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Laminar to transitional wakes occur in slow, quasi-steady flows past cylinders at low cylinder Reynolds numbers (R ed ≤ 250). Inviscid numerical solvers of the depth-averaged shallow water equations (SWE) introduce numerical dissipation that, depending on Red , may imitate the mechanisms of viscous turbulent models. However, the numerical dissipation rate in a second-order finite volume (FV2) SWE solver is so large at a practical resolution that this can instead hide these mechanisms. The extra numerical complexity of the second-order discontinuous Galerkin (DG2) SWE solver results in a lower dissipation rate, making it a potential alternative to the FV2 solver to reproduce cylinder wakes. This paper compares the DG2 and FV2 solvers, initially for wake formation behind one cylinder. The findings confirm that DG2 can reproduce the expected wake formations, which FV2 fails to capture, even at a 10-fold finer resolution. It is further demonstrated that DG2 is capable of reproducing key features of the flow fields observed in a laboratory random cylinder array. [ABSTRACT FROM AUTHOR]

Oscillatory flows past two identical circular cylinders are investigated by two-dimensional direct numerical simulations in the parameter space of gap ratio (0.5 ≤ G ≤ 4.0), angle of flow incidence (0° ≤ α ≤ 90°) and Keulegan–Carpenter number (4 ≤ KC ≤ 12) with a constant Reynolds number Re = 150, where G = L/D, KC = UmT/D and Re = UmD/υ with D being the dimeter of the identical cylinders, L the shortest surface-to-surface distance between the two cylinders, Um and T being the velocity amplitude and period of the sinusoidal oscillatory flow, respectively, and α is defined as the angle between the flow direction to the line connecting the centers of the two cylinders. Comparing with the tandem or side-by-side arrangements of twin circular cylinders in oscillatory flows, the staggered twin cylinders (0° < α < 90°) involve more diverse flow regimes, including the periodic, quasi-periodic and chaotic flow states, due to the inherent asymmetric flow interference around the cylinder pair. In addition to introducing four flow regimes for the tandem and side-by-side arrangements, this study newly identifies 11 flow regimes for the staggered twin cylinders. The newly reported flow regimes in this work are collaboratively identified through the flow visualizations, steady streaming, frequency spectra of fluid forces and Lissajous phase diagrams, as well as the temporal-spatial symmetry features of the wake flows. Connecting with the previous work by Zhao and Cheng ["Two-dimensional numerical study of vortex shedding regimes of oscillatory flow past two circular cylinders in side-by-side and tandem arrangements at low Reynolds numbers," J. Fluid Mech. 751, 1–37 (2014)], this study presents overall regime maps in the KC-α plane for varied gap ratios. It is found that the flow regimes previously and presently identified for the tandem and side-by-side arrangements may also appear for the staggered twin cylinders. The present numerical results suggest the sensitive dependence of the flow regimes on the parameters of KC, α, and G. It is also found that a specific flow regime with narrow parameter bands may appear within another flow regime, forming the abnormal regime hole in the regime map. To understand the profound influence of the control parameters on the flow regime transition, and the relevant temporal-spatial symmetry breaking, the linear Floquet stability analysis is conducted in this work. It was confirmed that the variation of the KC number may result in the Ky symmetry breaking over several periodic flow regimes, while the change of the angle of flow incidence may account for the H2 symmetry covering various periodic and quasi-periodic flow regimes. The stability analysis also explains the temporal flow transition and the abnormal occurrence of the regime holes within either quasi-periodic or chaotic flow regimes. [ABSTRACT FROM AUTHOR]

The proposed work focusses on the 2D design and modelling of integrated microcantilever in microchannel for the analysis of fluid pressure. In order to compute the microfluidic pressure in microchannel at various angles, fluid structure interaction is analyzed using the finite element method. With a fluid flow rate of 4.33 cm/s, a 2D integrated microcantilever can optimize both fluid pressure and microcantilever deflection. The novelty of the microfluidic pressure sensing mechanism allows pressure of fluid to be sensed in a microchannel without connecting any electrical method such as piezoresistor or piezoelectric approach. The objective of the research is to integrate a microcantilever into a microchannel to reduce setup complexity and procedure cost. Maximum deflection of the 2D T-microcantilever achieved 10.30µm at, pressure at the tip of T-microcantilever 10.89 Pa, fluid velocity 0.00309 m/sec, and Reynolds number is 1.22 [ABSTRACT FROM AUTHOR]

