Design and simulation of Open Circuit Blowdown type Wind Tunnel Sanjeev Kumar Gupta a, V.K.Dwivedi b, Jitendra Kumar Chauhan c, and Rahul Goswami c a Assistant Professor, Department of Mechanical Engineering, GLA University, Mathura, U. P. India, 281406. b Associate Professor, Department of Mechanical Engineering, GLA University, Mathura, UP, India, 281406 c Student, Department of Mechanical Engineering, GLA University, Mathura, U. P. India, 281406. Email: sanjeev.mnnita@gmail.com, vijjay_mirz@yahoo.com, rrg18070@gmail.com Abstract Wind tunnel is an aerodynamic test facility. It is mostly used to study flow patterns around bodies and measure aerodynamic forces on them. In the present paper an open circuit blow down type wind tunnel has been designed using standard design rules given by R. D. Mehta for Fluid Mechanics Laboratory, Department of Mechanical Engineering, GLA University, Mathura, India. The speed in test section is 40 m/s and static pressure drop is 30mm of water column. The design has been validated using commercial CFD code FLUENT 6.3. Keywords: Aerodynamics, Wind Tunnel, Contraction wall, Turbulence, Diffuser etc. 1. Introduction Wind tunnel is an aerodynamic test facility. It is mostly used to study flow patterns around bodies and measure aerodynamic forces on them. The bodies (called models) are usually scaled down but geometrically similar versions of bodies of interest like an airplane or an automobile. The results from wind tunnel tests can be scaled to the actual velocity and actual body size using suitable scaling laws. A typical wind tunnel consists of a test section in which the model is kept, a contraction section and settling section before the test section, and a diffuser after the test section. Contractions sections (also known as effuser) are located between the settling chamber and the test sections and serve to both increase mean velocities at the test section inlet and moderate inconsistencies in the uniformity of the flow. Large contraction ratios and short contraction lengths are generally more desirable as they reduce the power loss across the screens and the thickness of boundary layers. Small tunnels typically have contraction ratios between 6 and 9. From an engineering perspective, the wind tunnel is a data gathering device for aeronautical design purposes that has given rise to a great variety of testing facilities (Heinzerling, 1990). The air flow around a test model in a wind tunnel, however, cannot be compared to full scale motion in free flight without further assumptions derived from fluid mechanics. The so-called Reynolds number (a dimensionless term involving pressure, viscosity and a characteristic length) has to be the same in both situations. Pressure and length enter the Reynolds number as a product, so that, for example, a tenfold reduction of the length leaves the Reynolds number unchanged if the pressure is increased tenfold. Already such problems of scaling hint at a fundamental tension between theory and practice embodied in wind tunnels (Bushnell, 2006). But model testing entailed more problems and thus motivated further air flow studies. Beyond its use in aeronautical engineering the wind tunnel assumed an epistemic role for the development of basic concepts of twentieth century fluid dynamics. Currently, more flexibility in the design of wind tunnel contractions can be exhibited, with the use of CFD to enable rapid testing of designs to optimize contractions of arbitrary cross-section and wall profile. The use of CFD allows for the use of higher order polynomials, and non-zero curvature or slope at inlet to the contraction. This paper describes the design of open circuit blow down type wind tunnel. The speed in the test section is 40 m/s and static pressure drop is 30 mm of water column. This design is further validated using commercial CFD code fluent 6.3. 2. Design of Proposed Open Circuit Blow Down Type Wind Tunnel The proposed wind tunnel is designed using design rules given by R. D. Mehta for design of low speed wind tunnel. The technical specification of the proposed wind tunnel is given below.
