Multi-reference Frame Research Articles

Aerodynamic Investigation on a Coaxial-Rotors Unmanned Aerial Vehicle of Bionic Chinese Parasol Seed.

Aerodynamic investigation of a bionic coaxial-rotors unmanned aerial vehicle (UAV) is performed. According to Chinese parasol seed features and flight requirements, the bionic conceptual design of a coaxial-rotors UAV is described. A solution procedure for the numerical simulation method, based on a multi-reference frame (MRF) model, is expressed, and a verification study is presented using the typical case. The aerodynamic design is conducted for airfoil, blade, and coaxial-rotors interference. The aerodynamic performance of the coaxial rotors is investigated by numerical simulation analysis. The rotor/motor integrated experiment verification is conducted to assess the performance of the coaxial-rotors UAV. The results indicate that the UAV has excellent aerodynamic performance and bionic configuration, allowing it to adapt to task requirements. The bionic UAV has a good cruise power load reach of 8.36 kg/kw, and the cruise flying thrust force is not less than 78 N at coaxial-rotor and rotor-balloon distance ratios of 0.39 and 1.12, respectively. It has the "blocks stability phenomenon" formed by the rotor downwash speed decreases and the balloon's additional negative pressure. The present method and the bionic configuration provide a feasible design and analysis strategy for coaxial-rotors UAVs.

Aerodynamic Performance Assessment of Distributed Electric Propulsion after the Wing Trailing Edge

Distributed electric propulsion (DEP) with four propellers distributed along the rear edge of the wing (pusher DEP configuration) promote aerodynamic interactions to a higher level. To study the aerodynamic performance of DEP with the rear wing through simulations and experiments, the multi-reference frame (MRF) with sliding grid is combined with wind tunnel tests. The obtained results demonstrate that the lift and drag of DEP increase with the angle of attack (AoA) and are related to the relative position of the propellers and wing. The propeller has no significant effect on the lift of the wing, and the lift and the AoA remain linear when the AoA is less than 16°. By contrast, the lift coefficient is much higher than the baseline (isolated wing), and the lift is greatly improved with the increasing drag when the AoA is greater than 16°. This is because the flow around the wing of the pusher configuration remains attached due to the suction of the inflow of the propeller on the trailing edge vortex. In addition, the acceleration effect on the free flow improves the kinetic energy of the airflow, which effectively delays the separation of the airflow in the slipstream region.

Investigation of coupled aerodynamic characteristics in channel wing propeller configuration

This work studies the lift enhancement effect with a channel wing propeller configuration. Numerical simulations and flow field analysis are performed to investigate the influence (the relative position and spacing, angles of attack (AoA), propeller speed, incoming flow speed) on the lift enhancement effect. The aerodynamic performance is numerically simulated by the method of multi-reference frame (MRF). Numerical results demonstrate that the propeller suction process has a very significant lifting effect. The lifting effect can reach 5 times contrast to the single channel wing. The propeller located at 100% of the chord length of the channel wing achieves the best lift enhancement effect. The intervention of the propeller delays the flow separation at the trailing edge of the channel wing. It holds a high lift coefficient at a high AoA. The lift line slope remains essentially unchanged in the range of 0–30°. It is roughly the same as that of the single channel wing configuration before stall. With the increase of propeller speed or incoming flow speed, the lift coefficient increases significantly. The lift line slope is approximately the same at different speeds. The regularity functions of different coefficient are obtained through mechanism analysis and least squares data fitting methods. The lift coefficient changes with the AoA in a linear relationship and with the propeller speed or incoming flow speed in a quadratic relationship.

Performance analysis and flow mechanism of channel wing considering propeller slipstream

The development of the Urban Air Mobility concept has proposed stringent requirements for the fixed-wing vehicle’s Short/Vertical Take-Off and Landing (S/VTOL) performance. With its lift-enhancing impact, the channel wing, can improve aircraft low-speed performance and Short Take-Off and Landing (STOL) capability. It is critical for the performance analysis and flow mechanism study of a channel wing that considers the influence of propeller slipstream to guide the design of S/VTOL aircraft. The law and mechanism for the effect of the enclosing angle of the arc wing, the phase angle of the propeller, the tip clearance, the rotational speed and the chordwise position of the propeller on the channel wing are explored utilizing the quasi-steady multi reference frame method. A channel wing with a larger enclosing angle has a better ability to enhance lift and reduce drag using propeller slipstream. The effect of propeller phase angle on channel wing aerodynamic forces is periodic and weak. Increasing propeller rotational speed is helpful to enhance lift and resist flow separation for channel wing. It can reduce drag for tractor configuration but increase drag for pusher configuration. However, the nose-down pitching moment of a channel wing will grow dramatically, making longitudinal trimming in aircraft layout design challenging.

