Experimental Research on Flow Separation Control using Synthetic Jet Actuators

Airplane wings can suffer from flow separation, which greatly decreases their aerodynamic per-formance. The flow separates due to the bound-ary layer possessing insufficient momentum to engage the adverse pressure gradient along the airfoil surface. Flow separation control actively influences the flow such that flow separation is delayed and airfoil performance is improved. In this research flow separation control is ap-plied on a 2D wing with a NACA0018 section with a 0.165 m chord using tangentially direct-ed synthetic jets. The actuators are located in-side the wing and the jet exits from a slot in the upper wing surface. The synthetic jet inhales the low momentum air in the boundary layer during instroke and during the outstroke the air adds momentum to the boundary layer. The actuator, with a piezo-electric diaphragm, has a slot width of 0.25 mm. With this design jet velocities up to 65 m/s have been achieved at an optimum actuation frequency of 900 Hz. A spanwise row of ten actuators is placed inside the wing, such that the slots cover 66% of the wing's span. During wind tunnel experiments forces have been measured using a balance. The tests have been performed at a fixed free stream velocity of 25 m/s (Rec = 2.73×105) and for various actuation frequencies and jet velocities. It is shown that for given actuation frequency a higher jet velocity results in a higher maximum lift coefficient and a corresponding higher stall angle. However, for the performance of the syn-thetic jet actuation, actuation frequency proves to be of greater importance than jet velocity. The best actuation frequency in combination with the maximum jet velocity possible with the present actuator is a dimensionless frequency F+ of 5.9 (1300 Hz) and a momentum coefficient cμ of 0.0014 (maximum jet velocity 32.9 m/s and Velocity Ratio of 1.32). Using these actuation parameters the lift coefficient is increased by 12% and the stall angle by 22%

Numerical Simulation of SLD Ice Accretions

In this study, computational methods are presented that compute ice accretion on multiple-element airfoils in specified icing conditions. The ¿Droplerian¿ numerical simulation method used is based on an Eulerian method for predicting droplet trajectories and the resulting droplet catching efficiency on the surface of the configuration. Flow field and droplet catching efficiency form input for Messinger's model for ice accretion. The droplet trajectory method has been constructed such that the solution of any flow-field simulation (e.g., potential-flow, Euler equations) can be used as input for the finite-volume solution method. On an unstructured grid the spatial distribution of droplet loading and droplet velocity are obtained. From these quantities the droplet catching efficiency is derived. Of special interest in this study are the Supercooled Large Droplets (SLD). The simulation of SLD requires a specific splashing model.For a single-element airfoil a good agreement is found with the Lagrangian method 2DFOIL-ICE and with experimental results. The comparison of the catching efficiency predicted by both simulation methods is good for the smaller droplets. For larger (SLD) droplets the splashing and rebound models are a significant improvement to the catching efficiency results when compared with the experimental results

Numerical Method for Ice Accretion on 3D Wings

Computer simulations of the ice accretion process provide an attractive method for analyzing a wide range of icing conditions at low cost. An ice accretion model that accurately predicts ice growth shapes on arbitrary airfoils sections is valuable for the analysis of the sensitivity of airfoils for ice accretion. Furthermore the analysis of the influence of flow variables such as airspeed and angle of attack, pressure, temperature and humidity on ice accretion is studied easily. Such an approach can also be used to assess the energy requirements necessary to prevent and/or remove ice from an airfoil. Once the method has been validated, it will provide a cost-effective means of performing icing research studies which now rely, for an important part, on experimental techniques. In this paper, a computational method is presented that computes three dimensional ice accretion on multiple-element airfoils in specified icing conditions. The main part of the method is the method to compute the distribution of the supercooled water impinging on the wing surface, which is a challenge especially for so-called super-cooled large droplets (SLD). To this aim, for a given flow field solution, the numerical method ( Droplerian) uses an Eulerian method to determine the spatial distribution of the LiquidWater Content (LWC) and the droplet velocities. To solve the equations for the droplet velocities and liquid water content distribution, Droplerian uses a Finite Volume Method for unstructured grids. Through the droplet velocities and LiquidWater Content at the surface of the airfoil the droplet catching efficiency is calculated. The method can handle a multi-disperse droplet distribution with an arbitrary number of droplet classes (bins) and contains a droplet splashing and droplet rebound model. The splashing and rebound models are indispensable for correctly treating impingement of SLD's. Once the dropletcatching efficiency and droplet impact velocity are determined, they are used as input for the icing model, which is based on Messinger's model for ice accretion

Aircraft icing in flight: Effects of impact of super cooled large droplets

In this study a computational method is presented which simulates the presence of a liquid layer on an airfoil and its effect on splashing of Supercooled Large Droplets (SLD). The thin liquid film is expected to have a significant influence on the impact behaviour of SLD. It will arise when the impacting droplets freeze only partially and leave behind a layer of runback water on top of the ice layer. The liquid film is modelled using the wall shear stress and by assuming a linear velocity profile within the water layer. The shear stress is calculated by coupling an integral boundary-layer method to a potential flow method. The SLD splashing model is extended with a deposition model that accounts for impact on a liquid film and includes the solidification time of the droplets. This solidification time is obtained using multiple approaches which are based on either planar solidification or dendritic solidification. Planar solidification is controlled by diffusion and based on the Stefan problem for heat conduction. Dendritic solidification is more rapid and mostly governed by kinetics. The comparison of the catching efficiency with experimental results for a NACA-23012 airfoil shows a significant improvement employing the new deposition model. Also, good agreement is found with the experimental results for the ice accretion on a NACA-0012 airfoil