Traditionally, wind tunnels have been used to investigate the effect of wind on buildings and structures in terms of loading and response and to study micro-scale meteorological phenomena like exhaust gas dispersion. Over the last decades numerical methods to simulate fluid dynamical processes have been advanced and provide through increasing performance of affordable computers an attractive alternative to wind tunnel testing. 

Figure 1: Transient calculation of the turbulent flow field around a floor-mounted cube in a
turbulent boundary-layer (FLUENT-Ansys simulation on embedded LES).

At CEAero we study the application of Computational Fluid Dynamics (CFD) for the prediction of the wind load process varying over time. As any other simulation technique CFD has its strengths and limitations. A key parameter in CFD is the choice and implementation of algorithms and boundary conditions that allow solving the Navier-Stokes equations forming the core of CFD.

By paralleling the numerical simulation with wind tunnel testing the influence of specific methods and assumptions on the results from CFD can be evaluated in large detail. Presently, our research focuses on the

• simulation of the turbulent flow and the
  
• flow-induced pressure fluctuations on the surface of a body exposed to the turbulent flow.

Backbone of our research is a 24 core server with 64 GB RAM and 8x300GB hard disk storage. To use the full capacity of the server and hence not to be dependent on license limitation the main part of our work is conducted with the open source software OpenFOAM. Additionally, ANSYS-Fluent is used in some studies (e.g. in Figure 1 on embedded LES).

Comparison and validation studies are the cornerstones in simulation and have been conducted on characteristic cases of body shapes and flow conditions. An extensively studied case for comparing wind tunnel tests is the floor-mounted cube. With the development of the new simulation technique the vast experience from validating wind tunnel tests is applied to numerical simulation. The Closed-Circuit Wind Tunnel at DTU-Byg provides the ideal conditions to setup a validation case. The relatively small cross-sectional size and the sufficient length to create a proper turbulent boundary-layer flow allow studying the properties of the airflow and the induced loading on the floor-mounted cube. 

Figure 2: Sketches for experimental setup to study the turbulent airflow to and around the cube and the resulting wind load. The wind tunnel test provides a comprehensive data base allowing to assess statistical significance, similarity and stability of measured or calculated quantities.

Since we are primarily interested in the time-varying process in the air flow and wind load process the calculation uses the so-called Large Eddy Simulation (LES) requiring significantly higher resolution of the computational domain and hence more computational power than the widely used time-averaged approach (Reynolds Averaged Navier-Stokes - RANS).