2.1 Technical Specification of Proposed Wind Tunnel Table 1. Technical specification of wind tunnel Specification Total length of the tunnel Length of the settling chamber Size 5000 mm 1500 mm Cross sectional area of settling chamber 750 x 750 mm 2 Length of honeycomb 150 mm Major and minor diameter of honeycomb 25 mm and 22 mm Number of honeycomb 1023 Number of screen 6 Length of contraction cone 1000 mm Contraction ratio 6 Length of wide angle diffuser 1501 mm Area ratio of diffuser 4.76 Angle of diffuser 2.25 Maximum air velocity in the test section 40 m/s Maximum discharge through the tunnel 20 m 3 /s 2.2 Design of Settling Chamber The settling chamber is constant, square cross sectional chamber located between the fan or wide angle diffuser and the contraction. It contains the honeycombs and screens used to moderate longitudinal variations in the flow. Optimum distance between last screen and contraction entry should be 0.2 times of cross sectional diameters apart so that flow disturbed by the first screen can settle before it encounters the second. Honeycombs are the mass of hexagonal tiling located in the settling chamber and are used to reduce nonuniformities in the flow. For optimum benefit, honeycombs should be 6-8 cell diameters thick and cell size should be on the order of about 150 cells per settling chamber diameter. Figure 1. Honeycomb
Screens are typically located just downstream of the honeycomb and sometime at the inlet of the test section. Screens create a static pressure drop and serve to reduce boundary layer size and increase flow uniformity. A screen is characterized by its open-area ratio, which is defined in the equation below where d is the wire diameter and L is the length of the screen. At least one screen in the settling chamber (ideally the last) should have an open-area ratio of β<0.57, as screens with lower ratios are known to produced nonuniformities in the flow. This is presumable due to the formation of small vortices created by the random coalescence of tiny jets emitted from the screen. The pressure drop across a screen depends upon the open-area ratio of the screen and the density, kinematic viscosity, and mean velocity of the fluid. Screens create static pressure drop and serve to reduce boundary layer size and increases flow uniformity. Optimum distance between last screen and contraction entry should be 0.2 times of cross sectional diameter. Distance between two successive screen increases by 25 mm. Figure 2. Arrangement of Screen in Settling Chamber 2.3 Design of Wide Angle Diffuser Figure 3. Wide angle Diffuser
Diffusers are chambers that slowly expand along their length, allowing fluid pressure to increase and decreasing fluid velocity. Angles slightly larger than 5 degrees do increase pressure recovery, but can also lead to boundary layer separation and thus flow unsteadiness. 2.4 Design of Contraction Wall Profile In the design of flow condition, contraction for low speed wind tunnels several desirable characteristics of the wall profile are identified, including: a wall profile having first and second derivative equal to zero at the inlet and outlet, and inlet and outlet profile radii roughly proportional to the area, that is, the inlet radius is greater than the outlet radius. The result is hoped to be most favourable combination of flow uniformity, thin boundary layer and negligible losses. Over the year, many authors have been interested in methods of designing low speed wind tunnel contraction (Jardision 1961, Morel 1975, Downie 1984). But, a fifth order polynomial developed by Bell and Mehta (1988) is most widely used for the design of low speed wind tunnel contraction profile in two and three dimensions which is given below. 3 4 5 h 10 15 6 H H H. [1] i X... [2] L Where H i and H O are the height of contraction wall at the inlet and outlet respectively from the datum at the axis of symmetry. Using a transfer function like equation 1 to transform Bell and Mehta s polynomial to arbitrary inlet and outlet heights, while incorporating the change in shape provided by raising the polynomial to a power less than unity. The following transfer function is provided by Brassard and Dr. Mohsen Ferchichi (2005) as 3 4 5 1/ 1/ 15 6 H H o 1/ h 10 i O H i. [3] Where α is some function of ξ defined for 0 < ξ <1. This function will be referred to as α function throughout. Equation number 3 is obviously very similar to Bell and Mehta s transfer function. It is functional nature of α which provides interesting result. It can be shown that any function α chosen for use in equation 3, normalized to vary between 0 and 1, will result in smooth function, maintaining first and second derivative at inlet and outlet of the resulting contraction profile. In order to demonstrate the effect of α on h some sample α functions are presented. i Figure 4. Contraction Wall Profile when α=1
Figure 5. Contraction Wall Profile when α=0.5 Figure 6. Contraction Wall Profile when α=ξ 2 It can be seen by inspection that the wall profile of figure 5 is distorted. Therefore we can select α as a function of ξ rather than a constant. In the design of proposed wind tunnel α is chosen as a sine function of ξ, because to generate large radius at the inlet, a smaller radius at outlet while maintaining a longer transition of the outlet radius to the test section. The final design of contraction wall profile is shown in figure 8.
