DOCSLIB.ORG

Introduction to Aerospace Engineering

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Introduction to Aerospace Engineering Introduction to Aerospace Engineering Lecture slides Challenge the future 1 Introduction to Aerospace Engineering Aerodynamics 7&8 Prof. H. Bijl ir. N. Timmer 7 & 8. Laminar and turbulent flows Anderson 4.15 - end of chapter 4. Osborne Reynolds Ludwig Prandtl 1842-1912 1874-1953 Subjects lecture 7 & 8 • Viscous flows • Laminar boundary layers • Turbulent boundary layers • Transition • Separation 3 Viscous flow Up till now we have only dealt with frictionless flow. What is the effect of friction ? …. D Inviscid flow (No friction) Viscous flow (friction) NO DRAG FINITE DRAG 4 Viscous flow In real life the flow at the surface adheres to the surface because of friction between the gas and the solid material: Right at the surface the velocity is zero 5 Boundary layer Friction force Boundary layer (exaggerated) V In the vicinity of the surface there is a thin region of retarded flow: the boundary layer The pressure through the boundary layer in a direction perpendicular to the surface is constant 6 Viscous flow Inside the boundary layer Bernoulli’s law is not valid!!!!!! 7 Viscous flows Shear stress can be Boundary layer written as : velocity profile τ = μ dU w boundary layer dy y=0 thickness, δ shear stress, τw (schuifspanning) skin friction drag μ = absolute viscosity coefficient or viscosity Air at standard sea level : μ=1.789*10-5 kg/ms) 8 Viscous flows, some definitions δ V∞ x ==ρ∞∞ V x V ∞ x Reynolds number : Re x μ∞∞ν dimensionless, and varies linearly with x Laminar flow : streamlines are smooth and regular and a fluid element moves smoothly along a streamline Turbulent flow : streamlines break up and a fluid element moves in a random irregular way 9 Laminar boundary layer, boundary layer thickness Consider flat plate flow. What is boundary layer thickness δ and skin friction drag Df at location x? V∞ δ x δ = 5.2 x From laminar boundary layer theory : Rex Thus δ is proportional to : √x (parabolically) 10 Laminar boundary layer, skin friction drag τw x dx L 1 (unit width) width) Total force = total pressure force + total friction force Total friction force on element dx is: τw(x)⋅dx⋅1 = τw(x) dx L τ Total skin friction drag is: D f = w dx o 11 Laminar boundary layer, skin friction drag For the skin friction coefficient we find from laminar boundary layer theory : τ w τ w 0.664 c f = = = x 1 2 q∞ Rex ρ ∞V ∞ 2 C τ x Thus fx and w decrease as increases The skin friction at the beginning of the plate is larger than near the trailing edge. To calculate the total aerodynamic force we must integrate! 12 Laminar boundary layer, skin friction drag L L L dx 0.664 q∞ dx D f = c f . q∞ dx = 0.664q∞ = x V ∞ /ν x o o Rex o dx = x -1/2 = 2 x x 0.664 q∞ 1.328q∞ L Df = 2 L = V∞ / ν∞ V∞ L/ ν∞ Df Define total skin friction drag coefficient as Cf = q∞S 1.328 L 1.328.L 1.328 = = C = Cf ⋅ f ReL S ReL L 1 ReL 13 Results for a turbulent boundary layer δ turbulent δ laminar Due to the action of turbulence : no exact solution for turbulent boundary layers ! From experiments : 0.37x δ = Note : C varies as L-1/5 for turbulent flow 0.2 f Rex while it changes as L-1/2 for laminar flow. 0.074 C = Thus the friction in a turbulent boundary layer f 0.2 ReL is larger than in a laminar flow 14 Transition Osborne Reynolds (1842-1912) Pipe flow experiment 15 Transition Re > 2300 : transition from laminar to turbulent flow Osborne Reynolds (1842-1912) Development of turbulent flow in pipes observed and sketched by Reynolds (from his original paper) 16 Transition Cf transition turbulent laminar Re 17 Transition Flat plate flow 18 Transition Flow visualization experiment 19 Transition Tollmien- Three-dimensional Fully Schlichting vortex turbulent waves breakdown flow Stable U∞ laminar flow Growth of Generation of Span wise vorticity turbulent spots 20 Ludwig Prandtl (1875-1953) •Boundary Layer theory (1904) •Wing theory (1918 - 1919) •Contributions to the theory of supersonic flow & turbulence •Development of wind tunnels and other aerodynamic equipment •Theory of plasticity and of meteorology. 