Unsteady Computational Fluid Dynamics Research Articles

Atrial fibrillation increases thrombogenicity of LVAD therapy.

This study investigates the hypothesis that presence of atrial fibrillation (AF) in LVAD patients increases thrombogenicity in the left ventricle (LV) and exacerbates stroke risk. Using an anatomical LV model implanted with an LVAD inflow cannula, we analyze thrombogenic risk and blood flow patterns in either AF or sinus rhythm (SR) using unsteady computational fluid dynamics (CFD). To analyze platelet activation and thrombogenesis in the LV, hundreds of thousands of platelets are individually tracked to quantify platelet residence time (RT) and shear stress accumulation history (SH). The irregular and chaotic mitral inflow associated with AF results in markedly different intraventricular flow patterns, with profoundly negative impact on blood flow-induced stimuli experienced by platelets as they traverse the LV. Twice as many platelets accumulated very high SH in the LVAD + AF case, resulting in a 36% increase in thrombogenic potential score, relative to the LVAD + SR case. This supports the hypothesis that AF results in unfavorable blood flow patterns in the LV adding to an increased stroke risk for LVAD + AF patients. Quantification of thrombogenic risk associated with AF for LVAD patients may help guide clinical decision-making on interventions to mitigate the increased risk of thromboembolic events.

Lift tailoring on unsteady airfoils with leading-edge vortex shedding using an inverse aerodynamic approach

In this paper, we present a physics-informed approach to tailor the lift profile of an unsteady airfoil through the execution of an appropriate maneuver. In previous research, a low-order aerodynamic model based on the unsteady thin airfoil theory was developed for predicting the flowfield and loads on airfoils undergoing arbitrary motions. The theory was phenomenologically augmented using the concept of leading edge suction parameter (LESP) to incorporate the capability to predict intermittent leading edge vortex (LEV) shedding. The criticality of LESP was used to predict the onset and termination of LEV shedding and thus model the effect of LEVs on the flowfield and loads for a prescribed motion. In the current work, an inverse aerodynamic formulation is developed based on this framework for tackling the inverse problem: to obtain the motion kinematics required for generating a prescribed lift profile for an airfoil operating in the dynamic-stall regime. The LEV-modeling capability of the aerodynamic model enables the motion-design algorithm to take into account the effect of complex phenomena, such as dynamic stall and LEV shedding, which are not taken into account in previous research approaches. Several case studies are presented to demonstrate various scenarios such as lift tracking using pitching and heaving motions, lift cancellation during unsteady motion, and the generation of a given lift profile using two equivalent motions. The kinematic profiles generated by the inverse formulation are also simulated using a high-fidelity unsteady computational fluid dynamics solver to validate the predictions.

Data-Driven Unsteady Aerodynamic Modeling for Fluid–Structure Interaction

The piston theory class of simplified aerodynamic models that link body deformation to localized pressure fluctuation with respect to a mean steady state is attractive for modeling purposes as they constitute a computationally efficient model to calculate the unsteady pressure response on deforming bodies. However, as piston theories are valid for flow regimes where M>1.5, they are inaccurate for modeling the aerodynamic response associated with aeroelastic phenomena in subsonic or low-supersonic flow regimes. Unsteady computational fluid dynamics (CFD) simulations are used to learn the unsteady pressure response, and dynamic mode decomposition (DMD) is applied to the error between the CFD-computed pressure fluctuation and the piston-theory-predicted pressure fluctuation. This process is performed on two-dimensional and three-dimensional flow domains over various flow regimes. For both scenarios, the DMD analysis learns dominant spatial modes in the error that constitute low-order models, which may be used to improve the pressure response and are confirmed to be physically relevant. A method for computing aeroelastic stability with these leading modes is discussed and applied to fluid–structural beam and panel configurations. The results are compared against prior numerical and experimental investigations to quantify the improvement in prediction obtained by the DMD-augmented piston theory.

