Motorcycle Cornering Improvement: An Aerodynamical Approach based on Flow Interference A Master Thesis in Fluid Mechanics
Author: Vojtech Sedlak
Supervisor/Examiner: Alessandro Talamelli Technical Advisor: Stefan Wallin
Department of Mechanics and Department of Aeronautical and Vehicle Engineering
Royal Institute of Technology KTH 2012 Contents
Nomenclature 1
1 Introduction 3 1.1 Early History of Motorcycle Aerodynamics ...... 3 1.2 Focus on cornering ...... 4 1.3 Serious Attempts on Wing Use ...... 6
2 Project Overview 8 2.1 Concept ...... 8 2.2 Mechanical aspects ...... 10 2.2.1 Calculations ...... 11 2.2.2 Evaluating the Effect of Interference ...... 12 2.2.3 Anhedral angle effect on Vertical and Horizontal Forces ...... 14 2.2.4 Overall performance ...... 14 2.3 Speed estimation ...... 17 2.3.1 Airfoil Selection: NACA 23015 ...... 18
3 Problem Specification 19 3.1 Identifying Variables - The Buckingham Pi Theorem ...... 19 3.2 Problem Overview ...... 20 3.3 Predicting Near-wall Cell Size ...... 21
4 Numerical Approach 23 4.1 Work Flow ...... 23 4.2 Meshing ...... 24 4.3 Numerical Solver ...... 25
5 Preliminary 2D test-case 26 5.1 Geometrical Setup ...... 26 5.2 Mesh ...... 27 5.3 Solver ...... 28 5.4 Results ...... 29 5.4.1 Airfoil Properties ...... 29 5.4.2 Interpretation of Interfered Airfoil results ...... 30 5.4.3 Interfered Airfoil at α = 0◦ ...... 31 5.4.4 Interfered Airfoil at α = 4◦ ...... 32 5.4.5 Interfered Airfoil at α = 8◦ ...... 34
6 Simple 3D case 38 6.1 Geometrical Setup ...... 38 6.2 Mesh ...... 39 6.3 Solver ...... 40 6.4 Results ...... 41
7 Final Concept 43
1 8 Discussion 45 8.1 Additional Work ...... 45 8.2 Further improvements ...... 45 8.2.1 Higher top speed ...... 45
9 Conclusion 47
Acknowledgments 48
A Appendix: Data 49 A.0.2 Data for 2D cases ...... 49 A.0.3 Data for 3D cases ...... 50
B MotoGP Regulations 2012 52
2 Abstract
A new aerodynamic device, based on flow interference effects, is studied in order to signifi- cantly improve the cornering performance of racing motorcycles in MotoGP. After a brief overview on why standard downforce devices cannot be used on motorcycles, the new idea is introduced and a simplified mechanic analysis is provided to prove its effec- tiveness. The concept is based on the use of anhedral wings placed on the front fairing, with the rider acting as an interference device, aiming to reduce the lift generation of one wing. Numerical calculations, based on Reynolds-averaged Navier-Stokes equations, are performed on simplified static 2D and 3D cases, as a proof of concept of the idea and as a preparation for further analysis which may involve experimental wind-tunnel testing. The obtained re- sults show that the flow interference has indeed a significant impact on the lift on a single wing. For some cases the lift can be reduced by 70% to over 90% - which strengthens the possibility of a realistic implementation.
