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# Aeroacoustics - Science topic

Explore the latest questions and answers in Aeroacoustics, and find Aeroacoustics experts.
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Please see attachment, here two terms appeared in the inhomogeneous Helmholtz equation. What is the use of these two terms for a propagating wave in a particular direction.
This form will be union.
I have one more question . I want to calculate the transmission loss through a structure having a membrane and cavity and supported by an acrylic plate.
I am simulating using comsol multiphysics.
Over membrane I have chosen solid mechanics. The air cavity is assigned pressure acoustics but what about that backing acrylic plate.
I have to assign it as pressure acoustics or solid mechanics.
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Hi,
I am now writing a bachelor thesis on the aeroacoustics of muffler. My muffler consists of one mass-flow inlet, one pressure outlet, with a few chamber in it and it is connected to a 3 cylinder 4 strokes engine running at a constant 1500rpm. I am interested in the noise level at the outlet and also the backpressure caused by the muffler. I am using the software Ansys Fluent.
So I have a few question regarding its simulation of aeroacoustics.
1. For my mass-flow inlet, do I use constant mass flow rate or fluctuating mass flow rate? How does this affect the sound pressure level-frequency graph? By fluctuating mass flow rate, I mean the almost instantaneous release of gas during exhaust phase which I will further simplify as half of a sine wave, followed by zero mass flow, for every exhaust of each cylinder.
2. As my muffler consists of a few expansion chamber, there is a big pressure difference(~2000Pa) in the narrow tube connecting them. So I am setting my air density as ideal gas, even though it doesn't exceed 0.3 Mach. In this case which method should i use to model the acoustics, direct method or using FW-H model? I will be getting the value right at the outlet boundary, as i dont feel the need to go into far-field.
3. For the transient simulation, how should I set the time step size and the frequency of data export to CFD-Post if i am interested in frequency range up to 250Hz? Here's what i think, time step size should be 0.0004s (1/10 of period), data export frequency should be every 10 step size (every period). Is that right? In CFD-post, i wil be plotting a graph of pressure gasint time and use FFT to transform it into sound pressure level againt frequency graph.
4. Which properties should i monitor to know if my transient simulation has converged? Until now, i have only been using the residual chart as monitor, so not sure if thats the only way.
Any help or further suggestion or correction is very much welcomed. Thanks in advance.
Kai Han Goh
well Mr. Kai Han Goh, because you dealing aeroacoustic you may consider high and low frequencys then you have to reach 3000 Hz, it take time but you have to calculate.
check the setting for 40Hz.
BR
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In the simulation tutorial of the aeroacoustics around a cylinder by ANSYS, recievers are placed at a location where mesh isn't present, how does the solver predict the acoustics at that location and what material constrains does it consider? http://dl.mr-cfd.com/tutorials/ansys-fluent/FLUENT-acoustics-Tut2-FWH.pdf
I have applied FWH model to calculate transmission loss [dB] of muffler using Ansys solver, you can find the procedure and entire article in my profile.
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I am not so clear the use of freestream velocity in FWH equation when helicopter rotor was in forward flight condition and observer position was fixed relative to the rotor.
The description of the problem is that when i use the CFD information on blade surface as the input for non-porous FWH equation, where is the freestream velocity used, is it used for the correction of surface velocity?
Dear Fu Jinbin,
The speed of sound difference is there. Please imagine a source and an observer (both stationary in space). If you put them in an external "wind", the disturbances will travel downstream faster than upstream. This is because the speed of sound is local and relative to the medium that may be stationary or in movement.
With my best regards,
Oskar
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Dear all,
I want to download some papers from the following International symposium " 14th International Symposium on Unsteady Aerodynamics, Aeroacoustics & Aeroelasticity of Turbomachines (ISUAAAT14) ". I don't know how to download papers as they are not available online and no DOI number. Does anybody know how to download papers from this? thanks in Advance
Thank you Yeasir Mohammad Akib for the help
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Does anyone know how to set boundary conditions for aeroacoustic analysis in Ansys CFX for axial flow compressor?
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Dear All,
I need a tutorial of aeroacoustic of Ansys CFX for the turbomachinery components, Like an axial compressor or turbine.
As far as I know, there are no tutorials. However, you may find useful information on the Academia site where plenty of papers related of your problem are provided.
