Showing papers by "Julio Soria published in 2018"

Upstream-travelling acoustic jet modes as a closure mechanism for screech

Abstract: Experimental evidence is provided to demonstrate that the upstream-travelling waves in two jets screeching in the A1 and A2 modes are not free-stream acoustic waves, but rather waves with support within the jet. Proper orthogonal decomposition is used to educe the coherent fluctuations associated with jet screech from a set of randomly sampled velocity fields. A streamwise Fourier transform is then used to isolate components with positive and negative phase speeds. The component with negative phase speed is shown, by comparison with a vortex-sheet model, to resemble the upstream-travelling jet wave first studied by Tam & Hu (J. Fluid Mech., vol. 201, 1989, pp. 447–483). It is further demonstrated that screech tones are only observed over the frequency range where this upstream-travelling wave is propagative.

Dynamic stall in vertical axis wind turbines: scaling and topological considerations

Abstract: Vertical axis wind turbine blades are subject to rapid, cyclical variations in angle of attack and relative airspeed which can induce dynamic stall. This phenomenon poses an obstacle to the greater implementation of vertical axis wind turbines because dynamic stall can reduce turbine efficiency and induce structural vibrations and noise. This study seeks to provide a more comprehensive description of dynamic stall in vertical axis wind turbines, with an emphasis on understanding its parametric dependence and scaling behaviour. This problem is of practical relevance to vertical axis wind turbine design but the inherent coupling of the pitching and velocity scales in the blade kinematics makes this problem of more broad fundamental interest as well. Experiments are performed using particle image velocimetry in the vicinity of the blades of a straight-bladed gyromill-type vertical axis wind turbine at blade Reynolds numbers of between 50 000 and 140 000, tip speed ratios between to , and dimensionless pitch rates of . The effect of these factors on the evolution, strength and timing of vortex shedding from the turbine blades is determined. It is found that tip speed ratio alone is insufficient to describe the circulation production and vortex shedding behaviour from vertical axis wind turbine blades, and a scaling incorporating the dimensionless pitch rate is proposed.

The size dependant behaviour of particles driven by a travelling surface acoustic wave (TSAW)

Abstract: The use of travelling surface acoustic waves (TSAW) in a microfluidic system provides a powerful tool for the manipulation of particles and cells. In a TSAW driven system, acoustophoretic effects can cause suspended micro-objects to display three distinct responses: (1) swirling, driven by acoustic streaming forces, (2) migration, driven by acoustic radiation forces and (3) patterning in a spatially periodic manner, resulting from diffraction effects. Whilst the first two phenomena have been widely discussed in the literature, the periodic patterning induced by TSAW has only recently been reported and is yet to be fully elucidated. In particular, more in-depth understanding of the size-dependant nature of this effect and the factors involved are required. Herein, we present an experimental and numerical study of the transition in acoustophoretic behaviour of particles influenced by relative dominance of these three mechanisms and characterise it based on particle diameter, channel height, frequency and intensity of the TSAW driven microfluidic system. This study will enable better understanding of the performance of TSAW sorters and allow the development of TSAW systems for particle collection and patterning.

A detailed comparison of single-camera light-field PIV and tomographic PIV

Abstract: This paper conducts a comprehensive study between the single-camera light-field particle image velocimetry (LF-PIV) and the multi-camera tomographic particle image velocimetry (Tomo-PIV) Simulation studies were first performed using synthetic light-field and tomographic particle images, which extensively examine the difference between these two techniques by varying key parameters such as pixel to microlens ratio (PMR), light-field camera Tomo-camera pixel ratio (LTPR), particle seeding density and tomographic camera number Simulation results indicate that the single LF-PIV can achieve accuracy consistent with that of multi-camera Tomo-PIV, but requires the use of overall greater number of pixels Experimental studies were then conducted by simultaneously measuring low-speed jet flow with single-camera LF-PIV and four-camera Tomo-PIV systems Experiments confirm that given a sufficiently high pixel resolution, a single-camera LF-PIV system can indeed deliver volumetric velocity field measurements for an equivalent field of view with a spatial resolution commensurate with those of multi-camera Tomo-PIV system, enabling accurate 3D measurements in applications where optical access is limited

Coupling Modes of an Underexpanded Twin Axisymmetric Jet

Abstract: An experimental investigation into the coupling behavior of screeching axisymmetric twin supersonic jets is presented. Acoustic measurements and schlieren photography are used to identify four dist...

