Sh. Shams, R. Esbati Lavasani,
Volume 19, Issue 6 (6-2019)
Abstract
In this study, we derived the rotating airfoil system of equation considering Loewy aerodynamics. To this end, we define the local coordinate system on airfoil and reference coordinate on the hub. We define the free air velocity vector and the airfoil rotating speed vector according to the reference coordinate. So, the Kinetic and Potential energies are derived based on linear stiffness and linear damping according to the Hamiltonian principle. Wakes behind the rotating blades form into the helix. Therefore, we the equation of motion with Loewy aerodynamic which compensates the wake effects. Stability analysis is performed by the well-known P-K method. Flutter speed and stability boundary are estimated. Comparing the results of stability analysis and the reference validates the applied method. Furthermore, we proposed the PID Control to suppress the flutter speed. the PID controller input and command. The desired time and error tolerance are selected to design PID controller. Unit step response shows that pitch angle response is under-damped. However, step response tracks input well. Besides, disturbance rejection by considering the gain from input to output to remain below the gain value is analyzed.
M. Tahani , M. Kazemi , Z. Babaie ,
Volume 19, Issue 9 (9-2019)
Abstract
Today, one of the useful methods of flow control, especially external aerodynamics, is plasma DBD actuators. In this study, the effect of plasma DBD actuators on cylinders in tandem arrangement is investigated. The actuators are considered on upstream cylinder. The cylinders are placed in distance (L/D) relative to each other. Investigation is done at two Reynolds number (100 and 200) with two different conditions of applying actuators. Cases with Vp-p=55kv and Vp-p=1kv are selected from references. The results of the present study are validated against the previous available experimental and numerical data and close agreement is found. Finite volume method is applied to solve equation of motion. Plasma actuators caused downstream cylinder experience upper values of drag coefficient and Nusselt number in all cases of study. Also, the growth of drag coefficient and Nusselt number are decreased by rising the Reynolds number, so that increasing the Nusselt number is 2% more at cases with Re=100 compared to cases with Re=200.
E. Bakhtiari ,
Volume 19, Issue 9 (9-2019)
Abstract
A wind turbine airfoil was analysed, using computational fluid dynamics (CFD) to study the oscillating effects and slip boundary conditions. The slip boundary condition is due to applying superhydrophobic surface. Fluids on these surfaces are repelled. The superhydrophobic surface can delay the icing on blades. The surfaces is assumed at the leading edge; the icing can occur on this region. The chosen oscillation parameters was enough for modelling dynamic stall. The dynamic stall cause a severe loading on the blade. This phenomenon is depicted by two vortices: leading edge vortex and trailing edge vortex. Three reduced frequencies are considered:
in a range of
slip lengths. In this regard, the Transition-SST model is applied for SD7037 airfoil with
. The results showed that applying a superhydrophobic surface with low values of the slip length cannot be appropriate during the oscillating motion; but at the slip lengths larger than 100 microns, the aerodynamic coefficients are significantly changed. At the highest reduced frequency, the lift and drag coefficients are reduced about 12% and 40%, respectively. Increasing the slip length postponed the vortex formation and stall angle.
Sh. Shams, M. Ramezani, A. Molaei,
Volume 20, Issue 5 (5-2020)
Abstract
The aerodynamic of Vertical Axis Wind Turbine (VAWT) is more complex than a horizontal axis wind turbine. In the present research, the combination of the Wagner unsteady aerodynamic model, static stall and Double Multiple Stream Tube (DMST) aerodynamic model have been used to investigate the aeroelastic behavior of VAWT. For this purpose, the DMST aerodynamic model, which is related to the vertical axis wind turbine aerodynamics model, has been used to obtain two parameters of the angle of attack and relative velocity. Then these two parameters have been applied to the Wagner nonlinear aerodynamics, which considers the effect of the static stall. This flexible nonlinear presented model based on DMST is called NFDMST aerodynamic model. One-degree of freedom of typical section and two-degree of freedom model have been investigated for static aeroelasticity and dynamic aeroelastic behavior, respectively. The VAWT blade experiences a variety of attack angles and relative velocity in a spin, so the goal is to obtain the instability velocity in a different position and consider the effect of aerodynamic and structure nonlinearity. The results show that the nonlinear aerodynamic model has accurate results and the aeroelastic design condition associated with -90degree azimuth angle, in which the minimum instability velocity is 45.2m/s. In addition, the change of instability speed of rotating airfoil in a spin is about 6%.
