scholarly journals Trailing-edge flow manipulation using streamwise finlets

2019 ◽  
Vol 870 ◽  
pp. 617-650 ◽  
Author(s):  
Abbas Afshari ◽  
Mahdi Azarpeyvand ◽  
Ali A. Dehghan ◽  
Máté Szőke ◽  
Reza Maryami
Keyword(s):  
Boundary Layer ◽  
Surface Pressure ◽  
Pressure Fluctuations ◽  
Control Technique ◽  
Aerodynamic Noise ◽  
Trailing Edge ◽  
Low Frequencies ◽  
Passive Flow ◽  
Edge Surface ◽  
Flow Manipulation

The use of streamwise finlets as a passive flow and aerodynamic noise-control technique is considered in this paper. A comprehensive experimental investigation is undertaken using a long flat plate, and results are presented for the boundary layer and surface pressure measurements for a variety of surface treatments. The pressure–velocity coherence results are also presented to gain a better understanding of the effects of the finlets on the boundary layer structures. The results show that the flow behaviour downstream of the finlets is strongly dependent on the finlet spacing. The use of finlets with coarse spacing leads to a reduction in pressure spectrum at mid- to high frequencies and an increase in spanwise length scale in the trailing-edge region due to flow channelling effects. For the finely distributed finlets, the flow is observed to behave similarly to that of a permeable backward-facing step, with significant suppression of the high-frequency pressure fluctuations but an elevation at low frequencies. Furthermore, the convection velocity is observed to reduce downstream of all finlet treatments. The trailing-edge surface pressure spectrum results have shown that, in order to obtain maximum unsteady pressure reduction, the finlet spacing should be of the order of the thickness of the inner layer of the boundary layer. A thorough study is provided for understanding of the underlying physics of both categories of finlets and their implications for controlling the flow and noise generation mechanism near the trailing edge.

scholarly journals Effect of Boundary Layer and Rotor Speed on Broadband Noise from Wind Turbines

2019 ◽  
Author(s):  
Vasishta Bhargava ◽  
Rahul Samala
Keyword(s):  
Boundary Layer ◽  
Power Level ◽  
Broadband Noise ◽  
Aerodynamic Noise ◽  
Sound Power ◽  
Rotor Speed ◽  
Trailing Edge ◽  
Wind Speeds ◽  
Edge Surface ◽  
Blade Tip

Trailing edge surface of aerofoil is an important source of broadband aerodynamic noise production. In this paper, three aerofoil self-noise mechanisms from turbulent boundary layer near trailing edge surface are studied. Numerical computations were performed for a three bladed 2 MW horizontal axis upwind turbine of blade length 37 m and source height of 80 m, for wind speeds of 8-15 m/s. A weighted 1/3rd octave band sound power levels (SPL) are evaluated for receiver located at distance of total turbine height and at 2 m above ground. The results obtained for sound power level using baseline models showed maximum values occurring between 300 Hz and 1 kHz region of spectrum. The trends for BPM model showed a reduction of ~2 dBA near 1 kHz region of spectrum at 10 m/s, but Grosveld’s and Lowson model were identical and agreed over the entire spectrum. The effect of rotational speed on sound power levels using three baseline models are illustrated at a wind speed of 8 m/s for 2 MW turbine. Results showed that for a change of ±10% rotor speed from the rated value, there is an increase of 2 to 6 dBA over the entire sound spectrum due to differences in blade tip speed.

Mass transfer between a plane surface and an impinging turbulent jet: the influence of surface-pressure fluctuations

1982 ◽  
Vol 119 ◽  
pp. 91-105 ◽  
Author(s):  
K. Kataoka ◽  
Y. Kamiyama ◽  
S. Hashimoto ◽  
T. Komai
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Flat Plate ◽  
Stagnation Point ◽  
Surface Pressure ◽  
Velocity Gradient ◽  
Large Scale ◽  
Pressure Fluctuations ◽  
Wall Region ◽  
Local Measurement

