Evaluation of aerodynamic performance enhancement of Risø_B1 airfoil with an optimized cavity by PIV measurement
01 Aug 2020-Journal of Visualization (Springer Berlin Heidelberg)-Vol. 23, Iss: 4, pp 591-603
TL;DR: A cavity on a Risø_B1_18 airfoil, which is used as a wind turbine airfoils, was optimized at an off-design angle of attack by incorporating a genetic algorithm into a RANS flow solver and showed that the optimized cavity traps a vortex, which postpones the stall.
Abstract: Airfoils are mostly inefficient in their off-design conditions. In order to improve the aerodynamic performance of airfoils in these conditions, using an optimized cavity on airfoils as a passive method can be useful. In this study, a cavity on a Riso_B1_18 airfoil, which is used as a wind turbine airfoil, was optimized at an off-design angle of attack by incorporating a genetic algorithm into a RANS flow solver. For the cavity optimization, the geometry and downstream suction surface were defined by 16 parameters, and the lift-to-drag ratio was considered as the cost function at 14° angle of attack. The numerical solution showed that the optimized cavity traps a vortex, which postpones the stall. Due to the uncertainty of CFD especially at off-design conditions, it was necessary to evaluate the performance of the optimized cavity in a wide range of angles of attack. This study used the particle image velocimetry (PIV) measurement method to evaluate the improved flow structures over the optimized cavity. Two models of airfoils with and without the cavity were made of aluminum and installed inside the test section of an open-jet wind tunnel with an air speed of 30 m/s and a cross section of 30 × 30 cm2. The air flow on the suction side of the airfoils was measured at 7°–15° angles of attack by PIV. A comparison between the measured flow fields over the two airfoils showed that the optimized cavity postpones the stall angle by 3°. Furthermore, the cavity increases the momentum behind the airfoil at the angles of attack greater than 9°. After this angle, a further increase in the angle of attack increases the difference between the momentums behind the airfoils with and without cavity. The Riso_B1_18 airfoil with the optimized cavity can be used as a wind turbine airfoil at high angles of attack to increase the stall angle and decrease the instability and fluctuation at off-design conditions.
TL;DR: In this article, a code was developed to solve the boundary layer equations using the integration method, and then incorporated into a genetic algorithm to optimize the wall pressure distribution to achieve the maximum pressure recovery without separation occurrence.
Abstract: The inverse design is one of the aerodynamic design methods, in which the pressure distribution along the wall is known, and the duct geometry is unknown. To obtain the best geometry by the inverse design, the target pressure distribution along the walls should be optimum. This paper presents an aerodynamic design of diffusers by optimizing the wall pressure distribution and applying it to the ball-spine inverse design method. A code was first developed to solve the boundary layer equations using the integration method, and then incorporated into a genetic algorithm to optimize the wall pressure distribution to achieve the maximum pressure recovery without separation occurrence. Depending on the type of the duct, a series of constraints was applied to the wall pressure distributions during the optimization process. The optimized pressure distribution was considered as the target pressure distribution for the inverse design problem. The duct geometry changes during the inverse design process to reach one satisfying the target pressure distribution. An offline connection was observed between the ball-spine inverse design method and the genetic algorithm. The boundary layer code was the medium for this offline connection. The optimized wall pressure distribution and inverse design process were evaluated for a straight diffuser and three S-shaped diffusers with different height to length ratios. The results revealed the robustness of the offline link of the inverse design and the genetic algorithm for the optimal aerodynamic design of ducts.
08 May 2017
TL;DR: In this article, a novel passive flow control concept based on the local modification of an airfoil's surface is proposed and examined via CFD for the mitigation of the negative effects of dynamic stall, i.e. for the reduction of peak negative pitching moment as well as negative aerodynamic damping while not deteriorating significantly the original lift and drag characteristics.
