NUMERICAL STUDY OF FLOW SEPARATION CONTROL OVER A NACA 2415 AIRFOIL

This study involves numerical simulation of the flow around a NACA2415 airfoil, with a 18°angle of attack, and flow separation control using a rod, It involves putting a cylindrical rod -placing upstream of the leading edge- in vertical translator movement in order to accelerate the transition of the boundary layer by interaction between the wake behind the rod and the boundary layer. The viscous, non-stationary flow is simulated using ANSYS FLUENT 13. The rod movement is reproduced using the dynamic mesh technique and an in-house developed UDF (User Define Function).The frequency varies from 75 to 450 Hz and the considered amplitudes are 2%, and 3% of the foil chord. The frequency chosen closed to the natural frequency of separation, and the rod diameter is equal to 2% the foil cord. Our results show a substantial modification in the structure of the flow and a maximum drag reduction of 61%

NUMERICAL STUDY OF FLOW SEPARATION CONTROL OVER A NACA 2415 AIRFOIL

This study involves numerical simulation of the flow around a NACA2415 airfoil, with a 18°angle of attack, and flow separation control using a rod, It involves putting a cylindrical rod -placing upstream of the leading edge- in vertical translator movement in order to accelerate the transition of the boundary layer by interaction between the wake behind the rod and the boundary layer. The viscous, non-stationary flow is simulated using ANSYS FLUENT 13. The rod movement is reproduced using the dynamic mesh technique and an in-house developed UDF (User Define Function).The frequency varies from 75 to 450 Hz and the considered amplitudes are 2%, and 3% of the foil chord. The frequency chosen closed to the natural frequency of separation, and the rod diameter is equal to 2% the foil cord. Our results show a substantial modification in the structure of the flow and a maximum drag reduction of 61%

Optimisation du rendement propulsif d'une aile battante par la Méthode de Surface des Réponses

Dans la présente étude, le rendement propulsif d’un profil d’aile NACA0012 en mouvement de battement est optimisé à un nombre de Reynolds de Re = 1.1 × 10^4. Il s’agit d’une étude numérique 2D réalisée caractériser l’évolution du rendement propulsif en fonction des paramètres cinématiques de aile battante. Pour résoudre les équations de Navier-Stokes autour de l'aile battantes nous avons utilisé un solver 2D instationnaire avec un couplage Pression-Vitesse SIMPLEC. Pour la discrétisation spatiale on a utilisé le schéma convectif MUSCL de 3ème ordre avec une discrétisation temporelle du premier ordre. L'amplitude du mouvement de pilonnement, l’amplitude maximale du mouvement de tangage, la fréquence de battement et l’angle de phase entre ces deux mouvements sont considérés comme variables d’optimisation. Le battement du profil d’aile est réalisé grâce à l’utilisation des fonctions UDF et du module maillage dynamique, disponible dans Ansys Fluent. La Méthode des Surfaces des Réponses (MSR) est utilisé pour l’optimisation du rendement propulsif en fonction des paramètres cinématiques de l'aile. Un plan d’expérience complet avec 5 évaluations pour chaque variable a été élaboré pour conduire les simulations et obtenir les différentes combinaisons des paramètres de contrôle. Pour la prédiction de la réponse nous avons calculé les coefficients du méta-modèle de la MSR en utilisant des fonctions polynomiale . Le processus d’optimisation du méta-modèle est piloté par la technique du recuit simulé disponible sous MATLAB. Les résultats montrent que la méthode des surfaces de réponses est suffisamment robuste et donne, approximativement, les mêmes résultats que la méthode de montée de gradient. L’erreur relative entre le rendement propulsif obtenu par approximation en utilisant la RSM et celui obtenu par simulation numérique est très petite. Cela justifie très largement le recours à cette méthode. A la lumière des résultats obtenus, en plus de sa rapidité, la méthode des surfaces de réponse présente l’avantage d’être facile à implémenter, cependant, l’approximation quadratique qu’elle utilise est limitée à un certain nombre de variables d’optimisation

Numerical investigation of the effect of motion trajectory on the vortex shedding process behind a flapping airfoil

The effect of non-sinusoidal trajectory on the propulsive performances and the vortex shedding process behind a flapping airfoil is investigated in this study. A movement of a rigid NACA 0012 airfoil undergoing a combined heaving and pitching motions at low Reynolds number (11 000) is considered. An elliptic function with an adjustable parameter S (flatness coefficient) is used to realize various non-sinusoidal trajectories. The two-dimensional unsteady and incompressible Navier-Stokes equation governing the flow over the flapping airfoil is resolved using the commercial so ware STAR CCM+. It is shown that the combination of sinusoidal and non-sinusoidal mapping motion has a great effect on the propulsive performances of the flapping airfoil. The maximum propulsive efficiency is always achievable with sinusoidal trajectories. However, non-sinusoidal trajectories are found to considerably improve the propulsive force up to 52% larger than its natural value. Flow visualization shows that the vortex shedding process and the wake structure are substantially altered under the non-sinusoidal trajectory effect. Depending on the nature of the flapping trajectory, several modes of vortex shedding are identified and presented in this paper

Experimental investigation of an actively controlled automotive cooling fan using steady air injection in the leakage gap

ne pas mettre sur hal c'est déjà faitIn an axial fan, a leakage flow driven by a pressure gradient between the pressure side and the suction side occurs in the gap between the shroud and the casing. This leakage flow is in the opposite direction to the main flow and is responsible for significant energy dissipation. Therefore, many authors have worked to understand this phenomenon in order to reduce these inherent energy losses. Up to now, most of the studies reported in the literature have been passive solutions. In this paper, an experimental controlling strategy is suggested to reduce the leakage flow rate. To this end, a fan with hollow blades and a specific drive system were designed and built for air injection. Air is injected in the leakage gap at the fan periphery. The experiment was performed for three rotation speeds, five injection rates and two configurations: 16 and 32 injection holes on the fan's circumference. The experimental results of this investigation are presented in this articl

