Search

This study proposes a new approach that leverages deep learning to the study of flow-induced vibration (FIV), specifically to automate flow regime classification and to visualize the transitions between these regimes. Using previously obtained data on the amplitude response of an elastically mounted cylinder as a function of reduced velocity for a range of structural damping ratios, the time trace of the body displacement and fluid driving forces are first converted into a frequency-time representation using continuous wavelet transforms before being input to several pre-trained convolutional neural networks for feature extraction. When utilizing the outputs of each convolutional neural network for regime classification, we found that almost all the machine learning approaches had high cross-validation accuracy that was statistically insignificant from each other. The five best-performing classifiers were then used as an ensemble method, yielding a weighted accuracy of 99.1% on the test data. The FIV response regimes were further investigated by projecting the outputs of the pre-trained convolutional neural networks onto the first three modes identified with principal component analysis (PCA). The PCA plots indicated that, among all the models considered, Xception showed superior capability in delineating distinct locations for different FIV response regimes, based on the lock-in frequency and the presence of harmonics in the driving fluid forces. Moreover, the PCA plots also showed that increasing the structural damping ratio resulted in a diminished disparity in the dynamics of the identified FIV response regimes, leading to a less discernible separation between the regimes in the plots. [ABSTRACT FROM AUTHOR]

Natural hydro environments such as rivers and ocean currents provide abundant and essentially free sources of kinetic energy. Although significant research has been devoted to harnessing this power through traditional turbine systems, there are also studies on the novel use of flow-induced vibration of a bluff body. The large oscillations of an elliptical cylinder resulting from the newly-discovered hyper branch have raised interesting questions about its potential as a source of effective renewable energy. This study investigates the power extraction characteristics of the hyper branch over a range of flow velocities. A factor-of-ten increase in the maximum time-mean power coefficient relative to an established flow-induced vibration system — the vortex induced vibration for aquatic clean energy converter is demonstrated. This suggests the possibility of a new and effective strategy to utilise flow-induced vibrations for energy generation and could help drive the commercial implementation and uptake of oscillating energy converters. [Display omitted] • Thin elliptical geometry is inherently aerodynamically unstable. • Elastically mounted thin elliptical cylinder yields large crossflow oscillations. • Electromagnetic damper attached to cylinder is used to simulate generator loads. • Elliptical cylinder yielded a maximum power coefficient of 3.31. • Factor-of-ten improvement in power coefficient compared to other similar devices. [ABSTRACT FROM AUTHOR]

The effect of angle of attack on the flow-induced vibration (FIV) response of an elastically mounted thin elliptical cylinder has been investigated by measuring the structural displacement and fluid forces acting on the body in water-channel experiments. Specifically, an elliptical cylinder with a cross-sectional elliptical ratio of ɛ = b / a = 5 was chosen due to the presence of a region of vibration response associated with the combined effect of vortex-induced vibration (VIV) and galloping, where large vibration amplitudes nearly eight times the cross-flow dimensions can be sustained. Here, a and b are the semi-minor axis (aligned with the streamwise direction) and the semi-major axis, respectively. The present experimental results demonstrated that the large vibration amplitudes (i.e. where the maximum observed value was approximately 6 b) generally decrease with the angle of attack, resulting in substantial reductions for α ≳ 2 ∘ (with α = 3. 5 0 ∘ corresponding to a ∼ 60% decrease in the maximum vibration amplitude). Particle image velocimetry (PIV) measurements revealed that the dominant vortex shedding mode consists of two single opposite-signed vortices shed per body vibration cycle. The presence of additional vorticity regions that were absent in the zero angle of attack case was also observed, including crescent-shaped wake structures and secondary inline vortices. This study shows the importance of maintaining axial symmetry in such an FIV system, and that the flow incidence angle is an essential consideration for efficient energy harvesting using this elliptical geometry. [ABSTRACT FROM AUTHOR]

This study experimentally investigates the influence of aspect ratio on cross-flow flow-induced vibration (FIV) of elastically mounted oblate spheroids. The aspect ratio (ϵ = b / a) of an oblate spheroid, defined as the ratio of the major diameter (b) in the cross-flow direction to the minor diameter (a) in the streamwise direction, was varied between 1.00 and 3.20. The FIV response was characterized over a range of reduced velocity, 3. 0 ⩽ U ∗ = U / (f n w b) ⩽ 12. 0 , where U is the free-stream velocity and f n w is the natural frequency of the system in quiescent water. The corresponding Reynolds number varied over the range 4730 ⩽ R e ⩽ 20 120. It was found that in addition to the vortex-induced vibration (VIV) Mode I and Mode II responses observed for a sphere, on increasing the aspect ratio to ϵ = 1. 53 and 2.0, a galloping-dominated response, denoted by G-I, was encountered at high reduced velocities. With a further increase in aspect ratio to ϵ = 2. 50 , the body vibration exhibited an additional VIV-like response (V-I) following the sequential appearance of Mode I, Mode II and G-I, with smooth transitions between these modes. In the case of the largest aspect ratio considered in the present study, ϵ = 3. 20 , the spheroid intriguingly exhibited only a pure VIV Mode I before transitioning to a VIV-dominated mode, namely V-II. The largest vibration amplitude observed was 2. 17 b , occurring at the highest tested reduced velocity of U ∗ = 12. 0 for ϵ = 2. 5. Furthermore, the maximum time-averaged power coefficient was observed to be 0.165 for the thinnest oblate spheroid tested, ϵ = 3. 20 , approximately 660% higher than that observed for VIV of a sphere. This shows the relevance of geometry for FIV energy harvesting from oblate spheroids. The findings highlight the distinctive nature of FIV responses of 3D oblate spheroids compared to 2D bluff bodies such as elliptical, D-section, and square cylinders. [ABSTRACT FROM AUTHOR]

In April 2016, Crown Prince Mohammed bin Salman unveiled Saudi Vision 2030, and the accompanying National Transformation Programme. To mark the fifth anniversary of the launch of Saudi Vision 2030, the Kingdom's economic and diversification blueprint, Crown Prince Mohammed appeared on a popular Saudi talk show to discuss the progress of the Vision's National Transformation Programs and address the concerns of average citizens in relation to national development. Following the launch of the Vision, my 2017 paper Saudi Vision 2030: A Viable Response to Youth Aspirations and Concerns? documented young male perceptions of Vision 2030 including their expectations and aspirations for the future. This paper revisits the themes discussed in the earlier one and asks to what extent Vision 2030 has impacted Saudi youth mindsets and behaviour. What have been the significant transformations from the perspectives of Saudi youth of both genders, and are these transformations perceived as desirable, personally beneficial, and long lasting? It also documents that whilst there is widespread support for the Vision, there are concerns about the pace of change, as well as the perception that to date, there has been an over-emphasis on elite interests. [ABSTRACT FROM AUTHOR]

