## Publications

124 results found

Zhu T, Rigas G, Morrison J, 2022, Near Wake Coherent Structures of a Turbulent Axisymmetric Wake

The coherent structures of a turbulent axisymmetric bluff body wake are analysed using proper orthogonal decomposition (POD) and conditional POD based on synchronized near-wake velocity and base pressure measurements. The analysis confirms the persistence of the laminar unstable eigenmodes at high-Reynolds numbers (here ReD = 1.88 × 105). These correspond to a steady spatial-symmetry breaking mode and asymmetric unsteady vortex shedding modes in the near wake. Additionally, an unsteady axisymmetric bubble pumping mode representing the streamwise pulsation of the wake is found in the turbulent wake. The asymmetric modes are extracted in the wake of the axisymmetric body by performing conditional POD. The asymmetry of the wake, quantified using the centre of pressure (CoP), is correlated to the base pressure using conditional averaging. At the limits of the highly asymmetric wake or a perfectly symmetric wake, a high average pressure coefficient (low drag) is obtained. For the most probable state of the wake, a low-pressure coefficient (high drag) is obtained. The above results provide insight on the coherent structures for canonical 3D bluff body geometries and guidance for drag reduction strategies.

Basley J, Gouder K, Morrison JF, 2022, MIXING AND LARGE-SCALE MODULATION OF A TURBULENT BOUNDARY LAYER PERTURBED BY AN EFFUSION FILM

The large-scale modulation of an effusion film injected into a high Reynolds-number boundary layer is investigated for a wide range of film-velocity to freestream-velocity ratios. Particle Image Velocimetry (PIV), visualisations and hot-wire measurements are used to characterise the momentum effects underlying the mixing of the effusion film. For this experiment, the geometry of a turbine blade is idealised in a large-scale facility with a canonical upstream boundary layer where the effusion film and the boundary layer are isothermal. Spectral analysis shows that the injection of the effusion film first promotes near-wall turbulence until the film breaks away from the surface when the velocity ratio reaches a critical value. For higher velocity ratios, large-scale motions from the incoming boundary layer no longer penetrate the sublayer. The associated spectral signature is accompanied by enhanced inter-scale exchanges, as shown by the skewness of the streamwise velocity component and the amplitude modulation coefficients. Use of seeding in the film flow enables the estimation of the concentration of injected fluid in the laser sheet. Joint momentum-concentration data show that the mixing of the film is strongly modulated by the large-scale motions of the turbulent boundary layer, although in different ways depending on the velocity ratio. For very low velocity ratios, large-scale motions are largely unaffected by the film. They encompass the boundary layer down to the injected sublayer and modulate locally the concentration of injected film. For higher velocity ratios, mixing intensifies as the effusion film is injected with momentum and reaches further from the wall. As a result, the concentration of injected fluid in the near-wall region is remotely, yet more intensively modulated by the large-scale motions above the sublayer.

Zhu T, Morrison JF, 2021, Simulation of the turbulent axisymmetric bluff body wake with pulsed jet forcing, *Physical Review Fluids*, Vol: 6, ISSN: 2469-990X

The turbulent axisymmetric bluff body wake is studied with a large eddy simulation (LES). The effect of pulsed jet forcing on the wake is investigated. For completeness, the pulsed jet actuator is included in the computational domain. Spectral proper orthogonal decomposition (SPOD) is applied to analyze the wake and the effect of pulsed jet forcing. The numerical results show good agreement with the experimental results and successfully reproduce the dominant modes of the axisymmetric wake. SPOD identifies the axisymmetric bubble pumping mode (StD=0.06), axisymmetric-breaking vortex shedding mode (StD=0.22), and an axisymmetric-breaking mode close to the subharmonic of the shedding mode (StD=0.1). When the high-frequency pulsed jet (Stθ=0.107) is applied to the wake, the pressure fluctuation on the base and the azimuthal modes are globally weakened without any mode selection. The mean flow properties show that the inlet flow is driven towards the wake centerline by the high-frequency jet, and the shear layer deviates, forming a concave separation streamline. The entrainment in the wake is suppressed, and the wake is narrowed, leading to a global pressure rise. The high-frequency pulsed jet is concentric and greatly enhances the entrainment in the vicinity of the separation point. A low-pressure region is generated there, which drives the inlet flow towards the wake. Low-frequency forcing (Stθ=0.029) generates a diffusive pulsed jet which enhances the mixing and entrainment in its trajectory. Low-frequency forcing reduces the wake length and accelerates recirculating flow near the base, leading to a reduction in base pressure, and the vortex shedding mode (StD=0.2) is amplified.

Diwan S, Morrison J, 2021, Intermediate scaling and logarithmic invariance in turbulent pipe flow, *Journal of Fluid Mechanics*, Vol: 913, Pages: 913 R1-1-913 R1-12, ISSN: 0022-1120

A three-layer asymptotic structure for turbulent pipe flow is proposed, revealing in terms of intermediate variables, the existence of a Reynolds-number invariant logarithmic region for the streamwise mean velocity and variance. The formulation proposes a local velocity scale (which is not the friction velocity) for the intermediate layer and results in two overlap layers. We find that the near-wall overlap layer is governed by a power law for the pipe for all Reynolds numbers, whereas the log law emerges in the second overlap layer (the inertial sublayer) for sufficiently high Reynolds numbers (Reτ ). This provides a theoretical basis for explaining the presence of a power law for the mean velocity at low Reτ and the co-existence of power and log laws at higher Reτ . The classical von K´arm´an (κ) and Townsend-Perry (A1) constants are determined from the intermediate-scaled log-law constants; κ shows a weak trend at sufficiently high Reτ but falls within the commonly accepted uncertainty band, whereas A1 exhibits a systematic Reynolds-number dependence until the largest available Reτ . The key insight emerging from the analysis is that the scale separation between two adjacent layers in the pipe isproportional to √Reτ (rather than Reτ ) and therefore the approach to an asymptotically invariant state can be expected to be slow.

Garland MGC, Santer M, Morrison JF, 2021, Control of Cellular Separation Using Adaptive Surface Structures, Notes on Numerical Fluid Mechanics and Multidisciplinary Design, Pages: 73-80

The three-dimensional separation that gives rise to the formation of stall cells is shown to consist primarily of two discrete frequencies. The higher is the well known vortex shedding mode. However, at frequencies roughly ten times lower, the whole cell oscillates. Both features are clearly evident in both modal decomposition of the velocity field and surface pressure spectra.

Oxlade AR, Morrison JF, 2020, Open-loop control of an axisymmetric turbulent wake using high-frequency periodic jet blowing

Copyright © ETC 2013 - 14th European Turbulence Conference.All rights reserved. We show that high-frequency periodic jet blowing can be used to increase the base pressure of a bullet-shaped body with a turbulent axisymmetric wake by as much as 35%. A detailed investigation of the effects of forcing is made using random and phase-locked 2C PIV, and modal decomposition of dynamic pressure measurements on the base of the model. In contrast to other studies using periodic jet forcing, for example those discussed in [1], this control strategy does not target specific local or global wake instabilities. Instead, the high-frequency jet creates a row of closely spaced vortices which appear to act as a buffer between the wake and separating flow, thereby inhibiting the entrainment of fluid from the separating boundary. The resulting pressure recovery is proportional to the strength of the vortices produced by the jet, and is accompanied by a broadband suppression of base pressure fluctuations associated with all modes. We will show that the optimum forcing frequency is roughly six times the frequency of the shear layer mode, where excitation of the shear layer mode approaches unity gain. We also observe that despite being subject to an axisymmetric perturbation, the forced wake does not exhibit statistical axisymmetry.

Oxlade AR, Morrison JF, 2020, Open-loop control of an axisymmetric turbulent wake using high-frequency periodic jet blowing

We show that high-frequency periodic jet blowing can be used to increase the base pressure of a bullet-shaped body with a turbulent axisymmetric wake by as much as 35%. A detailed investigation of the effects of forcing is made using random and phase-locked 2C PIV, and modal decomposition of dynamic pressure measurements on the base of the model. In contrast to other studies using periodic jet forcing, for example those discussed in [1], this control strategy does not target specific local or global wake instabilities. Instead, the high-frequency jet creates a row of closely spaced vortices which appear to act as a buffer between the wake and separating flow, thereby inhibiting the entrainment of fluid from the separating boundary. The resulting pressure recovery is proportional to the strength of the vortices produced by the jet, and is accompanied by a broadband suppression of base pressure fluctuations associated with all modes. We will show that the optimum forcing frequency is roughly six times the frequency of the shear layer mode, where excitation of the shear layer mode approaches unity gain. We also observe that despite being subject to an axisymmetric perturbation, the forced wake does not exhibit statistical axisymmetry.

