Tahir Yavuz

Control of the Flow Around Square Cylinders at Incidence by Using a Rod

Aerodynamic characteristics of a square cylinder at incidence were investigated experimentally in the wake of a small rod at a Reynolds number of 3.4 x 10 4 . The dimensionless gap L/D was varied from 1.71 to 8.00. Depending on the spacing L/D, two flow patterns with and without vortex shedding from the rod were observed. Pressure measurements on the rod and square cylinder and hot-film measurements in the wake of the cylinder were carried out The combined influences of the rod and angle of incidence on the pressure distributions and vortex-shedding phenomenon were investigated

Subcritical Flow Around Bluff Bodies

Results of experimental investigations of vortex-shedding frequencies and surface pressures of circular and square cylinders with the same hydraulic diameter are described. Pressure measurements on circular and square cylindersandhot-e lm measurementsinthewakeswereconducted. TherangeofReynoldsnumberswas1 :3 £ 10 4‐ 2 £ 10 5 , which is characterized as subcritical e ow regimes. Also, to reveal the combined ine uence of Reynolds number and angle of incidence on the pressure and spectral density distributions of velocities, experiments were carried out on square and rectangular cylinders for the angle of incidence in the range 0 < ®< 45 deg. At zero incidence,pressurecoefe cientsonthefrontandsidefacesofthesquarecylinderchangeconsiderablywithReynolds number, whereas as the angle of incidenceincreasesto 30 deg, theeffectoftheReynolds numberseemsto diminish. Spectral density distributions in the wake and surface pressures depend on Reynolds number and the angle of incidence.Thevariationsofthestrengthofthespectraldensitieswith Reynoldsnumberinthewakesofcircularand square cylinders show different characteristics. Variations of spectral density distributions and vortex shedding with Reynolds number, geometric dimensions of models, and angle of incidence are presented.

Flow Around a Rotatable Circular Cylinder-Plate Body at Subcritical Reynolds Numbers

The behavior of a stationary circular cylinder with an attached plate, under conditions where the entire cylinder‐ plate body rotates about the cylinder axis, has been investigated experimentally for Reynolds numbers between 8 × 10 3 and 6 × × 10 4 .T osee the effect of the plate inclination on the pressure distributions and vortex shedding, the cylinder‐plate body was rotated from 0 to 180 deg, unlike freely rotatable cases in previous studies. The plate was located at the center plane of the cylinder, upstream of the cylinder, at the beginning. The diameter of the cylinder and the width of the plate were both chosen to be 35 mm. Measurements of shedding frequency and pressures on the surface of the cylinder were obtained. The results indicate that the shedding frequency was nearly constant in the range of 50‐120 deg and, by further increasing the angle from 120 to 160 deg, it strikingly increases and then again decreases at angles larger than 160 deg. The plate also causes important changes in pressures on the surface of the cylinder with increasing inclination angle. For different plate angles, five different types of pressure distributions have been observed. Characteristics of the vortex formation region and location of flow attachments, reattachments, and separations were observed by means of the flow visualizations. The drag coefficient of the cylinder has a maximum value at approximately θ =7 5deg, whereas it has a minimum value at θ =1 5deg. The lift coefficient has two maximums, at θ =1 5and 165 deg, depending on the plate position. The values of CL at about θ =4 5and 160 deg are zero as in the case of the cylinder without a plate.