This review surveys the dramatic variations in wake structures and flow transitions, in addition to body forces, that appear as the motion of bluff bodies through a fluid occurs increasingly closer to a solid wall. In particular, we discuss the two cases of bluff bodies translating parallel to solid walls at varying heights and bluff bodies impacting on solid walls. In the former case, we highlight the changes to the wake structures as the flow varies from that of an isolated body to that of a body on or very close to the wall, including the effects when the body is rotating. For the latter case of an impacting body, we review the flow structures following impact and their transition to three-dimensionality. We discuss the issue of whether there is solid–solid contact between the bluff body and a wall and its importance to body motion. [ABSTRACT FROM AUTHOR]

This paper endeavours to answer several key questions: Firstly, why do many young Saudis have a continued preference for public sector employment over private sector jobs despite the government's promotion of the private sector as a key pillar of Saudi Vision 2030? Secondly, what are the reasons behind a widespread perception among Saudi undergraduates that the current education system, spanning elementary to high school, does not prepare school pupils for entry into tertiary education? Thirdly, why do young Saudis maintain that the education system frequently fails to prepare them for future jobs or, indeed, does not provide them with adequate opportunities to think about their future jobs? Primary data gathered for this paper demonstrate that there is still a marked preference for public sector employment among young Saudis due to a widespread belief that the public sector offers more 'job security'. In addition, the paper finds that the failure of the education system to prepare young nationals for entry into the labour market is considered to be particularly true for private sector employment. [ABSTRACT FROM AUTHOR]

The term globalization, encompassing politico-economic and socio-cultural aspects, is widely used to describe the results of rapid modernization on Saudi Arabia in an era of rapid societal transformation. Whilst the outward signs of increased globalization in the Kingdom are highly visible, the underlying effects particularly in terms of the politico-economic and socio-cultural less so. Indeed, globalization is often characterized as inevitable or unstoppable. Yet, in a 'globalized' Saudi Arabia the impact of globalization is not always perceived as being positive, in particular its impact on notions of individual, national or Islamic identities. Research conducted for this article demonstrates that young Saudi men identify clearly both positive and negative effects of globalization on identity narratives, whether individual, national or Islamic. Indeed, the issue of how young Saudi men negotiate their identity in light of increased globalization raises important questions regarding the consequences of increased globalization on young Saudi men's sense of their own identity, and by extension the government's promotion of an officially sanctioned Saudi national identity, in an era of rapid societal transformation. [ABSTRACT FROM AUTHOR]

In published literature of the vortex-induced vibration of a sphere, limited attention has been devoted to spheres having three degrees of freedom (DOF) motion and the effect on the vibration response. In this study, the vortex-induced vibration response of a 3-DOF elastically-mounted sphere was examined numerically. The response of a sphere allowed to oscillate in all three spatial directions (3-DOF motion) was compared with the response of a sphere whose motion was restricted to only the transverse direction (1-DOF). Simulations were conducted over the reduced velocity range U ∗ ≡ U ∕ (D f n) ∈ [ 3. 5 , 16 ] , using a sphere of mass ratio m ∗ ≡ ρ s ∕ ρ = 3 , at a fixed Reynolds number of 2000, where U is the free-stream velocity of the flow, f n is the natural frequency of the system, ρ s and ρ are the solid and fluid densities, respectively. When the sphere was allowed 3-DOF movement, it was initially excited to vibrate along a linear path in the transverse direction, synchronized with the two-sided shedding of vortex loops behind the sphere. As the simulation time advanced, the sphere trajectory eventually converted into a circular orbit with a spiralling wake behind it. The transition time between these two modes was found to increase with decreasing Reynolds number. The vibration amplitude was significantly smaller when the sphere motion was in all three spatial directions than when it was restricted only to the transverse direction. The maximum vibration amplitudes were approximately 0.6 and 0.8 diameters, for 3-DOF and 1-DOF motion, respectively. In the synchronization regime, the maximum time-averaged drag coefficient was approximately 100% greater than that of a stationary sphere, when the sphere had 3-DOF, similar to previous observations for a tethered sphere. However, the maximum change to the time-averaged drag coefficient was only 76% when the motion was restricted to the transverse direction. [ABSTRACT FROM AUTHOR]

The article focuses on role of Gulf Cooperation Council (GCC) in promoting a better understanding of the complexity of decision making in the Gulf. It is mentioned that the rise in prominence of individual Gulf states like Kuwait, Qatar, Saudi Arabia and the United Arab Emirates and the growth of the GCC as a collective entity. The article also discusses the lack of the attention paid to the actual mechanics of policy-making in the region.

Saudi socio-politics is being affected by societal transformation; one that is being driven by a potent combination of demographics, improved education, wider access to the Internet and burgeoning new media usage. Whilst Saudi government decision-making remains ‘top-down’, the dynamic within this approach has shifted as the top-down system incorporates a consultation process that includes newly established civil society institutions. Nonetheless, of particular significance in Saudi Arabia is growing public awareness, particularly amongst young educated Saudis, of the need for government accountability, transparency and best-practices. This paper discusses the views of a group of young men who represent an academic elite. Saudi-related literature frequently centers on the status of Saudi women, but the perspective of young Saudi men is generally overlooked even though this constituency is going to be of critical importance to the Kingdom's future social and political stability. The author recognizes that this elite sample does not represent Saudi youth in its entirety, but the diversity of student backgrounds and access to their extended networks can act as a barometer of educated youth opinion regarding many of the pressing politico-economic and socio-cultural issues facing Saudi Arabia today. [ABSTRACT FROM PUBLISHER]

Since Deputy Crown Prince Mohammed bin Salman Al Saud unveiled Saudi Vision 2030 (The National Transformation Programme, 25 April 2016) a great deal of analysis has focused on the potential impact of Vision 2030 at the national level. Significantly, Saudi Vision 2030, which promises a better future and better governance, is raising hopes and expectations amongst young well-educated Saudis. Nevertheless, how has Saudi Vision 2030 been interpreted by young educated male Saudis many of whom are struggling to enter the labour market at a time of austerity and economic uncertainty? This paper discusses young male perceptions of Saudi Vision 2030 including their expectations and hopes for the Vision. In fact, an understanding of how young Saudis perceive Vision 2030 is not only pertinent in the Saudi national context (or indeed multiple Saudi domestic contexts), but also has wider regional relevance considering oil-market developments and economic policy. [ABSTRACT FROM AUTHOR]

