Porous Airfoils Research Articles

Numerical simulation study of the effect of porous media passive flow control on the propulsion performance of airfoils in kármán vortex streets

This study investigates the influence of porous media on the propulsion performance of airfoils in eddy currents. The passive motion of the airfoil was simulated using overlapping grid technology and the six degrees of freedom method. Research has found that porous media reduces the traction of fixed airfoils and reduces surface turbulence intensity. In the released state, the porous airfoil horizontal movement is slower and the lateral displacement decreases. Combining the two results in a longer forward distance for the NACA0024 thick airfoil, but the effect depends on the release time. This study may inspire the design of underwater robots.

Effects of a Nonlinear Boundary Condition on the Steady Aerodynamics of Porous Airfoils

This paper considers the lift coefficient of an airfoil with nonzero prescribed thickness and camber, along with a prescribed porosity distribution in a steady incompressible flow. In similar prior work, considering a Darcy-type porosity condition on the airfoil surface provides a Fredholm integral equation that can be solved exactly for Hölder-continuous porosity distributions. However, the generated lift predicted by the model diverges from the experimental data for porous airfoils with large values of the porosity parameter. This indicates a fundamental characteristic is missing in the mathematical model. Consequently, in this paper, the (linear) Darcy porosity condition is replaced by the (nonlinear) Forchheimer porosity condition. The Forchheimer porosity condition is decomposed into linear sections and furnishes a modified system of Fredholm integral equations, enabling an approximate solution by superposition for Hölder-continuous porosity distributions. The solution is then verified against the SD7003 airfoil results, and the comparison shows better agreement than considering the Darcy porosity condition. The methodology and results presented here may be combined with previous work on the aeroacoustics of porous airfoils with a Forchheimer boundary condition to address the conflicting aims of improving aerodynamic performance while reducing unwanted aeroacoustic emissions.

Large-Eddy Simulation of Low-Reynolds-Number Flow Around Partially Porous Airfoils

This paper uses the large-eddy simulation framework in OpenFOAM to investigate the flow characteristics around two-dimensional partially porous airfoils with symmetrical flat geometries, which were used in a previous experiment. The effects of a porous medium near the trailing edge are investigated at a chord Reynolds number of 350,000. Two porous media are modeled to investigate how the porous medium influences the flow around the airfoil and aerodynamic performance: One is a normal porous medium, defined as a homogeneous and isotropic material with identical properties in all directions. The other medium consists of straight holes perpendicular to the airfoil surface, allowing fluid to pass through unidirectionally. It is observed that drag and lift forces vary, depending on the internal structure of the porous medium. The size of the laminar separation bubbles also varies, depending on the parameters of the porous medium, such as the internal structure, the length of the porous section, and the porosity. It is shown that an airfoil with a low-porosity unidirectional porous medium increases the lift-to-drag ratio at high angles of attack as compared with the nonporous airfoil.

Numerical Study on Flow and Noise Characteristics of an NACA0018 Airfoil with a Porous Trailing Edge

An airfoil with a porous trailing edge has a low noise emission; thus, using a porous medium is a good technique for further reduction of wind turbine noise. In this paper, to reduce airfoil trailing edge noise while minimizing the negative influence of a porous medium on aerodynamic performance, a new filling method is proposed such that a porous medium is only used in the suction side half of the trailing edge, which is more sensitive to the noise generation. The large eddy simulation (LES) technique for flow and the Ffowcs Williams and Hawkings (FW-H) method for acoustics are used. At a Reynolds number of 2.63 × 105 and various angles of attack, an NACA0018 airfoil profile with a porous trailing edge covering 20% of the chord is studied under two porous configurations, namely a fully porous and a suction-side porous trailing edge type. The results show that the flow direction, velocity magnitude, and their distributions along the boundary layer of the two porous airfoils are significantly modified due to the presence of the porous medium. The fluctuation of the pressure coefficient and the increase in the boundary layer thickness are significant at low angles of attack. As compared to the solid airfoil counterpart, the noise radiation from the newly proposed suction-side porous airfoil achieves a noise reduction of 4.3 dB at an angle of attack α = 0°, and a noise reduction of 4.07 dB at an angle of attack α = 2°.

Experimental investigation of turbulent coherent structures interacting with a porous airfoil

The flow field on solid and porous airfoils subjected to turbulence shed by an upstream cylindrical rod and the corresponding far-field noise radiations are studied through particle image velocimetry (PIV) and microphone measurements, respectively. Three different Reynolds numbers based on the rod diameter are considered in a range between $${2.7\times 10^4}$$ and $${5.4\times 10^4}$$ , and two porous airfoil models are tested to analyze the influence of the design elements of the permeable treatment. A standard proper orthogonal decomposition (POD) algorithm is employed to band filter the different length scales that characterize the turbulent flow, making it feasible to determine which turbulence scales are affected by porosity. The aeroacoustic results indicate that the porous treatment of the wing profile leads to a noise reduction at low frequencies and a noise regeneration at high frequencies due to surface roughness. The investigation on the flow field shows that the main effect of porosity is to mitigate the turbulent kinetic energy in the stagnation region, attenuating the distortion of turbulence interacting with the airfoil surface. The application of the POD algorithm indicates that this effect acts mainly on the largest scales of turbulence.