21 • Prandtl and his water tunnel (TU Hannover 1904) 22 • FLOW OVER A PLATE WITH A SEMI-CIRCULAR LEADING EDGE Zhiyin Yang and Peter Voke Fluids Research Center, University of Surrey, U.K. The image represents the results of a hybrid DNS/LES (by Zhiyin Yang and Peter Voke) computation of separation, transition and reattachment of the flow over a plate with a semicircular leading edge 23 Transition Skin friction y ∂ (mm) τ = μ u ∂y laminar turbulent u/V transition V Flat plate flow 24 Transition transition V turbulent laminar xcr The critical Reynolds number at which transition occurs is difficult to find. It should be found from experimental data applicable for the given problem 25 Laminar-Turbulent Transition on airfoils Transition “point” Laminar Turbulent B.L. B/L V Laminar boundary layer: thin, low skin friction drag Turbulent boundary layer: thick, high skin friction drag 26 transition turbulent laminar V Airfoil model vertically spanning the wind tunnel 27 Laminar flow favourable τ < τ We have seen that : wla minar wturbulent Vast majority of flows is TURBULENT ! => We may adapt the geometry of the airfoil such that it favors laminar flow. We then have Laminar flow airfoils. Favorable pressure gradient laminar flow airfoil Favorable pressure gradient 28 Flow Separation Why is it important? When does it occur ? CFD example 29 Alternating separating vortices on a cilinder (Karman street) 30 Unsteady behavior of construction due to separation 31 Flow separation separation point Separation due to a positive pressure gradient 32 Separation From: Van Dyke: “An Album of fluid motion” aluminum powder in water 33 Flow Separation Effect of pressure pressure distribution without separation distribution strong adverse -Cp separated flow pressure gradient moderate adverse pressure gradient attached flow -1.0 34 DU 91-W2-250 35 -4 C p α = 0 degr. -3 -2 -1 0 1 x/c (%) 0 20406080100 36 -4 C p α = 2.6 degr. -3 -2 -1 0 1 x/c (%) 0 20406080100 37 -4 C p α = 5.6 degr. -3 -2 -1 0 1 x/c (%) 0 20406080100 38 -4 Cp α = 7.2 degr. (l/d)max -3 -2 -1 0 1 x/c (%) 0 20406080100 39 -4 Cp α = 9.2 degr. (Cl,max) -3 -2 -1 0 1 x/c (%) 0 20406080100 40 -4 Cp α = 9.2 degr. (Cl,max) -3 transition -2 -1 Separation at x/c=92% 0 1 x/c (%) 0 20406080100 41 -4 Cp -3 α = 9.2 degr. (Cl,max) -2 α = 7.2 degr. (l/d)max -1 0 1 x/c (%) 0 20406080100 42 -4 Cp -3 -2 Separation at x/c=35% 18.2 degr. -1 0 1 x/c (%) 0 20406080100 43 Flow Separation Why is it important ? Loss in lift (airfoil) Increase in pressure drag Generation of unsteady loads Example : Interaction of separated vortex flow with vertical stabilizer (F18) 44 Flow Separation • Effect of turbulence u/U Turbulent boundary layer has more flow kinetic energy near the surface. Thus flow separation may be postponed. laminar turbulent Boundary layer 45 Viscous drag Drag due to viscous effects = friction drag + pressure drag = profile drag C = C + C Dp Dpressure Dfriction 46 Influence of the Reynolds number a Re=300000 Cd high b Re=650000 Cd lower c ≈ Re=1200000 Cd Cd(b) 47 Effect on CD of shape and Re-no. 48 Pressure drag Drag distribution Pressure drag on Cylinder Friction drag V θ 49 Two objects with the