Analysis of Coherent Structures in an Under-Expanded Supersonic Impinging Jet Using Spectral Proper Orthogonal Decomposition (SPOD)

Abstract: The spatiotemporal dynamics of the coherent structures in an under-expanded supersonic impinging jet are studied using a spectral proper orthogonal decomposition technique. For this analysis, a large eddy simulation of an under-expanded supersonic impinging jet at a pressure ratio of 3.4 and a stand-off distance of 2 jet diameters at a Reynolds number of 50,000 is performed. The mean flow fields illustrate some striking features of this flow, such as an oblique shock, a stand-off shock, a Mach disk, and a recirculation bubble. The spectral proper orthogonal decomposition method is applied to time-resolved three-dimensional flow fields. The accumulative energy of modes within each azimuthal mode number reveals that the first three azimuthal modes contain most of the energy of the flow. The spectra of these azimuthal modes show that the flow exhibits a low-ranked behaviour with discrete frequencies at the optimal symmetric azimuthal mode while other two azimuthal modes have negligible contributions in this behaviour. Three peaks are observed in the spectra of the optimal symmetric azimuthal mode. The spatial fields of the streamwise velocity and pressure of these peaks show that the complex structures are consequences of the under-expansion, Mach disk, and the impingement. Strong hydrodynamic instabilities exist in the shear layer of the jet in the optimal azimuthal mode at each of these dominant frequencies. High-amplitude acoustic waves are also present in the near-field of the jet. These acoustic waves are strong at the nozzle lip, suggesting that a feedback loop linking these two processes exists for dominant frequencies in the optimal mode. High cross-spectrum density of near-field pressure fluctuations and streamwise velocity fluctuations near the nozzle lip at these frequencies confirms the hydro-acoustic coupling, which is necessary to close the feedback loop.

An experimental investigation of coupled underexpanded supersonic twin-jets

Abstract: High-resolution particle image velocimetry measurements of coupled underexpanded twin-jets are presented. Two nozzle pressure ratios are examined, which are selected due to a change in coupled plume mode indicated by a discontinuous jump in screech frequency. Estimates of the turbulent flow statistics, shear-layer thickness, merge point, inter-nozzle mixing, and integral length scales are provided. The higher nozzle pressure ratio case shows a strong standing-wave present in the velocity fluctuation amplitude and integral length scale. The ratios of standing, acoustic, and hydrodynamic wavelength are compared and find a close fit to Panda’s relation for screech. This indicates that screech in the twin-jet system operates with similar length-scale and frequency characteristics to single jets and provides evidence to suggest screech is an integral part of the twin-jet coupling process. Second-order spatial velocity correlation maps reveal the larger modal structure. A symmetric mode is found for the higher pressure ratio and a weakly symmetric mode for the lower. Comparison is made between where the standing-wave is present and where it is not. It is found that the standing-wave, not the shock structure, is the driver of turbulence coherence modulation near the jet. In regions that are affected only by the standing-wave, it is found that it contributes to both the turbulence intensity and coherence modulation.