The enhancement of flow induced vibration of a circular cylinder by a rotating control rod is investigated through two-dimensional numerical simulations. The Reynolds number, diameter ratio, and gap ratio are 150, 0.2, and 0.2, respectively. Simulations are conducted for two rod position angles of β = 90° and 135°, rotation rates ranging from 0 to 6, and reduced velocities ranging between 1 and 20. The response of the cylinder–rod system at the rotation rates 0 and 1 has a lock-in regime where the vibration amplitude is high and the vibration frequency stops increasing with the increase in reduced velocity linearly. For rotation rates exceeding 2, the response amplitude increases with the increase in reduced velocity and enters the lock-in regime at the lower boundary reduced velocity. It remains high until the largest studied reduced velocity of 20; as a result, the higher boundary reduced velocity of the lock-in regime cannot be determined. The vibration with large amplitudes and large rotation rates repeats cyclically after every two or more vibration periods. As a result, two combined wake modes are found: 2S/P + S and 2P/P + S. In a combined mode, the vibration changes from one mode to another within each cycle. The cylinder receives power from the fluid, and the rotating rod gives power to the fluid although the net power exchange between the whole system and the fluid is zero. [ABSTRACT FROM AUTHOR]

A two-dimensional stabilized space-time finite-element-based in-house solver is used to explore flow around an elastically mounted rigid diamond oscillator undergoing undamped vortex-induced vibrations in transverse and stream-wise directions simultaneously. The computations are carried over a reduced velocity ( U * ) range of 1–12 keeping the Reynolds number (Re) and the mass ratio ( m * ) fixed at 100 and 10, respectively. In the current investigation, a five-branch response behavior with the presence of an "upper branch" is identified, which is absent for transverse-only oscillations. A shift in normalized time-averaged phase difference (ϕ / π) from 0 to 1 is related to the transition from the upper to the lower branch. The non-dimensional oscillation frequencies in both streamwise and cross-stream directions (Fx and Fy, respectively) collapse on the upper branch, matching the oscillator's non-dimensional natural frequency (FN). This leads to periodic single-looped "raindrop-shaped" cylinder trajectories. On this branch, the vortex-shedding mode is asymmetric, and the mean lift is non-zero ( C l avg ≠ 0). For all other response branches, C l avg ≈ 0 and the vortex-shedding modes are symmetric. The presence of multiple frequencies in the in-line oscillations makes the initial branch quasi-periodic, whereas all other response branches are periodic. The addition of in-line motion modifies the fluid loading on the oscillator significantly. [ABSTRACT FROM AUTHOR]

The cross-stream flow-induced vibrations of a square cylinder of mass ratio, m * = 10 , are studied numerically at a fixed Reynolds number, Re, of 250. The reduced speed, U * , is varied from 1 to 10 independent of Re. The flow-induced vibrations of a square cylinder have been previously investigated either by decoupling U * from Re or by coupling U * with Re. While most of the studies available in the literature follow the former approach, those dealing with m * = 10 do not provide a detailed account of the branches of dynamic response, hysteresis, and wake patterns. The current effort aims at contributing to these research gaps. The vibrations are purely vortex-induced, and the dynamic response within synchronization is found to be composed of an initial branch, its extension or extended initial branch, and lower branch. For a square-section oscillator, the extended initial branch is resolved for the first time. The most noteworthy outcome of this work is perhaps the resolution of asymmetric as well as one-sided wake modes at certain reduced speeds. The one-sided shedding occurs either from the top or bottom surface of the oscillator. At U * = 4.6 , the non-zero mean lift changes sign in successive oscillation cycles, indicating that the solutions are bistable. The resolved asymmetric and one-sided modes are associated with positive and negatives values of mean lift, respectively. A very interesting result of this study is the mismatch of wake modes obtained at non-hysteretic U * = 4.6 using forward and backward computations. [ABSTRACT FROM AUTHOR]