Aerodynamic design of a high-efficiency two-stage axial turbine, using streamline curvature method and performance optimisation by clocking of stator blades

Abstract Aerodynamic design of a high-efficiency two-stage axial turbine is carried out using a hybrid method through implantation of a two-step design procedure. In the first step, the well-known streamline curvature (SLC) and free vortex (FV) methods are properly combined to establish three-dimensional geometries of the blades at each row and to obtain the flow field properties. The second step is provided to obtain the highest aerodynamic efficiency by optimum clocking of the second stator blades relative to the first ones through executing steady and unsteady computational fluid dynamics (CFD) of three-dimensional viscous flow. Slight discrepancies were observed between gas dynamics results of the SLC and those of CFD. Total pressure and temperature at the turbine outlet, obtained from SLC method, differed from those obtained by 3D-CFD technique by 13.06% and 1.88% respectively. Aerodynamic efficiency of the turbine is obtained about 91.83%, based on 3D-CFD. Time-averaged results showed that under the optimum clocking of the second row stator blades, inlet total pressure and output power of the second rotor increase by 0.23%, and 0.93%, respectively, in comparison to the worst clocking case. These augmentations resulted in increased total to total efficiency of the second stage by 0.444%. Additionally, the total output power of the two stages increased by 0.71% through the optimum clocking. Modeling the unsteady wake flow trajectory within the blades passages confirmed that all of these beneficial effects happen if the upstream wake impinges on the leading edge region of the second stator blades.

Investigation on unsteady characteristics of flow separation in a supercritical carbon dioxide centrifugal compressor

As one of the core components of supercritical carbon dioxide (S-CO2) closed Brayton cycle, the efficiency of S-CO2 centrifugal compressor plays a crucial role. Affected by the special thermophysical properties of S-CO2 fluid, the flow mechanism of S-CO2 centrifugal compressor is different from the centrifugal compressor of conventional fluid, such as air. Previous studies have found that the flow separation within the flow domain of this kind of compressor is easily to occur downstream the pressure surface. Many steady computational fluid dynamics (CFD) studies have been conducted on the S-CO2 centrifugal compressors, but few studies focused on the unsteady evolution of this flow separation. In this paper, the unsteady CFD simulation is carried out in the flow passage of a S-CO2 centrifugal compressor. The solution domain of CFD simulation includes compressor blades, diffuser and volute. The performance and the unsteady flow behavior of S-CO2 centrifugal compressor is obtained. Under low flow rate conditions, the flow separation on the pressure surface of the blade of S-CO2 centrifugal compressor is more likely to occur, causing the decrease of compressor performance. This flow separation has strong unsteady characteristics, which deteriorates several times during a rotation cycle, meanwhile the vortex shedding happens. At the time steps of vortex shedding, the pressure near the trailing edge of the impeller fluctuates greatly, indicating that this unsteady flow separation brings a large flow loss to the S-CO2 centrifugal compressor.

Effect of blade number on rotor efficiency and noise emission at hovering condition

The configuration of rotors significantly impacts the aerodynamic efficiency and noise emission of multicopters. To date, there are no general guidelines regarding how many blades a rotor should use for optimal aerodynamic performance and minimum noise emission. From the perspectives of aerodynamics and acoustics during the hovering condition, two key parameters, i.e., figure of merit (FM) and overall sound pressure level (OASPL), are evaluated to determine the optimal blade number (BN). The number of blades chosen in this study is BN = 2–6, which is largely observed in commercial multicopters. A genetic algorithm was developed to optimize blade design for each BN-rotor configuration. The individuals are evaluated by steady computational fluid dynamics (CFD) simulations and acoustic analogy for optimizations, and the detailed analyses of optimal ones are further explored by unsteady CFD simulations. The planform of the baseline blade is maintained, and the radial distribution of twist angles is the parameter for optimization. While generating the same thrust, the value of FM keeps increasing as the number of blades increases from 2 to 4, after which the FM value reaches a plateau. The value of OASPL keeps decreasing as the number of blades increases. The reason for the FM and OASPL value trends vs blade number is explained with the numerical simulation results, and a general design rule is suggested at the end.