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I am interested in the numerical characterisation (3D-CFD) of aerodynamics and flow-induced noise (near-field) generated during the operation of a standalone horizontal axis wind turbine. Since I am attempting this activity for the first time, I was wondering if there are any publicly available CAD and performance data of any test wind turbine to validate the numerical methodology.
On a side note, if anyone could recommend me some pivotal pertinent literature, it'll be a great help.
Also, as my knowledge on the state-of-the-art is limited, I was wondering about the practices for modelling impeller rotation. I understand that specifying an angular velocity to the rotating domain should be relatively straightforward but are there any literature/methodologies available to model the motion of the blades based on air-velocity boundary conditions.
Thank you for reading and contributing.
Olexiy Shynkarenko Thank you for the information Olexiy. Would it be possible to share some weblink or author information as i tried to google but couldn't get any relevant hit.
Thank you,
Sid
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I am looking for material which can replace aluminium in aerospace industry as i want material which will be having high melting point and must be light in weight. As i am looking at this from commercial point of view so if that material is easily available and cheap then it would be better. Especially Aluminium 7075
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I want to developed an aeroacoustics lab at the second floor of the building but the blower and the 20 HP motor is causing too much noise which dominate over the airfoil noise so we like to arrest this vibration. Is any body tell me how to arrest the vibration. Is spring based damping system is helpful in such case?
Kindly suggest some way to arrest the vibration produce by the Motor and blower which are connected using pulley arrangement and sound generated in the duct.
Hocine Bendjama : Kindly suggest what kind of damping is required?
Sven Rechenberger : Kindly give some way to solve the problem....
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I have a 76/40 degree double delta wing. I want to find the bevel angle for the leading edge of the wing configuration. The root chord of the wing is 191 mm and wing span is 195 mm. Is there any literature available on this? If yes Please advise.
Attached is the definition of bevel in the case of Delta Wings and the reference.
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Hi,
I am going to simulate a 2D aeroacoustics simulation in Fluent using a DES model on a stator. Actually, the simulation should be performed on a complete 3D rotor-stator setup but due to some limitations, I don’t have access to the rotor geometry and computing power. A simplified model of my setup is shown in the attached picture.
I have been searching for parameters that I can extract from the DES simulations. These are the following parameters that I can think of:
1)A common parameter is the FFT of pressure data recorded in the wake region of the blade. Using FFT, I want to see if I can see tonal noise peaks or broadband peaks. Based on these observations I can characterize the sound sources such as vortex shedding or just random turbulence.
2) Another parameter that I have come across is the sound pressure which can be calculated by subtracting the instantaneous pressure from the average pressure. How this can be done in Fluent, I am not sure.
3) The other is " divergence of velocity" or dilatation contours. I found it in this article
I am not able to understand what's the significance of this parameter.
4) Another quantity that can be visualized is the normalized Q-criterion, but this will be good to see the turbulence sources and not aeroacoustics as such.
5) Also, I was wondering if one can actually extract the noise from a pressure signal at a point so that you can hear it. I came across this in the STAR-CCM+‘s user guide some time ago and they say that it can be done. The guide mentioned that that one needs to perform inverse FFT of the signal.
Perhaps, people with experience in aeroacoustics can suggest me any additional parameters that can be used. Although the near-field region sound is of interest and not far-field, I am open to using the FW-H model if it provides additional insights.
Dear Ishan Nande,
Here are my suggestions:
1) a 2d simulation of turbulence might be questioned by many people
2) If you extract the pressure signals from the wake this will not be purely noise, but noise+flow. To talk about pure acoustics you need to study it out of the flow, in the far-field.
3) a calculation of the Overall Sound Pressure Level (OASPL) is based on a value of the Root Mean Square (RMS) of pressure fluctuations p'rms [Pa] (referenced to the treshhold of human hearing 2*10^-5 Pa): OASPL dB = 10 log (p'rms^2/p'ref^2) dB. The Sound Pressure Level is a frequency dependent parameter. To get it you need to make a DFFT of p'(t) [Pa] first and transfer the result to the logarithmic scale and get power spectrum of the signal (exact formulas to do that you can get from Matlab examples). As a result you get SPL(f). If you integrate SPL(f) you should get OASPL dB (valuable check).
4) FW-H model is designed for MOVING (e.g. rotating) surfaces in open space...
I hope it was usefull.
Oskar S.
