Publications
60 results found
Gramola M, Bruce P, Santer M, 2022, Passive control of 3D adaptive shock control bumps using a sealed cavity, Journal of Fluids and Structures, Vol: 112, Pages: 1-19, ISSN: 0889-9746
This paper presents a Fluid-Structure-Interaction study of a novel passive adaptive shock control bump concept. A flexible plate, clamped on all sides and placed above a sealed cavity, was tested beneath a Mach 1.4 normal shock in the Imperial College London supersonic wind tunnel. The plate was actuated into the shape of a 3D shock control bump by passively controlling the cavity pressure through an array of breather holes. Preliminary experiments were performed with active control of cavity pressure (via a vacuum tank) at Mach 1.4 and 2 to illustrate the potential of this concept. Full-field surface measurement techniques, namely photogrammetry and pressure sensitive paint, were employed in addition to static pressure tappings and schlieren photography. Results confirmed that cavity pressure plays a key role in determining the aerostructural behaviour of the flexible plate. In addition, it was found that carefully placed breather holes allowed the plate to deform into a 3D shock control bump when a shock was on the flexible region and remain flat otherwise. This shows significant potential for improving the off-design behaviour of adaptive shock control bumps.
Thillaithevan D, Bruce P, Santer M, 2022, Robust multiscale optimization accounting for spatially-varying material uncertainties, Structural and Multidisciplinary Optimization: computer-aided optimal design of stressed solids and multidisciplinary systems, Vol: 65, Pages: 1-18, ISSN: 1615-147X
In this work we demonstrate a methodologyfor performing robust optimization using multivariableparameterized lattice microstructures. By introducingmaterial uncertainties at the microscale, we are able tosimulate the variations in geometry that occur duringthe manufacturing stage and design structures which aretolerant to variations in the microscale geometry. Weimpose both uniform and spatially-varying, non-uniformmaterial uncertainties to generate structures which, interms of standard deviation, are up to 77% more robustin the non-spatially uncertainty varying case, and 74%more robust in the spatially-varying case. We also explore the utility of imposing spatially-varying materialuncertainties compared to using homogeneous, uniformmaterial uncertainties, which are much less computationally expensive. It is found that when designs thathave been optimized assuming uniform uncertainties aresubject to spatially-varying uncertainties, their standarddeviations of compliance are similar to designs optimizedassuming spatially-varying uncertainties. However, theirmean compliances are far higher in comparison to designs generated by assuming spatially-varying materialuncertainties.
Montgomery K, Bruce PJK, Navarro-Martinez S, 2022, Dynamics of varying thrust coefficients for supersonretro-propulsion during Mars EDL
This paper presents the results from a computational study of a supersonic retro-propulsion geometry with various adaptations to account for differing nozzle configurations. The study considers a central configuration with different diameter linearly diverging nozzles and focusses on the effects of different thrust coefficients on the drag of the craft, as well as the unsteady behaviour that arises from the jet-bow shock interaction. Results show both the long penetration mode and blunt penetration mode, as well as the transition between the two modes. The results presented in this paper are consistent with previous studies, which showed that axial force augmentation occurs for large thrust coefficients, while low values reduce axial force by changing the shape of the interaction. For the large diameter nozzle presented, there is no value of (formula presented) that decreases axial force below that of drag alone.
