Flap Side Edge Research Articles

A Numerical Study on the Flap Side-Edge Noise Reduction Using Passive Blowing Air Concept

The flap side-edge is a vital contributor to airframe noise. In this study, we propose a novel flap side-edge noise reduction method based on the concept of active blowing air. A long slot is opened from the flap’s lower surface to the tip surface to induce a secondary jet flow, which is driven by the local pressure difference between the flap’s lower surface and the tip surface. The unsteady flow field around the flap side-edge was computed by the lattice Boltzmann solver PowerFLOW, and the far-field noise was predicted by the FW-H equation. It is demonstrated that the dominant features of the flap side-edge flow are the double vortex structures, and the new passive blowing air reduction method can achieve about 3.3 dB noise reduction. Moreover, the underlying noise reduction mechanism has been analyzed and revealed. It is shown that the secondary jet flow from the long slot on the flap side-edge would dissipate the flap side-edge vortex and displace the flap side-edge vortical structure away from the flap surface, thus resulting in a decrease in the pressure fluctuations on the flap side-edge surface. As a result, the flap side-edge noise was reduced. In contrast to the current active air blowing technique, the newly proposed blowing air technique is passive and quite simple and does not require an extra air source or control system. This novel flap side-edge noise reduction technology provides a new flow control strategy and noise reduction methodology and can be further optimized.

Identifying the flap side‐edge noise contribution of a wind turbine blade section with an adaptive trailing edge

AbstractActive trailing‐edge technology is a promising application for localized load alleviation of large‐diameter wind turbine rotors, accomplished using one or more control surfaces in the rotor blade's outer region. This work focuses on identifying noise contributions from the flap side‐edge and the trailing edge in a laboratory condition. Measurements were conducted in the Acoustic Wind Tunnel Braunschweig (AWB) at the German Aerospace Center's (DLR) Braunschweig site. The small‐scale model has a span of 1,200 mm and a chord length of 300 mm. The control surface, a plain flap, has a span of 400 mm and a chord length of 90 mm. Far‐field noise was measured using a phased‐microphone array for various flow speeds, angles of attack, and flap deflection angles. Due to the size of the model and assumed closeness of the sound sources, two noise reduction addons were installed interchangeably: trailing‐edge brush and flap side‐edge porous foam for sound source identification. Analysis of the far‐field noise reveals that, while changes to the flap deflection angle alter the far‐field noise spectra, the trailing‐edge noise remains the predominant noise source at deflection angles and . No additional noise level was observed from the flap side edge within the measurable frequency range at these angles. The flap side‐edge noise has an increased role for frequency larger than 2 kHz for the larger flap deflection angles of and .

Identification of Flap Side-Edge Two-Source Mechanism Based on Phased Arrays

The airflow past flap side edges is an important contributor to airframe noise. It has been found that the dominant features of the flow at the flap side edges are the double-vortex structures and the far-field noise features as two humps in the noise spectrum. Flap side-edge noise is commonly assumed to be linked to the double-vortex. However, whether the two humps in the noise spectrum are directly related to the double-vortex has not been fully confirmed. In this study, the flap side-edge noise sources are computed and analyzed based on phased microphone arrays. The CLEAN based on Source Coherence method is applied to the airframe noise benchmark test DLR1. It is shown that the acoustic sources have different frequencies and amplitude scaling laws in low- and high-frequency ranges. In contrast to existing studies, this study demonstrates that the high-frequency spectrum follows the Helmholtz number scaling law rather than the Strouhal number scaling law. Furthermore, the source strength distributions indicate that low-frequency noise sources are mainly located on the aft half-chord of the flap side edge, and high-frequency noise sources are located on the forward half-chord. Finally, two subregions at the flap side edge are identified as source locations for the low and high frequencies, respectively. The two-source noise mechanism revealed in this study will aid the establishment of a physics-based flap side-edge noise prediction model.

