Müller, W

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The physics of ski jumpingW. MüllerDepartment of Biophysics, Medical University of Graz, Austria1 IntroductionThe Olympian ideal of going faster, jumping further and leaping higher than the opposition is central tocompetitive sports. Winning or losing in sport is related to a number of factors, and biomechanics, an-thropometrics, and aerodynamics play a major role in many sports. This lecture focuses on ski jumping.Performance in ski jumping is determined not only by the motor abilities of the athlete, but alsoto a large extent by the aerodynamic features of the equipment used and by a low body weight. Manyski jumpers were extremely underweight to the point of having a body mass index (BMI = m/h2 of16.4 kgm−2 (height h = 1.73m, body massm = 49 kg).Severe eating disorders (e.g., anorexia nervosa, bulimia; [1]) were health problems of major con-cern in this sport. Strategies for improving the health, fairness, and safety of the athletes by modifyingthe regulations have been developed by the lecturer and his research team in close co-operation withthe International Ski Federation (FIS), the International Olympic Committee (IOC), and the AustrianResearch Funds (FWF). Based on our scientific studies the FIS has passed changes to the ski jumpingregulations which relate relative body weight (in terms of BMI) to the maximum ski length permitted.Shorter skis (i.e., ‘smaller wings’) compensate for the advantage of very low weight and thus it is notattractive for the athletes to be underweight any more [2].Our analyses of contemporary ski jumping employ field studies during World Cup and OlympicGames competitions, wind tunnel measurements, computational fluid dynamic (CFD) modelling of aero-dynamic forces and torques, computer simulations of the flight trajectory, and computer-modelling-baseddesign of jumping hills [3–7].2 The dynamics of ski jumping: a brief descriptionSki jumping puts high demands on the athlete’s ability to control posture and movement. During thein-run the athlete tries to maximize acceleration by minimizing both the friction between skis and snowand the aerodynamic drag in order to obtain a maximum in-run speed v0, which has a high degree ofinfluence on the jump length. The friction between skis and snow is not well understood. The physicstext book solutions to this problem do not reflect reality. The theoretical as well as the empirical basis forthese complex problems are not sufficiently developed. The reduction of aerodynamic drag in the in-runphase is primarily a question of the athlete’s posture and his dress. Owing to the curved form of the in-runjust before the ramp, the athlete has to counteract the centrifugal force acting on him (as seen from theathlete’s point of view) and this phase is immediately followed by the athlete’s acceleration perpendicularto the ramp due to the muscular forces exerted. During this decisive phase of approximately 0.3 s durationthe athlete has to produce a maximal momentummvp0 perpendicular to the ramp (m: mass of the athleteplus equipment) through which an advantageous take-off angle has to be attained. The take-off velocityvector ~v00 is given by ~v00 = ~v0 + ~vp0 with ~vp0 being the velocity perpendicular to the ramp due to theathlete’s jump. Simultaneously, the athlete must produce an angular momentum forwards in order toobtain an advantageous angle of attack as soon as possible after leaving the ramp. During the jumpphase the athlete must anticipate the magnitude of the backward torque due to the air-stream so that theforward rotation will be stopped at the right moment. If the forward angular momentum is too low, adisadvantageous flight position reduces velocity and, therefore, results in bad competitive performance.Worse is the production of too much forward angular momentum because this substantially increases thetumbling risk. During the flight the gravitational force Fg, the lift force Fl, and the drag force Fd actupon the athlete:269Fg = mg ; Fl =ρ2v2clA =ρ2v2L ; Fd =ρ2v2cdA =ρ2v2D .The velocity of motion along the flight path v has the components x˙ and y˙v2 = x˙2 + y˙2 .The athlete can strongly influence the aerodynamic forces by changing his posture. He can affect thedrag force, the