Unsteady CFD Research Articles

Unsteady Aerodynamics of Delta Kites for Airborne Wind Energy Under Dynamic Stall Conditions

ABSTRACTThree unsteady aerodynamic tools at different levels of fidelity and computational cost were used to investigate the unsteady aerodynamic behavior of a delta kite applied to airborne wind energy. The first tool is an in‐house unsteady panel method that is fast but delivers low to mid fidelity predictions. The second tool uses the open‐source CFD code SU2 to solve the unsteady Reynolds‐averaged Navier–Stokes equations with the SST turbulence model. At an intermediate level of fidelity, a semiempirical dynamic stall model that combines the panel method with a phenomenological dynamic stall module is proposed. The latter has free parameters that are fine‐tuned with CFD results from the second tool. The research on the dynamic stall model has been inspired by two flight test campaigns suggesting dynamic stall phenomena possibly driven by the periodic variation of the angle of attack (aerodynamic pitching motion) during crosswind maneuvers. The recorded inflow along the flight path was prescribed in the three aerodynamic tools. As expected, the price to pay for the low computational cost of the panel method is its inability to capture the dynamic stall phenomenon. The results from unsteady CFD qualitatively matched the experimental data identifying a leading‐edge vortex that forms and detaches cyclically during the pitching motion. Using RANS data, the semiempirical tool was fined‐tuned to reproduce the dynamic stall behavior, becoming an accurate and fast aerodynamic tool for coupling with any kite flight simulator. Further discussions on the effects of kite aerostructural deflections are included.

Unsteady RANS CFD Simulation on the Parametric Roll of Small Fishing Boat under Different Loading Conditions

Unsteady RANS CFD Simulation on the Parametric Roll of Small Fishing Boat under Different Loading Conditions

Attenuation of Inlet Distortion Effects on Fans Using Asymmetric Inlet Guide Vanes

Abstract This research proposes and preliminarily analyzes a novel concept to passively reduce the negative impact of deep inlet distortion on the fan of an aero-engine. It consists of placing a row of non-axisymmetric inlet guide vanes (IGVs) just upstream of the fan rotor to induce a spatially varying swirl distribution. The swirl distribution is tailored so as to reduce flow incidence in the distorted flow region and increase it in the undistorted flow region to decrease the fluctuation in aerodynamic force on the fan blades under large inlet distortion that can lead to blade failure, as well as attenuate the negative effect of flow non-uniformity on fan/engine aerodynamic performance. A computational study is carried out on a high-speed (transonic) fan rotor (NASA Rotor 67) from a published distortion study using full-annulus unsteady 3D computational fluid dynamics (CFD) simulations. The asymmetric IGV is designed through a process of manual iterations and CFD simulations to take into account the change in flow redistribution with IGV geometry. The asymmetric IGV design, though not optimized, reduces the aerodynamic force variation amplitude by around two-thirds. Moreover, it allows the fan to recover over half of the loss in total pressure rise due to inlet distortion. The asymmetric IGV is also able to reduce the total pressure distortion at the fan rotor exit. Spanwise analysis indicates that the effectiveness of the asymmetric IGV can be improved on all three metrics if better 3D IGV shaping is performed.

Concept and Design of a Velocity Compounded Radial Four-Fold Re-Entry Turbine for Organic Rankine Cycle (ORC) Applications

The energy sector faces a pressing need for significant transformation to curb CO2 emissions. For instance, Czechia and Germany have taken steps to phase out fossil thermal power plants by 2038, opting instead for a greater reliance on variable renewable energy sources like wind and solar power. Nonetheless, thermal power plants will still have roles, too. While the conventional multistage axial turbine design has been predominant in large-scale power plants for the past century, it is unsuitable for small-scale decentralized projects due to complexity and cost. To address this, the study investigates less common turbine types, which were discarded as they demonstrated lower efficiency. One design is the Elektra turbine, characterized by its velocity compounded radial re-entry configuration. The Elektra turbine combines the advantages of volumetric expanders (the low rotational speed requirement) with the advantages of a turbine (no rubbing seals, no lubrication in the working fluid, wear is almost completely avoided). Thus, the research goal of the authors is the implementation of a 10 kW-class ORC turbine driving a cost-effective off-the-shelf 3000 rpm generator. The paper introduces the concept of the Elektra turbine in comparison to other turbines and proposes this approach for an ORC working fluid. In the second part, the 1D design and 3D–CFD optimization of the 7 kW Elektra turbine working with Hexamethyldisiloxane (MM) is performed. Finally, CFD efficiency characteristics of various versions of the Elektra are presented and critically discussed regarding the originally defined design approach. The unsteady CFD calculation of the final Elektra version showed 46% total-to-static isentropic efficiency.