A. Sharafi, D. Mokhtari,
Volume 20, Issue 5 (5-2020)
Abstract
In this research, the effect of several unconventional obstructions with cubic, spherical, cylindrical, and cone geometries on the propulsion vector of a convergent-divergent micro nozzle as a new method in propulsion vector control is experimentally investigated. For this purpose, a convergent-divergent nozzle was designed and constructed in small dimensions. This nozzle is such that the Mach number is its nominal output in full expansion conditions 2. The wall of this nozzle is designed to measure pressure variations with pressure holes. Also, in the nozzle wall, a duct has been created to apply a bulge inside the nozzle. Pressure sensors and the shadograph system have been used to pressure measurement and check the outlet flow field respectively. The total pressure of the calming chamber is constant in all experiments and is equal to 5.5 times. The results of this study show that the maximum deviation is related to an obstruction with a cubic geometry which is 2.1 degrees. Also, the geometries that have sharp corners are more shock-shaped and hit the opposite wall. In this research, the shock formed by a cubic barrier has hit the opposite wall, but with a spherical shaped and cone-shaped barrier, the shock comes out from the nozzle. Also, these results indicate that the axial force of the nozzle has been reduced to a very small extent.
A. Mehrabi, A. Davari,
Volume 20, Issue 6 (6-2020)
Abstract
In this study, multipurpose testing equipment with a sub-scale model of a specific tandem rotor helicopter constructed to conduct a number of experiments to accurate understanding from the tandem rotor's outwash in ground effect. The experiments conducted as measuring rakes were positioned at two distances equal to 1.5 R and 3R from the rotor(s). Unlike the experiments that have been performed in wind tunnels or in special hover chambers, these experiments were performed in an open environment with fewer side walls effects. The results show that when the single rotor operates in a fixed altitude, and the blades tip velocity of 0.2M, the outwash velocities reduces as the flow moving away and vice versa, but for tandem rotors, increasing the rack distance from the model does not have a noticeable effect on the average values of the flow velocity. A comparison of the results of these measurements with the CH-47D helicopter outwash patterns confirms the accuracy of the obtained patterns and showed that the overlap between the rotors increases the velocity values and causes to the occurrence of maximum outwash velocity at lower altitudes. No overlap between the tandem rotors makes the outwash flow pattern of each of them similar to a single rotor. Increase in ground effect as the height of the rotor(s) decreases to 1R, changes the flow pattern in the forward and aft of the model helicopter. In this altitude, unlike their operation in altitude of 2R, the outwash flow increases when moving away from the rotor(s).
A. Taban, A. Jalali, M. Zamani,
Volume 20, Issue 7 (6-2020)
Abstract
Humans are always looking for ways to produce cheap and permanent electricity. One of these ways is to use wind turbines. The vertical axis wind turbines are less sensitive due to the problem of the setup and low efficiency compared to the horizontal axis turbines. One way to improve the performance of VAWTs is to change the angle of attack of the wind turbine blade. In this study, the computational fluid dynamics method is used to solve the finite volume flow equations. Different angles of attack range from -12 to +10 degrees and wind speeds of 10m/s and density of 1.225kg/m3 and constant dynamic viscosity of 1.825psi were used. The calculations showed that by increasing the angle of attack of the blade to +10 degrees Cp and Torque decreased, by decreasing angle of attack of the blade to -4 degree, Cp and Torque increased, but by more decreasing AOA of -8 to -12 degrees Cp and torque decreased.