An R-adaptive algorithm based on self-organizing maps for solving incompressible flows with high-order discontinuous Galerkin methods

Mesh quality is critical for the numerical accuracy of CFD (Computational Fluid Dynamics). Although various techniques have been developed to improve mesh applicability to complex flows, r-adaptive methods have received far too little attention. This paper introduces an r-adaptive algorithm based on the self-organizing maps (SOM) of Kohonen and applies it to unsteady CFD applications using the Discontinuous Galerkin (DG) method. Inspired by the properties of the DG method, the numerical discontinuity over the element interface is quantified as an adaptation indicator. The method requires only one adaptation, followed by solution projection onto the new mesh to save computational costs in unsteady cases. A significant advantage of this method is its higher efficiency without solving complex equations during the adaptation. The performance of the developed mesh adaptation algorithm is tested on the steady laminar viscous flow past a circular cylinder at Re=40 and the unsteady laminar viscous flow past a circular cylinder at Re=100 and two circular cylinders in a side-by-side arrangement at Re=200. Significant improvements in flow contours and coefficients are obtained after adaptation.

Toward Airframe Noise Reduction for Passenger Aircraft

Airframe noise is one of the main noise sources of the latest passenger aircraft during the approach phase. To demonstrate noise reduction technology based on unsteady Computational Fluid Dynamics (CFD) and wind tunnel tests, JAXA conducted flight tests using the JAXA Experimental Aircraft "Hisho" in 2016 and 2017 in cooperation with industries in Japan. The noise data obtained from flyover noise source measurements successfully showed consistent characteristics with the noise reduction designs for the flaps and the main landing gear. The noise measurement data from the flight test 2017 were used to validate the wind tunnel tests and CFD used in the design phase. Subsequently, the research activities proceeded towards the research and development targeting regional jet aircraft, aiming for practical application in passenger aircraft. This paper overviews these research activities toward developing airframe noise reduction technology for passenger aircraft.

Sensitivity of compressor aerodynamic noise prediction to time setup of CFD simulations using hybrid CAA method

This paper numerically studies the aerodynamic noise of a centrifugal compressor in a vehicle engine turbocharger. The hybrid CAA approach based on computational fluid dynamics (CFD) and acoustic boundary element method (BEM) in the prediction of aerodynamic noise is validated. The impacts of the time setup of unsteady CFD simulations on the prediction of compressor flow and aerodynamic noise are discussed. The results show that a large time step would result in unreliable numerical results, in which severe reverse flow and flow separation can be observed. The middle time step 2 deg. can get accurately predicted results while reducing the computational efforts. It is also found that a large total simulation time with a reasonable time step is helpful to improve predicted noise spectra. Moreover, the overall sound pressure level discrepancy of aerodynamic noise between numerical prediction and experimental measurement is less than 0.5 dB.

Study of unsteady flow-induced vibration of centrifugal pump volute casing using fluid-solid coupling

The paper studies the flow-induced vibration of the volute of a centrifugal pump using the coupled approach of computational fluid dynamics (CFD) and finite element method (FEM). In the present paper, a numerical model of the centrifugal pump for fluid-solid coupling solution was established, and the unsteady flow of the centrifugal pump under different working conditions was simulated by unsteady CFD calculations. Then, the fluctuation pressure on the volute inner surface was extracted and used as the fluid load that induces the volute vibration. The volute vibration response including displacement, and acceleration was discussed based on the results of fluid-solid coupling solutions. The results show that the non-uniform distribution of fluctuation pressure along the volute, and the dominant vibration frequencies of the pump volute are blade frequency and its harmonics, and the minimum vibration and stress of the volute appear at the design condition. Overall, this research gives insight into the flow-induced vibration of centrifugal pump volute casing.

Aerodynamics and bird ingestion characteristics of a bulge-adjustable turboprop engine inlet

Turboprop aircraft plays an important role in the field of regional airliners and military transport. During take-off or landing, turboprop aircrafts frequently encounter bird flocks that can be ingested into the engine inlet, thereby posing a significant threat to flight safety. Consequently, the aerodynamic characteristics and bird ingestion characteristics of turboprop inlet are investigated in this article. An unsteady computational fluid dynamics (CFD) approach has been proposed to calculate bird ingestion characteristics in a turboprop inlet with different bulge heights under propeller interference by combining a self-developed collision-rebound model with the overset mesh method and 6-degree of freedom (6-DOF) motion method. Considering the characteristics of bird strikes, a parameterized controlled bulge of turboprop inlet with a scavenge duct has been designed. Moreover, the trajectory characteristics of the bird, bird-inlet coupling flowfield characteristics and the aerodynamic characteristics are analyzed in depth. The bulge of the inlet plays a positive role in preventing the bird object from entering the engine duct at a 23° incidence angle. The bird rebounds from the upper bulge and is discharged through the scavenge duct at MaAIP=0.45. The results show that the growth of inlet bulge height has the minimal influence on the total pressure recovery coefficient σ, with only approximately 0.3% variation within the bulge height range of interest. However, it significantly affects the total pressure distortion DC60 with an increase up to 84%.

