Overview
Fluid dynamics is a branch of fluid mechanics that describes the flow of fluids and gases. It has a wide range of applications such as determining the mass flow rate of any liquid, prediction of weather patterns, and measurement of forces on the aircraft.Computational fluid dynamics is a subdivision of fluid mechanics that utilizes different techniques such as numerical analysis and data structure processing to examine and solve the problem related to the flow of fluid. Different software like scSTREAM, ScFLOW, etc. are used to analyze and simulate the fluid flows at the desired location and conditions.
This article broadly explains the two types of flows: Steadystate flow and transient state flow. Different aspects and causes of these flows are also covered along with prominent differences and advantages and disadvantages.
This article broadly explains the two types of flows: Steadystate flow and transient state flow. Different aspects and causes of these flows are also covered along with prominent differences and advantages and disadvantages.
SteadyState flow
A steadystate flow generally refers to a condition where the properties of a fluid at a particular point in the system remain the same. These properties contain temperature, pressure, and velocity. The most important and notable property that remains unchanged in the steadystate flow is the mass flow rate of the system. This shows that there is no accumulation of mass within any element of the system. If a system is in a steadystate, then the recently notice behavior of the system will continue in the future. Mathematically, the properties in this system at any point do not depend upon the time. The below equation will help you to understand the phenomena,
Where, P represents any property like velocity, pressure, and density, etc.
Example
1
In many cases, the steadystate is not achieved until passing a certain time after the system initiated. The initial state is generally transient or you can say a warmup interval. Let's consider a flow of water through a tube. At starting it makes turbulence flow but after some time the flow becomes steady if the input remains at the same speed and pressure. Similarly, current pass through a wire is much higher at the beginning when we turn on a fan or motor due to the requirement of high initial torque but become stable when fan or motor reaches its nominal speed.
2
Here we consider a water circulation system for explaining the steadystate in terms of water flow rates (Q), inletoutlet concentration (C), and production of a hormone (P) in the water tank for fishes inside the tank. The inlet water comes from the unit process is equal to the outlet water which enables the system to maintain flow rate, concentration, and hormone production. It will also maintain the level of water inside the tank and provide required oxygenation continued for the fishes. Figure 1 clearly shows the abovementioned situation:
Figure 1: Water Circulation Process
Transient State Flow
Within a system, it takes a while for the output to settle and enter its final state when certain input changes. This short term phase is called the transient phase. The final state becomes the stable state and the system stays there forever until the feedback changes again. Fluid flow is transient or unstable because its flow parameters ( i.e. speed and pressure) depend not only on the location in the coordinate system used to define the flow field but also on time. There is a difference between three distinct types of transient flow phenomena.
Fluid flow is transient or unstable because its flow parameters ( i.e. speed and pressure) depend not only on the location in the coordinate system used to define the flow field but also on time. There is a difference between three distinct types of transient flow phenomena.
Fluid flow is transient or unstable because its flow parameters ( i.e. speed and pressure) depend not only on the location in the coordinate system used to define the flow field but also on time. There is a difference between three distinct types of transient flow phenomena.
 Phenomena which are stochastically unusual, e.g. Turbulent fluctuations
 Phenomena of runup, or rundown, e.g. If a centrifugal pump starts or ceases
 Phenomena Periodic, e.g. Pulsations induced by surge pressure in tubing, distorted H / Q curves of centrifugal pumps or spinning impeller effects
Example
1
In strict terms, the flow through a revolving series of vanes (e.g. Impeller) is always a transient fluid as seen from a fixed coordinate system, all velocity and pressure change regularly at a constant location in space as the vanes travel. The flow in an impeller and its immediate vicinity can, however, be called steady as long as it is represented using a coordinate system that rotates with the impeller
2
When a switch is flipped into an appropriate electrical circuit containing a condenser or inducer, the device draws out the resultant voltage or current shift (respectively), which allows the system to take a significant period of time to enter a new stable state. We may describe a transient by stating that a transient state has arisen while a substance is at rest or in constant motion and a shift in time occurs. When a SCR is switching on (a fourlayer PNPN Device) the question of transients arises as a result of high current and voltage values oscillating across the point until normal rates are again obtained.
Difference between steady and transient state:
Only a steadystate solution can be obtained by the steadystate analysis. However, analysis cannot predict about the process which takes place for achieving steadystate condition. If we terminate the thermal fluid analysis before reaching the steadystate solution, the result may not have any physical meaning.
On the other hand, a transient analysis predicts accurately the system conditions as it varies with time. That's why the transient analysis measures the time alteration phenomena. If we allow the transient analysis to run for a long time, it will eventually obtain a steadystate solution. However, the steadystate solution can be achieved by utilizing a steadystate analysis is much faster than transient analysis.
Consider a situation shown in figure 2, where water is poured into a container that has an outlet at one side. At first, when the water level is below the outlet then the poured water is greater than the discharged water showing turbulence or transient state. However, after some time when the water level reaches a certain point then the poured water is equal to the discharged water showing uniform or steadystate and water level remains constant. Figure 2 helps you to understand the phenomena.
On the other hand, a transient analysis predicts accurately the system conditions as it varies with time. That's why the transient analysis measures the time alteration phenomena. If we allow the transient analysis to run for a long time, it will eventually obtain a steadystate solution. However, the steadystate solution can be achieved by utilizing a steadystate analysis is much faster than transient analysis.
Consider a situation shown in figure 2, where water is poured into a container that has an outlet at one side. At first, when the water level is below the outlet then the poured water is greater than the discharged water showing turbulence or transient state. However, after some time when the water level reaches a certain point then the poured water is equal to the discharged water showing uniform or steadystate and water level remains constant. Figure 2 helps you to understand the phenomena.
Figure 2: Water Level Changes
Advantages and Disadvantages:
Advantages  Disadvantages  

