Fluid Dynamics

Fluid Dynamics

  • Aditya Prakash Singh added an answer:
    What is the value of drag coefficient in the case of hypersonic flow for sphere and hemisphere configuration?

    I want to know the formulation as well as the range of drag coefficient values for sphere and hemisphere. Thank you

    Aditya Prakash Singh · Indian Institute of Technology Bombay

    Check my answer in the link.

    For hemisphere with the half sphere facing the upstream, according to Newton theory would give the same drag coefficient as full sphere. Newton theory is valid after Mach 8

  • Kirill Tsiberkin asked a question:
    Can anybody point to the experimental studies of instability of flow over a saturated porous medium or flow in channel with porous walls?

    There are a lot of theoretical models, and it is necessary to verify them.

  • Siddesh Desai added an answer:
    How does vorticity affect the pressure and entropy in the shock layer in case of hypersonic flow?

    How vorticity effects shock structure in hypersonic flow case?

    Siddesh Desai · Indian Institute of Technology Guwahati

    @Julien Laurenceau i want to know how does it reduces the drag in hyper-sonic flow situation and how pressure and entropy get effected by generation of vorticity in shock layer.....

  • Dr. Ravindran S. added an answer:
    How can one design an Air Electric Heater to heat air up to 700C?

    I tried to heat air with flow of 5L/sec from 30oC up to 700oC through a tubular electric heater, as illustrated in the pictures. I used kanthal wires three coils, with 150V*15A*3=6.75kw.

    But the max. Temp. was 200oC.
    Do you think increasing the pathway of air as illustrated in the attached drawing to increase the pathway 3 times could enable to achieve 700oC.

    Any, suggestion, Please.

    Best Regards!


    Dr. Ravindran S. · Bharath University

    As Sameh Nada, Benha faculty of Engineering, Benha University, said you have poor bypass factor.  

    1. Try to have red hot condition of the heater (select a suitable kental or nicrothal wire for the purpose).  
    2. Ensure the surface temperature is at least 1000 deg C.  (you may use a thermal imager to measure surface temperature of the heater wire).  
    3. As the flow velocity of air is high, try to keep the wires wound around an insulating material, to cover full area of flow.  
    4. Test without any beads over the wires (this will lead to direct contact of air with heater wires)
    5. try with six heaters fixed zig-zag manner in the duct.  
    6. insulate the duct with suitable insulation to minimize heat losses (you can think of guard heaters on the down stream side of duct wall)

    If you have tried all these already, think of having heat exchanger - gas from a burner and air heat exchanger



  • Pooyan Nikeghbali added an answer:
    What value of gamma should be given in the equation of state (Tait equation) for oil?

    There is a well-known equation of state between pressure and density of fluid written as P=B[(rho/rho0)^gamma-1]. Gamma in the equation is different for each material, e.g. water=7.
    I am recently working on sunflower oil dynamic simulating. But I couldn't find what value of gamma should be taken. What I have merely known is that 10 is a suitable value of gamma for castor oil. But I'm not sure whether the value could be used in sunflower oil simulating.

    Could anyone provide me some information on the value of gamma for different materials.

    Thank you very much!

    Pooyan Nikeghbali · University of Sistan and Baluchestan

    in this thesis, he's written the gamma is usually between 1-7. If I'm in mistake I glad to say it and guidance me. thank you

    best regards

  • Bladimir Ramos-Alvarado added an answer:
    How does the velocity field around a quadrilateral doublet panel look like?

    I am writing a panel code using the method suggested by Katz and Plotkin. When checking the velocity field around a single quadrilateral doublet panel using their formulation I was a bit confused. I thought the velocity field would be symmetrical but I cannot find any reference confirming this. 

    I attached a plot of the velocity field. Is this correct or should I expect a vortex-ring like velocity distribution.


    Bladimir Ramos-Alvarado · Georgia Institute of Technology

    The book can give you an idea of what to expect if you can figure out a way to translate your hypothetical problem into one of flow around a real blunt body. The streamlines between potential flows and real flows are similar in the regions away from the wall.

