applied sciences Article The Impact of Skinsuit Zigzag Tape Turbulators on Speed Skating Performance Nando Timmer * and Leo Veldhuis Faculty of Aerospace Engineering, Delft University of Technology, 2629HS Delft, The Netherlands; [email protected] * Correspondence: [email protected] Abstract: At the 1998 Nagano Winter Olympic Games, zigzag tape was introduced on the race suit lower legs and cap of speed skaters. Application of these zigzag devices on live skaters and cylinders in the wind tunnel showed large improvements in the aerodynamic drag. These wind-tunnel results were unfortunately not widely published, and the impact of the zigzag strips in a real skating environment was never established. This paper aims to show the background of the application of the zigzag tape and to establish the impact it may have had on speed-skating performance. From comparisons of 5000 m races just before, during and just after the Nagano Olympics and an analysis of historic world record data of the 1500 m men’s speed skating, the impact of the zigzag tape turbulators on average lap times on 1500 and 5000 m races is calculated to be about 0.5 s. Keywords: speed skating; aerodynamic drag; zigzag tape; drag reduction; skating skinsuits 1. Introduction During the past decades, research and innovations in the field of sport apparel and Citation: Timmer, N.; Veldhuis, L. materials have helped to increase skating speed tremendously. The introduction and fine- The Impact of Skinsuit Zigzag Tape tuning of the clap skate mechanism, the thickness and metal compound of blades and Turbulators on Speed Skating the application of water-additives to reduce ice-friction all had their impact on improved Performance. Appl. Sci. 2021, 11, 988. speed-skating performance [1]. Skating suits evolved from the knitted outfits used during the early 1970s to the present suits with distributed rough patches [2,3]. https://doi.org/10.3390/app11030988 In the almost 25 years following the introduction of the one-piece skinsuit by the Aus- trian Franz Krienbühl, in 1974/75 until February 1998, the suits worn by elite speed skaters Academic Editor: Takeshi Asai developed gradually, mostly aiming at a better fit and at improved cooling characteristics. Received: 31 December 2020 While until 1998 the trend was to use increasingly smooth fabrics, at the 1998 Nagano Accepted: 20 January 2021 Published: 22 January 2021 Olympics, foam zigzag tape strips on the lower legs and the cap of the skating suit were introduced, aiming at significant aerodynamic drag reduction, Figure1. The application Publisher’s Note: MDPI stays neutral of the zigzag tape to racing suits by the Dutch team followed earlier measurements by with regard to jurisdictional claims in Eise Dobbinga, a Delft University of Technology (TU Delft) aerodynamics professor who published maps and institutional affil- experimentally investigated the impact of trip wires and seams on the aerodynamic drag iations. of cylinders and a mannequin leg covered with a skinsuit fabric [4]. He found that the drag coefficient of such a mannequin leg could be reduced by 22% when the entire leg was equipped with a 0.8 mm thick trip wire on both sides of the leg front end. These results unfortunately lay dormant for over 10 years, until in 1997 and January 1998, wind-tunnel measurements at TU Delft were performed on live skaters and cylinders, this time using Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. zigzag tape as a turbulator. Zigzag tape proved to be an efficient means to trip the laminar This article is an open access article boundary layer on a sailplane wing, thus avoiding drag rise due to laminar separation at distributed under the terms and low Reynolds numbers. This flow phenomenon is typically also present around the body of conditions of the Creative Commons speed skaters with smooth skinsuits. Total drag area reductions on static male skaters with Attribution (CC BY) license (https:// zigzag-tape strips on lower legs and cap were found up to 9%. Although some results were creativecommons.org/licenses/by/ presented in low-circulation media [5–8], unfortunately little of this work on the impact of 4.0/). zigzag tape, specifically on speed-skating performance, has been widely published. Appl. Sci. 2021, 11, 988. https://doi.org/10.3390/app11030988 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 20 Appl. Sci. 2021, 11, 988 2 of 18 work on the impact of zigzag tape, specifically on speed-skating performance, has been widely published. FigureFigure 1. TheThe foam foam zigzag zigzag tape tape for for the the lower lower legs legsand andcap introduced cap introduced at the 1998 at the Nagano 1998 Nagano Olym- Olympics. pics. The Dutch team was quite successful in Nagano; they gathered more gold medals thanThe ever Dutch before. team Although was quite the successful wind-tunnel in