3D CFD model for the analysis of the flow field through a horizontal axis wind turbine (HAWT)

With the world’s growing demand for energy, renewable energy production has become important in providing alternative sources of energy and in reducing the greenhouse effect. This study investigates the aerodynamics and performance of the WG/EV100 micro–Horizontal Axis Wind Turbine (HAWT) using Computational Fluid Dynamics (CFD). The complexity of VAWT aerodynamics, which is inherently unsteady and three-dimensional, makes high-fidelity flow models extremely demanding in terms of computational cost, limiting the analysis to mainly 2D Computational Fluid-Dynamics (CFD) approaches. This article explains how to perform a full 3D unsteady CFD simulation of HAWT. All main parts of the WG/EV100 HAWT were designed in SOLIDWORKS. Only the blade design was reverse engineered due to the unavailability of the CAD model and the complexity of its geometric characteristics. The impeller blade is scanned using a Coordi-nate Measuring Machine (CMM), and the obtained 3D scan data are exported from the PC-DMIS software to GEOMAGIC design X to obtain a CAD model of the blade.

High-fidelity modeling of a vertical axis tidal turbine model under realistic flow conditions

In the context of the global energy crisis, developing renewable energies is of primal importance. Among the renewable resources available are tidal currents. Tidal currents can be exploited with tidal turbines. Different concepts of tidal turbine exist, we here investigate a vertical axis tidal turbine or VATT prototype made by HydroQuest and CMN. More precisely, we carry on numerical investigations on a Hydroquest/CMN tidal turbine model. Experimental studies are limited to turbine models and thus suffer from scale effects. In situ measurement are still expensive and difficult to acquire. Computational Fluid Dynamics (CFD) is an interesting alternative in order to go beyond what experiments can offer. One major drawback of CFD is that it requires validation of the models used. A validation of our CFD tool is included in this paper. Because the hydrodynamic of vertical axis tidal turbines is highly unsteady, Large Eddy Simulation (LES) are well adapted here. Blade-resolved Computational Fluid Dynamics (CFD) approaches are appropriate to study vertical axis tidal turbines, but their cost is still prohibitive for real size tidal turbine. Simplified approaches are preferred solutions. Literature showed that Actuator Line Model (ALM) LES is well suited to model vertical axis turbine. Most tidal sites are characterized with strong turbulence intensities. The presence of turbulence in most tidal sites has led companies and academics to study the effects of ambient turbulence over tidal turbines. It has been shown that turbulence can have a non-negligible influence on the turbine performances and wake. To account for the effects of turbulence, synthetic turbulence is introduced at the inlet of the simulation. The LBM is an unsteady weakly-compressible CFD approach. It solves the Boltzmann equation using an explicit time discretization and a uniform Cartesian grid called lattice. It has been proven to be an efficient approach for modeling tidal turbines using LES and ALM, Grondeau et al. (2019). In this paper, we study a tidal turbine model of the Hydroquest/CMN VATT using an ALM-LBM-LES approach. This model was tested at the Ifremer Boulogne-sur-Mer testing facilities. We first compare our results with the experimental ones in a scenario without turbulence and for flood and ebb tide configurations. A numerical investigation of the influence of upstream turbulence on the turbine performances and wake is then realized. Prospects of this study include among others the effects of turbines interactions and waves.

Impacts of Inlet Circumferential Distortions on the Aerodynamic Performance of a Transonic Axial Compressor

Distortion, such as total pressure distortion, is a common phenomenon at the inlet of an axial compressor. Nonuniform inflow can greatly affect the aerodynamic performance and the stability of the compressor. In this paper, a sinusoidal distortion model is used at the inlet. Then, a series of unsteady computational fluid dynamics (CFD) simulations with a full-annulus model is conducted. Three kinds of the total pressure distortions, such as circumferentially covering 60, 120, and 180-degs sectors above 70% span of the blade at the inlet section, are adopted. Based on the results, how the rotating stall cells can be induced and developed under the different inlet conditions is understood during the stall process. It is found that the secondary stall cell is more easily triggered when the circumferential range of total pressure distortion is increased. Meanwhile, the influence of the rotating distortion on the compressor performance is also studied and a dangerous distortion rotating speed is observed.