The steady separated flow past a diamond cylinder at low Reynolds numbers, Re, is associated with diverse separation topologies not resolved for a circular or square cylinder. The present study, conducted for R e ≤ 150 , also uncovers three unique separation topologies for the time-averaged flow. In this regard, the most striking observation is the formation of a small sub-wake around the base of the cylinder at certain Re between 80 and 90. While two of these structures were previously captured by some recent studies, these studies did not investigate their origin or kinematics. In the present study, conducted via stabilized finite-element computations in two-dimensions, these wake topologies are analyzed in detail. For secondary separation, the pressure at the reattachment point on the cylinder rear surface exceeds the pressure at the corresponding separation point located upstream. In a similar manner, for primary separation, the pressure at the wake stagnation point(s) surpasses the one at the separation points. Via direct steady and unsteady computations, the value of the critical Re indicating the onset of vortex-shedding is found to be 41, approximately. At this Re, the flow quantities, such as wake length, drag, and pressure coefficients, obtained from the steady and unsteady computations are found to diverge. The presentation of wake length and drag as a function of base suction establishes that the drag and wake length share an inverse relationship. [ABSTRACT FROM AUTHOR]

Flow-induced bidirectional vibrations of two in-line square cylinders of same size and mass ratio (= 10) are analyzed at Reynolds number, Re = 100. The cylinders are located in the co-shedding regime, i.e., they are separated by a normalized center-to-center spacing of 5. The reduced speed, U * , is varied from 3 to 15 keeping Re constant. The upstream and downstream cylinders display identical frequency characteristics with U * . Accordingly, the cylinders share identical decomposition of dynamic response. The response is composed of the desynchronization regimes and lower branch; an initial branch does not exist. The vibrations are hysteretic at the lock-in boundaries whereas for a single square oscillator, hysteresis is identified only near the onset of lock-in. Hysteresis in the solutions for U * = 7 within the lower branch is reflected in the wake mode of the rear cylinder whereas for the upstream cylinder, wake mode remains identical. The vortex-induced vibration (VIV) of the upstream cylinder is qualitatively similar to that of an isolated square oscillator. Despite the range of lock-in of the cylinders being identical, the VIV of the downstream cylinder departs significantly from its upstream counterpart. The shear layers separated from the front cylinder impinge on the rear cylinder and alter its flow field. The rear cylinder executes high amplitude VIV over the entire lower branch. The symmetry of the drag-lift phase diagrams does not necessarily translate to symmetric phase plots of in-line and cross-stream response. The drag and in-line response of the cylinders are out of phase throughout. [ABSTRACT FROM AUTHOR]

We analyze a suspension of deformable particles in a pressure-driven flow. The suspension is composed of neutrally buoyant initially spherical particles and a Newtonian carrier fluid, and the flow is solved by means of direct numerical simulations, using a fully Eulerian method based on a one-continuum formulation. The solid phase is modeled with an incompressible viscous hyperelastic constitutive relation, and the flow is characterized by three main dimensionless parameters, namely the solid volume fraction, the Reynolds and capillary numbers. The dependency of the effective viscosity on these three quantities is investigated to study the inertial effects on a suspension of deformable particles. It can be observed that the suspension has a shear-thinning behavior, and the reduction in effective viscosity for high shear rates is emphasized in denser configurations. The separate analysis of the Reynolds and capillary numbers reveal that the effective viscosity depends more on the capillary than on the Reynolds number. In addition, our simulations exhibit a consistent tendency for deformable particles to move toward the center of the channel, where the shear rate is low. This phenomenon is particularly marked for very dilute suspensions, where a whole region near the wall is empty of particles. Furthermore, when the volume fraction is increased this near-wall region is gradually occupied, because of higher mutual particle interactions. Deformability also plays an important role in the process. Indeed, at high capillary numbers, particles are more sensitive to shear rate variations and can modify their shape more easily to accommodate a greater number of particles in the central region of the channel. Finally, the total stress budgets show that the relative particle-induced stress contribution increases with the volume fraction and Reynolds number, and decreases with the particle deformability. [ABSTRACT FROM AUTHOR]

