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CFD aerodynamic design of an ultra-light amphibious PrandtPlane aircraft

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Chapter 3

The flap design

In order to reach the necessary Lift in both take-off and landing conditions, the most efficacious flap system was chosen. In fact, the fowler flap system is used when high performance airfoil is required. Through this system, enough  in the low speed conditions will be generated.

The characteristics of the fowler flap are several. First of all, like all the other flap systems, it is able to generate a raise of  by increasing the airfoil chamber. Moreover, like the slotted flap system, it generates a gap between the wing and itself in order to keep the airflow tied to the upper flap surface. Furthermore, it is able to produce a significant increase of the wetted surface area of the wing, which is the main advantage of the fowler flap.

To begin, the geometry of the fowler flap was generated from the wing geometry. During its generation, the most important constraint taken into consideration was that when the flap is retracted, the whole airfoil – flap included – must be as similar as possible to the GOE398. Particular attention to flap extraction and flap housing shape is paid.

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2 Before running CFD simulations, a sensitivity analysis of the mesh was performed, and to reach as fast as possible the best flap position with respect to the wing airfoil, 2D CFD simulations were made.

3.1 The flap geometry

It is necessary that the front wing airfoil in cruise conditions (flap retracted) is as similar as possible to the GOE398. This requirement is to be considered as one of the main crucial points of the flap shape design.

The section of the front wing with the chord equal to 1   1 was taken as a reference. To generate the profile of the flap, the reference section was scaled to 30%, a percentage resulted from a bibliographic research on fowler flap wind tunnel experience. In fact, the fowler flap system reaches its highest efficiency when the flap’s chord is the 30% of the wing’s one. In order to avoid additional design problems regarding the kinematic of the flap extraction, it was decided to utilize a constant flap chord, so the optimum percentage of flap chord was obtained only for the reference section. The tip and root sections of the flap are shown in Figure 3.1

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3 A quasi GOE398 was created to satisfy the request explained before: in cruise conditions the front wing sections must be as similar as possible to the GOE398 chosen. The 3D flap was generated, as shown in Figure 3.2.

The end part of the front wing was cut by a line started at one particular point and parallel to the trailing edge. This particular point corresponds to the projection, along the z axes with respect to the wind system axes, of a point at 85% of the wing airfoil chord with respect to the tip flap section. The portion of the anterior wing, without the flap, is shown in Figure 3.3 and Figure 3.4.

Figure 3. 2: Flap

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4 The reference section of the flapped front wing was the one for which the chord of the unflapped front wing is equal to 1. Furthermore, the direction of this section is perpendicular to the isobar of the wing, so the angle between the section and the freestream flow direction is similar to the sweep (at ¼ of the wing chord) . The section of the flapped wing is shown in Figure 3.5 and Figure 3.6.

Figure 3. 4: Particular of the flap housing

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5

Figure 3. 6: Section of the flapped wing

3.2 The mesh selection

3.2.1 Mesh configurations

The mesh sensitivity analysis was made to define dimensions and positions of the internal control surfaces (used to refine the mesh locally).

Eight different mesh configurations were created by the software ANSA® and will be presented in the following figures.

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6

The mesh configuration 1 is characterized by an internal control surface around the gap between the wing airfoil and the flap airfoil, and another one just behind the flap whose task is to take the wake effect. Because of the great difference in the dimension of the elements around the airfoil, the CFD simulation presents a convergence problem.

In this mesh configuration, the internal control surface surrounds both the wing and the flap, resulting more uniform than the first one. Regarding the control surface outline (outline of the domain), the size of the elements is equal to that of mesh configuration 1.

Figure 3. 8: Detail of mesh configuration 1

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7 The difference between this mesh and the previous one is that the size of the elements of the control surface outline is higher, so to prove whether the CFD simulation is sensitive to their dimension or not.

The mesh configuration 4 presents a mesh with the same size of the elements in the control surface outline as the first two configurations but has an additional internal control surface around the wing-flap system. Its aim is to prove the relevance of a better discretization of the area around the airfoils.

