Instantaneous Force Coefficients Research Articles

Investigation on the unsteady aerodynamic performance of the Drela AG 08 and Boeing B29 airfoils subjected to heaving motion

This article aims to investigate the unsteady flow characteristics of Drela AG08 and Boeing B29 airfoils when subjected to heaving motion. The influence of heave amplitude, heave frequency in terms of instantaneous force coefficients of the airfoil is investigated at Re = 5 × 104. Furthermore, the flow field investigations were carried out to analyze the Leading Edge Vortex (LEV) formation and its convection airfoil’s suction side. From the findings, a variation in the oscillation amplitude of force coefficients was observed and a non-sinusoidal behavior was also noticed. Additionally, a remarkable change in the instantaneous force coefficients (both qualitatively and quantitatively) is also observed with respect to the slight change in aforementioned parameters. A transition in the wake pattern from drag inducing to thrust producing is observed under certain conditions. It is observed that the flow separates at the leading edge by forming LEV on both airfoil’s suction side and pressure side. It is also noticed that whenever the flow departs the trailing edge, these vortices convect downstream of the airfoil and form alternating pattern of coherent vortices. With the increase in number of cycles, these vortices interfere with the previously shed vortices and affects the airfoil’s aerodynamic characteristics. From the investigations, it is observed that, by controlling the system parameters, the adverse effects of the LEV can be minimized.

Comparative Assessment on Unsteady Aerodynamics of Thin and Thick Airfoils Subjected to Pitching Motion

This article presents the comparison between the unsteady flow characteristics of thick (t/c ≥ 6%) and thin (t/c ≤ 6%) airfoils performing pitching motion at Reynolds number Re = 3.6 × 10 5 . For the analysis, the considered airfoils are NACA 0012 as thick and symmetric airfoil and GOE 417A as thin and highly cambered airfoil. The analysis in this study is restricted to only two airfoils because from the preliminary investigations, similar aerodynamic characteristics with slight variation in their flow field development under same conditions were observed. The influence of reduced frequency, pitching amplitude and mean angle of attack on the airfoil’s aerodynamic performance in terms of instantaneous force coefficients is thoroughly investigated. Further, the flow surrounding the airfoils is also investigated. It is noticed that the aforementioned parameters substantially affect the instantaneous force coefficients (both qualitatively and quantitatively). Under certain conditions, the formation of reverse Karman vortex street was observed indicating airfoil’s thrust force generation. It must be noted that the conditions where the thrust force generation occurs for thick and symmetric airfoils like NACA 0012 are quite different from that of thin and cambered airfoils like GOE 417A. Additionally, a drastic change in the flow field around the airfoils is observed under similar conditions. For NACA 0012 airfoil, the formation and convection of the Leading Edge Vortex (LEV) under certain conditions has detrimental effects on its aerodynamic characteristics as it encourages early flow separation. However, the same cannot be true for thin and highly cambered airfoils like GOE 417A. Under the similar conditions with negligible variation, the LEV contributes toward providing extra suction over the GOE 417A airfoil’s suction side. This significantly improves airfoil’s aerodynamic characteristics.

Unsteady aerodynamics of plunging cambered foil at low Reynolds number

The present work focuses on the investigation of unsteady aerodynamics of plunging cambered foil with conditions corresponding to that of a Micro Air Vehicle. The effect of non-dimensional plunge amplitude, plunging frequency and the dimensionless plunge velocity on the propulsive performance of a harmonically plunging cambered foil are studied by varying Reynolds number from 2.5 × 104 to 7.5 × 104. Further, the flow field around the foil is analysed to understand the influence of the above-mentioned parameters and the reasons behind the positive thrust force generation. It is observed that these investigated parameters remarkably influence the instantaneous force coefficients. It is also noticed that these parameters have a significant effect on the topology of the leading edge vortex which can dramatically change the aerodynamic performance of the foil. It is observed that the Reynolds number also has a significant influence on the drag-thrust transition.

Numerical Investigation of Flow Over Oscillating Cambered Foil at Low Reynolds Number

Abstract This paper aims to investigate the effect of pitching amplitude, mean angle of attack and frequency of oscillation on the propulsive characteristics of a harmonically oscillating cambered airfoil at Reynolds number of 5×104. The flow field around the foil is analyzed to study the influence of the above parameters and to understand the reasons behind the positive thrust force generation. It is observed that the instantaneous force coefficients of the airfoil vary both qualitatively and quantitatively with respect to change in the above-mentioned parameters. A transition of wake from Karman vortex to reverse Karman vortex is observed. It is also observed that the formation and convection of the leading edge vortex over the airfoil surface has a strong impact on the drag-thrust transition. It is also noticed that the change in the location of the pitching axis has a significant influence over thrust force generation and propulsion efficiency of the airfoil.

Insights into the physics of dominating frequency modes for flow past a stationary sphere: Direct numerical simulations

Direct numerical simulations are carried out for an incompressible flow past a stationary sphere, in the range of 100 ≤ Re ≤ 1000. It is found that the first instability occurs as the axisymmetric wake undergoes breakage at Re ≥ 250. Adding small perturbations to the flow showed that the preferred direction of breakage of the axisymmetric wake and the corresponding contribution of the y and z-direction lift coefficients are highly sensitive and get randomly affected even due to slightest perturbations that might get induced. The second instability arises at Re = 300 as large-scale hairpin shaped structures are formed and shed periodically at frequency StVS = 0.134. At Re = 350, the highly regular hairpin shedding pattern undergoes a quasiperiodic change. From the Q-criterion isosurface, we observed that the quasiperiodicity is induced due to the formation and shedding of secondary hairpin structures which are alongside the primary ones. These secondary hairpin structures are of discernable orientations and are shed 4 times slower as compared to the primary hairpins at Re = 350. Identification of these secondary hairpin structures confirms the hypothesis of wake modulation. The low-frequency mode (Stm) is captured when energy spectral analysis is performed on the surface integrated instantaneous force coefficients and on the radial velocities. The low-frequency mode further exists at all higher Re, exhibiting a gradual increase in Stm. At Re ≥ 800, shear layer instabilities are manifested, demonstrating a characteristic peak at StKH = 0.32 in the energy spectra, rendering the mean lift coefficients to become zero again.