The flow-induced vibrations of an elastically mounted circular cylinder, free to oscillate in the streamwise and cross-flow directions, and forced to rotate about its axis, are investigated via two- and three-dimensional simulations. The Reynolds number based on the body diameter and inflow velocity is equal to $100$. The impact of the imposed rotation on the flow–structure system behaviour is explored over wide ranges of values of the rotation rate (ratio between the cylinder surface and inflow velocities, $\unicode[STIX]{x1D6FC}\in [0,5.5]$) and of the reduced velocity (inverse of the oscillator natural frequency non-dimensionalized by the inflow velocity and body diameter, $U^{\star }\in [1,25]$). Flow-induced vibrations are found to develop over the entire range of $\unicode[STIX]{x1D6FC}$ , including in the intervals where the imposed rotation cancels flow unsteadiness when the body is rigidly mounted (i.e. not allowed to translate). The responses of the two-degree-of-freedom oscillator substantially depart from their one-degree-of-freedom counterparts. Up to a rotation rate close to $2$ , the body exhibits oscillations comparable to the vortex-induced vibrations usually reported for a non-rotating circular cylinder: they develop under flow–body synchronization and their amplitudes present bell-shaped evolutions as functions of $U^{\star }$. They are, however, enhanced by the rotation as they can reach $1$ body diameter in each direction, which represents twice the peak amplitude of cross-flow response for $\unicode[STIX]{x1D6FC}=0$. The symmetry breaking due to the rotation results in deviations from the typical figure-eight orbits. The flow remains close to that observed in the rigidly mounted body case, i.e. two-dimensional with two spanwise vortices shed per cycle. Beyond $\unicode[STIX]{x1D6FC}=2$ , the structural responses resemble the galloping oscillations generally encountered for non-axisymmetric bodies, with amplitudes growing unboundedly with $U^{\star }$. The response growth rate increases with $\unicode[STIX]{x1D6FC}$ and amplitudes larger than $20$ diameters are observed. The cylinder describes, at low frequencies, elliptical orbits oriented in the opposite sense compared to the imposed rotation. The emergence of subharmonic components of body displacements, leading to period doubling or quadrupling, induces slight variations about this canonical shape. These responses are not predicted by a quasi-steady modelling of fluid forcing, i.e. based on the evolution of the mean flow at each step of body motion; this suggests that the interaction with flow unsteadiness cannot be neglected. It is shown that flow–body synchronization persists, which is not expected for galloping oscillations. Within this region of the parameter space, the flow undergoes a major reconfiguration. A myriad of novel spatio-temporal structures arise with up to $20$ vortices formed per cycle. The flow three-dimensional transition occurs down to $\unicode[STIX]{x1D6FC}\approx 2$ , versus $3.7$ for the rigidly mounted body. It is, however, shown that it has only a limited influence on the system behaviour. [ABSTRACT FROM AUTHOR]

The Den Hartog stability criterion tests for galloping of an oscillator. A square cylinder satisfies this and is susceptible to galloping. This criterion being necessary for occurrence of galloping appears insufficient as certain parameters, i.e., angle of incidence, α; mass ratio, m*; damping ratio, ζ; reduced speed, U*; and Reynolds number, Re; also assume key roles in determining if the oscillator motion is vortex-induced vibrations (VIV) or galloping. At Re ≈ 150 and U* ≈ 10 or smaller, a square cylinder does not gallop despite satisfying the Den Hartog criterion. By coupling U* and Re, K. Sourav and S. Sen ["Transition of VIV-only motion of a square cylinder to combined VIV and galloping at low Reynolds numbers," Ocean Eng. 187, 106208-1–106208-19 (2019)] two-degrees-of-freedom motion over Re ≤ 250, the minimum m* or m tr * as 3.4 below which galloping cannot develop. For transverse-only motion, X. Li et al. ["Mode competition in galloping of a square cylinder at low Reynolds number," J. Fluid Mech. 867, 516–555 (2019)] considered U* = 40 only and determined m tr * ≈ 4 at the least permissible Re of 150. For α = 0° and ζ = 0, we determine the m tr * numerically at Re = 150. By analyzing the transverse response and oscillation frequency over an extended U* range of 10–60, a novel "VIV-galloping transition map" is generated in the m*–U* plane. From this map, the value of m tr * converges to 3.4. The m tr * decays as U * − 2.36 . The conditions leading to "VIV forever" of a square cylinder are also identified. [ABSTRACT FROM AUTHOR]