same drag force 5 (ReD=10 ) diameter D V 10D 50 51 Artificial transition Flow over a sphere free transition artificial transition Re = 15000 Re = 30000 52 Artificial transition 53 Use in sports: example 1 :speed skating Effect of zigzag strips 54 lower leg 1.4 Cd 1.2 smoothsmooth cilinder cylinder 1 0.8 0.6 coolmax 0.4 0.2 0 0 100,000 200,000 300,000 400,000 Reynoldsgetal Measured drag characteristics of different fabrics on a cylinder 55 lower leg 1.4 Cd 1.2 smooth cilinder 1 0.8 0.6 coolmax 0.4 0.2 0 0 100,000 200,000 300,000 400,000 Reynoldsgetal 56 lower leg 1.4 Cd 1.2 smooth cilinder 1 0.8 0.6 coolmex 0.4 0.2 0 0 100,000 200,000 300,000 400,000 Reynoldsgetal 57 lower leg 1.4 Cd 1.2 smooth cilinder 1 0.8 0.6 coolmax+strips coolmax 0.4 0.2 0 0 100,000 200,000 300,000 400,000 Reynolds no. 58 59 Position of the strips 60 Gianni Romme World records speed skating men 5000 m: 8.36.6 Jaap Eden Hamar 1894 6.34.96 J. Olav Koss Hamar 1994 6.30.62 Gianni Romme H’veen 1997* 6.22.20 Gianni Romme Nagano 1998 6.21.49 Gianni Romme Galgary 1998 Current: 6.03.32 Sven Kramer Galgary 2007 Result: 8.4 seconds off WR * First on clap skates 61 Experimenting with aerodynamics Cathy Freeman, 400 m. gold medalist, Sydney Olympics 2000 62 Example 2 Case study : Usain Bolt 100m men world record holder 63 Measured effect of strips on the total aerodynamic drag: • strips on lower legs -3 to -11 % • strips on the cap -2 to -6 % 64 Location Berlin air density 1.20 kg/m^3 parameters Bolt length 1.96 m mass 86 kg Cd*S-total 0.82 Cd*S-legs 0.246 distance 100 m min sec 09.58 V-average 10.44 m/s 3 P = Cd.S.½.ρ.V First calculate the required power for the time of 9.58 s P = 560 Watts propulsion (V=10.44 m/s) 65 Pprop = 560 Watts Location air density 1.20 kg/m^3 parametersBolt length 1.96 m mass 86 kg Cd*S-total 0.806 Cd*S-legs 0.231 -6% V3 = P /C .S.½.ρ.
Load more

Recommended publications
  • Brief History of the Early Development of Theoretical and Experimental Fluid Dynamics
    Brief History of the Early Development of Theoretical and Experimental Fluid Dynamics John D. Anderson Jr. Aeronautics Division, National Air and Space Museum, Smithsonian Institution, Washington, DC, USA 1 INTRODUCTION 1 Introduction 1 2 Early Greek Science: Aristotle and Archimedes 2 As you read these words, there are millions of modern engi- neering devices in operation that depend in part, or in total, 3 DA Vinci’s Fluid Dynamics 2 on the understanding of fluid dynamics – airplanes in flight, 4 The Velocity-Squared Law 3 ships at sea, automobiles on the road, mechanical biomedi- 5 Newton and the Sine-Squared Law 5 cal devices, and so on. In the modern world, we sometimes take these devices for granted. However, it is important to 6 Daniel Bernoulli and the Pressure-Velocity pause for a moment and realize that each of these machines Concept 7 is a miracle in modern engineering fluid dynamics wherein 7 Henri Pitot and the Invention of the Pitot Tube 9 many diverse fundamental laws of nature are harnessed and 8 The High Noon of Eighteenth Century Fluid combined in a useful fashion so as to produce a safe, efficient, Dynamics – Leonhard Euler and the Governing and effective machine. Indeed, the sight of an airplane flying Equations of Inviscid Fluid Motion 10 overhead typifies the laws of aerodynamics in action, and it 9 Inclusion of Friction in Theoretical Fluid is easy to forget that just two centuries ago, these laws were Dynamics: the Works of Navier and Stokes 11 so mysterious, unknown or misunderstood as to preclude a flying machine from even lifting off the ground; let alone 10 Osborne Reynolds: Understanding Turbulent successfully flying through the air.
