![]() As α increases further, the drag decreases again, just as abruptly. when Re<10 5), the lift:drag polars are characterised by a`tongue', where at moderate angles of attack, α, the drag increases abruptly with little increase in lift. When Re falls significantly below the design space for this aerofoil (i.e. The aerofoil section is the Eppler 387, designed for sailplanes in what is termed low-speed flight in engineering literature. 2 is taken from a comprehensive collection of aerofoil data( Lyon et al., 1997), and shows an example of the measured performance of a two-dimensional airfoil section at Re from 6×10 4 to 3×10 5. 1, relevant wing performance data are more scarce, and the few reliable sources (such as Althaus, 1980 Schmitz, 1945 Hoerner, 1965 Laitone, 1997) are limited in scope. Indeed they are in the wake of the wings and body, but paradoxically enough, in the design and analysis of wing performance, the huge values of Re make it possible to completely ignore viscous terms, and a large and successful body of engineering literature makes accurate calculations of the aerodynamic performance based on inviscid (no viscosity)theories.īecause aeronautics is usually practised at much higher Re than appears in Fig. This is a large number, and one might expect the air motions to be characterised by enormously complex,turbulent flows. For example, for a large passenger plane (such as the Boeing 747-400) with cruising flight speed U=250 m s –1 and mean chord length c=8 m, a Reynolds number that uses c as the length scale is approximately 10 8. Re is a large number in many human-engineering applications. Inertial forces tend to destabilise a flow, while viscosity tends to smooth it out, and so high Reynolds numbers are frequently associated with complex, and possibly turbulent, flows. The numerator depends on the mass, size and speed at which fluid is moving, and one may think of Re as a measure of the relative importance of inertial and viscous forces in a flow. \ (1)where u is a flow speed, l is a characteristic length scale and μ is the fluid viscosity. ![]() Together, the measurements imply a fine control of boundary layer separation on the wings, with implications for control strategies and wing shape selection by natural and artificial fliers. Indeed, a commonly cited measure of the relative flapping frequency, or wake unsteadiness, the Strouhal number, is seen to be approximately constant in accordance with a simple requirement for maintaining a moderate local angle of attack on the wing. A comparison of measurements of fixed wing performance as a function of Re, combined with quantitative flow visualisation techniques, shows that, surprisingly, wakes of flapping bird wings at moderate flight speeds admit to certain simplifications where their basic properties can be understood through quasi-steady analysis. ![]() Bird flight occurs over a range of Reynolds numbers ( Re 10 4⩽ Re⩽10 5, where Re is a measure of the relative importance of inertia and viscosity) that includes regimes where standard aerofoil performance is difficult to predict, compute or measure, with large performance jumps in response to small changes in geometry or environmental conditions. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |