Complex Unsteady Flow Field Research Articles

Sparsity Promoting Dynamic Mode Decomposition for Data-Driven Modeling of Wind Turbine Wake

High-fidelity numerical simulation is suitable for analyzing the complex unsteady flow field dynamics of wind turbines. For a better understanding of these flow characteristics, the dynamic mode decomposition method can be used to carry out a reduced-order model study on the wakefield of wind turbines based on large-eddy simulations (LES) numerical simulation. In this paper, we abstract material dynamic information from the wakefield of the wind turbine by applying the sparsity-promoting dynamic mode decomposition (SPDMD) method, and the decomposition results are contrasted with the standard dynamic mode decomposition (DMD) method. Indicated that both mode decomposition methods can abstract the dynamic characteristics of wake and reveal the development and variation law of wind turbine wake. However, the frequency and spatial structure of the selected modes are different. For the purpose of demonstrating the extraction impact of the DMD/SPDMD method on the wakefield of wind turbines, DMD/SPDMD reduced-order models are established respectively. The result indicated that the relatively limited number of SPDMD modes is adequate to validly rehabilitate the wakefield of the unabridged wind turbine while standard DMD methods prerequisite more decomposition modes. Therefore, compared with the standard DMD method, the SPDMD method has strong robustness in mode selection, eliminates the feature information that contributes weakly to the flow, and has a smaller performance loss in the reconstruction of the wakefield of the wind turbine. The consumption of computing resources is greatly reduced.

Parallel Numerical Simulation of Complex Unsteady Multi-Component Three-Dimensional Flow Field of Nonequilibrium Chemical Reaction

Parallel Numerical Simulation of Complex Unsteady Multi-Component Three-Dimensional Flow Field of Nonequilibrium Chemical Reaction

Research on Unsteady Wake Characteristics of the NREL 5MW Wind Turbine Under Yaw Conditions Based on a LBM-LES Method

The yaw of a large-scale wind turbine will change the aerodynamic performance of its substructures. In view of this, this article applied a LBM-LES method to numerically simulate the yawed MEXICO wind turbine and compared the numerical results with the experimental data collected in the New MEXICO experiment to verify the reliability of the method. A rapid dynamic yaw is defined with a yaw speed of 5°/s in yaw control, while a slow dynamic yaw is with a yaw speed of 0.3°/s. The NREL 5 MW offshore wind turbine was used to explore the constant wake characteristics under the conditions of a rapid dynamic yaw and of a slow dynamic yaw. It can be seen from the results that the LBM-LES method captures the detailed characteristics of the complex unsteady flow field of the wind turbine. In the rapid dynamic yaw process, the transition area between the far wake and the near wake shows a large deflection. However, in the process of the slow dynamic yaw, the far wake deflects forward relatively to the yaw. The dynamic yaw wake forms a kidney-shaped cross-section similar to the one in a steady yaw. The yaw wake recovers faster than the wake without yaw, and the rapid dynamic yaw wake has an asymmetric structure. The thrust of the wind turbine drops rapidly during the rapid dynamic yaw, while the thrust of the slow dynamic yaw decreases gradually. The thrust drops three times per rotor revolution, and the time of thrust decrease is delayed as the yaw angle increases.

Three-dimensional unsteady aerodynamics of a H-shaped vertical axis wind turbine over the full operating range

Vertical-axis wind turbines have great potential for harvesting energy with high conversion efficiency both in small-scale distributed installations and in the emerging floating offshore technology; nevertheless, their complex unsteady flow field makes design and optimization very challenging. This paper documents an extensive CFD study on a H-shaped VAWT over the entire range of operation. First, a direct 2D-3D comparison of the prediction capability of CFD models over the entire turbine operational curve is presented, highlighting the range of operation in which 3D effects become crucial and, instead, the range of TSR for which a 2D approximation is acceptable. The simulations are systematically compared to a wide and accurate experimental data set of wind-tunnel experiments in controlled conditions. The combination of CFD results with measured data allowed to assess the turbine performance and time-resolved velocity measurements in the wake. Furthermore, the paper proposes a totally novel approach in analysing the computed non-periodic unsteadiness in the wake. Finally, a tracking of the unsteady tip vortex is also proposed, with remarkable similarity to experimental results documented in the literature. This fact suggests that URANS discretization permits to approximate the dynamic and evolution of the tip vortex, at least in the near wake region.