Acoustic sources mapping based on the non-negative [formula omitted] regularization

Phased microphone array techniques for estimating source locations and strengths have been widely employed in aeroacoustic applications. Many source localization algorithms by conducting the convex L1 norm optimization have been proposed by assuming a spatially sparse distribution of the sound sources in recent years. Nevertheless, L1 norm optimization has been shown to be suboptimal and cannot enforce further sparsity. This paper proposes a nonconvex L1/2 regularization approach for solving the inverse problem of deconvolution approach to improve its spatial resolution and robustness. Thus, the iteratively reweighted least squares method is employed to solve the L1/2 regularization problem. The capabilities of the proposed algorithm are demonstrated using various synthesized simulations to compare the results with other algorithms and it is found that multiple closely spaced sound sources with unequal powers can be identified. It indicates that the L1/2 regularization method can provide significant improvement with high spatial resolution and reduced sidelobes compared to the L1norm regularization method, and the computation cost is not increased significantly. The proposed method is applied in the experimental applications of benchmark test DLR1. Two incoherent sources are discerned near the flap side edge at particular frequency.

Retraction notice to: “Flap side edge noise modeling and prediction” [J. Sound Vibr. 332 (2013) 3846–3868

Retraction notice to: “Flap side edge noise modeling and prediction” [J. Sound Vibr. 332 (2013) 3846–3868

Investigation of Noise Generation from Bluff Flap Side-edge of a High-Lift Wing Model

Investigation of Noise Generation from Bluff Flap Side-edge of a High-Lift Wing Model

Flap side edge noise modeling and prediction

This paper presents a model for aircraft flap side edge noise prediction. The flap side edge noise is modeled as two components, respectively for the low and high frequency domain and corresponding to two noise generation mechanisms in the flap side edge flows. The high frequency noise mostly comes from the flow separation near the sharp corners of the flap, mainly in the forward half of the flap chord, and the low frequency component is largely due to the interactions between the large scale vortex structure and the sharp corners of the flap, occurring in the mid and aft half chord region of the flap where the rollup vortex is well established with significant vertical energy to be scattered into sound by the corners. The prediction model is developed from the theory of aerodynamic sound generation, relating the noise spectrum to the statistics of the surface pressure fluctuations in the vicinity of the flap side edges, the characteristic length and time scales in the flow, and the Green's function that accounts for the sound-flow coupling and propagation effects. For both the low and high frequency components, the individual physical mechanisms for the noise generation and propagation are analyzed to develop prediction models for the features of the radiated noise, such as the spectral shapes, the Mach number dependencies and the far field directivities. The individual models are validated by experimental data, showing good agreements between the developed prediction models and data.

