Search Results

Numerical simulations are performed to investigate noise generated by flow in automotive HVAC ducts. A hybrid computational method for analyzing flow noise is applied: Large Eddy Simulation (LES) for predicting flow fields and Multi-domain boundary element method for predicting acoustic propagation. LES gives time-resolved solutions of flow velocity and pressure fields. By applying the acoustic analogy theory, the unsteady flow parameters are translated into sound source in evaluating the acoustic propagation. The computational result shows the noise caused by the HVAC ducts is strong. The noise is of broadband with a peak value at 370Hz. A major contribution of the noise generation is from the center ducts. Two design modifications of the center ducts are explored to regulate the flow structures with the ducts for reducing noise generation. Test results demonstrate the effectiveness of the modifications.

The identification of the propulsion noise of turbofan engines plays an important role in the design of low-noise aircraft. The noise generation mechanisms of a typical turbofan engine are very complicated and it is not practical, if not impossible, to identify these noise sources efficiently and accurately using numerical or experimental techniques alone. In addition, a major practical concern for the measurement of acoustic pressure inside the duct of a turbofan is the placement of microphones and their supporting frames which will change the flow conditions under normal operational conditions. The measurement of acoustic pressures on the surface of the duct using surface-mounted microphones eliminates this undesirable effect. In this paper, a generalized acoustical holography (GAH) method that is capable of estimating aeroacoustic sources using surface sound pressure is developed.

The object of the work reported here was to relate the acoustic intensity level measured near the contact patch of a driven tire on a passenger vehicle with the passby noise levels measured at a sideline microphone during coast and cruise conditions. Based on those measurements it was then possible to estimate the tire noise contribution to the passby level measured when the vehicle under test was accelerating. As part of this testing program, data was collected using five vehicles at fourteen passby sites in the United States: in excess of 800 data sets were obtained.

The Porous Ceramic Tube - Nutrient Delivery System (PCT-NDS) offers means to control water availability to plants under non-standard gravities. It is hypothesized that control can be obtained by applying suction pressure within the ceramic tubes. The research objectives include verifying the presented control equation for the PCT-NDS under micro-(less than 1 g) and hyper- (greater than 1 g) gravities. Experiments were conducted on a KC-135 subjecting the system to near-zero to 2 g's and to sustained hyper-gravities upto 10 g's using a centrifuge. Results indicated that the water availability can be controlled through applied suction pressure.

A feedback controller bas been developed using robust control techniques to control the sound radiated from turbulent flow driven plates. The control design methodology uses frequency domain loop shaping techniques. System uncertainty, sound pressure level reductions, and actuator constraints are included in the design process. For the wind noise problem, weighting factors have been included to distinguish between the importance of modes that radiate sound and those that do not radiate. The wind noise controller has been implemented in the quiet wind tunnel facility at the Ray W. Herrick Laboratories at Purdue University. A multiple-input, multiple-output controller using accelerometer feedback and shaker control was able to achieve control up to 1000 Hz. Sound pressure level reductions of as much as 15 dB were achieved at the frequencies of the plates modes. Overall reductions over the 100-1000 Hz band were approximately 5 dB.

An analytical model based on “vortex sound” theory was investigated for predicting the frequency, the relative magnitude, the onset, and the offset of self-sustained interior pressure fluctuations inside a vehicle with an open sunroof. The “buffeting” phenomenon was found to be caused by the flow-excited resonance of the cavity. The model was applied to investigate the optimal sunroof length and width for a mid-size sedan. The input parameters are the cavity volume, the orifice dimensions, the flow velocity, and one coefficient characterizing vortex diffusion. The analytical predictions were compared with experimental results obtained for a system which geometry approximated the one-fifth scale model of a typical vehicle passenger compartment with a rectangular, open sunroof. Predicted and observed frequencies and relative interior pressure levels were in good agreement around the “critical” velocity, at which the cavity response is near resonance.

Structural sound transmission through primary bulb (PB) sealing systems was investigated. A two-degrees-of-freedom analytical model was developed to predict the sound transmission characteristics of a PB seal assembly. Detailed sound transmission measurements were made for two different random excitations: acoustic and aerodynamic. A reverberation room method was first used, whereby a seal sample installed within a test fixture was excited by a diffuse sound field. A quiet flow facility was then used to create aerodynamic pressure fluctuations which acted as the excitation. The space-averaged input pressure within the pseudo door gap cavity and the sound pressure transmitted on the quiescent side of the seal were obtained in each case for different cavity dimensions, seal compression, and seal designs. The sound transmission predictions obtained from the lumped element model were found to be in reasonable agreement with measured values.

The flow-induced pressure fluctuations in a door gap cavity model were investigated experimentally using a quiet wind tunnel facility. The cavity cross-section dimensions were typical of road vehicle door cavities, but the span was only 25 cm. One cavity wall included a primary bulb rubber seal. A microphone array was used to measure the cavity pressure field over a range of flow velocities and cavity configurations. It was found that the primary excitation mechanism was an “edge tone” phenomenon. Cavity resonance caused amplification around discrete frequencies, but did not cause the flow disturbances to lock-on. Possible fluid-elastic coupling related to the presence of a compliant wall was not significant. A linear spectral decomposition method was then used to characterize the cavity pressure in the frequency domain, as the product of a source spectral distribution function and an acoustic frequency response function.

Grazing flows over open windows or sunroofs may result in “flow buffeting,” i.e. self-sustained flow oscillations at the Helmholtz acoustic resonance frequency of the vehicle. The associated pressure fluctuations may cause passenger fatigue and discomfort. Many solutions have been proposed to solve this problem, including for example leading edge spoilers, trailing edge deflectors, and leading edge flow diffusers. Most of these control devices are “passive” i.e. they do not involve dynamic control systems. Active control methods, which do require dynamic controls, have been implemented with success for different cases of flow instabilities. Previous investigations of the control of flow-excited cavity resonance have used mainly one or more loudspeakers located within the cavity wall. In this study, oscillated spoilers hinged near the leading edge of the cavity orifice were used. Experiments were performed using a cavity installed within the test section wall of a wind tunnel.