Renewable energy sources are likely to become essential due to continuously increasing energy demands together with the depletion of natural resources that are currently used for power generation, such as coal and gas. They are also advantageous in terms of their reduced environmental impact. Here, the generation of electrical power from Vortex-Induced Vibration (VIV) of a cylinder is investigated numerically. The cylinder is free to oscillate in the direction transverse to the incoming flow. The cylinder is attached to a magnet that can move along the axis of a coil made from conducting wire. The magnet and the coil together constitute a basic electrical generator. When the cylinder undergoes VIV, the motion of the magnet creates a voltage across the coil, which is connected to a resistive load. By Lenz's law, induced current in the coil applies a retarding force to the magnet. Effectively, the electrical generator applies a damping force on the cylinder with a spatially varying damping coefficient. For the initial investigation reported here, the Reynolds number is restricted to Re ≤ 200 , so that the flow is laminar and two-dimensional (2D). The incompressible 2D Navier-Stokes equations are solved using an extensively validated spectral-element based solver. The effects of the electromagnetic (EM) damping constant ξ m , coil dimensions (radius a , length L ), and mass ratio on the electrical power extracted are quantified. It is found that there is an optimal value of ξ m ( ξ opt ) at which maximum electrical power is generated. As the radius or length of the coil is increased, the value of ξ opt is observed to increase. Although the maximum average power remains the same, a larger coil radius or length results in a more robust system in the sense that a relatively large amount of power can be extracted when ξ m is far from ξ opt , unlike the constant damping ratio case. The average power output is also a function of Reynolds number, primarily through the increased maximum oscillation amplitude that occurs with increased Reynolds number at least within the laminar range, although the general qualitative findings seem likely to carry across to high Reynolds number VIV. [ABSTRACT FROM AUTHOR]

Abstract: Bénard convection around a circular heated cylinder embedded in a packed bed of spheres is studied numerically. The Forchheimer–Brinkman–extended Darcy momentum model with the Local Thermal Non-Equilibrium energy model is used in the mathematical formulation for the porous layer. The governing parameters considered are the Rayleigh number (103 ≤ Ra ≤5×107) and the thermal conductivity ratio (0.1≤ kr ≤10,000). The structural properties of the packed bed are kept constant as: cylinder-to-particle diameter ratio D/d =20 and porosity ε =0.5, while the Prandtl number is fixed at Pr =0.71. It is found that the presence of the porous medium suppresses significantly the strong free convection produced in the empty enclosure, and reduces considerably the high intensity of the pair of vortices generated behind the cylinder. Also, the results show that the porous medium can play the role of insulator or enhancer of heat transfer from the heat source, depending mainly on their thermal conductivities regardless of the Rayleigh number. [Copyright &y& Elsevier]

Abstract: This paper examines the changes to the flow and heat transfer induced by a non-zero mean sinusoidally varying flow past a cylinder. This is done by simulating steady and pulsatile forced convective flows over a circular cylinder placed in either a horizontal empty or a porous medium filled channel. The incompressible Navier–Stokes equations are used for the empty channel, and the Darcy–Brinkmann–Forchheimer momentum and the two-equation energy LTNE models in the porous-material filled channel. The effects of pulsation frequency St and amplitude A on heat transfer are quantified, as the Reynolds number (Re D =1–250) and type of porous material (k r =0.1, 1.0, 10, 100) are varied, whilst keeping the structural properties of the porous medium constant. In the empty (non-filled) channel, initially steady and then unsteady wakes evolve with an increase in Reynolds number. For the time-dependent case, two kinds of wake structure, namely fully periodic and quasi-periodic shedding, are observed depending on the values of Reynolds number, oscillation amplitude and forcing frequency. For the porous-medium filled channel simulations, a highly stable flows results, i.e., without forming extended wakes in the regions behind and in front of the cylinder, due to the damping from the porous medium. This is true even at high amplitudes, e.g., A >1.0, which generates reverse flow in the channel. In general, using porous media promotes much higher heat transfer enhancement from the cylinder than that promoted by using pulsating flow, particularly at higher Reynolds number. But, a significant thermal benefit can be achieved by combining both schemes; however, only at the higher Reynolds numbers studied. [Copyright &y& Elsevier]

This paper studies the fluid–structure interaction of an elastically mounted square cross-section cylinder immersed in a free stream. The cross-section is mounted such that its sides are at 45° to the free stream direction, in a “diamond” configuration, and its motion is constrained to the transverse direction relative to the flow direction. Apart from the cross-section, this setup is the same as the majority of single-degree-of-freedom vortex-induced vibration studies of cylinders. Two-dimensional direct numerical simulations of this system have been performed. The Reynolds number based on the point-to-point distance of the cross-section has been fixed at Re=200). Simulations at this Reynolds number allow a direct comparison with previous results from circular cylinders, and therefore focus directly on the impact of the geometry. The sensitivity of the flow, and therefore the motion of the cylinder, to geometrical effects is considered. This is done by rounding the two side corners (those pointing across the flow) at a given radius. For well-rounded corners, the flow behaviour resembles that of a circular cylinder undergoing vortex-induced vibration. However, below a critical radius, the dynamics are considerably altered. Highly disordered and irregular wakes and body motions are observed, as well as a synchronized, periodic P+S wake mode (Williamson and Roshko, 1988), which consists of a pair of vortices on one side, and a single vortex on the other side, shed per oscillation cycle, which results in a non-zero mean lift force. A period-doubled version of this P+S wake is also presented. The spatial structure, and the spatio-temporal symmetries of each of these modes is reported. The results show that even though the spatio-temporal symmetry of the flow is unaffected by the geometry when the body is rigidly mounted (the flow always saturating to a Kármán vortex street) geometric features such as sharp corners can induce a number of spontaneous symmetry breaking bifurcations when the body is elastically mounted. Which of these various modes is observed is shown to be a function of both the corner radius and the spring stiffness, expressed through the reduced velocity. [ABSTRACT FROM AUTHOR]