Rees TW, Bruce PJK, Fisher TB, et al., 2021, Numerical and experimental studies of the hypersonic flow around a cube at incidence, ACTA ASTRONAUTICA, Vol: 183, Pages: 75-88, ISSN: 0094-5765
In order to improve predictions of the on-ground casualty risk associated with the uncontrolled atmospheric reentry of satellites from Low Earth Orbit, there is significant research interest in the development of engineeringmodels of hypersonic heating rates to faceted shapes. A key part of developing such models is generating accuratedatasets of the heat fluxes experienced by faceted shapes at various orientations in hypersonic flows. In this work,we use wind tunnel experiments and CFD simulations to study the hypersonic flow around a cuboid geometry at5◦ incidence in a Mach 5 flow at Reynolds numbers of 79.5 × 103, 109 × 103 and 148 × 103. The wind tunneldata are obtained in the University of Manchester’s High SuperSonic Tunnel and consist of schlieren images andtemperature histories collected using infrared thermography. These temperature histories are used to calculateexperimental heat fluxes by solving a three-dimensional inverse heat conduction problem. CFD simulationsaround the same geometry at equivalent free-stream conditions are calculated with the DLR-TAU code. Theexperimental and CFD results show good agreement both in terms of heat fluxes as well as flow structure. Notableflow structures include wedge-shaped regions of high heat flux which emanate from the windward corners of thecube. Analysis of numerical Q-criterion contours show that these high heat flux regions are caused by vortexstructures generated by the expansion at the cube corner. Analysis of the numerical skin friction coefficientshows that even at incidence there is no breakaway separation from the expansion edges of the cube and the flowremains attached throughout. We show that although there is little change in the average heat flux experiencedby a cube at 5◦ incidence to the free-stream compared to one at 0◦ incidence, there are significant changes in theheat flux contours over the cubes at these two incidences. Finally, we calculate a number of heating shape fact
Zheng S, Bruce P, Cuvier C, et al., 2021, The non-equilibrium dissipation scaling in large Reynolds number turbulence generated by rectangular fractal grids, Physical Review Fluids, Vol: 6, ISSN: 2469-990X
In this paper, the turbulence fields generated by a group of modified fractal grids, referred to as the rectangular fractal grids (RFGs), are documented and discussed. The experiments were carried out using hot-wire anemometry in three facilities at Imperial College London and the Laboratory of Fluid Mechanics in Lille, France. Due to the large Reynolds number of the resulting turbulence, several data processing methods for turbulence properties are carefully evaluated. Two spectral models were adopted, respectively, to correct the large and small wave-number ranges of the measured spectrum. After the technical discussion, the measurement results are presented in terms of one-point statistics, length scales, homogeneity, isotropy, and dissipation. The main conclusions are twofold. First, the location of maximum turbulence intensity xpeak is shown to be independent of the inlet Reynolds number but dependent on the ratio between the lengths of the largest grid bars in the transverse and vertical directions. This is crucial to the production of prescribed features of turbulent flows in laboratory. Second, these RFG-generated turbulent flows are shown to be quasihomogeneous in the decay region for x/xpeak>1.5, but the isotropy is poorer than that of the previous studied fractal square grid-generated turbulence. In the beginning of the decay region, a decreasing pattern of the integral length scale Lu and Taylor microscale λ was observed, yet the ratio Lu/λ remained roughly constant along the centerline, so Cε∼Re−1λ, complying with the nonequilibrium scaling relation reported in previous studies for various turbulent flows.
O'Driscoll D, Santer M, Bruce P, 2021, Design and dynamic analysis of rigid foldable aeroshells for atmospheric entry, Journal of Spacecraft and Rockets, Vol: 58, Pages: 741-753, ISSN: 0022-4650
A novel rigid deployable aeroshell architecture has been developed, where rigid panels with a thermal protectionsystem layer are connected between retractable ribs. Following origami principles, an optimal fold pattern is selectedand imposed on the panels to ensure efficient flat stowage during launch and repeatable deployment. The designprocess includes minimizing the number of folds to reduce stacking height and maximizing the angles between eachfold line to avoid an unfavorable aerothermodynamic response. The dynamic behavior of the optimal design isanalyzed with the aid of a dynamic multibody analysis model. Results from the dynamic model show that the processof deployment is highly sensitive to panel geometry (especially panel thickness and hinge design). Robust, repeatable,and controllable deployment is most readily achieved with a small (but nonzero) panel thickness and selection ofinterpanel hinges, which allow a degree of over-rotation, avoiding a premature hard stop, which would otherwiseprevent full deployment. Modeled results have been verified through experimental testing of a 0.4-m-diam scalemodel.