The classical resonance branches that construct the response of a freely vibrating circular or elliptic cylinder at low Reynolds numbers, Re, are "initial" and "lower." The existence of additional response branches, if any at low Re via alteration of controlling parameters, is unavailable in the literature. In this computational work, relating to a low mass ratio (m* = 1) and zero damping, i.e., m*ζ = 0 transverse-only vortex-induced vibrations of an elliptic cylinder over Re = 50–180, four response branches that are unreported in the literature are identified. The lock-in at such a low mass ratio is non-classical, and the new response branches are resolved close to the lock-in boundaries. These additional branches are designated as extended initial branch, extended lower branch, terminal branch, and quasi-periodic desynchronization branch. The method proposed by Kumar et al. ["Identification of response branches for oscillators with curved and straight contours executing VIV," Ocean Eng. 164, 616–627 (2018b)] has been employed to identify the branches by locating the Re region concerning the change of slope and discontinuous jumps of oscillation frequency. It is further shown that branching at a low mass ratio depends on structural damping, oscillator shape, and degree-of-freedom. [ABSTRACT FROM AUTHOR]

The steady separated flow past a square cylinder at 45° incidence and having a sharp/rounded base is a unique kind of separated flow around a symmetric bluff obstacle. At low Reynolds numbers (Re ≈ 6–8), this flow displays diverse features that are not known to be displayed by common symmetric bluff bodies, such as circular/square cylinders at zero incidence. The uniqueness of this flow was first highlighted by Kumar et al. ["Steady separation of flow from an inclined square cylinder with sharp and rounded base," Comput. Fluids 171, 29–40 (2018)] in their recent numerical investigation concerning the onset of initial separation. They reported detailed flow kinematics resulting from separation of the laminar boundary layer via secondary mode, primary mode, and their combination. In this work, based on stabilized finite-element computations, the fluid dynamic aspects of primary and secondary separation are delved. The parameters considered are the normalized corner radius at the cylinder base and Re. Exploration of the influence of base radius on the occurrence/absence of the secondary separation is the main objective. The occurrence of secondary separation is a function of base radius. It occurs only with small or no rounding at the base, i.e., when the pressure gradient close to the base is favorable or mildly adverse. Irrespective of the base radius, the onset of secondary separation occurs at a constant Re value of 7.3. The drag decays as Re−0.62; it becomes invariant when Re is held constant. A new nondimensional parameter η accommodating the effects of pressure recovery and radius of curvature is introduced. The drag exhibits no sensitivity to η. [ABSTRACT FROM AUTHOR]

A numerical analysis of flow around a circular cylinder oscillating in-line with a steady flow is carried out over a range of driving frequencies (fd) at relatively low amplitudes (A) and a constant Reynolds number of 175 (based on the free-stream velocity). The vortex shedding is investigated, especially when the shedding frequency (fs) synchronises with the driving frequency. A series of modes of synchronisation are presented, which are referred to as the p/q modes, where p and q are natural numbers. When a p/q mode occurs, fs is detuned to (p/q)fd, representing the shedding of p pairs of vortices over q cycles of cylinder oscillation. The p/q modes are further characterised by the periodicity of the transverse force over every q cycles of oscillation and a spatial-temporal symmetry possessed by the global wake. The synchronisation modes .p/q/ with relatively small natural numbers are less sensitive to the change of external control parameters than those with large natural numbers, while the latter is featured with a narrow space of occurrence. Although the mode of synchronisation can be almost any rational ratio (as shown for p and q smaller than 10), the probability of occurrence of synchronisation modes with q being an even number is much higher than q being an odd number, which is believed to be influenced by the natural even distribution of vortices in the wake of a stationary cylinder. [ABSTRACT FROM AUTHOR]

This paper presents a three-dimensional (3D) parallel multiple-relaxation-time lattice Boltzmann model (MRT-LBM) for Bingham plastics which overcomes numerical instabilities in the simulation of non-Newtonian fluids for the Bhatnagar-Gross-Krook (BGK) model. The MRT-LBM and several related mathematical models are briefly described. Papanastasiou's modified model is incorporated for better numerical stability. The impact of the relaxation parameters of the model is studied in detail. The MRT-LBM is then validated through a benchmark problem: a 3D steady Poiseuille flow. The results from the numerical simulations are consistent with those derived analytically which indicates that the MRT-LBM effectively simulates Bingham fluids but with better stability. A parallel MRT-LBM framework is introduced, and the parallel efficiency is tested through a simple case. The MRT-LBM is shown to be appropriate for parallel implementation and to have high efficiency. Finally, a Bingham fluid flowing past a square-based prism with a fixed sphere is simulated. It is found the drag coefficient is a function of both Reynolds number (Re) and Bingham number (Bn). These results reveal the flow behavior of Bingham plastics. [ABSTRACT FROM AUTHOR]