  • Rickard E. Bensow added an answer:
    Can somebody outline the advantages of utilizing a hexahedral mesh as compared to a tetrahedral mesh?

    why is it that hexahedral meshes are more accurate that tetrahedral meshes while still having a lower cell count. Why are they more preferred, i am a beginner to cfd and anybody who could explain this to me in simple term is greatly appreciated.

    Rickard E. Bensow · Chalmers University of Technology

    Solvers today are generally better at giving good accuracy on tet meshes than they used to be a couple of years ago. Moreover, in a complex geometry it can be more difficult to create a conforming hex mesh leading to worse mesh quality and difficulties to converge the pressure solver. But if the geometry allows for it, conforming hex has advantages as has been outlined above.

    And if one would like to do mesh refinements studies it's of course also much more consistent.

  • Barış Biçer added an answer:
    How to Simulate Micronozzle Using DSMC (dsmcFoam) ?

       I'm trying to simulate micronozzle using dsmcFoam. All the folders like 0, constant and system are set. To create vacuum at the exit I'm providing high velocity at the exit and initializing, but I'm not able to see any property difference (Pressure , velocity ....). If anybody knows simulating DSMC using OpenFOAM, please suggest me.

    Thank you 

    Ram PSV

    Barış Biçer · Kobe University


    Have you ever looked at this paper http://www.scirp.org/journal/PaperInformation.aspx?PaperID=29420#.VPHPyFphyfQ

    and this website  http://www.physimmasters.com/wordpress/?page_id=187.

    these may help you and let you find something useful.



  • Alliche Mounir added an answer:
    What distance occurs after sudden expansion flow is fully developed?

    Is there any relation for calculation of x(in the picture)?

    Alliche Mounir · Dr. Yahia Fares University of Médéa

    Hi Ouchiha

    Firstly the distance of 5 to 10 times the diameter duct is not sufficient to reach fully developped flow. I think that   Avraham Hirschberg proposition is very reasonnable and very practice.. I tested 20 to 30 times and it gives acceptable results

  • Abolore Abdulahi added an answer:
    How does pressure effect flow behaviour in two-phase flows?

    I am investigating the effect of pressure on two-phase oil-gas flow behavior using CFD techniques for my university dissertation. I am looking for any articles or information which may aid me in my literature review, specifically on how this may affect the transitions between separated and intermittent flow or how the intermittent flow region is affected. Is anyone able to recommend any particularly good articles?

    The justification for my research is due to the high pressures found in deepwater reservoirs.

    Abolore Abdulahi · University of Nottingham

    A paper by Abdulahi A. et. al. (2013) on the "Effect of pressure on a vertical two-phase flow with a high viscosity liquid" can be a useful source for your Literature Review. Let me know if you need further information or help regarding this

  • James Garry added an answer:
    Are there mathematical equations which show when a vacuum is created?

    As per my understanding vacuum is a state when or where the pressure inside a system goes so low (or equal to zero) that it creates a vacuum.

    I needed book (PDf or name of book) or papers which states that when a vacuum will be created with numerical assessments. ?

    What are conditions that a vacuum to be sustained which is  assisted with mathematical derivation or formula?

    Geometrical factors deciding vacuum creation?

    Any other definition of vacuum is welcomed. 

    James Garry · Red Core Consulting ltd.


    I add to Alfonso's valuable comment, from the perspective of having used ultra-high vacuum systems.

    In general, once you have removed obvious leaks from your system, the pumps will be fighting the mass-flow created from the sublimation and desorption of matter from the system's components. Water adheres well to stainless steel, and at pressures below 10^-5mbar, at room temperature, you will find water vapour as the major component of a UHV system's gas. So, the equation you need to find, to model the partial pressure of this is the Anroine equation.

    At a given temperature, for a given material, this equation tells you what the partial pressure of the compound is - and if you know the area exposed, and the pumping rate of the system, then you can calculate the equilibrium pressure that will exist.

    Note, pumps work on different molecules in different ways.

    A rotary pump evacuates all species - without favour.

    A turbopump is not able to pump hydrogen well - and so the exact outcome depends on the pumping method you have available.