Numerical Study on Flow and Noise Characteristics of a NACA0018 Airfoil with a Porous Trailing Edge

In this paper, the effects of porous trailing edge on airfoil aerodynamic and aeroacoustic characteristics are investigated. In order to reduce the airfoil trailing edge noise and in the mean while to minimize the negative influence on aerodynamic performance, a new filling method is proposed such that porous medium is only used in the suction side half of the trailing edge which is more sensitive to the noise generation. The Large Eddy Simulation (LES) for flow and Ffocws-Williams and Hawkings (FW-H) methods for acoustics are used. At a Reynolds number of 2.63×105 and various angles of attack, the NACA0018 airfoil profile with 20% chords porous trailing edge is studied under two porous configurations, namely a fully-porous and a suction-side-porous trailing edge types. The results show that the flowfield of the porous airfoils are also influenced due to the presence of the porous medium. The noise radiation from the newly proposed porous airfoil achieves a noise reduction of 4.3dB at an angle of attack α=0°, and 4.07dB at an angle of attack α=2°. On the other hand, the loss of lift force is about 0.8%-1.5% in the angle of attack range of 0~10°.

Self Noise Reduction and Aerodynamics of Airfoils with Porous Trailing Edges

The application of open-porous materials is a possible method to effectively reduce the aerodynamic noise of an airfoil. However, the porous consistency may have a negative effect on the aerodynamic performance of the airfoil, since very often the lift is decreased while the drag increases. In a recent investigation, the generation of trailing edge noise of a set of airfoil models made from different porous materials was examined experimentally. The materials were characterized mainly by their airflow resistivity. Besides the material, the chordwise extent of the porous material was varied, which was done by covering the front part of the porous airfoil with a thin, impermeable adhesive foil. Acoustic measurements were performed in an open jet wind tunnel using microphone array technology, while the aerodynamic performance was measured simultaneously using a six-component balance. In general, both the airflow resistivity and the extent of the porous material have an influence on the trailing edge noise. However, if a suitable material is chosen, the results show that a noticeable reduction of trailing edge noise is possible even with only a small chordwise extent of the porous material.

Aerodynamics of porous airfoils and wings

This paper presents novel wind tunnel test results on the aerodynamics of a symmetric thin porous airfoil and a porous rectangular half wing using a symmetric thin airfoil as its cross section, obtained by a six-component force balance. The variation of lift coefficient, drag coefficient, pitching moment, lift versus drag, the gradient of lift, and location of the aerodynamic center with respect to the angle of attack are presented as a function of the porosity. The data, where possible, are compared with the analytical results. The trend of the experimental results behaves in the same manner as the analytical solution. The measured drag coefficients of the airfoil and wing are also presented. The applicability of the standard equation relating the lift coefficient of a non-porous wing with that of a non-porous airfoil to the case of porous wings is verified by applying the equation to porous wings and validating the results using experimental data. Lift slope decreases as the porosity increases. The drag decreases at a low value of porosity and then increases as porosity increases. The standard equation for obtaining the lift coefficient of a wing from the lift coefficient of an airfoil is applicable and valid for porous wings and airfoils.

Symbolic regression modeling of noise generation at porous airfoils

Abstract Based on data sets from previous experimental studies, the tool of symbolic regression is applied to find empirical models that describe the noise generation at porous airfoils. Both the self noise from the interaction of a turbulent boundary layer with the trailing edge of an porous airfoil and the noise generated at the leading edge due to turbulent inflow are considered. Following a dimensional analysis, models are built for trailing edge noise and leading edge noise in terms of four and six dimensionless quantities, respectively. Models of different accuracy and complexity are proposed and discussed. For the trailing edge noise case, a general dependency of the sound power on the fifth power of the flow velocity was found and the frequency spectrum is controlled by the flow resistivity of the porous material. Leading edge noise power is proportional to the square of the turbulence intensity and shows a dependency on the fifth to sixth power of the flow velocity, while the spectrum is governed by the flow resistivity and the integral length scale of the incoming turbulence.

Porous airfoils: noise reduction and boundary layer effects

Porous airfoils: noise reduction and boundary layer effects

Porous Airfoils: Noise Reduction and Boundary Layer Effects

The present paper describes acoustic and hot–wire measurements that were done in the aeroacoustic wind tunnel at the Brandenburg University of Technology Cottbus on various SD7003–type airfoils made of different porous (flow permeable) materials. The objective of the research is the analysis of the turbulent boundary layer properties of porous airfoils and, subsequently, of the noise generated at the trailing edge. The influence of the porous materials, characterized by their air flow resistivity, is discussed. The acoustic measurements were performed using a planar 56–channel microphone array and the boundary layer properties were measured using constant temperature anemometry. The recorded acoustic data underwent further processing by application of advanced beamforming algorithms. A noticeable reduction of the emitted trailing edge noise was measured for the porous airfoils over a large range of frequencies. At high frequencies, some of the porous airfoils were found to generate more noise than the reference airfoil which might be due to the surface roughness noise contribution. It is found that the turbulent boundary layer thickness and the boundary layer displacement thickness of the airfoils increase with decreasing flow resistivities for both suction and pressure side. Both boundary layer thickness and displacement thickness of the porous airfoils are greater than those of a non-porous reference airfoil

