Prediction of Noise in HVAC Ducts - POWERLAB -...
Transcript of Prediction of Noise in HVAC Ducts - POWERLAB -...
Prediction of Noise in HVAC Ducts
Dr Karamjit Sandhu
Climate Control SystemsJaguar Land Roverg
Open Source CFD International Conference 2008Open Source CFD International Conference 2008
Summary
• Introduction
• Benchmark LES Flow Verification
• HVAC Aero-Acoustic Noise Source Calculations
• HVAC Aero-Acoustic Noise Propagation Prediction
• Conclusions
• Acknowledgements
Open Source CFD International Conference 2008
Introduction
• Current CAE methods are not mature enough to predict noise in ducts d t d i t th i t i hi l biand propagated into the interior vehicle cabin
• Eliminate late changes and costs in vehicle programmes to reduce HVAC noise
• Complex problem and methods required to predict noise in simple ducts
• Method needed to identify potential noise sources early
• Aim is to develop a CAE method to predict far field noise in a HVAC duct
• Advanced CFD and CAA methods will be introduced to tackle the blproblem
Th k f d i j ti ith ICONThe work was performed in conjunction with ICON
Open Source CFD International Conference 2008
Benchmark LES Flow Verification
• For the initial studies a mapped inlet duct LES flow solution was bt i d t t bli h f ll t b l t i l t t ti diti f th f llobtained to establish a fully turbulent inlet starting condition for the full
configuration
• Validation using experimental outlet velocity profilesg y
• Verification of the basic meshing and flow solution methodology
M d i l tMapped inlet
Outlet
Open Source CFD International Conference 2008
Inlet Mapped Boundary Condition
Benchmark LES Flow Verification
Instant velocitiesInstant velocities
Shaded contours of the instantaneous velocities in the inlet section
Comparison of mean velocities across the outlet
Open Source CFD International Conference 2008
velocities in the inlet section the outlet
HVAC Aero-Acoustic Noise Source CalculationsCalculations
Geometry used to calculate the flow through the duct
Open Source CFD International Conference 2008
y g
HVAC Aero-Acoustic Noise Source CalculationsCalculations
• Auto-Hex Mesh (AHM) for rapid high lit h tiquality mesh generation
– Hex dominant– Surface layers
6 illi C ll– 6 million Cells
• Inlet mapping – based on the a fixed total pressure inlettotal pressure inlet
• Advanced LES solver
• OpenFOAM LES solver, oodles
Di i i i h C k• Discretisation using the Crank-Nicholson for time and central-differencing Surface Mesh of the Duct
Open Source CFD International Conference 2008
HVAC Aero-Acoustic Noise Source CalculationsCalculations
Instantaneous velocity at 0.2s Mean Streamlines at 0.4s
Open Source CFD International Conference 2008
HVAC Aero-Acoustic Noise Source CalculationsCalculations
• Results show a re-circulation at the entry of the duct
• Major source of the noise generated
• Air borne noise is mainly caused by pressure fluctuations at the wall
• This is illustrated in the SPL values below
Mean Surface Sound Pressure Level(SPL)
Open Source CFD International Conference 2008
Mean Surface Sound Pressure Level(SPL)
HVAC Aero-Acoustic Noise Source CalculationsCalculations
• To validate the results a comparison was made with test data
• Fourier transforms(FFT) were employed to convert the time dependent pressure to frequency dependent domain
Open Source CFD International Conference 2008
Comparison of Simulation and Test Data
HVAC Aero-Acoustic Noise Source CalculationsCalculations
• Overall comparison of the CFD Equivalent SPL was good
• There were a couple of large discrepancies (due to experimental error)
• SPL mean error of 3.9dB over all sampling points130
100
110
120
130
70
80
90
100
dB)
40
50
60
70ExperimentCFD
SPL
(d
10
20
30
40
Open Source CFD International Conference 2008B1 B2 B3 B4 B5 B6 F1 F2 F3 F4 F5 L1 L2 L3 L4 L5 L6 L7 R1 R2 R3 R4 R5 R6 R7 R8
0