O'Driscoll D, Bruce PJ, Santer MJ, 2021, Hypersonic foldable Aeroshell for THermal protection using ORigami (HATHOR): evaluation of deployable structural rigidity during descent, AIAA Scitech 2021 Forum, Publisher: American Institute of Aeronautics and Astronautics
Rees TW, Fisher TB, Bruce PJK, et al., 2020, Experimental characterization of the hypersonic flow around a cuboid (vol 61, 151, 2020), Experiments in Fluids: experimental methods and their applications to fluid flow, Vol: 61, Pages: 1-1, ISSN: 0723-4864
Thillaithevan D, Bruce P, Santer M, 2020, Stress constrained optimization using graded lattice microstructures, Structural and Multidisciplinary Optimization: computer-aided optimal design of stressed solids and multidisciplinary systems, Vol: 63, Pages: 721-740, ISSN: 1615-147X
In this work we propose a novel method for predicting stress within a multiscale lattice optimization framework. On the microscale, a scalable stress is captured for each microstructure within a large, full factorial design of experiments. A multivariate polynomial response surface model is used to represent the microstructure material properties. Unlike the traditional solid isotropic material with a penalisation based stress approach of penalising stress values or using the homogenized stress, we propose the use of real microscale stress components with macroscale strains through linear superposition. To examine the accuracy of the multiscale stress method, full-scale finite element simulations with non-periodic boundary conditions were performed. Using a range of microstructure gradings, it was determined that 6 layers of microstructures were required to achieve periodicity within the full-scale model. The effectiveness of the multiscale stress model was then examined. Using various graded structures and two load cases, our methodology was shown to replicate the von Mises stress in the centre of the unit lattice cells to within 10\% in the majority of the test cases. Finally, three stress-constrained optimization problems were solved to demonstrate the effectiveness of the method. Two stress constrained weight minimization problems were demonstrated, alongside a stress constrained target deformation problem. In all cases, the optimizer was able to sufficiently reduce the objective while respecting the imposed stress constraint.
Rees TW, Fisher TB, Bruce PJK, et al., 2020, Experimental characterization of the hypersonic flow around a cuboid, Experiments in Fluids: experimental methods and their applications to fluid flow, Vol: 61, Pages: 1-22, ISSN: 0723-4864
Understanding the hypersonic flow around faceted shapes is important in the context of the fragmentation and demise of satellites undergoing uncontrolled atmospheric entry. To better understand the physics of such flows, as well as the satellite demise process, we perform an experimental study of the Mach 5 flow around a cuboid geometry in the University of Manchester High SuperSonic Tunnel. Heat fluxes are measured using infrared thermography and a 3D inverse heat conduction solution, and flow features are imaged using schlieren photography. Measurements are taken at a range of Reynolds numbers from 40.0×103 to 549×103. The schlieren results suggest the presence of a separation bubble at the windward edge of the cube at high Reynolds numbers. High heat fluxes are observed near corners and edges, which are caused by boundary-layer thinning. Additionally, on the side (off-stagnation) faces of the cube, we observe wedge-shaped regions of high heat flux emanating from the windward corners of the cube. We attribute these to vortical structures being generated by the strong expansion around the cube’s corners. We also observe that the stagnation point of the cube is off-centre of the windward face, which we propose is due to sting flex under aerodynamic loading. Finally, we propose a simple method of calculating the stagnation point heat flux to a cube, as well as relations which can be used to predict hypersonic heat fluxes to cuboid geometries such as satellites during atmospheric re-entry.
Threadgill J, Bruce P, 2020, Unsteady flow features across different shock/boundary-layer interaction configurations, AIAA Journal: devoted to aerospace research and development, Vol: 58, Pages: 1-13, ISSN: 0001-1452
An experimental study has been conducted to investigate unsteady flow phenomena observed within various two-dimensional configurations of shock/boundary layer interactions.Six configurations have been tested in Mach 2 flow: ϕ1 = 14◦ and 20◦compression ramps,and incident shock reflections from ϕ1 = 7◦, 8◦, 9◦, and 10◦shock generators; Reynoldsnumbers in each case are Reθ ≈ 8350. The flow is assessed using an array of fast-responsepressure transducers in conjunction with a high-repetition rate PIV system. Developmentof the mean flow structures early in each interaction is observed to be consistent withthe Free Interaction concept. Unsteady wall-pressure energy content at frequencies abovethose associated with the characteristic low-frequency shock motion also show significantsimilarities in the vicinity of the shock foot. Results confirm that this low-frequency peakis not associated with a narrow-band forcing mechanism from either upstream or downstream, but rather a characteristic frequency that varies with interaction strength, whichdescribes the flow’s dynamic response. These findings support various models published inliterature that have sought to explain the source of low-frequency unsteady shock motion.