Garland MGC, Santer MS, Morrison JF, 2019, Control of cellular separation using adaptive surfaces, *Journal of Fluids and Structures*, Vol: 91, ISSN: 0889-9746

We report results from an experimental investigation of the three-dimensional separation produced by a high-lift aerofoil at moderate incidence, with constant section, where the separation is controlled by the implementation of an adaptive surface. Mean and time-resolved measurements are made using a NASA GA(W)-1 aerofoil with AR=6 at Re c =3.5×10 5 . Surface oil visualisation and stereo Particle Image Velocimetry (PIV) are used to explore the flow field. The mean topology of the flow identifies characteristic spanwise periodic behaviour, “stall cells” along the surface of the model. Analysis of the time-dependent surface pressure shows two distinct frequencies within the flow field. The higher frequency appears at a Strouhal number, St≈0.2, representative of vortex shedding, and the typical von Kármán vortex street. The lower frequency appears at St≈0.02, observed as a global fluctuation in stall-cell extent. This lower frequency is apparent in many separated flows, but in the present context, appears to have received only little attention. It correlates with widely observed low-frequency unsteadiness in the wing loading around stall. While this mode is analogous to that observed in other types of separation, here the streamwise extent of the separation varies because the flow is separating from a curved surface rather than from a sharp edge; the width of the separated region also varies. We show that fully-reversible point actuations of an actuated surface with auxetic structure, introduced immediately upstream of the saddle point at the leading edge of the stall cell, reduce the extent of the separated region.

Saeed TI, Morrison JF, 2019, 2D roughness effects on crossflow instabilities

© 2019 International Symposium on Turbulence and Shear Flow Phenomena, TSFP. All rights reserved. A crossflow transition experiment is performed for a 40º swept-back wing with moderate free-stream turbulence, Tu = 0.13%, at Rec ≃ 1 × 106. Periodic Discrete Roughness Elements (DRE), spaced at the critical crossflow wavelength, are used to excite the crossflow disturbance. The subsequent interaction with a 2D roughness strip of variable height and chordwise location is investigated through boundary-layer hot-wire measurements. When the 2D roughness is located near the neutral point, stationary crossflow amplification is observed for strip heights up to 80% of δ99, above which it is attenuated. Thereafter, the unsteady disturbances are rich in high-frequency fluctuations, similar to those reported for a backward facing step. Moving the strip downstream results in a lower critical roughness height and earlier transition. In the absence of DRE forcing, a strip located near the neutral point amplifies the stationary crossflow, selecting the critical crossflow wavelength, albeit at a much lower amplitude.

Saeed TI, Morrison JF, 2019, 2D roughness effects on crossflow instabilities

A crossflow transition experiment is performed for a 40º swept-back wing with moderate free-stream turbulence, Tu = 0.13%, at Rec ≃ 1 × 106. Periodic Discrete Roughness Elements (DRE), spaced at the critical crossflow wavelength, are used to excite the crossflow disturbance. The subsequent interaction with a 2D roughness strip of variable height and chordwise location is investigated through boundary-layer hot-wire measurements. When the 2D roughness is located near the neutral point, stationary crossflow amplification is observed for strip heights up to 80% of δ99, above which it is attenuated. Thereafter, the unsteady disturbances are rich in high-frequency fluctuations, similar to those reported for a backward facing step. Moving the strip downstream results in a lower critical roughness height and earlier transition. In the absence of DRE forcing, a strip located near the neutral point amplifies the stationary crossflow, selecting the critical crossflow wavelength, albeit at a much lower amplitude.

