Aerodynamics Assignment
assignment2014.dvi AERO4260 Assignment - Supersonic Intake Design Background and Design Specifications Australia has a unique facility in the Woomera Test Range. It has been used to test long range supersonic air vehicles such as the Sea Wolf, Rapier, Sea Dart, and Bloodhound surface-to-air missiles, the Black Knight research rocket, the Blue Steel nuclear stand-off missile, the Malkara anti-tank missile, the Ikara anti-submarine missile, and the HyShot scramjet series. Recently, it has been employed to test UAVs, most notably the BAe Systems Taranis. Figure 1: Grollo Aerospace’s ramjet powered drone (left), and the ramjet on a testbed (right) [Credits: Grollo Aerospace website] To exploit this unique facility, Australia has decided to invest in the capability to design and develop it’s own supersonic target drones which will be able to operate at a range of Mach numbers. This is an extension of systems under development for a specific Mach number, for example Grollo’s ramjet powered target drone shown in Figure 1. A key challenge here is to design the intake for a range of different Mach numbers, and the prime contractor would like to investigate the feasibility of operating several different intake designs for the same vehicle. The engine designers require a mass flow rate of 40kg/s, with a target inlet Mach number of 0.6 at the compressor face (this may be difficult to achieve exactly). The intake and subsequent diffuser will have a square cross-section. To avoid flow separation in the diffuser, the diffuser should not expand at more than 5o (typical limit for a subsonic diffuser is around 7o [1]). The prime contractor has approached you for an initial design for the intake. They would like to understand the trade-offs to be made between a simple two-dimensional Pitot intake, a two-shock and a three-shock external compression systems. To give design certainty, they want to see your analytical solutions validated numerically using Computational Fluid Dynamics (CFD). Individual Project As an engineer in the sub-contracting firm, your tasks are: 1. Design three different intakes using external compression ramp/s coupled to an internal subsonic divergent section to satisfy the mass flow rate at your specific operating Mach 1 number. The Mach number which you need to design for is Mach 2. This includes [35 marks total]: • a pitot intake • a single ramp external compression design (two-shock) • a two-ramp external compression design (three-shock) Your report needs to detail for each of your designs: • a schematic of your designs with dimensions and ramp angles fully labelled [10 marks] • the predicted Mach numbers, static pressures and total pressures behind the oblique shock/s, normal shocks and at the engine face [10 marks] • the reasoning behind your design - why is each design optimal for your case? [10 marks] 2. Verify that your design is going to perform as expected by running a CFD simulation of the intakes you designed. In the report [35 marks]: • detail and justify the numerical methods, boundary conditions chosen and why you are confident your results are correct.[15 marks] • discuss visualisations of Mach number and static pressure in the intake [5 marks] • compare the predicted flow properties at the fan face with your analytically computed results. What differences are there and why? [15 marks] 3. During flight, variations of angle of attack of the order of ±5o are expected. Assuming that the intake geometry is similar to that of the HIFIRE/Grollo configurations in Figures 1 and 2 where the two intakes are attached back-to-back, assess the change in performance at the maximum and minimum angles of attack for the two ramp case in terms of total pressure recovery, static pressure, and flow structures present. [10 marks] Figure 2: The HIFiRE back-to-back configuration [2] 4. You are convinced that your proposal can be better. To win the lucrative contract for the detailed design and manufacture, propose and evaluate a further design based on a smooth Prandtl-Meyer compression. Detail clearly the design choices you make, present a design drawing, calculate the total pressure recovery expected, and demonstrate through a CFD simulation what your design achieves [20 marks] Report and Hand-in Write a report summarising your findings in a Word Document or PDF. Structure your report as a publication with Abstract, Introduction, Results and Conclusions sections. The report should 2 not exceed 2500 words, any additional words will not be marked. It can contain any number of figures, but figures and tables in the report must be numbered and discussed. An electronic copy of the report should be submitted electronically to
[email protected] with compliance statement by 5pm October 25th 2018 This assignment should take the average student 20 hours to complete. Note however that indi- vidual simulations may take 10+ hours to converge, and you will need to do several simulations. Hints In FLUENT you will need to create the two dimensional geometry in Design Modeler. For this, you will largely be able to follow the tutorial on the supersonic ramp which you undertook earlier. The key differences now are that you will have two potential places that the flow can exit, either through the engine face or around the cowl and out of the domain past the intake. Intake flows are notoriously difficult to simulate, they are very sensitive to the back pressure specified (engine operating point), hence I have uploaded an example ANSYS workbench analysis onto Blackboard for you to use as a reference. Note that I expect you to start your own design from scratch, not to use this poor design as an example! Key hints: 1. Start with a coarse mesh which runs quickly simply to verify your boundary conditions. 2. Boundary conditions are tricky. Specify ‘pressure farfield’ for the external boundaries, and ‘pressure outlet’ for the engine face. 3. There are two approaches to the ‘pressure outlet’ (i) specify a back pressure which you guess initially and then refine later on, (ii) specify a target mass flow rate and run the simulation. For option (i), to achieve a converged mass flow you will first have to ‘start’ your intake - begin with a lower specified ‘back pressure’ in the boundary condition specification and converge the solution. Note that by ’lower’ I mean perhaps 25% lower than you expect to have behind the normal shock. Then increase the back pressure gradually until you reach your design point. For option (ii) note that in 2D you assume that the geometry extends 1m into the third dimension when calculating the mass flow rate. e.g. if your exit is 0.5m x 0.5m and your target ρu = 200kg/m2s then you will input a target mass flow rate of 100kg/s. 4. To ensure that you have subsonic flow inside the intake, it is prudent to set the initial conditions so that the external flow is at the freestream Mach number (supersonic), but the internal flow is subsonic. To do this in Fluent, you need to first define a ‘region’ where the flow will be subsonic. This is done by selecting ‘Adapt/Region’ from the top menu, then inputting the coordinates of the bounding box around your expected internal subsonic region (lower right/upper left x and y). Click ‘mark’. For the initialisation select the far field condition from the ‘Compute From’ drop down box. Click ‘initialize’. Then click ‘patch’. This opens a dialogue box where the zone that you ‘Marked’ is now listed under ‘Registers to Patch’. Click on that register, the change the velocity to a subsonic value in the interior, and I’d recommend also changing the pressure to the pressure you specify at the pressure outlet. You can confirm that your initialisation has worked by plotting the contour flood from the ’Graphics and Animations’ menu. 5. Set autosave (under ‘Calculation Activities’) to a sensible number - if the simulation crashes you can then go back to a previous time step by going to File/Import/Data. Saving every 1000 iterations may save you some pain. 3 6. FLUENT’s implicit steady state solver has a standard CFL of 5. This may be too high, so you might need to reduce this. In my example case, a CFL of 1-2 worked well. 7. Begin with a 1st order simulation to gain an initial converged solution - setup a mass flow monitor at the engine face, which you can find under ‘monitors’. References [1] Mattingly, J.D., Heiser, W.H. and Pratt, D.T., Aircraft Engine Design, AIAA Education Series, 2nd ed., 2002 [2] Smart, M.K. and Suraweera, M.V., HIFiRE 7 - Development of a 3-D Scramjet for Flight Testing, 16th AIAA/DLR/DGLR International Space Planes and Hypersonic Systems and Tech- nologies Conference, AIAA Paper 2009-7259, 2009 4