Figure 3. 10: Mesh configuration 3

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8 As the mesh configuration 4, this configurations was generated to investigate the discretization of the area around the wing-flap system. Particular attention to the wake area was paid.

The discretization studied in the mesh configuration 6 concerns the gap between the wing and the flap, in order to analyze the importance of the elements size of this area.

Figure 3. 12: Detail of mesh configuration 5

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9 In the mesh configuration 7, the mesh around the wing-flap system is similar to the one of mesh configuration 4. The variable analyzed is the size of the control surface, in fact this one is bigger than the that presented in mesh configuration 4.

This configuration incorporates the variables previously analyzed. Thus, it has the control surface as big as the one in configuration 7, presents the additional internal

Figure 3. 14: Mesh configuration 7

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10 control surface around the wing-flap system, and also that one of the gap between the wing and the flap.

For each mesh configuration the same CFD analysis was performed in order to understand the sensitivity of the CFD results to the mesh configuration.

3.2.2 CFD simulations

After the mesh generation, the geometries were imported into STARCCM+®, the CFD simulation program used for this thesis, as shown in Figure 3.16.

The results given by the CFD simulations are resumed in Table 3.1 (the values of the coefficients are obtained through normalization of the forces with unitary surface).

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11  [-]  [-] Lift [N] Drag [N] Simulation 1 2,238 0,036 444,17 7,22 Simulation 2 2,274 0,035 451,37 6,984 Simulation 3 2,272 0,036 451,02 7,231 Simulation 4 2,275 0,034 451,62 6,809 Simulation 5 2,273 0,033 451,15 6,636 Simulation 6 2,285 0,034 453,54 6,804 Simulation 7 2,273 0,034 451,11 6,793 Simulation 8 gfhfghfgh gfdfgnhfghn fghndfgfg sdhgsgs

Table 3. 1: Results of CFD simulations of the sensitivity analysis of the mesh

Simulation 1 presents different values of Lift and Drag with respect to the other simulations, and is characterized by a non-steady convergence of the forces, due to the fact that corresponds to the mesh configuration 1 which is the less uniform. For these reasons, the mesh configuration 1 will not be taken into consideration in the research of the optimum position of the flap. Also in Simulation 3, the value of Drag is a little bit different from the others, underling the importance of the mesh element size in the control surface outline. Concerning the other simulations, the values of Lift and Drag are very similar to each other, except for Lift of Simulation 6 which is the highest. Probably, this fact is due to a better discretization of the gap area between the wing and the flap, leading to a better description of the flow behavior in the channel. The detail of the flow through the gap is shown in Figure 3.17.

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12 To choose the better mesh configuration, a certain flapped wing already experimentally studied was investigated. The aim of this study is to compare the values given by the CFD simulations with those experimentally found. The mesh configurations used to discretize the area of the control surface around the airfoil are 4, 5 and 6 ones. In order to explain such a choice, it is important to note that the mesh configuration 6 is the only one with the better discretization of the gap. Instead, the choice for the mesh configurations 4 and 5 is simply based on the fact that 2D CFD simulations need less time to run, so it is possible to adopt mesh configurations with more elements than in 2. However, as it can be seen in Table 3.1, it is useless to take into account the large number of mesh elements of configuration 8.

3.2.3 ClarkY experimental investigation

This study involves an experimental investigation of the aerodynamic characteristics of the ClarkY airfoil. The geometry of the airfoil was generated following the dimensions presented in the article about the experimental investigation [4], and then the mesh configurations were created using ANSA®. Precisely, mesh configurations 1, 2 and 3 correspond respectively to configurations 4, 6 and 5 previously studied (ref. Figures 3.11, 3.13 and 3.12).

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13

Figure 3. 19: Mesh configuration 1 of the flapped ClarkY airfoil

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14 After the mesh generation, the CFD simulations were run at the same aerodynamic conditions explained in the article [4]. The results and the visualization of Cp and velocity vectors are shown in Table 3.2.