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Hi All,
I wanna use the mentioned formulation for computing noise radiation from a fluid solid interaction system. I faced with some difficulty in my mind due to lack of enough knowledge, so I would like to have your support; Furthermore, any collaboration would be so pleasant.
Sincerely,
Hamed
@ Rashid Naseer,
Hi, I just clarified my plan for you through private messaging.
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I have artificial reflections at the freestream boundary and contaminating results at certain frequency range. Specially at corners of a spherical domain. I use a commercial CFD software.
If anyone can provide me some damping functions it would be helpful...
Hello Filippo Maria Denaro,
I agree that every code has it's own methods to do!
With the available options I still get reflections... I want to add artificial viscosity with grid stretching to damp... Mach numbers are ~0.3-0.4.
Regards,
Abhijit
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I am working on computing noise due to HVAC system. Noise is generated due to aerodynamics, turbulence (aeroacoustic) and structural vibration (vibroacoustic).
So I am having a problem with solving aeroacoustics and vibroacoustics simultaneously in ANSYS. Aeroacoustics and vibroacoustics  can be solved individually but how to do both together is a problem.
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I use an in-house CFD code to simulate the flow over Idealized Side Mirror. The reference paper is Caraeni2011. To compare with the reference paper, I obtain the pressure history at the monitor points. By subtracting the mean of the pressure history, the fluctuation pressure is obtained.
Sound Pressure Level is defined as follows,
Now, my question is: is the fluctuation pressure the same as the sound pressure? Can I directly use my fluctuation pressure to calculate the SPL?
I know SPL is a single value and it requires Root Mean Square of the pressure history. What I want to calculate is actually the spectra of SPL. Just like the following pic. The unit of frequency is dB, and I guess it's typo.
In order to get the spectra of SPL, I apply FFT on the fluctuation pressure history and then feed the absolute value of the FFT result as input into the SPL formula to get the spectra of SPL.
I cannot believe that Researchgate provides no function of editing scientific formula and inserting any inline image! Only text!
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I am trying to model a Doppler shift induced in a microwave signal by a flowing liquid in order to measure the flow speeds. I currently have 2 problems. The first is getting the plane wave to be incident on my flowing liquid at an angle. The plane wave seems to behave when modeled separately but not when combined with the flow block (even when the interaction isn't accounted for in the study).
The second problem is how model the Doppler interaction within the flowing medium. (I can't see how to adapt the acoustic Comsol Doppler example. Similarly, I can't see how to adapt the radar Comsol example).
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In CFD of reacting flows, the low-Mach number formulation is often used in LES or DNS as the time-step requirement is much more relaxed compared to the full compressible formulation. In turn, this allows for the consideration of more complex kinetics, which can lead to better prediction of the flame itself and turbulence-flame interactions. However in return, the acoustics are neglected.
Are the benefits of such simulations purely fundamental (i.e. better description of physical phenomena for various reasons?) For most of applications I can think off, either the incompressible assumption breaks down (i.e. piston engines, high speed flows) or the acoustic term is essential to ensure flame stability (jet combustion engine).
For engineering design applications on the other hand computational models have been developed to approximate underresolved physics. But these models are incomplete, do not have general applicability, and certainly provide no means of exploring fundamental fluid/chemistry interactions.
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I am planning to use it for determining flame transfer function.
The seminal paper on this topic was written by Seybert & Ross (1977):
The Journal of the Acoustical Society of America 61, 1362 (1977); http://dx.doi.org/10.1121/1.381403
This paper has a nice introduction and is easy to follow.
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I am trying to build several cases for solver and CAA-library code validation. There are breathing, trembling sphere, baffled piston.
In all cases we can see phase difference between pressure signals. I'm explaining it by inertia like in the mass-spring system. Is it correct?
How can i modify wave equation results or my CFD solution to clear it from such kind of discrepancy? I need to clear it because i'm trying to validate very different solvers (for example with mesh motion) and already observe reflections from boundaries (so then i will need to test some types of non-reflecting) and i suppose that this phase difference could make further analysis harder.
Thank you for any help ("to read" recommendations are also appreciated).
Last figure just for reference (baffled piston, 2d-axissymmetric case).
You can get dispersion error from your numerical scheme. Try decreasing your mesh spacing to see if the dispersion reduces. You could try using dispersion-relation-preserving (DRP) schemes. Another reason for inaccuracy is your boundary condition. When comparing against the analytical solution, do it at an instant when the wave has not reached the boundary, so you can isolate the effect of boundary conditions.
