On the other hand, the turbine airfoil presents several regions of high levels of unsteady pressure, namely the leading-edge, the suction side peak Mach number region, and the pressure side where the flow starts to accelerate. The regions with high-pressure RMS are located in the leading-edge, as predicted by quasi-steady aerodynamic theories. At low reduced frequency, f red = 3, the flat plate clearly presents a dipole pattern in the leading-edge, justifying the observations performed in Figure 5c,d. Anyhow, the pressure RMS can provide valuable information to understand the effect of the airfoil on noise generation, compared to flat plate geometries. However, a note of caution is sounded as a high pressure RMS region is not directly equivalent to a sound source because the hydrodynamic pressure does not lead to sound generation. This figure highlights the different acoustic sources present in both geometries. This is, upstream travelling modes are reflected towards the outlet, analogously as trapped modes, leading to an increase of downstream noise.įigure 6 shows the pressure root mean square value (RMS) at three different frequencies for both the airfoil and flat plate geometries, where a different colour scale is used in each column for enhanced visualisation.

At f r e d ≃ 5, the pressure level clearly increases through the cut-on-cut-off transition of the inlet ( m ≃ 20 ). It turns out that the cut-off transition of upstream-travelling waves has an effect on the downstream radiated noise because the pattern of the inlet cut-on line at positive m is seen also in Figure 5b. At low frequencies, f red < 10, the directivity pattern is quite uniform while at higher frequencies there are very low-pressure levels close to the cut-on line, for positive m. Contrarily, the cut-on region exhibited by the actual geometry is broader and tilted towards positive directivities due to the effect of the swirl. The observed behaviour is analogue to that in the inlet, but with opposite directivities. The results obtained with the equivalent flat plate, (d), have the same cut-on region as in the inlet since the flow properties are uniform. Regarding the downstream noise ( Figure 5b,d), the differences are even greater. This is likely to be produced by the presence of additional noise sources that do not arise in flat plates and also by the reflection of acoustic waves between neighbouring blades. At low frequency, f red < 10, noise is radiated fairly homogeneously along the whole cut-on region. On the other hand, the simulations accounting for the actual geometry do not show such preferred directions. The highest noise levels arise close to the cut-off condition for positive m because the regions of maximum radiation are normal to the plate. The latter exhibits a straight line of extremely low pressure, caused by the dipole behaviour of the flat plate, corresponding to a directivity − θ, because the plate’s dipole cannot radiate in directions parallel to itself. Figure 5a,c show the results obtained at the inlet with the actual and flat plate geometry. The y axis corresponds to the reduced frequency, while the x-axis represents the azimuthal mode index: m = β V s / ( 2 π ). To provide a more detailed broadband noise footprint, the NSPL modal decomposition is depicted in Figure 5. Turbulent wakes generated by a turbine row (see Figure 1) impinge on the downstream one, giving rise to acoustic waves with a broadband energy content. quantified experimentally the relative importance of turbine broadband noise, which can result in being dominant in many operating points and designs and hence, there is a need of industrial methods to predict it accurately. Regarding turbine noise sources break-down, Serrano et al. Nesbitt claimed that it is necessary to develop tools that can predict turbine noise with sufficient accuracy to understand its sources and reduce them in the future. Namely, their reduced blade-count and solidity, the removal of acoustic liner treatments to reduce engine weight, and the reduction of noise emissions by other engine components. Nesbitt described what he considered a ’turbine noise storm’ caused by the confluence of several design choices. Since the last decade, there has been an increasing concern regarding the impact of turbine-generated broadband noise in the context of civil aviation.

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