Publications
125 results found
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.
Morrison JF, Vallikivi M, Smits AJ, 2016, The inertial subrange in turbulent pipe flow: centreline, Journal of Fluid Mechanics, Vol: 788, Pages: 602-613, ISSN: 0022-1120
The inertial-subrange scaling of the axial velocity component is examined for the centreline of turbulent pipe flow for Reynolds numbers in the range 249⩽Reλ⩽986. Estimates of the dissipation rate are made by both integration of the one-dimensional dissipation spectrum and the third-order moment of the structure function. In neither case does the non-dimensional dissipation rate asymptote to a constant; rather than decreasing, it increases indefinitely with Reynolds number. Complete similarity of the inertial range spectra is not evident: there is little support for the hypotheses of Kolmogorov (Dokl. Akad. Nauk SSSR, vol. 32, 1941a, pp. 16–18; Dokl. Akad. Nauk SSSR, vol. 30, 1941b, pp. 301–305) and the effects of Reynolds number are not well represented by Kolmogorov’s ‘extended similarity hypothesis’ (J. Fluid Mech., vol. 13, 1962, pp. 82–85). The second-order moment of the structure function does not show a constant value, even when compensated by the extended similarity hypothesis. When corrected for the effects of finite Reynolds number, the third-order moments of the structure function accurately support the ‘four-fifths law’, but they do not show a clear plateau. In common with recent work in grid turbulence, non-equilibrium effects can be represented by a heuristic scaling that includes a global Reynolds number as well as a local one. It is likely that non-equilibrium effects appear to be particular to the nature of the boundary conditions. Here, the principal effects of the boundary conditions appear through finite turbulent transport at the pipe centreline, which constitutes a source or a sink at each wavenumber.
Saeed TI, Mughal MS, Morrison JF, 2016, The Interaction of a Swept-Wing Boundary Layer with Surface Excrescences, 54th AIAA Aerospace Sciences Meeting, AIAA SciTech, Publisher: American Institute of Aeronautics and Astronautics
The influence of steps and gaps on swept-wing crossflow development is an emergingarea of interest. An experiment is performed on a 40◦swept-wing model in a facility witha turbulence level of 0.10%. Periodic discrete roughness elements are spaced at the criticalcrossflow wavelength and used to excite the crossflow disturbance. The subsequent interactionwith a two-dimensional roughness strip of various height and chordwise location isinvestigated. Naphthalene flow visualisation is used to help understand the global transitionfeatures, whilst detailed boundary layer measurements are conducted using hot-wireanemometry. Excrescences located closest to the neutral point are seen to have the biggestinfluence on stationary-crossflow disturbance amplitude. There appears to be a thresholdheight below which the excrescence has no significant influence on the boundary layer development.Excrescences located further downstream appear to generate greater unsteadinessin the boundary layer for a given excrescence height, leading to earlier transition.
Hultmark M, Marusic I, McKeon BJ, et al., 2015, Introduction to topical issue on extreme flows, Experiments in Fluids, Vol: 57, ISSN: 1432-1114
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.
Jones BL, Heins PH, Kerrigan EC, et al., 2015, Modelling for robust feedback control of fluid flows, Journal of Fluid Mechanics, Vol: 769, Pages: 687-722, ISSN: 0022-1120
This paper addresses the problem of designing low-order and linear robust feedback controllers that provide a priori guarantees with respect to stability and performance when applied to a fluid flow. This is challenging, since whilst many flows are governed by a set of nonlinear, partial differential–algebraic equations (the Navier–Stokes equations), the majority of established control system design assumes models of much greater simplicity, in that they are: firstly, linear; secondly, described by ordinary differential equations (ODEs); and thirdly, finite-dimensional. With this in mind, we present a set of techniques that enables the disparity between such models and the underlying flow system to be quantified in a fashion that informs the subsequent design of feedback flow controllers, specifically those based on the H∞ loop-shaping approach. Highlights include the application of a model refinement technique as a means of obtaining low-order models with an associated bound that quantifies the closed-loop degradation incurred by using such finite-dimensional approximations of the underlying flow. In addition, we demonstrate how the influence of the nonlinearity of the flow can be attenuated by a linear feedback controller that employs high loop gain over a select frequency range, and offer an explanation for this in terms of Landahl’s theory of sheared turbulence. To illustrate the application of these techniques, an H∞ loop-shaping controller is designed and applied to the problem of reducing perturbation wall shear stress in plane channel flow. Direct numerical simulation (DNS) results demonstrate robust attenuation of the perturbation shear stresses across a wide range of Reynolds numbers with a single linear controller.