Active control of the leakage flow by air injection into the rotational shroud or the fixed carter of an axial fan composed of hollow blades

In axial fan, the static pressure difference between the suction and the pressure side of the impeller produces a leakage flow through the blade and the casing. This secondary flow occurs in the opposite direction of the working flow and has a negative impact on the overall performances. It tends to reduce the pressure coefficient, the efficiency and the fan operating range while increasing the noise level. That is why many studies dealt with ways to reduce this leakage flow. In this paper, the study focusses on the control of the secondary flow by air injection. Two ways to control this flow are compared. In a first case, the air is ejected from the fixed casing and in a second case the air exit from the rotating shrouds. In both configurations, the ejected air has a direction to counter the leakage flow. To realize the second configuration, a new method to build fan with hollow blades was developed. This new kind of fan allows having internal flows which could exit by any area of the fan. The results obtained by the active controls on the fan characteristic and the efficiency are presented in this article

Effects of Non-Sinusoidal Motion and Effective Angle of Attack on Energy Extraction Performance of a Fully- Activated Flapping Foil

Flapping foil energy harvesting systems are considered as highly competitive devices for conventional turbines. Several research projects have already been carried out to improve performances of such new devices. This paper is devoted to study effects of non-sinusoidal heaving trajectory, non-sinusoidal pitching trajectory, and the effective angle of attack on the energy extraction performances of a flapping foil operating at low Reynolds number (Re=1100). An elliptic function with an adjustable parameter S (flattening parameter) is used to simulate various sinusoidal and non-sinusoidal flapping trajectories. The flow around the flapping foil is simulated by solving Navier–Stokes equations using the commercial software Star CCM+ based on the finite-volume method. Overset mesh technique is used to model the flapping motion. The study is applied to the NACA0015 foil with the following kinetic parameters: a dimensionless heaving amplitude h0 = 1c, a shift angle between heaving and pitching motions f = 90 , a reduced frequency f = 0:14, and an effective angle of attack amax varying between 15 and 50 , corresponding to a pitching amplitude in the range q0 = 55:51 to 90:51 . The results show that, the non-sinusoidal trajectory affects considerably the energy extraction performances. For the reference case (sinusoidal heaving and pitching motions, Sh = Sq = 1), best performances are obtained for the effective angle of attack, amax = 40 . At small effective angle of attack amax 40 ), non-sinusoidal pitching motion has a negative effect. Performance improvement is quite limited with the combined motions non-sinusoidal heaving/sinusoidal pitching

Energy extraction performance improvement of a flapping foil by the use of combined foil

In this study, numerical investigations on the energy extraction performance of a flapping foil device are carried out by using a modified foil shape. The new foil shape is designed by combining the thick leading edge of NACA0012 foil and the thin trailing edge of NACA0006 foil. The numerical simulations are based on the solution of the unsteady and incompressible Navier-Stokes equations that govern the fluid flow around the flapping foil. These equations are resolved in a two-dimensional domain with a dynamic mesh technique using the CFD software ANSYS Fluent 16. A User Define Function (UDF) controls the imposed sinusoidal heaving and pitching motions. First, for a validation study, numerical simulations are performed for a NACA0012 foil undergoing imposed heaving and pitching motions at a low Reynolds number. The obtained results are in good agreement with numerical and experimental data available in the literature. Thereafter, the computations are applied for the new foil shape. The influences of the connecting area location between the leading and trailing segments, the Strouhal number and the effective angle of attack on the energy extraction performance are investigated at low Reynolds number (Re = 10 000). Then, the new foil shape performance was compared to those of both NACA0006 and NACA0012 baseline foils. The results have shown that the proposed foil shape achieves higher performance compared to the baseline NACA foils. Moreover, the energy extraction efficiency was improved by 30.60% compared to NACA0006 and by 17.32% compared to NACA0012. The analysis of the flow field around the flapping foils indicates a change of the vortex structure and the pressure distribution near the trailing edge of the combined foil compared to the baseline foils

Numerical Investigation of the Effects of Nonsinusoidal Motion Trajectory on the Propulsion Mechanisms of a Flapping Airfoil

The effect of nonsinusoidal trajectory on the propulsive performances and the vortex shedding process behind a flapping airfoil is investigated in this study. A movement of a rigid NACA0012 airfoil undergoing a combined heaving and pitching motions at low Reynolds number (Re¼11,000) is considered. An elliptic function with an adjustable parameter S (flattening parameter) is used to realize various nonsinusoidal trajectories of both motions. The two-dimensional (2D) unsteady and incompressible Navier–Stokes equation governing the flow over the flapping airfoil are resolved using the commercial software STAR CCMþ. It is shown that the nonsinusoidal flapping motion has a major effect on the propulsive performances of the flapping airfoil. Although the maximum propulsive efficiency is always achievable with sinusoidal trajectories, nonsinusoidal trajectories are found to considerably improve performance: a 110% increase of the thrust force was obtained in the best studied case. This improvement is mainly related to the modification of the heaving motion, more specifically the increase of the heaving speed at maximum pitching angle of the foil. The analysis of the flow vorticity and wake structure also enables to explain the drop of the propulsive efficiency for nonsinusoidal trajectories