Wind Tunnels and Experimental Fluid Dynamics Research 108 Fig. 9. Flow visualization inside of wake using smoke wire technique α = 28° Fig. 10. Flow visualization inside of wake using smoke wire technique α = 32° Flow Visualization in Wind Tunnels 109 Fig. 11. Flow visualization using smoke wire technique α = 40° Fig. 12. Conjectured flow field on wake side of flat plate at sub critical incidence Wind Tunnels and Experimental Fluid Dynamics Research 110 Fig. 13. Conjectured flow field on wake side of flat plate at super critical incidence On the portion of the wing near the leading edge, on the suction side, heavy cross flows are induced which move to the side edges and bend into the stream direction. The axes of these side-edged vortices when traced reach the leading edge. The angle it makes is about half the angle of attack α which the plate makes with the stream direction. The core of the three dimensional bubble is near to the leading edge at a distance of about 0.25 C. There was no trailing edge vortex shedding observed for these incidences. The side-edge vortices extend downstream at least six to seven times chord. The diameter of these side edges vortices increases considerably with a little change in angle from α = 26.5° to α = 28°. The side-edge vortex core axis moves straight up to trailing edge when moving upstream, where it bends a little and again moves straight up to leading edge. From the outer wake flow pictures it is observed that the flow at the center seems to be just as big bubble trying to close about 0.1C to 0.2C from the trailing edge which is quite different from the angle α = 32° where the bubble closes well away from the trailing edge. Also the wake boundary is wavy for α = 28° where as for α = 32° it is quite sharp as seen in the Figs. 9 and 10 Flow Visualization in Wind Tunnels 111 Fig. 14. Flow visualization on wake side using tuft probes. Wind Tunnels and Experimental Fluid Dynamics Research 112 Fig. 15. Conjectured flow field on wake side of flat plate at sub critical incidence Flow Visualization in Wind Tunnels 113 The result of the variation flow visualization techniques have been used to construct the flow field as is shown in Figs12-13 and Fig.15 (“Flow visualization details”). This is what has been observed it is not an interpretation how the actual flow structure is. A first attempt has been made to get an idea of the flow field in the following way. 4.2 α ≥32° For these incidences and higher up to α = 50, it was observed that the wing centre flow is dominating the total span and the tip vortices are not present on the wing as shown in Fig.10, but appear further down the stream. The three dimensional bubble covers almost whole of the span with the appearance of trailing edge vortex shedding, and the flow is rotating inside this bubble. The flow near the plate surface is forward and spreading outward which is also noted from surface flow visualization pictures. Trailing-edge vortex shedding and re-circulating flow was also observed by Calvert [9]. In the region near the leading edge on the suction side, the flow was observed as moving towards the plate centre from sides, and the flow which is moving forward is joining the main stream flow reaching from sides to form rotary motion like side edge vortices, the concentration of which is observed little downstream of the trailing edge. This distance increases gradually as the angle of incidence is increased. Also, it was found in the surface flow pictures that the two “eyes” had disappeared and with them the additional two vortices must have vanished there. For all these angles α ≥32° trailing-edge vortex shedding was observed, and the frequency of vortex shedding was observed to increase as the angle of attack increases. 5. Conclusions Flow visualization is considered an important tool to understand the nature of the flow field. Its proper utilization will provide reasonable information that will help in influencing flow characteristics. The above methods mentioned are not the only ways of understanding the flow but the results obtained using above methods highlight the usefulness of these flow visualization techniques. 6. Acknowledgements The author is grateful for the facilities provided by King Fahd University of Petroleum and Minerals (KFUPM) and University of Sydney, Australia. The author also acknowledges the support from Dr. K Srinivas from the School of Aerospace, Mechanical & Mechatronics Engineering, University of Sydney Australia. 7. References Batill, S.M.,& Mueller,T.M. (1981).Visualization of Transition in the Flow over an Aerofoilusing Smoke wire technique , AIAA journal,Vol.19, pp.340-345 Bienkiewicz, B.,& Sun, Y .(1992). Local Wind loading on the Roof of a Low-rise Building, JWEIA,Vol 45, pp.11-24 Calvert,J.R.( 1967). Experiments on the flow past an inclined disk, Journal of Fluid Mechanics,vol.29, Pt.4, pp.691-703 Wind Tunnels and Experimental Fluid Dynamics Research 114 Fiddes, S.P.,& Williams,A.L.