    [Show full text]
  • In Classical Fluid Dynamics, a Boundary Layer Is the Layer I
    Atm S 547 Boundary Layer Meteorology Bretherton Lecture 1 Scope of Boundary Layer (BL) Meteorology (Garratt, Ch. 1) In classical fluid dynamics, a boundary layer is the layer in a nearly inviscid fluid next to a surface in which frictional drag associated with that surface is significant (term introduced by Prandtl, 1905). Such boundary layers can be laminar or turbulent, and are often only mm thick. In atmospheric science, a similar definition is useful. The atmospheric boundary layer (ABL, sometimes called P[lanetary] BL) is the layer of fluid directly above the Earth’s surface in which significant fluxes of momentum, heat and/or moisture are carried by turbulent motions whose horizontal and vertical scales are on the order of the boundary layer depth, and whose circulation timescale is a few hours or less (Garratt, p. 1). A similar definition works for the ocean. The complexity of this definition is due to several complications compared to classical aerodynamics. i) Surface heat exchange can lead to thermal convection ii) Moisture and effects on convection iii) Earth’s rotation iv) Complex surface characteristics and topography. BL is assumed to encompass surface-driven dry convection. Most workers (but not all) include shallow cumulus in BL, but deep precipitating cumuli are usually excluded from scope of BLM due to longer time for most air to recirculate back from clouds into contact with surface. Air-surface exchange BLM also traditionally includes the study of fluxes of heat, moisture and momentum between the atmosphere and the underlying surface, and how to characterize surfaces so as to predict these fluxes (roughness, thermal and moisture fluxes, radiative characteristics).
    [Show full text]
  • Chapter 5 Frictional Boundary Layers
    Chapter 5 Frictional boundary layers 5.1 The Ekman layer problem over a solid surface In this chapter we will take up the important question of the role of friction, especially in the case when the friction is relatively small (and we will have to find an objective measure of what we mean by small). As we noted in the last chapter, the no-slip boundary condition has to be satisfied no matter how small friction is but ignoring friction lowers the spatial order of the Navier Stokes equations and makes the satisfaction of the boundary condition impossible. What is the resolution of this fundamental perplexity? At the same time, the examination of this basic fluid mechanical question allows us to investigate a physical phenomenon of great importance to both meteorology and oceanography, the frictional boundary layer in a rotating fluid, called the Ekman Layer. The historical background of this development is very interesting, partly because of the fascinating people involved. Ekman (1874-1954) was a student of the great Norwegian meteorologist, Vilhelm Bjerknes, (himself the father of Jacques Bjerknes who did so much to understand the nature of the Southern Oscillation). Vilhelm Bjerknes, who was the first to seriously attempt to formulate meteorology as a problem in fluid mechanics, was a student of his own father Christian Bjerknes, the physicist who in turn worked with Hertz who was the first to demonstrate the correctness of Maxwell’s formulation of electrodynamics. So, we are part of a joined sequence of scientists going back to the great days of classical physics.
    [Show full text]
  • The Atmospheric Boundary Layer (ABL Or PBL)
    The Atmospheric Boundary Layer (ABL or PBL) • The layer of fluid directly above the Earth’s surface in which significant fluxes of momentum, heat and/or moisture are carried by turbulent motions whose horizontal and vertical scales are on the order of the boundary layer depth, and whose circulation timescale is a few hours or less (Garratt, p. 1). A similar definition works for the ocean. • The complexity of this definition is due to several complications compared to classical aerodynamics: i) Surface heat exchange can lead to thermal convection ii) Moisture and effects on convection iii) Earth’s rotation iv) Complex surface characteristics and topography. Atm S 547 Lecture 1, Slide 1 Sublayers of the atmospheric boundary layer Atm S 547 Lecture 1, Slide 2 Applications and Relevance of BLM i) Climate simulation and NWP ii) Air Pollution and Urban Meteorology iii) Agricultural meteorology iv) Aviation v) Remote Sensing vi) Military Atm S 547 Lecture 1, Slide 3 History of Boundary-Layer Meteorology 1900 – 1910 Development of laminar boundary layer theory for aerodynamics, starting with a seminal paper of Prandtl (1904). Ekman (1905,1906) develops his theory of laminar Ekman layer. 1910 – 1940 Taylor develops basic methods for examining and understanding turbulent mixing Mixing length theory, eddy diffusivity - von Karman, Prandtl, Lettau 1940 – 1950 Kolmogorov (1941) similarity theory of turbulence 1950 – 1960 Buoyancy effects on surface layer (Monin and Obuhkov, 1954). Early field experiments (e. g. Great Plains Expt. of 1953) capable of accurate direct turbulent flux measurements 1960 – 1970 The Golden Age of BLM. Accurate observations of a variety of boundary layer types, including convective, stable and trade- cumulus.