Steady State  

 

 
 
 
Transient State  

 


Conclusions
In this article, a complete overview of a steadystate and transient state is given with proper examples. The main difference between steady and transient states is provided along with the benefits and as well as disadvantages of each state. We can simply conclude that the steadystate of a fluid is simply defined as the state of flow in which the fluid attributes such as velocity, density, pressure, etc. do not change over time at any particular moment. While the transient state of flow is defined as the state of flow in which the fluid attributes such as velocity, density, pressure, etc. change with respect to time at a point in time.
References
 Learn Mechanical."Types of Fluid Flow."https://learnmechanical.com/typesoffluidflow/ (accessed May 30, 2020).
 CFD Support."Transient or Steady State?"https://www.cfdsupport.com/OpenFOAMTrainingbyCFDSupport/node356.html (accessed May 31, 2020, 2020).
 "Transient flow."https://www.ksb.com/centrifugalpumplexicon/transientflow/328110/ (accessed May 31, 2020, 2020).
 Engineers Edge."Steady State Flow Fluids."https://www.engineersedge.com/fluid_flow/steady_state_flow.htm (accessed May 31, 2020).
 "Computational Fluids Dnamics."https://en.wikipedia.org/wiki/Computational_fluid_dynamics (accessed May 30, 2020).
Credits
Thana Pergchoei
CFD Engineer  Cradle Consulting Thailand
The author had experienced in wide researches of Aerospace engineering, and Airplane Aerodynamics related. He graduates with a Bachelor's degree in Aerospace Engineering Major from Rangsit University (College of Engineering) Bangkok, Thailand. He is currently pursuing his Master's degree at the University of South Wales, Wales, United Kindom. In addition, he has been trained and certified for EASA  European Union Aviation Safety Agency (EASA.147.0027) from Taikoo(Xiamen) Aircraft Engineering Co., Ltd.
The author had experienced in wide researches of Aerospace engineering, and Airplane Aerodynamics related. He graduates with a Bachelor's degree in Aerospace Engineering Major from Rangsit University (College of Engineering) Bangkok, Thailand. He is currently pursuing his Master's degree at the University of South Wales, Wales, United Kindom. In addition, he has been trained and certified for EASA  European Union Aviation Safety Agency (EASA.147.0027) from Taikoo(Xiamen) Aircraft Engineering Co., Ltd.