Imperial College London

DrGeorgiosRigas

Faculty of EngineeringDepartment of Aeronautics

Lecturer
 
 
 
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Contact

 

+44 (0)20 7594 5065g.rigas CV

 
 
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Location

 

327City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

23 results found

Pickering E, Rigas G, Schmidt OT, Sipp D, Colonius Tet al., 2021, Optimal eddy viscosity for resolvent-based models of coherent structures in turbulent jets, JOURNAL OF FLUID MECHANICS, Vol: 917, ISSN: 0022-1120

Journal article

Rigas G, Sipp D, Colonius T, 2021, Nonlinear input/output analysis: application to boundary layer transition, JOURNAL OF FLUID MECHANICS, Vol: 911, ISSN: 0022-1120

Journal article

Pickering E, Rigas G, Nogueira PAS, Cavalieri AVG, Schmidt OT, Colonius Tet al., 2020, Lift-up, Kelvin-Helmholtz and Orr mechanisms in turbulent jets, JOURNAL OF FLUID MECHANICS, Vol: 896, ISSN: 0022-1120

Journal article

Towne A, Rigas G, Colonius T, 2019, A critical assessment of the parabolized stability equations, THEORETICAL AND COMPUTATIONAL FLUID DYNAMICS, Vol: 33, Pages: 359-382, ISSN: 0935-4964

Journal article

Nogueira PA, Cavalieri AV, Schmidt OT, Jordan P, Jaunet V, Pickering E, Rigas G, Colonius Tet al., 2019, Resolvent-based analysis of streaks in turbulent jets, 25th AIAA/CEAS Aeroacoustics Conference, Publisher: American Institute of Aeronautics and Astronautics

Conference paper

Pickering EM, Rigas G, Sipp D, Schmidt OT, Colonius Tet al., 2019, Eddy viscosity for resolvent-based jet noise models, 25th AIAA/CEAS Aeroacoustics Conference, Publisher: American Institute of Aeronautics and Astronautics

Conference paper

Rigas G, Pickering EM, Schmidt OT, Nogueira PA, Cavalieri AV, Brès GA, Colonius Tet al., 2019, Streaks and coherent structures in jets from round and serrated nozzles, 25th AIAA/CEAS Aeroacoustics Conference, Publisher: American Institute of Aeronautics and Astronautics

Conference paper

Schmidt OT, Towne A, Rigas G, Colonius T, Bres GAet al., 2018, Spectral analysis of jet turbulence, Journal of Fluid Mechanics, Vol: 855, Pages: 953-982, ISSN: 0022-1120

Informed by large-eddy simulation (LES) data and resolvent analysis of the mean flow, we examine the structure of turbulence in jets in the subsonic, transonic and supersonic regimes. Spectral (frequency-space) proper orthogonal decomposition is used to extract energy spectra and decompose the flow into energy-ranked coherent structures. The educed structures are generally well predicted by the resolvent analysis. Over a range of low frequencies and the first few azimuthal mode numbers, these jets exhibit a low-rank response characterized by Kelvin–Helmholtz (KH) type wavepackets associated with the annular shear layer up to the end of the potential core and that are excited by forcing in the very-near-nozzle shear layer. These modes too have been experimentally observed before and predicted by quasi-parallel stability theory and other approximations – they comprise a considerable portion of the total turbulent energy. At still lower frequencies, particularly for the axisymmetric mode, and again at high frequencies for all azimuthal wavenumbers, the response is not low-rank, but consists of a family of similarly amplified modes. These modes, which are primarily active downstream of the potential core, are associated with the Orr mechanism. They occur also as subdominant modes in the range of frequencies dominated by the KH response. Our global analysis helps tie together previous observations based on local spatial stability theory, and explains why quasi-parallel predictions were successful at some frequencies and azimuthal wavenumbers, but failed at others.

