Qualitative fluid-dynamic analysis of wing profile by thermographic technique. M. Malerba, A. Salviuolo, G.L. Rossi

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1 Qualitative fluid-dynamic analysis of wing profile by thermographic technique M. Malerba, A. Salviuolo, G.L. Rossi Dipartimento di Ingegneria Industriale Università degli Studi di Perugia Via G. Duranti, Perugia - Italy Keywords: boundary layer, infrared thermography, subsonic flows, passive technique Abstract Thermographic systems have been proposed to analyse fluid-dynamic phenomena since about 40 years but nowadays high resolution and differential infrared thermographic measurement systems open up new possibilities. Temperature field that can be measured by a thermographic system on the surface of a solid body invested by a flow is determined by a lot of combined effects such as conversion of kinetic energy of the flow into thermal energy, flow temperature variation in time and space, convection heat transfer phenomena between flow and body, conduction phenomena inside the body and radiation heath exchange of the body surface with surroundings. By correspondence between convective heat transfer coefficient and local turbulence it s possible to carry out information about the boundary layer. In this work some tests have been performed using an high resolution thermographic system for fluid-dynamics analysis of a known test case, a wing profile, in a wind tunnel under variable and constant temperature condition at different air flow velocity. NOMENCLATURE Nu Nusselt number Nu x Local Nusselt number Re Reynolds number Re x Local Reynolds number Pr Prandl number C f Surface shear stress coefficient C fx Local surface shear stress coefficient ν Kinematic viscosity [m 2 s -1 ] h x Convective coefficient Φ Heat flux exchange T Temperature of the fluid T Temperature of the body surface ρ Density Φ cond. lat. Thermic stream due to lateral conduction Φ j Thermic stream due to per Joule s effect Φ rad Thermic stream due to radiation effect e Thickness

2 INTRODUCTION In fluid-dynamic phenomena analysis boundary layer plays an important role. To monitoring its pattern new measurement technique are in continuous development. Systems based on high resolution thermographic visualization systems seems to be very interesting measurement techniques. In general way those systems can measure superficial temperature distribution of a body. If the body is moving across a fluid, a correlation between its temperature and pressure distribution can be done. Using known hypothesis, a correlation between thermographic and fluid-dynamic pattern can be done. Reynold s analogy, is expressed by the following relation [1]: Nu x C = Re Pr 2 x fx (1) in alternative way it can be expressed as: C fx ν hx = 2 (2) c k This equation correlates heat exchange coefficient h x and local surface shear stress coefficient so it is possible to carry out information about boundary layer pattern. The above analogy is not sufficent for an exaustive analisys, it must be associated to the internal energy of the fluid flow that is strictly correlated with Mach number. Generally working with high Mach numbers (i.e. transonic flow or more) a direct reading of energy can be done [7]. Confining our measurement chain in subsonic flows (i.e. very low Mach number) this energy is not sufficient to produce a remarkable heat gradient on the body surface. Frequently this problem can be resolved in two different way: working by active technique, that needs to heat the body surface artificially or by passive technique, in witch is the fluid flow to be heated. Passive technique is preferred because it is less intrusive for the physical phenomenon. The goal of this work is to analyse the possibility to use direct reading of temperature in iposonic flow using an high resolution thermographic system MEASUREMENT PRINCIPLES Measurement principles adopted in this paper are based on the heat exchange that take place between a solid body and the surrounding fluid flow. In this situation, temperature pattern is due to three different phenomena: thermodynamic effects of fluid flow temperature change due to the friction, heat transmission phenomena by convection between flow and body and conduction phenomena inside the body. If we neglect conduction phenomenon, energy associated with convection between body and fluid, can be described by [2]: Φ = h x ( T T ) (3) where heat transfer coefficient x h change both if the flow is laminar or turbulent [2-8]. The trend of total temperature will depend by temperature difference between fluid and solid body. If the body is cooler then it will be characterized by increasing of temperature due to tangential conduction, followed by a first decrement in the flow acceleration zones, then the pattern change quickly due to C fx