Numerical analysis of flow instability with vortex cavitation in venturi tube

The swirling flow with cavitation can cause pressure pulsations in a draft tube of a hydro turbine at off-design operating conditions. Especially at the full load condition, the cavitation volume may fluctuate greatly, causing pressure pulsations with flow rate fluctuations known as cavitation surge throughout the hydraulic system. The cases that predicted the phenomenon qualitatively using the CFD analysis have been reported previously, whereas the methods to predict the cavitation volume and the fluctuation frequency in the cavitation surge condition quantitatively have not yet been established. Therefore, in this study, unsteady CFD analysis was conducted for the experimental apparatus that simulates the flow in the draft tube downstream of a runner using the stationary guide vane and the venturi tube. In CFD analysis, the influence of the turbulence models by using LES and SST and the influence of the number of meshes in LES simulation were investigated. It was founded that LES simulation captured the vortex generation finely and improved the prediction accuracy of the cavitation volume than SST simulation. On the other hand, it was founded that too fine mesh has a small effect on the prediction accuracy of the cavitation volume, although the resolution of the vortex is improved.

Investigation of radial force in a high-speed miniature pump

A high-speed miniature pump design is put forth for utilization in the aerospace sector, where the pump’s reliability holds critical importance under microgravity operational conditions. This study focuses on a dynamic bearing-equipped miniature centrifugal pump, chosen for its suitability in investigating radial forces. The prototype of the miniature pump is manufactured and tested by experiments at the rotational speed of 10,000r/min. A good agreement is observed between the numerical predictions and the experimental results. The radial force acting on different blade part has been analyzed for several flow rate conditions using the steady state CFD calculation. It has been found that the radial force exerted on the blade trailing edge exit is the main cause of the total radial force for the impeller. Moreover, using the unsteady CFD simulation, the evolution process of the radial force changing for both the direction and the magnitude with respect to the annular position due to the shaft rotation is discussed in details. The pressure pulsation is also monitored in impeller and volute casing. The results show that the amplitude of the unsteady radial force has a strong relationship with the time it takes for the blade trailing edge exit passing the volute tongue region, while the direction of the radial force hardly changes. The findings of this study contribute to facilitating the broader implementation of the high-speed miniature pump across diverse sectors of society.

MID-SPAN STALL INCEPTION IN LOW PRESSURE RATIO TRANSONIC FANS

Abstract In this paper, steady and unsteady CFD have been used to investigate stall inception for a modern low pressure ratio transonic fan. The computational results are validated against measurement data from a high-speed test facility. CFD validation was approached as a blind test case. The results shows good agreement between the experiments and computations. Stall is triggered by growth of a suction surface separation behind the shock around the mid-span of the rotor blade. As the fan is throttled the separation grows leading to increased blockage in the blade passages. At the point of instability the separation grows further, locally increasing incidence and leading to the formation of a stall cell. It is shown that changes to the tip leakage flow leave the stall inception mechanism unaffected. A computational case with a suction surface slip patch between 25-75% span shows that the reduction in blockage around the mid-span increases the stall margin by 25%. This demonstrates that for cases with mid-span initiated stall it is important to consider the flow away from the tip as well as the flow in the tip region. A redesigned fan is used to illustrate that design changes around the mid-span can be effective to improve flow range. The redesigned fan increases stall margin by 6.7% while maintaining the design point efficiency within 0.1%

Multi-scale analysis of respiratory droplets transport within the breathing cloud