Gomez-Vega N, Gramola M, Bruce PJK, 2020, Oblique shock control with steady flexible panels, AIAA Journal, Vol: 58, Pages: 2109-2121, ISSN: 0001-1452
Flexible panels deforming under pressure loads have been suggested as a passive form of adaptive oblique shock control. This study investigates oblique shock–boundary-layer interactions on a steady flexible panel in a Mach 2.0 flow. Experiments were performed in the Imperial College supersonic wind tunnel, where shock generators were used to produce an oblique shock followed by a corner expansion. A parametric study was conducted, exploring different shock impingement positions and shock–expansion distances. The steady aerostructural response is studied using schlieren photography, static pressure distributions, photogrammetry measurements, and surface oil flow visualization. Two-dimensional numerical simulations were performed to assess the effects of the flexible panel on downstream total pressure recovery. These were validated against experimental wall pressure distributions and measurements from a Pitot rake. Results show reductions in both separation length (of up to 40%) and stagnation pressure losses (of up to 10%) if the flexible plate is used. These improvements occur for a range of shock positions spanning approximately 50% of the panel length and for all the shock–expansion distances considered. A model that captures the flow physics responsible for these trends is proposed. The results highlight the potential of flexible panels for practical oblique shock control.
Gramola M, Bruce P, Santer M, 2020, Off-design performance of 2D adaptive shock control bumps, Journal of Fluids and Structures, Vol: 93, ISSN: 0889-9746
Adaptive shock control bumps can exploit the on-design drag-reducing potential of 2D bumps, while mitigating their off-design performance deterioration through geometric modifications. In this study, experiments and simulations have been employed to investigate the wave-drag reducing potential of (actuated and unconstrained) 2D adaptive shock control bumps over a wide range of shock positions. Experiments were carried out in the Imperial College supersonic wind tunnel, modelling the adaptive bump as a flexible surface placed beneath a Mach 1.4 shock wave. 2D RANS CFD simulations of the flow in a parallel channel with a solid bump complement experiments. Wave drag was demonstrated to be proportional to the ratio of inlet to exit stagnation pressure in a blow-down wind tunnel for a given shock position. The shock exhibits a hysteretic behaviour when travelling in the wind tunnel working section, governed by the wave drag reducing potential of the bump. The actuated adaptive bump tested reduces wave drag over a wider operational envelope than solid bumps as experiments revealed the presence of three preferred structural configurations, which lead to a significantly enlarged hysteresis region. Finally, tests on unconstrained bumps were shown to increase wave drag, both on- and off-design, due to the unfavourable bump shapes that result from (only) passive actuation, suggesting that some constraints are required to achieve desirable surface deformations.
O'Driscoll D, Bruce PJ, Santer MJ, 2020, Origami-based TPS Folding Concept for Deployable Mars Entry Vehicles, AIAA Scitech 2020 Forum, Publisher: American Institute of Aeronautics and Astronautics
Rabey P, Jammy SP, Bruce P, et al., 2019, Two-dimensional unsteadiness map of oblique shock wave/boundary layer interaction with sidewalls, Journal of Fluid Mechanics, Vol: 871, ISSN: 0022-1120
The low-frequency unsteadiness of oblique shock wave/boundary layer interactions (SBLIs) has been investigated using large-eddy simulation (LES) and high-frequency pressure measurements from experiments. Particular attention has been paid to off-centreline behaviour: the LES dataset was generated including sidewalls and experimental pressure measurements were acquired across the entire span of the reflected shock foot. The datasets constitute the first maps of low-frequency unsteadiness in both streamwise and spanwise directions. The results reveal that significant low-frequency shock motion (with St ≈ 0.03) occurs away from the centreline, along most of the central separation shock and in the corner regions. The most powerful low frequency unsteadiness occurs offcentre, likely due to the separation shock being strengthened by shocks arising from the swept interactions on the sidewalls. Both simulation and experimental results exhibit asymmetry about the spanwise centre. In simulations, this may be attributed to a lack of statistical convergence; however, the fact that this is also seen in experiments is indicative that some SBLIs may exhibit some inherent asymmetry across the two spanwise halves of the separation bubble. There is also significant low-frequency power in the corner separations. The relation of the unsteadiness in the corner regions to that in the centre is investigated by means of two-point correlations: a key observation is that significant correlation does not extend across the attached flow channel between the central and corner separations.
Peacocke L, Bruce P, Santer M, 2019, Coupled aerostructural modeling of deployable aerodecelerators for Mars entry, Journal of Spacecraft and Rockets, Vol: 56, Pages: 1221-1230, ISSN: 0022-4650
An analysis of deployable aerodecelerators has been performed using a developed six-degree-of-freedom entry trajectory simulator coupled with a structural model of the deployable structural members, or ribs, to investigate the effect of aerodecelerator flexibility on the trajectory and configuration design. The modified Newtonian method is used in the entry trajectory simulator, and the deployable ribs are modeled as Euler–Bernoulli beams. It is shown that, although flexibility is beneficial in reducing the mass and volume of the deployed ribs, an increase in peak heat flux will result. However, if mass savings from flexible ribs can be reallocated toward increasing the diameter of the entry vehicle, significant benefits can be gained.
