# On the Wake Properties of Segmented Trailing Edge Extensions

^{*}

## Abstract

**:**

## 1. Introduction

#### 1.1. Vortex Mitigation Techniques

## 2. Experimental Setup

#### 2.1. Wind Tunnel

#### 2.2. Test Model

#### 2.3. Force Based Experiment

#### 2.4. Force Transducer

#### 2.5. Particle Image Velocimetry (PIV) Setup

## 3. Influence of Reynolds Number

## 4. Results

#### 4.1. Force-Based Experimental Results

#### 4.2. Momentum Deficit

#### 4.3. Z-Vorticity

#### 4.4. Coherent Structures

#### 4.5. Reynolds Stress

#### 4.6. Root-Mean Square (RMS) Velocities

## 5. Conclusions

- The TE extensions had a minor effect on the coefficient of lift but had measurable impact on the coefficient of drag at high angles of attack. With the segmented TE extensions, the total drag coefficient reduced by 8% at an 8° angle of attack.
- Evidence for the cause of reduction in parasitic drag with TE extensions was supported by mean flow quantities, such as mean velocity and normalized vorticity. Both parameters showed measurable and significant reductions when compared to the baseline, especially in the vorticity case. The average reduction in vorticity is in the order of 40% at an 8° angle of attack.
- The reduction in vorticity behind TE extensions was further supported by determining the coherent structures in the wake. A comparatively lower correlation of the wake and the upper surface shear layer indicates lower velocity and pressure fluctuations behind the TE extensions when compared to the baseline.
- The lower pressure fluctuations can be supported by the changes observed in the Reynolds stress. On average, the magnitude of the Reynolds stress was reduced by 40% on the upper surface and by 55% on the lower surface.
- The reduction in fluctuations are further validated by determining ${U}_{RMS}$ and ${V}_{RMS}$, which showed an average decrease in magnitude of 15% and 57%, respectively.