Infectious diseases are transmitted primarily through pathogen laden droplets commonly exhaled during respiratory events such as breathing, coughing, or sneezing. The transport and evaporation of droplets are governed by the fundamental laws of fluid mechanics and convection-diffusion. From these, analytical models can be created helping in better understanding pathogens transmission from a mechanical perspective. The droplet transport within the humid air breath cloud and the local ventilation are crucial for accurately predicting the droplet fate. Different levels of complexity are possible for modelling the breath cloud, from simple 1D models to complex unsteady 3D simulations including discrete phase models for the droplets simulation. The former are too simple to capture the fluid dynamics of intermittent jets in detail, while the latter are too computationally expensive. The current work presents a novel multi-scale approach where an analytical model of the droplet transport and evaporation is coupled to unsteady CFD simulations of warm humid puffs of exhaled air. The proposed model has the advantage of the accuracy of an analytical model and the computational cost of a relatively standard unsteady CFD simulation, and can be used to predict the trajectory of the exhaled droplets for a variety of respiratory events.

Performance verification of pipe‐embedded wall system in school building and proposal of its optimal control strategy

AbstractIn this study, a thermally activated building system (TABS) installed on the exterior wall as the pipe‐embedded wall (PEW) system in a school building was investigated. The basic performance of the PEW system utilizing well water cascade is verified by unsteady CFD analysis and field measurements. Moreover, in order to improve the utilization efficiency of the thermal potential from well water, the MPC‐based optimal control method with the PEW system to maximize the heat extraction is proposed and verified by unsteady CFD analysis. The analysis results showed that: (1) The PEW system can reduce the peak load from the exterior wall facing different directions at different times. In addition, the PEW system has the effect of stabilizing the indoor thermal environment. (2) The proposed optimal operation strategy improves the energy efficiency, extracting up to 30% more heat and reducing the total heat transmission, which significantly develops the operation efficiency of the PEW system.

Passive flow control at impeller radial bend for stall delay in centrifugal compressors with fishtail pipe diffusers

This paper proposes a new strategy for extending centrifugal compressor stall margin through shroud passive flow control placed at the impeller radial bend. Relative to the usual leading-edge location, the radial bend presents fewer geometrical constraints for flow control devices and allows them to have potentially more impact on low-momentum flow structures that lead to stall. Two centrifugal compressors with fishtail pipe diffusers are chosen for study: a high-speed transonic design exhibiting diffuser stall and a low-subsonic design with exducer stall. Four passive flow control techniques placed on the impeller shroud at the radial bend are evaluated for each compressor, in terms of stall margin improvement and efficiency penalty at design mass flow, using unsteady RANS CFD simulations. These are circumferential groove, slots, impeller recirculation pipe and diffuser-impeller recirculation pipe. The results indicate that passive flow control at the impeller shroud radial bend can be significantly more effective than at the impeller leading edge. For each compressor, the flow mechanisms associated with stall initiation and its delay by the most effective studied flow control technique as well as the associated efficiency penalty are elucidated. The findings indicate that techniques having flow injection with high spanwise penetration and low relative streamwise momentum are most effective for delaying diffuser stall, while the opposite applies to exducer stall. The design point efficiency penalty is due to mixing/shear losses from the jet-in-cross flow phenomenon in the impeller and shear losses from flow redistribution in the diffuser.

Numerical Simulation of Aeroacoustics Generated by Flow around 30P30N High-lift Aerofoil using Hybrid CFD/BEM approach

In this study, we have performed the acoustic simulation generated by the flow around the 30P30N airfoil with a slat and a flap by means of Boundary element Method using quadrupole sources extracted from the unsteady CFD simulations. The CFD simulations are performed using Ansys Fluent with LES Dynamic Model, and compressible flow fields have been solved. The attack angles of the airfoil are 5.5 and 9.5 degree. When the BEM calculation for the acoustic propagation was performed, the FMM (Fast Multipole Method) and FMBEM were used to calculate quickly the contributions from the quadrupole sources and acoustic propagation, reflection and diffraction on the boundaries, those are the walls of the 30P30N airfoil. The results of the acoustic simulations show a relatively good agreement with the experimental data and other researchers' simulation results. The directivities of the acoustic pressure for the primary peak frequency are shown and they depend on the attack angle of the airfoil. The far-field SPL also depends on the attack angle of the airfoil and decreases as the attack angle increases. We have also investigated the quadrupole sources in frequency domain, and the results showed that the quadrupole sources at far-field SPL peak frequencies near the wake of slat and the suction side of the main element of the airfoil are stronger than those in other regions.