3 the change of h x and finally it starts to rise. The area of trend inversion is usually characterized by a transition (laminar to turbulent flow) or aerodynamic bubble. The maximum gradient of temperature appears is usually in a very small area positioned on the antinode of the profile. Figure 1: Classic temperature distribution on a wing The most used method to monitor motion field by thermographic measurement is an active stationary called heated-thin-foil technique [ ]. It consists of heating the observed body by conductor material metallic sheets, like copper or similar, or by printed circuit boards. By monitoring temperature and knowing thermal flow produced by Joule s effect, it is possible to found thermal convection coefficient between body and fluid using the energy balance expression: h = ( Φ Φcond lat Φ rad ) ( T T ) j. (4) An alternative way to proceed is to use transient flow condition and passive set-up. It is represented by the thin skin technique. In this case the body is considered whit reduced thermal capacity and resistance. Under determined assumption it is possible to suppose an instantaneous heat transfer inside the body until equilibrium and consequently it s possible to neglect internal conduction. Referring to the general equation of heat conduction without thermal source, this condition is represented by: T Φ = c ρ e (5) t As a result of this relation, it s sufficient to monitor the temporal development of superficial temperature to obtain informations about convection coefficient [9]: h= T c ρ e t ( T T ) (6) In another way it is possible admitting hypothesis of semi-infinite body involving a superficial thermal exchange. It generates internal conduction phenomena without alterate the internal energy of system. Body temperature is representable by the following relation [10]: ( x x ) * 2 a t = 1 4 T f ( x' ) e 2 π a t dx' (7)

direct vision with passive technique. Two type of test has been conducted.

4 The last two hypothesis expect flow heating, enforcing a passive technique work. EXPERIMENTAL TESTS The goal of this work is to investigated in qualitative way boundary layer pattern of an airfoil by connecting it to thermal maps in slow subsonic flow condition and to compare direct vision with passive technique. Two type of test has been conducted. The first one adopts an open circuit wind tunnel so that temperature pattern can be considered costant, the second set of test is done in a closed circuit wind tunnel so that passive method is adopted using own heating of the wind tunnel. Thermographic system used is an high sensitivity CCD thermocamera, manufactured by Stress Photonics, the Deltatherm 1560 system. OPEN CIRCUIT WIND TUNNEL An aluminium wing profile with length of 153 mm and chord of 100 mm was tested in a slow subsonic wind tunnel with a test section of 40*40 mm. Function typology is by aspiration. Figure 2 Aluminium wing profile Figure 3 Open circuit wind tunnel Temperature and wind speed were monitored by a thermal resistor and a Pitot s tube positioned inside the tunnel. Measured temperature gradient was minor than 0,1 K, therefore temperature is considered as a constant if compared to the gradient due to the flow compression. A first test was done simply covering the profile with black paint to avoid reflection effects and different emissivity values effects on its surface. The wing profile was fixed by one end using a 0 degrees angle of attach on the test section of the wind tunnel. Acquired thermographic images show that convection thermal exchange between flow and body propagate very quickly due to aluminium high conductivity inside the body. In addition it was covered by conduction thermal exchange inside the profile. Analysing temperature pattern in flow direction, we can only remark a temperature increment with thickness profile decrement.

5 Figure 4 Temperature distribution for aluminium airfoil black painted To take out high conductivity effects the profile was covered with a film of Mylar with a thickness of 36 µm. Mylar is an insulation, strong and durable polyester film. Thermal properties of Mylar are summarized in the following table 1. Table 1 Typical Value of Mylar film Property Typical Value Unit Specific Heat 1172 J/Kg/K Thermal Conductivity 1.5 x 10-4 W/mm*K To avoid reflection effects, it was also covered by a black film, with an high emissivity coefficient. Tests were accomplished modifying flow velocity that presented following values: 23 m/s, 22 m/s, 19 m/s, 10 m/s, 5 m/s e circa 2 m/s (Re between 1*10e5 1*10e6). Following images show temperature pattern on the surface of wing profile covered by film of Mylar and the different wind speed.

6 Figure 5 Airfoil covered with a film of Mylar with a thickness of 36 µm and black film Figure 6 Temperature distribution for aluminium airfoil black painted and Mylar 36 µm covered. Re 10 e 6 Figure 7 Temperature distribution for aluminium airfoil black painted and Mylar 36 µm covered. Re 10 e 5

Previous images show that for 23, 22 and 19 m/s velocity temperature pattern is the typical previously described: an increment to the transition zone followed by a decrement.

7 Previous images show that for 23, 22 and 19 m/s velocity temperature pattern is the typical previously described: an increment to the transition zone followed by a decrement. For lower velocity after the decrease to the aerodynamic bubble there is a new temperature increase, more marked when the velocity reduces. We can deduce that for the first three images there is a clearly transition area, while for the last three phenomenon is less marked (i.e. there is not a marked transition area or a fluid reattach is present). A second test section was done by covering the wing profile with a 12 µm thickness film of Mylar. Boundary layer pattern was the same of last test. The following image, obtained for a flow velocity of 23 m/s, shows temperature grown up and the decrease to the transition zone. Tests were realized with speed value of the other section, results are the same. Figure 8 Temperature distribution for aluminium airfoil black painted and Mylar 12 µm covered To confirm result another set of test was accomplished using a different angle of attach, rotating of 7,5 degrees clockwise the profile. Acquired thermographic images show a boundary layer pattern like the one obtained for 0 degree of incidence angle. Rotating of 7,5 degrees counter clockwise the profile, acquired thermographic images show a different boundary layer pattern, in fact there isn t flow detachment detected.