## Author Contributions

## Funding

## Conflicts of Interest

## References

- Theodorsen, T.; Stickle, G.W. Effect of a Trailing-Edge Extension on the Characteristics of a Propeller Section; National Advisory Committee for Aeronautics: Washington, DC, USA, 1944.
- Ito, A. The effect of trailing edge extensions on the performance of the Göttingen 797 and the Wortmann FX 63–137 aerofoil sections at Reynolds numbers between 3 × 10
^{5}and 1 × 10^{6}. Aeronaut. J.**1989**, 93, 283–289. [Google Scholar] - Yarusevych, S.; Sullivan, P.E.; Kawall, J.G. On vortex shedding from an airfoil in low-Reynolds-number flows. J. Fluid Mech.
**2009**, 632, 245–271. [Google Scholar] [CrossRef] - Huang, R.F.; Lin, C.L. Vortex shedding and shear-layer instability of wing at low-Reynolds numbers. AIAA J.
**1995**, 33, 1398–1403. [Google Scholar] [CrossRef] - Huang, R.F.; Lee, H.W. Turbulence effect on frequency characteristics of unsteady motions in wake of wing. AIAA J.
**2000**, 38, 87–94. [Google Scholar] [CrossRef] - Guan, Y.; Pröbsting, S.; Stephens, D.; Gupta, A.; Morris, S.C. On the wake flow of asymmetrically beveled trailing edges. Exp. Fluids
**2016**, 57, 78. [Google Scholar] [CrossRef] - Butler, S. Aircraft Drag Prediction for Project Appraisal and Performance Estimation. 1973. Available online: http://discovery.nationalarchives.gov.uk/details/r/C10818307 (accessed on 2 August 2018).
- Stanewsky, E. Adaptive wing and flow control technology. Prog. Aerosp. Sci.
**2001**, 37, 583–667. [Google Scholar] [CrossRef] - Neuhart, D.H.; Pendergraft, O.C., Jr. A Water Tunnel Study of Gurney Flaps; National Aeronautics and Space Administration (NASA): Washington, DC, USA, 1988.
- Jang, C.S.; Ross, J.C.; Cummings, R.M. Numerical investigation of an airfoil with a Gurney flap. Aircr. Des.
**1998**, 1, 75. [Google Scholar] [CrossRef] - Storms, B.L.; Jang, C.S. Lift enhancement of an airfoil using a Gurney flap and vortex generators. J. Aircr.
**1994**, 31, 542–547. [Google Scholar] [CrossRef][Green Version] - Traub, L.W. Examination of Gurney Flap Pressure and Shedding Characteristics. J. Aircr.
**2017**, 54, 1990–1995. [Google Scholar] [CrossRef] - Liu, T.; Montefort, J.; Liou, W.; Pantula, S.; Shams, Q. Lift enhancement by static extended trailing edge. J. Aircr.
**2007**, 44, 1939–1947. [Google Scholar] [CrossRef] - Lee, H.T.; Kroo, I.; Bieniawski, S. Flutter suppression for high aspect ratio flexible wings using microflaps. In Proceedings of the 43rd AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Denver, CO, USA, 22–25 April 2002; p. 1717. [Google Scholar]
- Spedding, G.; McArthur, J. Span efficiencies of wings at low Reynolds numbers. J. Aircr.
**2010**, 47, 120–128. [Google Scholar] [CrossRef] - Rodríguez, I.; Lehmkuhl, O.; Borrell, R.; Oliva, A. Flow past a NACA0012 airfoil: from laminar separation bubbles to fully stalled regime. In Direct and Large-Eddy Simulation IX; Springer: Berlin, Germany, 2015; pp. 225–231. [Google Scholar]
- McCormick, B.W. Aerodynamics, Aeronautics, and Flight Mechanics; Wiley: New York, NY, USA, 1995; Volume 2. [Google Scholar]
- Ngo, H.T.; Barlow, L.E. Lifting Surface with Active Variable Tip Member and Method for Influencing Lifting Surface Behavior Therewith. U.S. Patent 6,394,397, 28 May 2002. [Google Scholar]
- Anderson, J.D., Jr. Fundamentals of Aerodynamics; Tata McGraw-Hill Education: New York, NY, USA, 2010. [Google Scholar]
- Bendat, J.S.; Piersol, A.G. Random Data Analysis and Measurement Procedures; Wiley: New York, NY, USA, 2000. [Google Scholar]
- Mohsen, A.M. Experimental Investigation of the Wall Pressure Fluctuations in Subsonic Separated Flows; Technical Report; Boeing Commercial Airplane Co.: Renton, WA, USA, 1967. [Google Scholar]

**Figure 1.**Shedding of vortices from the trailing edge of NACA 0025 airfoil at different Reynolds numbers (

**a**) 50,000 (

**b**) 100,000 (

**c**) 150,000 (adapted from Yarusevych [3]).

**Figure 2.**(

**a**) NACA 0012 wing with static extended trailing edge (TE). (

**b**) Variation of the aerodynamic efficiency with the coefficient of lift for baseline, Gurney and static extended trailing edge (SETE) configurations. The SETE configuration yielded better aerodynamic efficiency than the Gurney flap (adapted from Lui et al. [13]).

**Figure 3.**Array of Miniature Trailing Edge Effectors (MiTEs) (adapted from Lee and Kroo [14]).

**Figure 6.**Schematic of the force based experiment test setup for the NACA 0012 semispan model with TE extensions. A similar setup was used for the NACA 0012 wing without TE extensions, as well.

**Figure 7.**(

**a**) Schematic of the PIV test setup for the NACA 0012 wing with TE extensions. A similar setup is used for the baseline wing. The PIV interrogation window for (

**b**) the baseline case was located at the TE and (

**c**) for the wing with the TE extension, it was location at the trailing edge of the TE.

**Figure 10.**(

**a**) Variation of Coefficient of Lift with angle of attack for the baseline wing and the wing with TE extensions. The lift curve slope shows similar variation with negligible differences between the two cases; (

**b**) Variation of the coefficient of induced drag for both cases.

**Figure 11.**(

**a**) Streamwise velocity contours in FSL behind the trailing edge of the NACA 0012 wing and behind the TE extension (

**b**) Momentum deficit profiles at different angles of attack for both cases. The momentum deficit behind the TE extension is lower at higher angles of attack when compared to the baseline.

**Figure 12.**Variation of coefficient drag estimation behind the trailing edge and behind the TE extension as a function of angle of attack. The estimated drag coefficient behind the TE extension is lower than the drag coefficient behind the hole.