Abstract: A novel passive flow control concept – based on the local modification of an airfoil’s surface is proposed and examined via CFD for the mitigation of the negative effects of dynamic stall, i.e. for the reduction of peak negative pitching moment as well as negative aerodynamic damping while not deteriorating significantly the original lift and drag characteristics. 2D CFD simulations of a NACA 0012 airfoil exposed to a freestream of Mach 0.3 and Re = 3.76×106 and undergoing a 15°±10° pitch oscillation with a reduced frequency of 0.101 were conducted. The baseline airfoil simulations carefully verified and validated, showing an excellent agreement with wind tunnel data. Twentysix different local geometry modifications were proposed and examined, all functioning as a trapped-vortex generator. The surface modifications were examined on both the upper and lower surfaces. In case of the upper surface modifications, the best geometries could reduce the peak negative pitching moment by as much as 37-63%, while sacrificing only 2-10% of peak lift and reducing drag by 14-38%. On the other hand, the lower surface modifications demonstrated the ability to increase lift by 4-16% with minor penalty in pitching moment and drag. BIOGRAPHY Name: Khider Al-Jaburi Education: Ph.D. Candidate. Mechanical and Aerospace Engineering, Carleton University, Present. M. Sc. in Mechanical Engineering/ Aircraft Engineering, University of Technology, 2003. B. Sc. in Mechanical Engineering/ Aeronautical Engineering, University of Technology, 2000. Research area: Aerodynamics of Rotary-wing and Fixed-wing Aircraft. Subsonic and Transonic Unsteady Flows. Computational Fluid Dynamics. Flow Control Techniques/Methods.
TL;DR: In this article, a new inverse design method was proposed for the full 3D inverse design of S-ducts using curvature-based dimensionless pressure distribution as a target function.
Abstract: In this study, a new inverse design method is proposed for the full 3-D inverse design of S-ducts using curvature-based dimensionless pressure distribution as a target function. The wall pressure distribution in a 3-D curved duct is a function of the centerline curvature and the cross-sectional profile and area. A dimensionless pressure parameter was obtained as a function of the duct curvature and height of the cross-sections based on the normal pressure gradient equation. The dimensionless pressure parameter was used to eliminate the effect of the cross-sectional area on the wall pressure distribution. Full 3-D inverse design of an S-shaped duct was carried out by substituting the 3-D duct with a large number of 2-D planar ducts. The ball-spine inverse design method with vertical spins was coupled with the dimensionless pressure parameter as a target function for the design of the planar ducts. The inverse design process was performed in two steps. First, the height of each cross-section was considered constant, and only the duct centerline was allowed to be deformed by applying the difference between the dimensionless pressure on the upper and lower lines of symmetry plane. Then, a constant curvature was considered for each centerline in the equation, and the difference between the current and the target dimensionless pressure was applied to each upper and lower line of the planar sections to correct the heights of the 2-D planar sections, separately. The method was validated by choosing a straight duct as an initial guess, which converges to the target S-shaped duct. The results showed that the method is an efficient physical-based residual-correction method with low computational cost and good convergence rate. The 3-D wall pressure distribution of a high-deflected 3-D S-shaped diffuser was modified to eliminate the separation, secondary flow, and outlet distortion. Finally, the geometry corresponding to the modified pressure was obtained by the proposed 3-D inverse design method, which revealed higher pressure recovery, lower total pressure loss, and lower outlet flow distortion and swirl angle.
TL;DR: In this paper, a combined cooling, heating, and power system powered by biogas is presented, which is suitable for small scale communities in remote locations, in order to obtain daily life essentials of electricity, hot water, and cooling, municipal waste can be considered as an option.
Abstract: This study presents a combined cooling, heating, and power system powered by biogas, suitable for small scale communities in remote locations. To run such a system, in order to obtain the daily life essentials of electricity, hot water, and cooling, municipal waste can be considered as an option. Furthermore, the organic Rankine cycle part of the organic Rankine cycle powered vapor compression chiller can be used in times of need for additional electric production. The system comprises a medium temperature organic Rankine cycle utilizing M-xylene as its working fluid, and the cooling was covered by an Isobutane vapor compression cycle powered by an R245fa employing organic Rankine cycle. The system analyzed was designated to provide 250 kW of electricity. The energetic and exergetic analysis was performed, considering several system design parameters. The impact of the design parameters in the prime mover has a much greater effect on the whole system. The system proposed can deliver cooling values at the rate between 9.19 and 22 kW and heating values ranging from 879 up to 1255 kW, depending on the design parameter. Furthermore, the second law efficiency of the system was found to be approximately 56% at the baseline conditions and can be increased to 64.5%.
TL;DR: In this paper , the effect of circular cavity on aerodynamic performance of the H-Darrieus rotor is investigated using a subsonic wind tunnel test facility to check which side cavity on the airfoil (inner or outer side) is beneficial in terms of the rotor's static and dynamic performances.
