Unsteady Vortex Research Articles

Improved weighted compact nonlinear scheme for implicit large-eddy simulations

Weighted compact nonlinear scheme (WCNS) is improved by replacing the nonlinear explicit interpolation with a nonlinear compact interpolation. The non-oscillatory behaviour of the weighting technique and the spectral resolution of the compact interpolation are maintained simultaneously. In order to investigate the performance of implicit large-eddy simulations (ILES) using WCNS with the new interpolation, several fundamental turbulent flows with relatively complex configurations are validated. Encouraging results are obtained in predicting mean flow profiles, unsteady vortex shedding and the frequency spectra, indicating that the improved WCNS is a promising algorithm for simulations of turbulent compressible flows.

An investigation of bluff body flow structures in variable velocity flows

The present study explores three-dimensional vortex-dynamics past a wall-attached bluff body kept in a variable velocity field with numerical simulations. A trapezoidal pulse of mean velocity, consisting of acceleration phase from rest followed by constant velocity phase and deceleration phase to rest, is imposed at the inlet of the computational domain similar to the experimental study of Das et al. [“Unsteady separation and vortex shedding from a laminar separation bubble over a bluff body,” J. Fluids Struct. 40, 233–245 (2013)]. For a wide range of Reynolds numbers (96≤Reb≤2390), acceleration Reynolds numbers (196≤Rea≤978), and deceleration Reynolds numbers (310≤Red≤1522), different stages of flow evolution are systematically analyzed. The flow evolution starts with the formation of a primary vortex followed by a two-dimensional circular array of spanwise vortex tubes by inflectional shear-layer instability. At a sufficiently high Reynolds number, the shear layer vortices originated from two-dimensional fluctuations deformed by three-dimensional instabilities, giving fragmented streamwise vorticity. In addition, long-wavelength “tongue-like structures” and short-wavelength “rib-like structures” are evident near the top wall and the bluff body, respectively. The streamwise vorticity generation equation indicates that the spanwise vortex tubes initially tilt, resulting in streamwise vorticity, further amplified by the vortex stretching process. The distinct flow features, including mode shape, frequency, and growth rate associated with the shear-layer instability, are identified using the dynamic mode decomposition (DMD) algorithm. Using the maximum growth rate criteria, the DMD technique successfully separates the coherent shear layer modes associated with two-dimensional shear layer instability from the flow field.

Three-dimensional deep learning-based reduced order model for unsteady flow dynamics with variable Reynolds number

In this article, we present a deep learning-based reduced order model (DL-ROM) for predicting the fluid forces and unsteady vortex shedding patterns. We consider the flow past a sphere to examine the accuracy of our DL-ROM predictions. The proposed DL-ROM methodology relies on a three-dimensional convolutional recurrent autoencoder network (3D CRAN) to extract the low-dimensional flow features from the full-order snapshots in an unsupervised manner. The low-dimensional features are evolved in time using a long short-term memory-based recurrent neural network and reconstructed back to the full-order as flow voxels. These flow voxels are introduced as static and uniform query probes in the point cloud domain to reduce the unstructured mesh complexity while providing convenience in the 3D CRAN training. We introduce a novel procedure to recover the interface description and the instantaneous force quantities from these 3D flow voxels. To evaluate the 3D flow reconstruction and inference, the 3D CRAN methodology is first applied to an external flow past a static sphere at the single Reynolds number of Re = 300. We provide an assessment of the computing requirements in terms of the memory usage, training, and testing cost of the 3D CRAN framework. Subsequently, variable Re-based flow information is infused in one 3D CRAN to learn a symmetry-breaking flow regime (280 ≤ Re ≤ 460) for the flow past a sphere. Effects of transfer learning are analyzed for training this complex 3D flow regime on a relatively smaller time series dataset. The 3D CRAN framework learns the flow regime nearly 20 times faster than the parallel full-order model and predicts this flow regime in time with a reasonable accuracy. Based on the predicted flow fields, the network demonstrates an R2 accuracy of 98.58% for the drag and 76.43% for the lift over the sphere in this flow regime. The proposed framework aligns with the development of a digital twin for 3D unsteady flow field and instantaneous force predictions with variable Re-based effects.

