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Airflow Modeling over a Flying Bird, CFD Simulation

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In this project, the flow field corresponds to a flying bird has been investigated.

This product includes a Mesh file and a comprehensive Training Movie.

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Project Description

In this project, the airflow field corresponds to a flying bird has been investigated. The noted problem has always been the center of attention from at least two perspectives. The study of bird aerodynamic and their flying mechanism has primarily been useful for companies involving the production of flying equipment. Furthermore, environmental scientists are also using these types of investigations to estimate the spread of birds’ illness during their migration period, in which they were flying in a group.


Mathematical Modeling (Airflow)

To study a bird flying and its corresponding aerodynamics, one must solve the flow equations in the differential form. By assuming an isothermal, incompressible, and steady-state condition for the air around the bird, the governing mass and momentum equations are written as follows:


Bird Geometry & Mesh

As a numerical study, the initial step towards the modeling is the CAD geometry production, which is depicted in following figures. The modeling step would then continue by discretizing the computational domain into a finite number of elements. The noted procedure is called the mesh generation step and has a direct impact on the solution accuracy. However, as an aerodynamic investigation, the bird’s flow field has the study’s interest, not the bird geometry. To this aim, an extra step in creating a computational domain must be carried out before importing the CAD file into the mesh generator software.

The blue face is considered as the inlet of the domain, while the red face on the other side is regarded as the outlet. It must be noted that the computational domain should be considered big enough so that no recursive flow would occur at the domain boundaries. The noted subject is of the highest priority since all other faces were considered as zero gradients. The noted assumption is a realistic flow condition for a flow field far from the bird’s distance. Afterward, the mesh generation process has been carried out. For the current problem, a mesh count of 1,123,235 elements was created to represent the geometry.

Bird Geometry & Mesh

Regarding the mesh’s quality, the maximum skewness of 0.95 with an average of 0.29 is a fine mesh for the current problem. Also, for an interested reader, the mesh’s quality distribution is shown. Also, five prism layers were added adjacent to the bird walls to calculate the boundary layer near the bird’s body accurately.


As a final note, due to the problem’s symmetric nature, only half of the domain is considered the potential computational domain. The noted decision allows one to lower the computational time. However, calculated parameters such as drag, lift, etc., should be multiplied by two.

Airflow over a Flying Bird CFD Simulation

By importing the mesh into the fluid solver, the calculation procedure is started. As discussed before, an incompressible, isothermal condition has found to be a valid assumption for the current simulation. However, gravity was not taken into account for two main reasons. First, the gravity source would produce equivalent force for the fluid cells if an isothermal condition is considered. Thus, it won’t affect the character of the fluid flow.

Moreover, the flow field is fully turbulent. Thus, the k-w-SST turbulent model was selected for the evaluation of eddies. The noted model is more accurate than any other eddy-viscosity variation due to a hybrid formulation taking care of both wall effects and the core flow strain rate. Details of the solution setup are as follows:

Solver settings: (Airflow)
Type: Pressure-based
Velocity formulation: Absolute
Time setting: Transient
Gravity: Off
Energy: Off
Model: k-w-SST
Zone: Single fluid zone
Boundary conditions: (Airflow) Walls: No-slip

Inlet: velocity inlet: 20 m/s

Outlet: pressure outlet

Other walls: symmetry

Operating Condition: Reference Pressure Point: 101325 Pa
Solution methods: SIMPLEC
Pressure interpolation scheme: Second-Order
Momentum: Second-Order
Turbulence: First-Order
Relaxation: Pressure: 1.00

Momentum: 0.8

Turbulent Intensity: 0.8

Turbulent dissipation rate: 0.8

Others: Default

Initialization: Hybrid
Material used : (Airflow)
Fluid: Air – constant properties

Density: 1.225 kg/(m3)

Viscosity: 1.7894×10-5 (Pa.s)

Monitor : (Airflow)
Drag Value of Bird wall in Z-direction

Results & Discussions


After the solution convergence, we observe the results through post-processing. Meanwhile, as an assurance for a valid convergence, we monitor the drag value during the solution iterations. In this study, the solution converged one when the drag force reached a constant rate, and the residuals were below 10-4 values. As an initial check, we evaluate the contour of Yplus in order to decide the consistency of the boundary layer mesh. Fortunately for this case, the maximum Y+ value was nearly 45, not far from the buffer layer. The majority of the facial elements were covered by a y plus below 10, indicating that the boundary layer is resolved in most places rather than being modeled.

Afterward, we depict the results regarding the pressure and the velocity field in Figures. The wall’s leading edge (bird’s head) suffers from the highest-pressure gradient, which is entirely logical since the velocity has just met zero.

Results & Discussions

For the velocity field, we present both contour and vector type results to give much insight into the problem. Briefly, the velocity field close to the bird’s body has the highest gradient, and the wake that arises from it stretches far behind the bird’s body. This could be, again, observed through the velocity vectors. Additionally, velocity vectors illustrate the quality of the flow streams. The flow streams close to the walls were parallel to the wall, which shows the effective boundary layer modeling, which is the core challenge of ant aerodynamic simulation.

Finally, we calculate the drag force 0.706 (N), which is accurate for a bird flying with 20 m/s speed.

There are a Mesh file and a comprehensive Training Movie that presents how to solve the problem and extract all desired results.


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