Nagano; tests were they very gathered promising, more gold the contributionmedals of thethan zigzag ever before. strips Although to reduce the the wind aerodynamic-tunnel tests drag were in very the actual promising races, the remained contribution unclear. The aimof the of zigzag this paper strips is to to reduc presente the results aerodynamic of wind-tunnel drag in the measurements actual races remained showing unclear the effects. of The aim of this paper is to present results of wind-tunnel measurements showing the ef- the strips and, in addition, to quantify the impact on speed skating performance the zigzag fects of the strips and, in addition, to quantify the impact on speed skating performance strips may have had in a real skating environment. the zigzag strips may have had in a real skating environment. 2. The Flow over the Lower Legs 2. The Flow over the Lower Legs The flow flow over over bluff bluff objects objects with with rounded rounded edges edges is typified is typified by the by Reynolds the Reynolds number number.. It describesIt describes the the ratio ratio of the inertia inertia forces forces (in (interms terms of flow of flowspeed speed V, characteristic V, characteristic length L length L andand densitydensity ρρ)) over over the the viscous viscous forces forces (in terms (in terms of the of dynamic the dynamic viscosity viscosity µ) in theµ )flow in the: flow: ρ. Vρ. L.V.L ReRe= = (1) (1) μ µ For cylinders and spheres, the characteristic length is the diameter, D. The Re-num- ber isFor closely cylinders related and to the spheres, laminar the-to- characteristicturbulent transition length location is the diameter,of the boundary D. The layer Re-number isflow closely along relatedthe surface to theof an laminar-to-turbulent object and the resulting transition drag. Transition location is a delicate of the boundaryprocess, layer flowwhich along is not the only surface influenced of an by object the shape and the of the resulting object and drag. its Transition orientation is towards a delicate the process, whichincoming is notflow only, but also influenced by its surface by the roughness shape of and the by object disturbances and its in orientation the flow, such towards as the incomingfree-stream flow, turbulence. but also For by bluff its bodies surface, such roughness as cylinders and and by spheres disturbances, there is in a signifi- the flow, such ascant free-stream drag reduction turbulence. (the “drag Forcrisis”) bluff at a bodies,certain Re such-number as cylinders, called the andcritical spheres, Reynolds there is a significantnumber, due drag to the reduction boundary (the layer “drag flow crisis”)becoming at turbulent a certain before Re-number, separation, called which the critical Reynoldsleads to a number,smaller drag due and to thea higher boundary wake static layer pressure flow becoming, compared turbulent to a situation before with separation, whichlaminar leads separation to a smaller. For lower drag (sub and-critical) a higher Reynolds wake numbers static pressure,, the flow is compared mostly laminar. to a situation withAlthough laminar a laminar separation. boundary For layer lower has (sub-critical)a lower friction Reynolds drag than a numbers, turbulent layer, the flow it has is mostly the disadvantage of losing its momentum close to the wall of the object relatively fast, laminar. Although a laminar boundary layer has a lower friction drag than a turbulent layer, resulting in separation of the flow from the object, even prior to reaching the thickest it has the disadvantage of losing its momentum close to the wall of the object relatively point. This leads to a large wake with a low pressure, which, combined with the stagnation fast,pressure resulting at the inleading separation edge of of the the object, flow results from thein a object, high pressure even prior drag. to reaching the thickest point.It Thisis long leads known to a that large this wake pressure with drag a low of pressure,circular cylinders which, in combined the sub-cr withitical theReyn- stagnation pressureolds number at the regime leading can be edge significantly of the object, reduced results by tripping in a high thepressure laminar boundary drag. layer, forcingIt is it long to become known turbulent that this pressure[9,10], thus drag adding of circular momentum cylinders to the in thelower sub-critical part of the Reynolds numberboundary regime layer, helping can be significantly it to flow against reduced the strong by tripping adverse thepressure laminar gradient boundary at the layer,back forcing itof to the become cylinder, turbulent which postpones [9,10], thus the separation adding momentum of the boundary to the layer, lower increases part ofthe the static boundary layer,pressure helping in the wake it to flowand reduces against the the size strong of the adversewake.