Imperial College London

DrGeorgiosRigas

Faculty of EngineeringDepartment of Aeronautics

Lecturer
 
 
 
//

Contact

 

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

 
 
//

Location

 

327City and Guilds BuildingSouth Kensington Campus

//

Summary

 

Publications

Publication Type
Year
to

30 results found

He X, Fang Z, Rigas G, Vahdati Met al., 2021, Spectral proper orthogonal decomposition of compressor tip leakage flow, PHYSICS OF FLUIDS, Vol: 33, ISSN: 1070-6631

Journal article

Callaham JL, Loiseau J-C, Rigas G, Brunton SLet al., 2021, Nonlinear stochastic modelling with Langevin regression, PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 477, ISSN: 1364-5021

Journal article

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, Pages: 1-34, ISSN: 0022-1120

Response modes computed via linear resolvent analysis of a turbulent mean-flow field have been shown to qualitatively capture characteristics of the observed turbulent coherent structures in both wall-bounded and free shear flows. To make such resolvent models predictive, the nonlinear forcing term must be closed. Strategies to do so include imposing self-consistent sets of triadic interactions, proposing various source models or through turbulence modelling. For the latter, several investigators have proposed using the mean-field eddy viscosity acting linearly on the fluctuation field. In this study, a data-driven approach is taken to quantitatively improve linear resolvent models by deducing an optimal eddy-viscosity field that maximizes the projection of the dominant resolvent mode to the energy-optimal coherent structure educed using spectral proper orthogonal decomposition (SPOD) of data from high-fidelity simulations. We use large-eddy simulation databases for round isothermal jets at subsonic, transonic and supersonic conditions and show that the optimal eddy viscosity substantially improves the agreement between resolvent and SPOD modes, reaching over 90 % agreement at those frequencies where the jet exhibits a low-rank response. We then consider a fixed model for the eddy viscosity and show that with the calibration of a single constant, the results are generally close to the optimal one. In particular, the use of a standard Reynolds-averaged Navier–Stokes eddy-viscosity resolvent model, with a single coefficient, provides substantial agreement between SPOD and resolvent modes for three turbulent jets and across the most energetic wavenumbers and frequencies.

Journal article

Rigas G, Sipp D, Colonius T, 2021, Nonlinear input/output analysis: application to boundary layer transition, Journal of Fluid Mechanics, Vol: 911, Pages: 1-42, ISSN: 0022-1120

We extend linear input/output (resolvent) analysis to take into account nonlinear triadic interactions by considering a finite number of harmonics in the frequency domain using the harmonic balance method. Forcing mechanisms that maximise the drag are calculated using a gradient-based ascent algorithm. By including nonlinearity in the analysis, the proposed frequency-domain framework identifies the worst-case disturbances for laminar-turbulent transition. We demonstrate the framework on a flat-plate boundary layer by considering three-dimensional spanwise-periodic perturbations triggered by a few optimal forcing modes of finite amplitude. Two types of volumetric forcing are considered, one corresponding to a single frequency/spanwise wavenumber pair, and a multi-harmonic where a harmonic frequency and wavenumber are also added. Depending on the forcing strategy, we recover a range of transition scenarios associated with K -type and H -type mechanisms, including oblique and planar Tollmien–Schlichting waves, streaks and their breakdown. We show that nonlinearity plays a critical role in optimising growth by combining and redistributing energy between the linear mechanisms and the higher perturbation harmonics. With a very limited range of frequencies and wavenumbers, the calculations appear to reach the early stages of the turbulent regime through the generation and breakdown of hairpin and quasi-streamwise staggered vortices.