Araz Nadi, Negar Nabatian, Pouyan Hashemi Tari, Shiva Asgari Marnani,
Volume 21, Issue 8 (8-2021)
Abstract
New generation of wind turbines, in comparison to the old versions, have been designed with colossal blades to produce larger amount of power output. However, this has led into some unpredictable challenges including their construction procedure and expenses and particularly blades’ transportation. To overcome these issues, multi-rotor wind turbines have been suggested. Aerodynamic performances of such turbines have been previously assessed by other investigators. However, the wake characteristics of these turbines have been less studied. The focus of the present research is on the assessment of these characteristics, which are crucial in the process of any wind farm design. For this purpose, wake flow of a small three-rotor wind turbine is numerically simulated using computational fluid dynamics. A numerical simulation has been conducted for a single-rotor wind turbine and three-rotor small horizontal axis wind turbine with the angle of 180⸰ arrangement. The results of single rotor wind turbine indicated that far downstream wake extended up to 8D, with Jensen-Gaussian model can be better predicted. The comparison between three bladed wind turbine and the results of wake models for the equivalent turbine showed that because of wake interactions in the downstream of the rotor, the loss of turbulent kinetic energy and recovery of the stream speed will be faster. As a result, in the wind farms, the turbines in closer distances around 4D of the equivalent signle-rotor wind turbine can be installed.
Mahdy Ahangar, Arefeh Hoseini,
Volume 22, Issue 6 (5-2022)
Abstract
In this research, the dielectric barrier discharge plasma driven channel flow with the applied magnetic field has been proposed for use as a thruster in propulsion applications and studied experimentally. Measurements of the thrust and consumed power of thruster for different values of the barrier thickness have been performed and the data have been compared with the corresponding ones without magnetic field. It is found that consumed power and thrust of the thruster in the presence of magnetic field are respectively little reduced and increased than that without the magnetic field. The measurements show that the effectiveness increases to a maximum and then drops as the operating voltage monotonically increases over a range from 12 to 26 kV. A power law analysis for revealing the relationships among the effectiveness, thrust, consumed power, and operating voltage has been presented for the thruster with and without the magnetic field. It is seen that the applied magnetic field and thicker dielectric barrier can lead to a higher effectiveness at the point of transition from the glow regime to the filamentary regime. The effects of micro-discharge channels on the effectiveness in the both regimes have been discussed. The observations indicate that in the presence of magnetic field, the additional micro-discharge channels are generated and develop along the magnetic field lines and the diffuse background emission of the discharge is stronger in plasma. The underlying physical mechanisms of mentioned phenomena have been explained and mainly ascribed to the enhanced ionization by applying the magnetic field.
Alireza Rabiee, Elyas Lekzian, Amirhossein Hossein, Farhad Ghadak, Mohsen Nahlegah,
Volume 22, Issue 9 (9-2022)
Abstract
In the current paper, downstream flow field of a propeller at low Reynolds numbers and at static conditions (zero flight speed) is investigated experimentally. This propeller can be utilized in UAVs. Propeller diameter is 56 centimeter and it is investigated at 2550 to 5670 rpm experimentally. Experiment results show that propeller rpm increasing, increases induction velocity. Flow swirl ratio and axial flow coefficient decrease along propeller radius at different propeller rpm. Experimental results of absolute velocity of swirl flow at the propeller airfoil trailing edge downstream is fairly similar to the free vortex flow theory at static condition along the blade radius. At static condition for r/R<0.8, semi-empirical equations are suggested for variation of flow swirl ratio and axial flow coefficient at downstream of propeller. The propeller is also simulated with numerical simulations. Relative standard deviation of numerical and experimental results for propeller thrust and power are 0.4 and 4.1, respectively. The exponential coefficient (n) which predicts numerical axial flow downstream of propeller for r/R<0.8 has a 7.7 relative standard deviation with experimental results at static condition.
Mohsen Nazemian Alaei, Mohammad Sadegh Valipour,
Volume 23, Issue 2 (1-2023)
Abstract
Wingsuit flying is one of the most popular flight disciplines in recent decades. In the aviation profession, efficiency and safety are paramount concerns for costume designers. An article in this issue examines how waveform changes to the wing surfaces of a wingsuit model improves aerodynamic performance. In order to increase performance, vortices are produced inside the boundary layer that improve the exchange of motion. In this experimental and numerical study, we investigate the formation and evolution of vortices in the Reynolds number range of 106 and provide insights into flow patterns on surfaces with geometric changes. A detailed study of flow structure can be obtained from experimental and numerical evaluations. According to the results, there are significant vortex generators near the backpack due to high pressure. Immediately after the creation of these vortices, the flow is drawn and spread on the surface of the wing in three dimensions. As a result of the angle of attack, the wing surface separates prematurely. Based on the lift and drag coefficients, the study model showed the best performance in flight at an angle of attack of 10 degrees for this flow regime.