lift force and the torque, and thereby significantly influence changes in his flight positionrelative to the air stream. The flight path is described by the following non-linear differential equationswhich can be solved numerically by using proper iterative procedures:v˙x =(−Fd cosϕ− fl sinϕ)mv˙y =(−Fd sinϕ− fl cosϕ)m− g .x˙ = vx y˙ = vyIn order to achieve highly realistic computer simulations, it is necessary to be able to simulate changesin posture and position during the simulated flight, i.e., changes in the resulting aerodynamic forces. Wedeveloped such a computer model of ski jumping in 1995 [3, 4].2.1 Basic aerodynamic problemsAerodynamic questions related to sports are complex and manifold. For this reason the influencingphenomena should be investigated by both theoretical and experimental approaches.2.1.1 Theoretical approachThe Navier–Stokes equations which describe the dynamics of Newtonian fluids have inherent majormathematical difficulties. Exact solutions are only possible in special cases involving objects with simplegeometries that do not exist in sports. The numerical solution becomes increasingly difficult as Reynoldsnumbers increase, even when supercomputers are used for the numerical solution of these non-linearpartial differential equations. Owing to the non-linearity of the equations, a variation of the geometricaland fluid mechanical parameters can result in bifurcations and the non-linear fluid system can displaydeterministic chaos. Computational fluid dynamics is proving to be invaluable at the early stage oftrend analysis prior to prototype testing in several kinds of sports (e.g., Formula 1, yachting). Initialstudies of ski jumping have also been made. However, the thin boundary layers around moving bodieshave to be resolved accurately and the associated physical effects are notoriously difficult to predictaccurately in CFD. A combination of measurement and CFD is the state-of-the-art approach necessitywhen appropriate predictions for sports aerodynamics are desired. The best configurations found in windtunnel tests still have to be tested in the field by the athletes before their competition debut. In skijumping (and many other sports as well) the characteristic dimensions of the body and/or equipment andthe typical velocities result in Reynolds numbers between Re = 104 and Re = 106 where pronouncedchanges in the drag coefficient may occur: cd = cd(Re). This was already shown for the sphere in1912 by G. Eiffel [8] and in 1914 by C. Wieselsberger [9]. The sensitivity of the transition from laminarto turbulent flow on the roughness of the surface (or on small surface obstacles) was also shown. Thetheoretical approach describing the lift forces is at least as difficult as the discussion of the drag, andscience still does not have a complete understanding when turbulence phenomena occur. Micro effectscan be the starting point for major flow changes in a system and to discount them in modern turbulencemodels or even to ignore them leads to completely inaccurate predictions of the whole process.2.1.2 Experimental approachSmall changes in the lift and/or drag coefficients can have pronounced consequences for the sport con-cerned. Therefore, accurate measurements in the wind tunnel are necessary when these data are to be2W. MÜLLER270used for subsequent mapping of ‘real world’ problems. A wind tunnel with a sufficiently large cross-section is necessary to minimize blockage effects. Because the athletes must be studied in various pos-tures in full gear, adequate positioning devices are required. However, these devices can cause secondaryerrors even when the forces acting solely on the devices are considered. Positioning devices that leadto small disruptions in the air flow around the athletes need to be designed. The experimental problemsare smaller for those questions where only relative changes in the aerodynamical parameters are to beconsidered. However, the design of experiments reflecting the special circumstances of different sportsis not trivial at all. A reliable interpretation of the effects associated with different aerodynamical charac-teristics usually necessitates a computer-based analysis of the experimental data. So, for instance, a givenincrease of the drag areaD = cdA during the initial flight phase reduces the jump length much more thanwould be the case during the final phase of the flight. Analogously, a change in the drag coefficient of theathlete in the crouched position may occur during the in-run phase due to a Reynolds-number increasebeyond the critical value (e.g., when the velocity increases). These phenomena may strongly dependon individual body dimensions of the athletes. Therefore, a computer-assisted analysis of the resultingeffects based on experimental aerodynamic data is the only way of appropriate treatment (based on theequations of motion for the in-run phase). For all kinds of sports where a minimal aerodynamic drag isimportant, the factors influencing the shift of the drop in the drag coefficient within the critical Reynolds-number range are most important. These factors have not been sufficiently understood in the context ofsports involving complex objects like human bodies. The surface structure (dress, jumping suit, down-hill suit, etc.), temperature effects, vibration of surfaces like human skin, tension of the surface materialetc. are all influential factors. Considering the significant influence of, for example, the geometry of asphere versus an aerodynamically well-shaped object on the lift and drag coefficients (the difference canbe a factor of 100; see, for example, H. Schlichting and K. Gersten [10]) the predominant importance ofimprovements in this direction is evident.Very little is known about the drag and lift forces acting on the complex structure of a human bodyin an air-stream. From the aerodynamic data according to flight positions it is evident that the lift forcesin ski jumping are of the same magnitude as the drag forces and that the flight length is very sensitive tochanges in both. It is well known that pronounced changes of the lift coefficients of wings occur in thecritical range of the Reynolds number, and it has been found from measurements in low-turbulence windtunnels that the degree of turbulence of the outer flow has an important influence on the lift coefficient ofa wing. Yet, no systematic study of these characteristics in relation to human bodies in the air-stream hasbeen made. Our knowledge of wings cannot be adequately transferred to the flow around human bodiesbecause the geometric form is not at all similar and wings usually do not work at angles of attack of upto 50 degrees.AcknowledgementsThis work was supported by the International Olympic Committee (IOC), the International Ski Federa-tion (FIS), and the Austrian Research Funds (FWF, P-15130 and P-14388).References[1] K. Sudi, D. Payerl, P. Baumgartl, K. Tauschmann and W. Müller, Anorexia athletica, Nutrition 20(2004) 657–661.[2] W. Müller, W. Gröschl, B. Schmölzer and K. Sudi, Body weight and performance in ski jumping:The low weight problem and a possible way to solve it, 7th IOC Olympic World Congress onSport Science, Athens, 2003, 43 D.[3] W. Müller, D. Platzer and B. Schmölzer, Scientific approach to ski safety, Nature 375 (1995) 455.[4] W. Müller, D. Platzer and B. Schmölzer, Dynamics of human flight on skis: Improvements onsafety and fairness in ski jumping, J. Biomech. 8 (1996) 1061–1068.3THE PHYSICS OF SKI JUMPING271[5] W. Müller, Biomechanics of ski-jumping: Scientific jumping hill design, in: Müller E.,Schwameder H., Kornexl E., Raschner C. (Eds.), Science and Skiing (Spon, London, 1997), pp.36–48.[6] B. Schmölzer and W. Müller, The importance of being light: Aerodynamic forces and weight inski jumping, J. Biomech. 35 (2002) 1059–1069.[7] B. Schmölzer and W. Müller, Individual flight styles in ski jumping: Results obtained duringOlympic Games competitions, J. Biomech. 38 (2005) 1055–1065.[8] G. Eiffel, Sur la resistance des sphères dans l’air en mouvement, Comptes Rendus 155 (1912)1597–1599.[9] C. Wieselsberger, Der Luftwiderstand von Kugeln, Z. Flugtech. Motor-Luftschiffahrt, 5 (1914)140–144.[10] H. Schlichting and K. Gersten: Grenzschicht-Theorie (Springer, 1997), p. 48.[11] E. Reisenberger, W. Meile, G. Brenn and W. Müller, Aerodynamic behaviour of prismatic bodieswith sharp and rounded edges, Exp. Fluids 37 (2004) 547–558.FiguresExamples of experimental resultsFig. 