TURBULENT FLOW AND CLINICAL APPLICATIONS

⚱️The flow pattern of a river running over rapids is very different to the steadily flowing river (laminar flow). Here, the water’s path of travel becomes far less predictable than for laminar flow. This is an example of turbulent flow. An intermediate example is water flowing near the bank of a steadily flowing river, which often tends to meander, turning round in gentle circles. This is an example of eddies, the forerunner to full-blown turbulence.

⚱️As flow is, by definition, unpredictable, there is no single equation that defines the rate of turbulent flow as there is with laminar flow.

⚱️But, in well controlled circumstances the point at which flow changes from laminar to turbulent flow can be estimated using the Reynolds number, Re, which is named after Osborne Reynolds (1842–1912) of Manchester University, an engineering professor.

⚱️The Reynolds number allows us to predict whether turbulent or laminar flow would occur in a given system. The Reynolds number is a dimensionless quantity, i.e. it has no units. It is defined as the ratio of inertial and viscous forces. 

⚱️A Reynolds number <2000, where viscous forces predominate, predicts flow to be laminar. Between 2000 and 4000, both laminar and turbulent flow are anticipated. Above 4000, flow is likely to be completely turbulent because inertial forces are dominant. Critical flow is the point above which turbulent flow commences, which occurs at a Reynolds number of around 2000.

⚱️Viscosity is the important property for laminar flow

⚱️Density is the important property for turbulent flow

⚱️Reynold’s number of 2000 delineates laminar from turbulent flow (Tim and Pinnock: Re < 1000 is associated with laminar flow, while Re > 2000 results in turbulent flow)

⚱️A high Reynolds number means that the inertial forces dominate, and any eddies in the flow will be easily created and persist for a long time, creating turbulence. In a given airway with a known gas and flow velocity, the likelihood of turbulent flow can be predicted from Re.

⚱️APPLICATIONS: Both laminar and turbulent flow exist within the respiratory tract, usually in mixed patterns. Turbulent flow will increase the effective resistance of an airway compared with laminar flow. Turbulent flow occurs at the laryngeal opening, the trachea and the large bronchi (generations 1–5) during most of the respiratory cycle. It is usually audible and almost invariably present when high resistance to gas flow is encountered

⚱️APPLICATIONS: The principal sites of resistance to gas flow in the respiratory system are the nose and the major bronchi rather than the small airways. Since the cross-sectional area of the airway increases exponentially as branching occurs, the velocity of the airflow decreases markedly with progression through the airway generations, and laminar flow becomes predominant below the fifth generation of airway