supersonic free jet issuing out from a convergent divergent nozzle on a flat plate. The plate is smeared with a thin oil film and kept parallel to the nozzle axis. For a thin oil film moving under the action of aerodynamic boundary layer, the shear stress at the air–oil interface is equal to the shear stress between the surface and air. A direct and dynamic measurement of the oil film slope generated by the shear force is done using a position sensing detector (PSD). The thinning rate of the oil film is directly measured which is the major advantage of the LRM. From the oil film slope history, calculation of the shear stress is done using a three-point formula. The range of Knudsen numbers investigated is from 0.028 to 0.516. Pressure ratio across the nozzle varied from 3,500 to 8,500 giving highly under expanded free jets. The measured values of shear, in the overlapping region of experimental parameters, show fair agreement with those obtained by force balance method and laser interferometric method.

1 citations

Journal ArticleDOI
TL;DR: In this paper, a new technique for wall friction measurement, non contact and instantaneous measurement, is developed, which is a kind of the so-called image measurement using oil flow on wall and image analysis technique.
Abstract: A new technique for wall friction measurement, non contact and instantaneous measurement, is developed. The method is a kind of the so-called image measurement using oil flow on wall and image analysis technique. Oil flow owing to wall friction is very small and also strongly depends on way of painting. The possibility of the measurement is examined in a flat plate flow.It is possible to measure the wall friction, if small displacement of oil can be measured. The spatio-temporal derivative method of image analysis is effective for this measurement. The ordinary oil film method for circulating water channel, based machine oil and liquid paraffin with carbon black powder, is employed here from the practical view point.The present method was applied to a juncture flow around a circular cylinder on the flat plate, the so-called horseshoe vortex, in which it is impossible to use any probe for wall friction measurement. The limiting streamlines on stern hull are also measured. The result is agree with the visualized one with milk painting method.
References
More filters
Journal ArticleDOI
TL;DR: In this article, the authors discuss force-measurement balances, the use of the velocity profile, pressure measurements by surface pitot tubes or about obstacles, and the analogies of heat transfer, mass transfer or surface oil flow.

337 citations

Journal ArticleDOI
TL;DR: In this paper, a simple relation is obtained between the film thickness variation and the skin friction distribution, and the results confirm the theory and show that the method gives reasonably accurate measurements of skin friction distributions in low speed flows.
Abstract: An oil film on a solid surface moves under the action of gravity or of the pressure gradient and skin friction resulting from the flow of air over the oil surface. Such oil flows are studied experimentally and theoretically. If the film is thin enough, the dominant force is the skin friction, and a simple relation is obtained between the film thickness variation and the skin friction distribution. The other forces give a perturbation which may be estimated and which decreases with time. The appropriate film thickness is of the order of 10 mu m and so is conveniently measured by interferometry. Experimental results confirm the theory and show that the method gives reasonably accurate measurements of skin friction distribution in low speed flows.

246 citations

Journal ArticleDOI
TL;DR: In this paper, a laser beam focused at position x, reflects partly from the oil surface and partly from a metal substrate, and the reflected beams are focused on a photocell and the pen recorder output gives an interferometric record of y against t.
Abstract: An oil film is placed on a surface over which gas is flowing, and moves under the influence of skin friction. The skin friction meter to be described makes use of the fact that, at time t after the start of the flow with skin friction tau , the thickness y of oil of viscosity mu at a point x close downstream of the leading edge is y= mu x/ tau t. A laser beam focused at position x, reflects partly from the oil surface and partly from the metal substrate. The reflected beams are focused on a photocell and the pen recorder output gives an interferometric record of y against t. To permit accurate setting of the position x, a second laser beam is used which is set at the upstream oil edge, x=0. The results obtained show good agreement with theory, repeatability and consistency. The meter can be used with any polished metal surface, whether flat or curved, and provides versatile and potentially accurate method for skin friction measurement.

59 citations

Journal ArticleDOI
TL;DR: In this article, a portable dual-laser-beam interferometer that nonintrusively measures skin friction by monitoring the thickness change of an oil film subject to shear stress is described.
Abstract: A portable dual-laser-beam interferometer that nonintrusively measures skin friction by monitoring the thickness change of an oil film subject to shear stress is described. The method is an advance over past versions in that the troublesome and error-introducing need to measure the distance to the oil leading edge and the starting time for the oil flow has been eliminated. The validity of the method was verified by measuring oil viscosity in the laboratory, and then using those results to measure skin friction beneath the turbulent boundary layer in a low speed wind tunnel. The dual-laser-beam skin friction measurements are compared with Preston tube measurements, with mean velocity profile data in a "law-of-the-well" coordinate system, and with computations based on turbulent boundary-layer theory. Excellent agreement is found in all cases. (This validation and the aforementioned improvements appear to make the present form of the instrument usable to measure skin friction reliably and nonintrusively in a wide range of flow situations in which previous methods are not practical.)

50 citations