Infrared imaging and tufts studies of boundary layer flow regimes on a NACA 0012 airfoil

Abstract

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 .

Twenty-five years of aerodynamic research with infrared imaging

“…This effect occurs in closedcircuit wind tunnels, where the air heats naturally during their operation. 62 It can also be observed in wind tunnels equipped with a cooling system. 61 Another option is to use electrical 19 ' 20 -63 or laser 64 heating of the model surface, or to blow hot air through a hollow model, 17 -18 and to observe the cooling effect of the flow.…”

“…17 " 20 -63 " 65 If the passive method is applied, the transition is identified visually at the location of the step change in the surface temperature. 7 ' 12 ' 55 - 61 ' 62 ' 66 ' 67 At higher incidence, the onset of turbulent separation is identified by the emergence of a second zone of increased heat transfer caused by local vorticity shedding. 62 The next step was to apply the IR imaging method to boundary-layer transition research that eventually would lead to "laminar" airplane designs.…”

“…7 ' 12 ' 55 - 61 ' 62 ' 66 ' 67 At higher incidence, the onset of turbulent separation is identified by the emergence of a second zone of increased heat transfer caused by local vorticity shedding. 62 The next step was to apply the IR imaging method to boundary-layer transition research that eventually would lead to "laminar" airplane designs. For this application, one has to identify on thermograms the various modes of boundary-layer transition, since the development of each mode can be attributed to a particular design feature of the wing under consideration.…”

Infrared thermography for shear stress field measurements in flows

“…These maps get information about boundary layer and can be reduced in convective coefficient distributions by an ad hoc thermal model or, using the following Reynolds' analogy, in any parameter of interest for aerodynamic aspects. In particular, as described in [6], assuming Pr = 1, eq. (X) gives rise to the following linear relationship between convective and skin friction coefficients.…”

“…As before mentioned, in order to monitor boundary layer by IR thermography, it is fundamental the presence of a thermal exchange between model and fluid [5,6]. One of the main aspects is represented by conversion of temperature from static to total values due to the interaction between body and fluid it is immersed in.…”