Journal article

Kamal O, Rigas G, Lakebrink MT, Colonius Tet al., 2021, Input/Output Analysis of Hypersonic Boundary Layers using the One-Way Navier-Stokes (OWNS) Equations

Accurate prediction of linear amplification of disturbances in hypersonic boundary layers is computationally challenging. While direct numerical simulations and global analysis can be used to compute optimal (worst-case) forced responses, their large computational expense render these tools less practical for large design parameter spaces. At the same time, parabolized stability equations can be unreliable for problems involving multi-modal and non-modal interactions. To bridge this gap, we apply an approximate fast marching technique, the One-Way Navier-Stokes (OWNS) Equations, in iterative fashion to solve for optimal disturbances. OWNS approximates a rigorous parabolization of the equations of motion by removing disturbances with upstream group velocity using a higher-order recursive filter. Using OWNS, we aim to characterize disturbances of flat-plate and complex-geometry hypersonic boundary layers over a range of Mach numbers, and find optimal disturbances under different cost functions that define corresponding receptivity problems. The calculation of optimal disturbances reveals multi-modal transition scenarios depending on the spatial support, frequency, and physical nature of the external disturbances.

Conference paper

He X, Fang Z, Rigas G, Vahdati Met al., 2021, SPECTRAL PROPER ORTHOGONAL DECOMPOSITION OF COMPRESSOR TIP LEAKAGE FLOW

Spectral proper orthogonal decomposition (SPOD) is performed on the near-stall tip leakage flow of a low-speed compressor rotor. The data used for the SPOD analysis is obtained by delayed-detached eddy simulation (DDES), which is validated against experimental data. The flow quantities of interest include the near-tip axial velocity and the blade surface pressure. Results show that the near-stall flow field of the investigated rotor is governed by two tip leakage vortices (TLV). The main TLV initiated from the leading edge exerts an unsteady force on the blade pressure surface. Its modal component is dominated by the leading modes at low frequencies. The secondary TLV originated from the mid-chord creates a weaker unsteady force on the blade suction surface, and its modal component has more high-frequency components due to its interaction with the suction surface boundary layer flows. These findings improve the understanding of the effects of tip leakage flow on compressor aerodynamic and aeroelastic stability.

Conference paper

Tan J, He X, Rigas G, Vahdati Met al., 2021, TOWARDS EXPLAINABLE MACHINE-LEARNING-ASSISTED TURBULENCE MODELING FOR TRANSONIC FLOWS

A machine-learning-assisted turbulence modeling framework is proposed to improve the prediction accuracy of the Spalart-Allmaras turbulence model. The case studied is the transonic bump flow, which partially resembles the flow physics of a transonic compressor. A random forest model is trained, cross-validated and tested to construct a mapping between the input features and the eddy viscosity discrepancy. These input features concern the physical effects of pressure gradient, strain versus vorticity, flow misalignment, wall proximity and viscosity ratio. Results show that the proposed approach predicts an interpolation and an extrapolation test case with L1-type errors of 11.1% and 16.5%, respectively. The Shapley additive explanations method is employed to investigate the global and local sensitivities of each input feature. The capability of these input features in identifying specific flow features is discussed. The methods and results of this work provide useful guidance for turbulence model developers.

Conference paper

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, Pages: 1-36, ISSN: 0022-1120

Three amplification mechanisms present in turbulent jets, namely lift-up, Kelvin–Helmholtz and Orr, are characterized via global resolvent analysis and spectral proper orthogonal decomposition (SPOD) over a range of Mach numbers. The lift-up mechanism was recently identified in turbulent jets via local analysis by Nogueira et al. (J. Fluid Mech., vol. 873, 2019, pp. 211–237) at low Strouhal number ( St ) and non-zero azimuthal wavenumbers ( m ). In these limits, a global SPOD analysis of data from high-fidelity simulations reveals streamwise vortices and streaks similar to those found in turbulent wall-bounded flows. These structures are in qualitative agreement with the global resolvent analysis, which shows that they are a response to upstream forcing of streamwise vorticity near the nozzle exit. Analysis of mode shapes, component-wise amplitudes and sensitivity analysis distinguishes the three mechanisms and the regions of frequency–wavenumber space where each dominates, finding lift-up to be dominant as St/m→0 . Finally, SPOD and resolvent analyses of localized regions show that the lift-up mechanism is present throughout the jet, with a dominant azimuthal wavenumber inversely proportional to streamwise distance from the nozzle, with streaks of azimuthal wavenumber exceeding five near the nozzle, and wavenumbers one and two most energetic far downstream of the potential core.