phenomenon the airfoil was tested also in a different subsonic wind tunnel, with

8 Figure 9 Airfoil with incidence of 7,5 clockwise Figure 10 Airfoil with incidence of -7,5 clockwise CLOSED CIRCUIT WIND TUNNEL To better understand the phenomenon the airfoil was tested also in a different subsonic wind tunnel, with a test section of 2200x2200mm and recirculating flow. Figure 11 Closed circuit wind tunnel

During the wind tunnel run, the air temperature increases by friction due the interaction with the honey-comb in the tunnel. Tests were accomplished by using passive technique.

9 During the wind tunnel run, the air temperature increases by friction due the interaction with the honey-comb in the tunnel. Tests were accomplished by using passive technique. Flow velocity during the test was 42 m/s. 17,4 17,2 17 Temperature ( C) 16,8 16,6 16,4 16, [Time (s)] Figure 12 Air temperature pattern in closed circuit wind tunnel The wing profile was covered by a film of Mylar with a thickness of 12 µm and a black film on it. It was positioned in the wind tunnel using 0 degrees angle. In this case airfoil is hotter than fluid so that when fluid arrives to the leading edge of the body starts to cool for tangential conduction phenomena. After fluid detaches, superficial temperature became constant. The gradient of temperature is the one that appears when boundary layer detaches. In this case it is reversed if compared with the previous because the body was cooled by the flow (Figure 13). Figure 14 Airfoil positioned in closed circuit wind tunnel

5 degrees clockwise. Results are similar to the last case (Figure 16).

10 Figure 15 Temperature distribution for airfoil tested in closed circuit wind tunnel In a second test the angle of attach was changed rotating the airfoil of 7.5 degrees clockwise. Results are similar to the last case (Figure 16). Figure 17 Temperature distribution in closed circuit wind tunnel and angle of attach of 7.5 degrees clockwise

11 CONCLUSION REMARKS In this work an high sensitivity thermographic system is used to perform a qualitative inspection of boundary layer in low speed flow and a comparison between passive technique and direct visualization is made. Tests performed on a wing profile test case allows to get the following conclusions: tests in wind tunnel with and without temperature increasing of the flow get the same results in terms of temperature distribution on the wing profile surface, it is therefore possible to perform the future tests in the simplest wind tunnel at constant temperature; tests performed by covering the wing surface by Mylar foils of different thickness allows to choose the optimal solution for future tests. Results explained on this paper, demonstrate that the measurement technique can be used to obtain a fast qualitative evaluation of boundary layer also for iposonic flow. REFERENCES [1] F. Kreith, Principi di Trasmissione del Calore, Liguori Editore,1988 [2] E. Gartenberg, A. S. Roberts Jr., Twenty-Five Years of Aerodynamics Research with Infrared Imaging, Journal of Aircraft, Vol.29, No2, March-April [3] G. M. Carlomagno, L. De Luca, Infrared Thermography for Flow Visualization and Heat Transfer Measurement. [4] G. M. Carlomagno, G. Cardone, T. Astarita, Wall Heat Transfer in Static and Rotating 180 Turn Channels by Quantitative Infrared Thermography, Rev. Gen. Therm. (1998) 37, , Elsevier Science, Paris. [5] G. Cardone, T. Astarita, Thermofluidynamics Analysis of the Flow in a Sharp 180 Turn Channel, Experimental Thermal and Fluid Science20 (2000) , Elsevier Science. [6] T. Astarita, G. Cardone, G. M. Carlomagno, C. Meola, A survey on Infrared Thermography for Convective Heat Transfer Measurement, Optics & Laser Technology, Elsevier Science 2001 [7] J. E. Lamar, Flow Visualization Techniques Used at High Speed by Configuration Aerodynamics Wind-Tunnel-Test Team, NASA report, No TM , April [8] D. L. Balageas, A-M. Bouchardy, Application of Infrared Termography in Fluid Mechanics, Lecture Series on Measurement Techniques, Von Karman Institute for Fluid Dynamics,

12 [9] M. Carbonaro, P. Maggiorana, R. Marsili, An innovative methodology in the characterization of halogen lamp as reference source for Heat Flux Sensors calibration, Measurement Science and Technologies, June 02, Institute of Physics Publishing. [10] H. S. Carslaw, J.C. Jaeger, Conduction of Heat in Solids, Second Edition, Oxford Sciense Pubblications,1990

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