Flow Around a Rotatable Square Cylinder-Plate Body

The flow around a square cylinder with a plate attached to it has been investigated experimentally in the Reynolds number range between 7.5 × 10 3 and 5.5 × 10 4 . The plate was located at the center of the front face of the square cylinder at the beginning. To determine the effects of the incidence angle of the square cylinder‐plate body on the pressure distributions and vortex shedding, the square cylinder with the plate was rotated. The incidence angle was varied from 0 to 180 deg, corresponding to the position of the plate in the front and back face of the cylinder, respectively. The width of the cylinder and the plate were chosen to be the same at 28 mm. Measurements of shedding frequency and pressures on the surface of the cylinder were obtained. The results indicate that the Strouhal number based on D, the side length of the square cylinder, has a strong peak at θ = 12 deg. After that, it decreases and remains nearly constant in the range from 50 to 105 deg. Two weaker peaks have also been observed at θ = 112 and 162 deg. The positions of flow attachments, separations, and reattachments on the square cylinder due to the attached plate and the inclination of the body have been clearly obtained from the pressure distributions on the square cylinder and from the flow visualization photographs. Also, drag and lift coefficients of the square cylinder are calculated from the pressure distributions. Drag coefficient of the square cylinder has minimum and maximum values at approximately θ = 20 and 80 deg, respectively, whereas the lift coefficient has a minimum and a maximum at approximately θ = 110 and 150 deg, depending on the plate position. The reducing effects of the plate on the drag and lift forces are discussed.

Performance Analysis of a Hydrofoil with and without Leading Edge Slat

Operational effectiveness of the wind and hydrokinetic turbines depend on the performance of the airfoils chosen. Standard airfoils historically used for wind and hydrokinetic turbines had and have the maximum lift coefficients of about 1.3 at the stall angle of attack, which is about 12o. At these conditions, the minimum flow velocities to generate electric power are about 7 m/s and 3 m/s for wind turbine and hydrokinetic turbine, respectively. Using leading edge slat, the fluid dynamics governing the flow field eliminates the separation bubble by the injection of the high momentum fluid through the slat over the main airfoil-by meaning of the flow control delays the stall up to an angle of attack of 20o, with a maximum lift coefficient of 2.2. In this study, NACA 2415 was chosen as a representative of hydrofoils while NACA 22 and NACA 97, were chosen as slat profiles, respectively. This flow has been numerically simulated by FLUENT, employing the Realizable k-e turbulence model. In the design of the wind and hydrokinetic turbines, the performance of the airfoils presented by aerodynamics CL = f (a,d), CD = f (a,d) and CL/CD = f (a,d) are the basic parameters. In this paper, optimum values of the angle of attack, slat angle and clearance space between slat and main airfoil leading to maximum lift and minimum drag, and consequently to maximum CL/CD have been numerically determined. Hence, using airfoil and hydrofoil with leading edge slat in the wind and hydrokinetic turbines, minimum wind and hydrokinetic flow velocities to produce meaningful and practical mechanical power reduces to 3-4 m /s for wind turbines and 1-1.5 m/s or less for hydrokinetic turbines. Consequently, using hydrofoil with leading edge slat may re-define the potentials of wind power and hydrokinetic power potential of the countries in the positive manner.

Comparison of Qblade and CFD results for small-scaled horizontal axis wind turbine analysis

The software QBlade under General Public License is used for analysis and design of wind turbines. QBlade uses the Blade Element Momentum (BEM) method for the simulation of wind turbines and it is integrated with the XFOIL airfoil design and analysis. It is possible to predict wind turbine performance with it. Nowadays, Computational Fluid Dynamics (CFD) is used for optimization and design of turbine application. In this study, Horizontal wind turbine with a rotor diameter of 2 m, was designed and objected to performance analysis by QBlade and Ansys-Fluent. The graphic of the power coefficient vs. tip speed ratio (TSR) was obtained for each result. When the results are compared, the good agreement has been seen.

CFD vs. XFOIL of airfoil analysis at low reynolds numbers

For Blade Element Momentum (BEM) theory, the airfoil data needs to be as accurate as possible. Nowadays, Computational Fluid Dynamics (CFD) is used for optimization and design of turbine application. Lift coefficient, drag coefficient and lift coefficient over drag coefficient are significant parameters for turbine application. Selecting a suitable computational tool is crucial for design of the turbine. Panel method and an integral boundary layer formulation are combined in the XFOIL code for the analysis of potential flow around the airfoils. In this study, XFOIL code and Transition SST k-omega model were used to predict the aerodynamic performance at low Reynolds numbers (Re = 3×10⁁5, 4×10⁁5, 5×10⁁5). The results were compared and CFD results and XFOIL code result are compatible with each other until stall angle. Also XFOIL and CFD results were shown a good coherence with literature.

