Flapping Foil Suction & Blowing Control CFD Validation

Extracting power from a moving fluid requires an efficient mechanical system. A flapping foil with suction and blowing control acts as an advanced energy harvesting device. As the foil moves up and down while rotating, it creates complex aerodynamic forces. The fluid flow often separates from the surface during high-angle movements, which wastes kinetic energy. To solve this physical problem, engineers apply active flow control. This specific method uses air jets on the solid surface to manipulate the pressure field directly. By pulling fluid in or pushing fluid out, the system keeps the flow attached to the foil longer.

This study is a numerical validation of the paper by Yalei Bai et al. (2025). We evaluate the energy extraction efficiency of this active method. For more foundational knowledge regarding fluid forces on lifting surfaces, visit our aerodynamics and aerospace CFD simulation studies.

• Reference [1]: Yalei, Bai, Yao Huimin, and Min Zheng. “Numerical study of the use of a flapping foil in energy harvesting with suction-and blower-based control.” Aerospace8 (2025): 698.

Figure 1: Illustration of the heave and pitch flapping foil motions over a one-half period.

Simulation Process: Periodic Kinematics and Dynamic Mesh Layering

The physical fluid domain represents a 2D cross-section around a NACA0015 airfoil. The mathematical space utilizes a fully structured grid containing 123,301 quad cells. A structured mesh provides high accuracy near the solid walls where the fluid boundary layer forms.

Because the solid object moves through the fluid, the grid must adjust continuously. We apply physical kinematics using coupled equations for pitching rotation

\theta(t) = \theta_0 \sin(\gamma t)

and heaving displacement

h(t) = h_0 \sin(\gamma t + \phi)

which yield pitch velocity \omega(t) = \theta_0 \gamma \cos(\gamma t) and heave velocity V_y(t) = H_0 \gamma \cos(\gamma t + \phi) .

To calculate these moving boundaries without calculation errors, the solver uses specific dynamic mesh techniques. You can learn more about these moving boundary principles in our dynamic mesh CFD simulation library.

To control flow jets over time, we apply a custom UDF to govern transient suction and blowing control. The UDF uses internal time-step logic so that jets are not always active; blowing and suction activate on the lower surface during the downward stroke (0.0T to 0.5T) and shift to the upper surface during the upward stroke (0.5T to 1.0T)

Figure 2: Diagram of the suction and blowing control active airfoil configuration.

Figure 3: The fully structured grid designed to resolve the boundary layer and moving interfaces safely.

Post-processing:  Phase-Averaged Aerodynamic Forces and Leading-Edge Vortex Analysis

We analyze the instantaneous forces and the fluid velocity field to understand the physical energy transfer. The flapping foil produces lift and moment forces that change rapidly over time. We calculate these using the instantaneous lift coefficient (Cy) and moment coefficient (Cm):

C_y = \frac{F_y}{0.5 \rho U_\infty^2 c L}

C_m = \frac{M}{0.5 \rho U_\infty^2 c^2 L}

The validation plot compares our calculated data against the reference paper. The physical force creates a periodic wave. The data shows a deep force drop near the 0.5 time period. The flow then recovers rapidly, creating a strong peak approaching -2.9 near the 1 time mark. Our mathematical data accurately tracks the same phase, sign, and magnitude as the experimental reference. This confirms that the fluid forces are calculated correctly

Figure 4: Plot validation of the instantaneous force coefficient against the reference dataset, showing high accuracy.

To understand why these forces change, we analyze the velocity magnitude contours. As the foil pitches and heaves, the fluid accelerates sharply at the front tip. This creates a leading-edge vortex. Without flow control, this vortex detaches quickly and becomes a swirling low-pressure zone in the wake, which wastes energy. The velocity contours display how the suction and blowing control alters this behavior. The active jets supply kinetic energy to the fluid. This physical action delays the vortex separation. The high-speed flow (indicated by the red zones) stays attached to the foil surface longer during the stroke reversal.

Figure 5: Velocity magnitude contours over time, illustrating the delayed separation of the leading-edge vortex.

Because the flow stays attached, the mechanical power extraction increases. We calculate the instantaneous power (P(t)) by combining the heave velocity (Vy) and pitch angular velocity (ω):

P(t) = F_y(t) V_y(t) + M(t) \omega(t)

We average this power over one full cycle (P ̅ )to find the energy coefficient (CP):

\bar{P} = \frac{1}{T} \int_0^T P(t) dt

C_P = \frac{\bar{P}}{0.5 \rho U_\infty^3 c L}

Finally, to normalize this, the instantaneous energy coefficient is defined, and its time-averaged value is used to compute the overall energy-extraction efficiency :

C_{op}(t) = \frac{P(t)}{0.5 \rho U_\infty^3 c}

\eta = \bar{C}_{op} \frac{c}{d}

The table below proves the physical benefit of this active control. The baseline foil without jets achieves a 33.8% efficiency. By managing the leading-edge vortex properly, the SBC Case A2B3C6D1E1 reaches an efficiency of 40.7%. This shows only 15% deviation which is completely acceptable and our results our validated.

Frequently Asked Questions (FAQ)

• Why does a flapping foil generate complex vortex structures? When the solid foil pitches and moves vertically, it forces the incoming fluid to change direction suddenly. This high-angle movement creates extreme pressure differences. The fluid rolls up into a spinning leading-edge vortex before detaching into the wake.
• How does suction and blowing control improve the energy efficiency? Fluid separates from the wing when it loses forward momentum. Active blowing pushes fast air into the slow boundary layer. Active suction pulls the slow, dead air away. Both physical actions keep the main fluid flow attached to the solid surface, which increases the usable force.
• Why are the jets turned on and off during the flapping cycle? The physical angle of the foil changes continuously. The pressure required to hold the fluid attached moves from the lower surface to the upper surface as the stroke reverses. Timing the jets perfectly ensures energy is only spent when it physically prevents flow separation.