Computational characterization of vortexes generated by low-profile Vortex Generators

Department of Nuclear Engineering and Fluid MechanicsEngineering School, Vitoria-GasteizUniversity of the Basque Country (UPV/EHU)

Iosu Ibarra Udaeta, Iñigo Errasti Arrieta, Unai Fernández-Gamiz, Ekaitz Zulueta, Javier Díaz de Argandoña

2017 Pamplona-Iruña, 26-28 April 2017

DEVICES:-TE flaps- Fences

- Microtaps-Gurney flaps

- VGs- AJVGs

ACTUATORS:- Piezoelectric

- Motors- MEMs- Fluidics

CONTROLS:- Neural network

- Adaptive- Physical model based- Optimal control theory

SENSORS:- Conventional

- Optical- MEMS

FLOW CONTROL

PHENOMENA

Active/passive devices for flow control1

• Modifying the boundary layer (BL) motion

• Generation of longitudinal vortices

• Overturn of the BL flow via large scale motions

• Bringing high momentum fluid down into the near wall region of the boundary layer

• IN SHORT: separation of the flow is delayed

Boundary layer motion alteration by a rectangular VG

Evolution of boundary layer velocity profiles with adverse pressure gradient

What is a VG?. How does it work?1

These passive devices are used for flow control:

VG

VGs on airfoils

• Geometry: triangular or rectangular vanes• Dimensioned: to the local boundary layer thickness• Lay-out: in cascades of groups of two vanes

Ref.: G. Godard and Stanislas, 2006

Counter rotating passive device configuration

Ref.: Author design, 2013

VG geometry and lay-out1

VG1

VG2

VG3

VG4

• Requires (some) knowledge about the base flow

Ref.: Miller, 1995

Parametric studies on VGs1

• VG cascade geometry optimization:

– Many degrees of freedom (angle, interspacing, orientation, height, length ...)

– Very important for VG performance

• VG row positioning on blade:

– Chord/span

– Aeroelasticity considerations

VGs

VGs

VGs

VGs VGsVGs VGs VGsVGs

• Main functionality: to delay or prevent separation of the flow

(a) Flow across an airfoil

Ref.: J.D. Anderson Jr., Brief History of the Early Development of Theoretical and Exp. Fluid Dynamics, Willey & Sons, 2010

Effect of vortex generators VGs on the performance of DU 97-W-300 (commonly used in wind turbines)

Ref.: van Rooij R. P. J. O. M. and Timmer W A. Roughness Sensitivity Considerations for Thick Rotor Blade Airfoils. AIAA-paper 2003-0350

VG on airfoils1

(b) Separated flow over the top surface of an airfoil

VG on wind turbines1

VGs on Wind Turbines

Source: Pictures were taken by the author in EWEA Conference 2012, Copenhaguen

VG on offshore wind turbines. Detail #11

Ref.: S. Øye, The effect of Vortex Generators on the performance of the ELKRAFT 1000 kW Turbine, 9th IEA Symp. On Aerodynamics of Wind Turbines, 1995

(a) Effects of VGs on a 2.5 MW wind turbine performance

Ref.: Miller, G.E., Comparitive Performance Tests onthe Mod-2 2.5 MW Wind Turbine With and WithoutVortex Generators, NASA TM N95-27978, Presented at the DOE/NASA Workshop on Horizontal Axis WindTurbine Technology, May 8-10, Cleveland, OH, 1984

Increased wind turbine performance from implementing VGs on the blades has also been confirmed in certain field tests:

(b) Effects of VGs on a 1 MW wind turbine performance

VG on wind turbines1

• Efficient

• Easy and fast practical implementation

• Inexpensive

• Can be integrated as part of the blade design

• Can also be applied as retrofits to improve existing designs

Pros and cons of VGs1

• ‘Small’ parasitic drag

• Require detailed understanding to be correctly applied

• VGs needed to be optimized for every specific flow/geometry

• Not an universal VG solution: VGs should be designedaccording to the local wind resource and turbine type

