FACULTY of COMPUTING, ENGINEERING & SCIENCE Assessment Cover Sheet and Feedback Form 2016/17 Module Title: Further Computational Fluid Dynamics Assessment Title and Tasks: CFD Analysis of a Car Model Aerodynamics using ANSYS Software Assessment No. 2 No. of pages submitted in total including this page: Completed by student Word Count of submission (if applicable): 3000 words (±10%) Part A: Record of Submission (to be completed by Student) Extenuating Circumstances If there are any exceptional circumstances that may have affected your ability to undertake or submit this assignment, make sure you contact the Advice Centre on your campus prior to your submission deadline. Fit to sit policy: The University operates a fit to sit policy whereby you, in submitting or presenting yourself for an assessment, are declaring that you are fit to sit the assessment. You cannot subsequently claim that your performance in this assessment was affected by extenuating factors. 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You are required to acknowledge that you have read the above statements by writing your student number(s) in the box: Student Number(s): IT IS YOUR RESPONSIBILITY TO KEEP RECORDS OF ALL WORK SUBMITTED Part B: Marking and Assessment (to be completed by Module Lecturer) Assessment Task: Please refer to page 7 to 9. Learning Outcomes to be assessed (as specified in the validated module descriptor LO2: Understanding the use of numerical methods used in the analysis of fluid flows. Grading Criteria: Marking Criteria Marks Available Marks Awarded Introduction/Background Information:  How can CFD approach be used to support car design?  The main components in CFD solver and the motivating points to sue it as an effective tool. 10 Mathematical Model:  Explain the flow physics of this problem and stat the numerical parameters used.  Describe the solution procedures (governing equations – input physical parameters of the flow – turbulence model – flow problem assumptions – boundary conditions).  Discretization method and solution algorithm. 25 Computational Domain and Solution Set-up:  Create a hybrid grid with high quality to capture all relevant flow features.  Y-Plus calculations to resolve the BL flow.  Mesh quality criteria.  Solution permanents and solving the problem. 25 Numerical Solution Results and Discussion:  Convergence and efficiency.  Flow field presentation and analysis.  Comparison with experimental data. 25 Conclusions and suggested modification:  Your conclusion of using CFD approach as design tool.  Modification to the current design could improve the car performance. 15 Total Mark: Assessment Criteria: Performance Level Criteria . Lower 2nd Class / PASS (50% - 59%) Evidence of some understanding of the CFD modelling process. The presentation of relevant results with a limited explanation of what has been obtained. Upper 2nd Class / MERIT (60% - 69%) Evidence of good understanding of the CFD modelling process. A good presentation of relevant results with explanation of what has been obtained. 1st Class / DISTINCTION (70% +) Clear evidence of understanding of the CFD process with good detail and justification of the steps taken. A comprehensive review of the results with evidence of some critical analysis. Feedback/feed-forward (linked to assessment criteria): • Areas where you have done well: • Feedback from this assessment to help you to improve future assessments: • Other comments Mark: Marker’s Signature: Date: Work on this module has been marked, double marked/moderated in line with USW procedures. Provisional mark only: subject to change and/or confirmation by the Assessment Board Part C: Reflections on Assessment (to be completed by student – optional) Use of previous feedback: In this assessment, I have taken/took note of the following points in feedback on previous work: Please indicate which of the following you feel/felt applies/applied to your submitted work • A reasonable attempt. I could have developed some of the sections further. • A good attempt, displaying my understanding and learning, with analysis in some parts. • A very good attempt. The work demonstrates my clear understanding of the learning supported by relevant literature and scholarly work with good analysis and evaluation. • An excellent attempt, with clear application of literature and scholarly work, demonstrating significant analysis and evaluation. What I found most difficult about this assessment: The areas where I would value/would have valued feedback: CFD Analysis of a Car Model Aerodynamics using ANSYS Software Please read carefully before you start your assignment …….. I. INTRODUCTION: Due to considerable increase in the number of vehicles around the world and therefore increasing the CO2 emissions, manufacturers are looking for new ways and developing new technologies to reduce fuel consumption and improve vehicle efficiency. In terms of vehicle efficiency, drag is an important factor which is why vehicle aerodynamics is such an active area of research for automobile manufacturers. While wind tunnel testing was the most profound way of testing vehicle aerodynamics, recent growth in the available computational power has led to more and more adaptation of numerical simulations. Computational Fluid Dynamics (CFD) helps study the flow behaviour without having to create a physical model and thus helps reduce R & D costs while simultaneously saving time. Ahmed body is a benchmark model that is widely used in the automotive industry for validating simulation tools. The Ahmed body shape is simple enough to model, while maintaining car-like geometry features. The Ahmed Body was first created by S.R. Ahmed in his research “Some Salient Features of the Time-Averaged Ground Vehicle Wake” in 1984. Since then, it has become a benchmark for aerodynamic simulation tools. II. OBJECTIVE: The objective of this assignment is to perform both mathematical and CFD analysis of the air flow around a car model using ANSYS package software for mesh generation and flow solution. III. CREATING FLUID ENCLOSURE: In order to simulate the airflow around the vehicle, a fluid volume needs to be created which will encompass the vehicle. This was