Estimating flow fields with reduced order models

The estimation of fluid flows inside a centrifugal pump in realtime is a challenging task that cannot be achieved with long-established methods like CFD due to their computational demands. We use a projection-based reduced order model (ROM) instead. Based on this ROM, a realtime observer can be devised that estimates the temporally and spatially resolved velocity and pressure fields inside the pump. The entire fluid-solid domain is treated as a fluid in order to be able to consider moving rigid bodies in the reduction method. A greedy algorithm is introduced for finding suitable and as few measurement locations as possible. Robust observability is ensured with an extended Kalman filter, which is based on a time-variant observability matrix obtained from the nonlinear velocity ROM. We present the results of the velocity and pressure ROMs based on a unsteady Reynolds-averaged Navier-Stokes CFD simulation of a 2D centrifugal pump, as well as the results for the extended Kalman filter.

A Conceptual Model for the Flutter Mechanism in Interlocked Turbine Blades

Abstract Many turbine rotor blades adopt the interlock configuration; among its advantages, there is a large increase in the natural frequencies with respect to the simpler cantilever configuration. Most widespread assessment methods estimate that this frequency increase should lead to a large improvement to the flutter stability of the interlocked rotor rows; some well-known approaches, such as the Panovsky–Kielb stability maps, suggest that flutter should be very unlikely for interlock configurations. Nevertheless, both the experience and detailed simulations indicate that it is possible to find flutter in interlocked blades, even with moderate aspect ratio. In this paper, the authors study the underlying physics behind flutter for interlocked blades, and present a very simplified model that is able to reproduce the main modal characteristics of interlocked blades, including the frequency increase and change in mode-shape as a function of the nodal diameter. This model also identifies a flutter mechanism, which is dependent on the aerodynamic interaction between the bending and torsion motion of the blade; even if both basic motions are individually stable, their interaction may lead to a coupled aeroelastic instability. The results from this simplified model are fully consistent with detailed three-dimensional linearized unsteady computational fluid dynamics simulations of representative turbine rotor blades, and should be of application in many realistic cases of interlock flutter. Finally, the authors present a study on the influence of the main parameters of the simplified model on the flutter stability, and derive some conclusions about the implications on flutter-free interlock design.

An Aerodynamic Correction Technique for the Unsteady Subsonic Wing–Body Interference Model

This paper investigates a novel correction technique for the unsteady subsonic wing–body interference model. The correction technique considers the unsteady forces on the lifting boxes and the body elements of an idealized aircraft model. The chosen simulation model was a passenger aircraft, and the transonic unsteady aerodynamics in sinusoidal pitch motion at four different frequencies are analyzed. The unsteady aerodynamics of the uncorrected DLM (doublet lattice method), ECFT (enhanced correction factor technique) and the new unsteady wing–body correction method are compared to the unsteady CFD simulation results. The results show that when the frequency is small, the new unsteady wing–body correction method can obtain certain benefits in terms of accuracy, for the lifting boxes and the body elements as well.

An investigation of the effects of volute A/R distribution on radial turbine performance

In order to meet the higher design requirements of radial turbines, it is necessary to clearly understand the impact of different design parameters on turbine performance. The A/R distribution is one of the most important design parameters for the radial turbine volute, but there are few studies on it. To clarify the effect of A/R distribution on turbine efficiency and blade excitation force, a detailed comparative study is carried out. An experimentally validated numerical method is used, and steady and unsteady CFD simulations are conducted in this study. The results show that the volute A/R distribution has a significant effect on the turbine efficiency, while a linear distribution can provide a more uniform inlet condition and thus improve the rotor efficiency. The flow parameters fluctuated significantly at the volute tongue. Although the efficiency is not sensitive to this fluctuation, it directly affects the excitation force on the rotor blades. The volute with S-shaped A/R distribution produces the smallest excitation force due to the smallest fluctuation. Two new A/R distributions are presented which combine the advantages of linear and S-shaped or convex A/R distributions with high efficiency and low excitation force. It is suggested that the A/R should be close to the linear distribution and decreases rapidly before the tongue.

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.