Pros

Cons

Study overview2

Characterization of the primary vortex generated by rectangular VGs of different heights h (H1, H2, H3, H4, H, H5) for four orientations ( =10º, 15º, 18º,20º)

VG: passive flow control device

VG: rectangular vane (h x 2h)

Domain dimensions: 75 x 64 x 10

β

Flat plate

β

: angle of attack

Negligible pressure gradient

OUTLET

VG

Study overview2

δ

: local boundary layer thickness

: incident angle of attack

δ

• Vortex Generators VGs of different heights h are positioned at a distance on a flat plate where the local boundary layer thickness equals the conventional VG height H

Conventional VG height: h=H=0.25 mSix VG heights h respect to the local BL: h/δ=0.2, 0.4, 0.6, 0.8, 1, 1.2

Four orientations (incident angles of attack): 10º, 15º, 18º and 20º

Free stream velocity U=20 ms-1, RE=27000

near wake region

βFlat plate

z=r

• CFD code: OpenFOAM v2.4.0

= 20 ms-1

VG

Geometry/3D Meshing

- Multigrading feature

- 44/64 structured blocks

- Three or four levels (N0, N1, N2, N3)

• BlockMesh & blockMeshDict:

Meshing of a rectangular VG vane on a flat plate:

• External mesher or …

Proximity of the vane: Mesh containing around 6 x 106 cells

Vortex data (pressure, velocity and vorticicity)obtained in spanwise planes normal to the flow direction at distances 3 to 28 times the conventional VG height from its trailing edge (TE)

Wake region containing around 4 x 106 cells

Wake region

3

Normalized height of the closest cell

to the wall: y/H=1.5 x 10-6 y+<1�

VG

VG

near wake

3D Meshing3

- 44/64 structured blocks

- 5 blocks defining the VG:

A, A’

B, B’

C

N0

N2

N1

VG

VG

VG

near wake

- the primary vortex

Near wake region able to capture:

- a secondary vortex

3D Meshing3

VG

VG

near wake

Assumptions & Numerical setup. Standard BCs and solver4

Rectangular Vortex Generator (VG) of height H=h=

44 blocks of 163 cells: 44 blocks of 323 cells: 44/64 blocks of 64 3 cells. 12 x 10 6 cells:

Detailed mesh around VG

• Incompressible flow (air)

• Turbulent flow

• Negligible pressure gradient

• No heat transfer

• Steady-state case

Richardson extrapolation method

Assumptions:

δ

VG

VG

VG

(Ref.: Urkiola et al, 2017)

near wake

Setup:

• Numerical methods: RANS simulation

• Turbulence modelling (K-Omega SST model)

• Boundary condition at inlet: uniform U=20 ms-1

• RE=27000

• Averaged normalized wall distance y+<1

• Typical RANS simulation time (16 CPUs, 3.0 Ghz): 10 days

Mesh dependency study

Axial u x and azimuthal u theta velocity profiles5

CFD and analytical (Velte, 2013) results of the axial ux and azimuthal utheta velocity profiles at the same plane 5h past the VG for incident angles 10º, 15º, 18º and 20º:

Ref.: Urkiola et al, 2017 Case: conventional VG

Beta=

Vortex evolution. Beta=10º5

Ux (ms-1)

Vortex evolution. Beta=15º5

Ux (ms-1)

Vortex evolution. Beta=18º5

Ux (ms-1)

Vortex evolution. Beta=20º5

Ux (ms-1)

Wall shear stress past the VG. Beta=10º. Beta=15º5

dU

dyωτ µ=

212

fcU

ωτ

ρ ∞

=

BL detachment condition: 0dU

dy= Ref.: Godard and Stanislas (2006)

