1,721,087 research outputs found
Acoustic and hydrodynamic analysis of the flow around an aerofoil with trailing-edge serrations
No full textDirect numerical simulations of the flow around a NACA-0012 aerofoil are conducted, employing an immersed boundary method to represent flat-plate trailing-edge extensions both with and without serrations. Properties of the turbulent boundary layer convecting over the trailing edge are similar for both cases. For cases with serrations, the trailing-edge noise produced by the flow over the aerofoil is observed to decrease in amplitude, and the frequency interval over which the noise reduction occurs differs depending on the serration length. The directivity and spanwise coherence of the trailing-edge noise appears largely unaffected by the serrations. The hydrodynamic behaviour in the vicinity of the trailing-edge extensions is investigated. The streamwise discontinuity imparted upon the turbulent flow by the straight trailing edge can clearly be observed in statistical quantities, whereas for the serrated case no spanwise homogeneous discontinuities are observed. The trailing-edge serrations appear to break up the larger turbulent structures convecting into the wake, and to promote the development of horseshoe vortices originating at the serrations themselves
Direct numerical simulations of low Reynolds number flow over airfoils with trailing-edge serrations
No full textDirect numerical simulations (DNS) have been conducted of NACA-0012 with serrated and straight flat-plate trailing-edge extensions using a purposely developed immersed boundary method. For the low Reynolds number airfoil flows accessible by DNS, laminar separation bubbles involving laminar-turbulent transition and turbulent reattachment occurs. Comparing results from simulations with serrated and un-serrated trailing-edge extensions, noise reduction for higher frequencies is shown using power spectra and one-third octave averaged pressure contours. The effect of the trailing-edge serrations on an acoustic feedback loop observed in previous simulations and the subsequent effect on the laminar separation bubble is studied via cross-correlations, probability density functions of skin friction and spanwise wavenumber spectra. The results show that the presence of serrations leads to some spanwise variation of transitional structures in the separated shear layer, but does not significantly affect the overall hydrodynamic field on the airfoil upstream of the serrations. Two reasons for why the hydrodynamic field is not considerably affected by the presence of serrations are suggested.<br/
Numerical investigation of airfoil self-noise reduction by addition of trailing edge serrations
No full textDNS of the flow around a NACA-0012 airfoil are conducted, employing an immersedboundary method to represent flat-plate trailing-edge extensions both with and withoutserrations. Properties of the turbulent boundary layer convecting over the trailing-edgeare similar for all cases. For cases with serrations, the trailing-edge noise produced by theflow over the airfoil is observed to decrease in amplitude, and the frequency interval overwhich the noise reduction occurs differs depending on the serration length. The trailing-edge noise appears otherwise largely unaffected by the serrations in terms of its directivityand spanwise coherence. The hydrodynamic behaviour in the vicinity of the trailing-edgeextensions is investigated. The streamwise discontinuity imparted upon the turbulent flowby the straight trailing-edge can clearly be observed in statistical quantities, whereas forthe serrated case no spanwise-homogeneous discontinuities are observed. The turbulentflow through the serrations promotes the development of horshoe vortices originating atthe serrations themselves, which appear to promote a more rapid mixing within the airfoilwake
Stability and receptivity characteristics of a laminar separation bubble on an aerofoil
No full textStability characteristics of aerofoil flows are investigated by linear stability analysisof time-averaged velocity profiles and by direct numerical simulations with timedependentforcing terms. First the wake behind an aerofoil is investigated, illustratingthe feasibility of detecting absolute instability using these methods. The time-averagedflow around an NACA-0012 aerofoil at incidence is then investigated in terms ofits response to very low-amplitude hydrodynamic and acoustic perturbations. Flowfields obtained from both two- and three-dimensional simulations are investigated,for which the aerofoil flow exhibits a laminar separation bubble. Convective stabilitycharacteristics are documented, and the separation bubble is found to exhibit noabsolute instability in the classical sense; i.e. no growing disturbances with zero groupvelocity are observed. The flow is however found to be globally unstable via anacoustic-feedback loop involving the aerofoil trailing edge as a source of acousticexcitation and the aerofoil leading-edge region as a site of receptivity. Evidencesuggests that the feedback loop may play an important role in frequency selection ofthe vortex shedding that occurs in two dimensions. Further simulations are presentedto investigate the receptivity process by which acoustic waves generate hydrodynamicinstabilities within the aerofoil boundary layer. The dependency of the receptivityprocess to both frequency and source location is quantified. It is found that theamplitude of trailing-edge noise in the fully developed simulation is sufficient topromote transition via leading-edge receptivity
