Glider stall angle and pitch sweep
The goal of this tutorial is to complete a pitch sweep to assess the glider stall angle for the geometry already simulated in the previous tutorial.
In particular we will progressively increase the glider plane pitch angle and therefore the angle of attack of the wing profiles. This will progressively increase the wing load until the flow separates from the wing surfaces until we reach the glider stall angle. This is a very important aerodynamic parameter of any plane design as it defines which is the range at which the plane is safe to fly.
Create new project, geometry and simulation
SimWorks manager allows the handling and definition of multiple simulations at the same time by defining new projects, geometries and simulations:
- Click on the Create new project icon in the Simulation manager window
- Create a new geometry
- Create a new simulation
- Rename the new project as Glider project, the geometry as Pitch sweep and finally the simulation as Glider
Once opened please follow the exact same steps as in the Glider tutorial to complete the first simulation as SimWorks Manager includes all the features and commands of SimWorks.
To properly assess the glider stall angle it is important to define proper outer domain dimensions and local mesh parameters capable of capturing the correct boundary layer evolution on the glider profiles:
- The mesh base size and point in mesh parameters are the same as the previous tutorial
- We will increase the surface mesh level to 5, SimWorks Manager advanced meshing capabilities allows to define a minimum and a maximum refinement levels. In this case let’s define 5 5 to force the mesh to that minimum dimension. The edge level should also be 5
- To get the correct value of YPlus it is critical to get the first cell height correct, in our case we will define a First cell height of 0.001, Number of layers 3 and Expansion ratio 1.2.
- As we are going to increase the glider angle of attack until it reaches the stall angle it is necessary to increase the outer domain dimensions. Let’s define it in the Regions tab with Origin -12 0 0 and Size 60 40 20
Complete the Run setup and Mesh phases of the first geometry as already seen in the first tutorial, once the mesh is complete load it in the Fields viewer right clickinng on it → Fields → Load.
Looking in detail at the mesh image it is evident that the mesher has correctly defined 3 prism layers with the mesh parameters defined in the previous point.
It is also possible to automatically define a sweep of multiple simulations changing the base parameters and run new simulations to assess the results, the operation can be completed both before and after having run the simulation. We will complete a pitch sweep of our glider to analyse the stall point.
Once the first simulation is available the first step is to create a new sweep:
- Right click on the Simulation manager window → New
- A new popup window with the simulation parameters will appear, here it is possible to specify which are the parameters of the sweep. In our case we will define 3 new CFD cases by defining a rotation angle along Y which defines the pitch sweep in our coordinates system
- Define the starting point as 0 and the final angle as 25 degrees
- The total number of simulations is automatically calculated and it is 6 in our case, this includes the starting configuration of 0 angle of attack
- Click Create to complete the sweep definition
Once the sweep is defined 5 new CFD simulations will appear in the Simulation manager window for a total of 6, all the simulations will share the exact same parameters as the original simulation we started from a part of the parameter we want to sweep, in our case the angle of incidence.
Looking at the geometry it is evident that the glider is always in the same position across all the 6 simulations but the outer domain (OD) angle is progressively increased. This way it will be possible once all the simulations are completed to directly compare the surface results on the glider surface.
Run the simulations
- Complete the setup phase for all the simulations by selecting each simulation and by clicking on the Run setup button of each one in the relevant Simulation editor window. This procedure and the following ones are exactly the same as the base glider tutorial.
- Now it is possible to run the mesh phase for all the simulations in parallel by clicking on the Mesh geometry icon of each simulation after selecting it. This way the meshing happens at the same time for all the simulations
- Follow exactly the same procedure as the meshing phase but for the running phase by clicking on the Run simulation icon
- All the meshing phases and the running phases can happen in parallel and at the same time
Fields viewer - 3D Postprocessing
Once all the simulations are completed it is possible to visualise all the results at the same time in the Fields viewer window, this feature is only available in SimWorks Manager and allows to get the differential effect for each increase of incidence comparing the pressure field of each angle of attack with the horizontal flying case. It is also possible to plot the different between each single aerodynamic result and the original case (multiple simulations delta) by activating it with the Show/Hide delta in the Fields viewer.
- Right click on each simulation in the Simulation manager window and select Fields → Load as already done in the glider tutorial
- In this case multiple simulations will be loaded in the same Fields window
Select the variable pressure (p) with range from -500 Pa to +500 Pa and trigger the delta with range -150 Pa to +150 Pa. Looking in detail at the sweep it is evident that increasing the angle of attack:
- The glider wings are initially picking up suction in a consistent way both at 5 deg and 10 deg of incidence
- At 15 deg of incidence some portions of the wing are starting to struggle and we can see that for instance between the wings and the fuselage the suction is actually reduced
- Beyond that point both at 20 deg and 25 deg the suction is actually reduced and we are clearly beyond the stall point
From the analysis above we can come to the conclusion that the glider stall angle is 15 deg of pitch.
Looking now at a section normal to the glider wing and plotting Cp0 on it (Cp0 is an adimensional number defining the flow energy, when it is 1 the flow energy is maximum). Define a range from -1 to 1 and a delta range from -0.5 to + 0.5, it is evident from the delta plot that at 15 deg and beyond the Cp0 is showing a clear drop on the wings suction sides, with a progressively increasing area of low energy flow. Looking at the most extreme angle 25 deg it is evident that most of the wing surface is now separated:
Conclusions - glider stall angle of 15 deg
SimWorks Manager powerful sweep functionality can be used to sweep any angle parameter which allows to define a roll sweep or a yaw sweep in the same manner as the current pitch sweep shown in this tutorial. Once defined it is possible to check the aerodynamic delta between different angles as done in the previous example to ultimately complete an aerodynamic characterization of the given geometry.
The final glider stall angle from the analysis was 15 deg was in good agreement with typical average plane critical angle of attack. It is important to stress that this angle is a function of the wing aerodynamic profile and can be increased with devoted profiles or adding slats elements in front of the wings.
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