Abstract: This present investigation is carried out to improve the performance of H-Darrieus wind turbine in the built environment, where it mostly experiences low wind speed. Here the effect of circular cavity on aerodynamic performance of the rotor is investigated using a subsonic wind tunnel test facility to check which side cavity on the airfoil (inner or outer side) is beneficial in terms of the rotor’s static and dynamic performances. For this, S1046 and NACA 0021 airfoil blades are considered at various low wind speeds of 5, 6 and 7 m/s for different rotor aspect ratios. A Computational Fluid Dynamics (CFD) study is also simultaneously conducted to realize the intrinsic flow physics of the cavity airfoil blade profile. Results show that inner surface cavity on both the blades improves their self-starting ability but only at 5 m/s wind speed, which is not so when wind speed is 7 m/s at which NACA 0021 blade without cavity performs better. Again, NACA 0021 blade without cavity exhibits the highest performance of all the considered blade shapes, for which the highest power coefficient of 0.15 is achieved at a tip speed ratio of 1.25 and wind speed 6 m/s. At wind speed 7 m/s, the NACA 0021 blade rotor having outside cavity has a lower maximum power coefficient but wider operating range than that of NACA 0021 blade without cavity. CFD results show that H-Darrieus rotor having NACA 0021 blades at 30° azimuthal angle with circular cavity at 1/4th chord distance from its leading edge located at its inner surface, can generate higher lift force. However, circular cavity will be useful for starting performance of H-Darrieus rotor, which is not so for its dynamic performance, although operating range is improved.
TL;DR: The accuracy of several algorithms was determined and the best performing methods were implemented in a user-friendly open-source tool for performing DPIV flow analysis in Matlab.
Abstract: Digital particle image velocimetry (DPIV) is a non-intrusive analysis technique that is very popular for mapping flows quantitatively. To get accurate results, in particular in complex flow fields, a number of challenges have to be faced and solved: The quality of the flow measurements is affected by computational details such as image pre-conditioning, sub-pixel peak estimators, data validation procedures, interpolation algorithms and smoothing methods. The accuracy of several algorithms was determined and the best performing methods were implemented in a user-friendly open-source tool for performing DPIV flow analysis in Matlab.
01 Apr 1997
TL;DR: In this paper, the authors provide accurate numerical solutions for selected flow fields and to compare and evaluate the performance of selected turbulence models with experimental results, including free shear flows, boundary layer flows, and axisymmetric shockwave/boundary layer interaction.
Abstract: The primary objective of this work is to provide accurate numerical solutions for selected flow fields and to compare and evaluate the performance of selected turbulence models with experimental results. Four popular turbulence models have been tested and validated against experimental data often turbulent flows. The models are: (1) the two-equation k-epsilon model of Wilcox, (2) the two-equation k-epsilon model of Launder and Sharma, (3) the two-equation k-omega/k-epsilon SST model of Menter, and (4) the one-equation model of Spalart and Allmaras. The flows investigated are five free shear flows consisting of a mixing layer, a round jet, a plane jet, a plane wake, and a compressible mixing layer; and five boundary layer flows consisting of an incompressible flat plate, a Mach 5 adiabatic flat plate, a separated boundary layer, an axisymmetric shock-wave/boundary layer interaction, and an RAE 2822 transonic airfoil. The experimental data for these flows are well established and have been extensively used in model developments. The results are shown in the following four sections: Part A describes the equations of motion and boundary conditions; Part B describes the model equations, constants, parameters, boundary conditions, and numerical implementation; and Parts C and D describe the experimental data and the performance of the models in the free-shear flows and the boundary layer flows, respectively.
••29 Jun 1997
TL;DR: In this article, the performances of four turbulence models are evaluated against eight selected experimental flow fields, including freeshear flows, an incompressibl e boundary layer, and three complex flows with flow separation.
Abstract: The performances of four turbulence models are evaluated against eight selected experimental flow fields. The four models are the two-equation k-e model of Launder and Sharma, the two-equation k-a> model of Wilcox, the twoequation k-03 SST model of Menter, and the one-equation eddy-viscosity model of Spalart and Allmaras. The eight turbulent flows of the validation are four fully-developed freeshear flows, an incompressibl e boundary layer, and three complex flows with flow separation. The free-shear layer flows comprise a mixing layer, a round jet, a plane jet, and a plane wake flow. The three complex flows are comprised of an adverse-pressure-gradient boundary layer, an axisymmetric shock-wave/boundary-layer interaction, and a transonic RAE 2822 airfoil flow. The experimental data for these flows is well established and has been extensively used in model developments. The numerical predictions include mean velocity profiles, spreading rates, surface pressure coefficients, skin friction, and shear-stress profiles. Most significantly, this research includes a sensitivity study on the accuracy of the solutions with respect to the effects of freestream turbulence, grid resolution, grid spacing near the wall, initial conditions, numerical methods and codes, and free stream Mach number effects on incompressible flows.