Passive flow-field control using dimples for performance enhancement of horizontal axis wind turbine

This paper investigates the effectiveness of dimples on the aerodynamic performance of NREL Phase VI wind turbine. Computational fluid dynamics (CFD) was conducted to simulate the turbulent flow and all the forces acting on the rotor. k-ω shear-stress transport (SST) turbulence model was selected to complete the governing equations and multi-reference frame was used as the computational domain. The computational results were compared with the experimental data and previous studies to ensure its reliability. The effect of different dimple configurations, placed at the suction side of the rotor, on the aerodynamic performance of the rotor blade was investigated in terms of the output torque and axial thrust, coefficient of pressure, and the flow field around the blade cross section in the spanwise directions. The results revealed that, dimpled-surface blade could generate up to 8.41% higher output torque compared to the original blade. The results showed no significant deviation in the axial thrust with the addition of the dimples. The modified surface blade with dimples was proven to enhance the operating performance of a wind turbine.

Numerical research on biomass wastewater treatment using double-partition vessel with off-centered impellers

Limited by single function, it is difficult for the traditional stirred vessels to meet the requirements of mixing system in biomass wastewater treatment processes. The estimation of biomass wastewater stirring reactor performance by computational fluid dynamics (CFD) during multiphase reactions is important, due to the uncertainty in the numerical results. In this study, a novel double-partition stirred vessel with eccentrically located impellers was developed for the special subject. In addition, many simulations were carried out with the wastewater from biomass ethanol production as the medium to ensure the high reactor performance. The fluid flow was simulated and analyzed using the turbulent RNG k-ε model and multi reference frames. A good agreement is found between the simulation results and the confirmatory experiment. Moreover, the weir crest and interconnected pore were specially designed for the establishment of the circulation of fluid to maintain different technological conditions in the two regions. The distributions of radial velocities and tangential velocities were concentrated near the stirring blade. From the velocity profile, it is deduced that the flow pattern in the stirred vessel is insensitive to Reynolds number. Finally, this simulation study could contribute to the improvement and optimization of the structure, as well as the operation of the novel stirred vessel. Keywords: biomass, wastewater treatment, double-partition vessel, computational fluid dynamics DOI: 10.25165/j.ijabe.20231602.7285 Citation: Zhang W, Yang Y J, Qian W M, Ma L L, Chen L G, Liu N, et al. Numerical research on biomass wastewater treatment using double-partition vessel with off-centered impellers. Int J Agric & Biol Eng, 2023; 16(2): 232-240.

A Simulation Analysis of Ball Bearing Lubrication Characteristics Considering the Cage Clearance

Abstract Lubrication characteristics can greatly affect the service performances of ball bearings. To further clarify the variation law of oil–air distribution in bearing cavity, a new simulation method combining the dynamic and computational fluid dynamics (CFD) models is presented. The relative motion between the ball and the cage caused by the cage clearance is obtained by the dynamic model. The coupling model uses the multi-reference frame (MRF) method and local remeshing approach (LRA) to describe the motion of each bearing component. The oil–air two-phase (OATP) flow is described through the volume of fluid (VOF) numerical model. Note that the lubricating oil content in the bearing cavity decreases significantly with the increment of rotation speed. There is a sudden change of pressure at the contact area between the ball and inner raceway. The relative motion of ball caused by the cage clearance has a certain effect on the local lubrication; however, it has little effect on the overall lubrication characteristics.

Hydrodynamic Comparative Study on the Pumping Effects of a Square Tank Equipped with Single-Stage and Bi-Stage Impellers

Abstract A computational fluid dynamics simulation is done for comparative study from the pumping effect on the four surfaces of the stirred tank. The flow field generated by one-stage and bi-stage six-bladed Rushton turbine in the unbaffled square tank was studied. The Reynolds-averaged Navier-Stokes equation with steady-state multi-reference frame approach (MRF) is used to simulate hydrodynamic flow in the tank. The turbulent viscosity, the turbulent kinetic energy and mean velocity distributions obtained in vertical and horizontal plans are analyzed and discussed. We can deduce that the additional Rushton turbine in the upper part of the square tank improves the quality of the mixture.