Local measurement of the mass-transfer rate and velocity gradient when an axisymmetric jet impinges on a flat plate was carried out using an electrochemical technique. Local measurement of the surface pressure on the flat plate was carried out separately using piezoelectric pressure transducers. The stagnation-point mass-transfer coefficient reaches a maximum when the flat plate is placed at 6 nozzle diameters from a convergent nozzle. It has been confirmed that the mass transfer to the flat plate for a high Schmidt number is greatly enhanced owing to the velocity-gradient disturbances in the wall region of the boundary layer, while the momentum transfer is insensitive to such disturbances. The relative intensity of the velocity-gradient fluctuations on the wall has an extremely large value at and near to the stagnation point, and decreases downstream, approaching a large constant value.These velocity-gradient disturbances are not due to the usual interaction of Reynolds stress with the shear stress of the mean flow, but are due to the interaction with the surface-pressure fluctuations converted from the velocity fluctuations of the oncoming jet.The three co-ordinate dimensions of large-scale eddies are calculated from the auto- and spatial correlations of the surface-pressure fluctuations. It is considered that such large-scale eddies play an important role in the production of a velocity-gradient disturbance in the wall region of the boundary layer from the velocity turbulence of the approaching jet.

scholarly journals Pressure fluctuations induced by a hypersonic turbulent boundary layer

2016 ◽  
Vol 804 ◽  
pp. 578-607 ◽  
Author(s):  
Lian Duan ◽  
Meelan M. Choudhari ◽  
Chao Zhang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Surface Pressure ◽  
Free Stream ◽  
Spatial Scales ◽  
Pressure Fluctuations ◽  
Propagation Speed ◽  
Dominant Frequency ◽  
Mean Velocity ◽  
Taylor’S Hypothesis

Direct numerical simulations (DNS) are used to examine the pressure fluctuations generated by a spatially developed Mach 5.86 turbulent boundary layer. The unsteady pressure field is analysed at multiple wall-normal locations, including those at the wall, within the boundary layer (including inner layer, the log layer, and the outer layer), and in the free stream. The statistical and structural variations of pressure fluctuations as a function of wall-normal distance are highlighted. Computational predictions for mean-velocity profiles and surface pressure spectrum are in good agreement with experimental measurements, providing a first ever comparison of this type at hypersonic Mach numbers. The simulation shows that the dominant frequency of boundary-layer-induced pressure fluctuations shifts to lower frequencies as the location of interest moves away from the wall. The pressure wave propagates with a speed nearly equal to the local mean velocity within the boundary layer (except in the immediate vicinity of the wall) while the propagation speed deviates from Taylor’s hypothesis in the free stream. Compared with the surface pressure fluctuations, which are primarily vortical, the acoustic pressure fluctuations in the free stream exhibit a significantly lower dominant frequency, a greater spatial extent, and a smaller bulk propagation speed. The free-stream pressure structures are found to have similar Lagrangian time and spatial scales as the acoustic sources near the wall. As the Mach number increases, the free-stream acoustic fluctuations exhibit increased radiation intensity, enhanced energy content at high frequencies, shallower orientation of wave fronts with respect to the flow direction, and larger propagation velocity.

scholarly journals Turbulent boundary-layer noise: direct radiation at Mach number 0.5

2013 ◽  
Vol 723 ◽  
pp. 318-351 ◽  
Author(s):  
Xavier Gloerfelt ◽  
Julien Berland
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
High Speed ◽  
Stokes Equations ◽  
Wall Pressure ◽  
Aerodynamic Noise ◽  
Mean Flow ◽  
Low Frequencies ◽  
Frequency Space ◽  
Aerodynamic Pressure

AbstractBoundary layers constitute a fundamental source of aerodynamic noise. A turbulent boundary layer over a plane wall can provide an indirect contribution to the noise by exciting the structure and a direct noise contribution. The latter part can play a significant role even if its intensity is very low, explaining why it is difficult to measure. In the present study, the aerodynamic noise generated by a spatially developing turbulent boundary layer is computed directly by solving the compressible Navier–Stokes equations. This numerical experiment aims at giving some insight into the noise radiation characteristics. The acoustic wavefronts have a large wavelength and are oriented in the direction opposite to the flow. Their amplitude is only 0.7 % of the aerodynamic pressure for a flat-plate flow at Mach 0.5. The particular directivity is mainly explained by convection effects by the mean flow, giving an indication about the compactness of the sources. These vortical events correspond to low frequencies and thus have a large lifetime. They cannot be directly associated with the main structures populating the boundary layer such as hairpin or horseshoe vortices. The analysis of the wall pressure can provide a picture of the flow in the wavenumber–frequency space. The main features of wall pressure beneath a turbulent boundary layer as described in the literature are well reproduced. The acoustic domain, corresponding to supersonic wavenumbers, is detectable but can hardly be separated from the convective ridge at this relatively high speed. This is also due to the low frequencies of sound emission as noted previously.