  • Mladen Aleksa Tomić added an answer:
    Is anybody familiar with two-fluid settings in Cham Phoenics?

    I am working on a heat exchange problem between water and air. When I am modeling, both fluid simulations become unstable.

    Mladen Aleksa Tomić · Visoka tehnička škola strukovnih studija, Serbia, Niš

    Dear Abdul,

    Thank you for the answer and dr. Onde day maybe Ill be one :) The simullations I am running, are the steady one. And as matter of fact, I am very familiarwith one fluid simulations.

    For settings, I am using mostly VR for settings and smt q1 file. Usualy it is not the problem to start the simulation with two fluids, but the convergence gives me the "saw diagram".

    Of course, when I am running the one fluid equivalent of the simulation, the convergence is excellent. So, I think I am missing fine tunning somewhere.

  • Shiuh-Hwa Shyu added an answer:
    How can I calculate mean velocity for a given Reynolds number?

    Re=(Density*Mean velocity*hydraulic diameter)/(absolute viscosity) 

    For a given cold plate with multichannels. How can I calculate mean velocity for coldplate? Please note that cold plate has only one inlet and only one outlet and water flows only in these grooves.

    Shiuh-Hwa Shyu · WuFeng University

    1st off, that definition of Reynolds number stems from the pipe flow. Most people use that, so makes no wrong to follow that. If you can pursuade researchers that other definition makes more sense or more useful, you and change the definition. 2nd, use of mean velocity as characteristic velocity is that it is easy ti obtain experimentally. Simply measure volumetric flow rate then divide it by cross-section area. 3rdly, using hydraulic diameter has been validated experimentally and people found out that the correlation for circular pipe can be also apply to other shape of cross-section with acceptable accuracy. So, stick to this definition make a lot of sense.

    Now, back to your problem: How to calculate the mean velocity? How do we calculate mean velocity for circular duct should be adopted for any problem attempting to link to the circular pipe flow. So, divide volumetric flow rate with cross-section area is the one you should use.

  • Guimarães Osvaldo added an answer:
    Any approximate solution of highly nonlinear differential equations?

    Dear friends, I have worked out some mathematical models of Walter's liquid B fluid.I use change of variables and convert the system of equations to highly nonlinear differential equations. Know the problem is to approximate the solution .Can some one gave us the suggestion of how can we solve the problem. Or what method is efficient and implemented easily. Any suggestions will be appreciated.

    Guimarães Osvaldo · University of São Paulo

    Hello,Khalil. I have used the GA with orthogonal polinomial for this kind of problem instead of linearization's Newton Method, since this one use to diverge.

    I have the code in Matlab ready to work, if you wish.

    All the best,


  • Abdul aziz Jaafar added an answer:
    Is the given relationship valid for multichannel heat sink?

    Pump Power=pressure drop*Mass flow rate

    The above relationship was given for a multi-channel heat sink(cold plate). The argument was "when fluid(water) flows through many channels in a heat sink(cold plate) their is a pressure drop, and hence change in dischage" so pump of different power can be used or power of a single pump can be varied.

    All I want to know if the above equation is true or not?

    If it is true then how?

    If it is false then What is the reason?

    Abdul aziz Jaafar · Universiti Malaysia Pahang

    Dear Abisyek..

    From you three-dimesional diagram,  there is no thermodynamic device and I will assume no net heat transfer involve and the flow will be incompressible and isothermal.

    I will also assume that the gravitational acceleration vector  is in the thickness direction so that Ii can neglected the potential energy changes for fluid that enters and leavings the coils.  From thermodynamic point of view, I will assumed that the fluid is a steady flow devices and is an open thermodynamic system.   I will further assumed that the fluid is doing work to flow through the complex  cooling coil geometry.

    I will be referring the good book of Prof Cengel for giving my argument in which based on the above assumptions, the resulting energy balance will be deduced as the  frictional power (W_friction) = pressure change x time rate of mass flow / density  or flow power

    and the frictional power is equal to the instantaneous shaft power that a 'physical' pump should deliver assuming no mechanical loses.       Note that the pump should be installed upstream from the inlet of the coil section OR at the downstream location from the outlet location. It is all depends on you pump design and performances..