Journal article

Brouzet D, Haghiri A, Talei M, Brear MJ, Schmidt OT, Rigas G, Colonius Tet al., 2020, Role of Coherent Structures in Turbulent Premixed Flame Acoustics, AIAA JOURNAL, Vol: 58, Pages: 2635-2642, ISSN: 0001-1452

Journal article

Kamal O, Rigas G, Lakebrink MT, Colonius Tet al., 2020, Application of the one-way navier-stokes (Owns) equations to hypersonic boundary layers

Prediction of linear amplification of disturbances in hypersonic boundary layers is challenging due to the presence and interactions of discrete modes (e.g. Tollmien-Schlichting and Mack) and continuous modes (entropic, vortical, and acoustic). While DNS and global analysis can be used, the large grids required make the computation of optimal transient and forced responses expensive, particularly when a large parameter space is required. At the same time, parabolized stability equations are non-convergent and unreliable for problems involving multi-modal and non-modal interactions. In this work, we apply the One-Way Navier-Stokes (OWNS) equations to hypersonic boundary layers. OWNS is based on a rigorous, approximate parabolization of the equations of motion that removes disturbances with upstream group velocity using a high-order recursive filter. We extend the original algorithm by considering non-orthogonal body-fitted curvilinear coordinates and incorporate full compressibility with temperature-dependent fluid properties. We validate the results by comparing to DNS data for a flat plate and sharp cone, and to LST results for local disturbances on the centerline of the HIFiRE-5 elliptic cone. OWNS provides DNS-quality results for the former flows at a small fraction of the computational expense.

Conference paper

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

The parabolized stability equations (PSE) are a ubiquitous tool for studying the stability and evolution of disturbances in weakly nonparallel, convectively unstable flows. The PSE method was introduced as an alternative to asymptotic approaches to these problems. More recently, PSE has been applied with mixed results to a more diverse set of problems, often involving flows with multiple relevant instability modes. This paper investigates the limits of validity of PSE via a spectral analysis of the PSE operator. We show that PSE is capable of accurately capturing only disturbances with a single wavelength at each frequency and that other disturbances are not necessarily damped away or properly evolved, as often assumed. This limitation is the result of regularization techniques that are required to suppress instabilities arising from the ill-posedness of treating a boundary value problem as an initial value problem. These findings are valid for both incompressible and compressible formulations of PSE and are particularly relevant for applications involving multiple modes with different wavelengths and growth rates, such as problems involving multiple instability mechanisms, transient growth, and acoustics. Our theoretical results are illustrated using a generic problem from acoustics and a dual-stream jet, and the PSE solutions are compared to both global solutions of the linearized Navier–Stokes equations and a recently developed alternative parabolization.

Journal article

Rigas G, Pickering E, Schmidt O, Nogueira PAS, Cavalieri AVG, Brès GA, Colonius Tet al., 2019, Streaks and coherent structures in jets from round and serrated nozzles