Sound production by shock leakage in supersonic jet screech

Abstract: Damon Honnery is Deputy Dean Operations in the Faculty of Engineering, a Professor in the Department of Mechanical and Aerospace Engineering, and he jointly directs the Laboratory for Turbulence Research in Aerospace and Combustion. He obtained his undergraduate degree in mechanical engineering from the University of Sydney in 1985. Following this he was employed as a research fellow at the University of Sydney during which he obtained a MEngSc in 1987. In 1987 he was awarded a cadet research scientist position in the Aeronautical Research Laboratory (ARL-DSTO, now DSTG) during which he obtained his PhD in gas turbine related research from the University of Sydney in 1992. This was followed by an appointment as a research scientist at ARL in the Propulsion Branch where he undertook research on soot formation in gas turbines systems. In 1993 he took up a lectureship at Monash University at the Caulfield Campus, he then moved to the Clayton Campus in 1999 where he established the Monash Aerospace Engineering Degree. With Julio Soria he jointly established the Laboratory for Turbulence Research in Aerospace and Combustion in 1999. In the Department of Mechanical and Aerospace Engineering he has been Aerospace Course Director, Director of Undergraduate Affairs, Director of Research, and Deputy Head. In the Faculty of Engineering he has played a leading role in the development of the Monash Makerspace and was Interim Joint Director of the Woodside-Monash Energy Partnership. His research interests range from spray systems, particle flows, pollutant formation, renewable energy and climate change mitigation.

Signatures of shear-layer unsteadiness in proper orthogonal decomposition

Abstract: Proper orthogonal decomposition can be used to determine the dominant coherent structures present within a turbulent flow. In many flows, these structures are well represented by only a few high-energy modes. However, additional modes with clear spatial structure, but low-energy contribution can often be present in the proper orthogonal decomposition analysis, even for flows with a high degree of periodicity. One such mode has been observed in both free and impinging jets determined from particle image velocimetry. Both experimental and synthetic data are used to investigate the role of this particular mode, linking its existence to the unsteadiness of shear-layer large-scale coherent structures.

Linearised dynamics and non-modal instability analysis of an impinging under-expanded supersonic jet

Abstract: Non-modal instability analysis of the shear layer near the nozzle of a supersonic under-expanded impinging jet is studied. The shear layer instability is considered to be one of the main components of the feedback loop in supersonic jets. The feedback loop is observed in instantaneous visualisations of the density field where it is noted that acoustic waves scattered by the nozzle lip internalise as shear layer instabilities. A modal analysis describes the asymptotic limit of the instability disturbances and fails to capture short-time responses. Therefore, a non-modal analysis which allows the quantitative description of the short-time amplification or decay of a disturbance is performed by means of a local far-field pressure pulse. An impulse response analysis is performed which allows a wide range of frequencies to be excited. The temporal and spatial growths of the disturbances in the shear layer near the nozzle are studied by decomposing the response using dynamic mode decomposition and Hilbert transform analysis. The short-time response shows that disturbances with non-dimensionalised temporal frequencies in the range of 1 to 4 have positive growth rates in the shear layer. The Hilbert transform analysis shows that high non-dimensionalised temporal frequencies (>4) are dampened immediately, whereas low non-dimensionalised temporal frequencies (<1) are neutral. Both dynamic mode decomposition and Hilbert transform analysis show that spatial frequencies between 1 and 3 have positive spatial growth rates. Finally, the envelope of the streamwise velocity disturbances reveals the presence of a convective instability.

Flow Reversal in Turbulent Boundary Layers with Varying Pressure Gradients

Abstract: This contribution describes detailed PIV measurements obtained in turbulent boundary layers in order to capture rarely occurring flow reversals within the viscous sublayer, which previously have been observed in both experiments and direct numerical simulations. Due to their confinement to the viscous sublayer along with their rare occurrence on the order of 10^-4, any statistical investigation requires very large data sets exceeding 10^5 samples. In the present investigation, this was achieved by capturing long PIV records using image high magnification near unity. To investigate the influence of Reynolds number and pressure gradient, the measurements were performed in turbulent boundary layers in both zero pressure Gradient (ZPG) and adverse pressure gradient (APG) conditions. The measurements were conducted in the TBL wind tunnel of Lille which features a test section length of 20 m and cross-section of 2x1 m^2. A ramp model placed inside the tunnel introduced APG conditions. Both visual inspection of the raw data and a dedicated processing scheme to retrieve the unsteady wall shear stress were used to quantify the flow reversal events. The occurrence of the self-similar flow reversals was found to weakly depend on the Reynolds numbers in ZPG and roughly doubled in frequency in the APG condition.