On the aerodynamic forces on heaving and pitching airfoils at low Reynolds number

The influence that the kinematics of pitching and heaving 2D airfoils has on the aerodynamic forces is investigated using direct numerical simulations and a force decomposition algorithm. Large-amplitude motions are considered (of the order of one chord), with moderate Reynolds numbers and reduced frequencies of order $O(1)$, varying the mean pitch angle and the phase shift between the pitching and heaving motions. Our results show that the surface vorticity contribution (viscous effect) to the aerodynamic force is negligible compared with the contributions from the body motion (fluid inertia) and the vorticity within the flow (circulation). For the range of parameters considered here, the latter tends to be instantaneously oriented in the direction normal to the chord of the airfoil. Based on the results discussed in this paper, a reduced-order model for the instantaneous aerodynamic force is proposed, taking advantage of the force decomposition and the chord-normal orientation of the contribution from vorticity within the flow to the total aerodynamic force. The predictions of the proposed model are compared with those of a similar model from the literature, showing a noticeable improvement in the prediction of the mean thrust, and a smaller improvement in the prediction of the mean lift and the instantaneous force coefficients.

Numerical Investigations into the Nonsinusoidal Motion Effects on Aerodynamics of a Pitching Airfoil

The growing applications of wind turbine impose the need for accurate flow solutions of airfoils with different pitching motions. In this paper an adjustable parameter K is employed to realize various nonsinusoidal motions and the effects of nonsinusoidal motion on pitching airfoil aerodynamics of a 2-D NACA0012 airfoil are numerically studied at Re=1.35×105. The nonsinusoidal motion effects on the instantaneous force coefficients and flow patterns are studied at different unsteady parameters (amplitude of oscillation, θ; reduced frequency, k). The results reveal that the nonsinusoidal motion noticeably affects the instantaneous force coefficients, maximum lift and drag coefficients. Besides, the formation of leading edge vortices and flow structures are also affected by the nonsinusoidal motion.

Numerical investigations into the asymmetric effects on the aerodynamic response of a pitching airfoil

The effects of asymmetric sinusoidal motion on pitching airfoil aerodynamics were studied by numerical simulations for 2-D flow around a NACA0012 airfoil at Re=1.35×105. Various unsteady parameters (amplitude of oscillation, d; reduced frequency, k) were applied to investigate the effect of asymmetry parameter S on the instantaneous force coefficients and flow patterns. The results reveal that S has a noticeable effect on the aerodynamic performance, as it affects the instantaneous force coefficient, maximum lift and drag coefficient, hysteresis loops and the flow structures.

Numerical study of large amplitude, nonsinusoidal motion and camber effects on pitching airfoil propulsion

The effects of large amplitude and nonsinusoidal motion on pitching airfoil aerodynamics for thrust generation were numerically studied with a 2-D NACA0012 airfoil used, and various 2-D NACA asymmetric airfoils were applied for camber effect study. The large amplitude effect study has been undertaken over a wide range of reduced frequency k (from 6 to 18) and pitching amplitude θ (from 5° to 30°) at Re=1.35×104 with sinusoidal pitching profile used. It is shown that the large pitching amplitude results in much more thrust generated than that at low pitching amplitude and the increase of thrust with amplitude becomes slow when the amplitude reaches some degree. However, the propulsive efficiency noticeably decreases with the increase of θ at a fixed k.An adjustable parameter K was employed to realize various nonsinusoidal motions and the effect of nonsinusoidal motion was investigated with various unsteady parameters (θ, k) applied. The results reveal that nonsinusoidal motion has a noticeable effect on the aerodynamic performance, as it affects the instantaneous force coefficients, maximum thrust coefficients and flow structures. An increase in K results in a better thrust generation performance at fixed θ and k, especially for K>0. It is also shown that the larger K noticeably influences the wake pattern and induces a stronger reverse von Karman vortex street in the wake, which in turn leads to the increased thrust. The camber study was performed on various 2-D NACA airfoils with different cambers and camber locations undergoing sinusoidal pitching motion at θ=5° and Re=1.35×104. It is found that varying camber offers little improvement in thrust generation performance.

Flow field characteristics study of a flapping airfoil using computational fluid dynamics

The flow field of a flapping airfoil in Low Reynolds Number (LRN) flow regime is associated with complex nonlinear vortex shedding and viscous phenomena. The respective fluid dynamics of such a flow is investigated here through Computational Fluid Dynamics (CFD) based on the Finite Volume Method (FVM). The governing equations are the unsteady, incompressible two-dimensional Navier-Stokes (N-S) equations. The airfoil is a thin ellipsoidal geometry performing a modified figure-of-eight-like flapping pattern. The flow field and vortical patterns around the airfoil are examined in detail, and the effects of several unsteady flow and system parameters on the flow characteristics are explored. The investigated parameters are the amplitude of pitching oscillations, phase angle between pitching and plunging motions, mean angle of attack, Reynolds number (Re), Strouhal number (St) based on the translational amplitudes of oscillations, and the pitching axis location ( x/ c). It is shown that these parameters change the instantaneous force coefficients quantitatively and qualitatively. It is also observed that the strength, interaction, and convection of the vortical structures surrounding the airfoil are significantly affected by the variations of these parameters.