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How to set Mach number in FLUENT using species transport method for shock waves phenomena, in Pulse Detonation Engine
Dear
Noor Alam may be following two tutorial help you to solve your species transport with shock waves phenomena:
(1) Species transport-combustion in Fluent
(2) Shock wave propagation in Fluent
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I am doing a 1D fintie element anlysis of a simple expansion chamber in matlab, i want to apply a anechoic termination at the inlet and outlet of muffler so that no reflection occurs. But i am stuck on how to incorporate that into my code.
Hi
For 1D problems, the anechoic termination impedance is p/u = rho*c, where p is pressure, u is particle velocity, rho is density and c is wavespeed.
However, this never is the case in real life. You only approach this situation if you place a wedge absorber at the outlet.
Therefore, outlets usually are provided with an end correction and a radiation impedance, where the radiation impedance modelled as a rigid piston. Take a look here
For end correction, I guess this is a good place to look at
Some add a mass element at the pipe end, other modify the pipelength. Beauty lies in the eye of the beholder.
Have fun
Claes
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I mean if we would want to modify a muffler based on parametric design, which parameters should be evaluated in the study, for example the cross section of the tube, the number of mufflers (inline or parallel), etc.
What kind of engine specifications should be evaluated in the study?
Hi
For muffler design, all dimensions matter, though some decide the base performance more than other. These tend to decide the system fundamental resonances.
I believe it is worth your while taking a look here
and to take a look at what Prof. Åbom & Prof Boden have been doing for quite some time.
For design, perhaps some of my ramblings can be of interest though they only adress Test, Simulation, QA and Optimization in general rather than muffler design in particular?
For what it is worth. It is worthwhile to simulate muffler not only regarding acoustics as they should be able to withstand also fatigue from pressure pulsation loading, and if the muffler is extended (large with respect to system dimensions), you must identify also good suspension positions, mechanical resonance should not lie at bad motor RPMs or at typical cruise speed, fit within a limited space envelope, etc.
Sincerely
Claes
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Hi everybody,
I am a Young researcher in aeroacoustics and I work with Kirchhoff integral in supersonic flow, due to Mach cone I face with a singular integral, in simple form of these integrals are in attached figure form and i like to solve this problem,
who can help me?
With Best Regards for your time
Dear Dr.Bernard
Thank you so much for answer
but i think my problem is a little difference,
i suppose shock occur in behind the vibrating structure and i don't consider shock effect in my computation, i e i have completely uniform flow,
about FWH analogy, i think this problem exist in it too, because i have surface integral in solid surface and it appear again
Regards
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1. How one can measure/distinguish between different sources of sound?
2. In general combustion has mono pole source induced from mass and heat release, how can we differentiate between the same (monopole) source of sound, whether it is induced from mass or heat source?
3. Further it would be helpful if you can refer some reference about measuring them ?
Hi
Your question is very wide. I will try and answer a small portion.
1. For aeroacoustic sources, theory was proved using Lighthills scaling laws where experiments made at different speeds allowed discrimination between monopole, dipole and quadrupole type sources. A simplistic overview of these scaling laws is found here
One can look also at directivity patterns that differ between source types.
2. I do not know for a fact, I can only venture a guess. Again, I imagine one would measure at different conditions. Mass type source should be invariant with respect to geometry of flow outlet whereas a heat source, i.e. a thermoacoustic source such as a flame, couples differently and in a small portion of the flame. Modifying the heat transfer conditions should allow you to distinguish between the two. At one time, I managed to turn off a thermoacoustic source simply by inputting a bent wire forming a + across the engine exhaust port. Also, mass flow should scale with density, i.e. with absolute temperature.
3. Again, I do not know. I suggest you take a look in this excellent book by Rienstra and Hirschberg which covers many aspects of aeroacoustics.
Worthhile to read are also Ingaards book
and Howe's book which contains a lot of useful references
If your interest lies in source characterization for combustion engines, I suggest you take a look at the 2-port method developed by Boden and Åbom
There exists also a simulation/measurement package called SIDLAB that might interest you
Hope this helps
Claes
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Any references on this topic. I think it will resemble a dipole radiation.
I did a quick calculation assume it is dipole of a sphere with radius been the length of the beam. For a moderate frequency of 1KHz (about the resonance of the cantilever), I found that the damping mostly come from the acoustic radiation rather than from the cantilever's internal loss. Is it true ? Want to confirm...
Thanks all for your answers. I did find a report from freefullpdf.  It is experiment result, but at extremely low freq of 4Hz. In that case, a steel cantilever beam's loss comes mostly from its internal loss (hysteresis loss, joint friction etc..), acoustic radiation account for ~6% of the total loss.
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While modeling aeroacoustics of the fan using DDES, I have to use the transient formulation. Fluent is recommending to use Bounded second order implicit. After using this, I have observed lot of fluctuation in acoustic signal. Can anybody elaborate why it is so?
Hello Malik
I am studying the time step effect on noise. It does help but still can not dampen the rotational frequency. Further reducing time step is costing tremendous increase in simulation time (while looking at my limited CPU resource).
As you are also interested in this, let me know if you get any relation of compressibility and fan noise. I have a seen similar plot for supersonic tip speed. But for my case it is subsonic. Share  if you find any relevant link.
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Hello everyone,
Plinian and subplinian style explosive eruptions are aeroacoustic emittors. They radiate infrasounds. The aeroacoustic emission and spectral content is related to the physical parameters of the impulsively starting jet as well.
Aeroacoustic experimental studies suggest that turbulent jets can change their dominant aeroacoustic noise emission in response to a forcing vibration such as ground vibration or excitation by wind or as a result of changing vent geometry. This is also the basis for aeroacoustic noise reduction from the jet engines of jumbo jets.
In turn, these studies suggest that a lock-on effect, a sort of resonance may set in.
When it does entrainment is increased by up to 30% according to experimental studies on aeroacoustic forcing and turbulent jets and this, in the case of particle-laden turbulent jet flows, can influence mass distribution and particle ejection and reduce the mass loading in the turbulent and increasingly dilute jet flow.
Conversely, if aeroacoustic excitation is not at the resonance dominant frequency mode, entrainment may be reduced by up to 30% and in the case of an explosive eruption jet one may anticipate that this may result in a sudden change of style into fountain collapse. Particularly in the case where the mass flow rate is such that the eruption column is in the transitional regime, that between the stable plume and the collapsing fountain regimes.
Because an external excitation (noise intensity)  of 1 in a million of the characteristic jet noise intensity may result in lock-on or the opposite, this may lead to the possibility of artificially modulating the entrainment and eruption style of some explosive éruptions.
That is to a partial external control, potentially, on the evolution of some explosive eruptions, given the means to do so (eg. 3 "jumbo jet" or "anchored missiles" arranged in a triangle around the erupting volcano, as aeroacoustic external sources of forcing).
To my knowledge this has not been studied either by doing any field tests, which is understandable or by studying this possibility using analogue particle laden jet systems in the lab with the view of applications to volcanic eruptions.
I am wondering if anyone has carried out integrated time-dependent studies of subplinian style explosive eruption jets (eg. at Sakurajima or Etna  ?) with the view of assessing potential correlations between aeroacoustic emissions by the jet itself, time-dependent jet video images, radar/sodar imagery, continous volcanic gas flux monitoring (eg. DOAS), ground tremor characteristics, time-dependent wind intensity and/or obvious changes in vent shape over time ?
Looking forward to any info that may relate to this.
With best wishes and kindest regards,
Gerald
Hi Gerald,
did you see these papers?
Hope that helps
Federico
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In general, gasoline engines are inherently smoother and quieter than diesel engines due to the difference in combustion characteristics. Going by this principle, exhaust system of gasoline engines should need lesser muffler volumes as compared to the exhaust system of diesel engines to achieve similar noise reduction However, the muffler volumes of production gasoline engines are about 5 liters more than that of the corresponding diesel engines. What could be the technical explanation for this in terms of acoustics and vehicle level noise control?
IMO, in contrast to SI engine, diesel exhaust gases go through a relatively complex EAS (EGR, DPF, SDPF, etc.) which contributes to reduce the exhaust noise till it reaches the final muffler.
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I want to calculate noise shielding from monopole source in semi-infinite barrier shielding, but some previous work done are suggest to combine the resultant shielding at a given field point from all four sides of the quadrilateral . But I cant see a picture how noise traveling through each quadrilateral edge and diffracted to observer.