Garland M, Santer M, Morrison J, 2015, Adaptive Vortex Generator Structures for the Reduction of Turbulent Separation, AIAA SciTech: 23rd AIAA/AHS Adaptive Structures Conference, Publisher: American Institute of Aeronautics and Astronautics
The stereotypical separation control method is the vortex generator which, as a passivedevice, produces an influence on the flow at all times. We investigate the production ofa deployable compliant separation control system that can be formed from the continuoussurface of an aerofoil when and where required, and return to normal conditions whennot required. The limitations in the development of a 3D finite geometry change froma surface are investigated and a systematic search of the design space is conducted toidentify the ideal material parameters. Based on these results a novel method of producinga spatially finite morph is proposed through the use of a 2D lattice structure. Deficienciesare identified with the use of an objective function which reduces geometric error, basedon prior knowledge of a suitable geometry, for flow within the boundary layer and analternative objective is proposed.
Bird J, Santer M, Morrison J, 2015, Adaptive Kagome Lattices for Near Wall Turbulence Suppression, AIAA SciTech: 23rd AIAA/AHS Adaptive Structures Conference
Jones BL, Kerrigan EC, Morrison JF, 2015, A modeling and filtering framework for the semi-discretised Navier-Stokes equations, Pages: 1215-1220
© 2009 EUCA. Spatial discretisation of fluid mechanical systems typically leads to descriptor systems consisting of large numbers of differential algebraic equations (DAEs). In an effort to apply standard control theory to such systems, physical insight is often used to analytically reformulate the DAE as an ordinary differential equation (ODE). In general, this is a difficult process that typically requires expert insight into specific systems, and so in this work we consider a more flexible numerical method that is straightforward to implement on any regular DAE. The numerical procedure is outlined and a new method for computing one of the steps is presented.With respect to Kalman filtering of descriptor systems, it is known in general that 'process noise' can sometimes not be added to all states owing to a violation of causality. In this paper we present a new method for computing the subspace of causal disturbances, suitable for large DAEs. Finally, the techniques developed in this paper are applied to the specific case of plane Poiseuille flow and it is shown how a standard state-space system is easily obtained that possesses a similar spectrum and pseudospectrum to the Orr-Somerfeld-Squire system.
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.
Jones BL, Kerrigan EC, Morrison JF, 2014, A modeling and filtering framework for the semi-discretised Navier-Stokes equations, Pages: 1215-1220
Spatial discretisation of fluid mechanical systems typically leads to descriptor systems consisting of large numbers of differential algebraic equations (DAEs). In an effort to apply standard control theory to such systems, physical insight is often used to analytically reformulate the DAE as an ordinary differential equation (ODE). In general, this is a difficult process that typically requires expert insight into specific systems, and so in this work we consider a more flexible numerical method that is straightforward to implement on any regular DAE. The numerical procedure is outlined and a new method for computing one of the steps is presented.With respect to Kalman filtering of descriptor systems, it is known in general that 'process noise' can sometimes not be added to all states owing to a violation of causality. In this paper we present a new method for computing the subspace of causal disturbances, suitable for large DAEs. Finally, the techniques developed in this paper are applied to the specific case of plane Poiseuille flow and it is shown how a standard state-space system is easily obtained that possesses a similar spectrum and pseudospectrum to the Orr-Somerfeld-Squire system.
