3. Fluid-induced vibrations
If a forced-convection air system is used to cool the MVD electronics,
high-velocity (15 m/s or higher), turbulent flow will occur and circulate in
the ductwork. There is practical concern that this turbulent flow will cause
the plenum chambers themselves, and/or the MCM's imbedded in the Rohacell
structure to vibrate. Such vibration of the MCM cards might enlarge the
mounting grooves leading to more vibration and possibly noise pickup in the
electronics.
What will cause turbulence in the MVD system? The flow in the MVD cooling plenums will certainly be turbulent because the flow Reynolds number will be larger than 1100 to 2300 for velocities above 3.5 m/s. For these Reynolds numbers, inertial forces will predominate over viscous forces and flow instabilities will produce random fluctuations in velocity and pressure throughout the flow loop. In addition, vortices will be shed from the tips of the fan blades, and this contribution to the turbulence will propagate around the loop. Turbulence can also be produced by flow separation from the walls of the ductwork, for example, if diffusion takes place too rapidly. Finally, the turns or bends in the ductwork will cause velocity gradients (shear flow) and spiraling flow which can add to the turbulence intensity.
Why might turbulence be a problem in the MVD system? Turbulent flow may be a problem because the random fluctuations in pressure in the flow will produce random fluctuations in wall forces, which lead to structural vibrations and displacements.
How can turbulence be characterized? Since turbulence is a "random" process, it must be characterized using time series analysis. There are two important variables that can be measured in the MVD prototype plenum cooling flow tests, velocity (a vector quantity) and pressure (a scalar quantity). Both of these variables can be measured in the time domain (time histories) and frequency domain (power spectra), and should be measured, if in preliminary testing, it appears that flow-induced vibration will be a problem.
What will be done with this turbulence data? A COSMOS finite element structural model of the MVD cooling plenums has been developed. A dynamics analysis could be performed to determine the normal modes of vibration of the plenum. In addition, with measurements of wall pressure power spectra, it can be analytically determined if these pressure loads will couple in the normal modes to produce a resonance, and if so, a prediction of displacement can be made. Also, with baseline turbulence intensity and rms pressure fluctuation measured for a given flow loop configuration, corrective modifications to the loop to reduce vibration can be assessed for improvement by making additional measurements.
How is turbulence measured? Sensitive, high-frequency response, miniature pressure transducers can be used to measure pressure fluctuations in the plenum. Also, hot-film anemometer equipment can be used to measure velocity fluctuations in the flow. True-rms averages of the time data are used to characterize the intensity of the fluctuations, and power spectra are used to portray the frequency composition of the turbulence.
What methods can be used to reduce the turbulence intensity if it is causing vibration problems? It is known that the flow will be turbulent, not laminar. To some extent this is favorable, because heat transfer coefficients are higher for turbulent flow. However, a number of duct design techniques can be used to improve the flow quality if the turbulence is too high. These include a settling chamber, turning or straightening vanes, flow straightners, and wire or plastic mesh screens.