Grossman I, Bruce P, 2019, Sidewall gap effects on oblique shock wave-boundary layer interactions, AIAA Journal, Vol: 57, Pages: 2649-2652, ISSN: 0001-1452
Gramola M, Bruce PJK, Santer M, 2019, Experimental and numerical study of 2D adaptive shock control bumps, 3AF International Conference
Gramola M, Bruce PJK, Santer M, 2019, Photogrammetry for accurate model deformation measurement in a supersonic wind tunnel, Experiments in Fluids, Vol: 60, ISSN: 0723-4864
The interest in adaptive devices for high-speed applications leads to the need for an accurate and reliable technique to obtain model deformation measurements during experiments. Point-tracking photogrammetry has been applied to supersonic wind tunnel testing, using four Phantom high-speed cameras placed on either side of the working section, where coded targets were applied to the surface of interest. Calibration experiments on a solid plate beneath a =1.4 normal shock and a =2 oblique shock allowed the quantification of the sources of optical distortion, namely the wind tunnel glass windows and aerodynamic effects (the lower pressure in the working section and the interaction between shock waves and the boundary layer). A correction matrix was applied to account for the optical distortion due to the glass, and the root-mean-square error due to aerodynamic effects (<0.03 mm) is believed to be negligible for applications with significant displacements (of the order of 1 mm). The application of photogrammetry to a flexible shock control bump has shown that the bump shape can be detected accurately, while disclosing some complex 3D effects that could not have been revealed by spanwise-averaged techniques such as schlieren photography.
Grossman I, Bruce P, 2018, Confinement effects on regular-irregular transition in shock wave-boundary-layer interactions, Journal of Fluid Mechanics, Vol: 853, Pages: 171-204, ISSN: 0022-1120
An oblique shock wave is generated in a Mach 2 flow at a flow deflection angle of 12∘ . The resulting shock-wave–boundary-layer interaction (SWBLI) at the tunnel wall is observed. A novel traversable shock generator allows the position of the SWBLI to be varied relative to a downstream expansion fan. The relationship between the SWBLI, the expansion fan and the wind tunnel arrangement is studied. Schlieren photography, surface oil flow visualisation, particle image velocimetry and high-spatial-resolution wall pressure measurements are used to investigate the flow. It is observed that stream-normal movement of the shock generator downwards (towards the floor and hence the point of shock reflection) is accompanied by (1) growth in the streamwise extent of the shock-induced boundary layer separation, (2) upstream movement of the shock-induced separation point while the reattachment point remains nearly fixed, (3) an increase in separation shock strength and (4) transition between regular and irregular (Mach) reflection without an increase in incident shock strength. The role of free interaction theory in defining the separation shock angle is considered and shown to be consistent with the present measurements over a short streamwise extent. An SWBLI representation is proposed and reasoned which explains the apparent increase in separation shock strength that occurs without an increase in incident shock strength.
Zheng S, Bruce PJK, Graham JMR, et al., 2018, Weakly sheared turbulent flows generated by multiscale inhomogeneous grids, Journal of Fluid Mechanics, Vol: 848, Pages: 788-820, ISSN: 0022-1120
A group of three multiscale inhomogeneous grids have been tested to generate different types of turbulent shear flows with different mean shear rate and turbulence intensity profiles. Cross hot-wire measurements were taken in a wind tunnel with Reynolds number ReD of 6000–20 000, based on the width of the vertical bars of the grid and the incoming flow velocity. The effect of local drag coefficient CD on the mean velocity profile is discussed first, and then by modifying the vertical barsto obtain a uniform aspect ratio the mean velocity profile is shown to be predictable using the local blockage ratio profile. It is also shown that, at a streamwise location x = xm, the turbulence intensity profile along the vertical direction u0(y) scales with the wake interaction length x peak∗,n = 0.21g2n/(αCDwn) (α is a constant characterizing the incoming flow condition, and gn, wn are the gap and width of the vertical bars,respectively, at layer n) such that (u0/Un) 2β2 (CDwn/x peak ∗,n) −1 ∼ (xm/x peak ∗,n) b, where β is a constant determined by the free-stream turbulence level, Un is the local mean velocity and b is a dimensionless power law constant. A general framework of grid design method based on these scalings is proposed and discussed. From the evolutionof the shear stress coefficient ρ(x), integral length scale L(x) and the dissipation coefficient C (x), a simple turbulent kinetic energy model is proposed that describes the evolution of our grid generated turbulence field using one centreline measurement and one vertical profile of u 0(y) at the beginning of the evolution. The results calculated from our model agree well with our measurements in the streamwiseextent up to x/H ≈ 2.5, where H is the height of the grid, suggesting that it might be possible to design some shear flows with desired mean velocity and turbulence intensity profiles by designing the geometry of a passive grid.