Diwan SS, Morrison JF, 2019, Reynolds-number dependence of the Townsend-Perry ‘constant’ in wall turbulence

We address the question of Reynolds-number dependence of the “Townsend-Perry constant”, which is the slope of the logarithmic variation of the streamwise variance in wall turbulence. We make use of the turbulent pipe flow and boundary layer (TBL) data available in the literature. We find that using a wall-normal length scale, proportional to the square root of the friction Reynolds number (akin to the distance of the “mesolayer” from the wall) and an associated velocity scale, it is possible to obtain a Reynolds-number similarity for the streamwise variance in a region intermediate to the inner and outer layers. In this region, the intermediate-scaled variance follows a logarithmic variation for which the coefficients are independent of Reynolds number, and the extent of the log region increases with increase in Reynolds number. The intermediate-scaled log-law constants for the pipe and TBL are fairly close to each other, suggesting a plausible “universal” behaviour for the variance, in terms of the intermediate variables. The consequence of Re-number invariance of the intermediate-scaled log law is that the classical Townsend-Perry ‘constant’ shows a systematic variation with Reynolds number. For the pipe flow the Townsend-Perry ‘constant’ is seen to increase until the highest Reynolds number, whereas for the TBL it reaches a relatively constant value for sufficiently large Reynolds numbers. These are interesting findings, which can have important implications towards understanding the scaling and structure of the high-Reynolds-number wall turbulence; in particular, their implications for the attached-eddy modelling are briefly discussed.

Diwan SS, Morrison JF, 2019, Reynolds-number dependence of the Townsend-Perry ‘constant’ in wall turbulence

© 2019 International Symposium on Turbulence and Shear Flow Phenomena, TSFP. All rights reserved. We address the question of Reynolds-number dependence of the “Townsend-Perry constant”, which is the slope of the logarithmic variation of the streamwise variance in wall turbulence. We make use of the turbulent pipe flow and boundary layer (TBL) data available in the literature. We find that using a wall-normal length scale, proportional to the square root of the friction Reynolds number (akin to the distance of the “mesolayer” from the wall) and an associated velocity scale, it is possible to obtain a Reynolds-number similarity for the streamwise variance in a region intermediate to the inner and outer layers. In this region, the intermediate-scaled variance follows a logarithmic variation for which the coefficients are independent of Reynolds number, and the extent of the log region increases with increase in Reynolds number. The intermediate-scaled log-law constants for the pipe and TBL are fairly close to each other, suggesting a plausible “universal” behaviour for the variance, in terms of the intermediate variables. The consequence of Re-number invariance of the intermediate-scaled log law is that the classical Townsend-Perry ‘constant’ shows a systematic variation with Reynolds number. For the pipe flow the Townsend-Perry ‘constant’ is seen to increase until the highest Reynolds number, whereas for the TBL it reaches a relatively constant value for sufficiently large Reynolds numbers. These are interesting findings, which can have important implications towards understanding the scaling and structure of the high-Reynolds-number wall turbulence; in particular, their implications for the attached-eddy modelling are briefly discussed.

Brackston RD, Wynn A, Morrison JF, 2018, Modelling and feedback control of vortex shedding for drag reduction of a turbulent bluff body wake, *International Journal of Heat and Fluid Flow*, Vol: 71, Pages: 127-136, ISSN: 0142-727X

Three-dimensional bluff body wakes are of key importance due to their relevance to the automotive industry. Such wakes have both large pressure drag and a number of coherent flow structures associated with them. Depending on the geometry, these structures may include both a bistability resulting from a spatial symmetry breaking (SB), and a quasi-periodic vortex shedding. The authors have recently shown that the bistability may be modelled by a Langevin equation and that this model enables the design of a feedback control strategy that efficiently reduces the drag through suppression of asymmetry. In this work the stochastic modelling approach is extended to the vortex shedding, capturing qualitatively both the forced and unforced behaviour. A control strategy is then presented that makes use of the frequency response of the wake, and aims to reduce the measured fluctuations associated with the vortex shedding. The strategy proves to be effective at suppressing fluctuations within specific frequency ranges but, due to amplification of disturbances at other frequencies, is unable to give drag reduction.

