Turbulence
Most flows occurring in nature are turbulent. Turbulence occurs in many kinds of flows and is used in many processes for positive purposes. Laminar flow is always the exception.Turbulence is an eddying motion that exist at high Reynolds numbers. Turbulence has a wide spectrum of eddy sizes with a corresponding spectrum of fluctuation frequencies. Turbulence has prevailing rotational motion that can be thought of as a tangle of vortex elements with highly unsteady vorticity vectors that are aligned in all directions. The largest eddies have sizes on the same order of magnitude as the flow domain, have low frequencies, and are effected by the boundaries and the mean flow. The smallest eddies, on the other hand, are determined by the viscosity of the fluid and have high frequency fluctuations. As the Reynolds number of a given flow increases, the width of the spectrum, or the difference between the largest and smallest eddies, increases.
The large eddies extract kinetic energy from the mean motion and feed it to the large scale turbulent motion. The eddies may be considered as vortex elements that stretch each other. Due to this vortex stretching, energy is passed down the cascade to smaller and smaller eddies until viscosity causes the dissipation of the eddies. The rate of energy dissipated is determined by the large scale motion although dissipation occurs at the smallest scales. It is important to note that viscosity does not determine the amount of dissipated energy, but only the scale at which dissipation occurs.
Since the size of the large eddies is on the order of the flow domain, their motion strongly depends on the boundary conditions of a problem. The preferred flow direction of the mean flow is imposed on the large scale turbulent motion which makes the flow strongly anisotropic. With the cascading process, the direction sensitivity of the flow is diminished, and at high Reynolds number flows, the small scale dissipative motion is isotropic.
It is hard to give a precise definition of turbulence, therefore, turbulence is defined by its characteristics. The characteristics of turbulence are given as:
- Irregular or random flows: this is the reason why statistical methods are used in solving turbulence calculations.
- Diffusive flows: causes rapid mixing and increased rates of momentum, mass, and heat transfer.
- High Reynolds number flows: due to high inertia forces and relatively small viscous forces.
- Three dimensional vorticity fluctuations: turbulent flows are three-dimensional and rotational and have high levels of fluctuating vorticity.
- Dissipative flows: need continuous supply of energy.
- Continuum: turbulent flows are governed by equations of fluid mechanics and are not on a molecular level.
- Turbulent flows are flows: turbulence is not a feature of the fluid but of fluid flow.
Written by Hagop Barsamian