Light Weighting of Body Structure for Drive-Away Structure-Borne Noise Targets Bhaskar R Gangu Lead Engineer GMTCI ITPB Bangalore 560066 INDIA bhaskar.gangu@gm.com Varun Agarwal Technical Lead GMTCI ITPB Bangalore 560066 INDIA varun.agarwal@gm.com Soundararajan S Engineering Manager GMTCI ITPB Bangalore 560066 INDIA soundararajan.s@gm.com Keywords: Structure borne Drive away noise, Topology optimization, Shape optimization Abstract Environmental awareness in modern times led to evolution of stringent CO2 norms. This has forced automotive OEMs to design automotive structures with optimal mass. This along with other vehicle requirements such as increased engine power, fuel economy enablers like start/stop, displacement on demand, have significantly increased the challenge in the field of Noise and Vibration. This has pushed design engineers to explore all means to achieve Noise and Vibration specifications without sacrificing other metrics. CAE based Optimization is one such tool, which allows the body structure engineers to generate designs, which has ability to balance these conflicting requirements. It allows generation of manufacturable light-weight structure without compromising on Noise and Vibration performance. In this work, modifications to the body structure are made by combining topology and shape variables, while achieving same or improved noise performance in a drive away event. The topology optimization is applied to portions of the body, which do not belong to A-surfaces, while shape and size optimization is used to optimize dimensions of select body sections. This study leverages the recent capabilities in Altair OptiStruct optimization solver, which support the possibility of balancing such intricate performance targets and incorporation of multiple variables in single optimization schedule. Introduction: Automotive OEMs are looking for means to reduce mass without compromising on performance. CAE based Optimization plays an important role in achieving the reduced mass while keeping similar performance or even an improved performance in certain cases. This work presents how body rear end panels, mass reduction was achieved keeping the drive away noise performance within required targets. Conventional methods to reduce the mass while achieving desired noise performance needs many iterations. A combination of topology and shape optimization techniques was deployed to achieve most efficient distribution of material and an optimized cross section for the rear end back panels. Problem Definition / Background In a Drive away event the engine rpm is increased from idle to its maximum value. A real life vehicle may generate air borne and structure borne noises, which can be attributed to Body structure, powertrain, 1
wind etc. Using Finite element method, low frequency powertrain induced, structure-borne noise can be evaluated to good fidelity. For this purpose required loads are applied at crankshaft and noise is measured at driver ear and rear passenger ear. During development of one of the body frame integral small cars, a drive away noise issue (commonly known as boom) is observed in the band of 1800-2200 rpm. A boom (increase and decrease of noise level with increasing rpm) is an undesired noise characteristic as it is easily noticeable during drive away and is very annoying. Using modal participation technique, the source of the noise was zeroed down to body rear end back panel. The baseline design for the rear end is shown in Figure-1. Drive away noise results at driver ear (for baseline design) are not meeting the target as shown in Figure.2. Figure 1: Base body rear end panels Figure 2: Drive away noise results with base rear end panels The operational deflection shape at rear end back panel at 48Hz is shown in Figure-3. Operational deflection shows weak section of rear end back panel. Figure 3: Operational deflection shape at 48Hz 2
A closed rear end back panel design (figure: 4) was proposed to resolve this issue and also to achieve improved performance in some other vehicle performance areas. The closed section design improved the stiffness and improved noise level by 7dB (A) and below the desired target, but with a mass penalty. A comparison of pressure response for the two designs is shows in the figure 5 against the required target. Now the challenge was to reduce the mass of the rear end back inner panel without compromising on drive away noise performance. A combination of topology and shape Optimization approach was considered to solve the problem. Figure 4: Closed section rear end back panel Figure 5: Drive away noise with closed section rear end back panel Optimization: To find an optimum surface topology for the rear panel, the first step is to close all cutouts in the baseline design. The second important step is to divide this panel surface into two parts, a design space and a non-design space. Non-design space generally has areas associated with weld lines, fillets and bolt locations. Figure-6 shows the rear end inner back panel for topology optimization. In first trials, only topology optimization was performed on rear end back inner panel. Optimization setup was done using Altair HyperMesh 12.0 and Optistruct 12.0.212 as a solver. The target curve in the band of 1400-2300 rpm was used to define optimization constraints. As the noise target is generally defined in db (A) scaling (as shown in the Figure-7) it was a bit of a challenging task to pose the constraints in db(a). 3