Efficient collective swimming by harnessing vortices through deep reinforcement learning

Fish in schooling formations navigate complex flow fields replete with mechanical energy in the vortex wakes of their companions. Their schooling behavior has been associated with evolutionary advantages including energy savings, yet the underlying physical mechanisms remain unknown. We show that fish can improve their sustained propulsive efficiency by placing themselves in appropriate locations in the wake of other swimmers and intercepting judiciously their shed vortices. This swimming strategy leads to collective energy savings and is revealed through a combination of high-fidelity flow simulations with a deep reinforcement learning (RL) algorithm. The RL algorithm relies on a policy defined by deep, recurrent neural nets, with long-short-term memory cells, that are essential for capturing the unsteadiness of the two-way interactions between the fish and the vortical flow field. Surprisingly, we find that swimming in-line with a leader is not associated with energetic benefits for the follower. Instead, "smart swimmer(s)" place themselves at off-center positions, with respect to the axis of the leader(s) and deform their body to synchronize with the momentum of the oncoming vortices, thus enhancing their swimming efficiency at no cost to the leader(s). The results confirm that fish may harvest energy deposited in vortices and support the conjecture that swimming in formation is energetically advantageous. Moreover, this study demonstrates that deep RL can produce navigation algorithms for complex unsteady and vortical flow fields, with promising implications for energy savings in autonomous robotic swarms.

Identification and quantification of losses in a LPT cascade by POD applied to LES data

A POD based procedure has been developed to identify and account for the different contributions to the entropy production rate caused by the unsteady aerodynamics of a low-pressure (LP) turbine blade. LES data of the extensively studied T106A cascade have been used to clearly highlight the capability of POD to identify deterministic incoming wake related modes, stochastic fine-scale structures embedded within the bulk of the wake carried during migration, and coherent structures originating in the boundary layer as a consequence of the wake-boundary layer interaction process. The POD modes computed by a kinematic kernel generate a full and complete basis, where both the velocity and enthalpy fields have been projected through an extended POD procedure to determine the relative coefficients. This allows to separately compute orthogonal sets of contributions to turbulent kinetic energy production, enthalpy-velocity correlation and turbulent dissipation of resolved structures, thus clearly identifying the dominating modes (i.e. phenomena) responsible for the overall entropy production rate. Moreover, low-order truncation of these different contributions have been grouped into three different parts: those arising from the deterministic incoming wake, those due to the turbulence carried by the wakes and its interaction with the boundary layer, and those related to boundary layer events. The spatial integration of these low-order truncations restricted to the time-mean boundary layer, wake mixing and the potential flow regions of the blade passage allows gathering further information on the unsteady loss generation mechanisms, and where they mainly act. Particularly, results show that the procedure is able to decompose losses into the dominant contributions, thus providing a new tool for a rapid and clear identification of the different sources of losses in complex unsteady flow fields.

Large eddy simulation of the influence of synthetic inlet turbulence on a practical aeroengine combustor with counter-rotating swirler

To study the influence of inlet turbulence on the prediction of flow structure in practical aeroengine combustor, large eddy simulation with dynamic Smagorinsky subgrid model is used to explore the complex unsteady flow field in a single burner of a typical aeroengine combustor with two-stage counter-rotating swirler. The complex geometric configuration including all film cooling holes is fully simulated without any conventional simplification in order to reduce the modeling errors. First, unsteady process that flow developing from static to statistically stationary state is fully simulated under laminar inlet condition to obtain a fundamental understanding of flow characteristics in the combustor. Afterwards, synthetic eddy method is utilized to generate a turbulent inlet condition so that a perturbation with about 5% turbulence intensity is superimposed to the inlet plane. Simulation result shows that for the laminar inflow case, flow separation occurs in the near-wall region of the diffusion section, inducing a boundary layer transition and consequently introducing turbulence with nonuniformity in space before the swirler. In contrast, synthesized inflow generated under turbulent inlet condition by synthetic eddy method is more spatially homogeneous. Time-averaged flow field inside the swirler cup reveals that turbulent inflow ultimately causes the swirling flow with higher rotating speed in central region and more uniform distribution along the circumferential direction. It also enhances the transverse jet flow from primary holes and reverse flow in the central recirculation zone, and makes streamlines corresponding to the recirculation vortices more symmetrical on central profile. Maximum recirculating velocity predicted in central recirculation zone is −27.65 m/s and −17.86 m/s in turbulent and laminar case respectively, and corresponding total pressure recovery coefficient is 96.03% and 96.81%.