done by creating an enclosure around the vehicle and subtracting the vehicle body. This enclosure acts as the computational domain. To reduce the overall computational cost and time, the vehicle was considered symmetric laterally. The simple geometrical shape has a length of 1.044 meters, height of 0.288 meters, and a width of 0.389 meters. It also has 0.5-meter cylindrical legs attached to the bottom of the body and the rear surface has a slant that falls off at 40 degrees as shown in the figure below. The total length of the domain is 6 car model lengths (L). The inlet is positioned 1 L from the front of the model and the outlet is 4 L from the rear. The free-stream domain side is 1 L from the side of the car and the domain height is 1 L from the ground. For greater accuracy the domain should be extended (especially after the model), but to minimize computational time the domain is kept relatively small. If you want, you are of course free to investigate how the domain size influences the results. IV. MESH GENERATION: While generating your mesh, sizing functions should be used wherever necessary in order to obtain accurate lift/drag parameters. Try to control your mesh around the car body to properly capture the flow in the region closest to the vehicle and also capture the flow in the wake region. Since boundary layer separation has a significant effect on drag, five layers of inflation can be added to the vehicle surface to properly resolve the boundary layer. V. BOUNDARY CONDITIONS: The following BC can be applied to your simulation:  The enclosure inlet plane will be assigned as “velocity-inlet”.  The road and the vehicle body will both be assigned as “walls”.  The surrounding enclosure surfaces will all be assigned as “symmetry” planes having a “no slip” condition.  The outlet will be assigned as “pressure-outlet” with its pressure set constant and equal to atmospheric pressure.  Air coming through the inlet has the following parameters:  Kinematic viscosity of air: 15 × 10-6 m2/s.  Dynamic viscosity: 1.789× 10-5 kg/m.s.  Density: 1.225 kg/m3.  Bulk velocity: U = 40 m/s.  Reynolds number based on height of body: Re = 768,000.  Average turbulence intensities of less than 0.25%. VI. SOLVER: For this analysis, you are required to perform steady state and transient flow analysis. The following numerical schemes may be recommended during your steady state flow simulation  Pressure based solver.  Realizable k- epsilon model with non-equilibrium wall functions.  Y-plus value on the vehicle surface should lie between 25-300.  2nd order solution accuracy is recommended.  Model frontal area is used as reference area to determine drag and lift coefficients. VII. REQUIRED: The complete report should cover the following points: 1. Brief introduction to how CFD approach can be used as powerful design tool. 2. The mathematical model of the flow problem (numerical parameters and solution procedures). 3. Creating high quality solution domain with good resolution at the BL flow region. 4. The numerical parameters used to control the solution convergence and accuracy. 5. Predicted values of drag and lift coefficients. 6. Visualize the mean velocity contours & pressure contours. 7. Plot the axial velocity and compare with experimental data. 8. Plot the pressure coefficient & skin friction coefficient over the body. 9. Conclusion and recommended modification to the studied vehicle geometry, which could improve its lift and drag characteristics making the vehicle handle better at cruise speeds and improve its fuel efficiency based on your numerical results. VIII. TRANSIENT FLOW ANALYSIS: Transient flow is the flow, wherein, the flow velocity and pressure are changing with time. Often, transient flow conditions persist as oscillating pressure and velocity waves for some time after the initial event that caused it. As the flow begins to transition to turbulence, it is no longer possible to assume that the flow is invariant with time. Such problems can be solved by transient flow analysis which solves it by considering a time based domain. It will solve the problem in different time steps with variation of flow. You can apply the same setting as in case of steady state simulation such as boundary and initial conditions. For this analysis, a pressure based transient state solver can be applied. The recommended solution methods are listed below:  2nd order implicit equations for pressure, momentum, turbulence K.E. and turbulence Dissipation Energy can be applied.  PISO scheme can be for better convergence.  A Coupled scheme can be used for the solution.  Time step size (Δt) must be small enough to resolve time-dependent features observed in transient flow and to make sure convergence is reached within the number of max iterations per time step. IX. REFERANCES: [1] S.R. Ahmed, G. Ramm, Some Salient Features of the Time-Averaged Ground Vehicle Wake, SAE-Paper 840300, 1984 [2] H. Lienhart, S. Becker, Flow and Turbulence Structure in the Wake of a Simplified Car Model, SAE 2003 World Congress, SAE Paper 2003-01-0656, Detroit, Michigan, USA, 2003 [3] H. Lienhart, C. Stoots, S. Becker, Flow and Turbulence Structures in the Wake of a Simplified Car Model (Ahmed Model), DGLR Fach Symp. der AG STAB, Stuttgart University, 15-17 Nov., 2000 [4] C. Hinterberger, M. García-Villalba, W. Rodi, Large Eddy Simulation of flow around the Ahmed body. In "Lecture Notes in Applied and Computational Mechanics / The Aerodynamics of Heavy Vehicles: Trucks, Buses, and Trains", R. McCallen, F. Browand, J. Ross (Eds.), Springer Verlag, ISBN: 3-540-22088-7, 2004 [5] S. Krajnovic, L. Davidson, Large eddy simulation of the flow around a simplified car model, SAE 2004 World Congress, SAE Paper 2004-01-0227, Detroit, Michigan, USA, 2004 [6] M. Minguez, R. Pasquetti, E. Serre, High-order large-eddy simulation of flow over the “Ahmed body” car model, Phys. Fluids, 20, 9, 2008 [7] Venning, J., Lo Jacono, D., Burton, D., Thompson, M., and Sheridan J., The effect of aspect ratio on the wake of the Ahmed body. Experiments in Fluids 56, 2015. [8] Venning, J., Vortex locations for the longitudinal structures in the wake of the Ahmed body. DOI: 10.13140/RG.2.1.1301.4882 .