Numerical study of acoustic radiation due to a supersonic turbulent boundary layer

2014 ◽  
Vol 746 ◽  
pp. 165-192 ◽  
Author(s):  
Lian Duan ◽  
Meelan M. Choudhari ◽  
Minwei Wu
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Numerical Simulations ◽  
Surface Pressure ◽  
Free Stream ◽  
Acoustic Radiation ◽  
Numerical Study ◽  
Near Field ◽  
Pressure Fluctuations ◽  
Spatial Coherence

AbstractDirect numerical simulations are used to examine the pressure fluctuations generated by fully developed turbulence in a Mach 2.5 turbulent boundary layer, with an emphasis on the acoustic fluctuations radiated into the free stream. Single- and multi-point statistics of computed surface pressure fluctuations show good agreement with measurements and numerical simulations at similar flow conditions. Consistent with spark shadowgraphs obtained in free flight, the quasi-homogeneous acoustic near field in the free-stream region consists of randomly spaced wavepackets with a finite spatial coherence. The free-stream pressure fluctuations exhibit important differences from the surface pressure fluctuations in amplitude, frequency content and convection speeds. Such information can be applied towards improved modelling of boundary layer receptivity in conventional supersonic facilities and, hence, enable a better utilization of transition data acquired in such wind tunnels. The predicted acoustic characteristics are compared with the limited available measurements. Finally, the numerical database is used to understand the acoustic source mechanisms, with the finding that the supersonically convecting eddies that can directly radiate to the free stream are confined to the buffer zone within the boundary layer.

Pressure fluctuations on an oscillating trailing edge

1989 ◽  
Vol 203 ◽  
pp. 307-346 ◽  
Author(s):  
Thomas Staubli ◽  
Donald Rockwell
Keyword(s):  
Surface Pressure ◽  
Vortex Shedding ◽  
Pressure Fluctuations ◽  
Resonant Peak ◽  
Trailing Edge ◽  
Vortical Structures ◽  
Instantaneous Pressure ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency ◽  
Edge Displacement

Turbulent boundary layers separating from a blunt trailing edge give rise to organized vortical structures in the downstream wake. The perturbation of this inherent flow-instability at f0 by controlled oscillations of the edge at fe produces corresponding, organized components of unsteady surface pressure along the edge. For edge excitation near the ‘natural’ vortex shedding frequency f0, the phase between the local pressure fluctuations and the edge displacement shows large changes for small changes in excitation frequency. Moreover, in this range of excitation, there is quenching (or attenuation) of the surface pressure component at f0 and resonant peaking of the component at fe. These phenomena are related to the change in sign of the energy transfer between the fluid and the body. Integration of the instantaneous pressure distributions along the surfaces of the edge leads to the instantaneous lift at fe and f0 acting upon the oscillating trailing edge. The location of the lift varies as the cotangent of the dimensionless time during an oscillation cycle. When the edge is excited near, or at, the natural vortex shedding frequency, there is a resonant peak in the amplitude of oscillation of the lift location at fe; that at f0 is invariant. Moreover, the mean location of the lift at fe undergoes abrupt changes in this region of excitation. Flow visualization allows determination of the phasing of the organized vortical structures shed from the trailing edge relative to the edge displacement. Modulation of the flow structure at the frequencies f0 and fe, as well as interaction of small-scale vortices at high excitation frequencies, was observed. These aspects of the near-wake structure are related to the instantaneous pressure field.

Numerical Simulation of Turbulent Flow Past a Serrated Airfoil

2011 ◽  
Author(s):  
Changhwa Han ◽  
Takeshi Omori ◽  
Takeo Kajishima
Keyword(s):  
Turbulent Flow ◽  
Pressure Fluctuations ◽  
Density Variation ◽  
Aerodynamic Noise ◽  
Experimental Investigations ◽  
Frequency Noise ◽  
Trailing Edge ◽  
Eddy Simulation ◽  
Naca0012 Airfoil ◽  
Edge Side

Despite a lot of experimental investigations, the effect of airfoil serrations on the reduction of discrete frequency noise (DFN) is not fully understood. We apply the large-eddy simulation (LES) to the turbulent flow around the NACA0012 airfoil without angle of attack in a uniform stream. In this case, a major source of aerodynamic noise is quasi two-dimensional spanwise vortices, which take place near the trailing edge. We therefore investigate the effect of serration in the trailing edge side. The depth of the serration is 10% of chord length. To take into account the weak compressibility at low Mach number, we made a particular modification to the pressure equation. One equation dynamic model for the subgrid scale stress is used for LES. These techniques have originally been developed in our research group. The serration successfully reduced the pressure fluctuations on the surface of the airfoil near the trailing edge. The observed structure of the density variation suggests that this modification contributes to the reduction of sound source.

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