Exhaust Noise & Vibration System-level structure borne vibration problem Loud noise in rear passenger seat at cruising speed is believed to be due to exhaust resonance. Neon RT exhaust test bed as an example of a complex system-level N&V problemAn increase in the spring rate of the Neon suspension introduces a system-level noise problem in the 300-350 Hz frequency range. - Why does a change in the suspension subsystem create a vibration problem in the exhaust subsystem in the first place? - How and why does a stiffness change in the exhaust system inlet pipe affect the vibration problem? Exhaust Noise & Vibration Contradictions in diagnosing the problemAn increase in the spring rate of the vehicle suspension subsystem causes the exhaust subsystem resonances to shift upward due to perturbed boundary condition. Certain modes are affected by increase in suspension stiffness, K2 Increase in K2 Exhaust Noise & Vibration Simplified modelElectro-dynamic 50-lb skewed shaker excitation at bellows (to excite bending and torsional modes of exhaust) Neon RT exhaust subsystem (10 tri-axial accelerometer measurements and impedance sensor) Agilent E1432 51.2 kHz DAQ with MATLAB-based MIMO/MRIT testing interface (51.2 kHz clock speed, 800 Hz bandwidth, 2048 sampling freq., 4096 BS, 50% overlap, 0.5 Hz Δf) PCB 288D01 (102.24 mV/lbf, 98.36 mV/g) 356A08/A356B18 (92~102, 907~1042 mV/g) Test Setup Electro-dynamic shaker testingMode near 312 Hz appears in all three directions with strongest motion in Y. Modal Deflection Degree-of-freedom ___ X-direction - - - Y-direction - - - Z-direction Experimental Modal Analysis Modal frequencies and shapesAccel/Force Accel/Force Problem mode in the 300-350 Hz range is displaced by the reduction in stiffness at the inlet as shown below. Frequency [Hz] Frequency [Hz] Results of Design Change Removal of problem frequencyStructural ModificationStructural Modification Windowed 41 Hz Windowless 90 Hz Concentrate inertia; reduce stiffnessFor linear steady, isentropic flow w/o viscosity or body forces Suction Gas Pulsation Model Annular volume w. const. cross-section Homogeneous ↔ Inhomogeneous ↔ Model manifold approx. with annular volumeNatural Frequencies Baseline modal model selection via finite element modeling and analysis r=50mm, D=12mm, b=28mm and h=12mm Baseline parameters Mode (Hz) Mode (Hz) Mode (Hz) 1 503.36 1 1004 1 1466 2 512.82 2 1014 2 1487 h Mode 1(0o) Mode 1(90o) Mode 2(0o) Mode 2(90o) 8 511.546 511.546 1023 1023 10 511.860 511.860 1024 1024 12 512.236 512.236 1024 1024 14 512.692 512.692 1025 1025Repeated Roots (Natural Frequencies) Symmetry causes repeated natural frequencies, each of which exhibits a different modal deflection shape. 512 Hz 512 Hz 1023 Hz 1023 Hz Md 1 and Md 2 shapes Mode k freq and phase Two periods…How do draw a pressure mode shape? When we draw the pressure mode shape in the volume, we must go through zero twice over 360 degrees and return to the original pressure. + 0 - 0 correctTennis RacquetTennis RacquetVehicleVehicleMackey