Square back bluff body LES computation Flow control ...€¦ · Geometry description and flow...

Square back bluff body LES computationFlow control applications

11.2014 ● GDR

Y.Eulalie – P.Gilotte – I.Mortazavi – S.Edwige

Orsay– 18 November 2014

Uref

- All rights reserved -18.11.2014 ● GDR

� Outlines� Computational hypothesis and comparison with experiments :

o Geometry description and flow control conditionso LES model, simulation parameters o Cd curve and table, Cp fields

� Analysis of reference wakeo Average field of pressure in the wake o Cross correlation in the shear layer, in the wake and with the back o Coherent structures identified

� Computational Flow control on the wakeo Results of DOEo Average field of pressure, TKEo Effect on the Q criterion average field

Square back bluff body LES computation and flow control

2

Experimental results obtained in partnership with Renault in the TIGRE project sponsored by ADEME

TIGRE project involves Renault Truck and LMFA, Renault and Plastic Omnium


� Geometry description and flow control conditions :� Orléans PRISME wind tunnel at U

∞=30m.s-1 � ReH= 412 000

� Ahmed body with square back (731mmx272mmx201mm)

� Blowing slot on the square back along the top edge, Surface S=220x0,5mm²

� Flow control DOE on frequency, flow rate and angle and location

3

(0,0,0)

(0,0,0)

Q=230L/min

Square back bluff body LES computation


� Computational hypothesis and LES model� 120 million unstructured tetrahedral cells mesh� Computational time step 1/∆t=8000Hz� Final time T=1,5s for 12000 time steps� LES computation, Smagorinski subgrid model� Finite element 2nd order solver AcuSolve™ � 1000Hz sampling outputs fields, averaging over 1000 fields� Running on 36 processors for 20 days – 1Tb database

4



� Average results and comparison with experiments : Cd values� Cd computed with and w/o control close to experiments� Maximum difference with experiments is 3% at 30Hz� 60% of Cd contribution due to the square back (except 30Hz)

5


St = f.H/V


� Average results and comparison with experiments : Rear pressure� Reference Cp closed to the experiments � Cp at 400Hz shows a dissymmetry (Bi-stable effect ?)

6

REF Flow control at 400Hz

Average Cp=-0,208

Average Cp=-0,222

Average Cp=-0,201

Average Cp=-0,224

Experiments

Computations


Cp [-]


� Average computational result : mean flow characteri stics� Reference : maximum value of TKE in the shear layer� TKE at 400Hz shows a dissymmetry in the Z=H/2 plane

7

REFFlow control

400Hz

PlaneY=0

PlaneZ=H/2 WW

H H


TKE [m²/s²]


� Average results : Pressure map and TKE in the reference wake

� Cd values are linked to low pressure centers� Classical 0-ring shape of the pressure field

8

PlaneZ=H/2

WidthW=272mm

PlaneY=0

HeightH=201mm

d=0,78H


Cp [-]

Iso surface of CpCp = - 0,29

H

Recirculation average centers

TKE [m²/s²]

- All rights reserved -18.11.2014 ● GDR9


� Identification of main wake phenomena� Cross correlation of pressure in the symmetry plane� Identification of delay characteristic time

PlaneY=0

H

H/2

HH/2

t1caract

t2caract

��

� . ��

� ��. � ��

τ [s]

Cro

ss c

orre

latio

n[-

]

Cp [-]

Cross correlation :

� � �̅ � �′



� Identification of main wake phenomena� Spectral coherence� 20Hz (St=0,13) strong in the wake� 26Hz (St=0,18)

PlaneY=0

H

H/2

HH/2

Cp [-]

Frequency [Hz]

Cro

ss s

pect

ra [-

]

Frequency [Hz]

Cro

ss s

pect

ra [-

]

Frequency [Hz]

Cro

ss s

pect

ra [-

]

20Hz

26Hz

20Hz26Hz

Shear layer frequencies


� Identification of main wake phenomena� Decomposition of the pressure fields� POD Snapshot method (Sirovitch ,1987)� 256 fields, sampled at 1000Hz - Spectral domain [4Hz - 500Hz]

11


20Hz (St=0,13)

U∞

Plane Y0

H

∑ Φ=N

kki

ki xtaxtp )().(),('

rr

20HzU∞

� � �̅ � ��

1st mode (12%)

5th mode (8%)

26Hz (St=0,18)

Plane Y0

H



φ=πφ=0

� Identification of main wake phenomena� Phase average at 20Hz� Phase shift of pressure minima� Accordance to the fifth mode

�� Test of more flow control cases



� Extended flow control parameters :� Pulsed and synthetic jet actuation

� Frequency range [0;800Hz]� Blowing angle� Flow rate : square or sinusoid signal� Continuous/discontinuous slot

� With deflector� 12° angle for roof� 6° angles for sides

� Coupling deflector and periodic jet

Periodic actuation

θθθθ



� Flow control : DOE with spoiler and periodic jet

� 7,3% of Cd gain with synthetic jet w/o spoiler

No blowing


� Flow control : Time average Cp maps on the back� Reference : square back� -5,9% : side deflector 6°and top spoiler 12°� -8,5% : side deflector 6°and top spoiler 12° + pulsed jet 400Hz

15

Reference

Space averageCp=-0,208


Cp [-]