Figure 6: Optimization and non-optimization zones Figure 7: Optimization target For this the acoustic pressure response at each frequency was extracted using DRESP1 card. The acoustic pressure was then converted from db and to db (A) using the following A-weighting formula (1), f is the frequency. Convertion was done using the DRESP2 card. 12200. 20.6 107.7 737.9 12200 2.02010. (1) The objective was set up as minimization of rear end back inner panel mass. Topology optimization results are shown in Figure-8. Figure-9 shows the optimization results against target. Topology results show the regions where material not needed with lower elemental density and regions where material needed with higher elemental density. Optimization results shows 50% mass saving compared to initial mass. This saving may be less when a manufacturable design developed out of the optimized design. Figure8. Topology optimization results Figure9. Topology end results against optimization target 4
Topology optimization results show a redistribution of mass and the blanked out regions to achieve the target performance in the least mass solution. Directionally the topological solution has ability to meet noise targets. A focused look into the topological results given in Figure-8 highlighted the need to question the usability of material in the encircled portion and point towards low need of material in this region. So it was decided to perform topology and shape optimization together. To accomplish this shape variables were added to the optimization setup for both inner and outer rear panels to enable section changes in vertical and fore aft direction. The shape variables were created using HyperMorph. Increased morphing capability in HyperMorph helped in fast generation of the shape blocks and creation of shape variables. The shape variables are bounded so that in vertical direction the section can change from 0 mm to -40 mm. Similarly in fore-aft direction the section can change from 0 mm to +10 mm as shown in Figure-10. A negative value here denotes that the section is contracting. Figure10. Shape variables in vertical and fore-aft directions Unlike the previous study, the existing surface topology is taken as the starting point for topology optimization (with-out filling openings). Combined topology and shape optimization results are shown in figures 11, 12 & 13. Results shown that the section changed considerably in vertical direction (around 60% of maximum allowable range). Figure-14 shows the optimization results against the optimization target. All constrained are within the given limit. Optimization results shows 35% mass saving compared to the initial mass. This mass saving combined mass saving compared to both inner and outer panels. 5
Figure11: Topology results Figure12: Shape change Figure13: Shape change section view Figure14: Optimization end results Vs optimization target Results and Discussion: Combined topology and shape optimization results considered and applied results to base panels. Figure 15 shows the optimization to design. Design shows both shape change and material removal. Major advantage of the free topology optimization, got material saving in-between welds as shown in figure15. Design saves 14.5% mass compared to base inner rear end back panel. Structural borne drive away noise results with final design shown in figure 16. Drive away noise results shows similar results as base results. Results are well below the target and improved drive away noise results in between 1800-2300 rpm. 6
Figure15: Optimization to design Figure16: Drive away noise results with final design Conclusions: - Optimization approach used to save the material from base panels, without compromising on the structure borne drive away noise performance and without going through many manual iterations. - The challenge was to define the optimization targets in db(a) scaling and over a frequency range, which were effectively implemented. - Initially applied topology optimization, which hinted for the combined shape & topology optimization. - Combined shape and topology optimization performed on body rear end back panels. - Shape and topology optimization results applied to get the final design of rear end back panel. - Final design validated, which shows similar drive away noise results as base and with mass saving. - Saved 14.5% mass for rear end inner back panel with improved drive away noise results. Acknowledgements Authors would like to thank management of GMTCI, for their support and allowing publication of this work. Authors would like to thank you Mr. Sangyun Lee from GM-Korea and grateful to TCI N&V colleagues. [1] Altair optistruct user and reference guide. References 7