Publications
45 results found
Brès GA, Bose ST, Emory M, et 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.
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.
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.
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.
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.
Rigas G, Schmidt OT, Colonius T, et 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.
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.
Brackston RD, Garcia de la Cruz Lopez JM, Wynn A, et 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.
Rigas G, Jamieson NP, Li LKB, et al., 2016, Experimental sensitivity analysis and control of thermoacoustic systems, JOURNAL OF FLUID MECHANICS, Vol: 787, ISSN: 0022-1120
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Rigas G, Morgans AS, Brackston RD, et 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.
Oxlade AR, Morrison JF, Qubain A, et 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.
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
Rigas G, Oxlade AR, Morgans AS, et 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.
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
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