CFD Simulation Model for Optimum Design of B-Series Propeller using Multiple Reference Frame (MRF)

Propulsion system is one of ship systems which require more attention, especially on propeller design. The propeller design greatly affects the ship speed. It is expected to be able to have maximum value of thrust coefficient and efficiency. Hence, the optimum design of propeller can be obtained by multi objective optimization process. In this study, a preliminary optimization is applied to B-series propeller with the Non-dominated Sort Genetic Algorithm-II (NSGA-II). The purpose of this study is to find out the optimum performance of B-series propeller. The thrust coefficient and open water efficiency are maximized in the optimization process which are then subjected to constraint function imposed by required thrust. The optimization is carried out to blade number Z=3 and Z=5. The population of design space is obtained after running the optimization program. The final optimum design parameter is considered using crowding distance value in the population. The result obtained by NSGA-II showed that the optimum design for Z=3 are B3-787, B3-314, B3-560, and Z=5 are B5-416, B5-501, B5-476 respectively. In addition, the Computational Fluid Dynamics analysis (CFD) is employed to investigate the characteristic of each propeller model by using Multi Reference Frame (MRF) approach. The CFD results showed that the highest thrust value of the Z=3 is 172.38 kN generated by the B3-787 whereas the highest thrust value of Z=5 is 168.80 kN generated by the B5-501 model.

Research on the Hydrodynamic Performance of a Horizontal-Axis Tidal Current Turbine with Symmetrical Airfoil Blades Based on Swept-Back Models

For the design of a horizontal-axis tidal current turbine with adaptive variable-pitch blades, both numerical simulations and physical model experiments were used to study the hydrodynamic performance of symmetrical airfoil blades based on backward swept models. According to the lift–drag ratio of symmetrical airfoils, variable airfoil sections were selected for each part of the blade in the spanwise direction. Then, three kinds of blades were designed by using different swept-back models from wind turbines. A rotation model with a multi-reference frame was employed to conduct a three-dimensional steady numerical simulation of the turbine model based on the CFD method. The axial thrust and energy-capturing efficiency under different tip speed ratios, as well as the corresponding starting torque under different flow rates, were analyzed. The simulation results indicate that model 2 has optimal start-up performance, and model 3 has the largest power coefficient. The thrust coefficient of model 1 is the smallest. In all, model 2 has better comprehensive performance. The experiments of model 2 show that it has suitable hydrodynamic performance to capture bidirectional energy via passively variable pitch. This research provides an important solution for the design and optimization of horizontal-axis turbines to harvest bidirectional tidal current energy.

A novel unibody axial flow pump for the lubrication of inverter type hermetic reciprocating compressors

Lubrication at low speeds is a general problem in the hermetic reciprocating compressors for household refrigerators. The aim of this article is to present a new method using a novel 3D printed unibody axial flow pump for the lubrication oil supply system of an inverter type hermetic reciprocating compressor during a low speed (< 2000 rpm) operation. The system is based on a 3D printed unibody bladeless impeller axial flow pump attached to the bottom end of the vertical rotating crankshaft partially immersed in the oil sump inside hermetic reciprocating compressor sealed casing. A Computational Fluid Dynamics (CFD) simulation besides the experiment is used to simulate the flow inside the pump to calculate the mass flow rate of the lubrication oil and to optimize the helix angle of the pump. Volume of Fluid (VoF) and Multi Reference Frame (MRF) methods are used for modeling the two-phase flow (air and lubrication oil) and rotary domain respectively. The mass flow rate and climbing time of the lubrication oil in the unibody axial pump are analyzed. The results show that the designed pump is capable to supply the lubrication oil at a low speed i.e., 1400 rpm. Moreover, results indicate that the mass flow rate of the lubrication oil increases as the viscosity decreases. The average climbing time is observed to be 1 s at 1400 rpm and 0.4 s at 2000 rpm.

Rheological and flow characteristics of fresh concrete in the rotational rheometer

The coaxial cylinder rheometer is widely used to measure the torque and rotational speed (T-N) of fresh concrete. T-N can be transformed into the corresponding rheological parameters via the solution of the ‘Couette inverse problem’. However, the solution of the ‘Couette inverse problem’ is a theoretical solution, which may produce errors due to the complex geometric shape of the coaxial cylindrical rheometer and the possible plug or secondary flow. The purpose of this study is to investigate the rheological and flow behaviors of fresh concrete in the rotational rheometer by coupling CFD (Computational Fluid Dynamics) simulations and experiments. The efficient high-fidelity 3-D CFD simulations of fresh concrete flowing in the ConTec Viscometer 5 are conducted using the Reynolds Average Navier–Stokes (RANS) model with the Multi-Reference Frame (MRF). Both Bingham and Herschel–Bulkley (H–B) models are adopted, and the numerical simulations are validated against by experiments, which enhanced the confidence that RANS simulation with MRF could supplement the experiments and elucidate the rheological and flow behaviors of fresh concrete with the whole-flow domain information.