The transient response of optimal linear perturbations of liquid metal flow under a strong axial magnetic field in an electrically insulated rectangular duct is considered. The focus is on the subcritical regime, below the onset of von Kármán vortex shedding, to determine the role of optimal disturbances in developing wake instabilities. In this configuration, the flow is quasi-two-dimensional and can be solved over a two-dimensional domain. Parameter ranges considered include Reynolds numbers 50≤Re<=2100, modified Hartmann numbers 50≤Ha[closed_star]<=500, and blockage ratios 0.1 <= β <= 0.4. In some instances, the optimal disturbances are found to generate energy growth of greater than four orders of magnitude. Variation in the wake recirculation length in the steady flow regime is determined as a function of Reynolds number, Hartman number, and blockage ratio, and a universal expression is proposed. For all β, the energy amplification of the disturbances is found to decrease significantly with increasing Hartmann number and the peak growth shifts towards smaller times. The optimal initial disturbances are consistently located in the vicinity of the boundary layer separation from the cylinder, and the structure of these disturbances is consistent for all Hartmann numbers and blockage ratios considered in this study. The time evolution of the optimal perturbations is presented, and is shown to correspond to sinuous oscillations of the shear layer downstream of the wake recirculation. The critical Reynolds number for the onset of growth at different Hartmann numbers and blockage ratios is determined. It is found that it increases rapidly with increasing Hartmann number and blockage ratio. For all β, the peak energy amplification grows exponentially with Re at low and high Hartmann numbers. Direct numerical simulation in which the inflow is perturbed by a random white noise confirms the predictions arising from the transient growth analysis: that is, the perturbation excites and feeds energy into the global mode. [ABSTRACT FROM AUTHOR]

Abstract: The fluid flow and heat transfer of a liquid metal past a circular cylinder in a rectangular duct (width-to-height aspect ratio of 2) under a strong transverse magnetic field is studied numerically using a quasi-two-dimensional model. Transition from steady to unsteady flow regimes is determined as a function of Hartmann number and blockage ratio, as are Strouhal number, and the heat transfer from the heated wall to the fluid. Downstream cross-stream mixing induced by the cylinder wake was found to increase heat transfer by more than a factor of two in some cases. [Copyright &y& Elsevier]

• Flapping wings of smaller drones are inspired from insect wings. • The curved leading edge (LE) of a wing enhances the wing performance. • The LE curvature increases the spanwise flow, stabilising the LE vortex. The aerodynamic performance of an insect wing is largely dependent on the leading-edge vortex (LEV) on the wing formed during the rotational translation phase of its flapping motion. The geometry of the wing can directly influence the formation and strength of the LEV. The wing geometry is broadly defined by the aspect ratio and the radii of the wing's moment of inertia, which can be optimised to obtain the highest possible lift at a given Reynolds number. However, the shape of the leading edge can also affect the LEV and the tip vortex structure. A straight leading edge and a sharp wingtip can affect the spanwise vorticity flux responsible for a stable LEV, thereby affecting the lift and drag. To investigate this in the current study, the wing shape is initially approximated by the Beta function with a straight leading edge and is varied systematically by increasing the leading-edge curvature. The lift and power economy are observed to be enhanced with the leading-edge curvature. The reasons behind the lift enhancement are further investigated by analysing the LEV structures over the wings of various leading-edge curvatures. The curved leading edge is observed to enhance lift at both low as well as high Reynolds numbers relevant to insects. [ABSTRACT FROM AUTHOR]

Abstract: The problem of a particle impacting on a wall, a common phenomenon in particle-laden flows in the minerals and process industries, is investigated computationally using a spectral-element method with the grid adjusting to the movement of the particle towards the wall. Remeshing is required at regular intervals to avoid problems associated with mesh distortion and the constantly reducing maximum time-step associated with integration of the non-linear convective terms of the Navier–Stokes equations. Accurate interpolation between meshes is achieved using the same high-order interpolation employed by the spectral-element flow solver. This approach allows the full flow evolution to be followed from the initial approach, through impact and afterwards as the flow relaxes. The method is applied to the generic two-dimensional and three-dimensional bluff body geometries, the circular cylinder and the sphere. The principal case reported here is that of a particle colliding normally with a wall and sticking. For the circular cylinder, non-normal collisions are also considered. The impacts are studied for moderate Reynolds numbers up to approximately 1200. A cylindrical body impacting on a wall produces two vortices from its wake that convect away from the cylinder along the wall before stalling while lifting induced wall vorticity into the main flow. The situation for a sphere impact is similar, except in this case a vortex ring is formed from the wake vorticity. Again, secondary vorticity from the wall and particle plays a role. At higher Reynolds number, the secondary vorticity tends to form a semi-annular structure encircling the primary vortex core. At even higher Reynolds numbers, the secondary annular structure fragments into semi-discrete structures, which again encircle and orbit the primary core. Vorticity fields and passive tracer particles are used to characterize the interaction of the vortical structures. The evolution of the pressure and viscous drag coefficients during a collision are provided for a typical sphere impact. For a Reynolds number greater than approximately 1000 for a sphere and 400 for a cylinder, the primary vortex core produced by the impacting body undergoes a short-wavelength instability in the azimuthal/spanwise direction. Experimental visualisation using dye carried out in water is presented to validate the predictions. [Copyright &y& Elsevier]

A reinterpretation is made of previously published data concerning the frequency of the instability waves in the separated shear layer from a circular cylinder for Reynolds numbers in the range 103–105. An accurate fit to the observed variation can be achieved using a piecewise fit based on theoretical and empirical arguments. A logical conclusion is that the ratio of the frequency of the instability waves to the Kármán vortex shedding frequency is indeed determined by the boundary-layer properties at separation, as suggested by Bloor. [ABSTRACT FROM AUTHOR]

The vortex structures in the wake of 2D elliptical cylinders at low Reynolds numbers are investigated for a Reynolds numbers range of 75 to 175 using direct numerical simulation. By varying the aspect ratio of an elliptical cylinder, the geometry is varied between the extremes of a circular cylinder and a flat plate normal to the flow. The power spectrum analysis of the vertical velocity along the horizontal centreline shows the presence of secondary and tertiary frequencies in the far region of the wake. As the Reynolds number is increased and/or the aspect ratio is decreased, the lower frequencies in the far wake become more dominant and their inception point occurs closer to the elliptical cylinder. This research suggests that the low frequency unsteadiness behind normal flat plates does not need to be due to the vortex interaction, but, rather, can result from the presence of a two-dimensional instability of the mean wake. [Copyright &y& Elsevier]

Following previous experimental and computational studies, this article further investigates the applicability of the Stuart–Landau equation to describe the Hopf bifurcation occurring for flow past a circular cylinder. It is shown that when the amplitude variable is taken as the transverse velocity component at a point in the wake, the so-called Landau constant varies considerably with position and importantly is generally far from constant during the saturation phase of wake development. The variation with downstream distance is quantified. However, it is found that the Landau constant at saturation is indeed a position-independent constant and this value is close to that generally measured previously both experimentally and numerically. It is shown that if the amplitude variable is taken as the lift coefficient of the cylinder (a global variable) then the same Landau constant is measured at saturation and the zero amplitude Landau constant corresponds to that for the transverse velocity at the back of the cylinder. These findings are used to interpret the wake behaviour of a transversely oscillating circular at subcritical Reynolds numbers. [Copyright &y& Elsevier]

