Maziar Arjomandi

Resonance responses of geometrically imperfect functionally graded extensible microbeams

This paper aims at analyzing the size-dependent nonlinear dynamical behavior of a geometrically imperfect microbeam made of a functionally graded (FG) material, taking into account the longitudinal, transverse, and rotational motions. The size-dependent property is modeled by means of the modified couple stress theory, the shear deformation and rotary inertia are modeled using the Timoshenko beam theory, and the graded material property in the beam thickness direction is modeled via the Mori - Tanaka homogenization technique. The kinetic and size-dependent potential energies of the system are developed as functions of the longitudinal, transverse, and rotational motions. On the basis of an energy method, the continuous models of the system motion are obtained. Upon application of a weighted-residual method, the reduced-order model is obtained. A continuation method along with an eigenvalue extraction technique is utilized for the nonlinear and linear analyses, respectively. A special attention is paid on the effects of the material gradient index, the imperfection amplitude, and the length-scale parameter on the system dynamical response.

Tubercles and Their Applications

AbstractThe implementation of tubercles on foils has demonstrated significant benefits, with the most evident occurring during post-stall. However, the flow mechanism(s) responsible for these benefits is currently unknown, and several possibilities have been proposed. These include compartmentalization, vortex lift, varying effective angle of attack, and boundary layer momentum exchange. Currently, it is only known that tubercles create pairs of streamwise, counter-rotating vortices. By determining how tubercles work, the effects of their addition to untested foils in untested conditions can be hypothesized. This paper reviews the current status of the field of tubercles, comparing hypotheses with published results. The effects of tubercles on the principal components of drag are conjectured from consideration of similar flow control devices. Current applications of tubercles are detailed, and potential applications are suggested.

An Investigation on the Effect of the Hot End Plugs on the Efficiency of the Ranque-Hilsch Vortex Tube

The phenomenon of temperature distribution in confined steady rotating gas flows is called as Ranque-Hilsch effect. The simple counter-flow vortex tube consists of a long hollow cylinder with tangential nozzles at one end for injecting compressed air. Rotating air escapes the tube through two different outlets-a central orifice diaphragm placed near the inlet (cold end) and a ring-shaped peripheral outlet at the opposite end of the tube (hot end). Open image in new window Vortex tube and air flow in the vortex

The response of a flat plate boundary layer to an orthogonally arranged dielectric barrier discharge actuator

The jetting characteristics of dielectric barrier discharge (DBD) actuators make these devices suitable for augmenting boundary layer flows. The associated change to the hydrodynamic stability of the fluid arising from the actuator provides a mechanism through which a DBD-based laminar flow control (LFC) system can be developed. Historically, DBD actuators with electrodes arranged parallel to each other have been used for LFC with mixed results. An alternative is to use an actuator with electrodes placed orthogonally to each other. Orthogonally arranged actuators exhibit different jetting characteristics to conventional ones, and as such understanding the effect that these actuators have on the mean velocity profile within a flat plate boundary layer is of significant interest to the development of DBD-based LFC technology. In this investigation, the velocity distribution within a flat plate boundary layer in a zero pressure gradient is measured in response to the operation of an orthogonally arranged actuator. The results suggest that significant thinning of the boundary layer can be realized with an orthogonally arranged actuator, over a short distance downstream of the device, and used in conjunction with a subtle suction effect, this thinning can be exacerbated. However, further downstream, rapid thickening of the layer, supported by a decrease in the shape factor of the flow suggests that the layer becomes unstable, in an accelerated fashion, to the presence of the actuator. Hence the stability of the layer is found to be significantly altered by the presence of the orthogonally arranged actuator, a requisite for a LFC system. However, since the actuator produces a destabilizing effect, the development of a successful LFC system based on orthogonal actuators will require further work.

Investigation of the effect of dielectric barrier discharge plasma actuators on the radar cross section of an object

The application of dielectric barrier discharge (DBD) plasma as an electromagnetic absorber was investigated by determining the radar cross section (RCS) of a rectangular, flat plate with a DBD plasma actuator array installed on one of its sides. In order to justify the experimental results, the expected effect of plasma actuation on RCS was analysed by determining the attenuation effect of the plasma with the Lorentz model. Due to the very limited life time of the free electrons and the small extent of the plasma sheath, the attenuation was found to be only minimal.The theoretical results have been verified by comparing the measured RCS values of a plate with and without plasma actuation applied on it in a high-frequency anechoic lab. As expected, no significant influence of DBD plasma on RCS was detected. In addition, it was found that the high voltage power supply used as a part of DBD circuitry produced a high level of disturbance even in the microwave range.