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Bird J, Santer M, Morrison J, 2018, Compliant kagome lattice structures for generating in-plane waveforms, *International Journal of Solids and Structures*, Vol: 141-142, Pages: 86-101, ISSN: 0020-7683

This paper details the design, manufacture and testing of an adaptive structure based on the kagome lattice geometry – a pattern with well documented interesting structural characteristics. The structure is used to produce in-plane travelling waves of variable length and speed in a flat surface. The geometry and dimensions, as well as the location and compliance of boundary conditions, were optimized numerically, and a pneumatically-actuated working demonstrator was manufactured. Static and dynamic photogrammetric and force measurements were taken. The structure was found to be capable of producing dynamic planar waveforms of variable wavelength with large strains. The lattice structure was then surfaced with a pre-tensioned membrane skin allowing these waveforms to be produced over a continuous plane.

Bird J, Santer M, Morrison J, 2018, Experimental control of turbulent boundary layers with in-plane travelling waves, *Flow, Turbulence and Combustion*, Vol: 100, Pages: 1015-1035, ISSN: 1386-6184

The experimental control of turbulent boundary layers using stream-wise travelling waves of spanwise wall velocity, produced using a novel activesurface, is outlined in this paper. The innovative surface comprises a pneu-matically actuated compliant structure based on the kagome lattice geometry,supporting a pre-tensioned membrane skin. Careful design of the structureenables waves of variable length and speed to be produced in the flat surfacein a robust and repeatable way, at frequencies and amplitudes known to havea favourable influence on the boundary layer. Two surfaces were developed,a preliminary module extending 152 mm in the streamwise direction, and alonger one with a fetch of 2.9 m so that the boundary layer can adjust to thenew surface condition imposed by the forcing. With a shorter, 1.5 m portionof the surface actuated, generating an upstream-travelling wave, a drag re-duction of 21.5% was recorded in the boundary layer withReτ= 1125. Atthe same flow conditions, a downstream-travelling produced a much smallerdrag reduction of 2.6%, agreeing with the observed trends in current simula-tions. The drag reduction was determined with constant temperature hot-wiremeasurements of the mean velocity gradient in the viscous sublayer, while si-multaneous laser Doppler vibrometer measurements of the surface recorded thewall motion. Despite the mechanics of the dynamic surface resulting in someout-of-plane motion (which is small in comparison to the in-plane streamwisemovement), the positive drag reduction results are encouraging for future in-vestigations at higher Reynolds numbers.

Vemuri H, Bosworth R, Morrison JF,
et al., 2018, Real-time feedback control of 3D Tollmien-Schlichting waves using a dual-slot actuator geometry, *Physical Review Fluids*, Vol: 3, ISSN: 2469-990X

The growth of Tollmien-Schlichting (TS) waves is experimentally attenuated using a single-inputand single-output (SISO) feedback system, where the TS wave packet is generated by a surfacepoint source in a flat-plate boundary layer. The SISO system consists of a single wall-mountedhot wire as the sensor and a miniature speaker as the actuator. The actuation is achieved througha dual-slot geometry to minimise the cavity near-field effects on the sensor. The experimentalset-up to generate TS waves or wave packets is very similar to that used by Li and Gaster [1]. Theaim is to investigate the performance of the SISO control system in attenuating single-frequency,two-dimensional disturbances generated by these configurations. The necessary plant models areobtained using system identification, the controllers are then designed based on the models andimplemented in real-time to test their performance. Cancellation of the rms streamwise velocityfluctuation of TS waves is evident over a significant domain.

Lavoie P, Morrison JF, Jakirlic S, 2017, Preface, *International Journal of Heat and Fluid Flow*, Vol: 67, Pages: 1-2, ISSN: 0142-727X

Garland M, Santer M, morrison J, 2017, Optimal aero-structural design of an adaptive surface for boundary layer motivation using an auxetic lattice skin, *Journal of Intelligent Material Systems and Structures*, Vol: 28, Pages: 2414-2427, ISSN: 1530-8138