A CFD study of coupled aerodynamic‐hydrodynamic loads on a semisubmersible floating offshore wind turbine

AbstractThe prediction of dynamic characteristics for a floating offshore wind turbine (FOWT) is challenging because of the complex load coupling of aerodynamics, hydrodynamics, and structural dynamics. These loads should be accurately calculated to yield reliable analysis results in the design phase of a FOWT. In this study, a high‐fidelity fluid‐structure interaction simulation that simultaneously considers the influence of aero‐hydrodynamic coupling due to the dynamic motion of a FOWT has been conducted using computational fluid dynamics based on an overset grid technique. The DeepCwind semisubmersible floating platform with the NREL 5‐MW baseline wind turbine model is considered for objective numerical verification with the NREL FAST code. A state‐of‐the‐art computational model based on the coupled computational fluid dynamics and dynamic structure analysis is constructed and analyzed to solve multiphase flow, 6 degrees of freedom motions of OC4 semisubmersible FOWT. A quasi‐static mooring solver is also applied to resolve the constraint motion of floater because of a 3‐line mooring system. The influence of tower shadow on the unsteady aerodynamic performance and loads is also demonstrated. Finally, complex unsteady flow fields considering blade and tower interference effects among blade‐tip vortices, shedding vortices, and turbulent wakes are numerically visualized and investigated in detail.

Investigation on the unsteady aerodynamics of cycloidal propeller in hovering flight

The aerodynamics of a cycloidal propeller operating in hovering flight is systematically studied using numerical method. By solving the time-dependent incompressible Navier–Stokes equations and using a dynamic mesh technique, the complex unsteady flow field around the cycloidal propeller and the time-varying aerodynamic forces are investigated. The wake flow deflection can be observed in the flow field of the cycloidal propeller, which results in a deflection of the thrust. Strong downwash around cycloidal propeller and unsteady interference between blades are induced by the rotation and pitch motion of the blade. The instantaneous vertical, horizontal and resultant thrusts fluctuate cyclically with the azimuths in a pattern similar to sinusoid, and although the direction of instantaneous resultant thrust varies in a comparatively large region during rotating, the direction of averaged resultant thrust produced in each revolution is almost constant at different rotation speeds. This is beneficial to the flight stability and maneuverability of a cyclogyro. When one blade moves upward, the aerodynamic forces of the blade are much higher than that produced when moving downward. With the increase of the blade number, the unsteady interference between blades and the wakes of other blades tend to be stronger, producing great effects on aerodynamic forces of the blades as well as the aerodynamic performance of cycloidal propeller.

Airfoil Dynamic Stall and Aeroelastic Analysis Based on Multi-frequency Excitation Using CFD Method

This paper conduct a comprehensive research on time varying pitching, plunging (flapping) motion and Periodic alternating oncoming flow for NACA0012 airfoil in order to accurately capture the real aerodynamic characteristics and airfoil responses of the rotor blades in complex unsteady flow field. When simulating the pitch motions, the excitation of the second and third harmonic were got into consideration. The result shows that: (1) the aerodynamic coefficient generated for the steady stall and pitching motion are compared well with the experimental data; (2) when the second and third harmonic were got into consideration for the pitching motion, the aerodynamic loads and response show significant difference which would be used for active control of helicopter.

1619 トルクコンバータモデル内部流れのダイナミックステレオPIV計測と速度場の空間再構築

Unsteady flow field inside a torque converter model made of acrylic resign is measured using a dynamic stereo PIV system. The index-matching technique is used to visualize the flow field in the gap between an impeller and a turbine. The rotation speed of the impeller is 250rmp while those of the turbine are 25rmp and 150rpm, giving the speed ratios of 0.1 and 0.6, respectively. The instantaneous velocity maps obtained from the stereo PIV analysis are phase averaged according to the relative angles of the impeller, the turbine and the stator. Those phase averaged three-component velocity maps are reconstructed to visualize 3-D characteristics of this complex unsteady flow field.

A Graph-Based Interface for Visual Analytics of 3D Streamlines and Pathlines.

Visual exploration of large and complex 3D steady and unsteady flow fields is critically important in many areas of science and engineering. In this paper, we introduce FlowGraph, a novel compound graph representation that organizes field line clusters and spatiotemporal regions hierarchically for occlusion-free and controllable visual exploration. It works with any seeding strategy as long as the domain is well covered and important flow features are captured. By transforming a flow field to a graph representation, we enable observation and exploration of the relationships among field line clusters, spatiotemporal regions and their interconnection in the transformed space. FlowGraph not only provides a visual mapping that abstracts field line clusters and spatiotemporal regions in various levels of detail, but also serves as a navigation tool that guides flow field exploration and understanding. Through brushing and linking in conjunction with the standard field line view, we demonstrate the effectiveness of FlowGraph with several visual exploration and comparison tasks that cannot be well accomplished using the field line view alone. We also perform an empirical expert evaluation to confirm the usefulness of this graph-based technique.