Reynolds stress structures in a self-similar adverse pressure gradient turbulent boundary layer at the verge of separation

Abstract: Mean Reynolds stress profiles and instantaneous Reynolds stress structures are investigated in a self-similar adverse pressure gradient turbulent boundary layer (APG-TBL) at the verge of separation using data from direct numerical simulations. The use of a self-similar APG-TBL provides a flow domain in which the flow gradually approaches a constant non-dimensional pressure gradient, resulting in a flow in which the relative contribution of each term in the governing equations is independent of streamwise position over a domain larger than two boundary layer thickness. This allows the flow structures to undergo a development that is less dependent on the upstream flow history when compared to more rapidly decelerated boundary layers. This APG-TBL maintains an almost constant shape factor of H = 2.3 to 2.35 over a momentum thickness based Reynolds number range of Re δ 2 = 8420 to 12400. In the APG-TBL the production of turbulent kinetic energy is still mostly due to the correlation of streamwise and wall-normal fluctuations, 〈uv〉, however the contribution form the other components of the Reynolds stress tensor are no longer negligible. Statistical properties associated with the scale and location of sweeps and ejections in this APG-TBL are compared with those of a zero pressure gradient turbulent boundary layer developing from the same inlet profile, resulting in momentum thickness based range of Re δ 2 = 3400 to 3770. In the APG-TBL the peak in both the mean Reynolds stress and the production of turbulent kinetic energy move from the near wall region out to a point consistent with the displacement thickness height. This is associated with a narrower distribution of the Reynolds stress and a 1.6 times higher relative number of wall-detached negative uv structures. These structures occupy 5 times less of the boundary layer volume and show a similar reduction in their streamwise extent with respect to the boundary layer thickness. A significantly lower percentage of wall-attached structures is observed in the present case when compared with a similar investigation of a rapidly decelerating APG-TBL, suggesting that these wall-attached features could be the remanent from the lower pressure gradient domain upstream.

Time-averaged three-dimensional density and temperature field measurement of a turbulent heated jet using background-oriented schlieren

Abstract: Three-dimensional time-averaged density and temperature fields of a heated air jet from a circular 10 mm diameter nozzle are reconstructed using tomographic background-oriented schlieren. A hybrid filtered back-projection–algebraic reconstruction (FBP-ART) technique reconstructs the refractive index gradient field in the flow. The number of ART iterations is varied to observe the effect on reconstruction artefacts. The largest improvement is observed after 1 ART iteration with filtering of the initial FBP solution. Subsequently, the gradient field is used to form a Poisson equation solved in a quasi-3D manner using successive over-relaxation. Convergence of the refractive index field is studied with iteration number and the effect this has on the temperature and density profiles, after applying the Gladstone-Dale relation and ideal gas law. The quasi3D solution parallelises well, and converges rapidly.

A novel method for determination of convective velocity of coherent structures in high speed flows

Abstract: A novel method to determine the average convection velocity of coherent structures in the shear layer of supersonic jets is presented in this paper. The method is based on the Proper Orthogonal Decomposition (POD) of a flow field and the standard advection equation. Substitution of a function satisfying the advection equation with its POD representation followed by the utilization of the fundamental properties of POD modes and coefficients yields an expression for the convection velocity in terms of the derivatives and the inner products of modes. The method is suitable for experimental data (like PIV) as it does not require the data set to be time resolved. For validation and practical applicability of the method, a sensitivity analysis has been performed using a synthetic data set followed by the application of the technique on the simulation data of a realistic research problem.

Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: Characterisation of the flow-field

Abstract: This study examines the effects of angle of attack on the characteristics of the laminar separation bubble (LSB), its associated low-frequency flow oscillation (LFO), and the flow-field around a NACA-0012 aerofoil at $Re_c = 5\times 10^4$ and $M_{\infty} = 0.4$. In the range of the investigated angles of attack, statistics of the flow-field suggest the existence of three distinct angle-of-attack regimes: 1) the flow and aerodynamic characteristics are not much affected by the LFO for angles of attack $\alpha 9.6^{\circ}$, the flow-field and the aerodynamic characteristics are overwhelmed by a quasi-periodic and self-sustained LFO until the aerofoil approaches the angle of full stall. A trailing-edge bubble (TEB) forms at $\alpha > 9.25^{\circ}$ and grows with the angle of attack. The LSB and TEB merge and continue to deform until they form an open bubble at $\alpha = 10.5^{\circ}$. On the suction surface of the aerofoil, the pressure distribution shows that the presence of the LSB induces a gradual and continues adverse pressure gradient (APG) when the flow is attached. The bursting of the bubble causes a gradual and continues favourable pressure gradient (FPG) when the flow is separated. This is indicative that a natural forcing mechanism keeps the flow attached against the APG and separated despite the FPG. The present investigation suggests that most of the observations reported in the literature about the LSB and its associated LFO are neither thresholds nor indicators for the inception of the instability, but rather are consequences of it.

Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: The underlying mechanism

Abstract: The present investigation shows that a triad of three vortices, two co-rotating vortices (TCV) and a secondary vortex lies beneath them and counter-rotating with them, is behind the quasi-periodic self-sustained bursting and reformation of the laminar separation bubble (LSB) and its associated low-frequency flow oscillation (LFO). The upstream vortex of the TCV (UV) is driven by the gradient of the oscillating-velocity across the laminar portion of the separated shear layer and is faithfully aligned with it. The secondary vortex acts as a roller support that facilitates the rotation and orientation of the TCV. A global oscillation in the flow-field around the aerofoil is observed in all of the investigated angles of attack, including at zero angle of attack. The flow switches between an attached-phase against an adverse pressure gradient (APG) and a separated-phase despite a favourable pressure gradient (FPG) in a periodic manner with some disturbed cycles. When the direction of the oscillating-flow is clockwise, it adds momentum to the boundary layer and helps it to remain attached against the APG and vice versa. The transition location along the separated shear layer moves upstream when the oscillating-flow rotates in the clockwise direction causing early-transition and vice versa. The best description of the mechanism is that of a whirligig. When the oscillating-flow rotates in the clockwise direction, the UV whirls in the clockwise direction and store energy until it is saturated, then the process is reversed. The present investigation paves the way for formulating a time-accurate physics-based model of the LFO and stall prediction, and opens the door for control of its undesirable effects.

Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: The dynamics of the flow-field

Abstract: Detailed flow dissection using Dynamic Mode Decomposition (DMD) and Proper Orthogonal Decomposition (POD) is carried out to investigate the dynamics of the flow-field around a NACA-0012 aerofoil at a Reynolds number of $5\times 10^4$, Mach number of $0.4$, and at various angles of attack around the onset of stall. Three distinct dominant flow modes are identified by the DMD and the POD: 1) a globally oscillating flow mode at a low-frequency (LFO mode 1); 2) a locally oscillating flow mode on the suction surface of the aerofoil at a low-frequency (LFO mode 2); and 3) a locally oscillating flow mode along the wake of the aerofoil at a high-frequency (HFO mode). The LFO mode 1 features the globally oscillating-flow around the aerofoil, the oscillating-pressure along the aerofoil chord, and the process that creates and sustains the triad of vortices. The LFO mode 2 features the expansion and advection of the upstream vortex (UV) of the TCV. The HFO mode originates at the aerofoil leading-edge and features the oscillating mode along the aerofoil wake. The HFO mode exists at all of the investigated angles of attack and causes a global oscillation in the flow-field. The global flow oscillation around the aerofoil interacts with the laminar portion of the separated shear layer in the vicinity of the leading-edge and triggers an inviscid absolute instability that creates and sustains the TCV. When the UV of the TCV expands, it advects downstream and energies the HFO mode. The LFO mode 1, the LFO mode 2, and the HFO mode mutually strengthen each other until the LFO mode 1 and 2 overtake the HFO mode at the onset of stall. At the angles of attack $9.7^{\circ} \leq \alpha \leq 10.0^{\circ}$ the low-frequency flow oscillation (LFO) phenomenon is fully developed. At higher angles of attack, the HFO mode overtakes the LFO mode 2, and the aerofoil undergoes a full stall.