Increasing efficiency of an existing francis turbine by rehabilitation process

As a renewable energy type, hydraulic energy has an important role in power generation. Francis turbines are the most common turbine type in use for hydraulic power generation. A francis turbines main components are spiral case, stay vanes, guide vanes, runner and draft tube. As the design processes of components differ with head and discharge values for each turbine, the process requires much effort and time. In recent years, Computational Fluid Dynamics (CFD) are commonly used to predict hydraulic performance of turbines since it is more economical and effective in time management. In this study, a Francis turbine is designed based on head and discharge values of an existing turbine at Kesikköprü, Ankara with an efficiency about %89 and analysed with commercial CFD codes. Pressure and velocity distributions of turbine components and turbine efficiency are examined. As a result of this study, it is obtained that there is an increment about %5 in planted turbines efficiency. Thus, the francis turbine can generate more power with a rehabilitation process.

Wind power electrical systems integration and technical and economic analysis of hybrid wind power plants

Energy is one of the major component of the economical and social development. Since fossil fuels are both limited source and environmentally hazardous, people have turned towards usage of renewable and clean energy sources recently. Wind energy, with its applications, became prominent within the renewable energy sources. In the scope of this study, not only wind energy and electricity production by wind turbines are explained but also main challenges and adverse impacts of wind energy integration to the grid are explained. Hybrid renewable power plant which is a method used for reducing the disadvantages of wind energy integration to the grid was modelled for Başkent University by the HO MER software. According to the current load and wind-solar characteristics, the optimum configurations are pv-wind grid connected system and pv-wind-fuel cell stand-alone system. Aiming of maximum renewable fraction and lowest cost, results gave wind — pv grid connected system is a clean and cost effective option with comparable energy costs which has 1,28 Million US Dollar for whole system and 0,107 USD per kWh, in addition to this the load is supplied 56% by PV, 24% by wind turbines and 20% by grid.

Parametric Analysis of Pumped Storage Hydropower-Coupled Wind Turbine Plants

The concept of a hybrid system consisting of a wind turbine plant and a pumped storage hydropower plant is a promising idea with respect to a number of important factors. Wind turbine plant power output has a fluctuating and interrupted nature, and the penetration of the electrical energy to the grid usually poses problems to the steady operation of the system. Pumped storage hydropower is a mature technology in the energy storage sector. Pumped storage hydropower has advantages compared to other energy storage technologies such as CAES, batteries, etc. due to its short discharge time, high energy storage capacities, and high power output. The first objective of this study is to time-shift the power output of the wind turbine plant to the peak-demand periods by means of storage and thus realizing increase in the penetration level to the grid. The second objective is to obtain a more stable power output from the wind turbine plant that inevitably has a variable power output. A 1-year operation simulation of the hybrid system was carried out under peak tariff strategy within the scope of these two objectives. An operation simulation was also made covering only the wind turbine plant with same power output data and same demand strategy and the results were compared. Without storage, the wind turbine plant feeds 6,350.30 MW h of electrical energy into the grid; this value increases to 7,656.00 MW h for the hybrid system, and this means that there is 17.05 % increase in penetration and the system is technically applicable for the first objective. Annual stable power output ratio that shows the fulfillment of demand is approximately 72.49 % for wind turbine plant without storage, and this value is approximately 87.40 % for the hybrid system. This means that there is a 14.91 % increase and that the system is technically applicable for the second objective. This paper provides the details of the year-long analysis, draws generalized conclusions, and provides recommendations for future studies.