Vibroacoustic analysis of a submarine-propeller system under self-propulsion condition from the perspective of approximate configurations

As a particular instance of the complete acoustic radiation problem for an underwater structure, structure-borne noise radiated from a combined submarine-propeller system under self-propulsion condition is investigated. The analysis procedure applies a set of numerical methods to capture different aspects of the problem: unsteady computational fluid dynamics to express the hydrodynamic excitation, finite element analysis to obtain the dynamic characteristics of the combined elastic system, and boundary element method to assess the radiated noise owing to structural vibrations. The structural and acoustic responses rely on the modal descriptions. The study has three objectives: employing the methodology in a realistic setting, investigating the validity of the proposed approximate configurations, and studying the influence of propeller blade flexibility on noise radiation. The DARPA-SUBOFF submarine and INSEAN E1619 propeller models are adopted for the application. It is found that approximate configurations provide accurate estimations for limited frequency bands and should be employed with caution. In addition, the flexibility of the propeller plays a major role in noise generation, not only by yielding higher noise levels but also by increasing the amplitude of the peak responses in the low- and high-frequency regions.

Accelerating unsteady aerodynamic simulations using predictive reduced-order modeling

Unsteady computational fluid dynamics (CFD) simulations are essential in aerospace engineering because they can provide high-fidelity flow fields to better understand transient physics, such as vortex shedding. However, unsteady full-order CFD simulations must repeatedly march the flow solution with a small time step and are computationally expensive. Reduced-order modeling (ROM) is a powerful approach to alleviate the above issue by decomposing the unsteady flow solutions into spatial modes and temporal coefficients, making the unsteady flow easier to simulate. Existing ROM studies mostly focused on parametric problems that use a large number of simulation samples to train an offline model (parametric ROM). Although the trained model can quickly predict any flow fields within the parameter space, the computational cost for generating the massive unsteady simulation samples is still high, especially when the number of parameters and their ranges increase. To further address the high-cost issue, we develop an efficient predictive ROM approach to accelerate individual unsteady aerodynamic simulations. We use the Galerkin projection approach to reduce the Reynolds-averaged Navier–Stokes equations, along with the discrete empirical interpolation method (DEIM) for decreasing the computation cost for nonlinear terms. In addition, we develop an efficient ROM formulation that correlates the temporal coefficients between the momentum, pressure, and turbulence equations, allowing solving fewer ROM equations at the prediction stage. The intrusive nature of the predictive ROM enables using the first portion of unsteady flow data to accelerate the rest of the simulation; no massive offline samples are needed. We use the stalled turbulent flow over the NACA0012 airfoil as the benchmark and evaluate the predictive ROM's speed and accuracy for challenging non-equilibrium scenarios caused by a large sudden change in flow conditions. The run time ratios between CFD and ROM (including the calculation of basis vectors and projection matrices) range between 13 and 45. The pressure, drag, lift, pitching moment, and flow fields simulated by the ROM approach agree reasonably well with the CFD references at various times. The proposed predictive ROM approach is, in principle, applicable to different airfoil geometries and flow conditions and can be integrated into a design optimization process for accelerating unsteady aerodynamic simulations.

Systematic investigation of power enhancement of Vertical Axis Wind Turbines using bio-inspired leading edge tubercles