Gramola M, Bruce P, Santer M, 2018, Experimental FSI study of adaptive shock control bumps, Journal of Fluids and Structures, Vol: 81, Pages: 361-377, ISSN: 0889-9746
The shock stabilisation and wave drag reduction potential of a two-dimensional adaptive shock control bump has been studied in the Imperial College supersonic wind tunnel. The bump was modelled as a flexible aluminium alloy plate deformed through spanwise actuation, and several bump heights were tested beneath a Mach 1.4 transonic shock wave. Schlieren images and static pressure readings along the flexible plate allowed the study of the λλ-shock structure generated by the bifurcation of the normal shock for a range of shock positions. All bumps tested were found to increase shock stability, but wave drag reduction was only observed for shocks close to the leading edge of the flexible plate. Positive deformations of the flexible plate for downstream shocks are believed to reduce supersonic flow reacceleration, and hence the strength of the rear leg of the λλ-shock and wave drag, in comparison to a solid bump with the same shape. The position of the rear leg of the λλ-shock was found to exhibit a bistable behaviour, and this is hypothesised to be caused by a complex coupling of aerodynamic and structural instabilities.
Jinks E, Bruce P, Santer M, 2018, Optimisation of adaptive shock control bumps with structural constraints, Aerospace Science and Technology, Vol: 77, Pages: 332-343, ISSN: 1270-9638
This paper presents the results from a study to design an optimal adaptiveshock control bump for a transonic aerofoil. An optimisation frameworkcomprising aerodynamic and structural computational tools has been used toassess the performance of candidate adaptive bump geometries based on a novelsurface-pressure-based performance metric. The geometry of the optimal resultantdesign is a unique feature of its adaptivity; being strongly inuencedby the (passive) aerodynamic pressure forces on the exible surface as well asthe (active) displacement constraints. This optimal geometry bifurcates theshock-wave and carefully manages the recovering post-shock ow to maximisepressure-smearing in the shock-region with only a small penalty in L=D for theaerofoil. Short adaptive bumps (with small imposed displacements) generallyperform better than taller ones, and maintain their performance advantage fora wide range of bump positions, suggesting good robustness to variations inshock position, which are an inevitable feature of a real-world ight application.Such devices may o er advantages over conventional ( xed geometry) shockcontrol bumps, where optimal performance is achieved with taller devices, atthe expense of poor robustness to variations in shock position.Keywords: Shock Control Bumps; Aeroelastic Optimisation
Melina G, Bruce PJK, Nedić J, et al., 2018, Heat transfer from a flat plate in inhomogeneous regions of grid-generated turbulence, International Journal of Heat and Mass Transfer, Vol: 123, Pages: 1068-1086, ISSN: 0017-9310
Experiments on the convective heat transfer from a flat plate, vertically mounted and parallel to the flow in a wind tunnel, were carried out via Infra-Red thermography and hot-wire anemometry. The Reynolds number based on the inflow velocity and on the length of the plate was about 5×1055×105. A step near the leading edge of the plate was used to promote transition to turbulence, with tripping effects on the heat transfer coefficients shown to be negligible for more than 90% of the plate’s length. Different types of grids, all with same blockage ratio σg=28%σg=28%, were placed upstream of the plate to investigate their potential to enhance the turbulent heat transfer. These grids were of three classes: regular square-mesh grids (RGs), single-square grids (SSGs) and multi-scale inhomogeneous grids (MIGs). The heat transfer coefficients at the mid-length of the plate were correlated with the mean velocity and the turbulence intensity of the flow at a distance from the plate at which the ratio of the standard deviations of the streamwise and wall-normal velocity fluctuations began to increase. However, the heat transfer was shown to be insensitive to the turbulence