Lift Generation and Streamline Curvature

Lift Generation and Streamline Curvature Department of Aeronautics and Astronautics Massachusetts Institute of Technology 1 Objective The objective of these notes is to explain how an airfoil generates lift using a streamline curvature analysis. The analysis will show in a qualitative manner how the shape of an airfoil influences the pressure distribution, and therefore the lift. 2 Inviscid Momentum in Natural Coordinates The steady, two-dimensional momentum equations in (x, y) coordinates is, ∂p , ∂x ∂p = − , ∂y ~ · ∇u = − ρV (1) ~ · ∇v ρV (2) where V~ = u~i+v~j. Instead of an (x, y) coordinate system, these equations can be written in a stream-aligned coordinated system (r, s). As shown in Figure 1, r is the direction normal to the local streamwise direction and s is the local streamwise direction. In this coordinate system, the inviscid momentum equations are, ∂V ∂s V2 ρ R ρV ∂p , ∂s ∂p , ∂r = − (3) = (4) r s R R ~ V s r Figure 1: Natural coordinate system (r, s). r is the direction normal to the streamwise direction (along the ~ ). R is the radius of radius of curvature), s is in the streamwise direction (tangent to the velocity vector, V curvature. 1 pu pl R Figure 2: Airfoil with a circular arc camber line with radius R and zero thickness. pu is the upper surface pressure, pl is the lower surface pressure. ~ | is the magnitude of the velocity vector (i.e. the speed). where V = |V For incompressible flow, we note that Equation (3) is equivalent to Bernoulli’s equation, ρV ∂V ∂s  ∂ 1 2 ρV ∂s 2   1 ∂ p + ρV 2 ∂s 2 1 p + ρV 2 2 ∂p , ∂s ∂p = − , ∂s = − = 0, = constant. However, the key to understanding lift generation is not Bernoulli’s equation (contrary to popular believe) but rather is the normal momentum equation, Equation (4). 3 Impact of Camber The impact of camber on the pressure distributions can be demonstrated most simply by considering an airfoil with a circular arc camber line and zero thickness as shown in Figure 2. Far away from the airfoil, the pressure returns to the freestream pressure p∞ . On the surface of the airfoil (which must be a streamline), we know that ∂p/∂r > 0 from Equation (4). Thus, above the airfoil the pressure increases as the distance from the airfoil increases. Since the pressure must eventually return to p∞ , this implies that pu < p∞ . Summarizing the logic chain, V2 ∂p =ρ > 0 ⇒ p ∞ − pu > 0 ⇒ p u < p∞ . ∂r R Similarly, on the lower surface, ∂p V2 =ρ > 0 ⇒ p l − p∞ > 0 ⇒ p l > p∞ . ∂r R Combining these results which are solely based on the curvature of the surface, we see that p u < p∞ < pl . Thus, this airfoil will generate lift since the pressure is lower on the upper surface than on the lower surface. As an example of the pressure distribution on a thin, circular arc airfoil, consider the NACA 4502 airfoil. This airfoil has a maximum thickness which is 2% of the chord length. The maximum camber is 4% of the chord and occurs at x/c = 0.5. Note: the NACA 4-digit series airfoils have camberlines which are two circular arcs that meet at the maximum camber location. Thus, when the maximum camber is at x/c = 0.5, the two circular arcs have the same radius. The cp distribution for the 4502 at a cl = 0.5 is shown in Figure 3. Recall the definition of cp is, p − p∞ . (5) cp ≡ 1 2 ρ ∞ V∞ 2 Thus, when the pressure is below p∞ , cp < 0 and vice-versa. The cp distribution for the 4502 shows that the pressures are below p∞ on the upper surface, and above p∞ on the lower surface. Furthermore, the decrease in pressure on the upper surface is nearly equal to the increase in pressure on the lower surface which is reasonable since the radius of curvature is essentially the same on both the upper and lower surface. Note that near the leading edge, the flow will have a stagnation point, V = 0. This corresponds to cp = 1. However, in Figure 3, the cp at the leading edge has a maximum value of about cp = 0.15. The reason the stagnation point is not observed is purely numerical; the panel method does not have enough panels to resolve the leading-edge region. If the number of panels at the leading edge were increased, or the thickness of the airfoil were increased, then the leading-edge stagnation point would be better resolved. As another example, Figure 4 shows the pressure distribution for a NACA 4202. This airfoil has a camber line with two different radius of curvatures; one from the leading edge until the maximum camber at x/c = 0.2 and the other from the maximum camber until the trailing edge. The radius of curvature is smaller from x/c < 0.2, thus the normal pressure gradients are generally expected to be larger in this region than for x/c > 0.2. This behavior is clearly observed in the cp distribution. The magnitude of the cp ’s drop abruptly for x/c > 0.2. 