13 Feb Calculate Lift & Drag with Paraview
The relative motion between an object and the air generates a load applied by the latter on the former called aero-dynamic force. When the mean, instead of air, is water or oil the same load is called hydro-dynamic or oleo-dynamic force. It is the very same phenomenon, that is exploited by a lot of different technologies and products to perform their task. Computational Fluid Dynamics simulations were adopted since their very first years to estimate aerodynamic forces, it is a typical/classical application. In this post we will show you how to manipulate a CFD solution file provided by CONSELF platform in order to estimate aerodynamic forces acting on a plane flying at a given speed.
Aerodynamic forces: what are they? What is their definition?
Aerodynamic forces are often encountered in everyday life: planes fly using the aerodynamic force their wing generates, race cars are equipped with multiple aerodynamic surfaces that, pushing the car against the ground, allow for a a better power transmission from the wheels to the track, catamarans have immersed wings that allow the boat to soar and “fly” above the water surface.
The aerodynamic force is typically decomposed in Lift and Drag:
- Lift is the projection of the aerodynamic force vector along the direction normal to the undisturbed velocity direction
- Drag is the projection of the aerodynamic force vector along the direction of the undisturbed velocity
So Lift is the force that keeps a plane in the air when it is cruising 9 km above the ground, acting against its weight, while drag acts in the opposite direction with respect to the thrust provided by engines. In race cars the Downforce is the Lift again, but this time it is directed as the car weight, because its goal is to keep the car pushed towards the track, while Drag acts in the same direction of car motion but against it, slowing it down.
Logical steps to calculate aerodynamic forces
When the fluid interacts with solid surfaces of the model, it is deviated from the undisturbed trajectory. To impose this variation of velocity (i.e. acceleration), the solid body imposes a force on the fluid. By the action-reaction principle, the fluid itself imposes an equal and contrary force on the solid body: the aerodynamic force. This force is applied in the form of a pressure force. The solid body will see a pressure distribution on its surface resulting from this interaction. If one integrates this pressure on the whole body, the net result will be the aerodynamic force vector.
The pressure acts always normally to the surface. Considering the X axis aligned with the wind velocity, Y orthogonal to it and Z in the span-wise direction, in order to evaluate Lift & Drag we need to project the pressure along Y and X respectively and then integrate it on the whole body.
There exists another contribution to the aerodynamic force calculation, it is given by the skin friction. It is a vector that, in order to be taken into account, has to be integrated on the body surface too. This result then has to be (vectorially) added to the pressure-related part.
“Import CONSELF solution file in Paraview and with some simple steps you can obtain an accurate estimation for Lift and Drag acting on your model in that particular condition”
How to calculate Lift and Drag with Paraview: the video tutorial
The tutorial video on the right shows you how to import CONSELF solution file in Paraview and, applying a sequence of simple filters, obtain an accurate estimation of aerodynamic forces acting on the model. In this specific case the X axis is aligned with the wind axis and Y is orthogonal with respect to it, so Lift & Drag will be oriented along Y and X respectively. In addition, for the sake of conciseness, only the pressure contribution will be considered, being the integration of the skin friction completely similar. In CONSELF complete solution file the skin friction is represented by the variable called Wall Shear Stress.
You can download the solution file to perform the same procedure clicking on this link.
Do you want to do the same thing with your model? You can use CONSELF 100% free WELCOME plan, you can activate it in few clicks here.
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