Majid Rashidi

The effect of variable viscosity on flow and heat transfer to a continuous moving flat plate

Abstract The influence of variable viscosity on laminar boundary layer flow and heat transfer due to a continuously moving flat plate is examined. The fluid viscosity is assumed to vary as an inverse linear function of temperature. By means of the similarity solutions and deviation of the velocity and temperature fields as well as of the skin friction and heat transfer results from their constant values are determined.

Mixed Convection to Power-Law Type Non-Newtonian Fluids from a Vertical Wall

Abstract The mixed convection boundary layer flow on a vertical stationary or moving plate to a power-law non-Newtonian fluid is analyzed. An exact similarity solution is derived for the case when the surface temperature is inversely proportional to the distance from the leading edge of the plate. A discussion is provided on the effects of the flow index and buoyancy parameter on the velocity and temperature fields.

Vibration Analysis of a Split Path Gearbox

Abstract : Split path gearboxes can be attractive alternatives to the common planetary designs for rotorcraft, but because they have seen little use, they are relatively high risk designs. To help reduce the risk of fielding a rotorcraft with a split path gearbox, the vibration and dynamic characteristics of such a gearbox were studied. A mathematical model was developed by using the Lagrangian method, and it was applied to study the effect of three design variables on the natural frequencies and vibration energy of the gearbox. The first design variable, shaft angle, had little influence on the natural frequencies. The second variable, mesh phasing, had a strong effect on the levels of vibration energy, with phase angles of 0 deg and 180 deg producing low vibration levels. The third design variable, the stiffness of the shafts connecting the spur gears to the helical pinions, strongly influenced the natural frequencies of some of the vibration modes, including two of the dominant modes. We found that, to achieve the lowest level of vibration energy, the natural frequencies of these two dominant modes should be less than those of the main excitation sources. (AN)

Design of a hydraulic actuator for active control of rotating machinery

A hydraulic actuator was designed and is described herein. This actuator consists of: a pump, which generates the nominal pressure, a hydraulic servovalve, and a thin elastic plate, which transduces the generated pressure variations into forces acting on a mass, which simulates the bearing of a rotor system. An actuator characteristic number is defined to provide a base for an optimum design of force actuators with combined weight, frequency, and force considerations. This characteristic number may also be used to compare hydraulic and electromagnetic force actuators. In tests this actuator generated 182.3 N force at a frequency of 100 Hz. and a displacement amplitude of 5.8 [times] 10[sup [minus] 5] m.

A computational strategy for active control of dynamic systems via minimizing the displacement magnitudes of dominent harmonics of vibration

An active control strategy is proposed to attenuate the undesirable vibrations of dynamic systems. In this approach the dynamic system is treated as a black-box, whose modal characteristics are not included in the control process. Instead, the control of the system is pursued via utilizing optimization techniques to minimize an objective function formulated based on the response of the system at several locations on the vibrating system. The decision variables of the formulated optimization problem are the magnitudes and phase angles of alternative force components to be exerted at proper location of the system. The proposed method is applied to simulated active control of a rotor system whose unbalance response is to be minimized.

Performance of a Rooftop Wind Turbine System Having a Wind Deflecting Structure, Experimental Results

This work presents the results of an experimental work on a novel design for wind tower system. The system is intended for areas of low wind speeds. The wind tower consists of four rooftop turbines mounted alongside a cylindrical structure that acts as a Wind Deflecting Structure (WDS). This arrangement results in lowering of the cut-in wind speed for the turbine; the system allows the turbine rotors to operate in the areas that have normally low wind speeds, as low as 3 m/s. The nameplate rating of each of the turbines is 1.65. The four-turbine system was installed on the rooftop of a five story building in downtown Cleveland, Ohio. A fifth stand-alone wind turbine was installed on the rooftop of the same building as the reference in order to examine. Orientation of the four wind turbines is controlled by a close-loop active direction control sub-system using a double worm-gear reduction as its actuator. The output of the worm-gear unit has a spur gear pinion that meshes with an external gear that is an integral part of the outer race of the bearing that in turn supports the arm to which the four turbines are attached to. The wind power data from five turbines (four turbines on the tower & one reference turbine) were recorded by PLC’s. The data was updated and recorded every 10 seconds. This means that in every hour there were 360 data points for each of the turbines. For each day the number of data points for each turbine is 24 × 360 = 8640. The other information that is recorded every 1 second includes: the wind direction, wind speed, time, date and an index number. The experimental results show that the power output of the each of the turbines installed on the proposed system may be increased by an average factor of about 4 times compared to the power generated by the stand-alone turbine under the same wind flow condition.Copyright

The Effect of Number of Blades on the Performance of Helical-Savonius Vertical-Axis Wind Turbines