Unsteady Numerical Investigation on Vortex Interaction between Rim Seal Purge Flow and Upstream Stator

To assess unsteady vortex interaction between rim seal purge flow and upstream stator, numerical ‎investigations were conducted under different purge flow rates. The vortex distributions for the stator and ‎cavity were investigated and the interaction processes near the cavity exit, in particular the vorticity ‎change resulting from the ingress and egress, were analyzed. Results show the intensity of hub passage ‎vortex (HPV) and hub trailing shedding vortex (HTSV) at stator exit is decreased as a consequence of ‎enhancing blockage effects caused by the egress flow. However, when the purge flow rate increases, ‎from stator exit to downstream of cavity exit, the reduction in the intensity of two vortices is weakened as ‎the extrusion of egress flow thins their vortex tubes. The vortex inside the cavity is generated as the ‎combined effect of relative rotation of cavity walls and non-uniform circumferential pressure mainly ‎imposed by upstream stator. The ingress leads the positive axial vorticity near the stator hub to ingest into ‎the cavity and eject into the main passage due to the blockage of purge flow. Furthermore, the interaction ‎between the ingress of the mainstream and purge flow produces local negative axial vorticity. The egress ‎flow carries negative axial vorticity mainly originated from the rotational cavity wall, and enters into the ‎main flow passage near the rotating hub, then locations of HPV and HTSV move to the mid-span slightly ‎with the extrusion of egress flow‎.

A Hybrid Non-Linear Unsteady Vortex Lattice-Vortex Particle Method for Rotor Blades Aerodynamic Simulations

This study presents a hybrid non-linear unsteady vortex lattice method-vortex particle method (NL UVLM-VPM) to investigate the aerodynamics of rotor blades hovering in and out of ground effect. The method is of interest for the fast aerodynamic prediction of helicopter and smaller rotor blades. UVLM models the vorticity along the rotor blades and near field wakes with panels that are then converted into their equivalent vortex particle representations. The standard Vreman subgrid scale model is incorporated in the context of a large eddy simulation for mesh-free VPM to stabilize the wake development via particle strength exchange (PSE). The computation of the pairwise interactions in the VPM are accelerated using the fast-multipole method. Non-linear UVLM is achieved with a low computational cost viscous-inviscid alpha coupling algorithm through a stripwise 2D Reynolds-averaged Navier–Stokes (RANS) or empirical database. The aerodynamics of the scaled S76 rotor blades in and out of ground effect from the hover prediction workshop is investigated with the proposed algorithm. The results are validated with experimental data and various high-fidelity codes.

Greedy Non-Intrusive Reduced-Order Model’s application in dynamic blowing and suction flow control to suppress the flow separation

The Greedy Non-Intrusive Reduced-Order Model (GNIROM) is applied in dynamic blowing and suction’s flow control to suppress the GAW1 airfoil’s flow separation. The effects of the uniform blowing and uniform suction on the flow fields and aerodynamic coefficients have been studied firstly. By comparing a uniform suction control with a dynamic blowing and suction control, it has been found that the dynamic blowing and suction can provide a higher lift coefficient and a lower energy loss. Finally, for the sake of achieving an optimal dynamic blowing and suction control, an optimization of the dynamic blowing and suction’s parameters is taken based on the GNIROM, consisting of a Non-Intrusive ROM (NIROM) built by the two-level POD and Radial Basis Function (RBF), and the relative greedy method. A cost function has been proposed by considering suppressing the flow separation, improving the lift-drag characteristics and reducing the energy loss together. The GNIROM has been built for unsteady vorticity fields with the parameters of the dynamic blowing and suction varying. Based on the GNIROM’s approximation, the optimal control parameters’ values have been determined quickly. Compared to the initial dynamic blowing and suction control, the optimal one has reduced the drag coefficient by 43.4 percents and the energy loss by 42 percents.

