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This paper numerically investigates the aerodynamic performance of the T106A low-pressure turbine based with different inflow conditions at moderate Reynolds number by using high performance computing based on high order unstructured methods. Two different inflow conditions respectively of uniform and disturbed are considered, while for the latter a small circular cylinder is placed upstream of the cascade to generate wake turbulence as a long-standing disturbance. A high order Fourier-spectral/hp element method is employed to solve the flow dynamics in the cascade of high complex geometries. Flow transition characteristics are quantified in terms of the distribution of cascade wall surface pressure and friction coefficient, the distribution of wake profile pressure loss and the evolution characteristics of boundary layer flow structures as well. The numerical results show that the current numerical simulations accurately predict the flow transition performance of low-pressure turbine cascades and capture the effects of wake-generated disturbance on the cascade, which is shown to effectively modify the flow transition performance as compared with the uniform inflow case. [ABSTRACT FROM AUTHOR]

Three-dimensional large-eddy simulations are carried out for flow past a cylinder beneath a deformable free surface at a fixed Reynolds number of Re = 7550. The results are examined for two Froude numbers of F r = 0.2 and 0.6 and a gap ratio of 0.4, to investigate the effects of the distortion of the free surface on the flow fields and hydrodynamics. At the low Froude number of F r = 0.2 , the deformation of the free surface is small with little influence on the wake characteristics, and an alternative vortex shedding modulated by the free surface is detected in the wake. As the Froude number increases to 0.6, intense interface distortion occurs, which can be divided into three different regions: a hydraulic jump in the region of overtopping, a well-defined long-wavelength wave generation region in the large-scale recirculation zone near the free surface, and a water level recovery slope further downstream. The sudden change in flow regime from locally supercritical to subcritical allows the occurrence of the hydraulic jump. The induced surface waves behind the cylinder are ascribed to the shedding process of three shear layers, two of which are separated from the cylinder surface and one from the free surface. In addition, a jet-like flow originated from the gap between the free surface and the top of the cylinder occurs, exerting a downward thrust on the cylinder and pushing the wake away from the free surface. The Kármán vortex shedding in the wake is suppressed due to the interruption of the jet-like flow. The fluctuations of the wake turbulence and hydrodynamic forces are also suppressed to a low level. [ABSTRACT FROM AUTHOR]

Two-dimensional numerical computations are carried out for two rigidly connected cylinders of unequal sizes undergoing vortex-induced vibrations (VIV) perpendicular to the free stream. Results are examined for Re = 250 and a fixed diameter ratio of d / D = 0.2. The VIV response of the system is investigated for various positions of the small cylinder, covering a fine grid of wide radial (r) and azimuthal (θ) ranges, relative to the origin of the main cylinder. It is shown that the structural dynamics and hydrodynamic forces are strongly dependent on the arrangements. Regions of VIV reduction and amplification are distinguished, and the highest and lowest oscillation amplitudes are, respectively, acquired at configurations of (r , θ) = (0.7 D , 90 °) and (r , θ) = (0.88 D , 130 °). A deeper analysis in terms of the wake topology and surface pressure is then provided for these two extreme cases, to figure out the underlying mechanisms that lead to such markedly distinct responses. For the former case, the shear layers from two cylinders intensely interact and amalgamate during the oscillation, setting off subsequent processes of shear layer reattachment and downflow that are responsible for the observed high-amplitude response, while for the latter case, the shear layers from the small cylinder are highly stretched and absent from direct interaction with that from the large cylinder, which is favorable for stabilizing the wake and maintaining the low-amplitude response. Proper orthogonal decomposition (POD) is further utilized to correlate the key features of the wake with the dominant coherent structures in the flow. [ABSTRACT FROM AUTHOR]

Three-dimensional numerical simulation for the three circular cylinders at Re = 3900 is carried out by using the large eddy simulation (LES) in this paper. The present numerical model and parameters are verified by solving single cylinder and two cylinders cases. Then, the parameters consisted of spacing ratio (L / D) and incidence angle (α) are checked in three circular cylinders array. The fluid force and near-wake behavior of the two- and three-cylinder systems are analyzed with emphasis. Under a tight arrangement, there are three flow patterns: reattachment flow pattern, large-incidence-angle flow pattern, and biased flow pattern, corresponding to cases with incidence angles of 0, 45, and 90 degrees, respectively. The Strouhal number (St) of the three cylinders is essentially overlapping at α = 0° and α = 90°, the distinctive secondary peaks can be observed at the small spacing ratio in power spectral density (PSD). The mean pressure coefficient of midstream cylinder (MC) influenced by the number of cylinders is negative in the tandem arrangement at the stagnation point, accompanied by reattachment phenomenon the upstream surface of MC with L/D = 1.5. Placing the third cylinder between the two cylinders in a tandem arrangement significantly suppresses their lift oscillations. The phase difference (φ um) between the fluctuating lift of the upstream two cylinders is highly sensitive to the incidence angle of the system. [ABSTRACT FROM AUTHOR]