Video Compressed Sensing Reconstruction Based on Multi-Dimensional Reference Frame Multi Hypothesis Rediction

In this paper, a video compressed sensing reconstruction algorithm based on multidimensional reference frames is proposed using the sparse characteristics of video signals in different sparse representation domains. First, the overall structure of the proposed video compressed sensing algorithm is introduced in this paper. The paper adopts a multi-reference frame bidirectional prediction hypothesis optimization algorithm. Then, the paper proposes a reconstruction method for CS frames at the re-decoding end. In addition to using key frames of each GOP reconstructed in the time domain as reference frames for reconstructing CS frames, half-pixel reference frames and scaled reference frames in the pixel domain are also used as CS frames. Reference frames of CS frames are used to obtain higher quality assumptions. The method of obtaining reference frames in the pixel domain is also discussed in detail in this paper. Finally, the reconstruction algorithm proposed in this paper is compared with video compression algorithms in the literature that have better reconstruction results. Experiments show that the algorithm has better performance than the best multi-reference frame video compression sensing algorithm and can effectively improve the quality of slow motion video reconstruction.

Multireference Frame Based Open-Phase Fault Modeling and Control for Asymmetrical Six-Phase Interior Permanent Magnet Motors

Multireference frame based asymmetric six-phase interior permanent magnet motor modeling and control strategy under single open-phase fault are investigated in this article. The motor modeling under healthy operation is firstly established based on a dual <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -axis synchronous rotating frame. With this dual <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dq</i> -axis frame, the motor modeling considering the magnetic saturation effect under open-phase fault has been derived. By introducing a dual-direction rotating frame, the stator current under single open-phase fault can be treated as the sum of a positive current sequence and a negative current sequence. With this dual-direction rotating frame, the two current sequences can be transformed into two dc components for fault-tolerant operation. Fault-tolerant control schemes with maximum torque per ampere for copper loss minimization and torque maximization are developed, respectively. Finally, the validity of the proposed modeling and control are evaluated by experiments under both steady and transient scenarios.

Effects of annular-cooler surface-roughness on wind resistance and thermal environment in a high-speed geotechnical centrifuge chamber

Numerical simulations were performed to illustrate the influences effects of annular-cooler surface roughness on wind resistance and thermal environment in a high-speed centrifuge chamber, with the use of multi-reference-frame (MRF) method and classical roughness-modified model. In the present study, the absolute roughness height was specified in a range form 0.05 mm–2.5 mm. With respect to the clearance between rotating-arm tip and stationary-cooler surface, the relative roughness height was changed from 0.0014 to 0.07 accordingly. The results show that the flow field inside the centrifuge chamber is of highly three-dimensional feature. With the increase of surface roughness, the wind resistance is increased monotonously. With regard to the thermal environment, the maximum temperature inside the centrifuge chamber takes on a non-monotonous variation along with the surface roughness height, due to the two contrary influencing roles of surface roughness on the viscous dissipation heat generation and the heat removal capacity. In general, the use of roughed cooler-wall is a promising scheme for reducing the environmental temperature in a high-speed centrifuge chamber. Under the present research conditions, a more favorable relative roughness height is identified in term of a comprehensive factor that reflects the benefit of roughed surface on the reduction of maximum environmental temperature at the pay of aerodynamic penalty.

Degradation of Fan Performance in Cooling Electronics: Experimental Investigation and Evaluating Numerical Techniques

This study reports on the essential forced air-cooled electronics issue of fan performance deterioration caused by the presence of obstructions inside Information Technology Equipment (ITE). Fan performance was characterized based on the fan's static pressure and flowrate. Three different experimental techniques (flow test chamber, pressure probe, and pressure taps) were used to measure the fan static pressure at different locations. Moreover, Computational Fluid Dynamics (CFD) models were built considering different fan working environments. Multi Reference Frame (MRF) and Lumped Fan (LF) model CFD techniques were employed. The experimental results were used to evaluate the modeling techniques whilst implemented in different working environments and to better understand how fans react to blockages inside ITE. Experiments showed that compared to the Free Environment (FE) readings, placing the fan inside a specific ITE reduced the flowrate delivered by the fan by 57.2% and decreased static pressure by 76.3%, which affects the thermal performance of the ITE cooling system. Moreover, comparing numerical results with the experimental ones showed that the MRF approach predicted the flowrate delivered by the fan with a relative error of 3.9%, while the LF approach overestimated the flowrate by 70.3%. The results and conclusions reported in this work can be expanded to cover many other applications in which fans are operating inside enclosed environments and surrounded by many obstructions.

