3.4 Various types of flow

3.4.1 Steady-state flow and transient flow

Let us observe a flow of a river at a certain point. If nothing causes a change on the river flow, the flow does not vary and keeps a constant state. This kind of flow that does not vary over time is called steady-state flow.

On the other hand, if a fish is swimming in the river, the river flow does not stay constant. This kind of flow that varies over time is called transient flow.

Figure 3.24 Steady-state flow and transient flow

As well as flow, other physical quantities such as temperature can be classified into two types; “its value does not vary over time” and “its value varies over time”. A state of a whole system containing a flow being balanced and that does not vary over time is called steady state. On the other hand, a state being unbalanced and that varies over time is called transient state.

 Want to know more  Cause of transient flow
Even if a flow is in a steady state, the flow has the transient state where it varies over time before it reaches the steady state, and this is a transient flow.

In addition, there is a flow which does not have a steady state. One example is shown in Figure 3.25 (a). The cause of flow variation varies over time. In a river flow, this corresponds to a situation where a sluice gate opens and closes. Because how much the sluice is open varies over time, the river flow controlled by the sluice also varies over time and this is a transient flow.

Another example is a greatly turbulent flow as shown in (b). For example, when wind blows into a tree, turbulence occurs in the wind flow and the flow becomes more uneven. A flow turbulent to some extent generates flows which vary over time in a micro scale separately from its left-to-right macro flow. This means that a greatly turbulent flow also is a transient flow in a micro-scale sight.

Figure 3.25 Examples of transient flow

 Want to know more  Quasi-steady-state flow
When an object is located in a uniform flow, vortices are generated behind the object as shown in Figure 3.26 in some cases. This kind of vortices is called Karman vortex and lining vortices as shown in the figure are sometimes called Karman vortex street.

Figure 3.26 Karman vortices generated behind a cylinder

This kind of flow varies over time and is classified into a transient flow; however, vortices are genereated from side to side backward from the cylinder and a periodic flow occurs. Among transient flows, a flow with periodicity is sometimes distinctively called quasi-steady-state flow.

About the Author
Atsushi Ueyama | Born in September 1983, Hyogo, Japan ）

He has a Doctor of Philosophy in Engineering from Osaka University. His doctoral research focused on numerical method for fluid-solid interaction problem. He is a consulting engineer at Software Cradle and provides technical support to Cradle customers. He is also an active lecturer at Cradle seminars and training courses and the author of serial articles Basic Course of Thermo-Fluid Analysis.