Abdullah O. Nuhait

Modeling and performance analysis of cambered wing-based piezoaeroelastic energy harvesters

We investigate the effects of aerodynamic loads on the performance of wing-based piezoaeroelastic energy harvesters. The rigid airfoil consists of pitch and plunge degrees of freedom supported by flexural and torsional springs with a piezoelectric coupling attached to the plunge degree of freedom. The effects of aerodynamic loads are investigated by considering a camber in the airfoil. A two-dimensional unsteady vortex-lattice method (UVLM) is used to model the unsteady aerodynamic loads. An iterative scheme based on Hamming?s fourth-order predictor?corrector method is employed to solve the governing equations simultaneously and interactively. The effects of varying the camber, its location, and the nonlinear torsional spring coefficient are determined. The results show that, for small values of the camber location, the flutter speed changes greatly on increasing the camber of the airfoil. On the other hand, for large values of the camber location, the variation of the flutter speed when changing the camber is very negligible. We demonstrate that the symmetric airfoil case is the best configuration to design enhanced wing-based piezoaeroelastic energy harvesters. Furthermore, the results show that an increase in the camber results in a decrease in the level of the harvested power. For cambered airfoils, we demonstrate that an increase in the camber location leads to an increase in the level of the harvested power. The results show that an increase in the airfoil camber delays the appearance of a secondary Hopf bifurcation.

Linear and nonlinear active feedback controls for vortex-induced vibrations of circular cylinders

We consider the problem of suppressing oscillations of an elastically mounted rigid cylinder undergoing vortex-induced vibrations by linear and nonlinear active velocity feedback controllers. Each controller relies on an actuator, which imparts an opposing force to the cylinder motion, thereby reducing its high-amplitude oscillations. A strongly coupled fluid–structure numerical model is used to solve the fluid–structure interaction equations. The results show that the choice of the active feedback controller depends on the allowable controlled amplitude of the cylinder. It is found that a cubic velocity feedback controller is more efficient than its linear velocity counterpart when very small controlled amplitudes are desired.

Unsteady aeroelastic behaviors of rigid airfoils with preset angles of attack

The effects of varying the angle of attack on the flutter speed and limit cycle oscillations of an aeroelastic system are investigated. This system consists of a plunging and pitching rigid airfoil supported by linear springs. The unsteady vortex lattice method is used to model the unsteady flow. The objective is to determine how the flutter boundary is affected by changing the angle of attack. To solve simultaneously and interactively the governing equations, an iterative scheme based on Hamming’s fourth order predictor–corrector model is employed. Several numerical simulations are conducted for various angles of attack to determine their effects on the dynamic behavior of the aeroelastic system and particularly on the dynamic stability or flutter speed and the nonlinear response of the system. The results show that the flutter speed increases as the angle of attack is increased. It is also determined that increasing the preset angle of attack results in a decrease in the dynamic amplitudes of the nonlinear response. In other words, increasing the angle of attack offers a way to control the system in terms of delaying flutter and reducing the limit-cycle oscillations amplitudes.

The Effect of Square Tube Location in a Vertical Array of Square Tubes on Natural Convection Heat Transfer

ABSTRACT Experimental investigation is reported on natural convection heat transfer from the outer surface of a vertical array of horizontal square tubes in air. Five tubes equally spaced are used with cross section 0.02 × 0.02 m2. The tubes are subject to constant heat flux boundary condition using internal constant heat flux heating elements in the range 46–510 W/m2. Experiment is done for arrays of 2–5 square tubes and for four center-to-center separation distance to hydraulic diameter ratios. Study is concentrated on the effect of tube location in the array and on the geometry of the array. Results show that the downstream tubes exhibit reduced Nusselt numbers than that of a single tube for small center-to-center separation ratio of 2.5. This reduction depends on the location of the tube in the array and the number of tubes in each array. Results also show that as the ratio increases, enhancement in heat transfer over that of a single tube is observed and critical ratio is obtained at a specified value of the modified Rayleigh number for the upper (downward) tubes in each array. Local circumference averaged correlations are proposed for the upper tubes in each array and for any other individual tube in each array geometry. An overall general averaged correlation is also reported for each tube in the array.

