Lecture # 08: Boundary Layer Flows and Controls

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1 AerE 344 Lecture Notes Lecture # 8: Boundary Layer Flows and Controls Dr. Hui Hu Department of Aerospace Engineering Iowa State University Ames, Iowa 511, U.S.A

2 Flow Separation on an Airfoil

3 Quantification of Boundary Layer Flow at y, u. 99U Displacement thickness: y Momentum thickness:

Boundary Layer Theory Y w X U y wall Re C f X p y U X 1.328 5.

4 Boundary Layer Theory Y w X U y wall Re C f X p y U X X Re Re Blasius solution for laminar boundary layer: X X X X 1.72X * Re.664X Re

5 Boundary Layer Theory Y X p y Turbulent boundary layer: w U y wall Re C f X U X.74 (Re ) X X.37X (Re ) 1/5 1/5

6 Boundary Layer Flows Y X w U y wall Which one will induce more drag? Laminar boundary layer? Turbulent boundary layer?

7 Flow Around A Sphere with laminar and Turbulence Boundary Layer

8 Boundary Layer Flows w U y wall Which one will induce more drag? Laminar boundary layer? Turbulent boundary layer?

9 CONVENTIONAL AIRFOILS and LAMINAR FLOW AIRFOILS Laminar flow airfoils are usually thinner than the conventional airfoil. The leading edge is more pointed and its upper and lower surfaces are nearly symmetrical. The major and most important difference between the two types of airfoil is this, the thickest part of a laminar wing occurs at 5% chord while in the conventional design the thickest part is at 25% chord. Drag is considerably reduced since the laminar airfoil takes less energy to slide through the air. Extensive laminar flow is usually only experienced over a very small range of angles-of-attack, on the order of 4 to 6 degrees. Once you break out of that optimal angle range, the drag increases by as much as 4% depending on the airfoil

10 Flow Control: Shark Skin

11 Vortex structure over rough surfaces with Riblet

12 Flow Control: Shark Skin

13 Shark Skin Swimming Suits

Micro Air Vehicles (MAVs) Micro Air Vehicles (MAVs) are commonly defined as an

) and capable of operating at speeds of about 1 m/s (~ 2 mph).

Militaristic Applications Surveillance Chemical/Radiation Detection

14 Micro Air Vehicles (MAVs) Micro Air Vehicles (MAVs) are commonly defined as an air vehicle with wingspan less than 15 cm (~ 6 in.) and capable of operating at speeds of about 1 m/s (~ 2 mph). There is a wide range of applications to motivate development of MAVs. Militaristic Applications Surveillance Chemical/Radiation Detection Communication Links Rescue and Life Detection Visual Inspections Toys Fixed-Wing MAV design Rotary-Wing MAV design Flapping-Wing MAV design

15 Introduction Re c 5, Re c 5,, Re c 1,, Re c 1, Re c < 5, MAVs

16 Micro-Air-Vehicles (MAVs) The airfoil and planform designs for most of the existing MAVs rely on scaled-down of those used by convectional, macro-scale aircraft. macro-scale aircraft: Re C = 1 6 ~ interest for MAV applications Streamlined airfoil MAVs: Re C = 1 4 ~ Scale-down of conventional airfoils could not provide sufficient aerodynamic performance for MAV applications. C C L D MAX 1 1 rough surface airfoil It is very necessary and important to establish novel airfoil shape and wing planform design paradigms for MAVs in order to achieve superb aerodynamic performances to improve their flight agility and versatility. Re (from McMaster and Henderson, 198) U C 7 1

17 Insects: Biological Micro-Air-Vehicles Dragonfly Cicada Bee Damselfly Chrysomya megacephala (fly) Locust

18 Close views of dragonfly wings Vein network Profiles taken from Kesel, A. B., Journal of Experimental Biology, Vol. 23, 2, pp Membranes Corrugated wing cross section!

19 Bio-Inspired Corrugated Wings A simple flat plate The flat plate has a rectangular cross section without any rounded treatment at the leading edge and trailing edge. a. Flat plate A bio-inspired corrugated airfoil The shape of the bio-inspired corrugated airfoil corresponds to the forewing of a dragonfly acquired at the mid section of the wing, which was digitally extracted from the profiles given in Kesel (Journal of experimental Biology, Vol. 23, pp3125~pp3135, 2). b. Bio-inspired corrugated airfoil A profiled airfoil The profiled airfoil was formed by tautly wrapping a thin film around the bio-inspired corrugated airfoil in order to produce a smooth surfaced airfoil. C. Profiled airfoil

20 Aerodynamic Force Measurement Results C L.4.2 Flat Plate Profiled Airfoil Corrugated Airfoil C L.6.4 Flat Plate Profiled Airfoil Corrugated Airfoil Angle of Attack (Deg.) Angle of Attack (Deg.).4.3 Flat Plate Profile Airfoil Corrugated Airfoil.3 Flat Plate Profiled Airfoil Corrugated Airfoil C D.2 C D Angle of Attack (Deg.) Angle of Attack (Deg.) Re=58, Re=125,