Direct numerical simulations of forced and unforced separation bubbles on an airfoil at incidence
Full text linkDirect numerical simulations (DNS) of laminar separation bubbles on a NACA-0012 airfoil at Re-c = 5 x 10(4) and incidence 5 degrees are presented. Initially volume forcing is introduced in order to promote transition to turbulence. After obtaining sufficient data from this forced case, the explicitly added disturbances are removed and the simulation run further. With no forcing the turbulence is observed to self-sustain, with increased turbulence intensity in the reattachment region. A comparison of the forced and unforced cases shows that the forcing improves the aerodynamic performance whilst requiring little energy input. Classical linear stability analysis is performed upon the time-averaged flow field; however no absolute instability is observed that could explain the presence of self-sustaining turbulence. Finally, a series of simplified DNS are presented that illustrate a three-dimensional absolute instability of the two-dimensional vortex shedding that occurs naturally. Three-dimensional perturbations are amplified in the braid region of developing vortices, and subsequently convected upstream by local regions of reverse flow, within which the upstream velocity magnitude greatly exceeds that of the time-average. The perturbations are convected into the braid region of the next developing vortex, where they are amplified further, hence the cycle repeats with increasing amplitude. The fact that this transition process is independent of upstream disturbances has implications for modelling separation bubbles
Direct numerical simulations of noise generated by the flow over an airfoil with trailing edge serrations
No full textDirect numerical simulations of airfoil self-noise
No full textDirect numerical simulations (DNS) of airfoil self-noise were conducted. For the low Reynolds number airfoil flows accessible by DNS, the occurrence of laminar separation bubbles involving laminar-turbulent transition and turbulent reattachment leads to additional noise sources other than the traditionally studied trailing-edge noise. Cross-correlations of acoustic and hydrodynamic quantities in conjunction with rayacoustic theory are used to identify the main source locations for a NACA-0006 airfoil. It is found that the contribution of trailing edge noise dominates at low frequencies while for the high frequencies the radiated noise is mainly due to flow events in the reattachment region on the suction side. DNS have also been conducted of NACA-0012 airfoils with serrated and straight flat-plate trailing-edge extensions using a purposely developed immersed boundary method. Noise reduction for higher frequencies is shown and the effect of the trailing edge serrations on the acoustic feedback loop observed in previous simulations and the subsequent effect on the laminar separation bubble is studied
Direct numerical simulations of tonal noise generated by laminar flow past airfoils
No full textA numerical investigation is presented of noise generated by flow past symmetric NACA airfoils with different thickness and at various angles of attack at M=0.4 and a Reynolds number based on chord of Re=50,000. Direct numerical simulations (DNS) are employed to directly compute both the near-field hydrodynamics and the far-field sound. The DNS data are then used to investigate whether the approach of determining tonal noise radiation based on the surface pressure difference, as done in the classical trailing-edge theory of Amiet, yields satisfactory results for finite thickness airfoils subject to mean loading effects. In addition, the accuracy of Amiet's surface pressure jump function is evaluated. Overall, increasing airfoil thickness to 12% chord, which corresponds to a trailing-edge angle of 16.8°, an unexpected phase change between the incident and scattered pressure is found at the frequency of the forced instability waves. This phase change is attributed to the flow oscillating around the trailing edge at a separate wake frequency. For the largest incidence investigated, Amiet's response function does not predict the total surface pressure difference as accurately as for zero of small incidence at the vortex shedding frequency, resulting in a poor prediction of the directivity and amplitude of the acoustic pressure. Moreover, predicting the airfoil self-noise based on the surface pressure difference appears not to be generally applicable at higher angles of attack because the radiated sound is only partly due to classical trailing-edge noise mechanisms. In these cases, it appears as if volume in the flow cannot be neglected
A zonal characteristic boundary condition for simulations of aerodynamic sound
No full textThis paper presents a non-reflecting boundary condition that significantly reduces the spurious pressure oscillations that are produced when vortical structures in a compressible flow cross the computational farfield boundaries. The method is based on commonly used characteristic boundary conditions. Here, incoming characteristics are ramped to zero in a buffer region as opposed to merely setting them to zero at the boundary. One of the key features of the approach is that it is free of coefficients requiring calibration. The equivalence of two formulations of the zonal boundary condition is verified with a model problem. Direct numerical simulations of trailing edges at different Mach numbers are conducted to demonstrate the effectiveness of the novel approach
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