Problem statement
A newly operational green field airport in India was setting up an MRO (Aircraft Maintenance, Repair and Operations) facility adjoining an aircraft fuel farm located within the airside of the operations. Since the MRO is to accommodate narrow and mid-size body aircrafts, concerns were raised on the high levels noise generated by the aircraft engines tested inside the MRO hanger that can potentially affect the ground level aircraft fuel storage tanks.
Understanding & Approach
Initial site survey indicated that both the MRO and the fuel storage areas are in typical acoustic free-field zones and that the aircraft engine noise is directional in its propagation. The fuel storage facility is about 90 meters away from the MRO and the height of the aircraft engine is approximately at 4 meters from the reference ground.
The fuel storage tanks are about 9 meters in height and are made of special grade steel with reinforced walls in cylindrical configuration. The tanks are very much in line-of-sight of emission of acoustic energy from the aircraft engines.
To assess and rate the severity of noise onto the fuel storage tank surface, a combined approach of test-based calculations and acoustic simulation were worked out.
Schematic representation of the proposed engine test area and fuel farm
Testing and simulation
An array of 16 high grade, free field microphones with adequate aerostatic pressure compensation were configured at multiple locations around the aircraft under test. Multiple ultra-low frequency seismic sensors were also configured to measure ground borne vibrations during engine throttling.
A simplified planar acoustic propagation simulation model was created considering the engines as point sources and both the ground and the aircraft body as major reflecting planes. To improve on the credibility of simulation results, much of the test data were used as normalized inputs for estimating near field and far field acoustic propagation behavior in the simulation model.
Noise test setup measuring different aircraft engine noise
Vibration measurements on the fuel tank surface
Results and inference
Considering the effective overall exposed area of the fuel storage tank to the line of sight of noise emission from the aircraft engine/s, a work back calculation to estimate force over unit area was arrived at; normalized sound pressure values originating from the engines were superposed on to the virtual model of the tank to determine if the acoustic energy and if its severity exceeds the design limits of the fuel storage tank in terms of its physical endurance.
The discrete frequencies from the noise emitted from the engine were checked with the natural frequency of the fuel storage tank body to ascertain if any resonance condition exists that are likely to cause non-linear excitations.
Noise radiated from engine throttling, full thrust condition
Conclusions
This task had a one-line definition of what the customer was looking for, to know if the noise radiated from the aircraft engine has any adverse impact on the fuel storage tank in terms structural cracking or failures. Configuring a reliable one-off method of test and simulation of acoustic pressure in terms of effective force acting on the fuel storage tank body was a challenge of sorts. Using the basic equations of force, pressure and acoustic quantities along with a very novel adaptation of applied techniques, results were derived and were validated through the calculation steps.
Such special tasks bring into perspective the overall knowledge required in niche consulting activities to arrive at specific outputs; NV Dynamics has time and again used this out of the box approach to solve some of the trickiest of the problems.