1: A ski jumper at the K = 120 m jumping hill in Park City. The nomenclature used for the positionangles is indicated, w is the resulting air stream vector.4W. MÜLLER272Fig. 2: Field research results obtained during the 19th Olympic Winter Games (Salt Lake City, 2002;venue in Park City). The histograms show the average values and standard deviations of position anglesfrom the best ten athletes in each of the five runs at the K = 120 m jumping hill. The number of anglemeasurements ranged from 18 to 50 at each position. The angle V of the skis to each other was determinedfrom digitized images taken from the end of the run out.5THE PHYSICS OF SKI JUMPING273Fig. 3: Schematics of the wind tunnel measurements. The figure shows the apparatus, which enabledalmost all postures of athletes and skis imaginable, and demonstrates the nomenclature used for theposition angles. This study used the large wind tunnel at Arsenal Research in Vienna. The tunnel has across-section of 5 × 5 m2. A 1.8 MW motor produces a maximum wind speed of 32 m s−1.Fig. 4: Wind tunnel measurements: The aerodynamic forces largely depend on the flight style. Large-scale wind tunnel in Vienna; Andreas Goldberger, wr 225 m (2000).6W. MÜLLER274(a) Different angles of attack α. The body position washeld constant (β = 9.5◦ and γ = 160◦) and the anglesof attack were 30◦, 35.5◦ and 40◦. The opening of theskis was held constant at V = 35◦. The interpolatingfunctions are: L = −0.43903 + 0.060743α − 7.192 ×10−4α2;D = −0.032061+0.01232α+2.283x10−4α2.(b) L and D values depending on the body-to-ski angleβ. The values shown have been taken at α = 35.5◦,γ = 160◦ and V = 35◦. The interpolating functionsare: L = −0.645718+0.0126185β−3.348×10−4β2;D = 0.408434 + 0.01364β + 3.9308× 10−5β2.(c) L and D values depending on the body-to-ski angleβ. The values shown here have been taken at α = 30◦,γ = 160◦ and V = 35◦. The interpolating functions are:L = 0.75037 + 8.86746x10−3β − 2.99665× 10−4β2;D = 0.578995 + 0.01201β + 2.91724× 10−5β2.(d) L and D values depending on the hip angle γ. Thebody-to-ski angle β and the angle of attack α were heldconstant (β = 9.5◦ and α = 35.5◦). The opening ofthe skis was held constant at V = 35◦. The interpolatingfunctions are: L = −2.442+0.04035γ−1.25×10−4γ2;D = 1.722− 0.01365γ + 4.5× 10−5γ2.Fig. 5: L and D values for model A (height h = 1.78 m)7THE PHYSICS OF SKI JUMPING275(a) Values are functions of time reflecting the ath-lete’s position changes during the flight(b) L/D ratio for the reference jump with modelAFig. 6: L and D values of reference jump of model AExamples of computer simulations of ski jumpingFig. 7: Results with the reference jump for model A. (a) shows the profile of the jumping hill in Sapporoand the trajectory y = y(x). Jumping hill parameters for Sapporo (K = 120 m): a = 11◦, b = −37◦,c = 35◦, H(K) = 59.449 m, N(K) = 103.391 m, r1 = 105 m, R2 = 120 m, M = 20 m, T = 7 m,S = 3.3 m. (b) is the velocity of motion v (solid line) and the horizontal component of this velocity vx(broken line). (c) shows the lift force Fl and drag force Fd acting on the athlete and his equipment. Theair density was set to 1.15 kg m−3; the mass of the athlete with equipment was 65 kg.8W. MÜLLER276(a) Jumping hill parameters for Park City (K = 120 m): α =−10.5◦, β = 35◦, βP = 38◦, βL = 37.77◦, γ = 35◦,h = 59.52 m, n = 103.51 m, r1 = 93 m, r2 = 105 m,rL = 356.5 m, l1 = 18.67 m, l2 = 13.90 m, t = 6.7 m,s = 3 m. For all jumping hills approved by the FIS the param-eters can be found in the FIS Certificates of Jumping Hills. Thetrajectories and velocities for three different masses (55, 65 and75 kg) are shown. The approach velocity v0 was 26.27 m s−1,the air density ρ = 1.0 kg m−3 and vp0 was 2.5 m s−1. The gustvelocity vg was set to 0.(b) Height above ground hg for three differ-ent masses as a function of the flight timet. The solid line shows a jump simulationusing a mass of m = 55 kg, the dotted lineform = 65 kg, and the broken line form =75 kg.(c) Analogous to Fig. (a), however, in thiscase a gust blowing constantly with vg =3 m s−1(ζ = 135◦) during the whole flightwas used.Fig. 8: Simulated jumps using the L and D tables from the reference jump for model A and the profileof the jumping hill in Park City9THE PHYSICS OF SKI JUMPING277

The physics of ski jumping

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