Effect of Mictrotab on Reduction of Noise Due to Aircraft High-Lift Devices

D EFINED as the “nonpropulsive noise of an aircraft in flight” in [1], noise produced by airframe components becomes significant during the approach-to-land phase of aircraft operations. Research efforts on identifying and understanding the noise sources associated with the individual airframe components, in particular, high-lift systems, have been undertaken by many researchers [2]. It has been confirmed experimentally in [3–5], and analytically by using empirical methods [6], that both leading-edge slats and trailing-edge flap side edges are the most important noise contributors from the high-lift systems to overall noise levels of an aircraft in approach. The high-lift noise, intuitively thinking, is configuration dependent. Any increase in the deflection angle of slat and flap will lead to an increase in overall noise level. The larger the deflection angles are, the higher the noise level will be. This is due to the fact that the highlift components are more loaded when their deflection angles are increased, generatingwhat is referred to as the loading noise. Further, higher deflection angles may induce stronger vortex shedding and turbulent wake downstream which causes higher noise. Such intuitions were supported by experiments conducted at NASA Langley Research Center in 1998 and 2002 [7], which revealed that slat noise is generated due to its configuration setting angles. Khorrami et al. [8] hypothesized with 2-D unsteady computation that the vortex shedding at the slat trailing edge was responsible for the additional highfrequency spike observed only at higher slat angles. An acoustic whistling mechanism at the slat gap due to vortex shedding was later proposed by Tam and Pastouchenko [9] and Agarwal [10]. The theory supported that the simple vortex shedding and feedback mechanism resonating with gap tone frequency caused noise amplification and propagation. Similar observations were made for flap noise, as reported in [11,12]. Both references reported that the measured noise level at higher flap deflection angles exceeded the predicted noise level, which suggests that there might be an additional source contributor, most likely vortex instabilities, as a result of its interaction with the nearby flap upper surface. It may be noted that such phenomenon was not seen at lower settings of flap angles. These observations may infer that high-lift device noise level is directly affected by the configuration setting angles of slats and flaps. In general, high-lift device noise sources are present at wide frequency ranges of low-to-mid andmid-to-high. At a fixed observer location, under a constant flow speed, slat noise is dominant in the low-to-midfrequency range, and flap side-edge noise is dominant at the mid-to-high-frequency range as shown by both Guo and Joshi [13] and Guo et al. [14]. In this study, the proposed technique uses reduced deployment angles of both high-lift devices simultaneously to achieve the most effective noise reduction in different frequency domains. Aerodynamics performance also has to be carefully monitored to ensure no significant loss is present. The objective of the current Note is to examine the effectiveness of a microtab device in airframe noise reduction. Reducing deployment setting angles of both slats and flaps, the associated noise can be alleviated. Loss in aerodynamic lift, as a result of the reduced highlift settings, is compensated by use of a microtab device. The microtab device can be viewed as a small spanwise strip located at the pressure side of flap near the trailing edge, similar to a Gurney flap (Fig. 1). While deployed normal to local airfoil surface, the microtab device alters the local flowfield which leads to increased effective camber, and hence improves lift-to-drag ratio, similar to that of a Gurney flap introduced by Liebeck [15]. In the present work, a threedimensional numerical analysis is presented and compared to sound pressure level (SPL) of the proposed microtab configuration at a specified far-field location to that of the baseline, that is, conventional configuration in approach, to see if any reduction in noise can be achieved.

Measurements of Flow Around a Flap Side Edge with Porous Edge Treatment

Wind-tunnel experiments were performed to investigate a flap side-edge vortex, which is a contributor to airframe noise. The flowfield investigation showed that the peak turbulent stresses were contained in the shear layer that rolled up to form the flap side-edge vortex. The wake from the main element was also entrained by the side-edge vortex. The near-field pressure fluctuations where the turbulent shear layer impinged on the flap side edge were broadband in nature from a Strouhal number of 10 to 50. Hot-wire measurements on the downstream vortex identified a broadband instability centered around a Strouhal number of 13.2. A porous side-edge treatment was applied to the half-span flap to modify the flap side-edge flowfield. The effect of applying a porous side edge was to reduce the Reynolds stresses contained within the vortex and the shear layer that formed it. The porous material also had the effect of displacing the vortex further away from the flap surface. This led to a reduction in the broadband pressure perturbations measured at the flap side edge. Compared with the accuracy of the measurements of the aerodynamic forces, the aerodynamic impact of the porous flap side edge was almost negligible.

On the reduction of flap side edge noise

Results from flap side edge noise reduction experiments are presented. The test model used was a NACA 632-215 main-element airfoil with a half-span Fowler flap. The flap side edge was flat for the baseline configuration. The effect that small geometric variations of the flap edge have on the noise spectra is shown, and noise measurements from a "continuous mold-line link" (CML) flap configuration are presented. These measurements are compared to those obtained from the baseline flap. The results indicate that a large level of noise reduction is achieved using the CML flap. Finally, Particle Image Velocimetry (PIV) measurements of the flow in the region of a flap side edge are presented for a blowing flap configuration. In this active control approach, air was blown from small slots located along the side edge of the flap to weaken the vortex system that is present in that region of the flap. These measurements are presented to show the effects that the flap tip jets have on the structure of the flap side edge flow, and the potential that this blowing flap configuration has in reducing flap side edge noise.

