For low speeds, the flow particles move according to clearly arranged layers; this is known as laminar flow. For high speeds, the flow is characterized by three-dimensional, stochastic movements, this is called turbulent flow. The speed can be measured as time-dependent but it varies around mean values. By increasing the speed of an originally laminar flow, it will become a turbulent structure after passing a determined value. Through the switching of fans, turbulent flow will rise due to the high speeds as well as the strong swirl.

Turbulent flow around a ball

Even high-powered computers cannot directly solve the flow equations of a turbulent flow for practical industrial applications. It is, therefore, necessary to simplify the governing equations. Different turbulence models can be used for the averaging of the nonstationary turbulent oscillations; the CFD-tools have different turbulence models and simplifications, especially for the velocity profile near the walls.

In a solid, the heat is transported through conduction. This is characterized by the conduction coefficient **λ**. In a flowing liquid or gas, the heat can also be transported through the flow. This is the so-called convection, which is characterized by the heat transfer coefficient α.

When the cooling medium is pushed through the machine, for example through the use of fans, the cooling phenomenon is called forced convection.

When the cooling medium is neither accelerated by fans nor by the rotation of a rotor, a
slow-moving laminar flow can rise due to temperature differences of the cooling medium,
which provoke density differences. The resulting heat transfer between the solids and the cooling medium is called free convection. Due to the time dependence of this phenomenon, the CFD calculations should be performed
time-dependently with the input of adequate time steps.

Surface temperatures of the E-Cooling tutorial electrical motor

With the convection, the temperature fields of the solid and of the
bordering wall are interdependent, therefore they need to be calculated
simultaneously, this is called conjugated heat transfer.

Through the setting of the heat transfer option, the energy equation will be solved in addition to the continuity and momentum equations. The energy equation makes the relation between temperature differences, heat fluxes, and velocity gradients; it takes into account the conduction as well as the convection.

Besides trough direct contact, the heat transfer can also take place by heat radiation, which is characterized by the surface emissivity. The heat density radiated from the surface takes into account the temperature differences to the power of 4. The heat radiation is effective for high temperatures or when there is no forced convection. The heat radiation can be calculated through the setting of the corresponding option in the CFD program.

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