4 Impact of Thickness The impact of thickness can also be explained qualitatively from streamline curvature arguments. Consider a symmetric airfoil with thickness. In this case, the curvature of the upper and lower surfaces are in opposite directions. Thus, the logic chain becomes, ∂p V2 =ρ > 0 ⇒ p ∞ − pu > 0 ⇒ p u < p∞ . ∂r R Similarly, on the lower surface, V2 ∂p =ρ > 0 ⇒ p ∞ − pl > 0 ⇒ p l < p∞ . ∂r R Thus, for a symmetric airfoil at zero angle of attack, the pressures on the surface are generally expected to be lower than p∞ . As examples of symmetric airfoils, the cp distributions for NACA 0002 and 00010 airfoils at zero angle of attack are shown in Figures 5 and 6, respectively. The low pressures are observed on both surfaces (note: the flow is symmetric since the geometry is symmetric and α∞ = 0, thus the cp on the upper and lower surfaces are the same). Also, the pressures are lower for the thicker airfoil as would be expected since the radius of curvature is small for the thicker airfoil. On a camber airfoil, the trends with thickness are similar to the trends on a symmetric airfoil. Specifically, the addition of thickness will tend to lower the cp on both size of the airfoil. Once again, this qualitative behavior can be motivated using streamline curvature arguments. Increasing the thickness on a cambered airfoil will tend to decrease the radius of curvature of the upper surface, and increase the radius of curvature of the lower surface. Thus, we have the following logic chain, on the upper surface, τ↑ R↓ V2 ∂p =ρ ↑ ∂r R p∞ − pu ↑ pu ↓ . R↑ ∂p V2 =ρ ↓ ∂r R pl − p∞ ↓ pl ↓ . Similarly, on the lower surface, τ↑ Since the addition of thickness to a cambered airfoil tends to lower both the upper and lower surface pressure and the lift is an integral of the upper and lower surface pressure difference, the resulting lift will be relatively unaffected by thickness. These trends in cp and cl can be observed by comparing the 10% thick cambered airfoils shown in Figures 7 and 8 to the 2% thick cambered airfoils shown in Figures 3 and 4. Note: the thicker airfoils were simulated at the same angles of attack for the corresponding thinner airfoils. For these conditions, the 5 times increase in thickness from 2% to 10% changes the lift by less than 10%. 3 Figure 3: NACA 4502, cl = 0.5. Figure 4: NACA 4202, cl = 0.5. 4 Figure 5: NACA 0002, α∞ = 0◦ . Figure 6: NACA 0010, α∞ = 0◦ . 5 Figure 7: NACA 4510, α∞ = −0.09854◦. Figure 8: NACA 4210, α∞ = 0.856◦. 6 5 Leading-edge Behavior Next, we will consider the behavior of the flow at the leading edge. As was noted above, the flow will stagnate near the leading edge which results in cp = 1. However, another effect often results in very low pressures at the leading edge. For example, the cp distribution around the NACA 4202 airfoil at cl = 0.5 shows cp < −2 at the leading edge. The cause of this can be explained also through the streamline curvature argument. In this case, the radius of curvature at the leading edge is very small. And, as R → 0, ∂p V2 =ρ → ∞. R→0 ∂r R lim Thus, the pressures at the leading edge will need to be very low to provide the required force to turn the flow around the leading edge. Question: Figure 4 and Figure 9 show the cp distributions for the NACA 4202 and NACA 0002, respectively, for cl = 0.5. Is the leading-edge stagnation point on the lower surface or upper surface for the NACA 4202? What about the NACA 0002? Question: airfoils. Explain the differences in the cp behavior at the leading edge for the NACA 4202 and 4210 7 Figure 9: NACA 0002, cl = 0.5. 8