Oscillations, flow time-delay

The second part of the work involves implementing a robust controller. The key issue in the controller design is the treatment of system dynamics uncertainties and rejection of exogenous disturbances, while optimizing the flow responses and control inputs. Parameter uncertainties in the wave equation and time delays associated with the distributed control process are formally included. Finally, a series of numerical simulations of the entire system are carried out to examine the performance of the proposed controller design. The relationships among the uncertainty bound of system dynamics, the response of flow oscillation, and controller performance are investigated systematically. 

The most important fact about piston flow is that disturbances at the inlet are propagated down the tube with no dissipation due to mixing. They arrive at the outlet t seconds later. This pure time delay is known as dead time. Systems with substantial amounts of dead time oscillate when feedback control is attempted. This is caused by the controller responding to an output caused by an input t seconds ago. The current input may be completely different. Feedforward control represents a theoretically sound approach to controlling systems with appreciable dead time. Sensors are installed at the inlet to the reactor to measure fluctuating inputs. The... 

In another approach in which heart rate variability, variations in cardiac cycle and arterial blood pressure are modeled, also flow elements are used, in addition, a baroreflex model is introduced, affecting the heart rate and the stroke volume of the heart. By introducing a time delay between the baroreflex input and output, an unstable system is created that continues to oscillate and explains the heart rate variability and variations in the cardiac cycle. Both modeling approaches will be briefly discussed in this chapter. 

The combination of a more oscillatory response and a larger period of oscillation could occur if the time delay associated with the composition measurement had increased. For example, the transport delay associated with the sampling line to the GC would increase if the flow rate in the sampling line decreased. A decrease could occur due to a partial blockage in the line, or perhaps due to the new filter in the sample line. Thus, the filter and the sample line should be inspected. 

The core inlet temperature affects the temperature and velocity distributions in the core. When it is lower, the coolant temperatures and density ratio between the core inlet and outlet become smaller which has a stabilizing effect. On the other hand, it also leads to lower coolant flow velocity and core pressure drop, resulting in a longer time delay which destabilizes the system. The net effect depends on the individual operating conditions. Here, the core power and flow rate are kept constant and the effect of inlet feedwater temperature on stability is investigated. As shown in Fig. 5.42 [11], under the operating conditions of the present parametric study, decreasing the inlet temperature leads to lower oscillation frequency and decay ratio which stabilizes the reactor. 

The study shed new light on the importance of dynamic interaction between flow structures and pulsed sprays in liquid-fueled ACC. The results provided valuable information on the fuel injection timing for desired outcome. In the present case, the fuel injection timing that was synchronized with the air vortex shedding led to the suppression of pressure oscillations. When the fuel injection timing was delayed a quarter cycle after the vortex shedding, combustor pressure oscillations reached the highest amplitude. The scale-up test revealed a critical role of the relative amount of modulated heat release from pulsed fuel injection. 

The flow step response of the model follows experimental studies quite precisely [Braakman et al., 1983 Shoukas et al., 1984 Van Huis et al., 1985]. Others have observed experimentally an initial autoregulation delay and a slow adjustment of the vascular smooth muscle in response to a step in flow. The time constants of the model were determined in accordance with previous observations. The time course of the flow step response was similar for a single vascular bed and for the entire systemic circulation. Experimental measurements found no oscillations in response to a flow step for the systemic circulation [ Shoukas et al., 1984]. Hence, employing a higher order model than was used here would be of little value. 

---