(1989).Recent developments in the study of Separated flows past Slender bodies at incidence, Conference proceedings Roy.Aeron.Soc.,London, pp 31.1-31.17. Mahmood, M.,”Low-Speed Experiments on a Flat Square at High Angles of Attack”, M.S.Thesis Dissertation, KFUPM, Dhahran, Saudi Arabia. Stahl,W.H, Mahmood,M and Asghar,A.,”Experimental Investigation of the Vortex flow on Delta wings at High incidences”AIAA journal, 30(4), April 1992, pp 1027-1032 Stahl,W.H., Mahmood,M.,Asghar,A.,”Experimental Investigation of the Vortex flow on a very slender Sharp-edged Delta wings at High incidence”, AIA Ajourn. vol 30,No. 4, pp1027-1032, April 1992. Stahl,W.H., Asghar,A.,Mahmood,M.,” Supression of Vortex Assymetry and side force on acircular cone”,DLR-IB222-A12, Cologne, Germany, April 1993. Terry, N.,Gangulee, D., “Secondary flow control on slender Sharp-edged Configurations”AIAA paper93-3470,CP,1993. 7 Components of a Wind Tunnel Balance: Design and Calibration Miguel A. González, José Miguel Ezquerro, Victoria Lapuerta, Ana Laverón, and Jacobo Rodríguez Escuela Técnica Superior de Ingenieros Aeronáuticos Universidad Politécnica de Madrid Spain 1. Introduction The aim of wind tunnel tests is the simulation of the flow around bodies or their scaled models. In aeronautical applications, the measurement of aerodynamic loads in a wind tunnel, forces and momentums, is a very difficult task due to the required accuracy. The wind tunnel balances, comprised by several hardware and software components, provides directly the pursued measurements, with high accuracy and reliability. For these reasons, among others, wind tunnel balances have become a common tool in testing facilities. This chapter starts with a general description of wind tunnel balances. The number of measuring components and the position of the balance with relation to the model and wind tunnel chamber determine the wind tunnel balances designs. The most flexible ones, in terms of usability, are the six components external balances, so these will be referenced for introducing the calibration process, which is one of the key points to achieve the required aerodynamic tests results accuracy and reliability. Because of its influence on the drag measurement accuracy, the coupling effect between lift and drag measurements is analysed very deeply as well. The analysis of the non-stationary effects are finally done taking into account the wind tunnel balance requirements and constraints, with special attention on an issue not commonly mentioned, the inertia forces generated on the balance by the model vibrations, and their influence on the aerodynamic forces to be measured. Several mentions to signal processing and acquisition are done, as this is the other key point on the measurements accuracy. However, it is easy to extrapolate these procedures to other types of balances, as the main intention is to show which are the critical points that make wind tunnel balances such a special and complex hardware. We do not intend here to describe the design and calibration procedures of the industrial manufacturers. This is the result of a work done in the Universidad Politécnica de Madrid (UPM), and the Instituto Tecnológico y de Energías Renovables (ITER, Tenerife, Canary Island, Spain, www.iter.es). Nevertheless, we do consider that is a good guide for developers of wind tunnel balances in institutions like UPM and ITER, where research and education are very important points. Wind Tunnels and Experimental Fluid Dynamics Research 116 2. Wind tunnel balance The wind tunnels main function is to provide flow simulation on a model introduced in a fluid flow. Global forces and momentums on the model are mainly obtained by using different wind tunnel balances; although in special tests, local balances or pressure distribution measurement can be used as well. Range, accuracy and response time of the measurements are the main parameters that define such systems. The wind tunnel balances are extensively used and are an accurate method for measurements acquisition, with a wide range of measures and a fast response to loads changes. This system requires an important initial calibration effort but once the measurements are probed to be correct, the system can be used to test several low cost models with a reduced effort. Other option for aerodynamic load measurements is the pressure measurement in several model points by means of a pressure scanner or scanivalve system. This system requires a very complex and expensive test model. The measurement points are built in the model surface by making holes and connecting them with the scanivalve by means of a tube that transfer the pressure. These holes introduce also modifications in the flow around the model thus modifying the real behaviour of the model. There are several types of wind tunnel balances. The most important are: - External balances: They are placed outside the model, inside or outside the wind tunnel chamber test section, but they always introduce some interference in the wind flow. However the possibility to change test models with almost no effort provides a high flexibility to the wind tunnel facility. There are several degrees of complexity for these balances, depending mainly on the number of measurement channels, which can vary between 1 and 6. - Internal balances: They are placed inside the model, thus no interferences are introduced in the wind flow by the balance components, but a mechanical support for the model is always needed to maintain it in the test chamber and change the model orientation if desired. The complexity of the test model is comparable or higher than the models for scanivalve systems, as the balance has to be installed inside. Thus this option does not provide flexibility in testing different models. These balances are normally supplied to the customer already calibrated and with the acquisition system. The number of measured components can also vary between 1 and 6. Figure 1 shows an example of internal balance. - Rotary balances: Used for propellers, helicopter blades and other rotating models. Fig. 1. Internal balance example. Image courtesy of STARCS. [...]