On the compressible biglobal stability of the mean flow motion in porous channels

This work relies on a compressible biglobal stability approach to describe the wave dynamics in a planar rocket chamber modeled as a porous channel. At first, the effectiveness of the compressible formulation is demonstrated by reproducing, in the absence of a mean flow, the Helmholtz frequencies and mode shapes. Next, the unsteady vorticity fluctuations, which intensify near the walls, are shown to be consistent with those associated with parietal vortex shedding. In this context, the penetration depth of vorticoacoustic waves is found to be strongly dependent on the penetration number. The latter gauges the cubic power of the injection speed to the product of kinematic viscosity, chamber half-height, and frequency squared. As for the strictly hydrodynamic modes, they seem to develop at the porous walls and grow in the core region, where the mean flow velocity is most appreciable. The ensuing modal analysis enables us to predict both longitudinal and transverse modes for several test cases, thus illustrating the tendency of hydrodynamic modes to intensify at higher injection speeds and longer chambers. Furthermore, by repeating the analysis with an active mean flow, one finds that successive increases in the injection speed gradually reduce the predicted frequencies relative to the eigenmodes obtained in a quiescent medium. Finally, recognizing that the spectral analysis is capable of recovering both longitudinal and transverse modes induced by acoustic and hydrodynamic disturbances, their coupled interactions, which often lead to specifically amplified frequencies in static tests, are robustly captured, namely, without resorting to any particular wave decomposition.

Aeroelastic Response of a Z-Shaped Folding Wing During the Morphing Process

This paper presents the development of an aeroelastic analysis approach for the dynamic response of a Z-shaped folding wing. The structural model is established by the finite element method and the component mode synthesis method by accounting for the configuration changing effects on the inertial and stiffness characteristics. The aerodynamic model is directly built using the continuous-time state-space unsteady vortex lattice method. The coupling relationship between structural and aerodynamics models is proposed innovatively with interpolation between structural generalized coordinates and incremental normal vectors on the collocation points of aerodynamic panels. The resulting aeroelastic model with the state-space equation can be used for flutter and dynamic response analysis by considering a rapid folding/unfolding process. The analysis results show that the folding and unfolding processes have opposite effects on both the aerodynamic load and aeroelastic characteristics. Moreover, the effects become more significant with an increasing morphing rate.

Experimental Investigation and Validation on Suppressing the Unsteady Aerodynamic Force and Flow Structure of Single Box Girder by Trailing Edge Jets

In the present investigation, a wind tunnel experiment was performed to evaluate the effectiveness of the trailing edge jets control scheme to mitigate the unsteady aerodynamic force and flow structure of a single box girder (SBG) model. The flow control scheme uses four isolated circular holes for forming the jet flow to modify the periodic vortex shedding behind the SBG model and then alleviate the fluctuation of the aerodynamic force acting on the test model. The Reynolds number is calculated as 2.08 × 104 based on the incoming velocity and the height of the test model. A digital pressure measurement system was utilized to obtain and record the surface pressure that was distributed around the SBG model. The surface pressure results show that the fluctuating amplitude of the aerodynamic forces was attenuated in the controlled case at a specific range of the non-dimensional jet momentum coefficient. The Strouhal number of the controlled case also deviates from that of the original SBG model. Except for the pressure measurement experiment, a high-resolution digital particle image velocimetry system was applied to investigate the detailed flow structure behind the SBG model to uncover the unsteady vortex motion process from the SBG model with and without the trailing edge jets flow control. As the jet flow blows into the wake, the alternating vortex shedding mode is switched into a symmetrical shedding mode and the width of the wake flow is narrowed. The proper orthogonal decomposition was used to identify the energy of the different modes and obtain its corresponding flow structures. Moreover, the linear stability analysis of the flow field behind the SBG model shows that the scheme of trailing edge jets can dramatically suppress the area of unsteady flow.

