1. Unione Industriale Torino
4 Settembre 2007
Instability and turbulence in flows of automotive and aeronautical interest
D. Tordella, M. Onorato, M. Iovieno, S. Scarsoglio, P. Bailey,
C. Tribuzi, C. Haigermoser, L. Vesely, M. Novara
Dipartimento di Ingegneria Aeronautica e Spaziale
Politecnico di Torino

2. Unione Industriale Torino Hydrodynamic stability in bluff-body wakes
Daniela Tordella, Stefania Scarsoglio
Dipartimento di Ingegneria Aeronautica e Spaziale
Politecnico di Torino

3. Motivation Stability analysis
To understand the reasons for the breakdown of laminar flow;
To predict the transition to turbulence
Two-dimensional wake past a circular cylinder
Circular cylinder is the quintessential bluff-body;
Important prototype of free shear flow for applications in fluid mechanics

4. Application Bluff-body wakes;
Sequence of bodies moving one respect to each other (unsteady motion):
automotive (highways);
aeronautical (runways)
Theory (initial-value problem) unsteady motion configurations where perturbations (bodies moving) can arbitrarily occur

5. Key points: Linear analysis in free flows:
Non linear aspects do not substantially affect the most unstable waves (true also for high Re);
Linear theory results (frequency and wavenumber) represent the large scale structures of the fully developed turbulent flow
For high Re, the most unstable waves are long ( 10 spatial scales D);
Long waves slowly dissipate in time;
Time scales order: ~ hours

6. Physical problem

7. 9/13/2018
R = 60
Linear Stability Analysis

8. 9/13/2018
Normal mode theory
Linear Stability Analysis

9. Initial-value problem
9/13/2018
Initial-value problem
click here
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(a)-(b): R=100, y0=0, x0=9, k=1.7, αi =-0.05, β0=1, symmetric initial condition, (a) Φ=π/8, r=0.0826, (b) Φ=(3/8)π, r= (c): R=100, y0=0, x0=11, k=0.6, αi=0.02, β0=1, asymmetric initial condition, Φ=π/4, r=
Linear Stability Analysis

10. 9/13/2018
Publications
A synthetic perturbative hypothesis for the multiscale analysis of the convective wake instability - D. Tordella, S. Scarsoglio and M. Belan - Phys. Fluids, Vol. 18, No. 5 - (2006)
22nd IFIP TC 7 Conference on System Modeling and Optimization - Analysis
of the convective instability of the two-dimensional wake (S. Scarsoglio, D. Tordella, M. Belan) - 18/22 luglio 2005 – Torino
6th Euromech Fluid Mechanics Conference (EFMC6) - A synthetic perturbative hypothesis for multiscale analysis of bluff-body wake instability (D.Tordella, S. Scarsoglio, M. Belan) - June 26-30, Stockholm, Sweden
59th Annual Meeting Division of Fluid Dynamics (APS-DFD) - Initial-value problem for the two-dimensional growing wake (S. Scarsoglio, D.Tordella and W. O. Criminale) – November 19-21, Tampa, Florida
11th Advanced European Turbulence Conference - Temporal dynamics of small perturbations for a two-dimensional growing wake (S. Scarsoglio, D.Tordella and W. O. Criminale) - June 25-28, Porto, Portugal
Linear Stability Analysis

11. Transient and asymptotic behaviour of small three-dimensional perturbations applied to a growing wake – S. Scarsoglio, D. Tordella and W. O. Criminale – submitted to J. Fluid Mech. 2007
Streamwise evolution of the entrainment in a steady two-dimensional bluff-body wake – D. Tordella, S. Scarsoglio – submitted to J. Eng. Math. 2007
Convective instability in wake intermediate asymptotics - M. Belan, D. Tordella - J. Fluid Mech., 552 : , 2006.
On the domain of validity of the near-parallel combined stability analysis for the 2D intermediate and far bluff body wake - D. Tordella, M. Belan - ZAMM, 85 (1):
A new matched asymptotic expansion for the intermediate and far flow behind a finite body – D. Tordella, M. Belan - Phys. Fluids, 15 (7):
Asymptotic expansions for two dimensional symmetrical laminar wakes - M. Belan, D. Tordella - ZAMM, 82 (4):

12. Turbulent transport - Trasporto turbolento
Prof. D Tordella
Michele Iovieno
Peter Bailey

13. Motivation Turbulence Scales: large (geometry); small (molecular)
Most energy at large scales

14. Motivation Turbulence Scales: large (geometry); small (molecular)
Most energy at large scales – let us ignore the small scale ?