This study investigates the flow behavior over roughened inline cylinders for postcritical flow, a parameter space with relatively little prior scrutiny. Two cylinders of the same relative surface roughness, k s / D = 1.9 × 10 − 3 , separated by a pitch (i.e., L, distance between the centers of two cylinders) between 1.175 ≤ L / D ≤ 10 are studied at Reynolds numbers from 3 × 10 5 to 6 × 10 5 using unsteady surface pressure measurements. As pitch ratio is increased from L / D = 1.175 , CD of the downstream cylinder increases sharply at (L / D) c = 3.25. This critical pitch ratio (L / D) c is toward the lower end of the reported range for subcritical smooth cylinders. Asymmetric mean gap flow along with alternating reattachment is found for 1.5 ≤ L / D < 2.25 (i.e., two asymmetric modes in the gap, mode 1 and mode 2, that are the reflections of each other), and symmetric gap flow with a continuous reattachment is found for 2.25 < L / D ≤ 3. The gap flow is also symmetric for the closest pitch ratio tested of L / D = 1.175. While the change in upstream cylinder drag coefficient with Reynolds number broadly follows that of an isolated cylinder, for the downstream cylinder, it is approximately independent. The critical separation is also insensitive to Reynolds number within 3 × 10 5 ≤ R e ≤ 6 × 10 5 . Transitions between the reattachment and the co-shedding flow are predominantly continuous over the spanwise planes tested. On the other hand, alternating reattachment occurs in spanwise cells, where one sectional measurement exhibits the asymmetric mode 1 while a spanwise-adjacent section exhibits the asymmetric mode 2 or even symmetric flow. Previously reported maxima in the fluctuating lift and drag coefficients of the downstream cylinder at L / D ≈ 2.4 at subcritical Reynolds numbers are absent in the current investigation. [ABSTRACT FROM AUTHOR]

Rotating bluff bodies are known to experience a side force due to a well-known phenomenon called the Magnus effect. The Magnus effect is evident in many sports (cricket, soccer, baseball, and football), where spinning balls are observed to change their trajectories dramatically. In this paper, we have experimentally investigated the effect of imposed transverse rotation on flow past a sphere in the intermediate Reynolds number range (1 0 3 < Re ≲ 5 × 1 0 4 ). The results show the considerable effects of Reynolds number and the rotation rate on the force coefficients. A relatively sudden drop was observed in both the lift and drag coefficients at a 'critical' rotation rate, before a recovery at higher values, similar to but lower in magnitude than that observed near the drag crisis Reynolds number range (R e ≈ 3 × 1 0 5 ). The change in lift was found to be correlated with the change in the separation angle on the advancing side of the sphere, which occurs when the boundary layer flow becomes unsteady and turbulent, presumably as a result of the shear-layer instability observed for rotating spheres. The situation appears to be similar at lower Reynolds number (R e < 1000), with early transition to boundary layer turbulence causing the advancing side separation angle to increase substantially at a critical rotation rate. However, the boundary layer thickness is considerably thicker at lower Reynolds number, which presumably limits its ability to remain attached to high angles thus limiting the magnitude of the effect. Of interest, the shear-layer instability that develops as the rotation rate is increased manifests even at very low Reynolds numbers, suggesting that the origins of the inverse Magnus effect may be observable at even lower Reynolds numbers than those studied here. • Investigated flow past transversely rotating spheres at intermediate Reynolds numbers. • Force coefficients are strongly related to Reynolds number and rotation rate. • Noted abrupt drop in lift and drag coefficients at a critical rotation rate. • Lift correlated with advancing side flow separation angle variation. [ABSTRACT FROM AUTHOR]

We experimentally investigated the potential of a trip wire as a passive control mechanism for controlling flow-induced vibration (FIV) of a three-dimensional bluff body. The effect of a surface trip wire was investigated for varying diameter ratio in the range 1. 3 × 1 0 − 2 ⩽ k / D ⩽ 6. 7 × 1 0 − 2 , where k is the wire diameter and D is the sphere diameter, and trip-wire location in the range 2 0 ∘ ≤ ϕ ≤ 6 0 ∘ , where the angle ϕ was measured from the leading stagnation point. The FIV response of the sphere is characterised for a wide range of reduced velocities 3 ⩽ U ∗ ⩽ 20 , defined as U ∗ = U / f n w D , where U is the free stream velocity and f n w is the natural frequency of the system in water. It was found that for a fixed trip-wire diameter ratio, the vibration amplitude decreased progressively with increasing ϕ. Furthermore, the synchronisation regime became narrower, and the mode II and the plateau branches of the FIV response occurred for lower reduced velocities, compared to those for a smooth sphere. The control was highly effective for high reduced velocities ( U ∗ > 10) with a maximum reduction of up to 97.8% for ϕ = 6 0 ∘ . Interestingly, thicker trip wires (k / D > 1. 3 × 1 0 − 2 ) were effective in disrupting Mode I, but induced a galloping-like response for higher reduced velocities. Tripping proved to be an effective passive control strategy for spheres, with the optimal trip-wire size varying across Mode I to Mode III. [ABSTRACT FROM AUTHOR]

While the flow-induced vibration (FIV) of a circular cylinder has been characterised in considerable detail, the FIV of elliptical cylinders has not received similar attention. This study investigates the dynamic response of elastically-mounted elliptical cylinders with various cross-sectional aspect ratios (or elliptical ratios) in a free stream. The elliptical ratio is defined by ϵ = b ∕ a , where a and b are the streamwise and cross-flow dimensions, respectively, of the cross section of a cylinder placed at zero incidence angle. The elliptical ratios tested were in the restricted range of 0. 67 ⩽ ϵ ⩽ 1. 50 , which was designed to achieve some but not too large a geometric deviation from a circular cylinder. The fluid–structure system was modelled using a low-friction air-bearing rig in conjunction with a free-surface recirculating water channel facility. This experimental set-up yielded very low mass and damping ratios. The FIV response was characterised as a function of reduced flow velocity. The results showed that the ellipses exhibited different response regimes as ϵ varied. Surprisingly, for the lowest elliptical ratio ϵ = 0. 67 , there existed two separated lock-in regimes. However, for ϵ ⩾ 0. 80 , lock-in occurred over only a single reduced velocity range that was characterised by different response regimes. As ϵ was increased above unity, lock-in tended to occur at considerably lower reduced velocities than for the circular cylinder case. Moreover, the peak vibration amplitude increased with the elliptical ratio. This means that the body vibration is enhanced, rather than attenuated, as the afterbody is reduced for an ellipse. [ABSTRACT FROM AUTHOR]