Hamidreza Kaviani, Ehsan Bashtalam,
Volume 23, Issue 8 (8-2023)
Abstract
| Icing is a common issue in blowers, wind turbines and flying vehicles. This phenomenon has a great impact on reducing aerodynamic performance, increasing noise pollution and imposing extra load on the structure. In this article, the effect of icing on the aerodynamic and aeroacoustic performance of the Naka-0012 airfoil has been studied. Transient and three-dimensional Navier-Stokes equations have been used for aerodynamic prediction. Sound wave is calculated using Fox-Williams and Hawkins equations. Simulation of eddies has been done using LES method and WALE subgrid scale model. First, all calculation methods have been validated using experimental data. Then the effect of icing on airfoil performance has been studied. Flow vortices have been studied and sound production mechanisms corresponding to these vortices have been identified. The results show that icing reduces the lift force by 9.7% and increases the drag force by 3.8 times. In the range of maximum human hearing sensitivity (one to five kHz), the average amount of sound increase is around 9 dB, which is a significant amount in terms of noise pollution. The increase in sound caused by icing can be used to identify and deal with this phenomenon faster and reduce its risks. |
Hamidreza Kaviani, Ehsan Bashtalem,
Volume 23, Issue 11 (11-2023)
Abstract
This study examines the accuracy of three different methods for calculating the aerodynamic noise of the NACA-0012 airfoil in a homogeneous shear flow. The strength of flow vortices is crucial in aeroacoustic calculations, and it can be modeled more cost-effectively using the k-ω SST method or directly simulated up to 90% using large eddy simulation (LES) with higher cost. Additionally, a hybrid method called IDDES, which offers moderate accuracy and cost, is also considered in this research. The primary sources of noise generation identified are vortex shedding from the laminar boundary layer and its interference with the trailing edge, as well as Tollmien-Schlichting waves. Experimental data of the sound pressure level (SPL) in 1/3 octave is used to validate the accuracy of the methods. The results indicate that LES and IDDES show the closest agreement with the experimental data, with LES showing better accuracy. Furthermore, when studying the intensity of sound attenuation with distance, it is observed that the rapid attenuation of small vortices in LES leads to similar SPLs as IDDES after a distance of 1.2 meters. Moreover, since IDDES does not require strict regulations for creating a near-wall grid, it reduces the computing mesh by approximately 41% with less than 2 dB of error. This finding suggests that in similar applications, the IDDES method can be used as a suitable approximation instead of LES to expedite calculations and conduct parametric studies
Mostafa Dehghan Manshadi, Vahid Esfahanian,
Volume 24, Issue 5 (4-2024)
Abstract
The main approach in the study of fluid flow instabilities is the theory of linear stability, which is based on linearizing the governing equations and finding unstable eigenvalues. In many flows, like shear flows, the results of linear stability theory fail to match most experiments. In a linear system, even if all the eigenvalues are stable, the perturbations can lead to instability, if the eigenfunctions are not orthogonal. The transient features of these non-normal dynamical systems, can be described with low-dimensional structures, i.e. a few modes. It is possible to suppress the asymptotic and transient growth by identification of time-dependent modes. In this paper, a method of order reduction based on optimally time-dependent modes has been implemented. This method identifies the growth behavior of disturbances in short and long times. Also, a control algorithm based on the above method has been implemented to stabilize the growth of disturbances. The DNS solution of the flow and the implementation of the reduction and control algorithms is based on the NEKTAR++ open-source solver. At first problem, to validate the solution method, the order reduction and control algorithm has been implemented on the flow over a cylinder with Re=50. At second problem, for the first time, the control algorithm is implemented on the flow over a cylinder subjected to persistent time-varying disturbances. The results show that by applying a control force, the Von-Karman vortices are stabilized and a constant lift is obtained and body vibrations are cancelled.