A novel multiblock immersed boundary method for large eddy simulation of complex arterial hemodynamics

Computational fluid dynamics (CFD) simulations are becoming a reliable tool to understand hemodynamics, disease progression in pathological blood vessels and to predict medical device performance. Immersed boundary method (IBM) emerged as an attractive methodology because of its ability to efficiently handle complex moving and rotating geometries on structured grids. However, its application to study blood flow in complex, branching, patient-specific anatomies is scarce. This is because of the dominance of grid nodes in the exterior of the fluid domain over the useful grid nodes in the interior, rendering an inevitable memory and computational overhead. In order to alleviate this problem, we propose a novel multiblock based IBM that preserves the simplicity and effectiveness of the IBM on structured Cartesian meshes and enables handling of complex, anatomical geometries at a reduced memory overhead by minimizing the grid nodes in the exterior of the fluid domain. As pathological and medical device hemodynamics often involve complex, unsteady transitional or turbulent flow fields, a scale resolving turbulence model such as large eddy simulation (LES) is used in the present work. The proposed solver (here after referred as WenoHemo), is developed by enhancing an existing in-house high-order incompressible flow solver that was previously validated for its numerics and several LES models by Shetty et al. (2010) [33]. In the present work, WenoHemo is systematically validated for additional numerics introduced, such as IBM and the multiblock approach, by simulating laminar flow over a sphere and laminar flow over a backward facing step respectively. Then, we validate the entire solver methodology by simulating laminar and transitional flow in abdominal aortic aneurysm (AAA). Finally, we perform blood flow simulations in the challenging clinically relevant thoracic aortic aneurysm (TAA), to gain insights into the type of fluid flow patterns that exist in pathological blood vessels. Results obtained from the TAA simulations reveal complex vortical and unsteady flow fields that need to be considered in designing and implanting medical devices such as stent grafts.

Numerical Analysis of Interaction Between Single-Pulse Laser-Induced Plasma and Bow Shock in a Supersonic Flow

The interaction of laser-induced plasma and bow shock over a blunt body is investigated numerically in an M∞ = 6.5 supersonic flow. A ray-tracing method is used for simulating the process of laser focusing. The gas located at the focused zone is ionized and broken down and transformed into plasma. In a supersonic flow the plasma moves downstream and begins to interact with the bow shock when it approaches the surface of the blunt body. The parameters of flowfield and blunt body surface are changed due to the interaction. By analyzing phenomena occurring in the complex unsteady flowfield during the interaction in detail, we can better understand the change of pressure on the blunt body surface and the mechanism of drag reduction by laser energy deposition. The results show that the bow shock is changed into an oblique shock due to the interaction of the laser-induced low-density zone with the bow shock, so the wave drag of the blunt body is reduced.

Two and Three Dimensional Simulations of Supersonic Cavity Configurations

:The flow over cavities can produce complex unsteady flowfields that are an important practical concern in aerospace applications. Understanding the flow field in the cavity can help to determine the driving mechanism of the oscillations to create effective control methods to avoid structural damage. In this study, both two and three dimensional time-dependent Reynolds-Averaged Navier-Stokes simulations are performed for the flow over an open cavity to demonstrate the capability of computational fluid dynamics to accurately predict the supersonic flow field inside a cavity and to assess the adequacy of two dimensional simulations in capturing cavity flow physics accurately. In this particular problem of a supersonic flow over a cavity, the three-dimensional effects cannot be neglected. A two-dimensional simulation would give an insight about the longitudinal mode of the cavity flow without any three-dimensional effects, but it is not enough to understand the real physics of the problem.