Saeed TI, Morrison JF, Mughal MS, 2014, Roughness effects on swept-wing crossflow transition in moderate free-stream turbulence, 29th Congress of the International Council of the Aeronautical Sciences, ICAS 2014
An experimental investigation of swept-wing crossflow transition is carried out, focussing on the effects of chord Reynolds number and direct roughness element height (spaced at the critical crossflow wavelength) in a moderate-level disturbance facility. Flow quality measurements performed in the wind tunnel indicate turbulence intensities of u'rms=U∞ = 0:14% and v'rms=U∞ = 0:20%. The Reynolds numbers are Rec = 1:3 × 106 and Rec = 1:6 ×106. Despite the disturbance levels being indicative of travelling wave dominated crossflow, flow visualisation reveals that stationary crossflow is the dominant mode. Increased DRE height is shown through flow visualisation to advance transition, with boundarylayer velocity profiles showing mean-flow deformation and fluctuating-velocity spectra containing a high-frequency mode normally associated with stationary crossflow related secondary instability. Nevertheless, spectral information and unsteady disturbance profiles do indicate the travelling crossflow modes are excited, and that their disturbance amplitude increases with Reynolds number (as expected from linear stability theory) and with roughness height.
Morrison JF, 2013, MEMS devices for active drag reduction in aerospace applications, MEMS for automotive and aerospace applications, Editors: Kraft, White, ISBN: 978 0 85709 118 5
Gouder K, Potter M, Morrison JF, 2013, Turbulent friction drag reduction using electroactive polymer and electromagnetically driven surfaces, Experiments in Fluids, Vol: 54
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
Oxlade AR, Valente PC, Ganapathisubramani B, et al., 2012, Denoising of time-resolved PIV for accurate measurement of turbulence spectra and reduced error in derivatives, EXPERIMENTS IN FLUIDS, Vol: 53, Pages: 1561-1575, ISSN: 0723-4864
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- Citations: 17
Birch DM, Morrison JF, 2012, An innovative low-profile monolithic constant-temperature anemometer, Journal of Wind Engineering and Industrial Aerodynamics, Vol: 100, Pages: 38-45
Jones BL, Kerrigan EC, Morrison JF, et al., 2011, Flow estimation of boundary layers using DNS-based wall shear information, International Journal of Control, Vol: 84, Pages: 1310-1325, ISSN: 1366-5820
This article investigates the problem of obtaining a state-space model of the disturbance evolution that precedes turbulent flow across aerodynamic surfaces. This problem is challenging since the flow is governed by nonlinear, partial differential-algebraic equations for which there currently exists no efficient controller/estimator synthesis techniques. A sequence of model approximations is employed to yield a linear, low-order state-space model, to which standard tools of control theory can be applied. One of the novelties of this article is the application of an algorithm that converts a system of differential-algebraic equations into one of ordinary differential equations. This enables straightforward satisfaction of boundary conditions whilst dispensing with the need for parallel flow approximations and velocity–vorticity transformations. The efficacy of the model is demonstrated by the synthesis of a Kalman filter that clearly reconstructs the characteristic features of the flow, using only wall velocity gradient information obtained from a high-fidelity nonlinear simulation.
Jones BL, Kerrigan EC, Morrison JF, et al., 2011, Flow estimation of boundary layers using wall shear information, IFAC Proceedings Volumes (IFAC-PapersOnline), Vol: 44, Pages: 13813-13818, ISSN: 1474-6670
This paper investigates the problem of obtaining a state-space model of the disturbance evolution that precedes turbulent flow and the associated increase in skin-friction drag on aircraft surfaces. This problem is highly challenging since the flow system is governed by nonlinear, partial differential-algebraic equations (the Navier-Stokes equations) for which there currently exists no efficient controller/estimator synthesis techniques. In this paper it is shown how a sequence of model approximations can be employed to yield a linear, low-order state-space model, to which the standard tools of control theory can be applied. One of the novelties of this paper is the application of a numerical routine that converts a system of differential-algebraic equations into one of ordinary differential equations. This enables straightforward satisfaction of boundary conditions whilst dispensing with the need for parallel flow approximations and velocity-vorticity transformations. The efficacy of the model is demonstrated by the synthesis of a Kalman filter that clearly reconstructs the characteristic features of the flow, using only wall velocity gradient (shear) measurements obtained from a high-fidelity nonlinear simulation. © 2011 IFAC.