    So in my opinion, the equation that you posed should have a density because it will not give a unit of watts.  and the shaft power to rotate the pump empeller is not directly come from energy balance but the value is based on the amount of friction power that need by the fluid so that it can flow at the designated mass flow rate.  The pump power in the above equation may not be referring to a physical pump but referring to  a mechanical work in the context of thermofluid engineering

    This equation is wrong if your problem does not satisfy the assumption that has been made, but sometime it is still acceptable because engineering assumptions are made because the designer feels that these contributing factor does not significantly affecting the frictional power.

    I might be wrong .. but hope this help.. wallahuaklam


  • Shuvayan Brahmachary added an answer:
    How to determine drag force over a sphere (2D axisymmetry confriguration) for hypersonic compressible flow condition?

    I want to calculate the drag force over a sphere i need a formulation to do that ..using CFD

    Shuvayan Brahmachary · Indian Institute of Technology Guwahati

    I agree with Aditya but since you are using CFD codes to evaluate Cd you can't be direct like the local inclination methods. Obtain the surface pressure distribution and then the surface pressure coefficient from it. Numerically integrate it taking infinitesimally small strips. Non dimensionalise  it with base are and viola!!! you have the coefficient of drag

  • Vijay Duryodhan added an answer:
    Do you have a paper or name of any book which specifies pressure drop, discharge according to geometry?

    Both magnitude of a pressure and mass flow rate of any fluid is affected by the geometry of flowing in and out.

    Any book or paper which has lots of examples to show pressure drop and discharge for various geometry will be of great help.

    Vijay Duryodhan · Indian Institute of Technology Bombay

    Hi Abhishek,

    Apart from the above mentioned reference book, I would also like to suggest research papers by Morini et al."Laminar liquid flow through silicon microchannels" on non-dimensional Poiseuille number for different shapes of geometry. Of course this is for microscale geometry but you will definitely get some idea.

  • Gergo Vassilev added an answer:
    Can such flow profiles develop in tightly packed bed of spheres?

    Spherical particles of silica and other materials are tightly packed at very high pressures (>10,000 psi) into narrow columns (< 5 mm i.d.) for separating mixtures. The smaller the width of the separated bands, the higher the quality of the packed bed in terms of micro-structure. In reality, it is always a random closed packing of hard spheres.

    Please see the attached file, where authors purpose different flow patterns which can develop in a packed bed. They do not show any calculations nor give a reference to flow profiles in a packed bed (which is quite wide in diameter).

    The question is related to figure 3 (c) and it says that exceptionally narrow separated bands can be seen as a result of this flow profile. Is anyone aware of oscillating flow profiles in packed beds or is it just authors' imagination?

    Thank you.

    Perhaps not quite scientific assumption, but if the particles are filled not when dry, but when there is significant flow through the column, the packing density will depend on the velocity. Also going from hard sphere model to real deformable particles might flatten the plug flow.

    Another modification might be achieved with heating/cooling the column, creating expansion/contraction profile and depending on the elastic properties to change the form and packing densities. So column manufacturing vs column working conditions might be hidden trick.

  • K. V Sharma added an answer:
    How should I do conduction, convection and radiation simulation all together to get a temperature distribution?

    I am performing simulation of X ray tube used in CT scan. Cathode and anode are both enclosed in vacuum glass, and oil cooling is applied to anode heat sink point. Now, when cathode bombards the electrons on anode, the heat is generated, which is taken away through vacuum (till glass envelope - Radiation), then through solid material attached to anode and afterwards by oil at sink (convection). What are the different methods available through which I can get temperature distribution in anode and also in x-ray tube.

    I have performed conduction simulations before in Ansys but in this situation all 3 together are acting, kindly guide me on this to achieve best results.

    K. V Sharma · Universiti Teknologi PETRONAS

    Heat is generated at one end (anode) of the solid cylinder.  It may be estimated by suitable means.  This heat is now conducted along the cylindrical rod by conduction and certain part leaves the surface of the rod by radiation.  This is a conjugate phenomenon. The process can be modeled as a one dimensional heat conduction using the fin equation with radiation alone (convection absent).  