Deflector only

Cp = -0,180∆Cp = +13,6%∆Cd = - 5,9%

Deflector with pulsed jet

Cp = -0,167∆Cp = + 20%∆Cd = - 8,5%

Cp=-0,195 Cp=-0,14

Cp [-]



� Flow control : Time average Cp fields in the wake� Decreased pressure level� O-ring shape preserved with control

PlaneY=0

Cp [-]

Min Cp = -0,24 Min Cp = -0,22Min Cp = -0,29

Cp = -0,24Cp = -0,22

Iso surface of CpCp = - 0,29

Reference Deflector only Deflector with pulsed jet



� Flow control : Q criterion and TKE in the wake� Source term of pressure� Modification of Q locations in symmetry plane

Q [s-²]Reference Deflector only Deflector with pulsed jet

� � ��

�

��

��

��

��

�

�Ω ²− � ² � … �

��

��

H

[m²/s²]



� Flow control : Iso contour of Q criterion� Q criterion decreased in the shear layer with flow control� Pulsed jets decrease Q criterion in the deflector area� Maxima remained on lateral flow separation

Iso value Q =30000= 1.4V²/H² , colored by X-velocity




� Summary of flow control� Possible Drag reduction up to 10,5% � Dominant effects on superior part of the wake� Increase of pressure in the wake and effect on the Q criterion� Perturbation in the shear layer and its vortex

� Further work� Optimization of flow control parameters in order to reduce the source

term of pressure in the shear layer� DOE to finalize, perform cross correlation and modal decomposition on

interesting cases

19


Thanks for your attention

Questions ?



APPENDIX



� Average results and comparison with experiments : Boundary layer� Profile variation at X=H/10 cut section � Turbulent ratio of 35% calculated in boundary layer� Strouhal StLR

= 0,6 and 0,8 (Kiya & al.,86 )where LR is the recirculation length of the bubble

22

Spectra of velocity at the end of the boundary layer

Velocity (up) and Turbulent kinetic energy (down) profile in Y0 cut plane

St=0,6St=1,3

St=0,8

LR=175mm

Velocity iso-surface at V=0,99U∞

in a X=-H/10 cut plane

Y0

Velocity iso-surface at V=0,99U∞

in a Y>0 fields colored by pressure

(X=H/10;Y=Y0)

Recirculation length of the bubble



� Average results and comparison with experiments : Cd values� Best Cd gain obtained in experiments is –4.6% for :

o ground clearance of 35mmo pulsed frequency of 400Hz

� Reduction of the Cd legs contribution linked to shorter ground clearance

23

-4,6%

-2%

Numerical points :

Experimental curves :

Third mode of pressure and related spectra



Σ[ ]24

),(')(),( xtuxuxtu ii

rrrrrr +=

∑ Φ=N

kki

ki xtaxtu )().(),('

rrrr )(xk

rrΦ )( i

k ta

U∞

⟩•⟨= ),('),('1

xtuxtuN

R jiX

ij

rrrr

iiiR Ψ=Ψrr

λ ⟩Ψ•⟨=Φ iik xtuxrrrrr

),(')( ⟩Φ•⟨= )(),(')( xxtut kiik

rrrrα

1/

2/

3/ are proper modes are modal coefficients

R is the correlation matrix ),( ii Ψr

λ are eigen values and eigen vectors of R

� Modal decomposition : method description� Snapshots method (Sirovitch,1987)� Pressure decomposition� 256 fields used, sampled at 1000Hz

� Spectral domain available : [ 4Hz - 500Hz ]

x

Proper modeModal coefficient

=


+

Average fieldReconstructed field at time ti

ti

k

)(xk

rrΦ)( i

k ta ),( xtu i

rr

N modes


� Modal decomposition :� Distribution is different between uncontrolled and controlled case, � The first mode is more contributive for controlled cases� More than 256 modes would increase decomposition accuracy

25

First modal coefficients for cases reference (left), flow control 30Hz (middle), flow control 400Hz (right)

Reference case Flow control 30Hz Flow control 400Hz)(1 ta )(1 ta )(1 ta

Correlation reference matrix colored by log(|Rij|)


41% contribution of first mode for 400Hz case

12% contribution of first mode for reference case

Contribution of firsts 20 modesCorrelation 400Hz matrix

colored by log(|Rij|)


� Modal decomposition : description of 1st mode� Frequencies of control appears in the spectra as the most energetic � Importance of the 5 legs in the decomposition of reference 1st mode� At 400Hz, convection velocity of blowing jet in the vicinity of the slot

26

REF Flow control 400Hzλ = 0,3Η λ = 0,17Η

Vmod = λ.f1 = 14m/s�Average blowing velocity

Plane Y0


= ∫

Tslot dttU

T 0

)(1

U∞ U∞

Plane Y0

HH

8Hz

24Hz48Hz

8Hz 400Hz

200Hz

Isocontour (left) and Y0 field (right) of 1st proper mode Isocontour (left) and Y0 field (right) of 1st proper mode

1st modal coefficient (left) and its associated spectra (right) 1st modal coefficient (left) and its associated spectra (right)

Isocontour of 1st proper mode in the ground clearance



� Flow control : Time average Cp fields in the wake� Decreased pressure level

PlaneY=0

PlaneZ=H/2

Cp [-]

Min Cp = -0,24 Min Cp = -0,22Min Cp = -0,29


Square back bluff body LES computation Flow control ...€¦ · Geometry description and flow...

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