Hydrodynamic instabilities are directly related to large-scale coherent structures that are correlated with jet noise emission. Unravelling and accurately predicting their fundamental dynamics shows a promising direction for designing quieter jet engines. In this study, we analyze high-fidelity large-eddy simulation data of a turbulent Mach 0.4 round jet and a Mach 1.5 chevron jet. Using spectral proper orthogonal decomposition we identify, beyond the well-known1 Kelvin–Helmoholtz and Orr mechanisms, elongated alternating streamwise streaks of high and low-speed fluid that have been associated with a non-modal lift-up effect in wall-bounded shear flows. In the global three-dimensional domain, the most energetic streaks manifest for azimuthal wavenumber m = 1 and frequency St → 0. Furthermore, for the chevron jet, streaks and streamwise vortices appear due to the presence of the serrated nozzle, and they inherit the periodicity of the nozzle geometry. Finally, local (planar) spectral proper orthogonal decomposition is used to analyze the coherent structures of the chevron jet flow. Near the nozzle exit, antisymmetric and symmetric modes appear to be amplified and linked to the presence of the chevrons/streaks. Further downstream, the most energetic modes share similar characteristics to the ones observed in round jets.

Conference paper

Nogueira PAS, Cavalieri AVG, Schmidt OT, Jordan P, Jaunet V, Pickering E, Rigas G, Colonius Tet al., 2019, Resolvent-based analysis of streaks in turbulent jets

Large scale, elongated structures, similar those ones widely studied in wall-bounded flows, are also present in turbulent jets. Several characteristics of these streaks can be identified via reduced order models such as resolvent analysis. The present work involves a resolvent-based study of these structures in turbulent jets. We focus on obtaining the optimal forcing that generates these energetic coherent structures. Results are compared with experimental data post-processed using spectral proper orthogonal decomposition, allowing us to draw conclusions about the nature of the non-linear forcing, since the two analyses should provide equivalent results if this term is modelled as spatially white. By identifying streaks in a global framework, we expect to better understand the mechanism by which they are generated.

Conference paper

Pickering E, Rigas G, Colonius T, Sipp D, Schmidt OTet al., 2019, Eddy viscosity for resolvent-based jet noise models

Response modes computed via linear resolvent analysis have shown promising results for qualitatively modeling both the hydrodynamic and acoustic fields in jets when compared to data-deduced modes from high-fidelity, large-eddy simulations (LES). For an improved quantitative prediction of the near-and far-field, the role of Reynolds stresses must also be considered. In this study, we propose a methodology to deduce an eddy-viscosity model that optimally captures the nonlinear forcing of resolvent modes. The methodology is based on the maximization of the projection between resolvent analysis and spectral proper orthogonal decomposition (SPOD) modes using a Lagrangian optimization framework. For a Mach 0.4 round, isothermal, turbulent jet, four methods are used to increase the projection coefficients: linear damping, spatially constant eddy-viscosity field, a turbulent kinetic energy derived viscosity field, and an optimized eddy-viscosity field. The resulting projection coefficients for the optimized eddy-viscosity field between SPOD and resolvent can be increased to over 90% for frequencies in the range St = 0.35 − 1 with significant improvements to St < 0.35. We find that the use of a frequency-independent turbulent kinetic energy turbulent viscosity model produces modes closely inline with optimal results, providing a preliminary eddy-viscosity resolvent model for jets.

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

Large eddy simulations are performed for a cold ideally-expanded dual-stream jet issued from cylindrical co-axial nozzles, with supersonic primary stream (Mach number M1 = 1.55) and subsonic secondary stream (M2 = 0.9). The geometry includes the internal screw holes used to fasten the two nozzles together and to the plenum chamber. These slanted cylindrical holes over which the secondary stream flows were not covered in the experiment and were seamlessly captured in the computational mesh thanks to a novel grid generation paradigm based on the computation of Voronoi diagrams. A simulation with the screw holes covered is also performed and the preliminary results tends to indicate that these features have minimal impact on the flow and acoustic fields for the present operating conditions. As expected, the present dual-stream configuration with subsonic annular stream surrounding the primary supersonic stream features a reduced shear-layer growth, a longer potential core and a lack of strong Mach wave radiation. A long LES database is currently being collected for analysis and modeling of wavepackets and noise sources in such complex turbulent jets.