    Since the other end is dipped in a liquid, the quantity of heat leaving is not known. This has to be estimated by determining the temperature gradient at the tip.  Hence convection need not be known or determined.  Since radiation is considered in the analysis, the problem has to be solved numerically.  Note that I have explained the solution methodology assuming that heat is generated at one end (anode tip) of the rod and not in the volume of the rod. 

  • Claus-Dieter Ohl added an answer:
    Are there any proper tube fitting/connections for a rectangular glass microcapillary?

    I have to connect a syringe to a squared glass microcapillary. I looked in many online shops but I've never found a proper fitting thus having always liquid leakage between the microcapillary and the connection.

    Claus-Dieter Ohl · Nanyang Technological University

    There is an epoxy putty which you can mix and shape and then harden in an oven. That worked well for any fluid and gas tight connection, however a permanent solution. 

  • Sourabh Singh added an answer:
    In the attached picture why does water take a spiral path when low frequency acts on it?

    I guess its because of a pressure difference as created by sound, as at one place there is compression and at another place there is expansion. Water prefers to go in lower pressure so while going forward it bends towards lower pressure. It happen at lower frequency because water have enough time to respond. For all this to imagine, just see at pipe , pipe will go forward and then backward , maybe its due to at one time there is low pressure at one place ,  then at place where there is low pressure , then it is shifted by high pressure , and pipe move forward so pipe could go at place where there is low pressure .may be its happening to every part , but why it is rotating ? Does anyone have an idea why it is happening?


    see this for viewing pic.
    How to use langrangian , or how waves are interfering with each other ? 

    Edit:  Not going to make pipe oscillate , because
    1)24hz is still large frequency and hence very less time period in order to whole pipe react. 
    2) The force applied by that pressure is very less , so whole pipe is not going to move .

    We can do little bit of maths to proof it .. so this idea which is given above is wrong. 

    wow , awesome video ,thank you for sharing . 

  • Lydia Dux-Santoy added an answer:
    What do you think is the "best" way to compute wall shear stress (WSS) in aorta?

    Has anyone experience in this sense?

    I've read several papers and the report different ways to compute WSS, with different assumptions. I would like, if possible, to know what do you think is the best method proposed for the moment.

    If possible, suggest bibliography to support this method.

    Thanks in advance.

    Lydia Dux-Santoy · VHIR Vall d’Hebron Research Institute

    I'm coming back to thank you again all of you, and briefly explain what I'm doing for the moment, after talking to cardiologists and considering different possibilities.

    I'm working with MRI 3-directional velocity encoding (i.e. 4d Flow MRI). Then, I've decided to compute WSS without using CFD (at least for the moment), so I'm going to implement Stalder's method, cause I want to obtain vectorial components of WSS.

    Although this method underestimates WSS, it seems good and is frequently used by other researchers. I'm aware spatial resolution it's a problem for me, but let's see.

    Have you implemented or worked with Stalder's method for computing WSS?


  • Ron Reiserer added an answer:
    Selection of appropiate blade profile for axial fan ?

    I am struggling with selection of blade profile for an axial fan. the data I have are the fixed rpm, flowrate, fan dia, no. of blades. Kindly help me out on which parameters should i select blade profile and how can I calculate relative blade angles.

    Ron Reiserer · Vanderbilt University



  • James M Buick added an answer:
    What software or techniques of visualization are you using with Lattice Boltzmann methods (LBM)?
    I implemented the D2Q9 and the D3Q19 LBM in C/C++ language and I am using OpenGL to visualize the macroscopic fields. But I am interested in a more realistic visualization.
    James M Buick · University of Portsmouth

    I find Tecplot good for this

  • Hasan Al Ba'ba'a asked a question:
    How are Froude and Reynolds numbers calculated for an aeration basin?


    I was reading the following paper:

    Oxygen transfer model development based on activated sludge and clean water in diffused aerated cylindrical tanks.

    and I found that the authors have calculated Reynolds number and Froude number using the equations attached. What they did is that they used the volumetric flow rate of the air instead of the slip velocity of the two phases to calculate the Reynolds and Froude number.