Numerical study of turbine rim seals performance with different sealing structures

The unsteady large-scale vortex near the turbine rim has an important influence on the sealing performance. Characteristics and performance of four sealing structures are researched in this paper. Three-dimensional unsteady numerical simulation was adopted to deeply reveal the characteristics of the rim sealing vortex and its influence mechanism on the rim sealing performance. The results show that the rim seal vortex structure induced by the interaction between ingested gas and sealing flow in the gap is the leading cause of unsteady flow in the rim. The vortex size is suppressed with the increasing seal flow rate or a Chute seal structure. However, the rim seal vortex exit in the cavity gap under a low seal flow rate can suppress the gas intrusion and improve the sealing efficiency of the turbine cavity even with a simple sealing structure. The Chute sealing structure achieves better performance among the four sealing structures studied in this paper. It can achieve complete sealing under a low sealing flow rate of 0.5% and has less impact on the aerodynamic performance of the mainstream even with high sealing flow rate. The research of this paper has guiding significance for further understanding the sealing mechanism and optimizing the design of the sealing structures.

Three-component diagnostics of swirling flow in the model of an improved four-vortex furnace

The spatial structure of a swirling turbulent flow has been investigated based on the three-component laser Doppler anemometry method in an isothermal laboratory model of a four-vortex furnace. The structure of the vortex cores of the flow with the shape of a deformed vertical elliptical cylinder is visualized using the ?minimum total pressure? criterion. The spectrum of velocity pulsations indicates the absence of unsteady periodic vortex structures, which means the occurrence of a stable vortex flow in the volume of the combustion chamber.

Identification on Vortex Rope in Francis Turbine Draft Tube Based on Convkurgram

Francis turbine is a core energy conversion part and low frequency vibrations caused by unsteady vortex rope in the draft tube may damage the unit through resonance. Therefore, a more comprehensive assessment and monitoring of the status of Francis turbine is of great significance for unit stability. In this hydraulic machinery, mechanical impulses and hydraulic excitation are co-existing. The former manifests as a narrow low frequency band with high kurtosis, while the latter represents a wide high frequency band with energy associated with the vortex rope. Based on that, a novel method coined Convkurgram is proposed to suppress the extreme high kurtosis mechanical components and extract the hydraulic carrier wave efficiently. Besides, it takes multiple information frequency bands into consideration in case of contamination waves exist. Finally, the method is evaluated by means of actual vibration signals collected from the hydroelectric generating unit under normal and cavitation conditions. The results show the proposed method recognizes the characteristic frequency bands of mechanical components and hydraulic carriers as well as extracts the shaft frequency and blade passing frequency successfully.

An experimental study of the unsteady vortex structures of a clapping-wing micro air vehicle

It is a challenge to explore the unsteady vortex structures of flexible flapping wings of X-shaped flapping-wing micro air vehicles (also known as clapping-wing micro air vehicles (CWMAVs)). The objective of this paper is to obtain the influence of wind speed, flapping frequency, and angle of attack (AoA) on the instantaneous and average force coefficients of CWMAVs, investigate flow visualization of the leading-edge vortex (LEV), trailing-edge vortex (TEV) and wake vortex (WV) structures and identify some novel flow mechanisms of flapping propulsion. This paper proposes a mechanics and particle image velocimetry (PIV) measuring platform in a low-speed tunnel. In addition, cross-correlation peak analysis and kriging image reconstruction are applied for postprocessing to enhance PIV image recognition. Combining the time evolution of the forces, the force coefficients, and the flow visualization, we find the evolution and effects of LEV, TEV and WV structures.

