- In comparison to the 3-fin configuration, the 4-fin configuration is expected to be slower because more drag force is produced, and the surfer will have to apply more work to perform a turn maneuver because of the higher lift force.
- Fluid flow velocity will have a significant influence on the lift and drag forces and will therefore have an immense effect on the surfer.
- The various positions of the rear fins along the axial and transverse direction of the surfboard potentially have a massive impact on the lift and drag forces of the entire configuration, on the stall point, and consequently on the surfing.
2. Materials and Methods
2.1. Surfing Hydrodynamics
- Front fins: the front fins in prevailing surfing direction.
- Rear fins: the fins next to the tail of the surfboard.
- Outside fins (OF): the fins next to the shore.
- Inside fins (IF): the fins next to the wave crest.
2.2. Geometric Dimensions of the Fins
- Both front fins are fixed.
- Just symmetric fin configurations are realized.
- The displacement of the rear fins is oriented on the positions of the intersection point board and leading edge.
2.3. Simulation Region and Boundary Conditions
2.4. Mesh Generation and Independence Study
2.5. Numerical Methods
3.1. Lift and Drag of the Commercial 4-Fin Configuration (fin Position T:80/A:150)
3.2. Lift and Drag for Four Different uin (5 m/s, 7.5 m/s, 10 m/s, and 18 m/s) at the Commercial 4-Fin Configuration
3.3. Rear Fin Position Study: Axial vs. Transverse Shift of the Rear Fins
4.1. Lift and Drag of the Investigated Commercial 4-Fin (T:80/A:150) Configuration
4.2. Comparison of 3- and 4-Fin Configurations Using FCS Accelerator Fins from This Study and Falk et al. 2019, and FCS k2.1 Fins from Gudimetla et al. 2009
4.2.1. Comparing Both 4-Fin Configurations
4.2.2. Comparing the 3- and 4-Fin Configurations
4.2.3. Summary of the Comparisons
4.3. Lift and Drag for Four Different Inflow Velocities (5 m/s, 7.5 m/s, 10 m/s, and 18 m/s) at the Investigated 4-Fin
4.4. Rear Fin Position Study: Axial vs. Transverse Shift of the Rear Fins
- The stall point is moving from an AoA of 30° (T:0) to 20° (T:120),
- the maximum of the lift coefficients decreases significantly from 0.87 (T:0) to approx. 0.7 (T:50–120), and
- the lift coefficients also decrease in the range of high AoA of 30°–45°.
- The stall point is moving from an AoA of 25° (A:0) to 20° (A:50–200),
- the maximum lift coefficient decreases slightly from 0.72 (A:0) to 0.69 (A:50–200), and
- the lift coefficients start to increase again at an AoA = 35° for the positions A:150 and A:200.
- The surfer has to apply much more work to perform a turn maneuver with a 4-fin configuration due to the higher lift forces and is losing more velocity during maneuvers because of the higher drag forces compared to a 3-fin configuration.
- The investigated surfing velocity has nearly no effect on the lift and drag coefficients, but the lift and drag forces increase nonlinearly with increasing velocity.
- At the 4-fin configuration, the front and rear fins of each side act like two large fins when the OFs and RFs are nearly aligned (T:0 A:150 and T:20 A:150) and therefore produce the highest lift forces with a later onset of force fluctuations.
- The maximum of the lift-to-drag ratio increases while shifting the rear fins in the transverse direction toward the longitudinal axis of the surfboard with a fixed axial position.
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