Mathieu Gruber

Self-noise produced by an airfoil with nonflat plate trailing-edge serrations

This paper represents the results of an experimental study aimed at reducing the airfoil self-noise by the trailing-edge serration of four different sawtooth geometries (defined in the serration angle and length). These serrations have a common feature: all of the sawtooth patterns are cut directly into the trailing edge of a realistic airfoil. This configuration offers better structural strength and integrity. For the sawtooth trailing edges investigated here, the radiation of the extraneous vortex shedding noise in a narrowband frequency due to the partial bluntness at the serration roots is unavoidable. However, this narrowband component tends to be less significant provided that the serration angle is large and the serration length is moderate. Sound power was measured, and some of the sawtooth geometries have been shown to afford significant boundary-layer instability tonal noise and moderate turbulent broadband noise reductions across a fairly large velocity range. This paper demonstrates that a non...

An analytical investigation of trailing edge noise reduction using novel serrations

Passive control of trailing edge noise using complex periodic trailing edge serrations is investigated. The airfoil is modelled as a semi-infinite flat plate with a periodic trailing edge, set at zero angle of attack to a low Mach number flow. Analytical expressions have been derived for the far-field acoustic frequency spectrum for different serrations, namely, sawtooth, sinusoidal, slitted, slitted-sawtooth and sawtooth-sinusoidal. Numerical results have been presented for these serrations over a wide range of frequencies. It has been shown that the noise reduction from serrated trailing edges is a sensitive function of the complexity of the serration geometry and that the noise generation efficiency can be significantly reduced by applying complex periodic serrations to the trailing edge of the airfoil. Our numerical investigations have also shown that the slitted-sawtooth serration is the most effective design for reducing the trailing edge noise, particularly at low and mid frequencies. The theoretical results presented in this paper complement the experimental study presented by M. Gruber et al. [1].

Noise reduction using combined trailing edge and leading edge serrations in a tandem airfoil experiment

This work reports the noise reduction obtained using simultaneously serrated trailing edges and serrated leading edges in a tandem airfoil con guration. Two types of serrated trailing edge treatments are applied to the upstream airfoil, i.e., a sawtooth serration and a slitted sawtooth serration. The leading edge serrated airfoil was designed and manufactured by ONERA. Broadband noise reductions of between 5 dB and 8.5 dB are reported. It is shown over most of the frequency range that overall noise radiation is dominated by interaction noise, while trailing edge noise dominates the high frequency power spectrum at low mean ow velocity. Therefore, the overall noise reduction is dominated by the reductions in interaction noise, and particularly by the reduction of the downstream airfoil response R(f) due to leading edge serrations. However, additional reductions of the overall noise radiation of up 3.5 dB are provided by the use of the slitted sawtooth trailing edge rather than the sawtooth trailing edge on the upstream airfoil, at the high ow velocity U0 = 80m=s. This is shown to be most likely due to modi cations of the wake parameters, i.e., notably a faster decay of the turbulence intensity behind the slitted sawtooth trailing edge.

On the Noise and Wake flow of an Airfoil with Broken and Serrated Trailing Edges

This paper represents the results of an experimental program aiming at exploring the effectiveness of several broken, saw-tooth shaped trailing edges to suppress the instability tone noise at acceptable aerodynamic loss at low Reynolds number flow. This paper also studies the noise sources through acoustic and wake velocity measurements. The test cases consist of several angles of attack from 0 o to 5 o at a fixed Reynolds number of 150,000. For an airfoil with a sharp trailing edge, instability tone noises are detected. The noise mechanism is identified as the aeroacoustic feedback loop which involves the diffraction of the Tollmien-Schlichting waves at the trailing edge. When the trailing edge is replaced with a broken and serrated type, most of these instability tone noises are suppressed, resulting in large noise reduction. The effectiveness of the tonal noise reduction also increases with the serration angle of the saw-tooth. The wake velocity spectra produced by a sharp trailing edge could be very different depending on the angle of attack. Laminar wake at the zero angle of attack ensures that no wake tones could be produced. At 3 o attack angle, the wake becomes very unstable with the presence of a spectra hump and multi-discrete components. It is proposed that these multi-discrete components could eventually produce wake tones in addition to the instability tones produced by the aeroacoustic feedback loop at the pressure side. Because of this, laminar airfoil at a moderate angle of attack always possesses serious noise issue. The wake velocity spectra for a broken, serrated trailing edge are characterized by vortex shedding at 0 o attack angle with no apparent bluntness-induced tone. At 3 o attack angle, the pressure imbalance between the suction and pressure side at each saw-tooth creates counter-rotating vortices. These vortices inhibit both of the spanwise coherence of the bluntness-induced vortex shedding and the multi-discrete components in the wake that would have been produced by a sharp trailing edge. The results in this paper demonstrate that a versatile and low noise laminar airfoil could be achieved when a broken, serrated type trailing edge is used.

On the airfoil self-noise reduction by trailing edge serrations of non-insertion type

This paper represents the results of a preliminary study aimed at reducing the airfoil trailing edge self noise by employing non-insertion type trailing edge serrations. This configuration offers better structural strength and integrity, as well as more straightforward manufacturing process compared to the flat plate type serrations. We found that the noninsertion type serration can reduce the broadband self noise significantly. However, due to the presence of partial bluntness at the serration root, unwanted by-product such as the lowfrequency vortex shedding tonal noise can also be produced. This extraneous noise is found to be less significant if a larger serration angle is employed when the airfoil is set at an angle of attack. We further tested a hybrid configuration composing a non-insertion serration and a woven-wire mesh screen from which the low-frequency vortex shedding tonal noise can be further reduced alongside the broadband self noise reduction.

Aeroacoustics of an axial ducted low Mach-number stage: numerical and experimental investigation

This paper presents an experimental and numerical investigation of a ducted low-speed axial fan. The stator row is heterogeneous with 3 thickened vanes expected to have a non-negligible impact on the radiated noise. A good agreement for pressure ratio / mass flow rate characteristics is obtained between numerical and experimental results up to the onset of rotating stall. Hot-wire velocity profiles at the inlet assessed the effectiveness of a turbulence control screen added to prevent from inflow distortions. Induct acoustic measurements allowed a detailed description from wall-pressure spectra up to modal decompositions upstream and downstream of the fan. Acoustic modes were identified at the first two blade passing frequencies, which should be cut-off with a homogeneous stator row.