Kinematic Optimization of a Flexible Wing Undergoing Flapping and Pitching

In recent years, there has been widespread interest in the design of microair vehicles (MAVs) for flapping flight with high-aspect ratio wings due to their high efficiency and energy savings. However, the flexibility of a flapping wing causes the aeroelastic effect, which remains a subject of investigation. Generally, existing research simulates active bending and twisting of flexible wings under the assumption of neglecting flapping inertia. In this research, the kinematic optimization of a bionic wing with passive deformation in forward flight while undergoing flapping and pitching is considered. To this end, a computational aeroelasticity framework, which includes the three-dimensional unsteady vortex lattice method (UVLM) and the Newmark-β method, is constructed for flapping flight. Under the assumption of linear elastic deformation, this tool is capable of simulating attached flows over a thin wing and capturing unsteady effects of wakes. A bionic numerical wing with an aspect ratio of 6.5, chord Reynolds number of 1.9 × 105, and reduced frequency less than 0.1 is investigated in kinematic optimization. The computational aeroelasticity framework is combined with a global optimization algorithm to identify the optimal kinematics that maximize the propulsive efficiency under the minimum average lift constraint. Two types of numerical wings, rigid wing and flexible wing, are considered here to compare the influence of deformation on the aerodynamics of the flapping wing. The results show that the aeroelastic effect, which increases the flapping amplitude, yields a significant improvement in terms of propulsive efficiency. In addition, the optimization algorithm maximizes the thrust efficiency while satisfying the required lift. Moreover, the optimal kinematics of both the rigid wing and the flexible wing reach the maximum flapping angle, which indicates that a larger range of motions is needed for optimal kinetics when loosening the boundary conditions.

Numerical Method for Determining Three-Dimensional Response of a Wing to Incident Turbulence

A method is developed to numerically determine the response of three-dimensional wings to turbulent upwash. The method uses a generalized spatial-domain gust response function in the place of the Sears function that differentiates the strip experiencing the gust and the strip producing force. This generalized gust response function is computed with an incompressible unsteady vortex lattice method. The vortex lattice operates in the time-domain to simulate the development and relaxation of the wake to steady state and then switches to a frequency-domain approach to evaluate the gust response. The total wing response to homogenous isotropic turbulent inflows is numerically integrated to produce stochastic turbulent lift spectra. The numerically generated transfer functions are compared to the Sears and Filotas gust response functions. The effects of finite span and sweep on both the transfer functions and lift spectra are investigated.

Internal flow phenomena of a Pump–Turbine model in turbine mode with different Thoma numbers

In the low-flow rates of off-design conditions in the pump–turbines, complicated flows can develop with vortices in the flow passages, which significantly increase the vibrations and noise, resulting in destabilizing the pump–turbine system. Under low-flow-rate conditions, an appropriate Thoma number is the most essential requirement for stable operation in turbine mode of the pump–turbine. Thus, this study elucidated to examine the internal flow and unsteady pressure phenomena in a laboratory-scale pump–turbine model operated in turbine mode with different Thoma numbers. The experimental investigations were conducted under 70% of the flow rate at the best efficiency point with different Thoma number to observe the unsteady pressures and vortex rope behavior. Then, the numerical validations were conducted by three-dimensional unsteady-state RANS analyses in a SAS–SST model. The Thoma number did not significantly affect the performance of the pump–turbine model or the unsteady flow phenomena in the vaneless and runner regions. However, the Thoma number affected the draft tube region at low flow rate condition. At low Thoma numbers, the irregular flow phenomena along the flow direction enhanced the unsteady pressure characteristics.

Effects of Rotation on Transient Fluid Flow and Heat Transfer Through a Curved Square Duct: The Case of Negative Rotation

The fluid flow and heat transfer through a rotating curved duct has received much attention in recent years because of vast applications in mechanical devices. It is noticed that there occur two different types of rotations in a rotating curved duct such as positive and negative rotation. The positive rotation through the curved duct is widely investigated while the investigation on the negative rotation is rarely available. The paper investigates the influence of negative rotation for a wide range of Taylor number (<i>−10 ≤ Tr ≤ −2500</i>) when the duct itself rotates about the center of curvature. Due to the rotation, three types of forces including Coriolis, centrifugal, and buoyancy forces are generated. The study focuses and explains the combined effect of these forces on the fluid flow in details. First, the linear stability of the steady solution is performed. An unsteady solution is then obtained by time-evolution calculation and flow transition is determined by calculating phase space and power spectrum. When <i>Tr</i> is raised in the negative direction, the flow behavior shows different flow instabilities including steady-state, periodic, multi-periodic, and chaotic oscillations. Furthermore, the pattern variations of axial and secondary flow velocity and isotherms are obtained, and it is found that there is a strong interaction between the flow velocities and the isotherms. Then temperature gradients are calculated which show that the fluid mixing and the acts of secondary flow have a strong influence on heat transfer in the fluid. Diagrams of unsteady flow and vortex structure are further sketched and precisely elucidate the curvature effects on unsteady fluid flow. Finally, a comparison between the numerical and experimental data is discussed which demonstrates that both data coincide with each other.