 

Case 1 1,722 0,0547

Case 2 1,729 0,0510

Case 3 1,709 0,0506

Table 3. 2: Results of CFD simulations of the flapped ClarkY airfoil

As it was expected, comparing these results with the value of  taken from the article [4], the better mesh configuration results to be the one in Case 2, because it presents a better discretization of the gap between the wing and the flap. For this reason, the mesh configuration which corresponds to Case 2 (mesh configuration 6, Figure 3.13) has been used to find the optimum flap position for the ID04-002-T2.

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15

3.3 Research of the optimum flap position

The efficacy of the fowler flap is strictly correlated to the gap dimension between the end of the wing and the leading edge of the flap itself. Particular maps, which represent iso- curves with respect to the relative position of the wing and the flap, are provided for several kind of flapped airfoils. Unfortunately, for the airfoil taken into consideration to develop the IDINTOS project, those curves were not generated. The purpose of this part of the master thesis is to try to understand how the aerodynamic characteristics of the flapped GOE398 are influenced by the gap dimension.

3.3.1 The gap geometry

The section of the flapped wing taken into consideration is the same section used to evaluate the mesh sensitivity. That flapped wing section is shown in Figure 3.5, and its 2D geometry is shown in Figure 3.23.

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16

In order to understand the aerodynamic characteristics of the flapped GOE398 with respect to the gap geometry, several configurations were analyzed. These configurations were created by moving the flap along both the x axis (that corresponds to the chord direction) and the z axis.

Figure 3. 23: One of the several configurations used to reach the optimum position of the flap

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17 Figure 3.24 shows five configurations, at 30° of the flap deflection, relative to five different positions of the leading edge of the flap with respect to the trailing edge of the wing. For example, the position of the yellow flap airfoil is at 2,5% of the unflapped wing chord along -z direction and at 1,5% of the chord along –x direction. As it can be seen, the gap geometry is strictly dependent from the leading edge position of the flap.

3.3.2 Mesh of the geometries

As shown in the following figures, the mesh chosen was characterized by internal control surface at the gap and two of such surface around the wing-flap system (ref. paragraph 3.2 The mesh selection).

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18

3.3.3 The most efficacious wing-flap configuration

More than 110 CFD simulations were run in order to understand the behavior of the  with respect to variables such as both the x and the z positions of the flap from the end of the wing airfoil, the angle of flap deflection and the angle of attack. The velocity of the freestream flow was set equal to 18 m/s, as imposed by the regulations. Indeed, the true set velocity was about 17,5 m/s, in order to take into consideration the fact that the section of the wing-flap system was defined not as parallel to the freestream flow, but sloped down at an angle equal to the sweep at ¼ of the wing chord. Atmosphere pressure and air density were taken at the sea level.

The results of the test campaigns were plotted for each geometry configuration at different angles of attack. It is important to take into consideration that, about the name of the configurations, the first number is referred to the distance, in percentage of the wing chord, along z axis of the leading edge of the flap from the trailing edge of the wing, and the second one to the same distance along x axis. Regarding the curves, the third number indicates the angle of deflection of the flap.

Figure 3. 26: with respect to the angle of attack of 1,0_0 configuration

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 1,0_0_20 Cl - 1,0_0_30

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19 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 1,25_0_20 Cl - 1,25_0_30

Figure 3. 28: with respect to the angle of attack of 1,25_0 configuration

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 1,5_0_20 Cl - 1,5_0_30

Figure 3. 27: with respect to the angle of attack of 1,5_0 configuration

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 2,5_0_20 Cl - 2,5_0_30

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20 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 2,5_1,25in_20 Cl - 2,5_1,25in_30

Figure 3. 30: with respect to the angle of attack of 2,5_1,25in configuration

Figure 3. 31: with respect to the angle of attack of 3,75_0 configuration

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 3,75_0_20 Cl - 3,75_0_30 Figure 3. 32: with respect to the angle of attack of 3,0_0 configuration