Journal article

Brès GA, Bose ST, Emory M, Ham FE, Schmidt OT, Rigas G, Colonius Tet al., 2018, Large-eddy simulations of co-annular turbulent jet using a Voronoi-based mesh generation framework, 2018 AIAA/CEAS Aeroacoustics Conference, Publisher: American Institute of Aeronautics and Astronautics

Conference paper

Jamieson NP, Rigas G, Juniper MP, 2017, Experimental sensitivity analysis via a secondary heat source in an oscillating thermoacoustic system, INTERNATIONAL JOURNAL OF SPRAY AND COMBUSTION DYNAMICS, Vol: 9, Pages: 230-240, ISSN: 1756-8277

Journal article

Rigas G, Schmidt OT, Colonius T, Brès GAet al., 2017, One Way Navier-Stokes and resolvent analysis for modeling coherent structures in a supersonic turbulent jet, 23rd AIAA/CEAS Aeroacoustics Conference, Publisher: American Institute of Aeronautics and Astronautics

Conference paper

Rigas G, Esclapez L, Magri L, 2017, Symmetry breaking in a 3D bluff-body wake

The dynamics of a three-dimensional axisymmetric blu -body wake are examined at low Reynolds regimes where transitions take place through spatio-temporal symmetry breaking. A linear stability analysis is employed to identify the critical Reynolds num- ber associated with symmetry breaking, and the associated eigenmodes, known as global modes. The analysis shows that the axisymmetric stable base ow breaks the rotational symmetry through a pitchfork m = 1 bifurcation, in agreement with previously reported results for axisymmetric wakes. Above this threshold, the stable base ow is steady and three-dimensional with planar symmetry. A three-dimensional global stability analysis around the steady re ectionally symmetric base ow, assuming no homogeneous direc- tions, predicts accurately the Hopf bifurcation threshold, which leads to asymmetric vortex shedding. DNS simulations validate the stability results and characterize the ow topology during the early chaotic regime.

Working paper

Rigas G, Morgans AS, Morrison JF, 2017, Weakly nonlinear modelling of a forced turbulent axisymmetric wake, Journal of Fluid Mechanics, Vol: 814, Pages: 570-591, ISSN: 0022-1120

A theory is presented where the weakly nonlinear analysis of laminar globally unstableflows in the presence of external forcing is extended to the turbulent regime. The analysisis demonstrated and validated using experimental results of an axisymmetric bluff bodywake at high Reynolds numbers,ReD∼1.88×105, where forcing is applied using aZero-Net-Mass-Flux actuator located at the base of the blunt body. In this study wefocus on the response of antisymmetric coherent structures with azimuthal wavenumbersm=±1 at a frequencyStD= 0.2, responsible for global vortex shedding. We foundexperimentally that axisymmetric forcing (m= 0) couples nonlinearly with the globalshedding mode when the flow is forced at twice the shedding frequency, resulting inparametric subharmonic resonance through a triadic interaction between forcing andshedding. We derive simple weakly nonlinear models from the phase-averaged Navier-Stokes equations and show that they capture accurately the observed behaviour forthis type of forcing. The unknown model coefficients are obtained experimentally byproducing harmonic transients. This approach should be applicable ina variety ofturbulent flows to describe the response of global modes to forcing.

Journal article

Rigas G, Colonius T, Beyar M, 2017, Stability of wall-bounded fows using one-way spatial integration of Navier-Stokes equations, 55th AIAA Aerospace Sciences Meeting, Publisher: American Institute of Aeronautics and Astronautics

Conference paper

Orchini A, Rigas G, Juniper MP, 2016, Weakly nonlinear analysis of thermoacoustic bifurcations in the Rijke tube, JOURNAL OF FLUID MECHANICS, Vol: 805, Pages: 523-550, ISSN: 0022-1120

Journal article

Brackston RD, Garcia de la Cruz Lopez JM, Wynn A, Rigas G, Morrison JFet al., 2016, Stochastic modelling and feedback control of bistability in a turbulent bluff body wake, Journal of Fluid Mechanics, Vol: 802, Pages: 726-749, ISSN: 0022-1120

A specific feature of three-dimensional bluff body wakes, flow bistability, is a subject of particular recent interest. This feature consists of a random flipping of the wake between two asymmetric configurations and is believed to contribute to the pressure drag of many bluff bodies. In this study we apply the modelling approach recently suggested for axisymmetric bodies by Rigas et al. (J. Fluid Mech., vol. 778, 2015, R2) to the reflectional symmetry-breaking modes of a rectilinear bluff body wake. We demonstrate the validity of the model and its Reynolds number independence through time-resolved base pressure measurements of the natural wake. Further, oscillating flaps are used to investigate the dynamics and time scales of the instability associated with the flipping process, demonstrating that they are largely independent of Reynolds number. The modelling approach is then used to design a feedback controller that uses the flaps to suppress the symmetry-breaking modes. The controller is successful, leading to a suppression of the bistability of the wake, with concomitant reductions in both lateral and streamwise forces. Importantly, the controller is found to be efficient, the actuator requiring only 24 % of the aerodynamic power saving. The controller therefore provides a key demonstration of efficient feedback control used to reduce the drag of a high-Reynolds-number three-dimensional bluff body. Furthermore, the results suggest that suppression of large-scale structures is a fundamentally efficient approach for bluff body drag reduction.