Characterization of a high Reynolds number turbulent boundary layer by means of PIV

Abstract: To characterize the turbulent boundary layer evolution in the LMFL wind tunnel, two experiments of velocity measurement by PIV were carried out. A first campaign was done based on a high magnification 2C2D PIV in the near wall region in order to capture the near wall scales and the mean gradient. A second experiment of stereo PIV using two systems was carried out to measure the full boundary layer with a good spatial resolution. By merging these two experiments, a good characterization can be made at 3 stations along the wind tunnel and for three free-stream velocities.

The influence of the cavity in the flow structures of a zero-net-mass-flux jet

Abstract: The main goal of this study is to obtain the dominant frequencies and wavenumbers related to the main flow structures of a zero-net-mass-flux (ZNMF) jet. For this purpose, numerical simulations have been carried out at Reynolds Number 1000. The influence of including a cavity for the ZNMF piston is also studied. With this aim, the data obtained from the numerical simulations are analysed using higher order dynamic mode decomposition. The method has been applied in both directions, time and space. The results show that the effect of including a cavity increases the flow complexity.

Stability of a self-similar adverse pressure gradient turbulent boundary layer

Abstract: Linear stability analysis (LSA) of a self-similar adverse pressure gradient (APG) turbulent boundary layer (TBL) is explored in order to identify coherent structures. An eddy viscosity model (EV) is implemented via the Boussinesq hypothesis [8] to model the nonlinear coherent-turbulent interactions. Direct numerical simulations (DNS) by Kitsios et al. [3, 6] are used for the database of this study. A weak APG and strong APG (on the verge of separation) are studied with dimensionless streamwise pressure gradients (β) of 1 and 39 respectively. Their Reynolds numbers based on the momentum thickness (δ2) within their respective regions of interest are 3,100− 3,400 and 10,000− 12,300. For the strong APG, the most unstable eigen-solution produces a wave resembling a Kelvin-Helmholtz (KH) instability located near the displacement thickness (δ1) height. This position coincides with the inflection point (IP) in the mean flow profile. The IP satisfies Rayleigh’s and Fjortoft’s criterion for the existence of an inviscid instability [9]. Positive growth rate is seen for non-dimensional angular frequencies of 0.08 ≤ ω̂ ≤ 0.51, with the maximum growth occurring at ω̂ = 0.26. The weak APG also contains a KH like wave, however for all ω̂, the growth rates are negative. Spanwise wavenumber ˆ kxr and phase velocity ĉr increase monotonically for both β cases. Comparisons with a quasi-laminar analysis are also made.

Investigation of the factors contributing to skin friction coefficient in adverse pressure gradient turbulent boundary layer flow using direct numerical simulation

Abstract: Adverse pressure gradient (APG) turbulent boundary layers (TBLs) are found when the flow takes place over the diverging part of curved surfaces, like flow over the leeward side of an aerofoil section. This paper reports on a study of the various factors contributing to the skin friction coefficient in such flows. Specifically, it deals with the contributions to the wall shear stress from Reynolds stress, viscous effects and pressure gradient in an incompressible turbulent boundary layer flow. The skin friction coefficient is expressed in terms of the decomposition suggested by Renard & Deck [6] for boundary layer flows.

