Simulation Flow Around a NACA 4415 Wing Model
At the Hochschule Karlsruhe, Germany, a NACA model 4415 was tested in a wind tunnel by students as part of a fluid mechanical lecture. The experiment was done for different approach angles of the NACA-model. The experiment was also simulated with CFD-tools such as NOGRID points. We outline the physical problem and results obtained by NOGRID points for an approach angle of 20°.
The wind tunnel is modeled by a sufficiently large cuboid flow domain with "open" faces: Except for the inflow face the other five faces are outflow faces with a zero Dirichlet boundary condition for the pressure and zero Neumann boundary condition for the velocity. Using a "closed" box with only a small outflow area, opposite to the inflow area, would require a much larger fluid domain as otherwise the dynamic pressure would considerably increase.
At the inflow face the air flows in at 35.7 m/s in a circular area with a diameter of 0.35 m, outside the area the velocity is set to zero. This reflects the blast nozzle of the wind tunnel sufficiently well.
The faces of the NACA-model carry a wall slip condition. Compared to a noslip condition this yields much better numerical results as it avoids very large gradients of the velocity especially at the front of the model and it introduces only a small error.
Figure 1: A cut through the cuboid flow domain with the NACA model, the inflow is at the right-hand side
Figure 2: The smoothing length (which determines the point density) in the flow domain
Figure 3: NACA model with stream traces
Figure 4: NACA model with stream traces seen from behind the model
Figure 5: Comparison of experiment and simulation
In this case study we've computed the air flow around a NACA wing. The preprocessing requires no volume mesh generation and therefore the modelling part is very easy. So, the User can directly start to import the model and the choose the smoothing length distribution, which effectively determines the density of points for the computation. As we are interested in the pressure profile at the surface of the NACA model we increase the density around the model. The high pressure gradient at the front of the model and the thinness at the back require an even higher density (or equivalently a smaller smoothing length). The distribution is shown in figure 2.
Comparison of experiment and simulation
In figure 3 and 4 path integration yields stream traces, which show the typical wake vortex generated by the positive and negative dynamic pressure below and above the NACA-model, respectively.
In order to verify the simulation quantitatively as well, the measured pressure profile along the surface of the NACA-model is compared to the simulation. The experimental data and the simulation data are presented in figure 5. They show a very good agreement of the experiment and the simulation. The small differences are probably due to small simplifications done in the model such as the rather small fluid domain, the omission of the suspension gear of the model and the simplified blast nozzle of the wind tunnel.
NOGRID unites abilities to handle air flows and allows the simulation of any conceivable wing geometry and operation modes such as
- computation is in 2D, axis-symmetric or full 3D solving complete Navier-Stokes-Equations
- easy and intuitive setup of the air flow case
- free definable material properties by equations or curves for each fluid
- large wing geometries with small gaps or holes
- open or closed domains including moving of additional parts
- any material combination for the fluids and for the wing material (in case of heat transfer)
Easy and fast modelling: Build geometry, mesh boundary, setup the case and start computation
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