... along the plane of symmetry 30.0 35.0 40 .0 45 .0 142 Wind Tunnels and Experimental Fluid Dynamics Research 2.1 .4 Conclusions A method of determining local skin friction developed by Lien and Ahmed (2006) using multi-hole pressure probe has been presented in this study The method is based on wall similarity law and the utilization of the logarithmic law of the wall Wind tunnel tests were conducted to obtain... of a Wind Tunnel Balance: Design and Calibration 129 Fig 9 Representation of channel 1 signal at a 2.500 Hz sampling rate of an aerodynamic model introduce in the ITER-LSWT, at a wind speed of 40 m/s and at angle of attach of 8º Time domain signal plot during 20 sec Frequency domain signal plots with and without the 0 Hz component of the signal 130 Wind Tunnels and Experimental Fluid Dynamics Research. .. direction (lift) and the y axis the measured in horizontal direction (drag) It shows that the coupling effect is almost linear, being the drag 0 ,46 % of the applied lift 122 Wind Tunnels and Experimental Fluid Dynamics Research Coupled drag due to lift 4. 0 D [N] 3.0 2.0 1.0 0.0 0 200 40 0 600 800 1000 L [N] Fig 5 Coupled drag due to lift Horizontal axis represents the applied vertical force, and vertical... measurements The supports 132 Wind Tunnels and Experimental Fluid Dynamics Research fairing geometry should be known and common, with an extensive and contrasted bibliography, so that the aerodynamic forces and momentums theoretic calculus will be reliable enough The support fairings airfoil should be a symmetric one, and its angle of attack should be 0º in order to generate only drag force and one momentum component... Scientist and Engineer's Guide to Digital Signal Processing, California Technical Publishing, Retrieved from < http://www.dspguide.com/> Unknown author (2010) Pressure Probes, Department of Engineering, University of Cambridge, Retrieved from http://www-diva.eng.cam.ac.uk/whittle/pressure html 1 34 Wind Tunnels and Experimental Fluid Dynamics Research Various authors (2010) Beginner’s Guide to Wind Tunnels, ... Wind Tunnels and Experimental Fluid Dynamics Research and other fluid mechanical applications The chapter will also demonstrate that various features and important findings from wind tunnel investigations have helped enhance the accumulation of new and significant knowledge, and validated the ‘concepts of proof’, that can, and also have lead to the development of practical devices such as wind driven rotating... with and without the 0 Hz component of the signal 128 Wind Tunnels and Experimental Fluid Dynamics Research Fig 8 Representation of a resultant force signal, combination of two drag cells, at a 2.500 Hz sampling rate of an aerodynamic model introduce in the ITER-LSWT, at a wind speed of 40 m/s and at angle of attach of 8º Time domain signal plot during 20 sec Frequency domain signal plots with and. .. for aerodynamic model tests 120 Wind Tunnels and Experimental Fluid Dynamics Research b The wind flow direction inside the test chamber has to be determined We cannot assume that the wind direction and test chamber horizontal direction are the same, as the risk of being committing a same order error as the presented in the previous step is not acceptable A correct wind tunnel balance calibration must... in aviation between1903 and 19 14 Although, primarily designed for aerodynamic studies, the wind tunnels of today are used in a multitude of fluid mechanical investigations including aerodynamic studies It is considered to be an essential experimental tool in fluids/aerodynamic laboratories to complement research and teaching activities around the world Today there are wind tunnels that are designed... the left Calibration facility example on the right Images courtesy of STARCS 118 Wind Tunnels and Experimental Fluid Dynamics Research Besides the model supports, a balance support is needed to fix the balance to the wind tunnel structure These supports are placed outside the wind tunnel test chamber, so their dimensions and weight are not important, the requirement imposed on them is an elevated stiffness . institutions like UPM and ITER, where research and education are very important points. Wind Tunnels and Experimental Fluid Dynamics Research 116 2. Wind tunnel balance The wind tunnels main function. on the flow past an inclined disk, Journal of Fluid Mechanics,vol.29, Pt .4, pp.691-703 Wind Tunnels and Experimental Fluid Dynamics Research 1 14 Fiddes, S.P.,& Williams,A.L.(1989).Recent. tests. Wind Tunnels and Experimental Fluid Dynamics Research 120 b. The wind flow direction inside the test chamber has to be determined. We cannot assume that the wind direction and test