Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
1Development of High-fidelity CFD Tool for Aeroacoustics

SHINSUKE NISHIMURA*1 KENTARO HAYASHI*2SHOEJI NAKAYE*3MINORU YOSHIMOTO*4To improve the environmental performance of our products, Mitsubishi Heavy Industries,
Ltd. (MHI) has taken on the challenge of reducing the noise from equipment. As part of theseefforts, we are developing large-scale aeroacoustic noise analysis technology that enables thelow-noise design of fluid equipment. Aeroacoustic noise analysis requires precise flow analysis andsound wave propagation analysis. From this perspective, the Lattice Boltzmann Method (LBM) has
been applied as a large-scale fluid analysis technology that has been rapidly advancing in recent
years and is of high potential. This report summarizes this analysis technology and presents theverification analyses of an aircraft airfoil during take-off/landing and axial fan noise.

|1. Introduction
There are many types of fluid-related products in our portfolio. The long-distancepropagation of pressure fluctuations caused by the interaction between these products and fluidmay become a source of aeroacoustic noise. In product design, from the perspective of improving
environmental performance, it is necessary to minimize the generation of such aeroacoustic noisefrom the product. Especially important in the design process is technology to predict the generationof aeroacoustic noise with high accuracy. In general, there are two prediction methods in thisregard: wind tunnel test and computational fluid dynamics (CFD) analysis. The former ischaracterized by a long lead time from the production of a test piece that is exactly simulated inshape until data measurement in wind tunnel testing, which indicates difficulty in using it on adaily basis in the design process. The latter requires highly-accurate, large-scale CFD analysis,
because unsteady turbulence can often be a source of aeroacoustic noise.
To enable highly-accurate, large-scale CFD analysis, MHI is developing a CFD analysis toolby employing the Lattice Boltzmann Method (LBM)(1). LBM is an analysis method in which thekinetic theory of gases is used as the analogy. It has been reported that, compared withconventional CFD analysis tools which are based on the Navier-Stokes equations, LBM requires ashort turnaround time and its accuracy is high(2),(3). This report summarizes our LBM-based CFD
analysis code and presents its characteristics and application examples.

|2. CFD analysis tool development based on Lattice Boltzmann
MethodLBM is a numerical algorithm in which fluid is approximated by a collection of many virtual
particles with a velocity. The velocity distribution function of particles is used to compute thecollision and translation of each particle and obtain the macroscopic flow field (e.g., flow velocityand pressure). Described below are the characteristics of our CFD analysis tool.

*1

Fluid Dynamics Research Department, Research & Innovation Center*2

Research Manager, Vibration Research Department, Research & Innovation Center*3

Vibration Research Department, Research & Innovation Center*4

Research Manager, Fluid Dynamics Research Department, Research & Innovation CenterMitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
2(1)Cumulant collision model
As described earlier, it has been reported that LBM can perform fluid analysis within ashorter turnaround time and with higher accuracy than conventional CFD tools. When it comesto high-Reynolds-number flows, however, LBM analysis has a disadvantageous tendency todiverge. The flow fields of our products often involve flows with a high Reynolds number. To
develop a practically-applicable CFD analysis tool, it is necessary to suppress numericalinstability phenomena at high Reynolds numbers.
In our CFD analysis tool, the instability phenomena are suppressed by using the cumulant
collision model(4). With this model, the numerical viscosity can be controlled at a level that
does not directly affect the fluid governing equations, i.e., Navier-Stokes equations. It has thus
become possible to analyze high-Reynolds-number flows in a stable manner.
(2) Wall model
Generally, the boundary layer near the wall surface is very thin. To resolve the boundary
layer through CFD analysis, it is necessary to create a very dense analytical lattice near the wall
surface. However, as LBM uses cubic lattice cells, it is difficult to locally change the lattice celldimensions near the wall surface of any shape, thus resulting in an insufficient number of latticecells near the wall surface.
As such, the equilibrium wall model(5) has been employed to capture the boundary layer
near the wall surface in our CFD analysis tool, because it does not require a dense lattice near
the wall surface.
(3) Moving boundary conditions
In consideration of application to rotating machinery such as fans, the boundary
conditions for moving boundaries using the Multi-Direct Forcing Method(6) has been
implemented in our analysis tool. In the Multi-Direct Forcing method, the wall surface of amoving body is regarded as a thin film and the external force term in the LBM equation iscontrolled in such a way that the desired moving velocity is realized on that thin film.

|3. Verification and application to low-noise product design
As the verification of our CFD analysis tool and the application to low-noise product design,
two case examples will be presented.
(1) Tool verification by aircraft airfoil analysis during take-off/landing
Described below are the results of aeroacoustic noise analysis of 30P30N three-element
airfoil. It is an airfoil proposed openly as a CFD analysis benchmark problem at the Workshop
on Benchmark problems for Airframe Noise Computations(7) (an international workshop to
promote the improvement of airframe noise prediction technologies).
Figure 1 shows an example of unsteady flow field obtained by the analysis, i.e., Mach
number distribution, which is a flow velocity indicator. In Figures 2 and 3, the wind tunnel test
results are compared with the analysis results in terms of the pressure coefficient around theairfoil and the spectrum of unsteady pressure fluctuations, respectively. These figures show thatthe analysis results captured the complex flow field with large vortices. It has also been verified
that pressure fluctuations, which will become a source of aeroacoustic noise, can bequantitatively assessed with high accuracy.
(2) Application to axial fanThe following are the results of aeroacoustic noise analysis with the use of USI7(8). It is a
widely-known type of axial fan designed by the University of Siegen.
Figure 4 gives an example of the unsteady flow field obtained by the analysis (i.e.,
distribution of the divergence of velocity field, which is strongly correlated with the soundwave density disturbance). Figure 5 shows the far-field Overall Sound Pressure Level(OASPL). It has thus been verified that aeroacoustic noise can be predicted to a high degree of
accuracy even in application to fan rotation.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
3

Figure 1
Example of flow field
analysis of aircraft airfoil during
take-off/landing - visualization of
unsteady flow field

Figure 2 Example of flow field analysis of aircraft
airfoil during take-off/landing - graph ofpressure coefficient around airfoil

Figure 3 Example of flow field analysis of aircraft airfoil during
take-off/landing - graph of unsteady pressure fluctuation spectrum

Figure 4 Example of flow field
analysis of axial fan visualization of aeroacoustic
noise propagation

Figure 5 Example of flow field analysis of axial fan OASPL graph|4. Conclusion
To improve the environmental performance of our products, we are working on the noise
reduction of each type of equipment. This report summarizes the large-scale aeroacoustic noise
analysis technology that enables the low-noise design of fluid equipment and presents theverification analyses of an aircraft airfoil during take-off/landing and axial fan noise. This analysis
technology can accurately predict the generation of aeroacoustic noise and therefore is of use in
examining noise reduction measures. By widely applying this technology to our products andtaking appropriate measures for each type of equipment, we will further contribute to the noise
reduction of each product.

Mitsubishi Heavy Industries Technical Review Vol. 57 No. 1 (March 2020)
4References
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