This work presents the results of a series of experiments conducted on three different scaled-down Helical-Savonius vertical axis wind turbines (VAWT) systems. The work was aimed at investigating how the number of blades may affect the performance of the Helical-Savonius VAWTs. The first turbine consisted of two helical blades, the second turbine had three blades, and the third turbines had four blades. The work included a design phase in which the three dimensional (3D) geometry of each of three VAWTs were developed using a 3D drawing software. The 3D models were then uploaded to a rapid-prototyping machine to fabricate the VAWTs. The projected areas of each of the VAWTs were that of a rectangle of 4″ × 6″. A test setup was designed and developed to examine the performance of the scaled-down turbines. A 1.1 KW floor fan was used to simulate wind flow in the laboratory for testing of the turbines. A flow straightener was also designed and developed in order to minimize the turbulent flow of the air at the discharge opening of the floor fan. The test results show that the 3-bladed rotor design performs better than the two and four bladed turbines. Under the same wind speed conditions the 3-bladed turbine produced 18% more power compared to the 2-bladed turbine, whereas the 3-bladed turbine produced 30% more power compared to the 4-bladed turbine.© 2012 ASME

Design and Analysis of a Large-Scale Positive Displacement Vane Pump for Wind Tower Application

An innovative combined hydraulic and gear-train power transmissions system for Mega-Watt scale wind turbines is proposed herein. The proposed concept targets large-scale wind turbines for an efficient and reliable conversion of the mechanical power of the rotating blades to electrical power. The novel hybrid system presented in this approach takes advantage of the benefits of both hydraulic and conventional gearbox systems, without introducing their potential inherent undesirable attributes at large scale. The proposed design first converts the mechanical power of the turbine blades to hydraulic power at a relatively high-pressure (about 2,500 psi) under a relatively low-speed (about 4 in/sec). The hydraulic fluid exiting the discharge port of the low-speed hydraulic pump is branched out into plurality of hydraulic lines for the purpose of dividing the total mechanical power of the wind turbine into multitude of lower hydraulic power lines. Each hydraulic line then delivers its hydraulic power into the corresponding intake port of a hydraulic motor having a low-speed-high-torque output shaft. The output shaft of each of the hydraulic motors then drives the input shaft of a mechanically matched gearbox to increase the rotary speed. Finally, the high-speed output shaft of each gearbox (about 1800 RPM) drives a corresponding matched electric generator. A preliminary design for a variable displacement vane pump has been proposed in this paper. This work includes a theoretical analysis of the overall efficiency of the system. The combined volumetric, mechanical, and overall efficiency of a typical proposed system was shown to be about 98%.Copyright

Wind Flow Regime Around a 3 Dimensional Helical Structure

A three dimensional heliacal structure is modeled as a wind deflecting structure in this work. The purpose of the structure is to increase the natural wind speed and direct the follow of the wind toward two columns of horizontal-axis rooftop-size wind turbines that are installed in the grooves of the helical structure, diametrically opposed to each other. Computational Fluid Dynamics (CFD) analyses were conducted to determine the influence of the helical structure on the wind speed reaching the turbines. A wind speed amplification coefficient was determined for a helical structure of 6.7 m outer diameter. The velocity profiles of the wind flow around the helical structure were determined under a postulated wind speed of 4.47 m/s. The flow was modeled as turbulent with a Reynolds Number of 2,052,167. Standard “k-ἐ” turbulent model with “near wall treatment” and “standard wall function” were adapted in all analysis. A “y+” value of 50 was held constant in all simulation. The grid-size effects on the accuracy of the results were examined. Convergence criterion was satisfied in each case. This study shows that the helical structure having an outer diameter of 6.7 m results in an average wind speed increase factor of 1.52.© 2012 ASME

Analysis of a Concept for a Low Wind Speed Tolerant Axial Wind Turbine

In this paper, the authors analyze a design for a wind tower intended for areas of low wind speeds. The wind tower consists of a combination of several rooftop size turbines arranged alongside a cylindrical structure that acts as a Wind Deflecting Structure (WDS). The WDS amplifies the effective wind speed thus allowing the turbine rotors to operate under lower ambient wind speeds. Analyses were performed using simple models as well as more sophisticated CFD methods employing Steady and Unsteady Reynolds Averaged Navier-Stokes methodology. The effect of the wind amplification was shown on a commercial small wind turbine power output map. Also, a wind turbine rotor flow was computed as operating alongside the WDS and compared to the computed operation of isolated turbines at equal effective and ambient wind velocities. The computational analyses of this work suggest that the power output of isolated rooftop wind turbines deployed at low to moderate wind speed may be matched by installing wind turbines alongside a cylindrical wind deflecting structure operating at lower wind speeds. Other benefits of the arrangement are also enumerated.© 2011 ASME