**3. Mesh independency study**

To acquire accuracy in attaining the CFD results, and keeping in mind the computational power, it is necessary to analyze the geometry for mesh independency study. For this the model's physical properties were defined with similar inlet velocities, and physical boundary conditions were set similar for the different cases. The model chosen was SST Kω model. Grid convergence analysis was conducted on coarse, medium, and fine mesh specifications at which *CD*, *CL*, and *CM* were analyzed. This is conducted to determine the effect of mesh quality on CFD results. The number of cells and simulation time for three different cases was simulated. First set was conducted with 0° angle of attack with all control surfaces non-deflected. The second set was with 0° angle of attack with elevator deflected by +5°. The results collected shown in **Table 4** depict that the number of cells has a huge impact on the mesh independence and time period required for conducting simulations. The six meshes shown in **Table 4** demonstrate that mesh independency was acquired as per deviation from coarse level mesh to fine settings has achieved a level of stagnation for estimated parameters of *CL***,** *CD***,** and *CM*. For the fine mesh, base size reduction for different geometrical parts was set around 6% of actual geometry.

Conducted CFD simulations for above mesh settings are demonstrated in **Figure 2**. It can be noted that the CFD results demonstrated significant velocity profiles and depiction of wake generation was considered but dominance was given to shearing stress, i.e., related to the near-wall stresses. Special focus was given to the surface shearing stress, as to capture the precise effect caused due to control surface deflections. Moreover, wake dominance can be optimized further by deploying more number of cells at the aft side of aircraft geometry with more computational power.

*Mesh independency study on a 0° AoA profile and 5° elevator deflection of the aircraft; (a1) coarse mesh, (a2) medium mesh, (a3) fine mesh, (b1) CFD output with coarse mesh, (b2) CFD output with medium mesh, (b3)*

*Development of the Flight Dynamic Model (FDM) Using Computational Fluid Dynamic (CFD)…*

The FDM file is then processed using the FlightGear flight simulating software; this file is in .xml format. Moreover similar procedure is followed for checking under the JSBSim stand-alone module designed by Jon Berndt in 2004 [10]; however, it is embedded to the external image generation tool for visual effects. Input devices for aircraft control loading system used with JSBSim simulations were similar to the actual flight control loading system; however, with FlightGear simu-

lation, Logitech extreme 3d edition flight joystick was used, this is further

**Properties Actual flight FlightGear JSBSim**

Altitude 5000 fts 5000 fts 4900–5300 fts

Cruise velocity 210–220 knots 210–220 knots 210–220 knots

*Control loading and basic settings set at different simulators and during actual flight mode.*

Logitech extreme 3d edition flight joystick

Real FlightGear CIGI

Max Max Max

Similar to aircraft control loading system

Actual control loading system of aircraft

**4. CFD to FDM integration**

**Figure 2.**

*CFD output with fine mesh.*

*DOI: http://dx.doi.org/10.5772/intechopen.91895*

aggregated in **Table 5**.

Control loading system

Image generation

Throttle condition

**Table 5.**

**153**


#### **Table 4.**

*Mesh independence test on different number of meshed cells, with CFD predicted results.*

*Development of the Flight Dynamic Model (FDM) Using Computational Fluid Dynamic (CFD)… DOI: http://dx.doi.org/10.5772/intechopen.91895*

#### **Figure 2.**

The CFD software as mentioned earlier requires sufficient computer hardware to function properly. The elapsed time for analysis was almost 6 hours for a highend desktop configuration machine in year 2017. Compared to this, same analysis was done within 3 h on a high-end machine in year 2018 with new specifications. For better and faster results, cluster computers and supercomputers are used to run

The results attained for aircraft aerodynamics from the CFD simulations are then used by the FDM's aerodynamic module. Results obtained are of forces and moment coefficients for six different axes, i.e., drag, lift, side force, roll, pitch, and

To acquire accuracy in attaining the CFD results, and keeping in mind the computational power, it is necessary to analyze the geometry for mesh independency study. For this the model's physical properties were defined with similar inlet velocities, and physical boundary conditions were set similar for the different cases. The model chosen was SST Kω model. Grid convergence analysis was conducted on coarse, medium, and fine mesh specifications at which *CD*, *CL*, and *CM* were analyzed. This is conducted to determine the effect of mesh quality on CFD results. The number of cells and simulation time for three different cases was simulated. First set was conducted with 0° angle of attack with all control surfaces non-deflected. The second set was with 0° angle of attack with elevator deflected by +5°. The results collected shown in **Table 4** depict that the number of cells has a huge impact on the mesh independence and time period required for conducting simulations. The six meshes shown in **Table 4** demonstrate that mesh independency was acquired as per deviation from coarse level mesh to fine settings has achieved a level of stagnation for estimated parameters of *CL***,** *CD***,** and *CM*. For the fine mesh, base size reduction for different geometrical parts was set around 6% of actual geometry.

**Mesh resolution Coarse mesh Medium mesh Fine mesh**

Number of cells 1,810,981 2,294,045 3,628,023 CFD simulation time 1 h 30 min 2 h 02 min 2 h 15 min Estimated *CD* 9.002894e02 9.662265e02 9.033574e02 Estimated *CL* 6.521853e02 6.139491e02 7.067754e02 Estimated *CM* 6.438546e02 6.970677e02 6.398511e02

Number of cells 1,781,326 2,314,142 3,760,216 CFD simulation time 1 h 2 min 1 h 11 min 1 h 30 min Estimated *CD* 8.790877e02 9.168335e02 8.853459e02 Estimated *CL* 1.435455e03 3.072429e03 8.159027e03 Estimated *CM* 7.883882e02 5.123151e02 5.492518e02

*Mesh independence test on different number of meshed cells, with CFD predicted results.*

CFD simulations for acquiring tremendous amount of data sets.

yaw axes, with deflected surfaces at different angles.

**2.3 Post-processing**

**3. Mesh independency study**

*Computational Fluid Dynamics Simulations*

*At +5° elevator on 0° AoA*

*At 0° elevator on 0° AoA*

**Table 4.**

**152**

*Mesh independency study on a 0° AoA profile and 5° elevator deflection of the aircraft; (a1) coarse mesh, (a2) medium mesh, (a3) fine mesh, (b1) CFD output with coarse mesh, (b2) CFD output with medium mesh, (b3) CFD output with fine mesh.*

Conducted CFD simulations for above mesh settings are demonstrated in **Figure 2**. It can be noted that the CFD results demonstrated significant velocity profiles and depiction of wake generation was considered but dominance was given to shearing stress, i.e., related to the near-wall stresses. Special focus was given to the surface shearing stress, as to capture the precise effect caused due to control surface deflections. Moreover, wake dominance can be optimized further by deploying more number of cells at the aft side of aircraft geometry with more computational power.

## **4. CFD to FDM integration**

The FDM file is then processed using the FlightGear flight simulating software; this file is in .xml format. Moreover similar procedure is followed for checking under the JSBSim stand-alone module designed by Jon Berndt in 2004 [10]; however, it is embedded to the external image generation tool for visual effects. Input devices for aircraft control loading system used with JSBSim simulations were similar to the actual flight control loading system; however, with FlightGear simulation, Logitech extreme 3d edition flight joystick was used, this is further aggregated in **Table 5**.


#### **Table 5.**

*Control loading and basic settings set at different simulators and during actual flight mode.*

After the hardware is set up and FDM .xml scripted file is tested, we are able to collect different flight data of interest for quantification of flight maneuvers. In the next section, results of responses generated for angular rates that are dependent on the aerodynamic coefficients plugged in FDM file are discussed.