The aero-structural design of an adaptive vortex generator for repeatable, elastic, deployment and retraction from anaerodynamically clean surface is presented. A multidisciplinary objective function, containing geometrically nonlinear nite element analysis and large eddy simulation, is used to derive the optimal adaptive geometry for increasing themomentum of the near wall uid. It is found that the rapid increase of in-plane membrane stress with de ection is asigni cant limitation on achievable deformation of a continuous skin with uniform section. Use of a 2D auxetic latticestructure in place of the continuous skin allows signi cantly larger deformations and thus a signi cant improvement inperformance. The optimal deformed geometry is replicated statically and the e ect on the boundary layer is validatedin a wind tunnel experiment. The lattice structure is then manufactured and actuated. The deformed geometry isshown to compare well with the FEA predictions. The surface is re-examined post actuation and shown to return tothe initial position, demonstrating the deformation is elastic and hence repeatable.

Garcia De La Cruz JM, Brackston RD, Morrison JF, 2017, Adaptive Base-Flaps under Variable Cross-Wind, *SAE Technical Papers*, Vol: 2017-August

© 2017 SAE International. Road vehicles usually operate within windy environments. The combination of typical wind distributions and vehicle speeds, imposes on such vehicles aerodynamic yaw angles, β, which are often almost uniform up to 6° and relevant up to 14°. Drag saving devices are often optimized for zero cross-wind scenarios, minimizing drag only around these design conditions. This work presents the drag saving increase that an adaptive system can provide over a classic boat-tail. In the experimental set up employed, two flaps are located at the rear lateral edges of an Ahmed body and respectively set at angles θ1 and θ2 with respect to the model. To evaluate the efficacy of different flap positioning strategies under cross-wind, the model was tested in a wind tunnel, ReW O(105), with and without flaps at yaw angles β = 0°, 3°, 6° or 9°. The flap sizes tested, δ, were 9% or 13% of the body width. For each β and δ, the maps of drag against the two flap angles were obtained. The minimum drag is generally not located at the symmetric flap deflection, defined by θ1 = θ2. Such a condition is characteristic of static positioning and is equally effective for positive and negative β. The flap positioning strategies considered for comparison are the static symmetric configurations minimizing drag at each β and the adaptive configuration providing the minimum drag at every β. Except for β = 0°, the adaptive strategy consistently provides less drag than each optimal symmetric flap deflection. Also, a static strategy cannot optimize the symmetric flap deflection at each β. When the average of each configuration drag, weighted with a realistic β distribution, is employed, the adaptive solution provides a drag reduction 40% to 70% higher than the best static positioning, depending on the flap length and exact yaw distribution.

Fournié M, Morrison J, 2017, Fictitious domain for stabilization of fluid-structure interaction, *IFAC-PapersOnLine*, Vol: 50, Pages: 12301-12306

© 2017 We study the numerical approximation of the fluid structure interaction for stabilization of the fluid flow around an unstable stationary solution in a two dimensional domain, in the presence of boundary perturbations. We use a feedback control law recently proposed in [Airiau et al. (2017)] which is able to stabilize the nonlinear semi-discrete controlled system and based on the fluid only. Using Dirichlet boundary feedback, we deduce a boundary structure displacement. The fluid structure closed loop feedback is tested numerically using a fictitious domain finite element method based on extended Finite Element.

Diwan SS, Morrison JF, 2017, Spectral structure and linear mechanisms in a rapidly distorted boundary layer, *International Journal of Heat and Fluid Flow*, Vol: 67, Pages: 63-73, ISSN: 0142-727X

The aim of the present work is to investigate the spectral structure of a rapidly distorted boundary layer that develops on a flat plate in presence of a localised patch of roughness or/and grid-generated freestream turbulence. We observe that, at a certain distance downstream of the roughness patch the boundary layer exhibits a bimodal shape in the energy spectrum of the streamwise velocity fluctuations, similar to that found in a fully-turbulent boundary layer at relatively high Reynolds numbers. The physical mechanism that gives rise to the low-wavenumber peak in the spectrum, which represents long streamwise motions or “superstructures”, is identified to be the interaction of the broadband disturbances with the region of high shear near the wall in the boundary layer. We next show that the flat-plate boundary layer combined with surface roughness and grid turbulence can serve as building-block elements towards synthesising the wall-normal structure of a canonical turbulent boundary layer, in the context of large-scale streamwise motions. The rapidly distorted (or “synthetic”) boundary layer presents a simpler environment in which the coherent motions can evolve and therefore can enable a better characterisation of these motions. To further illustrate the utility of the present approach we compare results from our measurements with the predictions of the Rapid Distortion Theory (RDT). We show that the streamwise turbulence energy in the near-wall region of the rapidly distorted boundary layer grows linearly with time consistent with the RDT results on the effect of pure shear on an initially isotropic turbulence. Moreover close to the edge of the boundary layer the large-scale fluctuations experience an enhancement in the streamwise turbulence energy in accordance with the linear blocking model in the RDT framework. The present work thus highlights the importance of linear processes in wall turbulence and can help us identify aspects of it