Journal article

Rigas G, Jamieson NP, Li LKB, Juniper MPet al., 2016, Experimental sensitivity analysis and control of thermoacoustic systems, JOURNAL OF FLUID MECHANICS, Vol: 787, ISSN: 0022-1120

Journal article

Rigas G, Brackston RD, Garcia de la Cruz Lopez JM, Morrison JF, Wynn Aet al., 2015, Drag Reduction Method

Patent

Rigas G, Morgans AS, Brackston RD, Morrison JFet al., 2015, Diffusive dynamics and stochastic models of turbulent axisymmetric wakes, Journal of Fluid Mechanics, Vol: 778, Pages: R2-1-R2-10, ISSN: 0022-1120

A modelling methodology to reproduce the experimental measurements of a turbulent flow in the presence of symmetry is presented. The flow is a three-dimensional wake generated by an axisymmetric body. We show that the dynamics of the turbulent wake flow can be assimilated by a nonlinear two-dimensional Langevin equation, the deterministic part of which accounts for the broken symmetries that occur in the laminar and transitional regimes at low Reynolds numbers and the stochastic part of which accounts for the turbulent fluctuations. Comparison between theoretical and experimental results allows the extraction of the model parameters.

Journal article

Oxlade AR, Morrison JF, Qubain A, Rigas Get al., 2015, High-frequency forcing of a turbulent axisymmetric wake, Journal of Fluid Mechanics, Vol: 770, Pages: 305-318, ISSN: 0022-1120

A high-frequency periodic jet, issuing immediately below the point of separation, is used to force the turbulent wake of a bluff axisymmetric body, its axis aligned with the free stream. It is shown that the base pressure may be varied more or less at will: at forcing frequencies several times that of the shear layer frequency, the time-averaged area-weighted base pressure increases by as much as 35 %. An investigation of the effects of forcing is made using random and phase-locked two-component particle image velocimetry (PIV), and modal decomposition of pressure fluctuations on the base of the model. The forcing does not target specific local or global wake instabilities: rather, the high-frequency jet creates a row of closely spaced vortex rings, immediately adjacent to which are regions of large shear on each side. These shear layers are associated with large dissipation and inhibit the entrainment of fluid. The resulting pressure recovery is proportional to the strength of the vortices and is accompanied by a broadband suppression of base pressure fluctuations associated with all modes. The optimum forcing frequency, at which amplification of the shear layer mode approaches unity gain, is roughly five times the shear layer frequency.

Journal article

Oxlade AR, Morrison JF, Rigas G, 2015, Open-Loop Control of a Turbulent Axisymmetric Wake, INSTABILITY AND CONTROL OF MASSIVELY SEPARATED FLOWS, Vol: 107, Pages: 137-142, ISSN: 0926-5112

Journal article

Rigas G, Oxlade AR, Morgans AS, Morrison JFet al., 2014, Low-dimensional dynamics of a turbulent axisymmetric wake, Journal of Fluid Mechanics, Vol: 755, Pages: 1-11, ISSN: 0022-1120

The coherent structures of a turbulent wake generated behind a bluff three-dimensional axisymmetric body are investigated experimentally at a diameter-based Reynolds number of ∼2×105 . Proper orthogonal decomposition of base pressure measurements indicates that the most energetic coherent structures retain the structure of the symmetry-breaking laminar instabilities and are manifested as unsteady vortex shedding with azimuthal wavenumber 𝑚=±1 . In a rotating reference frame, the shedding preserves the reflectional symmetry and is linked with a reflectionally symmetric mean pressure distribution on the base. Due to a slow rotation of the symmetry plane of the turbulent wake around the axis of the body, statistical axisymmetry is recovered in the time average. The ratio of the time scales associated with the slow rotation of the symmetry plane and the vortex shedding is of order 100.

Journal article

Rigas G, Morgans AS, Morrison JF, 2013, Stability and coherent structures in the wake of axisymmetric bluff bodies, Int. Conf. on Massively Separated Flows, Publisher: Springer

Conference paper

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