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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

AbstractA 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.

Topics: Supersonic speed (51%)

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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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
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!
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
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Journal ArticleDOI
Abstract: This paper reviews three relatively modern categories of skin-friction measurement techniques that are broadly classified as microelectromechanical systems (MEMS)-based sensors, oil-film interferometry, and liquid crystal coatings. The theory, development, limitations, uncertainties, and misconceptions of each of these techniques are presented. Current and future uses of the techniques are also discussed. From this review, it is evident that MEMS-based techniques possess great promise, but require further development to become reliable measurement tools. Oil-film techniques have enhanced capabilities and greater accuracy compared to conventional shear-stress measurement techniques (i.e., Preston tube, Clauser plot, etc.) and, as a result, are being employed with increasing frequency. Liquid crystal coatings are capable of making measurements of mean shear-stress vector distributions over a region of a model, but complex calibration and testing requirements limit their usefulness.

367 citations


01 Jan 1999
Abstract: This thesis deals with the problem of high Reynolds number zero pressuregradient turbulent boundary layers in an incompressible flow without any effects of heat-transfer. The zero-pressure gradient turbulent boundary layer is one of the canonical shear flows important in many applications and of large theoretical interest. The investigation was carried out through an experimental study in the MTL wind-tunnel at KTH, where the fluctuating velocity components and the fluctuating wall-shear stress in a turbulent boundary layer were measured using hot-wire and hot-film anemometry. Attempts were made to answer some basic and “classical” questions concerning turbulent boundary boundary layers. The classical two layer theory was confirmed and constant values of the slope of the logarithmic overlap region (i.e. the von Karman constant) and the additive constants were found and estimated to κ = 0.38, B = 4.1 and B1 = 3.6 (δ = δ95). The inner limit of overlap region was found to scale on the viscous length scale (ν/uτ) and was estimated to be y = 200, i.e. considerably further out compared to previous knowledge. The outer limit of the overlap region was found to scale on the outer length scale and was estimated to be y/δ = 0.15. This also means that a universal overlap region only can exist for Reynolds numbers of at least Reθ ≈ 6000. The values of the newly determined limits explain the Reynolds number variation found in some earlier experiments. Measurements of the fluctuating wall-shear stress using the hot-wire-onthe-wall technique and a MEMS hot-film sensor show that the turbulence intensity τr.m.s./τw is close to 0.41 at Reθ ≈ 9800. A numerical and experimental investigation of the behavior of double wire probes were carried out and showed that the Peclet number based on wire separation should be larger than about 50 to ensure an acceptably low level of thermal interaction. Results are presented for two-point correlations between the wall-shear stress and the streamwise velocity component for separations in both the wallnormal-streamwise plane and the wall-normal-spanwise plane. Turbulence producing events are further investigated using conditional averaging of isolated shear-layer events. Comparisons are made with results from other experiments and numerical simulations. Descriptors: Fluid mechanics, turbulence, boundary layers, high Reynolds number, zero-pressure gradient, hot-wire, hot-film anemometry, oil-film interferometry, structures, streak spacing, micro-electro-mechanical-systems.

315 citations


Journal ArticleDOI
Abstract: This survey covers recent developments and applications of four skin-friction measurement techniques (oil-film interferometry, wall hot wire, surface fence and wall pulsed wire). Comparisons of the techniques with each other and with other methods are presented. Applications in attached and separated fully turbulent boundary layers and in highly accelerated laminar-like flows will be shown to demonstrate the application range and the limits of the various techniques.

181 citations


Proceedings ArticleDOI
01 Jan 1993
Abstract: A new oil-fringe imaging fkin friction (FISF) technique to measure skin friction on wind tunnel models is presented In the method used to demonstrate the technique, lines of oil are applied on surfaces that connect the intended sets of measurement points, and then a wind tunnel is run so that the oil thins and forms interference fringes that are spaced proportional to local skin friction After a run the fringe spacings are imaged with a CCD-array digital camera and measured on a computer Skin friction and transition measurements on a two-dimensional wing are presented and compared with computational predictions

71 citations


Journal ArticleDOI
Abstract: A new technique for measuring skin friction was employed to help document the flow on an airfoil at angles of attack from -0.5 to 11.5 deg. Surface pressures were also measured on both the wing and wind-tunnel walls. The experiment was conducted at a freestream Mach number of 0.2 and Reynolds numbers of 0.6, 2, and 6 x 10 6 . The objective of the study was to provide data and boundary condition information sufficient for the validation of numerical simulations. Such a simulation of the experiment was conducted using the INS2D Navier-Stokes code with the shear-stress-transport turbulence model. The computations provide a good description of both laminar and turbulent shear levels, except for turbulent flow on the top surface of the wing at the higher angles of attack.

56 citations


References
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Journal Article

7,092 citations


Journal ArticleDOI
Abstract: The techniques covered include force-measurement balances, the use of the velocity profile, pressure measurements by surface pitot tubes or about obstacles, and the use of the analogies of heat transfer, mass transfer or surface oil-flow. Hot-wire or laser techniques for determining the shear stress within the fluid are not included. The sources of error and ranges of application of the various techniques are discussed.

328 citations


Journal ArticleDOI
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.

234 citations


Journal ArticleDOI
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.

58 citations


Journal ArticleDOI
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