A. Darabi

Bio: A. Darabi is an academic researcher from Tel Aviv University. The author has contributed to research in topics: Finite element method & Rotor (electric). The author has an hindex of 3, co-authored 3 publications receiving 756 citations.

Delay of Airfoil Stall by Periodic Excitation

Abstract: It was recently demonstrated that oscillatory blowing can delay separation from a symmetrical airfoil much more effectively than the steady blowing used traditionally for this purpose. Experiments carried out on different airfoils revealed that this flow depends on many parameters such as, the location of the blowing slot, the steady and oscillatory momentum coefficients of the jet, the frequency of imposed oscillations, and the shape and incidence of the particular airfoil. In airfoils equipped with slotted flaps, the flow is also dependent on the geometry of the slot and on the Reynolds number in addition to the flap deflection that is considered as a part of the airfoil shape. The incremental improvements in single element airfoil characteristics are generally insensitive to a change in Reynolds number, provided the latter is sufficiently large. The imposed oscillations do not generate large oscillatory lift nor do they cause a periodic meander of the c.p. C* C D = dp Ct =

Dynamic Stall Control by Periodic Excitation, Part 2: Mechanisms

Abstract: Dynamic e ow separation and its control over a stationary dee ected surface are used to demonstrate the timescale disparity between the process of dynamic stall, which is dominated by the dynamic stall vortex (DSV), and the excitation-induced large coherent structures that effect its control. Appreciation of this disparity provided a framework for analyzing dynamic stall control on a NACA 0015 airfoil, where leading-edge excitation had effectively eliminated the DSV and signie cantly attenuated trailing-edge separation. Within this framework, a comparisonofstaticandairfoilphase-lockeddynamicpressuredataacquiredin thevicinity ofmaximum incidence (® o 25 deg) revealed that chordwise pressure distributions were independent of the airfoil pitching frequency and that the generation and advection of LCSs were not signie cantly affected by the dynamic airfoil pitching motion.Furthermore,disparitiesbetweenstaticanddynamicdatadiminishedastheexcitationfrequencyincreased relative to the airfoil pitching frequency. Oscillations of the aerodynamic coefe cients induced by the excitations were negligibly small but served to regulateairfoil cycle-to-cycle disparities typical of the baselinepoststall regime.

Taguchi Method for Design and Optimization of a High-Speed Permanent Magnet Synchronous Generator Protected by Retention Sleeve

Abstract: High-speed permanent magnet synchronous generators (HS-PMSGs) suffer from mechanical stresses due to high speeds. With the predicted mechanical stresses that may occur in the rotor of the HS-PMSGs, the design of these machines should be very accurate. So, for the HS-PMSGs, a proper electromagnetic coupled with mechanical design is a critical issue. This paper presents a novel method for the electromagnetic and mechanical design of an HS-PMSG by finding an appropriate dimension of the retention sleeve and permanent magnets (PMs) based on the well-known Taguchi optimization method. A 40-kW, 60-krpm, 2-poles and 18-slots HS-PMSG is designed at the first step, and next, it has been optimized by the proposed optimization method, and finally modeled and analyzed through Finite-Element Method (FEM). Results obtained from the electromagnetic and mechanical simulations of the HS-PMSG show that in the optimized design of the HS-PMSG some parameters changed and the HS-PMSG has a better performance compared to the initial design. For example, The effective air gap has been reduced which leads to the better electromagnetic and mechanical performance of HS-PMSG compared to the initial design. By the reduction in the thicknesses of the retention sleeve and the PM, it can be concluded that the total size and dimensions of the HS-PMSG have been reduced. The weight of the PM and the retention sleeve are reduced by about 16.31% and 29.28% responsively, and as a result, the total weight of the HS-PMSG is reduced by approximately 1.94%, The Joule loss is reduced by about 9.80%, the HS-PMSG efficiency has been improved by 0.02%, and finally, the cogging torque is reduced by 27.87%, comparing with the initially designed. The FEM results ensure the electromagnetic and mechanical performance of the machine around the predicted speed of 60-krpm.

Online Interturn Short Circuits Fault Monitoring for Permanent Magnet Synchronous Machines

Abstract: This paper presents an unscented Kalman filter (UKF) based method to estimate the severity level of stator interturn short circuit fault in permanent magnet synchronous machines (PMSMs). A mathematical model is firstly built for PMSMs to represent the machine dynamic with inter-turn faults (ISF). The voltage imbalance, inherent asymmetry, and ISF effects are all differentiated analytically by the model. To quantify the fault severity in the presence of unmeasurable fault parameters, the UKF is employed to estimate the nonlinear fault-related quantities in PMSMs, such as the short circuit current, the percentage of shorted turns, and fault loop resistance. The effectiveness and reliability of the proposed method have been validated by extensive simulations.

On Dynamic Mode Decomposition: Theory and Applications

Abstract: Originally introduced in the fluid mechanics community, dynamic mode decomposition (DMD) has emerged as a powerful tool for analyzing the dynamics of nonlinear systems. However, existing DMD theory deals primarily with sequential time series for which the measurement dimension is much larger than the number of measurements taken. We present a theoretical framework in which we define DMD as the eigendecomposition of an approximating linear operator. This generalizes DMD to a larger class of datasets, including nonsequential time series. We demonstrate the utility of this approach by presenting novel sampling strategies that increase computational efficiency and mitigate the effects of noise, respectively. We also introduce the concept of linear consistency, which helps explain the potential pitfalls of applying DMD to rank-deficient datasets, illustrating with examples. Such computations are not considered in the existing literature, but can be understood using our more general framework. In addition, we show that our theory strengthens the connections between DMD and Koopman operator theory. It also establishes connections between DMD and other techniques, including the eigensystem realization algorithm (ERA), a system identification method, and linear inverse modeling (LIM), a method from climate science. We show that under certain conditions, DMD is equivalent to LIM.

Actuators for Active Flow Control

Abstract: Actuators are transducers that convert an electrical signal to a desired physical quantity. Active flow control actuators modify a flow by providing an electronically controllable disturbance. The field of active flow control has witnessed explosive growth in the variety of actuators, which is a testament to both the importance and challenges associated with actuator design. This review provides a framework for the discussion of actuator specifications, characteristics, selection, design, and classification for aeronautical applications. Actuator fundamentals are discussed, and various popular actuator types used in low-to-moderate speed flows are then described, including fluidic, moving object/surface, and plasma actuators. We attempt to highlight the strengths and inevitable drawbacks of each and highlight potential future research directions.

Aerodynamic Flow Control over an Unconventional Airfoil Using Synthetic Jet Actuators

Abstract: Control of flow separation on an unconventional symmetric airfoil using synthetic (zero net mass flux) jet actuators is investigated in a series of wind tunnel tests. The symmetric airfoil comprises the aft portion of a NACA four-digit series airfoil and a leading edge section that is one-half of a round cylinder. The experiments are conducted over a range of Reynolds numbers between 3.1 × 10 5 and 7.25 × 10 5 . In this range, the flow separates near the leading edge at angles of attack exceeding 5 deg. When synthetic jet control is applied near the leading edge, upstream of the separation point, the separated flow reattaches completely for angles of attack up to 17.5 deg and partially for higher angles of attack. The effect of the actuation frequency, actuator location, and momentum coefficient is investigated for different angles of attack. The momentum coefficient required to reattach the separated flow decreases as the actuators are placed closer to the separation point. In some cases, reattachment is also achieved when the actuators are placed downstream of the stagnation point on the pressure side of the airfoil