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The majority of flows in nature and in engineering applications are turbulent. Turbulent flow fields are three dimensional, chaotic, diffusive, dissipative, and random. These flows are characterized by velocity fluctuations in all directions with infinite number of scales. Exact analytical solution of Navier-Stokes equations for turbulent flows is not currently possible since these equations are elliptic, non‐linear, and coupled. Furthermore, direct numerical simulation (DNS) of turbulent flows is not currently practical due to significant computational resources required. So far, DNS approach has only been applied for a limited class of simple low Reynolds number applications.
Presently, turbulence modeling based on Reynolds-Averaged Navier Stokes (RANS) equations is the most common and practical approach for turbulence simulation. RANS are time-averaged modification of Navier-Stokes equations and turbulence models are semi-empirical mathematical relations that are used to predict the general effect of turbulence.
The objective of turbulence modeling is to develop equations that will predict the time-averaged velocity, pressure, and temperature fields without calculating the complete turbulent flow pattern as a function of time. Unfortunately, there is no single universally accepted turbulence model that works for all flows and all regimes. Therefore, users have to use engineering judgment to choose from a number of different alternatives sine the accuracy and effectiveness of each model varies depending on the application.
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