Laminar and turbulent flow
In terms of turbulence, flows are divided into two categories: Laminar flow and turbulent flow. In laminar flow, the fluid moves in layers, and the layers slide on each other and have a laminar and regular movement. But in turbulent flow, different fluid layers collide and have a turbulent and unpredictable movement. In turbulent flow, fluid layers, in addition to moving along the flow path, have a velocity component perpendicular to the flow path; turbulence generally is a three-dimensional phenomenon.
But what is turbulence, and what is turbulent flow called? Turbulence is a chaotic, random, unpredictable, and variable phenomenon concerning time and place, and a turbulent flow is a flow in which there is turbulence. Almost all air currents in nature are turbulent, and this is also true for water currents. Except for the flows caused by capillary effects, blood flow, and issues like this, the rest of the flows are included in the definition of turbulent flows. Turbulence can occur in the boundary layer, exit jet, free shear surface, or behind objects. In turbulent flows, we see the appearance of small and large eddies called large-scale or small-scale eddies.
In turbulent flow, the kinetic energy of molecules is lost due to the friction between fluid layers, and we have a higher pressure drop than in laminar flows. Laminar or turbulent fluid flow is measured by a dimensionless number called the Reynolds number. Reynold’s number shows the ratio of inertial forces to viscous forces. There are also small turbulences in laminar flow, but because viscous forces are more substantial than inertial forces, these turbulences disappear quickly.
As the flow speed increases, the inertial forces become more assertive, and the flow changes from calm to turbulent. The region where the flow is neither completely Laminar nor turbulent is called transient. The transition state in the internal flow starts from Reynolds 2300 and continues almost to Reynolds 4000, and in the external flow between Reynolds 10000 and 100000, the transition state occurs. Of course, it should be noted that the flow disturbance does not occur in a specific Reynolds number and is a transient phenomenon that factors such as the surface roughness, the flow geometry, and the intensity of the flow fluctuation affect the extent of the transition state.
In the transition area, some small turbulences are repelled, but others with higher kinetic energy continue. As the flow speed increases, the intensity of the turbulence increases, and the inertial forces overcome the viscous forces until finally reaching the region of we arrive thoroughly Turbulent. In turbulent flow, the fluid velocity decreases around a high average value and does not have a constant value.
Most of the currents we deal with and observe in nature are turbulent currents (such as atmospheric currents or currents in oceans and rivers). Generally, laminar currents are fictitious and visible in the laboratory. Flow turbulence is a complex and three-dimensional phenomenon. Many details of the turbulent flow are yet to be available. Turbulence is a still debated issue in fluid mechanics and has not been entirely resolved.
Simulation of laminar and turbulent flows using CFD
As it was said, turbulent flows in fluid mechanics are complex phenomena and do not have exact and analytical solutions. Therefore, we use CFD software to investigate and study these flows. Since turbulent flows are inherently three-dimensional phenomena and are always variable concerning time (so they must be transiently simulated), simulating them is a complex and costly task and requires powerful computers for this task. Developed turbulence models should be used to simulate turbulent flows.
Of course, to reduce the computational cost of simulating turbulent flows in some of these turbulence models, simplifications and assumptions have been made to solve fewer equations. These simplified turbulence models can be used in some physical phenomena that do not have high turbulence.
In CFD software, there are various turbulence models, each of which should be used in the appropriate phenomena for that model. Each of the turbulence models is designed for specific applications. For example, the Spalarat Almaras turbulence model is suitable for aerospace and aerodynamic applications. K-omega models perform better in internal flows with low Reynolds numbers, or K-epsilon models are ideal for external flows and high Reynolds numbers.
One of the challenges of turbulent flow simulation is choosing the appropriate turbulence model. There is no exact answer to that. For CFD simulation of the desired problems, we must select the turbulence model according to the laboratory work related to our situation that has been done before and validated with the appropriate turbulence model. If it is the first time we want to simulate a specific problem, we must verify the turbulence model by testing it ourselves.
Of course, this process is only sometimes correct. First, a series of simplifications are used in the laboratory environment to simulate experimental data. Second, there is no guarantee that turbulence models are accurate enough to simulate complex problems. In addition, comparing the performance of turbulence models is complicated, considering that each of these models requires different settings.
One of the essential points that should be paid attention to when choosing a turbulence model is the dominant flow phenomena in the main flow, such as separation, formation of eddies, secondary flows, etc., knowing the prevalent flow phenomena in the problem according to the characteristics of the turbulence model helps us in choosing the suitable model helps for simulation.
In the following, we will discuss some systems where flow turbulence significantly impacts their analysis.
Combustion occurs in the combustion chambers with the combination of fuel and air. Turbulent flow is one of the most critical parameters in reaction formation and combustion. Because the turbulence of the flow causes better mixing of fuel and air, the combustion completely takes place. Therefore, choosing the appropriate turbulence model in combustion chambers is very important, according to previous observations and investigations, K-epsilon turbulence models are suitable for simulating combustion chambers.