Measurement of the noise generation at the trailing edge of porous airfoils

Owls are commonly known for their quiet flight, enabled by three adaptions of their wings and plumage: leading edge serrations, trailing edge fringes and a soft and elastic downy upper surface of the feathers. In order to gain a better understanding of the aeroacoustic effects of the third property that is equivalent to an increased permeability of the plumage to air, an experimental survey on a set of airfoils made of different porous materials was carried out. Several airfoils with the same shape and size but made of different porous materials characterized by their flow resistivities and one non-porous reference airfoil were subject to the flow in an aeroacoustic open jet wind tunnel. The flow speed has been varied between approximately 25 and 50 m/s. The geometric angle of attack ranged from −16° to 20° in 4°-steps. The results of the aeroacoustic measurements, made with a 56-microphone array positioned out of flow, and of the measurements of lift and drag are given and discussed.

Computational analysis of buffet alleviation in viscous transonic flow over a porous airfoil

RANSONIC flow past an airfoil at a high freestream Mach number is traditionally defined as consisting of large supersonic regions embedded within a subsonic flowfield. The pressure fluctuations initiated by the shock/boundary-layer interaction may excite intense levels of buffeting. 1 Therefore, to extend the buffet boundaries for a particular airfoil, a technique is required which is capable of controlling either the boundary layer, or alternatively, the shock strength. Boundary-layer control may be performed by employing either blowing or suction within the interaction region. Blowing has a twofold effect, namely, an increase in the boundary-layer thickness coupled with a reduction in the skin friction, whereas suction induces a contrasting decrease in boundary-layer thickness and increase in skin friction. Passive control of the shock/boundary-layer interaction is a technique which provides a combination of blowing and suction forward and aft of the shock, respectively, without the need for a pump. A passive control device (Fig. 1) consists of a porous region and plenum which are positioned at the interaction region. The static pressure rise across the shock wave results in the flow traveling through the plenum to the region of lower pressure forward of the shock position. The increased communication across the shock stimulates the development of a thicker boundary layer forward of the shock. This process effectively changes the airfoil's geometry in a manner which produces a weaker lambda shock system. Moreover, passive control's inherent damping characteristics have been shown experimentally25 to reduce the pressure fluctuations initiated by the shock/boundary-layer interaction and, therefore, delay buffet onset. In the present study, time-accurate flow over an 18%-thick porous circular-arc airfoil is computed in an attempt to ascertain whether passive control will damp, or even eliminate, the flow periodicity detected in the corresponding solid airfoil (CSA) computation,6 thus extending the airfoil's buffet boundary. The following sections briefly describe both the numerical procedure and the results. Numerical Model An explicit Navier-Stokes finite volume scheme MGENS has been developed which is capable accurately predicting viscous flow over an arbitrary airfoil using an orthogonal boundary conforming hyperbolic grid MGHYPR. A detailed description of both the

Incompressible flow around porous two-dimensional thin sail wings.

A linearized analysis has been made of a two-dimensional aerodynamic characteristics of infinitely thin, perfectly flexible, possibly porous airfoils. Jacobi polynominals have been used to express pressure distributions and airfoil shapes, considering well over the leading edge singularity. It is found that airfoil shapes become stable with increasing porosity. Furthermore, the effects of porosity on lift and center of pressure are determined as functions of angle of attack and excess length of material over the chord length.

Viscous–Inviscid Computations of Transonic Separated Flows Over Solid and Porous Cascades

A complete viscous–inviscid interaction is performed that reliably computes steady two-dimensional, subsonic and transonic attached and separated flows for cascades of airfoils. A full-potential code was coupled with both a laminar/transition/ turbulent integral boundary-layer/turbulent wake code and the finite-difference boundary-layer code using the semi-inverse methods of Carter and Wigton. The transpiration coupling concept was applied with an option for a porous airfoil with passive and active physical transpiration. Examples are presented which demonstrate that such flows can be calculated with engineering accuracy by these methods. Carter’s update formula gives smoother solutions for a strong shock than Wigton’s update formulas, although Wigton’s formulas are preferred in the early coupling cycles. The computations show that passive physical transpiration can lead to a lower drag coefficient and higher lift coefficient, a weaker shock, and elimination of shock-induced separation. The extent of the porous region and permeability factor distribution of the porous region must be chosen carefully if these improvements are to be achieved.

Porous Airfoils in Transonic Flow

Analyse demontrant que l'intensite d'onde de choc sur des profils aerodynamiques epais dans le regime transsonique est considerablement reduite par etablissement d'un ecoulement secondaire au travers de la paroi poreuse du profil (entre la region externe et sa cavite interne), cet ecoulement secondaire etant soumis a la loi de Darcy