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WAVE_Acoustics_Training.pdf

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WAVE_ACOUSTICS_TRAINING
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www.ricardo.com ? Ricardo plc 2012 Ricardo Software WAVE Advanced Training Basic Acoustics Training 2 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Contents ? Introduction – Key Features – WAVE modelling for Acoustics – Understanding WavePost Acoustic Acquisitions – Steady or Transient ? Model Setup – Intake Noise – Airbox ? WavePost Acoustic Acquisitions – Sources, Microphones, Acquisitions – Plotting – Visualizing Resonances ? Further Investigation – Helmholtz resonator – Airbox size ? Audio file ? Model setup – Exhaust Noise ? Test benches 3 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Introduction ? One of WAVE’s key features is its ability to accurately predict intake and exhaust noise. ? By predicting gas velocities at the intake and exhaust orifices, WAVE is able to supply noise source data which can be post processed to give a predicted sound pressure level. ? WavePost acoustic acquisitions emulate a laboratory NVH analysis system, using standard DSP techniques to process WAVE-generated data in the same way as measured data, allowing acoustic problems to be identified. ? WAVE allows you to run a fully coupled simulation giving simultaneous prediction of noise and performance and can therefore demonstrate any tradeoffs. ? Interactions between the source and the noise are captured. ? This coupled with our advanced 3D pre processors and sophisticated acoustic post processor means that WAVE is unparalleled as an intake and exhaust noise analysis tool. ? WAVE can be used to simulate a transmission loss test bench. 4 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Key Features ? Post processing – WavePost acoustic acquisitions is one of WAVE’s dedicated post-processors. It is comprised of noise radiation models and acoustic processing and analysis tools. It is used to simulate noise measurement experiments using a WAVE model to provide the noise sources. ? Transmission loss test bench – The acoustic analysis capabilities within WAVE allow for modelling of an acoustic piston (speaker producing white noise) and determination of the transmission loss between two locations. ? Concentric tube resonator – A pre-processing capability allows the automatic modelling of a concentric tube resonator based on user-specified geometric and operating parameters. This feature significantly reduces the time and effort required to produce a WAVE model of a silencer. 5 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Key Features ? Advanced 3D Modelling – To obtain high frequency resolution for acoustic analysis, a silencer must be subdivided into smaller units, and details of internal pipes, baffles and perforates added. This is done using two- or three-dimensional arrays of complex Y-Junctions to form a mesh that defines each distinct volume of the silencer. The Y-Junctions are separated by zero-length ducts, which serve as boundary elements between the Y-Junctions. Open passages are created using ducts with the same diameter as the Y-Junctions while baffle holes or perforate sections are created with smaller diameter ducts. – Several years ago this would have been a very labour intensive process, with mufflers taking up to a week to create. Modification of the design would require major reworking of model which again would be very laborious. 6 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Key Features ? The pre-processors WaveBuild3D and WaveMesher have made it very straightforward to create and modify acoustics-quality models. 7 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution WAVE Modelling for Acoustics ? In this training we will take one of our earlier models as a starting point and after modifying it slightly, perform an acoustic analysis using WavePost acoustic acquisitions. ? We will then identify a possible problem area and use countermeasures to try and improve the acoustics of the intake system. ? Using WavePost we should be able to easily compare results from both models and assess the impact that the modifications have had on the engine performance and intake noise. 8 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Understanding WavePost Acoustic Acquisitions ? Before commencing it is important to get a basic understanding of WavePost acoustic acquisitions (also see Intermediate Tutorial ? WAVE Acoustics ? Acquisitions in WavePost) – A WAVE model is treated as a set of potential noise sources attached to a vehicle, which may be stationary or moving. There are two basic noise sources produced by a WAVE model: ? flow sources such as exhaust and intake orifices; pressure sources. – Flow sources radiate noise to the free field to be measured by all free-field microphones. Any number of flow sources may be activated from a single WAVE model and any number of free-field microphones may be used to measure the radiated noise. – Free-field microphones may be stationary or moving. Sources attached to a moving vehicle and/or moving microphones will give rise to Doppler effects; suitably positioned microphones will give rise to stereo effects. Combining a moving vehicle with suitably positioned microphones can give highly realistic simulations of vehicle pass-by noise. 9 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Understanding WavePost Acoustic Acquisitions ? … a basic understanding of WavePost acoustic acquisitions (cont.) – Pressure sources are measured by WAVE’s pressure sensors placed inside ducts or junctions in the WAVE model. Analysis of pressure sensor data is often carried out to provide validation data and as input to other analysis packages requiring processed pressure data as an input. – All microphones can have measured noise transfer functions activated. Measured noise transfer functions are normally used to replicate the acoustic characteristics of a vehicle body to allow interior noise prediction. 10 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Understanding WavePost Acoustic Acquisitions – Once measured, either by external (free-field) or internal (pressure sensor) microphones, all noise data can be processed and analyzed in the same way. The processing and analysis system is based on typical acoustic test and measurement systems currently available. It allows for processing of steady-state and transient WAVE data in a range of ways including frequency and order analysis; order tracking; speed-spectral mapping and frequency response analysis. Plotted data may have A/B/C acoustic weighting filters applied. Acoustic weighting filters simulate the human ear’s response to sound stimuli. – For steady-state processing each WAVE case is treated as a single measurement; for transient processing measurements are made at times controlled by a highly configurable transient acquisition controller. The user also has fine control over all relevant signal processing parameters such as frequency resolution, block size and data sampling rates. 