Abstract in Swedish: Sammanfattning
Ett nytt aerodynamisk koncept som nyttjar effekter av fl¨odesinterferenser ¨arutv¨arderati syfte att p˚aett noterbart s¨attf¨orb¨attraen roadracing-motorcykels kurtagningsm¨ojligheter. Efter en kort genomg˚angav varf¨ordiverse klassiska “downforce” l¨osningarej ¨arapplicerbara p˚amotorcyklar, presenteras det nya konceptet. Varp˚aen mekanisk analys genomf¨orsi syfte att se ¨over dess till¨ampbarhet.Konceptet bygger p˚aanhedrala vingar som placeras p˚aden fr¨amrek˚apan,d¨arf¨orarenagerar som ett interferensobjekt, och f¨ors¨oker st¨oraut lyftkraften som den ena vingen genererar. Numeriska ber¨akningarbaserade p˚aRANS-ekvationer ¨ar utf¨ordai f¨orenklade statiska 2D och 3D fall. Som ett vidare steg rekommenderas vindtun- neltester. Resultaten visar att fl¨odesinterferenser ¨arytterst m¨arkbaraf¨orvingar och i vissa fall kan lyftkraften reducerats med 70–90%. Detta f¨orst¨aker m¨ojlighetenf¨oren realistisk implementering. Nomenclature
Symbols
2 Aw area of a wing [m ] 2 bw half-span of a wing [m ] Cd, CD drag coefficient for 2D, 3D case [-] Cf friction coefficient [-] CI interference coefficient [-] Cl, CL lift coefficient for 2D, 3D case [-] cr airfoil chord length [m] dc diameter of interference device[m] F~ , F force vector, force [N] 2 g0 sea level gravity constant, 9.81 [m/s ] K turbulence kinetic energy [m2/s2] k number of fundamental dimensions [-] l∗ viscous length scale [m] M~ , M moment vector, moment [Nm] M∞ free stream Mach number [-] m mass [kg] m0 mass motorcycle [kg] mr mass rider [kg] N normal force [N] n number of independent physical variables [-] P mean static pressure [N/m2] p static pressure [N/m2] p0 fluctuating pressure part [N/m2] R~, R reaction force vector, reaction force [N] rc radius of a corner [m] ~r distance vector [m]
Recr Reynolds number for an airfoil chord[-] ReL Reynolds number for specific lenght[-] −1 Sij mean strain rate tensor [s ] T temperature [K] U mean velocity [m/s] u0 fluctuating velocity part [m/s] uτ friction velocity [m/s] v velocity [m/s] xc position of interference device in x-direction [m] yc position of interference device in y-direction [m] yn wall-distance [m] y+ normalized wall-distance [-] α angle of attack [◦] δij Kronecker delta [-] µ dynamic viscosity [kg/(m s)] µs static friction coefficient [-] ν kinematic viscosity [m2/s] 2 νt turbulence eddy viscosity [m /s] Π dimensionless product [-] Πcount number of dimensionless products [-] ρ density [kg/m3] 2 τw wall shear stress [N/m ] ◦ φwing anhedral angle of wing [ , rad] ◦ ϕlean lean angle of motorcycle [ , rad]
1 Acronyms
CAD Computer Aided Design CFD Computational Fluid Dynamics FIM F´ed´erationInternationale de Motocyclisme FL Finish Line RANS Reynolds-averaged Navier-Stokes equation VLM Vortex Lattice Method
Constant values
To avoid any misconceptions, due to different definition-style in various literature, following values yield throughout this document. They are mainly based upon the standard values provided by Ansys Fluent.
−2 g0 9.81 [m/s ] 3 ρ∞ 1.225 [kg/m ] µ∞ 1.7894e-05 [kg/(m · s)] T∞ 288.16 [K]
2 Chapter 1
Introduction
A first thing to identify is what motorcycle type should be subjected for improvement. Clearly when aerodynamics is the topic, the fast going road racing machines are the ones with most to gain. The road racing motorcycle is a definition that includes all types of motorcycles that may compete by doing laps or sprint races on paved, closed down, purpose built race tracks. These tracks have a high number of corners, thus making cornering speed and agility a key element for success. Within road racing, there are several different categories in which motorcycles may compete. Mainly there are two premier classes. One that is referred to as MotoGP - a category purely focused on prototype racing with less strict regulations in an attempt to encourage creative thinking and development. The other category is Superbike World Championship where the focus is to get as much racing for as small cost as possible. This results in production bikes that are heavily regulated. As this thesis aims to provide a plausible aerodynamic cornering improvement, the aim is to be within the more lenient MotoGP regulations. However, the aim is not to present a final concept, but merely to provide some basic analysis whether the idea is at all realistic or not.
1.1 Early History of Motorcycle Aerodynamics
In the early years much of the focus regarding aerodynamics for motorcycles, was simply focused on streamlining. And very much so, various concepts that were brought to light, would challenge different speed-records of the day. The idea was basically to create a tear- drop shaped faring that would cover the rider as much as possible. They also tried to build the motorcycle as low and narrow as possible to reduce the frontal area.