HVAC Aero-Acoustic Noise Propagation PredictionPrediction
• Two approaches tested for CAA computations:– Multi Domain Direct Boundary Element Method (MDDBEM) using
LMS Virtual Lab– Simplified form of the Ffowcs Williams-Hawkings (FW-H) equation
for line-of-sight noise propagation using OpenFOAM
• LES results for instantaneous pressure were used as source of aero-S esu ts o sta ta eous p essu e e e used as sou ce o ae oacoustic noise (surface dipoles)
• The two approaches were compared against experimental• The two approaches were compared against experimental measurements
Open Source CFD International Conference 2008
HVAC Aero-Acoustic Noise Propagation PredictionPrediction
LMS.VL MDDBEM ModelI l t I d
• MDDBEM solves two simultaneous problems:
Inlet: Impedance
Walls: aero-acoustic sourcesinterior problem with surface dipoles as acoustic sources (from LES calculation) is
Walls: aero acoustic sources applied as surface dipoles at inner walls
(from LES calculation) is coupled to external far field noise propagation problem at outletoutlet
• The solution was solved in the range 20Hz to2kHz Outlet: Coupling
Microphone locations
p ginner and outer acoustic meshes
Open Source CFD International Conference 2008
locations
HVAC Aero-Acoustic Noise Propagation PredictionPrediction
OpenFOAM FWH Model
• Simplified form of the Ffowcs Williams-Hawkings (FW-H) equation implemented in OpenFOAM
• Propagation method along line of sight provides estimates of upper and lower bounds on noise propagation results
• Sound reflection, deflection or transmission are not considered as with BEM/FEMBEM/FEM
• Results are obtained as part of the CFD calculation and are only of qualitative valuequalitative value
Open Source CFD International Conference 2008
HVAC Aero-Acoustic Noise Propagation PredictionPrediction
13
Open Source CFD International Conference 2008
HVAC Aero-Acoustic Noise Propagation PredictionPrediction
5
6
Open Source CFD International Conference 2008
HVAC Aero-Acoustic Noise Propagation PredictionPrediction
• LMS Virtual Lab results highly oscillatory: 15dB jumps between i hb i f ineighbouring frequencies
• Lighthill’s acoustic analogy provides upper and lower bounds on noise propagation resultspropagation results
• Level of background noise at low frequency comparable with experimental noise
• Microphone 5 has SPL higher by O(20dB) than surrounding microphone location, probably located in exit stream
• Variation of up to 3dB between microphones over the frequency space using Virtual Lab. Up to 15dB variation in experimental results (excluding Mic 5).( g )
Open Source CFD International Conference 2008
Conclusions
• CFD method to calculate aero acoustic noise based on LES methods h b f ll l d i O FOAMhas been successfully employed using OpenFOAM
• LES results showed good agreement with experimental data. Mean error in SPL was 3.9dB
• LES solutions can be used to perform SPL calculations to identify high SPL regions and compare alternative duct designs
• Two methods were compared to predict noise propagation based on acoustic sources from LES solutions
• The MDDBEM in LMS Virtual Lab was the more accurate method andThe MDDBEM in LMS Virtual Lab was the more accurate method and showed relatively good agreement in terms of trends
• The Ffowcs William-Hawkings method is limited since no reflection, d fl ti t i i i id d It d id d ldeflection or transmission is considered. It does provide upper and lower bounds which could be employed for engineering design purposes
• Further work is required in this area to provide more accurate solutions
Open Source CFD International Conference 2008
Acknowledgments
The following should be thanked for their effort in producing this piece of hresearch
Tayeb ZegeurTayeb Zegeur
Neil Beloe
Francisco Campos
Andrew JacksonAndrew Jackson
Eugene de Villierswww.iconCFD.com
Open Source CFD International Conference 2008