Flows through sudden enlargement(expansion)geometries is an important point of study in the field of Fluid Mechanics because of its application in heat exchanger, Nuclear reactor, pipe lines and Dump combustor. Axis-symmetric sudden enlargement geometry looks simple but internal flow involves complexities such as separation and reattaching flows. Over decades many research have been carried by researchers on investigating steady and unsteady recirculating flows at high Reynolds number in both numerical and experimental methods.
Losses due to sudden expansion
Due to abrupt deviation of the boundary, it is unable to follow the flow emerging from the small pipe. Flow takes diverging pattern as it gradually reaches the expansion point causing turbulent eddies/vortex in the corners as shown in Fig.1 resulting in mechanical energy dissipation into inter-molecular energy.
Fig.1 Flow in sudden expansion pipe (finite enlargement)
Understanding the flow mechanism from Fig.1, the inlet velocity V1 will be higher compared to the outlet velocity V2 as a outcome of continuity. Due to low kinetic energy near the walls, the fluid particles cannot overcome the adverse pressure in the flow path and hence causing the reverse flow under pressure gradient (from p2 to p1). This creates recirculating zones near the walls of a pipe at sudden expansion cross sections resulting in loss of total mechanical energy.
The loss of head in such pipes can be derived by applying Bernoulli's equation for the control volume of horizontal pipe considering flow to be incompressible. The loss of head (hL) obtained is as shown in equation below.
where,
V1 and A1 are upstream velocity and cross-sectional area
V2 and A2 are downstream velocity and cross-sectional area
g is acceleration due to gravity
With these understanding, problem was defined for two-dimensional flow with axis-symmetry. Geometry, Meshing and Analysis were carried using commercial Computational Fluid Dynamics (CFD) software. The meshing of the fluid domain chosen structured quad mesh is as shown in Fig.2.
Fig.2 Quad mesh
The meshed model was set for iteration after applying necessary boundary conditions. The results obtained from the analysis is as shown in Fig.3, it highlights high velocity zones, development of flow at boundary layers and recirculating/vorticity zones.
Fig.3 (a) Velocity contour (b) Velocity vector (c) Stream function and (d) Vorticity magnitude
Also velocity at different locations are studied as shown in Fig.4, it shows the variation in the center-line axial velocity.
Fig.4 Center-line axial velocity
Like wise, other cross-sections (Fig. 5) can also be studied to understand the complex flow phenomena involved due to sudden change in cross sections in flow through pipes.
Fig.5 Simulation- Flow at sudden bends
Conclusion
Sudden expansion or enlargement flow through pipe is a benchmark problem every fluid-thermal engineers should understand in order to investigate and predict complex flow phenomena in any relevant engineering design. Over the decades, every scientist and engineer involved in fluid research studied this problem from fundamentals to advance which led to innovative systems/structures involving complex pipe designs with varying cross-sections. Problem at fundamental levels can be understood through analytical method with some assumptions in the flow but complex flows to be understood through experimentation and numerical techniques for accuracy.
For more understanding on the problem, visit below video links.
Thanks for reading and have a great learning........ !!!☺
Lid-driven cavity is defined as one of the solid walls on cuboid/cube/square moves tangentially to itself. It is a standard problem studied for understanding tangential motion of a boundary wall in a confined volume with incompressible flow and validation of Computational Fluid Dynamics (CFD) codes.
Problem gained popularity due to its simple geometry, compact domain size and majorly it can be analyzed for wide range of Reynolds number. Though geometry looks simple, flow occurred inside the cavity involves many complexities such as turbulence, re-circulation, fluid trap in re-circulation zones, separation of flow, attachment with both moving and stationary boundary and other flow phenomena. Hence, understanding these complexities makes a good study.
Application
The study of lid-driven cavity gains importance in fluid research because of its application in transport processes in lakes, short-dwell coating, continuous drying and flow in gate slots of water-reservoir dam-gates.
Simulation
Problem was defined to understand the flow in a cavity as shown below in schematic. Modeled and simulated using commercial CFD software. Numerical solution for a 2D incompressible steady flow equation obtained choosing SIMPLE algorithm to decouple velocity and pressure. The results obtained from the simulation is as shown in Fig (a), (b) & (c) highlighting primary and secondary re-circulation zones (vortex) in vector and stream function plots.
Schematic
Fig (a) Velocity vector plot and Fig (b), (c) Stream function plot
Experimental demonstration on lid-driven cavity can be seen in videos below.
Experimental videos from YouTube: Lid-driven cavity flow
Conclusion
Lid- driven cavity is a very conventional problem and served as benchmark problem in validation of techniques and methods for numerical accuracy. Over the period this problem is studied by many researchers validating every numerical methods that exists. The problem helps to understand many complex flow phenomena occurring in a cavity over wide range of Reynolds number. Due to these still it is a popular problem. Problem can be highly recommended for beginning course of study in Fluid Mechanics/CFD, as myself found it's importance throughout my journey as a CFD Engineer.
Also, before getting into the simulation, theoretical understanding on flow circulation is necessary. Few videos below gives theoretical overview on the problem.
Thanks for reading and have a great learning............ !!! ☺
Any Electrical/Electronic system requires an enclosure, without which the
system is incomplete.
Enclosure helps in systematic
arrangement of electronic/electrical components, protects components causing
harm from environmental effects and systematic provision of cooling mechanism.
There is always a complexity in designing big enclosures consisting high heat
generating components in choosing design of the system, cooling mechanisms and
add-on features (Eco-friendly, easy access, aesthetic, compactness etc.)
lead to high cost.