Influence of Radial Installation Deviation on Hydraulic Thrust Characteristics of a 1000 MW Francis Turbine

In this paper, a 1000 MW Francis turbine was used to study the influences of uneven clearance distribution caused by various radial installation deviations on the hydraulic thrust of a runner under the rated operating condition with steady-state CFD analysis. Then, the influences of radial installation deviation on the pressure pulsations of the runner and the fluid domains most affected by the deviation were investigated via unsteady CFD calculation. The results show that the radial hydraulic force on the chambers increased linearly with the increase in the radial installation deviation. Additionally, the high-pressure zone was not located in the same position as the radial deviation. With increasing values of the radial installation deviation, the high-pressure zone rotated along the opposite direction of the rotating direction of the runner. This study also found that the flow in the upper crown chamber was most affected by radial installation deviation and that the percentage of high frequencies of the pressure pulsations increased with the flow in the clearances.

Dynamic stall characteristics of wind turbine airfoil in sand-wind environment

Although the traditional dynamic stall has been well studied, the effect of sand particle on dynamic stall characteristics is seldom considered in the previous studies. Therefore, a research about dynamic stall of the S809 airfoil is accomplished under a sand-wind environment. The unsteady CFD method and the discrete phase model (DPM) are adopted in this research to simulate the dynamic stall characteristic of the S809 airfoil. The influence of different sand particle diameters and different particle concentrations on airfoil dynamic stall characteristics is studied respectively. The simulated results indicate that the maximum lift coefficient and drag coefficient with particle diameter of 10 μm decrease about 2.18% and 2.52% respectively compared with that of the clear air. Meanwhile, the maximum lift coefficient decreases about 6.93% for the case of particle concentration with 62.6 g/m3. The simulated results also indicate that the lift coefficient and drag coefficient decreasing are enlarged with particle concentration increasing. This tendency indicated that the efficiency of a wind turbine would be reduced in sand storm weather.

The Influence of Rotor Overlapping Azimuth on Compound Coaxial Helicopter Performance Based on Unsteady CFD Simulation

In this paper, a computational fluid dynamics simulation method is developed to study the influence of the rotor overlapping azimuth on the aerodynamic performance of compound coaxial helicopter. The simulation method is verified by comparing the numerical simulation results with the wind tunnel experiment data of the NASA coaxial rotor. Two overlapping azimuths of the upper and lower blades are considered, and the aerodynamic performance of the isolated rotor and the compound coaxial helicopter in hover and forward are analyzed respectively. State 1 means the upper and lower blades overlap at azimuth 0/180° or 90/270°, state 2 means the upper and lower blades overlap at azimuth 45/225° or 135/315°. It is found that the performance of isolated rotors is not affected by rotor overlapping azimuth in hover, but the total thrust fluctuation amplitude of isolated rotors in state 2 is 76.3% smaller than that in state 1 in forward. In the hovering flight of compound coaxial helicopter, compared with state 1, the fluctuation amplitude of the lift of the wing in state 2 is 42.7% smaller; the lift fluctuation amplitude of the flat tail in state 2 is 52.4% smaller. In the forward flight of compound coaxial helicopter, compared with state 1, the total thrust fluctuation amplitude in state 2 is 83.5% smaller; the fluctuation amplitude of the lift of the wing in state 2 is 61.2% smaller. It can be concluded that the compound coaxial helicopter working in state 2 has better aerodynamic performance than the compound coaxial helicopter working in state 1; changing the rotor overlapping azimuth of the upper and lower rotors has a high engineering application value, which can increase aerodynamic stability and reduce lift fluctuations.

Subject experiment on personal air-conditioning airflow using a vortex ring

A personal air conditioning system using vortex rings, which are characterized by their linearity and low diffusivity, is developed in this study. The authors have clarified the characteristics of the vortex ring airflow by LES (unsteady CFD) and full‐scale experiments. However, the airflow type and worker's comfort using vortex rings as an air conditioning system has not been clarified yet. As a result of the subject experiments, it became clear that air conditioning with vortex rings cooled the subjects better than conventional air conditioning and improved their sense of comfort with respect to the airflow. In addition, it was found that airflow to the neck was more effective since the subjects were less likely to feel the airflow at the top of the head due to the resistance of the hair on the head. The effectiveness of the vortex ring as a personal air conditioning system was clarified, enabling relaxation of the room temperature setting. The comfortability of the airflow sensation provided by the vortex ring was also examined when further airflow was given.