The analytic picture of edge diffraction is treated at length in Eugen Skudrzyk's classic text "Foundations of Acoustics". As I recall he works from basic Helmholtz Integral Equation (he calls it the Helmholtz-Huygens equation) to the Rabinowitz equivalent line integral around the edge of a boundary, to equivalent diffraction point sources (stationary points in the Rabinowitz line integral) at isolated locations on the edge. It is rather involved, but well worth the effort to understand edge diffraction.
In brief, it is shown that the diffracted field owing to a boundary "looks" mathematically as if there is a line of monopole sources distributed along the edge of the boundary. Monopole sources radiate in all directions, to a receiver virtually anywhere in the acoustic medium, even into the shadow zone of the boundary. Since there is a distribution of monopoles in a line (along the edge), moreover, their total  effect is the constructive and destructive interference of all of the monopoles.
The strongest contribution from the distribution of monopoles comes at "stationary points" in the phase of the monopoles. These stationary points look like isolated point sources -- equivalent edge-diffraction sources. It can be shown that the location of these stationary points follow a "law of diffraction" that is analogous to the "law of reflection" for acoustic rays reflected from a rigid boundary: The angle of a three-dimensional cone of diffraction rays equals the incident angle of the ray illuminating the edge (the angles being measure with respect to the normal of the edge in the plane of the incident ray).
The more recent text Morse and Ingard's "Acoustics" may also have a treatment, but I am unsure.
I hope that helps.
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how to perform simple calculation in predicting noise propagation from engine and shielding effect ?
For reasonably high frequencies (wave length shorter than the tail width) you can use geometrical acoustics to estimate the shadow effect of the tail when the aircraft is flying sufficiently high. The sound source and its spectrum should be estimated from empirical correlations. This should provide you a first order estimate for the shielding effect.
When the aircraft is flying low the atmosphere is quite inhomogeneous and the propagation can be complex. Also reflection on the ground can be significant. At large distances sound can reach the observer through different path. This phenomenon is responsible for the rumble sound of guns (or thunder).
For larger wave length diffraction at the edges of the tail become important. At very low frequencies (wave length larger than the dimension of the tail) the tail is not shielding.
Numerical simulation of the engine flow is quite difficult and rather useless.
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In normal mode analysis, for example in instability problem in axisymmetric jet, what is the physical significance of the real and imaginary parts?
in orr-sommerfeld solution there is a normal mode present which is the combination of amplitude and phase, in the phase what is the physical meaning of exponential power?

thank you sir.
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I am in need to validate the codes for aeroacoutics computational simulation.
Hi Geen Paul V,
I have a PDF file that I think it could be useful for you.
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As for computational fluid dynamics analysis, can we analyze the voice predication?
If anyone knows, please recommend me some software and also on how to operate them.
Thanks!
I THINK lASER VELOSOMETRY  can be used
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I am in need to validate the codes for aeroacoutics computational simulation. What are the standard test cases in computational simulation of Aeroacoustics, whose experimental data will be available?
Did you have a look to the benchmark problems used for the computational aeroacoustics workshops? They are available on the NASA Technical Reports server. There are different test cases of increasing complexity. Analytical solutions are usually provided as well.
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I am working on an Aeroacoustics Competency Development Program.
As I don't have much solid experience on Aeroacoustics and its allied processes, I would like to know which will be the best way to start with.
Hello Green
This is my collection of aeronautical books:
I hope it will be helpful.
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I'm making a Jet Noise Analysis, I need to know how to simulate an aeroacoustic simulation in openFOAM for a conventional nozzle. Thanks.
Sebastian:
- Investigation of high frequency noise generation in the near-nozzle region of a jet using large eddy simulation. By: Ali Uzun · M. Yousuff Hussaini.  Theor. Comput. Fluid Dyn. (2007) 21: pp. 291–321. DOI 10.1007/s00162-007-0048-z.
- PREDICTION OF NOISE GENERATED BY A ROUND NOZZLE JET FLOW USING COMPUTATIONAL AEROACOUSTICS.  Authors: ALI UZUN and M. YOUSUFF HUSSAINI, J. Comp. Acous. 19, 291 (2011). DOI: 10.1142/S0218396X11004365.
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Using compressible CFD methods such as the lattice Boltzmann method, it is easy to set up initial conditions that result in sound waves.