Thermal management in high heat generating components and thus enclosure is
highly challenging, as heat transfer is multi-dimensional. Research has shown
the life of electronic components will be cut in half on every 10 degree
Celsius rise above normal temperature (22-24 degree Celsius). Therefore
cooling becomes necessary and cooling mechanisms chosen plays a important role
in reliability of electrical/electronic components.
Cooling mechanisms chosen always depends on amount of heat generated in an
enclosure. Fig(a), shows simple graph on how the device temperature increases
with time on cooling.
Fig (a): Device temperature increase with time on cooling
When the system is ON, the device absorbs heat and causing its
temperature rise. At some point of time as the heat generated and heat absorbed by cooling
becomes equal the temperature is said to be stabilized and the state is called
as steady state operation condition
and the temperature rise from the ambient is called as
transient state operation condition
as shown in Fig(a).
A typical electrical/electronic enclosure problem was studied comparing
analytical and
Computational Fluid Dynamics (CFD)
software analysis result. Problem was modeled
symmetrically using commercial CFD
software defining its dimensions and components position. The
electrical/electronic enclosure consisting Battery and Inverter as a major
heat generating components. The total heat generated in an enclosure was above
1000W and to dissipate that exhaust fan was chosen through analytical approach
based on CFM requirement.
Fig (b), shows the result obtained from the analysis highlighting hot-spot
zones and air flow streamlines and also contours are plotted for velocity and
temperature. Relied results are obtained from the meshed model undergone
mesh independence test.
Fig (b): Heat generating enclosure with Battery and Inverter
Interesting part of the problem was the optimization study by positioning
exhaust fan at different locations, positioning inlet and provision to
baffles with and without perforations to direct air to hot-spots.
Optimization study makes
important sense because over cooling and under cooling creates problem again
and hence for effective cooling (optimized flow of air), it is
necessary. Pressure drop in
an enclosure is also a concern, as it is directly proportional to the
pumping power cost of an enclosure.
Overall it was an interesting journey in my experience, solving this
problem.
Conclusion:
Cooling of an heat generating enclosure becomes important to keep the
components in optimized operating temperature range. Optimization study
helps in providing effective cooling for a system. Any system should be
tried for symmetric design for simplicity and ease in the system design,
with this aspect choosing cooling mechanism creates real challenge for
design and thermal engineers. Hence, from start of the design to deployment
of the system, if all the important influencing parameters is considered
then the system results in excellent reliability.
Interesting practical videos on Electrical/Electronics cooling are shown below.
Flow over a circular cylinder is one of the basic benchmark conventional problems over a range of bluff body problems because of its importance application in aerodynamic and hydrodynamic problems. The study gives important views on flow separation and wake around the cylinder and also formation of vortex shedding is studied as a function of Reynolds number.
One of the majorly focused results from the study is the Drag force. Practically, in external flow applications the resulted Drag force becomes a major challenge for design engineers in Automotive, Aerospace etc. applications, as built structures in reality are complex and understanding those streamline interfaces and variations in body curvature is a real challenge. One of the ways to solve this problem is to simplify the complex body structure to simple shapes and solve using analytical, experimental (Video shown below) and computational methods. Hence, flow over a circular cylinder problem finds application as one of the simple shapes.
Experimental Setup: Flow over a circular cylinder
The problem is studied opting computational technique using Computation Fluid Dynamics (CFD) software and the general steps involved in solving any CFD problem are:
1. Pre-processing: Modeling, Meshing, defining boundaries and Application of physics and Boundary conditions
2. Solver setup: Residual monitoring, Selection of Solving Schemes/Algorithms and iteration setup
3. Post-processing: Display of geometry, grid (mesh) display, Vector and Contour plots, Particle tracking and Graph plots
Initially, problem was defined for steady state and laminar flow application. Simple two dimensional circular cylinder geometry with circular fluid domain was created. Later, the problem was set for meshing.
Fig (a): Meshing
Fig (a), shows the meshing of circular fluid domain over the cylinder. The mesh is finer at the interface of fluid and cylinder surface and gradually becomes coarser as moved away from the cylinder. Why meshing is made finer at the interface ? is what to be understood by good meshing professional. Meshing technique is always meant as an art.
Fig (b) Velocity contour
Fig (c) Vorticity
Fig (d) Stream function
Fig (b), (c) and (d) shows the simulation results highlighting the Maximum and Average Pressure and Velocity zones, flow separation regions and wake vortex around the cylinder. Also results on Total Drag Force, Drag Coefficients, Vorticity magnitudeand Velocity stream function can be plotted/tabulated.
Similarly, the simulation can be carried for varied Reynolds number to understand the vortex shedding also known asVon Karman Vortex street as simulated and experimentally demonstrated in below shown videos and also steady state results can be compared with transient statestudy.
Simulation and Experimental video- Vortex shedding at varied Reynolds number
Likewise the above problem, square cylinder shaped object is studied. It's important application in Tall building, Skyscrapers, offshore structures etc. those are exposed to continuous wind loads.
Conclusion:
Continuous and rapid increasing innovative design market demands clear understanding of basic problems. Hence, before getting involved in any complex problem solving, it is necessary to start from basics. Flow over a cylinder is one such benchmark problem which helps us to understand various phenomenon and influencing parameters which is still important and applicable in designing many complex models. Theoretical, Conceptual and Physics understanding is must before proceeding to any software tools. This article gives general overview on a problem and application of CFD software for Beginners.
Finally to mention, this was the first problem solved during my UG program and presented here.
More theoretical videos can be found for understanding flow over a cylinder, few are below.
Thank you for reading and have great learning............... Cheers..!☺