    [Show full text]
  • Boundary Layers
    1 I-campus project School-wide Program on Fluid Mechanics Modules on High Reynolds Number Flows K. P. Burr, T. R. Akylas & C. C. Mei CHAPTER TWO TWO-DIMENSIONAL LAMINAR BOUNDARY LAYERS 1 Introduction. When a viscous fluid flows along a fixed impermeable wall, or past the rigid surface of an immersed body, an essential condition is that the velocity at any point on the wall or other fixed surface is zero. The extent to which this condition modifies the general character of the flow depends upon the value of the viscosity. If the body is of streamlined shape and if the viscosity is small without being negligible, the modifying effect appears to be confined within narrow regions adjacent to the solid surfaces; these are called boundary layers. Within such layers the fluid velocity changes rapidly from zero to its main-stream value, and this may imply a steep gradient of shearing stress; as a consequence, not all the viscous terms in the equation of motion will be negligible, even though the viscosity, which they contain as a factor, is itself very small. A more precise criterion for the existence of a well-defined laminar boundary layer is that the Reynolds number should be large, though not so large as to imply a breakdown of the laminar flow. 2 Boundary Layer Governing Equations. In developing a mathematical theory of boundary layers, the first step is to show the existence, as the Reynolds number R tends to infinity, or the kinematic viscosity ν tends to zero, of a limiting form of the equations of motion, different from that obtained by putting ν = 0 in the first place.
    [Show full text]
  • Historie Internationale Kampioenschappen
    update 14-02-2021 in rood de mutaties seizoen 2020/'21 Uitslagen EK allround .................................................................................................................. 2 Uitslagen EK Allround Heren ...................................................................................................... 4 Uitslagen EK sprint Dames ......................................................................................................... 9 Uitslagen EK sprint Heren ........................................................................................................... 9 Uitslagen EK Afstanden Dames................................................................................................ 10 Uitslagen EK Afstanden Heren ................................................................................................. 12 Uitslagen WK Allround Dames ................................................................................................. 15 Uitslagen WK Allround Heren ................................................................................................... 18 Uitslagen WK Sprint Dames ...................................................................................................... 23 Uitslagen WK Sprint Heren ....................................................................................................... 26 Uitslagen WK Afstanden Dames............................................................................................... 29 Uitslagen WK Afstanden Heren ...............................................................................................
    [Show full text]
  • Top 100 Van Het Schaatsen
    Top 100 van het Schaatsen De honderd beste schaatsers en schaatssters aller tijden sinds 1892 www.sporttop100.nl Eerder verschenen: Top 100 van de Nederlandse Sport, De 100 grootste sporters, trainer/coaches, bestuurders, teams uit de Nederlandse sportgeschiedenis, Uitgeverij Eigen Boek, 2013, €14,25, ISBN 978-94-6129-148-6 Top 50 van de Belgische Sport, De 50 grootste Belgische sporters uit de geschiedenis, Uitgeverij Eigen Boek, 2013, €9,75, ISBN 978-94-6129-142-4 Het Wereldkampioenschap Voetbal Aller Tijden, De beste voetballers ooit gaan met elkaar de strijd aan op het wereldkampioenschap voetbal aller tijden, Uitgeverij Mijnbestseller.nl, 2014, €13,95, ISBN 978-94-6254-460-4 Top 100 van het Wielrennen, De beste 100 wielrenners aller tijden op basis van uitslagen van bijna 8000 gereden wedstrijden, Uitgeverij Mijnbestseller.nl, 2015, €13,95, ISBN 978-94-6254-188-7 Alle rechten voorbehouden. Niets uit deze uitgave mag worden verveelvoudigd en/of openbaar gemaakt op enige wijze zonder voorafgaande schriftelijke toestemming. © 2015 Voorwoord Het moet rond de Olympische Spelen van 2010 in Vancouver zijn geweest, dat ik het boek “Schaatsen Top 100” bestelde op het internet. Ik ben namelijk dol op dergelijke boeken, waarin de beste sporters met elkaar vergeleken worden. Ik wil graag weten “Wie wordt gezien als de grootste aller tijden?” “Welke sporters behoren in hun sport tot de besten ooit?” De manier waarop de schrijvers Gerard Sierksma en Huub Snoep (in samenwerking met schaatscoach Henk Gemser) hun top 100 hadden samengesteld, was ingenieus. Ze vergeleken de schaatsers niet direct met elkaar, maar op basis van de onderlinge tijdsverschillen van hun directe concurrenten op dat moment.
