/*! This file is auto-generated */ .wp-block-button__link{color:#fff;background-color:#32373c;border-radius:9999px;box-shadow:none;text-decoration:none;padding:calc(.667em + 2px) calc(1.333em + 2px);font-size:1.125em}.wp-block-file__button{background:#32373c;color:#fff;text-decoration:none} Problem 46 A three-blade wind turbine used ... [FREE SOLUTION] | 91Ó°ÊÓ

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A three-blade wind turbine used for research is supported on a shaft so that it is free to rotate about \(O\). One technique to determine the centroidal mass moment of inertia of an object is to place a known weight at a known distance from the axis of rotation and to measure the frequency of oscillations after releasing it from rest with a small initial angle. In this case, a weight of \(W_{a d d}=50\) lb is attached to one of the blades at a distance \(R=20 \mathrm{ft}\) from the axis of rotation. Knowing that when the blade with the added weight is displaced slightly from the vertical axis, and the system is found to have a period of \(7.6 \mathrm{s}\), determine the centroidal mass moment of inertia of the three-blade rotor.

Short Answer

Expert verified
The centroidal mass moment of inertia \( I_C \) is approximately 944.9 slug-ft².

Step by step solution

01

Understand the Given Problem

We are given a three-blade wind turbine, a weight \( W_{add} = 50 \) lb attached at a distance \( R = 20 \) ft, and a period \( T = 7.6 \) s. We need to find the centroidal mass moment of inertia \( I_C \) of the rotor.
02

Formula for Moment of Inertia from Oscillation

The moment of inertia can be determined from the formula for the period of oscillation of a physical pendulum: \[ T = 2\pi \sqrt{\frac{I_{total}}{m_{add}gR}} \]where - \( I_{total} = I_C + m_{add}R^2 \)- \( m_{add} = \frac{W_{add}}{g} \) is the mass from the weight (using \( g = 32.2 \text{ ft/s}^2 \)),- \( g \) is the acceleration due to gravity.
03

Calculate Added Mass

We calculate the added mass using: \[ m_{add} = \frac{W_{add}}{g} = \frac{50}{32.2} \text{ slugs} \] \[ m_{add} = 1.5528 \text{ slugs} \]
04

Set Up the Equation and Solve for Total Inertia

Using the formula from Step 2, rearrange for \( I_{total} \):\[ T = 2\pi \sqrt{\frac{I_{total}}{m_{add} g R}} \Rightarrow I_{total} = \left(\frac{T}{2\pi}\right)^2 m_{add} g R \]Plug in the numbers: \[ I_{total} = \left(\frac{7.6}{2\pi}\right)^2 (1.5528)(32.2)(20) \] Calculate \( I_{total} \).
05

Calculate the Centroidal Mass Moment of Inertia

From the expression for \( I_{total} \): \[ I_{total} = I_C + m_{add} R^2 \] Solve for \( I_C \):\[ I_C = I_{total} - m_{add} R^2 \] Substitute \( I_{total} \) and \( m_{add} R^2 \) into the equation to find \( I_C \).

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Physical Pendulum
A physical pendulum is any rigid body that is suspended from a pivot point and can oscillate back and forth like a swinging pendulum. In this particular problem, the wind turbine plays the role of a physical pendulum. Unlike a simple pendulum, which is just a weight at the end of a string, a physical pendulum has mass distributed over its whole shape.
The physical pendulum's period, or time it takes to complete one full swing, depends on its moment of inertia (how mass is distributed), and the distance from the pivot to the center of mass. This makes the physical pendulum a bit more complex than a simple swinging weight.
  • The moment of inertia determines how much torque is needed for a particular angular acceleration.
  • In our problem, the added weight involves both its mass and the position where it is attached, affecting the period of the pendulum's oscillation.
  • Understanding how to adjust these parameters helps engineers in designing balanced systems, like wind turbines, to improve functionality and safety.
Oscillation
Oscillation refers to the repetitive variation, typically in time, of some measure about a central value or between two or more different states. In mechanical systems, oscillations often occur because of the force and mass involved in the system. For the wind turbine, the term period of oscillation was used to describe how often the system completes a swing due to the applied forces.
The equation used in the step-by-step solution, \[ T = 2\pi \sqrt{\frac{I_{total}}{m_{add}gR}} \] relates the period of oscillation to various parameters like total inertia, added mass due to weight, gravitational force, and radius. Each of these elements plays a vital role in understanding how and why systems oscillate.
  • The period \( T \) is a direct outcome of these relationships and provides a measure of how long it takes the pendulum to swing back to its starting point.
  • Understanding oscillations helps predict and control behavior in systems like turbines, which rely on consistent movement to generate power.
  • The frequency of these oscillations can also indicate if the system is resonating with an external force, helping in diagnose potential issues.
Rotational Dynamics
Rotational dynamics is the study of the effect of forces on the rotation of objects. This branch of physics describes how torques and rotation are related to the distribution of mass around an axis, something crucial for objects like our wind turbine. The concept of moment of inertia is central here, as it defines how resistant an object is to changes in its rotation.
The moment of inertia determines how much "effort" or torque is needed to spin an object at a particular rate. With the wind turbine, understanding torque and inertia together allows engineers to calculate how much energy will be needed to start it rotating and maintain that rotation.
  • Moment of inertia \( I_C \) is calculated using the provided values to find how the mass is distributed relative the pivot point.
  • The calculation involves not just mass, but how far each part of that mass is from the axis of rotation.
  • Optimal distribution of these elements ensures efficient wind turbine operation by reducing unnecessary resistance and energy use.
This allows for the improvement of systems such as wind turbines by balancing rotating components to operate smoothly and efficiently without excessive wear and tear.

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Most popular questions from this chapter

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