Humidification Technique Using New Modified MiniModule Membrane Contactors for Air Cooling

An experimental study is conducted to cool the ambient air using a new humidification technique. A wind tunnel is built with a test section formed by four modified MiniModule membrane contactors. An ambient air passes over the membrane contactors (cross flow) while water pumps through the contactors. Air temperature and relative humidity are measured upstream and downstream of the membrane contactors array which was used to humidify and cool the outdoor air. Five average air velocities (3.03, 3.33, 3.95, 4.52, and 5.04 m/s) and four water flow rates (0.0, 0.013, 0.019, and 0.025 kg/s) are used. Air velocity is measured at different locations along the centerline of the cross section. Using the modified MiniModule membrane contactors array dropped the air temperature by a maximum and minimum of 10.77°C and 3.44°C, respectively, depending on the outdoor air. The corresponding maximum increase of the relative humidity is 4.65% which depends on the ambient condition. It is noticed that the evaporation process does not follow the isenthalpic lines therefore; heat transfers from the air as latent and sensible heats.

Air cooling using a matrix of ceramic tubes

An experimental study is conducted to cool the outdoor air using a humidification technique. A wind tunnel was built with a matrix of ceramic tube test section. An outdoor air passes over the ceramic tube matrix (cross flow) where water passing through the ceramic tubes. Air temperatures and relative humidity are measured before and after the test section for several air and water speeds. Air speed is measured at different locations along the centerline of the cross section. Results show that the ambient temperature drops by about 10 °C when the relative humidity increases from 2% to 5.4%.

Unsteady Aeroelastic Response of Rigid Airfoils with Nonzero Angles of Attack

The effects of varying the angle of attack on the flutter speed and limit cycle oscillations of an aeroelastic system are investigated. This system consists of a rigid airfoil supported by linear springs that undergoes plunging and pitching motions. The Unsteady Vortex Lattice Method (UVLM) is used to model the aerodynamic loads. To solve simultaneously and interactively the governing equations, an iterative scheme based on Hamming’s fourth order predictor-corrector model is employed. The effects of the angle of attack on the dynamic response including the flutter speed and ensuing limit cycle oscillations are investigated. The results show that the flutter speed increases as the angle of attack is increased. It is also determined that increasing the preset angle of attack results in a decrease of the amplitudes of the limit cycle oscillations.

Piezoelectric energy harvesting from an oscillating wing

We investigate power levels that can be harvested from aeroelastic vibrations of an elastically-mounted wing that is supported by nonlinear springs. The energy is harvested by attaching a piezoelectric transducer to the plunge degree of freedom. A model that tightly couples the electromechanical model with the three dimensional unsteady vortex lattice method for the prediction of the unsteady aerodynamic loads is developed. The effects of the electrical load resistance, nonlinear torsional spring and eccentricity between the elastic axis and the gravity axis on the level of the harvested power are determined for a range of operating wind speeds. The results show that there is an optimum value of load resistance that maximizes the level of harvested power. The results also show that the nonlinear torsional spring plays an important role in enhancing the level of the harvested power. Furthermore, the harvested power can be increased by properly choosing the eccentricity. This analysis helps in the design of piezoaeroelastic energy harvesters that can operate optimally at prevailing air speeds.

Power Enhancement of Piezoelectric Energy Harvesters from Aeroelastic Vibrations

We investigate the level of harvested power from aeroelastic vibrations of an elasticallymounted wing supported by nonlinear springs. The energy is harvested by attaching a piezoelectric transducer to the plunge degree of freedom. A model that tightly couples the electromechanical model with the three dimensional unsteady vortex lattice method for the prediction of the unsteady aerodynamic loads is developed. The eects of the electrical load resistance and eccentricity between the elastic axis and the gravity axis on the level of the harvested power, pitch and plunge amplitudes are investigated for a range of operating wind speeds. The results show that there is an optimum value of load resistance that maximizes the level of harvested power. The results also show that the nonlinear torsional spring plays an important role in enhancing the level of the harvested power. Furthermore, the harvested power can be signicantly enhanced by properly choosing the eccentricity.