Simulation of embryo transport in a closed uterine cavity model

Objective Uterine peristalsis is induced by synchronized contractions of the non-pregnant myometrium. The resulting periodic motions of the anterior and posterior walls of the uterus generate intrauterine motions which serve as transport vehicles for crucial processes of the early stages of mammalian reproduction. Among other duties, these flow currents are responsible for transporting the embryo to a successful site of implantation when the time window for implantation is open. In this study we investigated the transport characteristics of embryos with a new computational model. Study design A computational model of idealized peristaltic flow within a two-dimensional channel with a closed end was implemented to study pre-implantation transport of embryos within the sagittal cross-section of the uterine cavity, which is closed at the fundal end and open towards the cervix. A commercial finite-element numerical package was implemented to solve the complex unsteady flow field. Results The numerical simulations revealed that the fluid flow field and the resulting transport characteristics of peristaltic flow in a closed channel are strongly affected by the closed end. The velocity profiles are also dependent on wall motility, level of asymmetry and frequency of the peristalsis. The trajectories of massless particles demonstrate periodic recirculation in small moving loops around their initial location along and across the channel. Particles which were initially separated by one wavelength along the channel length were transported in almost identical patterns. Particles which were initially located at full wavelengths experience very small velocities, and thus, their net axial displacement is negligible. Conclusions The major outcome of the model simulation is that particles at almost any location, except those at full wavelengths of the peristalsis, are most likely to recirculate in small moving loops around their initial location. Accordingly, embryos are expected to be implanted in the uterine wall in the vicinity where they were initially placed, either naturally or artificially, within the uterine cavity.

Shock tube flows past partially opened diaphragms

Unsteady compressible flows resulting from the incomplete burst of the shock tube diaphragm are investigated both experimentally and numerically for different initial pressure ratios and opening diameters. The intensity of the shock wave is found to be lower than that corresponding to a complete opening. A heuristic relation is proposed to compute the shock strength as a function of the relative area of the open portion of the diaphragm. Strong pressure oscillations past the shock front are also observed. These multi-dimensional disturbances are generated when the initially normal shock wave diffracts from the diaphragm edges and reflects on the shock tube walls, resulting in a complex unsteady flow field behind the leading shock wave. The limiting local frequency of the pressure oscillations is found to be very close to the ratio of acoustic wave speed in the perturbed region to the shock tube diameter. The power associated with these pressure oscillations decreases with increasing distance from the diaphragm since the diffracted and reflected shocks partially coalesce into a single normal shock front. A simple analytical model is devised to explain the reduction of the local frequency of the disturbances as the distance from the leading shock increases.

Experimental methods for multi-diagnostics of flow fields in wind tunnels

Advanced optical and acoustical measurement techniques as developed during the last decade allow new insight into complex unsteady flow fields and sound fields in aerodynamics. Parallel application of such experimental diagnostics methods allows a more complete description of complex flow phenomena and provides high quality data for validation of numerical calculations. In particular, three examples for multi-diagnostics of the flow field above a delta wing and within the slat of a high lift configuration by means of PSP and PIV, PIV and PDMS, and by means of PIV and the Acoustic Microphone Array Technique will be given during the presentation.

Investigation of the Unsteady Rotor Aerodynamics in a Transonic Turbine Stage

The paper focuses on the unsteady pressure field measured around the rotor midspan profile of the VKI Brite transonic turbine stage. The understanding of the complex unsteady flow field is supported by a quasi-three-dimensional unsteady Navier–Stokes computation using a k-ω turbulence model and a modified version of the Abu-Ghannam and Shaw correlation for the onset of transition. The agreement between computational and experimental results is satisfactory. They both reveal the dominance of the vane shock in the interaction. For this reason, it is difficult to identify the influence of vane-wake ingestion in the rotor passage from the experimental data. However, the computations allow us to draw some useful conclusions in this respect. The effect of the variation of the rotational speed, the stator–rotor spacing, and the stator trailing edge coolant flow ejection is investigated and the unsteady blade force pattern is analyzed.

Ecoulement instationnaire dans un diffuseur aubé d'une pompe centrifuge

This paper describes experimental and computational work to examine the unsteady flow field in the vaned diffuser of a medium specific speed pump. The aim of this project is to verify a commercialy available Navier-Stokes program with a k-E turbulence model and to improve our understanding of the complex unsteady three-dimensional flow field associated with the interaction between a pump impeller and its vaned diffuser. The paper provides the results of experiments carried out using Laser Doppler Anemometry (LDA), in which time-resolved details of the unsteady flow field in a vaned diffuser of a medium specific speed pump have been obtained as a function of the local position of the pump impeller blades. The static pressures were also measured at the hub and shroud side of the diffuser. The measurement results have been analysed to elucidate some interesting flow features observed in this typical pump diffuser. These include unsteady recirculation of the flow from the diffuser into the impeller at partload and flow separation in the diffuser channel. The unsteady flow simulation of a two dimensional diffuser showed the capabilities and the limits of a Navier Stokes program with k-E turbulence model. The comparisons of the simulation results with experimental data identify that this simple computational model with a time periodic inlet profile is able to show important flow phenomena. In terms of the flow field and pressure rise there is no substantial difference between the predictions of a steady simulation and the time-average of an unsteady simulation.