Even for the stationary airfoils, due to the boundary and shear layer interactions of upper and lower surface of the airfoils, alternating vortex patterns form and the flow becomes time dependent. In the current study, the unsteady behavior of the flow around a symmetric airfoil is considered as incidence angle increases. The flow patterns are presented for wide range of angles of attack values. The vortex pattern generated is analyzed numerically for different angles of attack at Re=1000 around NACA 0012 airfoil. At this Reynolds number, the flow is laminar and boundary layers are quite thick. Flow separation and unsteady vortex shedding is observed even at low angles of attack. For NACA 0012 airfoil, the unsteady vortex pattern is observed at about 8° angle of attack for Re=1000. Spectral analysis is performed for angles of attack ranging from 0° to 90°. It is presented that amplitude spectrum of lift coefficient (C1) start to shows a peak l at 8° for NACA 0012 and the aerodynamic forces presents oscillatory behavior afterwards. The effect of angle of attack to wake pattern and instantaneous and mean aerodynamic coefficients are discussed. The time-averaged streamlines, pressure and skin friction coefficients are analyzed to observe the vortex formation and separation from the airfoil upper surface at this low Reynolds number. [ABSTRACT FROM AUTHOR]

The present research deals with simulation of pulsatile flow through a tubular bulge filled with homogeneous, isotropic, saturated porous medium. For comparison, flow fields are also computed for the flow through clear medium in the same geometry. Both cases involve three-dimensional, unsteady, laminar, incompressible flow of Newtonian fluid. Flow through the porous medium is modeled via Forchheimer-Brinkman-extended Darcy model. While semi-analytical solutions are presented for the asymptotic cases, numerical techniques are used for the general solutions of the conservation equations. The governing equations, in both porous and clear media, are discretized by an implicit finite volume technique over an unstructured tetrahedral mesh. The resulting algebraic equations are solved using stabilized biconjugate gradient technique. Simulations include Darcy numbers of $$10^{-4}$$ and $$10^{-2}$$ at Reynolds numbers of 500 and 2000 based on the peak incoming average velocity and main tube diameter. The Womersley number is chosen to be 11.3 to mimic biomedical application. Results show that the porous medium naturalizes the recirculations and secondary flows while experiencing higher-pressure drop and wall shear compared to that of the clear medium. [ABSTRACT FROM AUTHOR]

The Kuroshio current-induced island wake downstream of Green Island, Taiwan, is studied using satellite imagery and a depth-averaged shallow-water model. Spatial–temporal scales, such as aspect ratio, dimensionless width, and Strouhal number, as well as propagation speed of the vortices, are quantified for Reynolds numbers 100, 200, and 500, respectively. It is found that the aspect ratio is between 1.76 and 2.72 obtained from satellite images and between 3.50 and 3.72 from numerical simulations. The dimensionless width is in the range of 1.84 to 1.92 from satellite images, and 1.29 to 1.33 from numerical modelling. The Strouhal number is between 0.17 and 0.20 from numerical simulations. Computed results are compared with theory, available satellite imagery, andin situmeasurements, and show reasonable agreement. The magnitude of the propagation speed of the vortices is found to be of the same order as that of the Kuroshio, implying non-linearity as well as interactions of vortices, and the Kuroshio is strong. The Coriolis force affects the free-surface distribution, but its effect on vortices and flow field is insignificant. [ABSTRACT FROM AUTHOR]

