A RANS CFD based VPP is used to study M5's helm balance.
The aerodynamic and hydrodynamic forces on M5 while sailing are modeled using a combination of CFD data and empirical force models. The aerodynamic and hydrodynamic forces are balanced to give predicted sailing states using a velocity prediction program (VPP). VPP data is used to study helm balance while sailing upwind and to evaluate possible downwind sail configurations.
The aerodynamic forces on the sails, rig and superstructure are modeled using steady state RANS CFD. RANS CFD accurately captures turbulent flow effects important for sailing such as separation, viscous drag and wakes. The aerodynamic model consists of four functions that use force coefficients to calculate the aerodynamic driving force, heeling force, heeling moment and yaw moment as functions of apparent wind angle, apparent wind speed and heel angle. CFD data is used to derive the force coefficients.
The hydrodynamic forces on M5’s hull are modeled using a combination of empirical formulas and RANS VOF CFD. RANS VOF simulates the turbulent flow field around the hull and appendages as well as the water surface by solving the RANS equations for a multi-phase fluid, in this case air and water. The VOF simulations capture many hydrodynamic phenomena important to sailing such as waves, wakes, surface pressure, viscous forces and the complex flow interaction between the hull, appendages and the free surface. RANS VOF simulations for a yacht the scale of M5 are computationally intensive so a limited number of VOF runs are used to calibrate an empirical force model of the hull and appendages. The hydrodynamic model uses empirical force equations calibrated with CFD to predict the total side force, resistance and yaw moment.
The VPP program predicts sailing performance by balancing the aerodynamic and hydrodynamic forces for a given TWS and TWA. The VPP solver balances four degrees of freedom Fx, Fy, Mx and Mz by adjusting four sailing state variables boat speed, yaw, heel and rudder (BS, γ, ϕ, θ) until the aerodynamic and hydrodynamic forces and moments are in equilibrium.