de la Cruz JMG, Oxlade AR, Morrison JF, 2017, Passive control of base pressure on an axisymmetric blunt body using a perimetric slit, *PHYSICAL REVIEW FLUIDS*, Vol: 2, ISSN: 2469-990X

The effect on the base pressure of a thin slit located at the base edge of a blunt axisymmetric body, communicating an internal cavity with the external flow, is investigated. A parametric study is performed of the effect on base pressure of changes in slit size and cavity depth. The base pressure increases initially with increasing cavity depth, but saturates at a depth which depends on the slit size. The base pressure increases monotonically up to 5% with increasing slit size for the geometries tested. An upper limit of base pressure recovery of 20% is extrapolated from the data. It is observed that the main effect of the slit is to reduce the instantaneous pressure asymmetry, which is linked to the total base pressure in a similar fashion for all the slit sizes. As a second-order effect, for highly asymmetric pressure distributions, the slit produces a base pressure increase not associated with the base pressure asymmetry. The results suggest a global effect of the slit on the wake due to a diametrical flow within the cavity driven by the pressure differences across the slit and regulated by the largest of the pressure drops between the slit and cavity. The slit also reduces the periodic base pressure fluctuations, corresponding mainly to the vortex shedding, and increases the rotational speed of the wake.

Diwan SS, Morrison JF, 2017, Spectral structure and linear mechanisms in a rapidly distorted boundary layer, *International Journal of Heat and Fluid Flow*, ISSN: 0142-727X

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.

Morrison JF, Diwan SS, 2017, Inner-outer interaction in a rapidly sheared boundary layer

In the present work we study the inner-outer interaction in wall turbulence using a novel experimental arrangement of first generating a shearless boundary layer over a moving ground plane in the presence of grid turbulence, which is then passed over a stationary floor downstream resulting in a rapidly sheared boundary layer. The velocity spectra in such a boundary layer are shown to mimic the spectral features typical of a canonical turbulent boundary layer over a range of Reynolds numbers. This suggests that the rapidly sheared boundary layer consists of coherent structures that are qualitatively similar to the large-scale motions and superstructures observed in a canonical turbulent boundary layer. Static pressure fluctuations measured using a specially-made "needle" probe reveal the variation of the pressure field inside the rapidly sheared boundary layer. The pressure fluctuations in the free stream are seen to be highly correlated with wall pressure, especially when the boundary layer is sufficiently thin, supporting the view that the pressure fluctuations can play an important role in coupling turbulent eddies in the inner and outer regions. Further, we show that the present experimental arrangement is well-suited to studying the relative importance of the "top-down" and "bottom-up" mechanisms in wall turbulence in a systematic manner. The results obtained so far suggest that the top-down mechanism is dominant near the leading edge of the stationary surface with the bottom-up mechanism becoming progressively important as the boundary layer grows downstream.

Brackston RD, Wynn A, De La Cruz JMG, et al., 2017, Modelling and feedback control of turbulent coherent structures for bluff body drag reduction

Three-dimensional bluff body wakes are of key importance due to their relevance to the automotive industry. Such wakes have both large pressure drag and a number of coherent flow structures associated with them. Depending on the geometry, these structures may include both a bistability resulting from a reflectional symmetry breaking (RSB), and a quasi-periodic vortex shedding. The authors have recently shown that the bistability may be modelled by a Langevin equation and that this model enables the design of a feedback control strategy that efficiently reduces the drag through suppression of asymmetry. In this work the modelling approach is extended to the vortex shedding, capturing both the forced and unforced behaviour. A control strategy is then presented that makes use of the frequency response of the wake, and aims to reduce the measured fluctuations associated with the vortex shedding. The strategy proves to be effective at suppressing fluctuations within specific frequency ranges but, due to amplification of disturbances at other frequencies, is unable to give drag reduction.