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 3,0_0_20 Cl - 3,0_0_30

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Figure 3. 34: with respect to the angle of attack of 2,5_1,5in configuration

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 2,5_1,5in_20 Cl - 2,5_1,5in_30

Figure 3. 33: with respect to the angle of attack of 2,5_1,25out configuration

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 2,5_1,25out_20 Cl - 2,5_1,25out_30

Figure 3. 35: with respect to the angle of attack of 2,5_1,5out configuration

0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 2 4 6 8 10 12 Cl - 2,5_1,5out_20 Cl - 2,5_1,5out_30

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22 Thus, for each configuration and angle of flap deflection, the behavior of  with respect to the angle of attack was plotted. By analyzing the trend of these curves, the wing-flap configuration chosen was the 1,5_0 one, with the flap deflection equal to 30°. This configuration was chosen simply because its  behavior is the most regular with respect to the others. In fact, as shown in Figure 3.36, both at low angles of attack and at high ones, the 1,5_0_30 configuration generates one of the highest  with respect to the other configurations.

Thus, the configuration 1,5_0_30 will be studied.

2,0 2,2 2,4 2,6 2,8 3,0 3,2 0 2 4 6 8 10 C l α Cl - 1,5_0_30 Cl - 1,0_0_30 Cl - 1,25_0_30 Cl - 2,5_0_30 Cl - 3,0_0_30 Cl - 3,75_0_30 Cl - 2,5_1,25in_30 Cl - 2,5_1,5in_30 Cl - 2,5_1,25out_30 Cl - 2,5_1,5out_30 Figure 3. 36: Comparison of all the configurations for the flap deflection equal to 30°

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23

3.4 CFD analysis of the optimum wing-flap configuration

Figures 3.37 and 3.38 show the 1,5_0_30 geometry.

Figure 3. 37: The optimum configuration 1,5_0_30

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24 Both the coefficients of Lift () and Drag () of this configuration, at different angles of attack α, will be presented in Table 3.3.

1,5_0_30_α 2,3162 0,0408 2,5203 0,0433 2,9753 0,0570 3,0348 0,0916 10° 3,0483 dfsfsfsdfdsfsd

Table 3. 3: Results of the chosen configuration 1,5_0_30 at different angles of attack α

The flow velocity vectors will be shown in Figure 3.39. As it can be seen, for an angle of attack equal to 0° (Figure 3.39a), the flow which wets the flap is separated for about the 35-40% of the chord. This behavior does not change if the angle of attack is equal to 2° (Figure 3.39b). Instead, when it is increased, it is possible to see a particular event. By analyzing the velocity vectors at the angles of attack of 6°, 8° and 10° (respectively Figure 3.39c-d-e), it is evident that the flow from which the flap is wetted is more attached as the angle of attack increases. In fact, when the angle of attack reaches 10° (Figure 3.39e), the upper flap surface results to be completely wetted by an unseparated flow. This fact is able to explain why the flap is more efficacious at high angles of attack than at low ones. At the same time, the flow which has wetted the upper wing surface tends to separate after its trailing edge.

Thus, at high angles of attack, it seems that the aerodynamic field around the wing-flap system is “divided” into two different flows. One of these flows wets the upper surface of the wing and separates in correspondence of the flap, while the other one wets the upper surface of the flap and is able to remain attached along all the flap itself.

In conclusion, the configuration 1,5_0_30 is able to work both in take-off and landing conditions.

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Figure 3. 39: Aerodynamic behavior of the configuration 1,5_0_30 at angles of attack equal to 0°(a), 2°(b), 6°(c), 8°(d), 10°(e)

Figura

Figure 3. 1: Tip and root sections of Fowler Flap airfoil
Figure 3. 4: Particular of the flap housing
Figure 3. 8: Detail of mesh configuration 1
Figure 3. 10: Mesh configuration 3
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