Birch DM, Morrison JF, 2011, Scaling of Turbulence Structures in Very-Rough-Wall Channel Flow, WALLTURB International Workshop on Understanding and Modelling of Wall Turbulence, Publisher: SPRINGER, Pages: 405-+, ISSN: 1382-4309
Birch DM, Morrison JF, 2011, Similarity of the streamwise velocity component in very-rough-wall channel flow, Journal Of Fluid Mechanics, Vol: 668, Pages: 174-201
Sharma AS, Morrison JF, McKeon BJ, et al., 2011, Relaminarisation of Re=100 channel flow with globally stabilisinglinear feedback control, Physics of Fluids, Vol: 23, Pages: 125105-1-125105-17, ISSN: 1070-6631
Potter M, Gouder K, Morrison JF, 2010, A numerical model for electro-active polymer actuators with experimental validation, Sensors and Actuators A-Physical, Vol: 170, Pages: 121-130
This paper presents a theoretical model for predicting the behavior of electro-active polymer actuators. This model takes the form of an array of masses with the interconnecting forces derived using Hooke's law coupled with an electrostatic force, and is solved using time-marching integration. Forming the model in this way allows the response of actuators of various sizes and pre-strains to be simulated with a range of electrode designs and mechanically stiffened sections. To validate the response of the model a series of Nusil MED4905 polydimethylsiloxane actuators were built and tested with differing electrode designs and with additional mechanical stiffening of the membrane. Through a comparison of maximum electrode displacement, with applied electrode voltage, tracking of a grid on the membrane surface and dynamic testing, the model was found to show excellent agreement with the experimental results.
Hanson RE, Lavoie P, Naguib AM, et al., 2010, Transient growth instability cancelation by a plasma actuator array, EXPERIMENTS IN FLUIDS, Vol: 49, Pages: 1339-1348, ISSN: 0723-4864
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- Citations: 46
Dearing SS, Morrison JF, Iannucci L, 2010, Electro-active polymer (EAP) "dimple" actuators for flow control: Design and characterisation, SENSORS AND ACTUATORS A-PHYSICAL, Vol: 157, Pages: 210-218, ISSN: 0924-4247
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- Citations: 17
Hanson RE, Lavoie P, Naguib AM, et al., 2010, Control of transient growth induced boundary layer transition using plasma actuators, Pages: 183-188, ISSN: 1875-3507
This study investigates an actuation scheme that can be readily implemented and integrated as part of a feedback control system in the laboratory for the purpose of negating the effect of the transient growth instability in order to delay boundary layer transition. The actuators investigated here consist of a spanwise array of symmetric plasma actuators, which are capable of generating spanwise periodic counter-rotating vortices. Two different actuator geometries are investigated, resulting in a reduction of the total disturbance energy by 46% and 68%. It is demonstrated that the control authority of an actuator can be significantly improved by optimizing the geometry of the array. © 2010 Springer Science+Business Media B.V.
Birch DM, Morrison JF, 2010, Large roughness effects in channel flow, Pages: 167-173, ISSN: 1875-3507
The streamwise velocity component in fully-developed turbulent channel flow is studied for two very rough surfaces and a smooth surface at comparable Reynolds numbers. One rough surface comprises sparse and isotropic grit with a non-Gaussian distribution. The other is a uniform mesh consisting of twisted rectangular elements which form a diamond pattern. The mean roughness heights (± the standard deviation) are, respectively, about 76 ± 42 and 145 ± 150 wall units. The mean velocity profile over the grit surface exhibits self-similarity (in the form of a logarithmic law) within the limited range of 0:03 ≤ y/h ≤ 0.05, but the profile over the mesh surface exhibits only a small region with a slope tangential to log-law slope scaled on outer variables. However, the mean velocity deficit and higher moments (up to the fourth order) all exhibit some degree of outer scaling over both surfaces. The distinction between self-similarity and outer similarity is clarified and the importance of the former is explained. Spatial correlations show that the dominant large-scale features are very large quasi-streamwise structures with circulation in the cross-flow plane, similar to those found in smooth-wall internal and external flows. However, in the present case, the spanwise length scales are considerably larger. © Springer Science+Business Media B.V. 2010.
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