Access Type

Open Access Thesis

Date of Award

January 2015

Degree Type

Thesis

Degree Name

M.S.

Department

Mechanical Engineering

First Advisor

CHIN-AN TAN

Abstract

Vehicle designs and developments must meet safety legislations along with market demands. Safety regulations are mainly concerned with how a vehicle body performs in crashes, during which pedestrian collision is the one of the most important issues. When a vehicle strikes a pedestrian, its engine hood will most likely hit the head of the pedestrian, causing serious injuries. To reduce the severity of the injuries, along with market demands to improve vehicle fuel economy and performance, engineers have considered the design solution of reduction of the vehicle hood thickness (and hence its weight) and the use of advanced light materials. As a result, the engine hood panel becomes much more susceptible to excessive vibrations. The sources of these vibrations are road irregularities which provide base excitations and aerodynamic fluid-structure interactions.

Engine hood vibrations due to aeroelasticity, called flutter, have been investigated extensively in the past using CAE, wind tunnel experiments, and modeling and simulations by computational fluids dynamics (CFD) techniques. This work studies engine hood vibrations due to excitations from road surface irregularities and explores a passive technique to suppress the levels of vibrations. Due to the unpredictable nature of surface irregularities, this problem is approached by random vibrations analysis. Validated CAE model, with experimental modal analysis, and random vibration analysis are employed to determine the system response under road excitations. The power spectral densities (PSD) of road irregularities are obtained by experiments and used as inputs to the vibratory system. Finally the effects of stiffening elastic bumpers on the engine hood are examined a possible passive technique to suppress vibrations. It is shown that stiffening of the elastic bumpers decreases the vibration levels for the first mode slightly but drastically in other modes. It is also observed that there is less than 2.5% shift in the frequency while increasing the stiffness of the elastic bumpers. Finally, although a suppression of less than 5% in the response amplitude of the first mode of the engine hood system might not seem significant, the proposed approach represents a plausible design solution as it is relatively simple cost-effective, and without much changes to the resonant frequencies.

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