15. Motivation Turbulence Scales: large (geometry); small (molecular)
Most energy at large scales – let us ignore the small scale ?
Practical reasons (Applications)
Effect on light and sound wave propagation
Combustion/chemical reactors – small scale level dynamics determine reaction rate
Parametrization of subgrid scale terms in LES, k-eps and other turbulence models

16. Motivation Turbulence Practical reasons (Applications)
Scales: large (geometry); small (molecular)
Most energy at large scales – let us ignore the small scale ?
Practical reasons (Applications)
Effect on light and sound wave propagation
Combustion/chemical reactors – small scale level dynamics determine reaction rate
Parametrization of subgrid scale terms in LES, k-eps and other turbulence models
Our study of shearless turbulent mixing allowed us to discover a set of general properties

17. Turbulent mixing
Homogeneous Isotropic Turbulence (HIT) is the simplest turbulence possible
A HIT mixing is the simplest turbulence mixing
We investigate this mixing without a mean shear and without a length scale
 to date this configuration is shown to lack intermittency

18. Turbulent mixing
Homogeneous Isotropic Turbulence (HIT) is the simplest turbulence possible
A HIT mixing is the simplest turbulence mixing
We investigate this mixing without a mean shear and without a length scale
 to date this configuration is shown to lack intermittency

19. State of the Art
Empirically investigated by Gilbert (1980) and Veeravalli & Warhaft (1989)
by passive grid generated turbulence. Grids of equal solidity but differing size
adjacent turbulence fields of differing kinetic energy and integral length scale, without mean shear

20. State of the Art
Empirically investigated by Gilbert (1980) and Veeravalli & Warhaft (1989)
by passive grid generated turbulence. Grids of equal solidity but differing size
adjacent turbulence fields of differing kinetic energy and integral length scale, without mean shear
in planes downstream of grid, no mean shear, we have a HIT mixing with different scales

21. State of the Art
Empirically investigated by Gilbert (1980) and Veeravalli & Warhaft (1989)
by passive grid generated turbulence. Grids of equal solidity but differing size
adjacent turbulence fields of differing kinetic energy and integral length scale, without mean shear
in planes downstream of grid, no mean shear, we have a HIT mixing with different scales
Analyze and determine time and spatial properties of the turbulence energy production as a function of these ratios

22. State of the Art
Empirically investigated by Gilbert (1980) and Veeravalli & Warhaft (1989)
by passive grid generated turbulence. Grids of equal solidity but differing size
adjacent turbulence fields of differing kinetic energy and integral length scale, without mean shear
in planes downstream of grid, no mean shear, we have a HIT mixing with different scales
Analyze and determine time and spatial properties of the turbulence energy production as a function of these ratios
 in computation we can go one step better. Empirically length and energy scales are intrinsically linked. However in the computation we are able to alter these independently and get a better insight into the interrelations

23. Analysis of Mixing
The principle means to identify turbulent intermittence here is through one point velocity statistics
We look at:
S (skewness) = the flow of kinetic energy
K (kurtosis) = the flow of S
S remains 0, K=3 in HIT

24. Flow of kinetic energy and penetration
S moves from 0 as the mixing begins, remains close to 0 outside the mixing
Penetration is from high energy field into the low energy side, shown as fraction of the mixing layer thickness
As energy ratio is increased we note a linear log relation before the energy flow asymptotes to its maximum
click here

25. Flow visualisation click here
Mixing enhancement (retardation) if the gradients of energy and length scale are concurrent (opposite) {Journal of Fluid Mechanics, 2006}
Scaling law for the mixing penetration with respect to the turbulent kinetic energy ratio {Journal of Fluid Mechanics, 2006}
Sufficiency of the presence of a gradient of kinetic energy for Gaussian departure in turbulence
{in review for American Physical Society Physical Review E, 2007}

26. Turbulent Cavity Flows Flussi della cavità turbulenta-
Flussi della cavità turbulenta
AeroTraNet network: (supported by Marie Curie Actions)
Prof. D Tordella
Prof. M Onorato*
Michele Iovieno
Peter Bailey
Christian Haigermoser*
Lukas Vesely*

27. Motivation

28. Statistical Results Time averaged flow
Shear layer impinges onto the forward facing step
Primary and secondary recirculation zones

29. Time-resolved Results
Vortex identification:
Vorticity ωz
λci-vortex-identification criteria; λci = swirling strength
Vortices – > Click here

30. Time-Resolved Drag Drag coefficient time history
CD reveals oscillation frequency of f≈2.8Hz (FFT), corresponding to StH=0.08
Highest contribution to CD from Reynolds stress term
Contributions to CD along x/H

31. DRAG FROM CONDITIONAL ANALYSIS
Low drag
High drag