Wind energy is employed as an effective source for catering to the increasing energy demands and depleting fossil fuels, while also counteracting their adverse environmental effects. VAWTs have numerous advantages over HAWTs. But, the relatively low power coefficient of VAWTs in comparison to HAWTs restricts their utilization. Bio-inspired leading-edge tubercles appear to be one of the potential design changes for enhancing the power performance of VAWTs. This study systematically investigated the relative influence of tubercle design variables and their geometrical trend subjected to power performance by employing a hybrid Design of Experiments (DoE) approach and Response Surface Methodology (RSM) rather than choosing random values of tubercle variables. Furthermore, the conflict existing in the literature about the improved or degraded power performance of VAWTS with tubercles is resolved by evaluating the power performance of VAWT at on-design and off-design conditions. For the computation of aerodynamic forces, unsteady Computational Fluid Dynamics (CFD) was utilized with a 4-equation transition SST turbulence model. Tubercle amplitude influences the aerodynamic efficiency of VAWTs relatively more than its wavelength. It is evident from the results that energy is harnessed efficiently at on-design conditions considering lower tubercle amplitude and higher wavelength. However, the reverse trend of variables is observed under off-design conditions. At on-design conditions, tubercles degraded the VAWTs performance in comparison to baseline VAWT configuration by a minimum of 13.55% among the 14 hybrid DoE generated cases, while the performance was enhanced by a maximum of 55% at off-design conditions of VAWT.

Study on aerodynamic performance of inverted cone vertical axis wind turbine with different rotor configurations

Offshore wind resources are abundant. Vertical axis wind turbines (VAWTs) are suitable for working in the sea environment because of their low cost and high reliability. In this paper, an offshore floating inverted cone vertical axis wind turbine (ICVAWT) is proposed. The 3D unsteady computational fluid dynamics (CFD) method is utilized to model the ICVAWT. The turbulence model SST k−ω is used to solve the Navier–Stokes equation, and the setting parameters of the solver and the independent grid are determined. The scale experimental prototype is designed using the similarity theory and the wind tunnel experiment is carried out. The experimental results verify the validity of the CFD model. The aerodynamic performance of nine ICVAWT configurations is analyzed by using the CFD model, and the effects of inverted cone angle and blade number on the power coefficient and operation stability of the ICVAWT are studied. It is found that when the inverted cone angle is 45°, and the number of blades is three (called the optimal configuration), the power coefficient is the highest at 0.309 (the optimal tip speed ratio is 3.5). With the increase in inverted cone angle and blade number, the fluctuation of wind turbine operating torque decreases. According to the distribution of each physical quantity in the flow field, the changes in aerodynamic performance are explained. Further research on the optimized configuration is carried out.

Dynamic Response of Circulation Control for Step and Sinusoidal Inputs

A numerical study of the dynamic performance of a circulation controlled (CC) airfoil is performed to explore the feasibility of using circulation control as means of gust suppression. Steady and unsteady two-dimensional computational fluid dynamics (CFD) simulations were initially validated using published data. The dynamic performance of a CC airfoil was subsequently assessed. The response of the CC airfoil to step and sinusoidal inputs were obtained from unsteady CFD simulations. It was found that the actuation speed of circulation control is significantly faster than a conventional mechanical flap at the same flight condition. Additionally, the lift response to a step input was found to be very similar to the Küssner function.

Effects of different inlet configurations on the performance of solar storage tanks: A three-dimensional unsteady CFD simulation

A validated three-dimensional unsteady computational fluid dynamics analysis is performed in this study to investigate the effects of the inlet flow conditions on the thermal performance of solar storage tanks. First, effects of different mass flow rates on the thermal performance of a simple tank (Model 1, single inlet and single outlet) are investigated. The results show that higher mass flow rates deteriorate thermocline and thermal stratification which is desirable. To reduce the unfavorable effects of inlet jet mixing with stagnant fluid, the mass flow rates are divided equally between two inlet ports in the next model (Model 2). The results show that this model acts better than previous model form heat transfer point of view. In the next model (Model 3), the simple inlet port is replaced by a circular truncated cone shaped diffuser to reduce the momentum of the jet and impede unwanted mixing in the tank. The effects of the diffuser aspect ratio are analysed at fixed mass flow rates. The results reveal that small aspect ratios improve the performance of the tank while higher aspect ratios deteriorate the performance because of strongly adverse pressure gradient occurred inside the diffuser, which consequently results in a high-speed core flow, that contradicts with the idea of employing a diffuser. The outcomes indicate that only the aspect ratios between 1.0 and 1.5 have positive effects. In the final section, the study continues by modifying Model 3 through examining different orientations of the inlet port with respect to horizon (Model 4). Different angels including −30°, −15°, 15°, 30° are considered and the results show that changing inlet orientation is not as effective as the previous modifications.