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An infrared imaging system was used to detect the thermal signature of boundary-layer flow regimes on a NACA 0012 airfoil from zero angle of attack up to separation. The boundary-layer transition from laminar to turbulent flow and the onset of separation could be seen on the airfoil thermograms. The findings were compared against the behavior of aluminum foil tufts observable both visually and with the infrared imaging system. This arrangement offers the option of using the infrared imaging system both for flow regime detection through surface thermograph} and flow visualization by the aluminum foil tufts. Ultimately the surface temperature changes due to variation in the angle of attack of a lifting surface provide a means for interpretation of the boundary-layer flow regimes.

This paper reviews the technique of boundary-layer transition detection using infrared (IR) imaging, emphasizing cryogenic wind-tunnel testing. With the exception of the low-temperature effects on the IR radiation, the discussion is relevant to conventional wind-tunnel and flight testing as well. At low temperatures, IR imaging encounters a reduction in the radiated energy throughout the IR spectrum, combined with a shift to longer wavelengths of the bulk of the radiation. This radiation behavior affects the minimum resolvable temperature difference (MRTD) of the IR imaging system because of its fixed wave band sensitivity. In the absence of commercial long wavelength IR imaging systems, operating at wavelengths longer than 13 jim, some measures can be taken to alleviate the problem caused by the MRTD limitation. The thermal signature of transition can be enhanced by allowing a small and controlled temperature increase of the wind-tunnel flow that induces a transient heat transfer to the model. This action temporarily reveals the model area under the turbulent regime through its higher heating rate compared with the laminar regime. The contrast between the areas exposed to the two regimes can be enhanced by subtraction of thermograms (the equilibrium thermogram from the transient thermogram). Further visual improvement can be obtained through shade stretching or binary shading. Nomenclature A = detector area c = speed of light D* = detectivity of IR detector e = emissive power h = Planck constant / = in-band detectable blackbody radiance, w/m 2 sr k = Boltzmann constant M = Mach number MRTD = minimum resolvable temperature difference at 50% signal modulation, see also appendix A N = photon emission number NEP = noise equivalent power of detector n = refraction index, c 0 /c P = Prandtl number (P = IR incident power at detector p = IR detectable power at detector R = Reynolds number S/N = signal-to-noise ratio T = temperature t = time U = velocity x = local coordinate, or unit length a = angle of attack, or thermal diffusivity 7 = specific heats ratio A/ = amplifier bandwidth d = velocity boundary-layer thickness e = emittance X = wavelength, or spectral property (subscript) /x = viscosity, or micro (10~6) Experimental Testing Technology Division, MS 234. r = transmittance, or imager optics transmission co = acceptance solid angle, sr Subscripts aw = adiabatic wall b = blackbody c = chord r = recovery value in the boundary layer ref = temperature reference conditions, 303 K t = total property t -I = difference between turbulent and laminar values 0 = vacuum conditions oo = freestream value

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Old Dominion U n i v e r s i t y N o r f o l k , VA 23529-0247 USA ABSTRACT A s t u d y of boundary l a y e r f l o w regimes on a NACA 0012 a i r f o i l from zero a n g l e o f a t t a c k up t o s e p a r a t i o n i s presented. The boundary l a y e r t r a n s i t i o n from t h e l a m i n a r t o t h e t u r b u l e n t regime and t h e onset o f t h e s e p a r a t i o n were d e t e c t e d by s u r f a c e thermography o f t h e a i r f o i l performed w i t h an i n f r a r e d imaging system. The f i n d i n g s were compared a g a i n s t o b s e r v a t i o n s o f aluminum f o i l t u f t s " v i s i b l e " w i t h t h e i n f r a r e d imaging system. T h i s arrangement a l l o w s t h e i n f r a r e d imaging system t o assume t h e d u a l r o l e o f f l o w regime d e t e c t i o n t h r o u g h s u r f a c e thermography and f l o w v i s u a l i z a t i o n t h r o u g h t h e o b s e r v a t i o n o f t h e aluminum f o i l t u f t s . U l t i m a t e l y t h e temperature h i s t o r y on an uncontaminated surface w i l l p r o v i d e an i n t e r p r e t a t i o n o f t h e s t a t e o f boundary l a y e r f l o w .