The origin of the secondary vortex street, observed in the far wake in the flow past a circular cylinder, is investigated. The Reynolds number, based on the diameter of the cylinder, is 150. The von Kármán vortex street, which originates in the near wake, decays exponentially downstream of the cylinder. Beyond the region of decay, a broad band of frequencies are selectively amplified, leading to the formation of a secondary vortex street consisting of packets of large-scale vortex structures. The streamwise location of the onset of the instability, frequency of the generation of packets and their convection speed are estimated via direct numerical simulation (DNS). Global linear stability analysis of the time-averaged flow reveals the presence of unstable convective modes that travel at almost the same speed and have a structure similar to the packet-like disturbances as observed in the DNS. Sensitivity analysis of the global convective modes to structural perturbations is carried out to locate the region of the wake that is most significant in generating the modes responsible for the appearance of the secondary vortex street. This information is utilized to control the flow. By placing a ‘slip’ splitter plate along the wake centre line, in the overlap region of the direct and the adjoint modes, the oscillations in the far wake are significantly reduced, though the oscillations related to the primary vortex shedding in the near wake are not. It is also found that suppression of the primary vortex shedding leads to annihilation of the secondary vortex street as well. Linear stability analysis of the steady-state flow does not yield any modes that can explain the appearance of the secondary vortex street. The steady and time-averaged wake profiles, for the $\mathit{Re}= 150$ flow, are compared to bring out the differences in the two. The effect of free-stream oscillations on the evolution of the secondary vortex street is investigated. By reducing the amplitude of inlet excitation, a gradual transition from ordered shedding in the far wake to the appearance of a broad-band spectrum of frequencies, as in the unforced wake, is observed. All the computations have been carried out using a stabilized finite element method. [ABSTRACT FROM PUBLISHER]

This paper investigates the vortex-induced vibrations (VIV) of a spring-mounted circular cylinder. We compute analytically the leading-order equations describing the nonlinear interaction of the fluid and structure modes by carrying out an asymptotic analysis of the Navier–Stokes equations close to the threshold of instability of the fluid-only system. We show that vortex-shedding can occur at subcritical Reynolds numbers as a result of the coupled system being linearly unstable to the structure mode. We also show that resonance occurs when the frequency of the nonlinear limit cycle matches the natural frequency of the cylinder, the displacement being then in phase with the flow-induced lift fluctuations. Using an extension of this model meant to encompass the effect of the low-order added-mass and damping forces induced by the displaced fluid, we show that the amount of energy that can be extracted from the flow can be optimized by an appropriate choice of the structural parameters. Finally, we suggest a possible connection between the present ‘exact’ model and the empirical wake oscillator model used to study VIV at high Reynolds numbers. We show that for the low Reynolds numbers considered here, the effect of the structure on the fluid can be represented by a first coupling term proportional to the cylinder acceleration in the fluid equation, and by a second term of lower magnitude, which can stem either from an integral term or from a term proportional to the third derivative of the cylinder position. [ABSTRACT FROM AUTHOR]

The hysteretic behaviour of a freely vibrating cylinder, near the low-Reynolds-number end of synchronization/lock-in, in the laminar regime is investigated. Computations are carried out using a stabilized finite-element method. The flow remains two-dimensional in this Reynolds number regime. This is verified via comparison of two- and three-dimensional computations. The cylinder is free to undergo crossflow as well as in-line vibrations. The combined effect of mass ratio (1 ≤ m* ≤ 100) and blockage (0.25% ≤ B ≤ 12.5%) is studied in detail. The existence of a critical mass ratio (m*cr = 10.11), below which hysteresis disappears for an unbounded flow situation, is identified. For higher mass ratio the hysteretic behaviour is observed for all blockage. However, the hysteresis loop width is found to vary with B; its variation with m* and B is studied. The concept of critical blockage Bcr is introduced. For B ≤ Bcr the response of the cylinder is virtually the same as that in an unbounded flow domain. The variation of Bcr with m* is investigated. Furthermore, Bcr is found to vary non-monotonically with m* for m* ≤ m*cr and is almost constant for m* ≥ 20. The effect of damping, as well as restricting the cylinder to undergo transverse vibrations only, on the hysteresis behaviour is studied. The transverse-only motion leads to a larger hysteresis loop width compared with the transverse and the in-line motion of the cylinder. An attempt is made to explain this by comparing the results from forced vibrations. [ABSTRACT FROM AUTHOR]