Figure 7. Contraction Wall Profile when α= sinξ 3. CFD Analysis of Proposed Wind Tunnel Figure 8. Contraction Wall Profile In order to conduct a CFD analysis, three main tasks must be completed: grid generation, or pre-processing, the actual computational processing of the analysis and visualization of the computational results or postprocessing. 3.1 Grid generation or Pre processing The final mesh profile shown below consists of 1,10,000 cells, 56,200 faces and 23,401 nodes. The faces were meshed with quadrilateral type of element for good capture of flow phenomenon with less number of cells. 3.1.1 Grid Independency Checks Any rigorous CFD investigation requires a grid independency check. In certain circumstances, coarse grids can lead to the conservativeness property not being fulfilled. Also, to ensure that a solution is fully converged it is necessary to solve a problem utilising successively refined grids until the solution is converged. Edges nodes are multiplied by different factors, 1.2, 1.1, and 1.05 to check the grid independency. Based on the relative error and CPU time, the initial grid size is selected i.e. 1.05.
Figure 9. Grid Generations in Contraction Cone of Proposed Wind Tunnel 3.2 Processing 3.2.1 Model Selection Figure 10. Grid independency The Reynolds number R e was found to be 2.384 x 10-6, indicating that the flow is steady state turbulent. Hence the flow is modelled using Turbulent RNG K-e model. The RNG k-epsilon model provides an alternative technique for deriving turbulence closure models. It was first introduced by Yakhot et al (1986) who applied a complex mathematical technique termed as Renormalisation Group Theory (RNG) to the Navier-Stokes equations and derived an alternative two-equation k-epsilon turbulence model. 3.2.2 Boundary Conditions and Scheme Inlet Boundary Condition Outlet Boundary Condition Wall 1,2 Velocity Inlet (6.67 M/S.) Outflow Stationary wall 3.2.3 Convergence Criteria The convergence criteria is set to 10-8 and convergence is achieved after approximately 50 iteration for turbulent k-epsilon model. The residual were plotted and the plot is shown below.
3.3 Post Processing Figure 11. Residual Plot using k-epsilon model The contour of velocity and pressure were plotted and analyzed. The plots are given below. The post-processor is then utilised in order to visualise the resulting fluid flow in terms of velocities, pressures and other flow variables. Once the computational analysis is complete, the results must be converted to a form that is easily understood. This can be accomplished through the use of commercial software such FLUENT 6.3. These programs use the computational grid and analytical results to produce still pictures, or in the case of unsteady data, animations, that illustrate the characteristics of the flow of interest. Data can be presented in a number of forms, including vector fields or streamlines to represent velocity, isometric surfaces to display regions where a given property is constant, and grid slices, which show property variation as a series of contours. Figure 12. Contour of Velocity Magnitude
Figure 12. Contour of Velocity Vector 4. Conclusion Figure 13. Contour of Static Pressure The open circuit blow down type wind tunnel is designed successfully using design rule of R. D. Mehta and further it is validated using commercial CFD code fluent 6.3. All considered flow quality factors such as mean flow variation, turbulence are well within the expected level. The desired speed 40 m/s at the outlet of test section is achieved. The contraction ratio of designed contraction wall profile is 6.007 which show very good design of contraction wall profile. References [1] Rae, W. H. & Pope, A., 1984, Low-speed wind tunnel testing, 2nd edn. John Wiley & sons [2] R. D. Mehta and P. Bradshaw, November 1979, Design Rules for Small Low Speed Wind Tunnels, Aeronautical Journal of the Royal Aeronautical Society, Page Number 442 449. [3] J. E. Sargison, G. J. Walker and R. Rossi, 13 17 December 2004, Design and Calibration of a Wind Tunnel with Two Dimensional Contraction, 15 th Australian Fluid Mechanics Conference, the University of Sydney, Sydney, Australia.
[4] M. Abbaspour, M. N. Shojaee, 10 December 2009, Innovative Approach to Design a New Low Speed Wind Tunnel, International Journal of Environmental Science and Technology, Vol. 6, No. 1. Page Number 23-34, ISSN: 1735 1472. [5] R. D. Mehta, September 1985, Turbulent Boundary Layer Perturbed by a Screen, AIAA Journal, Vol. 23, No. 9, Page Number 1335 1342. [6] J. H. Bell and R. D. Mehta, March 1989, Boundary Layer Prediction for Small Low Speed Contractions, AIAA Journal, Vol. 27, No. 3, Page Number 372 374 [7] Borger, G. G., 1976, The optimization of wind tunnel contractions for the subsonic range, Tech. Rep. TTF 16899. NASA. [8] Johansson, A. V., 1992, A low speed wind-tunnel with extreme flow quality-design and tests, Proc. the 18 th ICAS Congress pp. 1603-1611.