Spatial Stability Analysis of a Flap Side Edge Vortex

The flap side edge vortex is suspected to contribute to aerodynamic noise generation. Using a temporal stability analysis, Khorrami and Singer have shown that unstable modes could exist in this vortex. Due to the convective nature of this instability, a spatial analysis is more suitable. This is the subject of the present work. The mean flow past a 2D wing with a half-span flap has been computed with a steady 3D Navier-Stokes code. Then, local linear stability calculations are performed in several planes perpendicular to the vortex axis. The vortex is assumed axisymmetric and modelled with Batchelor's analytical vortex. Using Gaster's relation, the spatial amplification rate is calculated, giving by integration the relative amplitude of the fluctuations. Some low-frequency fluctuations are seen to be preferentially amplified by the vortex, but the amplifications remain small, so that this mechanism alone should not produce important noise in this particular configuration, where the flap deflection angle is moderate.

On noise reduction by flap side edge fences

This paper discusses the effects of fences on side edge cross flows. Experimental data are reviewed to show that fences can modify the cross flow in the side edge region of aircraft flaps in such a way that noise reduction can be achieved. Near field pressure data reveal that fences reduce the characteristic frequency of the flow fluctuations. This leads us to postulate that the fences make the far field noise spectra shift downward in frequency. Because airframe noise spectra typically have a negative slope in the most important mid to high frequency domain, this downward shift in frequency leads to an equivalent reduction of noise levels, when measured by conventional metric such as the effective perceived noise level in aircraft certification. To explain the decrease of the characteristic frequency of the flow fluctuations due to the fences, an analytical model is developed, based on vortex dynamics. The predictions from the model are shown to be in very good agreement with measured data, both for the dominant frequency in the baseline case without the fences and for the trends of the decreasing frequency with increasing fence height.

Noise Characteristics of Aircraft High Lift Systems

The characteristics of far-field noise from aircraft high lift systems are discussed. Analyses are made of the data from an airframe noise test conducted in the 40 by 80 ft wind tunnel at NASA Ames Research Center, using a 4.7% DC-10 aircraft model. Discussions are given on both far-field free microphone data and measurements from a phased microphone array. Major trends are revealed and discussed from the free microphone data, which include far-field frequency characteristics, dependence of the acoustic radiation on flow Mach numbers, effects of flap and/or slat deployment, and far-field directivity. Data from the phased microphone array are used to identify locations of major noise sources. Four subregions on the wing are identified as important source locations, namely, the leading-edge slat region, the inboard and outboard side edges of the outboard flap, and the inboard flap region close to the hub of the wing. The source strength distributions in these subregions are integrated to reveal dependencies of various noise sources on flow conditions and high lift system configurations. The effects of flap side edge fences on far-field noise are also discussed, which shows a reduction of a few decibels in flap-related noise.

Surface pressure fluctuations on aircraft flaps and their correlation with far-field noise

This paper discusses unsteady surface pressures on aircraft flaps and their correlation with far-field noise. Analyses are made of data from a 4.7% DC-10 aircraft model test, conducted in the 40 × 80 feet wind tunnel at NASA Ames Research Center. Results for various slat/wing/flap configurations and various flow conditions are discussed in detail to reveal major trends in surface pressure fluctuations. Spectral analysis, including cross-correlation/coherence, both among unsteady surface pressures and between far-field noise and near-field fluctuations, is used to reveal the most coherent motions in the near field and identify potential sources of noise related to flap flows. Dependencies of surface pressure fluctuations on mean flow Mach numbers, flap settings and slat angles are discussed. Dominant flow features in flap side edge regions, such as the formation of double-vortex structures, are shown to manifest themselves in the unsteady surface pressures as a series of spectral humps. The spectral humps are shown to correlate well with the radiated noise, indicating the existence of major noise sources in flap side edge regions. Strouhal number scaling is used to collapse the data with satisfactory results. The effects of flap side edge fences on surface pressures are also discussed. It is shown that the application of fences effectively increases the thickness of the flaps so that the double-vortex structures have more time to evolve. As a result, the characteristic timescale of the unsteady sources increases, which in turn leads to a decrease in the dominant frequency of the source process. Based on this, an explanation is proposed for the noise reduction mechanism of flap side edge fences.