The wake of wavy cylinder has been shown to exhibit bistability. Depending on the initial condition, the final state of the wake can either develop into steady flow (state I), or periodic shedding (state II). In this paper, we perform numerical simulations to reveal the Reynolds number effects (R e = 30 – 300) on these two wake states. With increasing Reynolds number, the steady vortical structures in state I wake sways back and forth in the spanwise direction, resulting in low-frequency fluctuations in drag forces, but not in lift. For state II, the increase in Reynolds number is associated with the emergence of another spectral peak in the lift coefficient. The secondary frequency is associated with highly three-dimensional vortical structures in the wake. For both states, the wakes transition to turbulent flows at higher Reynolds numbers, with the development of small-scale vortices. We further study the streamwise gust flows over the wavy cylinder. The time-varying inflow velocity results in a wide range of instantaneous Reynolds number spanning from the absolutely unstable flow regime to the bistable regime. Depending on the period of the inflow velocity variation, the wake perturbations grown at the absolutely unstable flow regime can be damped out in state I wake, or grow large enough to trigger the transition to state II, resulting in loss of flow control efficacy. The above analyses reveal novel flow physics of the bistable states at unexplored Reynolds numbers, and showcase the complex transition behavior between the two states in unsteady flows. The insights gained from this study improve the understanding of the wake dynamics of the wavy cylinder. • The Reynolds number effects on the bistable states in the wake of a wavy cylinder are revealed. • A novel unsteady flow which only induces variations in drag but not in lift is discovered. • Complex transition behaviors between the two wake states in streamwise gust flow is unveiled. [ABSTRACT FROM AUTHOR]

Previous studies have revealed that fluid induced forces and wake structure of a cylinder can be significantly modified by varying the cross-section size along its axis. However, the fluid dynamic characteristics behavior of a flexible cable with a sinusoidal wavy surface remains unclear. The current study endeavors to present a systematic investigation of flow-induced vibrations (FIV) of an infinitely long wavy cable with different wave amplitude (A = 0. 1 D to 0. 3 D) and a constant spanwise wavelength λ = 4 π , by using a highly resolved direct numerical simulation employing a high-order spectral/ h p element method at a Reynolds number of 100, corresponding to laminar flow state. A tensioned beam model with a tension value can trigger a single wave is selected to govern the dynamics of the wavy cables. In addition, the structural dynamics are initialized from a standing wave. The present study focuses on the effects of the wave amplitude on the cable wake and the structural dynamic response, including the transverse displacement, wake pattern, vortex formation length, energy transfer, vortex shedding frequency and flow separation angle. Two completely different vibration modes are identified from the numerical cases studied, and hence the wavy cable are divided into control failure (A < 0. 2 D) and optimal wavy cable (0. 2 D ⩽ A ⩽ 0. 3 D) classes, respectively. It is also revealed that the counter-rotating vortices observed in the optimal wavy cables play an important role in stabilizing the shear layer and elongating the vortex formation length. Furthermore, the three-dimensional wake patterns of the wavy cable are mainly caused by the variations of flow separation angle along its spanwise direction. For the optimal wavy cables, the separation point is almost fixed along its spanwise direction while is variable for the control failure cases. [ABSTRACT FROM AUTHOR]

This paper identifies and characterizes modes of the three-dimensional instability of the flow over a circular cylinder of diameter D near a plane moving wall with a gap height of G = 0. 1 D beyond the onset of vortex shedding through the Floquet stability analysis. The associated effects of the transitions on the hydrodynamic forces of the cylinder are examined by performing direct numerical simulations. Six distinct three-dimensional modes are identified and their characteristics are discussed in detail. It is found that the physical mechanism responsible for the synchronous mode 1 appears to be associated to an elliptic instability. The quasi-periodic mode 4 is observed to transform to a synchronous mode (mode 6) on increasing the Reynolds number, indicating that they are likely to be resulted from the same physical mechanism. For mode 6, its physical nature seems to be a hyperbolic instability. Fully three-dimensional simulations show that the wake undergoes a rapid transition to complex chaotic flow state after the most dominant mode starts to saturate, due to the strongly nonlinear interactions of many modes with different spanwise wavelengths. • The presence of a plane moving wall in close proximity to a circular cylinder fundamentally alters the dynamics and the 3D wake transitions. • Six distinct modes of 3D instability are identified, their characteristics are discussed and the underlying physical mechanisms are proposed. • The three-dimensionality of the flow is triggered by different 3D instabilities as the Reynolds number is varied beyond the onset of vortex shedding. • The wake undergoes a rapid transition to complex chaotic flow state after the most dominant instability mode starts to saturate. [ABSTRACT FROM AUTHOR]

In this paper, we study the transverse vortex-induced vibrations (VIV) of two rigidly connected circular cylinders of equal size, arranged in a tandem configuration, by means of two-dimensional numerical computations. Results are examined for R e = 250 and a fixed center-to-center separation of 2 D. The dynamic response of the two-cylinder system is investigated in detail over a domain of reduced velocities ranging from U r = 2 to 8. The diverse types of branches are identified, on the basis of their distinct characteristics in the amplitude and frequency responses. Among them, the lower branch is of particular interest as it is quasi-periodic in nature, instead of the periodic regime that is well documented for the isolated cylinder case. By scrutinizing the vorticity dynamics, it reveals that this quasi-periodicity arises from the switching between two flow states, which are essentially distinguished by whether an active gap flow is present. The dynamically rich response leads to a rich variety of phase dynamics, involving phase locking, trapping, slipping and drifting; in particular, phase drifting is indicative of a forthcoming phase jump of 180° between the lift force and the displacement. It is also found that in the initial branch, the excitation of the system is driven by the pressure lift force on the rear cylinder; by contrast, in the lower branch, it is the pressure lift force on the front cylinder that acts as a source of excitation. • The diverse types of response branches are identified. • A rich variety of phase dynamics are captured. • The mechanism for the quasi-periodic nature of the lower branch is proposed. [ABSTRACT FROM AUTHOR]