intensity of the incoming flow in close proximity of the tripping step. Furthermore, the integral length scale of the streamwise turbulent fluctuations was found not to affect the heat transfer results, both near the tripping step and in the well-developed region on the plate. For the smallest plate-to-grid distance, the strongest heat transfer enhancement (by roughly 30%) with respect to the no-grid case was achieved with one of the SSGs. For the largest plate-to-grid distance, the only grid producing an appreciable increase (by approximately 10%) of the heat transfer was one of the MIGs. The present results demonstrate that MIG design can be optimised to maximise the overall heat transfer from the plate. A MIG that produces a uniform transverse mean shear, which is approximat
Steiros K, Bruce PJK, Buxton ORH, et al., 2017, Effect of blade modifications on the torque and flow field of radial impellers in stirred tanks, PHYSICAL REVIEW FLUIDS, Vol: 2, ISSN: 2469-990X
We perform both high- and low-speed particle image velocimetry and torque measurements to characterize eight radial impeller types in an unbaffled stirred tank. The blade types consist of a set of regular flat blades, used as a baseline, regular blades of increased thickness, perforated blades, and fractal blades. We find a qualitative correlation between the blades' torque coefficient and both vortex coherence and turbulent kinetic energy, possibly explaining the torque differences of the tested impellers. Furthermore, we find that the proposed modifications increase the bulk turbulence levels and mass flow rates while at the same time reducing the shaft torque, showing promise for applications. Finally, we attempt a comparison between fractal and perforated geometries using data from this study and the literature.
Rodríguez-López E, Brizzi LE, Valeau V, et al., 2017, Aeroacoustic Characterization of Single- and Multiscale Porous Fences, AIAA Journal, Vol: 56, Pages: 264-278, ISSN: 0001-1452
An experimental characterization of the acoustic behavior and the flowfield past several wall-mounted porousfences is presented. Particle image velocimetry is conducted in the near field of the fences. It shows a spanwiseperiodicity (associated with the periodic geometry of the grid) of regions where well-defined wakes appear as opposedto regions with a lower mass flow rate influenced by strong recirculations. The acoustic behavior is characterizedusing a microphone array enabling the spatial locationof noise sourcesto bedetermined. Broadband noise is observedfor all grids except for the fractal square grid, where a clear peak appears. The intensity of noise spectra displaysscalabilitywith MachnumberM6(dipoles), and the preferentialfrequencyfor the fractal square case is shown to scalewith thefreestreamvelocity(suggestingasheddingmechanism).For certaingrids, themeanandinstantaneous spatiallocations of the noise sources also present a spanwise preferential arrangement coincident with regions where the flowpresents well-defined wakes. Regarding the intermittency of the unsteady sources, they present smaller temporalwidth, separation, and spatial extent for increasing freestream velocity. However, the ratio between these quantitiesseems to be constant to within 6% for different Mach numbers.
Prigent SL, Buxton ORH, Bruce PJK, 2017, Coherent structures shed by multiscale cut-in trailing edge serrations on lifting wings, Physics of Fluids, Vol: 29, ISSN: 1070-6631
This experimental study presents the effect of multiscale cut-in trailing edge serrations on the coherentstructures shed into the wake of a lifting wing. Two-probe span-wise hot-wire traverses are performedto study spectra, coherence, and phase shift. In addition, planar particle image velocimetry is used tostudy the spatio-temporal structure of the vortices shed by the airfoils. Compared with a single tonesinusoidal serration, the multiscale ones reduce the vortex shedding energy as well as the span-wisecoherence. Results indicate that the vortex shedding is locked into an arch-shaped cell structure. Thisstructure is weakened by the multiscale patterns, which explains the reduction in both shedding energyand coherence.
