Computational Fluid Dynamics Services

The Atmospheric Studies Group of TRC provides a full range of Computational Fluid Dynamics (CFD) services. In CFD modeling, the non-linear Navier-Stokes equations that govern fluid flow are numerically solved on a three-dimensional grid or mesh that represents a virtual prototype of the fluid system to be analyzed. This mesh contains millions of cells with finer resolution applied near surfaces or exhaust locations where flow properties change rapidly. By using billions of iterative steps on high-speed computers, TRC uses the FLUENT software from ANSYS to predict flow variables including wind, turbulence, heat and mass transfer and chemical reactions on scales of centimeters to kilometers.

Typical surface computational mesh

Some recent applications by TRC have included the potential effect of building wakes on the generation of wind power, wind loading on structures during hurricanes, plume rise from an air-cooled condenser, and the dispersion and rainout of a cloud containing a two-phase mixture of dense gases and droplets from accidental chemical releases.

Wind-induced pressures on building during Hurricane Katrina

Air flow under and through air cooled condenser fans

Other potential applications include indoor air pollution; HVAC design, re-entrainment of toxic releases from building vents, pedestrian winds in city centers, ventilation of train exhausts in tunnels, mitigation of odors from landfills by fences, berms or vegetation barriers, recirculation of mechanical draft cooling tower exhausts, the local transport and dispersion of pollutants through industrial complexes such as oil refineries, airflow over sharp-edged terrain, light wind speed dispersion, flare plume rise and radiative heating, and evaluating green building design parameters such as solar heating and ventilation.

Potential clients include the power generation industry, the oil, gas, chemical and transportation industries, wind energy developers, architects, research laboratories, remediation engineers, and waste treatment facilities. 

Wind intercepted by wind turbine in the wake of proposed building

Cooling tower recirculation and ground-level fogging

Different CFD models use different methods to solve the equations. FLUENT uses finite volumes that ensure the exact conservation of flow properties. The FLUENT solver offers both density-based and pressure-based solution methods. The best method depends on the flow problem.

In addition to the computational mesh and the numerical solver, a turbulence scheme is applied. FLUENT offers several turbulence schemes including multiple variations of the k-e models, as well as k-w models and Reynolds stress turbulence models. These turbulence models are part of the Reynolds Averaged Navier-Stokes (RANS) family which explicitly computes only the mean flow. Also available in FLUENT are a large eddy simulation (LES) model and a detached eddy simulation (DES) model that directly solve some of the larger scales of turbulence containing most of the turbulent kinetic energy and momentum and parameterize the smaller scale fluctuations which are more isotropic and homogeneous. The drawback of using LES or DES is that they are much more computationally demanding and are not practical for most applications.

FLUENT offers a wide range of options for modeling complex flow problems. Several of these involve the use of specific models within the FLUENT system For example, the gravitational fall and evaporation of droplets from a flashing liquid can be modeled with the discrete phase model (DPM). TRC has validated FLUENT for large releases of pressurized ammonia using the Desert Tortoise field studies. The results of this validation were presented at the Fifth International Symposium on Computational Wind Engineering (CWE2010) in May 2010.

Accidental dense gas spills in complex terrain

Flashing dense gas jet with a rainout of liquid droplets

Particles or droplets can undergo heat, mass, and momentum transfer with the background fluid. Also a compressible flow option is available for exhaust jets with speeds above about Mach 0.3. Above this speed, the approximation that local density is constant is not valid when solving the Navier-Stokes equations of motion. Reactive flows can also be modeled in FLUENT. Chemical reactions can be specified and, in conjunction with turbulence models, can be used to conduct an array of liquid or gaseous combustion simulations such as flaring. Other models are available which deal with specific situations.

FLUENT simulations can either be steady-state or transient. The steady-state solutions assume constant inlet and flow conditions. This can be used, for example, to simulate a release where the averaging time of the fluid property is shorter than the release time of the fluid. For problems with time-varying releases or steady releases that are shorter than the averaging time of the fluid property, a transient solution that steps forward in time is applied. For these problems the Navier-Stokes equations governing fluid flow are solved at each time step. Results can be presented both as a time series of monitored values at specific locations and as animations.

Neighborhood-scale wind flows with speeds shown near ground level

Flow over hypothetical terrain

For questions or additional information, please contact Dr. Lloyd Schulman