**Figure 13.**Section drag coefficient variation across the span for wing with seven TE extensions. The drag coefficient behind the TE extension is lower than the drag coefficient behind the trailing edge of NACA 0012.

**Figure 14.**Variation of (

**a**) net parasitic drag coefficient and (

**b**) the total drag coefficient of the baseline wing and wing with TE extensions. The wing with TE extensions shows an average decrease in the drag coefficient around 8%.

**Figure 15.**(

**a**) Z-vorticity contours in FSL behind the trailing edge of the NACA 0012 wing and behind the TE extension (

**b**) Vorticity profiles at different angles of attack for both cases. The local rotating velocity behind the TE extension is lower at higher and lower angles of attack when compared to the baseline.

**Figure 16.**Contours of two-point correlations of the streamwise velocity component for the baseline wing and for the wing with TE extensions. Weaker correlations are observed in the wake behind the TE extension, indicating lower length scales and velocity fluctuations.

**Figure 17.**Contours of two-point correlation of the transverse velocity component for the baseline wing and for the wing with TE extensions. Similar to ${\rho}_{{u}_{i}{u}_{j}}$, weaker correlations are observed in the wake behind the TE extension lower velocity fluctuations. Also, the decrease in wavelength of the correlation indicates a decrease in turbulent length scales.

**Figure 18.**(

**a**) Streamwise Reynolds stress contours in FSL behind the trailing edge of the NACA 0012 wing and behind the TE extension (

**b**) Reynolds stress profiles at different angles of attack for both cases. The Reynolds stress behind the TE extension is lower across all angles of attack when compared to the baseline.

**Figure 19.**(

**a**) Streamwise ${U}_{RMS}$ contours in FSL behind the trailing edge of the NACA 0012 wing and behind the TE extension. (

**b**) ${U}_{RMS}$ profiles at different angles of attack for both cases. Large decreases were observed in the ${U}_{RMS}$ of the TE extension.

**Figure 20.**Streamwise ${V}_{RMS}$ contours and profiles in FSL behind the trailing edge of the NACA 0012 wing and behind the TE extension at different angles of attack for both cases. Large decreases where observed in the ${V}_{RMS}$ of the TE extension.

Test Model | Reynolds Number | Angle of Attack (Degrees) |
---|---|---|

AR 4 NACA 0012 without TE Extensions | 200,000 | −15 to 15 |

AR 4 NACA 0012 with TE Extensions | 200,000 | −15 to 15 |

${\mathit{F}}_{\mathit{X}}$ (N) | ${\mathit{F}}_{\mathit{Y}}$ (N) | ${\mathit{F}}_{\mathit{Z}}$ (N) | ${\mathit{T}}_{\mathit{X}}$ (Nm) | ${\mathit{T}}_{\mathit{Y}}$ (Nm) | ${\mathit{F}}_{\mathit{Z}}$ (Nm) | |
---|---|---|---|---|---|---|

Range | 40 | 40 | 120 | 2 | 2 | 2 |

Resolution | 1/100 | 1/100 | 1/50 | 1/4000 | 1/4000 | 1/4000 |

Test Model | Angle of Attack (Degrees) | Interrogation Location |
---|---|---|

AR 4 NACA 0012 without TE Extensions | 0, 2, 4, 6, 8 | Behind TE |

AR 4 NACA 0012 with TE Extensions | 0, 2, 4, 6, 8 | Behind TE Extension |

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Gunasekaran, S.; Curry, D. On the Wake Properties of Segmented Trailing Edge Extensions. *Aerospace* **2018**, *5*, 89.
https://doi.org/10.3390/aerospace5030089

**AMA Style**

Gunasekaran S, Curry D. On the Wake Properties of Segmented Trailing Edge Extensions. *Aerospace*. 2018; 5(3):89.
https://doi.org/10.3390/aerospace5030089

**Chicago/Turabian Style**

Gunasekaran, Sidaard, and Daniel Curry. 2018. "On the Wake Properties of Segmented Trailing Edge Extensions" *Aerospace* 5, no. 3: 89.
https://doi.org/10.3390/aerospace5030089