Rodriguez-Lopez E, Bruce PJK, Buxton ORH, 2017, Flow characteristics and scaling past highly porous wall-mounted fences, Physics of Fluids, Vol: 29, ISSN: 1070-6631
An extensive characterization of the flow past wall-mounted highly porous fences based on single-and multi-scale geometries has been performed using hot-wire anemometry in a low-speed windtunnel. Whilst drag properties (estimated from the time-averaged momentum equation) seem to bemostly dependent on the grids’ blockage ratio; wakes of different size and orientation bars seem togenerate distinct behaviours regarding turbulence properties. Far from the near-grid region, the flowis dominated by the presence of two well-differentiated layers: one close to the wall dominated bythe near-wall behaviour and another one corresponding to the grid’s wake and shear layer, originatingfrom between this and the freestream. It is proposed that the effective thickness of the wall layer canbe inferred from the wall-normal profile of root-mean-square streamwise velocity or, alternatively,from the wall-normal profile of streamwise velocity correlation. Using these definitions of wall-layerthickness enables us to collapse different trends of the turbulence behaviour inside this layer. Inparticular, the root-mean-square level of the wall shear stress fluctuations, longitudinal integral lengthscale, and spanwise turbulent structure is shown to display a satisfactory scaling with this thicknessrather than with the whole thickness of the grid’s wake. Moreover, it is shown that certain gridsdestroy the spanwise arrangement of large turbulence structures in the logarithmic region, which arethen re-formed after a particular streamwise extent. It is finally shown that for fences subject to aboundary layer of thickness comparable to their height, the effective thickness of the wall layer scaleswith the incoming boundary layer thickness. Analogously, it is hypothesized that the growth rate ofthe internal layer is also partly dependent on the incoming boundary layer thickness.
Braun M, Bruce P, Levis E, 2017, Strategies to utilize advanced heat shield technology for high-payload mars atmospheric entry missions, Acta Astronautica, Vol: 136, Pages: 22-33, ISSN: 0094-5765
Present Entry, Descent and Landing (EDL) technology for interplanetary missions does not have the capabilities to meet the demanding requirements that come with future missions. A popular target for such missions is Mars and today efforts are made to send manned as well as sophisticated robotic probes to the Martian surface. Because present EDL technology has reached its limits, fundamentally new approaches are needed to significantly extend capabilities. Systematic evaluation of novel EDL technologies and optimization of EDL strategies are crucial needs for conceptual design. A computational framework will be presented tailored to enable systematic EDL analysis with special regards to novel EDL technology and event strategies. The benefits of flexible heat shield concepts that come with liberties in the choice of the ballistic coefficient will be shown in comparison with solid shield alternatives for payload classes of 2, 25 and 40 tonnes to show potential for manned and robotic missions. Furthermore, benefits of the new methodology for novel EDL event strategies are presented and discussed. The introduced methodology will help designers exploit new directions for conceptual design regarding EDL systems in terms of entry mass optimization and mission capabilities.
Melina G, Bruce P, Hewitt G, et al., 2017, Heat transfer in production and decay regions of grid-generated turbulence, International Journal of Heat and Mass Transfer, Vol: 109, Pages: 537-554, ISSN: 0017-9310
Heat transfer measurements around the centreline circumference of a cylinder in crossflow areperformed in a wind tunnel. The cylinder is placed at several stations downstream of threeturbulence-generating grids with different geometries and different blockage ratiosσg: a reg-ular grid (RG60) withσg= 32%, a fractal-square grid (FSG17) withσg= 25%and a single-square grid (SSG) withσg= 20%. Measurements are performed at 20 stations for 3 nominalReynolds numbers (based on the diameterDof the cylinder)Re∞= 11 100,24 500,37 900.Hot-wire measurements are performed along the centreline, without the cylinder in place,to characterise the flow downstream of the grids. The extent of the turbulence productionregion, where the turbulence intensityTuincreases with the streamwise distancexfrom thegrid, is higher for SSG and more so for FSG17 than for RG60. The angular profiles of theNusselt numberNuare measured in the production regions of these two grids and are com-pared to those obtained in the decay regions, whereTudecreases withx. This comparison ismade at locations with approximately sameTu. It is found that, for SSG,Nu/Re0.5on thefront of the cylinder (boundary layer region) is lower in the production region than in thedecay region. This is explained by the presence of clear and intense vortex shedding in theproduction region of SSG which reduces the turbulent fluctuations which are “effective” inenhancing the heat transfer across a laminar boundary layer. For higherRe∞, the values ofNu/Re0.5on the front of the cylinder are higher in the production region of FSG17 than inthat of SSG, despiteTubeing higher for SSG. This is consistent with a lower intermittencyof the flow for FSG17 caused by the presence of the fractal geometrical iterations. The recov-ery ofNuon the back of the cylinder (wake region) is appreciably higher in the productionregion than in the decay region for both FSG17 and for SSG. This can be due to the lowerint
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