Predictions from embedded-LES are presented of a model of a section of a double-stacked freight wagon subjected to different local loading configurations. In total, 15 different upstream ( G f r o n t ) and downstream ( G b a s e ) gap spacings were simulated to characterise the change in the flow topology. The mean flow fields indicate that the inter-wagon flow undergoes a significant topology change over the range of G f r o n t = 1.77 W – 3.23 W (W = wagon width). For G f r o n t ≤ 1.77 W , the mean recirculating flow in the gap covers its entire length. In contrast, for G f r o n t ≳ 3.23 W , the complete wake closure for the upstream wagon occurs enabling the upstream shear layers to impinge on the entire downstream surface, in turn, increasing the rate of change of drag force as G f r o n t is increased. The change in the wake shedding frequency, directly affecting the base pressure and consequently the drag, due to the variation in G f r o n t and G b a s e , is presented. [ABSTRACT FROM AUTHOR]

The effect of damping on vortex-induced vibration (VIV) of a circular cylinder with a fixed mass ratio ( m ∗ = 3 . 0 ) was studied through water-channel experiments. An eddy-current-based damping mechanism was constructed to provide controlled and adjustable damping values. It consisted of a permanent magnet connected to the cylinder that moves parallel to a copper plate at some predetermined gap, which determines the damping in the system. Increased damping was found to reduce the reduced-velocity range of the upper and lower branches, thus reducing the synchronization region. As the damping is increased, the lower branch remains easy to identify from the amplitude response curves, but the boundary between the initial and upper branch becomes less clear. However, the frequency response under higher damping shows similarities to that at the lowest damping and these similarities, for the first time, were used to delimit the different response branches. The existence of the upper branch was found to continue down to A ∗ ≈ 0 . 2 D . The experimental data was assembled to plot the peak amplitude response as a function of the mass–damping parameter in a “Griffin plot”. Due to a restricted variation in Reynolds number in the experiments, the measured data shows negligible scatter compared to the assembled literature data. Three sets of experiments using different sets of springs were conducted to quantify the Reynolds number effect previously established by Govardhan and Williamson (2006). An exponential fitting function was then used to successfully fit the data on the Griffin plot. Under higher damping, it was found that the total and vortex phases are no longer at either 0°or 180°, and take intermediate values throughout the response branches. The power extracted by the damping mechanism was also calculated. Maximum power extraction occurs for a combination of optimal damping and reduced velocity. The power was also found to increase with Reynolds number, correlated with the increase in vibration amplitude. At the highest Reynolds number examined, the dimensionless energy conversion ratio is 0.2, indicating that approximately 20% of the flow energy approaching the cylinder frontal cross-section can be converted to useful electrical energy. This factor increased substantially with Reynolds number from approximately 15 to 20% over the Reynolds number range considered ( R e ∼ 1700 –5900). The fit devised for the peak vibration amplitude was extended for expressing the average extracted power as a function of mass–damping and Reynolds number. [ABSTRACT FROM AUTHOR]

Fabric deformation alters its local permeability characteristics which, in turn, influences the resin infusion process. This work investigates the radial permeability test method of sheared carbon fiber textile reinforcement materials for the purposes of infusion modeling. A novel experimental approach is described, which utilizes a customized Matlab video processing code to track fluid flow and calculate permeability using data from the complete flow field. Results show principal permeability values, K 1 and K 2, to increase and decrease, respectively, from a near isotropic state as the fabric shear angle increases, yielding a K 1/ K 2 ratio of 3.74 at the maximum measured shear angle of 40°. Detailed statistical analysis revealed significant error for cases where fewer than 16 measurement directions were used for the permeability calculation, particularly in highly anisotropic samples. POLYM. COMPOS., 39:2287–2298, 2018. © 2016 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

This study presents an assessment of the capabilities of various turbulence modelling approaches —ELES, SAS, URANS and RANS—to predict the aerodynamic flow around a double-stacked freight wagon, both in isolation and within a train. The numerical predictions are compared with experimental measurements at the same Reynolds number to determine the accuracy of each model. Specifically, aerodynamic drag, front and rear surface pressures, planar velocity fields and skin friction lines are validated against the wind tunnel results. In particular, predictions from the ELES and SAS models show good agreement with the wind tunnel data, both qualitatively and quantitatively. Indeed, ELES predicts both the range and distribution of the rear-face surface pressure very closely, indicating that the separated flow is also likely to be well predicted. Both SAS and ELES predict the pressure drag of the multi-wagon configuration to within 2% of the experimental value. However, the steady RANS model predicts the trends in pressure drag in line with the experiments as the front and rear gaps are varied, even though individual drag predictions are considerably worse. Overall, the set of results establishes the benefits and deficiencies of using particular turbulence models to capture different aspects of freight train aerodynamics. [ABSTRACT FROM AUTHOR]

The increasing demand for large, complex and low-cost composite aerostructures has motivated advances in the simulation of liquid composite moulding techniques with textile reinforcement materials. This work outlines the development and validation of a multi-physics process model that better simulates infusion behaviour through a complex preform compared with traditional models used in industry that do not account for fabric deformation. By combining the results of a preform draping model with deformation-dependent permeability properties, the shape and local flow characteristics of a deformed textile reinforcement have been more realistically defined for infusion. Simulated shear deformation results were used to define the distributed permeability properties across the fabric domain of the infusion model. Full-scale vacuum infusion experiments were conducted for a complex double dome geometry using a plain weave carbon fibre material. The multi-physics process model showed significant improvement over basic models, since it is able to account for the change in flow behaviour that results from local fabric deformation. [ABSTRACT FROM AUTHOR]

The variation of Strouhal number with Reynolds number is quantified experimentally for a series of elliptical cylinders spanning aspect ratios between Ar=1, corresponding to a circular cylinder, and Ar=0, corresponding to a flat plate, over the Reynolds number range 100<=Re<=300. The widths of the spectral peaks in Fourier space at each Reynolds number, together with changes in the shape or continuity of the Strouhal number curves, provide information of underlying three-dimensional transitions. Whilst modified versions of the mode A and B transitions of a circular cylinder wake occur at aspect ratios above Ar≈0.4, one major difference is observed for Ar<=0.4. In a limited range of Reynolds numbers, the wake appears to re-laminarize after it has already undergone three-dimensional transition. This flow regime is characterized by a strictly periodic vortex shedding. [ABSTRACT FROM AUTHOR]