11 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Understanding WavePost Acoustic Acquisitions – Processed data may be interactively plotted and printed from a WavePost acoustic acquisitions session. Import and export of data to and from text files is also available. – Audio output is available in the form of binary audio files in a variety of file formats, mono or stereo, each with configurable sampling rates, output levels and other parameters. Any microphone can be used for either channel in the file. An engine speed signal can be encoded in the file if required for subsequent analysis using other audio software. ? In summary – A WAVE model can be used simulate the flow and pressure sources required for acoustic analyses. – WavePost acoustic acquisitions can be used to post process this data in much the same way as a typical NVH analyser. 12 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Steady-State or Transient ? As mentioned, WavePost acoustic acquisitions can post process both steady-state and transient WAVE data. ? When using steady-state data, much better clarity can be achieved with smaller engine speed steps (but at cost of run time). ? To obtain the most accurate results, Ricardo typically uses transient predictions for acoustic analysis, where possible. This is equivalent to having seamless engine speed steps and as we will see, it gives smooth continuous colormaps, whilst also allowing the generation of sound files. 250 rpm steps 125 rpm steps 13 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Model Setup – Intake Noise ? For these reasons we will use the transient model created previously as the starting point of the acoustics training. ? We will be analysing intake noise and for this reason we require a higher fidelity model of the airbox. – To create a new airbox model we can replace the basic one used in the 4 cylinder SI tutorial with a WaveBuild3D model available in the TAG files. 14 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Model Setup ? Start by opening WaveBuild and loading the tut_si4_16v_transient.wvm file. ? Save a copy in your working folder with a new name, e.g. tut_si4_16v_trans_AirBox3D.wvm (if you change folders note to copy the interpolation map text files also). ? Load the SI_AIRBOX component from the list of TAG files (double click). 15 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Airbox ? Firstly we must remove the air filter element – When modelling airboxes, Ricardo recommends that the filter is removed at the point where the model is to be used for acoustic work. This is because in reality the filter very close to being acoustically transparent. The current WAVE approach to modelling filters can lead to unwanted reflections in the airbox, which is why we recommend that the filter is removed. This will however lead to slightly reduced losses in the intake system. ? The filter can be removed by highlighting it and pressing delete – Use SHIFT middle-mouse to rotate the view – CTRL middle-mouse to zoom 16 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Airbox ? Set the cavity mesh sizes to Dx=Dy=60, Dz=70. Tools ? Mesh Parameters – Our aim is to create a model with at least 3 elements in each of the x, y and z directions, that will run in a short amount of time. – Generate the mesh ? Adjust the orientation so that the meshed component appears as you would like it to look on the WaveBuild canvas. – Close the window, save the component 17 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Airbox ? Place the component onto the WaveBuild canvas (1) – Drag a File Reference ? Merge it (2) – Right click on the airbox icon ? Merge Network ? Clear the prefix to shorten the part names ? Replace the simple airbox model (3) (1) (2) (3) 18 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution Transient ? The model will be run as a 2 case transient. The first case is steady-state and is used to supply converged initial conditions for case 2. ? Case 2 is the transient case and will be used to generate the noise data for WavePost acoustic acquisitions. ? For case 2 only, we need to ensure we have transient noise data enabled and that the plot storage interval is set to 1 (should be so already) ? Switch back to case 1 and run the model. 19 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution WavePost Acoustic Acquisitions ? Once the simulation has finished WavePost can be loaded to post-process the results. ? In WavePost – to create an acoustic acquisition, right-click on “Acoustic” in the Data Reference frame ? The 3 main areas to define are: – Sources – Acquisitions – Microphones 20 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution WavePost Acoustic Acquisitions – Sources ? To define a source first load in the newly created WAVE Output file e.g. tut_si4_16v_trans_AirBox 3D.wvd ? You will see INTAKE and EXHAUST flow sources available – Enable the INTAKE noise source ? Highlight INTAKE and check the Enable Source box 21 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution WavePost Acoustic Acquisitions – Sources ? The location of the noise source relative to the vehicle can be defined if required. In this case we have a single source only, so position is not important. ? There are a number of options the user can specify to allow physical effects to be modelled. In this case we will leave the settings at their defaults 22 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution WavePost Acoustic Acquisitions Microphone ? Next a microphone can be added. – For this example we are using an external microphone (it is also possible to add an internal one). ? The microphone must be positioned in relation to the noise source, as in your test data measurement setup – Here, the position is constant with time, and is at 0.1 [m] at 45 [deg] from the source ? It is possible to specify to use a measured noise transfer function to allow the prediction of interior vehicle noise. ? A microphone and a noise source cannot be coincident. 23 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution WavePost Acoustic Acquisitions ? Finally we have to set the parameters of the acquisition. – An acquisition is a single measurement experiment, using the data from a single WAVE run to simulate noise and/or pressure sources, which are then measured by microphones. ? The settings in the acquisition panel should be familiar to anyone who has used a typical NVH analyser. – In most cases the settings can be left at their default values. Details for each of the settings can be found in the online help 24 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution WavePost Acoustic Acquisitions ? Once the source, microphone and acquisition have been set-up you can start the acquisition. ? Right-click on the acquisition, and select “Acquire All” 25 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution WavePost Acoustic Plotting ? Once acquired, results can be presented through a variety of plots and audio files. ? In the first instance we will create a colormap of Sound Pressure Level SPL versus Frequency versus Engine Speed – Right-click on Acoustic plot ? 3D ? Colormap ? SPL vs. Frequency (X) vs. Engine Speed (Y) 26 ? Ricardo plc 2012 Ricardo Software July 2012 Open Distribution WavePost Acoustic Plotting ? This will allow us to plot the sound pressure level against engine speed and frequency. ? Select Add ? Data ? Click on the plot for the Add Data panel ? The sole Acquisition and Microphone are selected by default ? Exit OK creates the colormap 27 ? Ricardo plc 2012 Ri
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