Multi-hypothesis distributed video compression sensing based on key frame secondary reconstruction

Reconstruction algorithms are the key technology of distributed video compressed sensing. The research focus of traditional distributed video compressed sensing reconstruction algorithms is mostly on improving the reconstruction quality of non-key frames, ignoring the reconstruction quality of key frames, and the information of key frames are not Underutilized. In view of the above problems, a distributed video compression sensing algorithm based on secondary reconstruction of key frames is proposed. Firstly, the fractional order total variation algorithm is used for the initial reconstruction of the key frame, and the reconstructed frame is used as the reference frame to assist the secondary reconstruction of the key frame, which improves the reconstruction quality and reduces the calculation complexity. Then, a multi-reference frame bidirectional prediction hypothesis set optimization algorithm is proposed to increase the number of reference frames and improve the quality of the hypothesis set through optimization without expanding the size of the hypothesis set. Experimental results show that the overall performance of the proposed algorithm is better than the most advanced methods.

Numerical Simulation of the Mixing and Hydrodynamics of Asphalt and Rubber in a Stirred Tank

Computational fluid dynamics (CFD) has been used to analyze the mixing process of Asphalt and Rubber (AR) in a stirred tank with a six flat-blade disc turbine (Rushton), a down-pumping 45° pitched-blade turbine (PBTD-6) and a down-pumping propeller (TXL). The two-phase (solid-liquid) flow in the considered stirred tank has been modelled in the framework of an Eulerian-Eulerian approach, a laminar-flow assumption and a multi-reference frame strategy. The following effects have been investigated: The influence of the impeller speed, impeller type, crumb rubber (CR) particle diameter and initial CR particle loading on the quality of the CR particle’s suspension. The outcomes of the numerical simulations have been found to be in good agreement with the experimental results. The most important findings can be summarized as follows. The flow velocity on the lower part of the stirred tank, induced by the Rushton impeller, is the highest for the three considered mixing systems at the same impeller speed. The suspension quality of CR particles increases with the impeller speed. A larger particle diameter and a higher initial CR particle loading lead to an uneven distribution of particles. The degree of homogeneity in the Rushton impeller system is better than that of the PBTD-6 impeller system and the TXL impeller system at the same impeller speed, while the suspension uniformity of the TXL impeller system is the best possible one for the same power consumption.

CFD Investigation of Impact of Vessel configuration and Different Impeller Types in Stirred Tank

Computational Fluid Dynamics (CFD) study is carried out to simulate the flow characteristics of different impeller types in stirred tanks of baffled and un-baffled system. The hydrodynamic behavior induced by a Rushton turbine (RT6) and Curve Blade impeller (CD-6) is numerically analysed by solving the Navier-Stokes equations coupling with k-ε turbulence model with Multi-reference frame (MRF) impeller model. The predicted results were validated with available experimental published data and show a satisfactory agreement. The power characteristics for the two agitator systems (baffled and un-baffled) were obtained for the turbulent system of the fluid flow. In these conditions, it is found that the power number depended strongly on the length of the baffle when baffled is provided. Therefore, the presence of the baffles in the tank greatly improved the mixing performance of the system but consumes higher power than the un-baffled system. Similarly, mixing improves when Rushton impeller is employed but consumes higher power than the Curve Blade impeller. Hence, it was observed that overall efficiency of the system is better when baffled system of Curve Blade impeller is opted in any mixing operation.

Analyzing regularity of interpolar air motion and heat dissipation coefficient distribution of a salient pole synchronous generator considering rotary airflow

The extreme attention should be paid to cooling performance of air-cooled synchronous generators in order to ensure that the temperature rise of each part is within the allowable range. It is necessary to know losses distribution, cooling airflow rate distribution laws, heat dissipation coefficient distribution and temperature distribution characteristics. Considering the influence of rotary airflow and yoke ventilation duct structure on air motion, the physical parameters and the motion characteristics of interpolar air were analyzed in three cases. These cases were considered respectively when the rotor was in a rotating or non- rotating state and the yoke ventilation ducts were intake air, and when the rotor was in a rotating state and the yoke ventilation ducts were not intake air. Based on Multi-Reference Frame, the finite volume method was used to solve the fluid-solid coupling numerical simulation of the solving domain. Taking a 250 MW air-cooled hydro-generator as an example, the air velocity of each air cooler was measured and the inlet air velocity of the rotor support was calculated. By calculating the average temperature of the excitation winding in the steady state, the measured value and the simulation result were compared, and the correctness of the calculation method was verified.