An approach is studied for side-wall heat transfer enhancement in the magnetohydrodynamic flow of fluid in a rectangular duct that is damped by a strong transverse magnetic field. The mechanism employs the rotational oscillation of a cylinder placed inside the duct to encourage vortex shedding, which promotes the mixing of fluid near a hot duct wall with cooler fluid in the interior. The effectiveness of the heat transfer enhancement is investigated over a wide range of oscillation amplitudes and forcing frequencies. The motivation for exploring this mechanism is inspired by the transient growth response of this flow, which indicates that the optimal disturbances feeding the vortex shedding process are localized near the cylinder, and are characterized by an asymmetrical disturbance with respect to the wake centreline. The results show that a considerable increase in heat transfer from the heated channel wall due to rotational oscillation of the cylinder can be achieved, with the maximum enhancement of more than 30% over a zone extending 10d downstream of the cylinder. As the angular velocity amplitude of oscillation is increased, the range of oscillation frequencies for effective enhancement is widened, and the frequency at which the peak Nusselt number occurs is shifted slightly to lower frequencies. As the amplitude is increased, the formation of strong discrete wake vortices draws fluid from the wall boundary layers into the wake, enhancing heat transfer. The effect of oscillation amplitude on the distribution of local Nusselt number Nuw along the heated wall is significant. With an increase in Reynolds number, scope for additional heat transfer enhancement is possible. [ABSTRACT FROM AUTHOR]

For moderate Reynolds numbers, a sphere striking a wall in the normal direction leads to the trailing recirculating wake threading over the sphere and developing into a complex vortex ring system as it interacts with the wall. The primary vortex ring, which consists of vorticity from the wake, persists and convects slowly outwards away from the sphere due to the motion induced from its image. The structure and evolution of this vortex system is quantified through a combined experimental and numerical study. At higher Reynolds numbers a non-axisymmetric instability develops, which leads to rapid dispersion of the ring system. A comparison of the wavelength and growth rate, predicted from both linear stability theory and direct simulations, with idealized models indicates that the mechanism is dominated by a centrifugal instability at the edge of the primary vortex core. [ABSTRACT FROM AUTHOR]

Research in cycling aerodynamics is performed using mannequins of different geometries, which are usually not shared, thus hampering the advancement of our understanding of the flow around a rider on the bike. The primary outcome of this work is to introduce and openly share two anthropometrically realistic generic cyclist models, one in time-trial and one in sprint position. These two models are obtained by averaging the scans of 14 male elite cyclists. The average cyclist geometries are published and openly accessible, making them unique in the field of cycling aerodynamic research. The second objective of this work is to better understand how the difference between the sprint and time-trial position affects the velocity and vortex topology in the wake of a cyclist and, in turn, the aerodynamic drag. Robotic volumetric particle image velocimetry measures the time-average velocity for each mannequin within a wind tunnel. One meter downstream of the lower back, the wakes of the two mannequins are dominated by strong hip/thigh streamwise counter-rotating vortices, which induce a downwash behind the riders' backs. The strength of these vortices downstream of the sprint model is significantly larger than that of the vortices of the mannequin in the time-trial position. The same holds for a secondary vortex pair that originates from the upper arms and hips. In addition to the vortex strength, the aerodynamic drag area of the sprint model exceeds that of the time-trial model. Hence, it is presumed that stronger vortices relate to higher aerodynamic drag. In contrast to the drag area, the drag coefficient of the two models is the same. Further research is necessary to understand the relation between the cyclist position, the flow topology and the drag coefficient. Finally, the flow around the time-trial model is described in further detail to understand the origin of the different vortex structures. Through comparison to the literature, a vortex topology classification is postulated for the mid-wake and upper-wake. The arm spacing and shoulder width play a critical role in the development of this vortex system. • ·Two anthropometrically realistic generic cyclist models are introduced: A time-trial and a sprint model. • The models are openly accessible, making them unique in the field of cycling aerodynamic research. • The flow around the wind tunnel models, obtained through robotic volumetric particle image velocimetry, is published as well. • The arm spacing and shoulder width play a critical role in the development of the streamwise vortex system around a cyclist. • Stronger streamwise vortices seem to be related to higher aerodynamic drag. [ABSTRACT FROM AUTHOR]

In this wind-tunnel based experimental study, the flow topology of the near wake of a generic anatomically accurate model cyclist is mapped for a range of reduced pedalling frequencies. Wake flow fields for both static leg and pedalling cyclists are compared over the full 360° rotation of the crank using both time- and phase-averaging. The primary wake flow structures and aerodynamic forces are quantified and analysed under dynamic pedalling conditions representative of an elite-level time-trial cyclist. Over the range of reduced pedalling frequencies studied, only minor variation was detected between the instantaneous drag and primary vortical structures of a pedalling cyclist compared to a stationary cyclist with the pedals in the same position. A simplified model of the aerodynamic forces acting on the legs under motion is presented to provide insight into how the motion of the legs influences aerodynamic drag. A comparison of predicted forces from this model with those from experiments provides a new perspective on how the aerodynamics of cyclists may be optimised. [ABSTRACT FROM AUTHOR]

The validity of the local thermal equilibrium assumption in the free convective steady flow over a circular cylinder heated at constant temperature and embedded in a packed bed of spheres is investigated numerically. For this purpose, the Forchheimer – Brinkman - extended Darcy momentum model and the local thermal non-equilibrium energy model are solved numerically using a spectral element method. Numerical solutions obtained over broad ranges of representative dimensionless parameters, i.e. Rayleigh number (10 3 ≤ Ra ≤ 5 × 10 7 ), solid-to-fluid thermal conductivity ratio (0.1 ≤ kr ≤ 10 4 ), cylinder-to-particle diameter ratio (20 ≤ D / d ≤ 100), porosity (0.3 ≤ ε ≤ 0.7), are used to present conditions at which the local thermal equilibrium assumption can or cannot be employed. This study yields that the local thermal equilibrium LTE assumption is true at lower Rayleigh number, higher solid-to-fluid thermal conductivity ratio, higher cylinder-to-particle diameter ratio and lower porosity. In addition, it is found that the solid conductivity has the significant influence on satisfying the LTE , where it is completely satisfied in the system for all values of flow and structural parameters, i.e. Ra , D / d and ε , at higher kr , and vice versa, it cannot be satisfied at lower kr , for the whole ranges of other parameters. [ABSTRACT FROM AUTHOR]