Investigation of High-Lift Flowfield of an Energy Efficient Transport Wing

The high-lift flowfield around a multielement energy efficient transport wing is investigated both experimentally and numerically. Special emphasis is placed on resolving aeroacoustically relevant local features of the mean flowfield, e.g., separated flow regions including recirculation bubbles, free shear layers/wakes/jets, and vortices. Such features are typically present in slat and flap cove areas, flap side-edge regions, and slat-main-wing confluent boundary layer. The flow fluctuations sustained in these regions can generate significant noise, especially via interaction with nearby airframe structures. The experimental measurements and computed results presented here show excellent agreement for mean aerodynamic quantities and provide valuable physical insight into potential sources of flow unsteadiness, such as the vortex system near the flap side edge. Whereas the generic features of the computed flowfield are similar to other published studies, the specific details of the acoustically relevant flow features are found to depend on the geometry of the high-lift configuration

Stability Analysis for Noise-Source Modeling of a Part-Span Flap

The relevant and important flow features of the local mean flowfleld In the vicinity of a flap side edge are extracted from an extensive computational and experimental database. Based on the local flow features, possible mechanisms for flow-induced noise sources that are responsible for sound generation at flap side edges are conjectured. Relatively simple flow models for the dominant noise sources are proposed and developed. The models are based on the most amplified instability modes of the local shear flow. Stability analysis of the modeled flowfleld provides an estimate of the frequencies associated with the local flow unsteadiness. Computed frequencies show the proper trend and are in good agreement with NASA Langley Research Center's acoustic microphone array and unsteady surface-pressure measurements.

Reynolds-Averaged Navier-Stokes Computations of a Flap-Side-Edge Flowfield

An extensive computational investigation of a generic high-lift configuration comprising a wing and a half-span flap reveals details of the mean flow field for flap deflections of 29 and 39 degrees. The computational effort involves solutions of the thin layer form of the Reynolds Averaged Navier-Stokes(RANS) equations. For both flap deflections, the steady results show the presence of a dualvortex system; a strong vortex forming on the lower portion of the flap side edge and a weaker one forming near the edge on the flap top surface. Downstream, the vortex on the flap side edge grows and eventually merges with the vortex on the flap top surface. Comparison of on- and off-surface flow quantities with the experimental measurements of Radeztsky, Singer and Khorrami (AIAA Paper 98-0700) show remarkable agreement. For the 39 degree flap deflection, the calculation also reveals the occurrence of a vortex breakdown, which is corroborated by 5-hole probe velocity measurements performed in the Quiet Flow Facility at NASA Langley. The presence of the vortex breakdown significantly alters the flow field near the side edge.

Flap Side-Edge Noise: Acoustic Analysis of Sen's Model

The two-dimensional flap side-edge flow model developed by Sen (Sen, R., Local Dynamics and Acoustics in a Simple Model of Airfoil Lateral-Edge-Noise, AIAA Paper 96-1673, May 1996) is analyzed to reveal the noise-production potential of the proposed mechanism. The model assumes that a vortex will form at the equilibrium position off the side edge of the flap. The vortex is then perturbed away from the equilibrium position by incoming turbulence, causing it to oscillate and thus radiate sound. The noise field is calculated three-dimensionally by taking the flap to have a finite chord. Spectra and directivity of the far-field sound are presented. In addition, the effect of retarded time differences is evaluated. The parameters in the model are related to typical aircraft parameters and noise reduction possibilities are proposed.

Large scale model measurements of airframe noise using cross-correlation techniques

Cross-correlation techniques are used to measure the sound radiated by wing/flap airfoil configurations in the NASA–Ames 40- by 80- foot wind tunnel using a 6.7-m semispan model with three deployed flaps. The sound from the flap side edges exceeds other airframe noise sources by more than 10 dB, with the noise from the side edges of the leading flaps the strongest. The radiated sound is estimated using two formulas based on the standard aeroacoustic theory and one method using only the near and farfield cross correlation. The last method is essentially independent of such theory. The classic dipole angular distribution pattern is modified by wing diffraction effects to yield stronger radiation in the forward direction. The radiated intensity increases at slightly higher than the fifth power of the mean flow speed U0. The location of the flap track fairings is found to have a substantial effect on the radiated intensity: placing the fairings at or near the lateral edges of the flaps substantially reduces the radiated noise.