CDS 140a Winter 2014 Homework 9

From Murray Wiki
Revision as of 16:40, 2 March 2014 by Murray (talk | contribs)
Jump to navigationJump to search
R. Murray Issued: 25 Feb 2014 (Tue)
ACM 101b/AM 125b/CDS 140a, Winter 2014


Due: 5 Mar 2014 (Wed) @ noon


WARNING: This homework set is still in preparation. This banner will be removed when it is finalized.

Note: In the upper left hand corner of the second page of your homework set, please put the number of hours that you spent on this homework set (including reading).

  1. Perko, Section 4.4, Problem 1a
  2. Show that for $a + b \neq 0$ the system
       \dot x &= \mu x - y + a (x^2 + y^2) x - b (x^2 + y^2) y + O(|x|^4) \\
       \dot y &= x + \mu y + a (x^2 + y^2) x + b (x^2 + y^2) y + O(|x|^4)

    has a Hopf bifurcaton at the origin at the bifurcation value $\mu = 0$. Determine whether it is supercritical or subcritical.

  3. The Moore-Greitzer equations model rotating stall and surge in gas turbine engines describe the dynamics of a compression system, such as those in gas turbine engines. The three-state "MG3" equations have the form:
       \frac{d\psi}{dt} &= \frac{1}{4 B^2 l_c}\left(\phi - \Phi_T(\psi) \right), \\
       \frac{d\phi}{dt} &= \frac{1}{l_c}\left( \Psi_c(\phi) - \psi + \frac{J}{8} 
         \frac{\partial^2 \Psi_c}{\partial \phi^2} \right), \\
       \frac{dJ}{dt} &= \frac{2}{\mu + m} \left(
          \frac{\partial \Psi_c}{\partial \phi} + \frac{J}{8}
             \frac{\partial^3 \Psi_c}{\partial \phi^3} \right) J,

    where <amsmath>\psi</amsmath> represents the pressure rise across the compressor, <amsmath>\phi</amsmath> represents the mass flow through the compressor and <amsmath>J</amsmath> represents the amplitude squared of the first modal flow perturbation (corresponding to a "rotating stall" disturbance). For the Caltech compressor rig, the parameters and characteristic curves are given by:

         B &= 0.2, & \Phi_T(\psi) &= \gamma \sqrt{\psi},\\
         l_c &= 6, & \Psi_c(\phi) &= 1 + 1.5 (\phi-1) - 0.5 (\phi-1)^3, \\
         \mu &= 1.25, &\qquad\qquad m &= 2.

    The parameter <amsmath>\gamma</amsmath> represents the throttle setting and typically varies between 1 and 2.


    (a) Compute the bifurcation diagram for the system showing the equilibrium value(s) for <amsmath>J</amsmath> as a function of <amsmath>\gamma</amsmath>. Your answer should match what was shown in class (i.e., make sure to get capture the hysteresis loop).

    (b) Suppose that we can modulate the throttle, so that <amsmath>\gamma = \gamma_0 + u</amsmath>. Analyze the performance of the system using the Liaw-Abed control law <amsmath>u = k J</amsmath>. Show that if we choose <amsmath>k</amsmath> sufficiently large to cause the bifurcation to stall to be supercritical.

    (c) Suppose that we restrict the dynamics of the system to the invariant set given by $J = 0$. Show that the system undergoes a subcritical Hopf bifurcation (this phenomena is called "surge").