    The second question is:

    What is the significant of Froude number in an aeration tank?

    Thank you very much.

  • Indranil Banik added an answer:
    Experiments with non-Newtonian fluids?

    In your opinion what are the best fluids  to use to experiment with non-Newtonian fluid.
    I look for a shear-thinning fluid (without viscoelasticity).
    I am also looking for a viscoelastic fluid (without elongational properties).
    I want to work at room temperature.
    Transparent fluids are desirable
    The maximum shear rate is 1000 s-1

    Indranil Banik · University of St Andrews

    Glycerine has a much higher viscosity than xanthan & I think it's less shear-thinning. Depends exactly what you want. What'll be good is to estimate the timescale of the experiment and see if this is acceptable, though it isn't always possible to do this.

  • Kezhao Fang added an answer:
    Does anyone know the modern technique for measuring velocity profiles along water column in a wave flume?

    I would like to measure the vertical velocity profile along water column in a wave tank, where the water depth is quite shallow, approximately 10 cm. Under this circumstance, what is the best option to obtain the velocity profile? Thanks.

    Kezhao Fang · Dalian University of Technology

    Thanks for Nathan Wells  and Francesca De Serio  providing usful information on that issue and I would like to consider the mentioned methods.

  • A. S. Francisco added an answer:
    How does pressure and velocity vary in a pipe with constant diameter and fully developed flow?

    The gauge pressure between the two sections is desired to be 0 and the pumps are centrifugal pumps. There is also head loss due to friction.

    What I want to know is how velocity and gauge pressure vary along the suction and discharge lengths of the pipe. And also how the gauge pressure can be made zero at the intersection after exiting the first pump

    A. S. Francisco · Universidade Federal Fluminense

    The velocity and pressure can vary due to the viscosity effect of the fluid that causes the head loss of the pipe.

  • Tarik Ömer Oğurtani added an answer:
    How does the presence of corners affect the 3D boundary integral equation?

    It is well known that corners require a special treatment in the 2D boundary integral equaiton. The factor 1/(2pi) appearing in the equation should be replaced by 1/(2 alpha), where alpha is the corner angle (see the problem 2.6.4 in the book of Pozrikidis on the boundary integral methods for viscous flows). Here I am concerned with a 3D viscous flow problem where the geometry is composed with several planes that form sharp edges. Right now, I am excluding those nodes from the computation but I get dubious results. I wonder if the nodes at the corner need a special treatment as well. Any help will be highly appreciated. Thank you in advance.

    Tarik Ömer Oğurtani · Middle East Technical University

    Dear Dr. Popov, I thank you for your list.  I was just joking when I said 'no books left to be read!  I have a set of articles (Craster & Shain etc.) on the Laplace Operator, which are interesting in on the diffraction and scattering of  EW.  But how about the elasticity?  All they admit that up to now no acceptable or manageable solution of singularity appeared any where in the literature.  Best Regards

    Note: I have solved very important singularity problem nothing to with the Laplace or Helmholtz  but related to  dynamics of the grain boundary triple junctions motion formulated by the  irreversible thermodynamics. 

    I will check all those articles you have kindly commented above. Thanks again.

  • Miguel Coussirat added an answer:
    Is there an exact analytical solution of the Stokes equations for the fluid flow between two divergent planes?

    I would like to know whether exact analytical solutions exist for the fluid flow between two planes with a linearly increasing height, when the convective term is zero (please see the attached image). The flow is pressure driven flowing in the x direction (the gravity effect is neglected). Unlike a plane Poiseuille flow, the component w pointing toward the wall does not vanish. The geometry is infinite along the y direction. That is to solve for the two unknown velocities u and w, both are functions of x and z. Any hints / references would be highly appreciated. Thank you.

    Miguel Coussirat · National University of Technology, Mendoza, Argentina

    May be you can check this:

    P.G. Drazin, N. Riley;  The Navier-Stokes equation, a classifications of flows and exact solutions, Cambridge University press

About Fluid Dynamics

Fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids (liquids and gases) in motion.

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