Abstract: The validity of the local thermal equilibrium assumption in the forced convection steady and pulsatile flows over a circular cylinder heated at constant temperature and embedded in a horizontal porous channel is investigated. For this purpose, the Darcy–Brinkman–Forchheimer momentum and the local thermal non-equilibrium energy models are solved numerically using a spectral element method. Numerical solutions obtained over broad ranges of representative dimensionless parameters are utilized to present conditions at which the local thermal equilibrium assumption can or cannot be employed. For steady flow, the circumstances of a higher Reynolds number, a higher Prandtl number, a lower Darcy number, a lower microscopic and macroscopic frictional flow resistance coefficient, a lower Biot number, a lower solid-to-fluid thermal conductivity ratio, a lower cylinder-to-particle diameter ratio and a lower porosity, are identified as unfavorable circumstances for the local thermal equilibrium LTE condition to hold. For oscillatory flow, the degree of non-equilibrium can be decreased as pulsating amplitude increases or pulsating frequency decreases; however, such flows do not have a significant influence on satisfying the LTE. [Copyright &y& Elsevier]

Abstract: The experimental research reported here employs particle image velocimetry to extend the study of , recording detailed vorticity fields in the near-wake of a circular cylinder undergoing combined translational and rotational oscillatory motions. The focus of the present study is to examine the effect of the ratio between the cross-stream translational and rotational velocities and frequencies on the synchronization of the near-wake structures for multiple phase differences between the two motions. The frequencies are fixed close to that of the natural frequency of vortex shedding. The results are presented for a fixed amplitude of rotational oscillation of 1rad and a range of ratios between the translational and rotational velocities and for a range of frequency ratios . In particular, it was found that varying the V R value changed the near-wake structure. The results show that at the lower value of , for all of the phase differences examined, the vortices are shed in a single-row 2S mode aligned in the medial plane with a slight offset from the centreline and also synchronized with the combined oscillatory motion. As V R increases the vortex shedding mode changes from a 2S single-row to a 2S double-row structure and eventually back to the single-row (at . Increasing V R further resulted in the loss of lock-on over the range of negative phase angles and a transition from the 2S to P+S mode for the in-phase case. There was transition back to the 2S wake mode with a further decrease in . For higher V R the range of desynchronization increased. In the second and third parts of this paper it is shown that the occurrence of unlocked wake flow as the phase angle is varied is greater when the frequency ratio between the imposed oscillatory motions and the natural vortex shedding frequency is higher than unity. The vortices are synchronized in the near-wake at values less than unity and unlocked when . In particular, the near-wake structures have also been shown to be synchronized for and unlocked for . [Copyright &y& Elsevier]

Abstract: Time-dependent forced convection heat transfer from a single circular cylinder embedded in a horizontal packed bed of spherical particles under local thermal non-equilibrium condition is investigated numerically using the spectral-element method. The non-Darcian effects, i.e. inclusion of the effect of solid boundaries and inertia forces, and the effect of thermal dispersion, are taken into account. The influences of the presence of the porous material and its thermal properties: solid-to-fluid thermal conductivity ratio and Biot number , on the rates of heat transfer and the hydrodynamic and thermal responses, are examined for the Reynolds number range . These effects are quantified. Perhaps not surprisingly, the results show that the presence of the porous particles suppresses significantly the wakes behind the cylinder and enhances considerably the heat transfer. A comparison that is made between the one- and two-equation energy model predictions shows that the former model predicts a continuous increase in Nuf against kr ; however, the trend of Nuf with kr , for kr  > 10, is governed entirely by Bi when the latter model is used. Also, the increase in Bi decreases Nuf and increases Nus , and high values of kr or Bi lead to establishing a thermal equilibrium status in the porous bed. [Copyright &y& Elsevier]

Abstract: An investigation of the flow resulting from the collision of two spheres at low Reynolds numbers is presented. Each sphere starts from rest and traverses a distance of 5 sphere diameters to the point of contact. Experimental and numerical results are compared for a symmetric collision; that is, a collision between two spheres of the same diameter and travelling with the same velocity. The flow consists of two axisymmetric recirculation zones which become a pair of colliding vortex rings, expanding radially from the collision point. Several examples of unbalanced collisions are also presented numerically, with one or both of the velocity and diameter of the spheres altered. These collisions break the symmetry, altering the post-collision expansion of the vortex rings. [Copyright &y& Elsevier]

Abstract: A numerical investigation of the flow past a circular cylinder centred in a two-dimensional channel of varying width is presented. For low Reynolds numbers, the flow is steady. For higher Reynolds numbers, vortices begin to shed periodically from the cylinder. In general, the Strouhal frequency of the shedding vortices increases with blockage ratio. In addition, a two-dimensional instability of the periodic vortex shedding is found, both empirically and by means of a Floquet stability analysis. The instability leads to a beating behaviour in the lift and drag coefficients of the cylinder, which occurs at a Reynolds number higher than the critical Reynolds number for the three-dimensional mode A-type instability, but lower than a Reynolds number for any mode B-type instability. [Copyright &y& Elsevier]

The response of unconfined swirling jets undergoing vortex breakdown to axial pulsing is explored experimentally. In particular, the combination of particle visualization and particle image velocimetry has enabled the effect of axial pulsing on the development of shear-layer flow structures and vortex breakdown to be quantified, as a function of both pulsing frequency and amplitude. A range of Reynolds and swirl numbers is considered. A key result is that application of low-level forcing at the natural frequency intensifies the shear-layer vortices considerably and results in a substantial relative downstream shift of the mean breakdown position. [ABSTRACT FROM AUTHOR]

A comprehensive two-dimensional numerical investigation has been undertaken to calculate the energetic cost of propulsion and the various flow transitions of a fish-like body undulation mechanism based on a National Advisory Committee for Aeronautics 0012 hydrofoil. This covers a wide range of Strouhal (0 ≤ S t ≤ 1.4) and Reynolds (500 ≤ R e ≤ 5000) numbers from simulations based on a level-set function immersed-interface method. It is found that the time-averaged thrust coefficient displays a quadratic relationship with increasing St, and increases significantly with Re. Additionally, the time-averaged input power coefficient exhibits a cubic dependence with increasing St but is independent of Re. Both St dependences agree with those previously observed experimentally and numerically for an oscillating foil; however, for similar ranges of governing parameters, comparisons suggest that the body undulation mechanism possesses a higher propulsive efficiency. The S t ∝ R e − 0.19 scaling for the drag-to-thrust transition is consistent with that found for a wide variety of fish and birds in nature. Interestingly, for cases with an undulation wave-speed below the free-stream speed, the time-averaged drag coefficient is found to be higher than that of a stationary hydrofoil at the same Re. Furthermore, the time-averaged input power coefficient is negative, indicating the potential for the undulation mechanism to extract energy from the free-stream. Eight different wake patterns/transitions are documented for the parameter space; these have been assembled into a wake-regime parameter-space map. The present findings should aid in predicting and understanding different hydrodynamic forces and wake patterns for undulating kinematics. [ABSTRACT FROM AUTHOR]