Wall shear stress past the VG. Beta=18º. Beta=20º5

Vortex size. Half-life radius R 0.55

• Half-life radius: distance from the center of the vortex to the point where the axial vorticity wx is

half the peak vorticity wx,max measured on planes

normal to the streamwise direction

• R0.5 based on estimations on the maximum value of the axial vorticity (peak vorticity)

• Gaussian distribution fitting to the CFD results corresponding to the axial vorticity wx taken from a horizontal line probe passing through the vortex center on a plane normal to the streamwisedirection

Ref.: Martinez-Filgueira et al, 2017

( )2

5.02

1ln

= R

r

peaker ωω (Eq. 2)

z=r

y

x

Primary vortex

Secondary vortex!!

Half-life radius evolution. Beta=10º. Beta=15º5

Half-life radius evolution. Beta=18º. Beta=20º5

Best incident angle, according to Godard and Stanislas (2006)

Vortex center path. Vertical path. Beta=10º. Beta=18º5

Vortex center path. Lateral path. Beta=10º. Beta=18º5

Conclusions6

• Similar vortex parameter results that those obtained in experiments, analytical models and commercial CFD codes

• Vortex path: As expected, the vortex with highest vertical path is that which corresponds to the case 1.2H. Lateral path moves as expected

• Half life radius: Best cases: 0.6H and 0.8H

• Wall shear stress: Best cases: 0.6H and 0.4H

OpenFOAM

Vortex characterization study

• Work in progress …

• Considerable number of cases analysed: 6 VG heights (0.2H, 0.4H, 0.6H, 0.8H, H, 1.2H) and four incident angles (10º, 15º, 18º, 20º)

• Turbulence model & solver used are suitable for simulation & developing a computational model in order to characterize the primary vortex generated by rectangular VGs on a flat plate with negligible pressure gradient at Re=27000 based on the conventional VG height

• Appropriate tool used in the Department for Educational & Research purposes

References6

• A. Urkiola, U. Fernandez-Gamiz, I. Errasti and E. Zulueta. Computational characterization of the vortex generated by a vortex generator on a flat plate for different vane angles. Aerospace Science & Technology. 65, 18-25. 2017.

• P. Martinez-Filgueira, U. Fernandez-Gamiz, E. Zulueta, I. Errasti and B. Fernandez-Gauna. Parametric studyof low-profile vortex generators. International Journal of Hydrogen Energy. In press. 2017.

• U. Fernandez-Gamiz, G. Zamorano and E. Zulueta. Computational study of the vortex path variation with the VG height. J. Phys. Conf. Ser. 524. 012024. 2014.

• U. Fernandez-Gamiz, C.M. Velte, P.E. Réthoré, N.N. Sorensen and E. Egusquiza. Testing of self-similarity and helical symmetry in vortex generator flow simulations. Wind Energy. 2016.

• G. Godard and M. Stanislas. Part 1. Optimization of passive vortex generators. Aerosp. Sci. Technol. 10. 181–191. 2006.

• C. M. Velte. Vortex generator flow model based on self-similarity. AIAA J. 51(2), 526-529. 2013.

• C. M. Velte, M. O. L. Hansen and V. L. Okulov. Helical structure of longitudinal vortices embedded in turbulent wall-bounded flow. J. Fluid Mech. 619 pp. 167-177. DOI: 10.1017/S0022112008004588. 2009.

• J. C. Lin, F. G. Howard and G. V. Selby. Small submerged vortex generators for turbulent-flow separation control. J. Spacecraft Rockets 27(5), 503-507. 1990.

• J. C. Lin. Review of research on low-profile vortex generators to control boundary-layer separation. Prog. Aerospace Sci. 38(4-5), pp. 389. 2002

• P. Ashill, J. Fulker and K. Hackett. Research at DERA on sub boundary layer vortex generators(SBVGs). Presented at 39th AIAA Aerospace Sciences Meeting and Exhibit, AIAA Paper 2001-0887. 2001.