    [Show full text]
  • Analysis of Boundary Layers Through Computational Fluid Dynamics and Experimental Analysis
    Analysis of Boundary Layers through Computational Fluid Dynamics and Experimental analysis By Eugene O’Reilly, B.Eng This thesis is submitted as the fulfilment of the requirement for the award of degree of Master of Engineering Supervisors Dr Joseph Stokes and Dr Brian Corcoran School of Mechanical and Manufacturing Engineering, Dublin City University, Ireland. October 2006 DECLARATION I hereby certify that this material, which I now submit for assessment on the programme of study leading to the award of Degree of Master of Engineering is entirely my own work and has not been taken from the work of others save and to the extent that such work has been cited and acknowledged within the text of my own work Signed ID Number 99604434 Date 2&j \Oj 2QOG I ACKNOWLEDGEMENTS I would like to thank Dr Joseph Stokes and Dr Brian Corcoran of the School of Mechanical and Manufacturing Engineering, Dublin City University, for their advice and guidance through this work Their helpful discussions and availability to discuss any problems I was having has been the cornerstone of this research Thanks also, to Mr Michael May of the School of Mechanical and Manufacturing Engineering for his invaluable help and guidance in the design and construction of the experimental equipment To Mr Cian Meme and Mr Eoin Tuohy of the workshop for their technical advice and excellent work in the production of the required parts and to Mr Keith Hickey for his help with all my software queries and computer issues Thanks also to Mr Ben Austin and Mr David Cunningham of the School
    [Show full text]
  • Ludwig Prandtl and Boundary Layers in Fluid Flow How a Small Viscosity Can Cause Large Effects
    GENERAL I ARTICLE Ludwig Prandtl and Boundary Layers in Fluid Flow How a Small Viscosity can Cause large Effects Jaywant H Arakeri and P N Shankar In 1904, Prandtl proposed the concept of bound­ ary layers that revolutionised the study of fluid mechanics. In this article we present the basic ideas of boundary layers and boundary-layer sep­ aration, a phenomenon that distinguishes stream­ lined from bluff bodies. Jaywant Arakeri is in the Mechanical Engineering A Long-Standing Paradox Department and in the Centre for Product It is a matter of common experience that when we stand Development and in a breeze or wade in water we feel a force which is Manufacture at the Indian called the drag force. We now know that the drag is Institute of Science, caused by fluid friction or viscosity. However, for long it Bangalore. His research is was believed that the viscosity shouldn't enter the pic­ mainly in instability and turbulence in fluid motion. ture at all since it was so small in value for both water He is also interested in and air. Assuming no viscosity, the finest mathematical issues related to environ­ physicists of the 19th century constructed a large body ment and development. of elegant results which predicted that the drag on a body in steady flow would be zero. This discrepancy between ideal fluid theory or hydrodynamics and com­ mon experience was known as 'd' Alembert's paradox' The paradox was only resolved in a revolutionary 1904 paper by L Prandtl who showed that viscous effects, no matter how'small the viscosity, can never be neglected.