21 PIV Measurement Results ( AOA = 6. deg., Re=58,) 8 Spanwise vorticity (1*1/s) Spanwise vorticity (1 *1/s) spanwise vorticity (1*1/s) Y/C*1 2 Y/C*1 2 Y/C* m/s Shadow Region m/s Shadow Region m/s Shadow Region X/C* X/C*1 A. instantaneous results X/C*1 8 Velocity (m/s) Velocity (m/s) Velocity (m/s) Y/C* m/s Shadow Region Y/C* m/s Shadow Region Y/C* m/s Shadow Region X/C*1 X/C* X/C*1 B. ensemble-averaged results

22 PIV Measurement Results ( AOA = 12. deg., Re=58,) 6 Spanwise vorticity (1*1/s) spanwise vorticity (1*1/s) spanwise vorticity (1*1/s) Y/C*1 2 Y/C*1 2 Y/C* m/s Shadow Region m/s Shadow Region m/s Shadow Region X/C* X/C* X/C*1 A. instantaneous results Velocity 6 (m/s) Velocity (m/s) Velocity (m/s) Y/C*1 2 Y/C*1 4 2 Y/C* m/s Shadow Region m/s Shadow Region m/s Shadow Region X/C* X/C*1 X/C*1 B. ensemble-averaged results

23 PIV Measurement Results ( AOA = 6. deg., Re=58,) Y/C* Spanwise vorticity (1*1/s) Y/C* Spanwise vorticity (1*1/s) Y/C* X/C*1 Velocity (m/s) Y/C* X/C*1 Velocity (m/s) Y/C* X/C*1 T.K.E Y/C* X/C*1 T.K.E X/C* X/C*1

24 Y/C* PIV Measurement Results ( AOA = 12. deg., Re=58,) Spanwise vorticity (1*1/s) Y/C* Spanwise vorticity (1*1/s) Y/C* X/C*1 Velocity (m/s) Y/C* X/C*1 Velocity (m/s) Y/C* X/C*1 T.K.E Y/C* X/C*1 T.K.E X/C* X/C*1

25 Aerodynamic Performance of An Airfoil Lift Coefficient, C l L 1 V 2 C l 2 c C L =2 Experimental data Airfoil stall Y /C * m/s GA(W)-1 airfoil shadow region Before stall -6 vort: Angle of Attack (degrees) X/C * Drag Coefficient, C d D 1 V 2 C d 2 c Experimental data Y /C * m/s GA(W)-1 airfoil shadow region After stall.5 Airfoil stall -4 vort: X/C * Angle of Attack (degrees)

y AerE344 Lab4: Hot wire measurements in the wake of an airfoil x Pressure rake with 41 total pressure probes (the distance between the probes d=2mm) 8 mm Lab#3 Test conditions:

26 y AerE344 Lab4: Hot wire measurements in the wake of an airfoil x Pressure rake with 41 total pressure probes (the distance between the probes d=2mm) 8 mm Lab#3 Test conditions: Velocity: V=15 m/s Angle of attack: AOA=, and 12 deg. Date sampling rate: f=1hz Number of samples: 1, (1s in time) No. of points: 2~25 points Gap between points: ~.2 inches Lab#4 Hotwire probe

AerE344 Lab: Hot wire measurements in the wake of an airfoil 6 4 2 Y /C *1 GA(W)-1 airfoil Lab#4 Hotwire probe -2-4 -6 25 m/s shadow region vort: -4.5-3.5-2.5-1.5 -.5.5 1.5 2.5 3.

27 AerE344 Lab: Hot wire measurements in the wake of an airfoil Y /C *1 GA(W)-1 airfoil Lab#4 Hotwire probe m/s shadow region vort: X/C *1 FFT arbitary scale Force -Z component (N) time sequence with data sampling rate of 1 Hz Freqency (Hz)

28 AerE344 Lab: Hot wire measurements in the wake of an airfoil Lab Hotwire probe Required data for the lab report: 1. Wake velocity profiles at AOA = and 12 deg 2. Wake turbulence intensity profiles at AOA = and 12 deg. 3. Estimated drag coefficients at AOA=, and 12 deg. 4. FFT transformation to find vortex shedding frequency in the wake of the airfoil 5. Discussions based on the measurement results

Lecture # 08: Boundary Layer Flows and Drag

Lecture # 08: Boundary Layer Flows and Drag AerE 311L & AerE343L Lecture Notes Lecture # 8: Boundary Layer Flows and Drag Dr. Hui H Hu Department of Aerospace Engineering Iowa State University Ames, Iowa 511, U.S.A y AerE343L #4: Hot wire measurements