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A nonintrusive laser interferometer method for measurement of skin friction

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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.

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A :

dT/dt [Eq. (9)]

C f :

local skin-friction coefficient, τ/q

dp/dx :

external flow pressure gradient

g :

gravitational acceleration

H :

step height

I :

incidence angle for interferometer flat

i :

laser beam incidence angle measured from the normal to a surface

M :

Mach number

N :

fringe number

n :

coordinate perpendicular to oil-flow direction (Fig. 4)

n g :

interferometer flat index of refraction

n o :

oil index of refraction

q :

free-stream dynamic pressure

R :

Reynolds number; also, refraction angle for interferometer flat

r :

laser beam refraction angle within oil measured from the normal to a surface

S :

oil-viscosity/temperature-slope [Eq. (11)]

s :

coordinate along oil-flow direction (Fig. 4); also, delta wing semispan

T :

temperature; also, interferometer flat thickness

t :


V :

tunnel free-stream speed

W s :

transverse speed on surface of rotating cylinder

x :

coordinate parallel to line joining beams (Fig. 4); also, distance downstream from step

Y o :

tunnel height

y :

oil thickness; also, delta wing semispan distance

z :

coordinate perpendicular to line joining beams (Fig. 4) atunnel-wall deflection angle; also, delta-wing angle of attack

y :

local oil-flow angle with respect to the x coordinate (Fig. 4)

ΔN :

incremental change in fringe number

Δt :

incremental change in time

Δx :

beam spacing


boundary-layer thickness


pressure gradient and gravity-correction parameter [Eq. (8)]


surface inclination from horizontal laser wavelength


oil kinematic viscosity


oil density


local skin friction

( )′:

corrected or “effective” value

( ):

average value

L :

model length

x, z :

directions as shown in Fig. 4

1, 2, 3, 4:

refer to positions in Figs. 1 and 4, or to times in Fig. 3


free-stream conditions


momentum thickness


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  5. Monson, D.; Driver, D.; Szodruch, J. 1981: Application of a laser interferometer skin-friction meter in complex flows. ICIASF '81 Rec., IEEE Publ. 81CH1712-9, Dayton, Ohio, pp. 232–243

  6. Monson, D.; Higuchi, H. 1981: Skin friction measurements by a dual-laser-beam interferometer technique. AIAA J. 19, 739–744

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Monson, D.J. A nonintrusive laser interferometer method for measurement of skin friction. Experiments in Fluids 1, 15–22 (1983). https://doi.org/10.1007/BF00282262

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  • Shear Stress
  • Skin Friction
  • Surface Skin
  • Focus Laser
  • Separate Flow