    [Show full text]
  • Ice Skating: Sport with Natural Refrigerants
    NATURAL WORKING FLUIDS IN ARTIFICIAL SKATING RINKS E. BERENDS GEA-Grenco Refrigeration BV - Postbox 205 5221 AE ‘s Hertogenbosch – The Netherlands [email protected] ABSTRACT CO2 technology also used now in skating rinks. In a growing number of countries it is not allowed anymore to use big amounts ammonia in areas occupied by many people. So new skating halls with direct ammonia systems are not built anymore although those systems are the best solutions concerning ice quality and energy consumption! An indirect NH3 brine-glycol system uses 25 % more energy and by using (H)CFC’s instead NH3 it might even be more! During the last years CO2 has proven to be an excellent secondary refrigerant also in skating rinks! 1. INTRODUCTION Already in the 5th century people discovered that sliding on ice made more fun and went faster than walking. For long time they skated on animal bones on the frozen natural refrigerant - water. Around 1400 the bones changed in steel blades, but frozen water kept frozen water! On the most Dutch winter landscapes of famous painters there are people “playing” on the ice! Ice skating is a famous sport in Northern Europe, and in particular in the Netherlands. Most skaters prefer natural ice to make long trips over the lakes and canals. However with the nowadays winters and the changing climate this happens not as often as they like! A good replacement of course is skating on artificial ice - in fact a wrong name - for the ice is very natural but the way how it is produced is artificial (with mechanical cooling machines).
    [Show full text]
  • The Turbulence Structure in Rough- and Smooth-Wall Boundary Layers
    J. Fluid Mech. (2007), vol. 592, pp. 263–293. c 2007 Cambridge University Press 263 doi:10.1017/S0022112007008518 Printed in the United Kingdom Turbulence structure in rough- and smooth-wall boundary layers R. J. V O L I N O1,M.P.SCHULTZ2 AND K. A. F L A C K1 1Mechanical Engineering Department, United States Naval Academy, Annapolis, MD 21402, USA 2Naval Architecture and Ocean Engineering Department, United States Naval Academy, Annapolis, MD 21402, USA (Received 9 April 2007 and in revised form 18 July 2007) Turbulence measurements for rough-wall boundary layers are presented and compared to those for a smooth wall. The rough-wall experiments were made on a woven mesh surface at Reynolds numbers approximately equal to those for the smooth wall. Fully rough conditions were achieved. The present work focuses on turbulence structure, as documented through spectra of the fluctuating velocity components, swirl strength, and two-point auto- and cross-correlations of the fluctuating velocity and swirl. The present results are in good agreement, both qualitatively and quantitatively, with the turbulence structure for smooth-wall boundary layers documented in the literature. The boundary layer is characterized by packets of hairpin vortices which induce low- speed regions with regular spanwise spacing. The same types of structure are observed for the rough- and smooth-wall flows. When the measured quantities are normalized using outer variables, some differences are observed, but quantitative similarity, in large part, holds. The present results support and help to explain the previously documented outer-region similarity in turbulence statistics between smooth- and rough-wall boundary layers.
    [Show full text]
  • Chapter 9 Wall Turbulence. Boundary-Layer Flows
    Ch 9. Wall Turbulence; Boundary-Layer Flows Chapter 9 Wall Turbulence. Boundary-Layer Flows 9.1 Introduction • Turbulence occurs most commonly in shear flows. • Shear flow: spatial variation of the mean velocity 1) wall turbulence: along solid surface → no-slip condition at surface 2) free turbulence: at the interface between fluid zones having different velocities, and at boundaries of a jet → jet, wakes • Turbulent motion in shear flows - self-sustaining - Turbulence arises as a consequence of the shear. - Shear persists as a consequence of the turbulent fluctuations. → Turbulence can neither arise nor persist without shear. 9.2 Structure of a Turbulent Boundary Layer 9.2.1 Boundary layer flows (i) Smooth boundary Consider a fluid stream flowing past a smooth boundary. → A boundary-layer zone of viscous influence is developed near the boundary. 1) Re Recrit → The boundary-layer is initially laminar. u u() y 9-1 Ch 9. Wall Turbulence; Boundary-Layer Flows 2) Re Recrit → The boundary-layer is turbulent. u u() y → Turbulence reaches out into the free stream to entrain and mix more fluid. → thicker boundary layer: turb 4 lam External flow Turbulent boundary layer xx crit , total friction = laminar xx crit , total friction = laminar + turbulent 9-2 Ch 9. Wall Turbulence; Boundary-Layer Flows (ii) Rough boundary → Turbulent boundary layer is established near the leading edge of the boundary without a preceding stretch of laminar flow. Turbulent boundary layer Laminar sublayer 9.2.2 Comparison of laminar and turbulent boundary-layer profiles For the flows of the same Reynolds number (Rex 500,000) Ux Re x 9-3 Ch 9.
    [Show full text]