The steady flow is constant over the time. The numerical calculation solves the simplified Navier-Stokes equations to obtain the resulting pressure and velocity field.
When the wind blows across tall slender structures, such as chimneys, skyscrapers, or masts, transient (unsteady) flow may be generated. Under steady continuous or low turbulent winds at a critical velocity, vortex shedding phenomenon can occur behind the structure.
The vortices are shed alternately from one side to the other. This organized vortex pattern is referred to as Karman vortex street. As the vortices are shed, the alternating low-pressure zones are generated on the downwind side of the structure, and the fluctuating force acting perpendicular to the wind direction is generated, see Vortex Shedding. As a result, large vibrations may occur at moderate and frequent wind velocities, structures may undergo a great number of stress cycles that lead to fatigue damage and may determine structural failure without reaching the ultimate limit stress.
The periodic frequency of the vortex shedding can also lock in on the natural frequency of the structure. When these two frequencies equal, resonance sets in and the structure experiences large oscillations perpendicular to the wind direction.
To consider the potential vortex shedding induced damage, it is important for the structural design to simulate the transient wind flow. Changes of the structure geometry can interrupt coherent shedding and together with stiffness modifications, they minimize the wind effect problems. The transient wind flow and effects of the structure geometry can be simulated by the numerical CFD calculation in RWIND Pro without the need of expensive wind tunnel tests. The benefit of numerical simulation is that many scenarios and designs can be checked in a cost-effective way. [1] [2]
The transient wind behavior influences also the microclimate around the buildings. The pedestrian wind comfort in urban areas is affected by various wind effects, such as tunnel throttling or vorticity. Also, for the simulation of these problems, RWIND Pro is a suitable tool, see Wind Comfort in Pedestrian Areas in our Knowledge Base.
For simulations of transient flow, RWIND 2 uses a special solver ("BlueDyMSolver", developed by CFD Support from the standard OpenFOAM® solver called "PimpleFoam").