A Taylor-Couette system for the experimental study of the influence of surface roughness

Abstract: An optically accessible Taylor-Couette flow has been setup to enable experimental investigation of the influence of surface finish and roughness on the transfer of momentum between the wall and the bulk flow. Details of this facility and flow characterisation are presented, demonstrating the ability to explore flow regimes from laminar Taylor vortices to featureless turbulence. Introduction Naturally occurring or engineered surface roughness (riblets, superhyrophobic surfaces) can have a significant effect on wallbounded flows. Despite the tendency for the surfaces of many rotating flow system such as bearings, compressor and turbines to develop roughness due to wear and pitting, relatively little is known about how such roughness influences the structure of the boundary layers in these flows and how this affects the interaction with the bulk flow. In the more general case of streamwise developing boundary layers and finite length channel flows over rough surfaces, it is difficult to assess the extent to which the flow structure is a function of the local conditions (e.g. local surface roughness, temperature) and the degree to which it depends on the upstream evolution and history of the flow. One method of decoupling these effects is to study the flow in a selfsimilar state, such that the relative contribution of each term in the governing equations is independent of streamwise position. In the case of surface roughness such a state can potentially be achieved by considering a flow that is streamwise periodic and hence independent of streamwise position. While the later is readily done in direct numerical simulations of pipes and channels, obtaining the equivalent flow in an experimental facility is often hampered by limited streamwise extent. One practical flow that avoids streamwise dependence, is the flow between two-coaxial independent rotating cylinders, Taylor-Couette flow. This configuration results in a statistically homogeneous flow in the azimuthal direction once a statistical stationary state has been reached. Taylor-Couette flows exhibit significant variations in the nature of the flow depending on the Taylor number and the Reynolds numbers of the inner and outer cylinders, which both represent the ratio of rotational inertial force to viscous forces. In the case of a rotating inner cylinder and a stationary outer cylinder the definition of the Taylor, Ta, and Reynolds number, Rei, reduce to: Rei = riωi(ro − ri) ν (1) Ta = (1+η)4 64η2 (ro − ri)(ri + ro)ωi ν2 (2) where ri and ro are the radii of the inner and outer cylinders, ωi is the rotational speed of the inner cylinder, ν is the kinematic viscosity of the fluid and η = ri/ro. In the case of smooth inner and outer walls the flow regimes vary from laminar Couette flow, to turbulent Taylor vortices with laminar boundary layers, through to a featureless turbulence with turbulent boundary layers on both walls [4]. Surface roughness will modify the boundary layers and is therefore expected to modify the interaction between the boundaries and the bulk flow and the transfer of momentum from the inner wall to the flow. Cadot et al. [2] performed experiments on a Taylor-Couette flow, with η = 0.625, Rei = 1×104 to 3×105 consistent with featureless turbulence, for a smooth rotating inner and smooth stationary outer wall, and after adding axially aligned ribs on both the inner and outer cylinders acting as roughness with a height of 0.067d. At these conditions the addition of the riblets was found to increase the required driving torque and reduced the sensitivity to Reynolds number seen in the absence of the riblets, resulting in a flow where energy dissipation is dominated by the bulk flow. Changes in velocity profiles and structure of the flow were not captured. Van den Berg et al. [1] also studied the influence of rough walls on the torque required to drive a Taylor-Couette flow with η = 0.73, Rei = 1× 104 to 1× 106, considering cases where both walls are rough and when one of the two walls remains smooth. Again, the roughness took the form of square riblets aligned in the axial direction. Results showed that keeping at least one surface smooth preserved the Reynolds number dependence while the addition of each rough wall increased the relative contribution of the bulk flow to the total energy dissipated by the flow. Recently, Zhu et al. [7] performed direct numerical simulations to analyse the source of the greater energy dissipation for rough wall Taylor-Couette flows, with η = 0.714, Rei = 8×103 to 8× 104. Roughness again took the form of axially aligned square rods with height 0.1d, this time only on the inner rotating cylinder. Simulations showed that the increase in torque at the wall was due to the pressure drag associated with the roughness elements, with the increase in bulk dissipation being due to the vortices shed from the roughness elements that are then transported to the bulk flow by the Taylor vortices. In this paper we discuss the design and characterisation of a Taylor-Couette facility with a driven inner cylinder for the experimental investigation of the influence of different natural and engineered roughness and surface coatings on the energy dissipation, required driving torque, and flow structure. Taylor-Couette System To assess the influence of different surface roughness and coatings on the resulting flow structure an optically accessible Taylor-Couette system with a transparent acrylic stationary outer cylinder was established with the geometry summarised in table 1. The outer cylinder is housed within a water filled clear rectangular acrylic tank in order to reduce the influence of the changes in refractive index and the optical distortions that are created when imaging through the cylindrical surface (see figure 1). Both the top and bottom end caps of the cylinder Inner radius ri 80 mm Outer radius ro 95 mm Radius ratio η 0.842 Gap size d 15 mm Length L 1.2 m Kinematic viscosity ν 9.562×10−7 m2/s Table 1: Taylor-Couette system and flow parameters. are fully submerged in water with a flooded 1 mm gap between the cylinder end surfaces and those of the outer tank. The inner cylinder is driven by a rs43C Compumotor stepper motor driven by a JMC 2DM2280 motor driver with step timing provided by an in-house timing unit developed using an Beaglebone black board computer [3]. For the present measurements the motor was run with 1600 steps per revolution and accelerated exponentially from rest, asymptoting to a target inner cylinder rotation rate in two minutes for each case. An inline torque transducer is placed between the motor and the cylinder to directly measure the torque required to drive the inner cylinder. In this configuration the inner cylinder is able to achieve a rotational rate of up to 300 revolutions per minute (rpm). Beyond this the imperfect balance of the cylinder and the high centre of gravity associated with the top mounted motor, results in noticeable vibration of the facility. Alterations are being made to the setup to enable in-situ adjustment and balancing of the inner cylinders with different surface finishes and to enable the higher rotation rates needed to achieve truly featureless turbulence [4].