Three-dimensional flows over impulsively translated low-aspect-ratio flat plates are investigated for Reynolds numbers of 300 and 500, with a focus on the unsteady vortex dynamics at post-stall angles of attack. Numerical simulations, validated by an oil tow-tank experiment, are performed to study the influence of aspect ratio, angle of attack and planform geometry on the wake vortices and the resulting forces on the plate. Immediately following the impulsive start, the separated flows create wake vortices that share the same topology for all aspect ratios. At large time, the tip vortices significantly influence the vortex dynamics and the corresponding forces on the wings. Depending on the aspect ratio, angle of attack and Reynolds number, the flow at large time reaches a stable steady state, a periodic cycle or aperiodic shedding. For cases of high angles of attack, an asymmetric wake develops in the spanwise direction at large time. The present results are compared to higher Reynolds number flows. Some non-rectangular planforms are also considered to examine the difference in the wakes and forces. After the impulsive start, the time at which maximum lift occurs is fairly constant for a wide range of flow conditions during the initial transient. Due to the influence of the tip vortices, the three-dimensional dynamics of the wake vortices are found to be quite different from the two-dimensional von Kármán vortex street in terms of stability and shedding frequency. [ABSTRACT FROM AUTHOR]

The airflow, pollutant dispersion and penetration of outdoor pollutants indoor in an isolated urban building are investigated computationally. The computation involves solving the two-dimensional Navier–Stokes equations using large eddy simulations based on the Smagorinsky subgrid scale model. The simulations involve investigations of the phenomena of airflow and pollutant dispersion in both indoor and outdoor environments under different Reynolds numbers with distinct combinations of building height, floor height, and building width. Pollutants originate from outdoor, located at the datum level of the windward side of the building. The objective is to study how outdoor pollutants penetrate indoor and relate this pattern to different building geometric configurations. The gradual increase in Reynolds number significantly favors pollutant transportation into the indoor environment and thus eventually changes the indoor/outdoor (IO) air ratio. Moreover, the value of the IO air ratio is particularly found to be highly affected by building height due to the capability of climbing over and escaping from the roof of the building. Results also show that the IO air ratio is dependent on the position of the sampling, which has been ignored in many previous experimental works. The research work poses important questions of indoor air quality as well as field measurements of the IO parameter. [ABSTRACT FROM AUTHOR]

The velocity and pressure fields of the flow over street canyons formed by groups of buildings are studied numerically. The flow fields are computed by solving the time-dependent incompressible Navier–Stokes equations using the fractional step method. The numerical model is validated by simulating flows over a square cylinder at different Reynolds numbers. The Strouhal numbers, which reflect the dynamic flow characteristics, agree well with published experimental data over a wide range of Reynolds numbers. The wind field model is then applied to two street canyon configurations. First, flows inside street canyons formed by four identical buildings are simulated. The incidental flow is raised by the most upstream building and becomes parallel to the ground at the rooftop level of the fourth building downstream, resulting in a clockwise rotating vortex in downstream street canyons with an inflow from left to right. Second, flows inside street canyons formed by two identical buildings are simulated. In this case, a primary eddy that is counterclockwise rotating may be formed due to flow separation at the front corner of the upstream building. A clockwise rotating primary eddy is formed in the wake area of the separate zone above the street canyon, which drives the counterclockwise rotating eddy in the street canyon. The result indicates that rooftop level flows cannot be assumed parallel to the ground as some modelers have done in their studies. Studies also show that flow regimes in the street canyon will remain unchanged while the inflow velocity is greatly increased from 0.1 to 6.0 m/s. In addition, the wind velocities in the street canyon have a linear response to the inflow velocity. [ABSTRACT FROM AUTHOR]

Flow-induced vibration (FIV) of a triangular cylinder is numerically investigated at a Reynolds number of Re = 100. The four-step fractional finite element method is employed to solve the two-dimensional (2D) incompressible Navier–Stokes equations. The cylinder is endowed with a two-degree-of-freedom motion with the reduced mass ratio of Mr = 2. Three typical flow incidence angles, α = 0°, 30° and 60°, are examined to identify the effect of incidence angle on the vibration characteristics of the cylinder. For each α, computations are conducted in a wide range of reduced velocities 2 Ur ≤ 18. The numerical results show that at α = 0° and 30°, the responses of the cylinder are dominated by vortex-induced vibration which resembles that of a circular cylinder. At α = 0°, the peak amplitude of transverse vibration is the smallest among the three investigated α, and most of the cylinder motions exhibit a regular figure-eight trajectory. Some single-loop trajectories are observed at α = 30°, where the vibration frequency in the in-line direction is always identical to that in the transverse direction. At α = 60°, the triangular cylinder undergoes a typical transverse galloping with large amplitude and low frequency, and the vibration trajectories appear to be regular or irregular figure-eight patterns, which are strongly affected by the reduced velocity. [ABSTRACT FROM AUTHOR]