One example is a Gaussian density distribution in an otherwise quiescent fluid, which results in an outgoing sound pulse. Another example is Poiseuille flow initialisation of the flow field in a simple 2D channel with an obstacle; sound waves will propagate until the flow is stabilised.
My intuition suggests that if Ma << 1 then it should be possible to decompose a given initial condition into an incompressible flow part and a sound wave part. For instance, we know for an initial condition with ∇2p ≠ 0 that it must contain a sound wave component. This must be true because ∇2p = 0 is a defining characteristic of incompressible flow, and because the wave equation tells us that ∇2p ≠ 0 means that a wave will be propagating.
Understanding this decomposition would be essential for setting up pure sound simulations or pure flow simulations in compressible CFD methods. However, I have not been able to find anything about this in the literature. Would anyone here have any relevant references for me?
You may use an orthogonal decomposition of your pressure field into its mean value plus its fluctuation around the mean value, and the orthogonal Helmholtz decomposition of your velocity field into a divergence free component and a curl free component. Then the incompressible component will be the mean value (and thus constant) pressure and the divergence free velocity. And the acoustic part will be the fluctuation of the pressure and the curl free component of the velocity. This is a 'continuous' (non discretized) point of view. The discretized equivalent of this point of view depends on your numerical scheme. I don't know enough about lbm to help you on that, but we have looked at that topic in the context of finite volumes. If you are interested take a look at the publication I co-authored with S. Dellacherie and F. Rieper in the JCP in 2010. Best regards. Pascal
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Change of CLo with speed.
Are we mixing up a little bit the names? the "zero lift coefficient" CL0 is the lift coefficient at AoA=0 (it makes no sense to define the lift coefficient at CL=0!). This is different to the "zero lift drag coefficient" CD0, which is the drag coefficient at CL=0. So both values are not necessarily at the same incidence!
CL0 is only constant for an airfoil in incompressible inviscid theory. The effect of compressibility can be approxiated by the Karman-Tsien rule as long as the onflow is below M* (the onflow Mach number where first a sonic speed is achieved somewhere at the airfoil). The vicous effect is harder to approximate, since it strongly depends on which Reynolds number range you are interested (laminar, laminar/turbulent, fully turbulent flow)
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Our present aeroacoustic model for rotating blade aeroacoustics does not include thickness noise. What are the options? Wind turbine application.
Read the analytical acoustics by F. Stumpf
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Of course these flying objects must not just floating in air like fine solid particles or liquid droplets. Their motion must be controllable. What is limitation of the scale? Can anyone give some micro-scale or nano-scale existing examples and talk about how they are flying?
Many thanks for the above answers. I am asking for the possible smallest size of the "controllable: flying object. Can we learn from nature by taking a look at birds, insects, bugs, etc.?
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I want solve by shooing method but I am not getting it. We can also use F''(-infinty) =0;
and F''(+infinity) =0; can anyone help?
As I am sure you know, this is the Blasius equation. Below is a shooting method that I once wrote in matlab, for the typical case where you shoot from y=0 to infinity in order to get the ZPG boundary layer profile. It will be easy to adapt to your case.
Shooting from minus infinity to plus infinity sounds like a bad idea for stability issues - I suggest you shoot from y=0 in both directions, and you adapt the (unspecified but needed) BC for F'(0) iteratively.
To try this routine, pick a y0 not too far from 0.33:
======================
function [T,Ynew]=shootingBlasius(y0)
% by Lutz Lesshafft, 2011
% solves the ODE y''' + y*y'' = 0 with BC y(0)=0, y'(0)=0, y'(inf)=1.
% input parameter y0 is initial guess for y''(0).
% shooting algorithm:
[T,Y0] = ode45(@diffeqn,[0 20],[0 0 y0]);
y1=y0+0.01;
[T,Ynew] = ode45(@diffeqn,[0 20],[0 0 y1]);
while (abs(Ynew(end,2)-1) > 1e-15)
ynew=interp1([Y0(end,2) Ynew(end,2)],[y0 y1],1,'linear','extrap');
y0=y1; Y0=Ynew; y1=ynew;
[T,Ynew] = ode45(@diffeqn,[0 20],[0 0 ynew]);
disp(ynew);
disp([' error: ',num2str(abs(Ynew(end,2)-1))]);
end
end
function dy=diffeqn(t,y)
dy = zeros(3,1);
dy(1)=y(2);
dy(2)=y(3);
dy(3)=-0.5*y(1)*y(3);
end