PIV measurements of a wake from a rough flat plate

Abstract: The structure of the wake behind a submersed body greatly affects the performance and design of many engineering applications. The behaviour of the wake partially dictates the drag experienced by the body, therefore optimisation of this wake behaviour can have a significant effect on the energy required to move the body relative to the fluid. The formation of wakes over smooth bodies is well understood, however, the implications of surface roughness on the near wake structure are not well known. Surface roughness is known to greatly affect the behaviour of a turbulent boundary layer and the nature of the structures within. The subsequent interaction of these modified boundary layers in the wake modifies the formation of the wake. Roughness can be introduced into a system in a range of ways, such as manufacturing processes, surface finishes and degradation and accumulation of fouling. Therefore, the prevalence of roughness in combination with the importance of wake behaviour on performance is the motivation for the investigation of the implications of surface roughness on plane wake flows. To investigate these implications a series of 2-component 2dimensional particle image velocimetry (2C-2D PIV) measurements were performed on a smooth and rough flat plate in the LTRAC large horizontal water tunnel facility. The boundary layer was tripped at the leading edge and measurements were acquired in the near wake of the blunt trailing edge of the plate. The surface was roughened using commercially available sandpaper adhered to the plate surface.

Receptivity analysis in under-expanded supersonic impinging jets

Abstract: This paper presents the receptivity process at the nozzle-lip to an external forcing in the configuration of under-expanded supersonic impinging jets. The nozzle-lip is considered as a linear transformer. Therefore, the linearised three-dimensional Navier Stokes equations in cylindrical coordinate are considered. The mean flow fields are obtained from large-eddy simulations. An impulse response analysis of the linearised system is performed which allows a wide range of frequencies to be excited. The transfer function at the nozzle-lip is used to characterise the receptivity process. The sensitivity of the transfer function to the pulse location and the azimuthal wave number is performed. It is found that external forces at the vicinity of the infinite lip and with Strouhal numbers in the ranges of 0.7 to 6.5 have the highest amplification. This is consistent for all the azimuthal wave numbers in the nozzle-to-wall distance of 2d but not for the azimuthal wave number two for the nozzle-to-wall distance of 5d. For this case, external forces located at angles between 15° and 50° from the jet centreline also have high amplification in contrast to the nozzle-to-wall distance of 2d. Based on these results, the region close to the infinite lip is found to be a prominent candidate to control instabilities in this configuration.