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

In this article, we report the results of an experimental sensitivity analysis on a vertical electrically heated Rijke tube. We examine the stability characteristics of the system due to the introduction of a secondary heat source. The experimental sensitivity analysis is quantified by measuring the shift in linear growth and decay rate as well as the shift in the linear frequency during periods of growth and decay of thermoacoustic oscillations. Linear growth and decay rate measurements agree qualitatively well with the theoretical predictions from adjoint-based methods. A discrepancy in the linear frequency measurements highlight deficiencies in the model used for those predictions and shows that the experimental measurement of sensitivities is a stringent test of any thermoacoustic model. The findings suggest that adjoint-based methods are, in principle, capable of providing industry with a cheap and efficient tool for developing optimal control strategies for more complex thermoacoustic systems.

Journal article

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 flows using one-way spatial integration of navier-stokes equations

A method for constructing well-posed one-way equations for calculating disturbances of slowly-varying flows was recently introduced (Towne & Colonius, JCP, Vol. 300, 2015). The linearized Navier-Stokes equations are modified such that all upstream propagating modes are removed from the operator. The resulting equations, termed one-way Navier-Stokes equations, are stable and can be solved efficiectly in the frequency domain as a spatial initial value problem in which initial perturbations are specified at the domain inlet and propagated downstream by spatial integration. To date, the method has been used to predict large-scale wavepacket structures and their acoustic radiation in turbulent jets. In this paper, the method is extended and applied to wall-bounded flows. Specifically, we examine the spatial stability of two- and three-dimensional boundary layers, corresponding to the Blasius and the Falkner-Skan-Cooke flows, and predict the evolution of unstable Tollmien-Schlichting waves and crossflow vortices, respectively. The method is validated against well-known results from the literature.

Conference paper

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

A linear analysis of the mean flow of a turbulent Mach 1.5 high Reynolds number jet is conducted. Optimal response modes describing the fluctuating hydrodynamic and acoustic fields are obtained in a computationally efficient way by spatially marching the linearized One-Way Navier-Stokes equations. For this purpose, an adjoint-based optimization framework is proposed and demonstrated for calculating optimal boundary conditions and optimal volumetric forcing. These optimal modes are validated against modes obtained in terms of global resolvent analysis. Two scenarios are considered in the present analysis. In the first case, no restrictions are applied to the spatial forcing distribution. In the second scenario, the forcing is restricted to the nozzle plane. The resulting optimal and suboptimal modes are compared to spectral proper orthogonal modes obtained from a high-fidelity large eddy simulation. 1 The implications of these observations are discussed in detail.

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

In this study we present a theoretical weakly nonlinear framework for the prediction of thermoacoustic oscillations close to Hopf bifurcations. We demonstrate the method for a thermoacoustic network that describes the dynamics of an electrically heated Rijke tube. We solve the weakly nonlinear equations order by order, discuss their contribution on the overall dynamics and show how solvability conditions at odd orders give rise to Stuart–Landau equations. These equations, combined together, describe the nonlinear dynamical evolution of the oscillations’ amplitude and their frequency. Because we retain the contribution of several acoustic modes in the thermoacoustic system, the use of adjoint methods is required to derive the Landau coefficients. The analysis is performed up to fifth order and compared with time domain simulations, showing good agreement. The theoretical framework presented here can be used to reduce the cost of investigating oscillations and subcritical phenomena close to Hopf bifurcations in numerical simulations and experiments and can be readily extended to consider, e.g. the weakly nonlinear interaction of two unstable thermoacoustic modes.

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

This data is extracted from the Web of Science and reproduced under a licence from Thomson Reuters. You may not copy or re-distribute this data in whole or in part without the written consent of the Science business of Thomson Reuters.

Request URL: http://wlsprd.imperial.ac.uk:80/respub/WEB-INF/jsp/search-html.jsp Request URI: /respub/WEB-INF/jsp/search-html.jsp Query String: respub-action=search.html&id=00682264&limit=30&person=true