Numerical simulation of the flow around a square cylinder under plasma actuator control

Navier–Stokes computations are performed, and detailed results are documented on the vortex shedding, lift and drag fluctuations, and their spectra for flow past a square cylinder with the Reynolds number ranging from 1 to 300. A body force model for the dielectric barrier discharge (DBD) plasma actuator is used in the Navier–Stokes computations to study the effectiveness of flow control by a pair of the DBD actuators. Three installation configurations of the actuators are investigated. The effects of the actuator location, actuator input power, and the flow Reynolds number are studied. In the first case, two actuators have been installed on the top and bottom of the cylinder. In the second case, two actuators are installed on the front surface. In the last case, two actuators are installed on the rear surface. It is discovered that the best flow control effect is achieved in the last case. Both the unsteady vortex shedding and the average drag can be significantly reduced by the plasma actuators. Under best conditions, the vortex shedding in the wake can be completely suppressed.

Wind tunnel measurements of the aerodynamic characteristics of a 3:2 rectangular cylinder including non-Gaussian and non-stationary features

This paper presents a wind tunnel investigation of the aerodynamic characteristics (force and pressure coefficients) of a static 3:2 rectangular cylinder in smooth flow for angles of attack between −4° and 90° at various values of Reynolds number. In contrast to much of the existing literature, this study shows clear dependence of the mean drag coefficients on Reynolds number. In addition, the variation of aerodynamic parameters with angle of attack is fully mapped for a symmetric section revealing that the peak values of the mean values of drag, lift, moment and Strouhal number do not occur at the same critical angle of attack.The present study also presents the first map for identifying the locations of the reattachment and stagnation points as well as the zones of angle of attack where the flow is separated and attached on a face of the section. The phenomenon of switching flow is observed at the angle of attack 25°, leading to strong non-stationarity and non-Gaussian distributions of the aerodynamic forces and pressure. Another phenomenon, so-called unsteady low-frequency vortex shedding, is also observed at higher angles of attack. These phenomena need to be accounted for when estimating wind loading and aeroelastic instability for these sections.

Multi-scale nature of non-blade-order propagating flow disturbances in axial compressors

Non-blade-order flow disturbances, also referred to as pre-stall disturbances or tip flow unsteadiness, are closely related with compressor instabilities. The present work provides a comprehensive investigation on multi-scale nature of non-blade-order disturbances and the underlying flow physics in axial compressors. By applying full-annulus Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, along with space–time correlation and spatial Fourier decomposition, to the disturbed pressure, the propagating feature of the non-blade-order disturbances is obtained. Further, a bridge between non-blade-order disturbances and the evolution of unsteady vortex has been set up. The results show that non-blade-order disturbances, featured as short-length-scale (35 modes across annulus), first appear as the occurrence of tip leakage vortex fluctuation, while the compressor still operates far from stall. Leading-edge radial vortex appears at near stall condition, and its movement induces a circumferential propagating disturbance overlaying on the one induced by oscillating tip leakage vortex. The interaction of the short-scale disturbances with a low-amplitude long-scale (of circumference) disturbance is observed, which results in disturbances with multiple scales of consecutive spatial modes, along with multiple frequency peaks in spectra. The compressor falls into stall as the circumferential nonuniform scattering of the leading-edge vortexes occurs. The densely- and sparsely-scattered leading-edge radial vortexes induce a high-amplitude long-scale (of circumference) disturbance, i.e. stall disturbance.