Horizontal axis wind turbine dynamic stall predictions based on wind speed and direction variability

The onset of dynamic stall in horizontal axis wind turbines (HAWTs) is related to the rapid increase in the angle of attack caused by sudden changes in wind speed and direction. In order to relate the changes in wind speed and direction with the variations in the blade-section angle of attack, an analytical model is proposed to determine the regions of the blade affected by dynamic stall. The so-called threshold radius has been identified and defined as the percentage of the blade length from the horizontal axis wind turbines hub beyond which the probability of dynamic stall occurrence falls to zero. High quality wind data were acquired to determine the average wind conditions that serve as the model inputs. It is shown that the rate of change of wind speed, due to gusts or the average turbulence, can cause large regions of dynamic stall on the wind turbine blade. Other parameters, such as the yaw misalignment and the rate of change of yaw angle are shown to be the cause of asymmetrical distribution of threshold radius with azimuth and also serve to increase the affected regions. Finally it is shown that the type of airfoil used in the turbine blade also has a significant effect on the threshold radius due to the different limiting reduced frequencies.

Analysis of the turbulent boundary layer in the vicinity of a self-excited cylindrical Helmholtz resonator

This study investigates the changes in the structure of a turbulent boundary layer downstream of a flow-excited Helmholtz resonator. To this end, a fully developed turbulent boundary layer over a resonator mounted flush with a flat plate was simulated by implementing a large eddy simulation (LES). To assist in understanding the effect of the resonator on the flow structure, a sensitivity study was undertaken by changing the main geometrical parameters of the resonator. The results demonstrated that when the boundary layer thickness equals the orifice length, the cross-stream component of velocity fluctuations penetrates the boundary layer, resulting in a reduction of the turbulence intensity by up to 12%. Therefore, it is concluded that a Helmholtz resonator has the potential to reduce the instabilities within the boundary layer. These investigations also assist in identifying the optimal parameters to delay turbulence events within the grazing flow using Helmholtz resonators.

Force Measurements and Wake Surveys of a Swept Tubercled Wing

AbstractForce measurements and wake surveys have been conducted on two swept NACA 0021 wings. One wing had a smooth leading edge, while the other wing had a tubercled leading edge. The force measurements and the wake survey results were in good agreement. Between 0 and 8° angles of attack, tubercles reduced the lift coefficient by 4–6%. For the same range of angles of attack, tubercles reduced the drag coefficient by 7–9.5%. Tubercles increased the lift-to-drag ratio of this wing by 2–6%, and increased the maximum lift-to-drag ratio by 3%. At angles of attack higher than 8°, tubercles typically decreased the lift coefficient and the lift-to-drag ratio, while substantially increasing the drag coefficient. The wake surveys revealed that tubercles reduced the drag coefficient near the wingtip and that they also spatially modulated the drag coefficient into local maxima and minima in the spanwise direction. Typically, tubercles reduced the drag coefficient over the peaks where the tubercle vortices produced d...

Effect of a rigid wall on the vortex induced vibration of two staggered circular cylinders

Vortex Induced Vibrations (VIVs) play a key role in a wide range of engineering applications including the extraction of renewable energy. In this paper, numerical studies of the phenomenon of VIV were conducted to investigate the flow behaviour around two identical circular cylinders. The upstream cylinder was located in the vicinity of a rigid wall and downstream one was mounted on an elastic support with one degree of freedom. The Reynolds number based on the cylinders diameter was kept constant at 8700, while the separation between the upstream cylinder and the wall was varied. The results show that this separation distance known as the gap ratio has a significant effect on the dynamic behaviour of the upstream and downstream cylinders. Accordingly, the interaction of shear layers between the upstream cylinder and the rigid wall has a strong influence on the vortex dynamics of both cylinders, in particular, when the upstream cylinder was mounted close to the wall. In this arrangement, a jet flow produced in the wake of the upstream cylinder significantly affects the vortex shedding frequency, and the lift and drag coefficients of both cylinders. This can alter the dynamic response of the downstream cylinder and theoretical efficiency of the VIV power.

Understanding of the flow behaviour on a Helmholtz resonator excited by grazing flow

In this study, a large eddy simulation of the three-dimensional shear flow over a flow-excited Helmholtz resonator has been implemented. The simulations have been performed over a wide range of flow speeds to analyse the effect of the inlet flow properties on the excitation condition. For validation proposes, the results obtained from the numerical simulations have been compared with published experimental data and show that numerical modelling provides an accurate representation of the pressure fluctuations inside the cavity. The main objective of this paper is to gain an understanding of the flow features over a flow-excited Helmholtz resonator. To this end, using the numerical model, the interaction of a turbulent boundary layer with a Helmholtz resonator has been considered, and the characteristics of the flow inside the resonator and over the orifice for various flow conditions are also analysed.