Figure 1.1: Left: In early 1950’s the typical “dustbin” fairing were popular as shown by Giulio Carcano’s Moto Guzzi. Right: In 1957 FIM banned these types of fairings and the “dolphin” shape quickly became the norm. [5]
However, these massive fairings turned soon out to be dangerous in crosswinds and cumber- some when cornering. It soon became clear, that to make fast and safe motorcycles, which
3 can go around twisting race tracks, many other considerations had to be taken into account. This led to motorcycles with more open, yet sleek, fairings. A concept which seems to have stood the time, since the basics layout, appears to be similar to even todays machines. Knowing this, raises the question whether the industry is reluctant to change or if this basic concept is actually so good, that any radical changes will most likely fail. What is known, is that countless attempts have been made to improve on this classic design. How many and with which ideas, is something one can only speculate, since these sort of things are usually close guarded secrets.
1.2 Focus on cornering
It is clear that if one would find a way of how to improve cornering, the advantage would be substantial. A typical MotoGP track consists of a high number of sweeping corners. Even the majority of road sections that are between corners, that may look like straights, are actually no real straights since the motorcycle has to prepare for the next corner right away. In section 2.3, an example is given of the traditional racing track TT circuit Assen. Notice how turn 1 to 4 can be considered as a one sweeping corner. Cornering can obviously be improved by aerodynamic means. Placing a wing that creates negative lift (downforce) increases the normal force, thus enabling the static friction force to reach higher values.
In uence of Lift coe cient during Cornering on a vehicle for unbanked turns
600
2 1 2 mv equation: s ( mg 0 – 2 C L ρ ∞ v Aw ( = 500 rc
values: s = 1 m = 600 kg A = 1.47 m2 CL = –2 400 w
v [km/h]
300 CL = –1
200
CL = 0
100
50 100 150 200 250 300
rc [m]
Figure 1.2: Visualization of the great speed advantage a higher downforce can provide. In some cases the speed can be more than doubled.
In figure 1.2 an example is given where a simplified vehicle is cornering at various corner radii. The figure shows that if the downforce is increased (CL = -2) the vehicle may go more than twice as fast through corners with low curvature. There are some radical concepts that have been implement in the past, based on this way of thinking and some of them reached the public attention and curiosity. One of these concepts was conceived by a university student, Rodger Freeth in 1977 (figure 1.3). He added two horizontal wings, in the front and back with the hope that it would create extra downforce on the tires in mid-turn, to improve cornering speed. The largest wing was placed behind the rider, mounted on the back of the rear sub-frame and had a span of 700 mm with a chord of 245 mm. The front wing was attached to the lower fork sliders and had a span of 660 mm and chord of 130 mm. [10]
4 Figure 1.3: Rodger Freeth and his concept “Aerofoil Viko TZ750A” from 1977 [10]
Naturally, when a motorcycle is leaned into a corner, this wing will generate negative lift (downforce) at the angle at which the bike is leaning. This will not only generate a vertical force component which will make the bike stick to the ground. It will also add to the lateral force component, pushing the bike of the track. Perhaps back in 1977, when Freeth was racing his bike the lean angles were not so great, maybe not even scraping his knees. Today these lean angles can typically reach over 50◦. In such case the lateral force component becomes greater than the desired vertical one. On top of all that this concept got banned by the controlling body, due to the great risk of entanglement in close racing. As many have pointed out over the years a far better concept would be to mount the wings on a gyroscopic tilting device. Thus making sure that no matter what lean angle, the wings would always be parallel to the ground. This way the lateral force component would be eliminated. Even if such device would be allowed, the placement would remain a problem. It is important to place the wings in the undisturbed free-stream. That would mean either placing it above the bike or on the sides. The bike itself is usually about ∼500 mm wide (excluding handlebars), so to put them on the sides would add major width (since they have to be of a significant size). On top of all that they would have to be movable, which adds additional level of complexity to the design and makes them unusable in competitive racing as MotoGP due to regulations (Appendix B).
Lateral force Fx component Tilting Wing Wing
F Vertical force Vertical force y component Fy N component N