Gouder KA, Naguib AM, Lavoie PL, et al., 2017, Control of boundary layer streaks induced by freestream turbulence using plasma actuators, 10th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2017

Copyright © 2016 Zakon Group LLC. Previously, a systematic series of investigations, such as those of Hanson et al. (2010), Hanson et al. (2014) and Bade et al. (2016) were carried out aimed at assessing the capability of plasma-actuator-based Feedforward-Feedback control system to weaken streaks that were artificially induced into a Blasius boundary layer using dynamic roughness elements. In contrast, the current work builds on these previous works and drives towards the delay of bypass boundary layer transition, where in the presence of a freestream flow with turbulence intensity exceeding approximately 1%, streaks form naturally and stochastically in the underlying boundary layer. For the freestream velocity of the current experiment, turbulent spot formations were first observed at a streamwise location x ≈ 350 mm from the leading edge. Upstream of this location, the naturally-occurring high and low-speed streaks exhibit linear transient growth. Two wall-shear-stress sensors - one feed-forward (FF) and one feedback (FB) - and two plasma actuators capable of producing positive and negative wall-normal forcing to oppose high and low-speed streaks respectively were placed in the linear growth region. The output from the FF sensor was used in conjunction with single-point Linear Stochastic Estimation (LSE) and actuator-flow identified response models in order to generate a counter-disturbance, which, at the (downstream) FB sensor location, was equal in magnitude but opposite in sign to the natural streak estimate. The output of the FB sensor was fed to a PI controller to correct for any remaining, uncancelled disturbance resulting from, for example, inaccuracies in the LSE model of streak growth. Results, such as notable changes in the mean and rms wall normal velocity profiles and energy spectra, for FF only, and FF + FB control, provide an evaluation of the viability of the control approach to weaken boundary layer streaks and delay transition.

Bird J, Santer M, morrison J, 2016, The determination and enhancement of compliant modes for actuation in structural assemblies, *International Journal of Solids and Structures*, Vol: 106-107, Pages: 264-273, ISSN: 0020-7683

Linear algebra methods for determining modes of kinematic and static indeter-minacy in jointed frames are extended to reveal modes of compliance in oth-erwise rigid assemblies. These modes are extracted from a structural model,based on nite elements, via a singular value decomposition and yield the waysin which a structure can be most easily deformed. This modal approach alsoallows for the formulation of a reduced-order structural model, whereby relevantmodes are selected and used as the basis for the optimisation of a complaintstructure. The method detailed is shown to be a useful design tool, demon-strated by its application to a structure based on the Kagome lattice geometry.For certain frameworks, rst order e ects produce tightening under actuation.As a result, a scheme to adjust the modes to take nonlinear e ects into accountis also given.

Brackston RD, Wynn A, Morrison JF, 2016, Extremum seeking to control the amplitude and frequency of a pulsed jet for bluff body drag reduction, *Experiments in Fluids*, Vol: 57, ISSN: 1432-1114

Feedback control of fluid flows presents a challenging problem due to nonlinear dynamics and unknown optimal operating conditions. Extremum seeking control presents a suitable method for many flow control situations but involves its own challenges. In this paper we provide a brief analysis of the extremum seeking method, with attention to modifications that we find to be advantageous. In particular, we present an adaptation for optimisation ofthe frequency of a harmonic input signal, a common scenario for open-loop flow control systems. We then present results from the experimental implementation of our modified method to the open-loop control system of Oxlade et al (2015, J. Fluid Mech., vol. 770), an axisymmetric bluff bodywake, forced by a pulsed jet. We find that the system is able to achieve optimal operating conditions in both the amplitude and frequency of the harmonic input signal, and is able to largely reject the disturbances arising from measurements of a highly turbulent flow. We finally show the ability of the extremum seeking system to adapt to changing conditions.

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.

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