Fluid Dynamics

Fluid Dynamics

  • Dmitry Skiba added an answer:
    Is there any tool which can convert airfoil image or airfoil co-ordinates into mathematical equations(algebraic)?

    In the optimization process, airfoil co ordinates needed to simplify in terms of algebraic equation. Kindly tell me the tool or procedure to do it,.

    Dmitry Skiba · Ufa State Aviation Technical University

    see this they generate mesh on the base of bitmap picture It may be helpfull http://www.freefem.org/ff2a3/Stic-FF2A3-2010/edp/Chesapeake/

  • Martin Blasl added an answer:
    Could you give me a solution?

    I am working in fluid dynamics and facing a problem while running the model using time dependent solver. I am using COMSOL 4.3. The error shows are in attachment file. Please try to give a solution regarding the problem.

  • Daniele Bissacco added an answer:
    There are studies that analyze the dynamics of fluids in the carotid arteries?

    Even through computational analysis, comparing patients with or without carotid plaque? Thanks

    Daniele Bissacco · University of Milan

    Thanks for all answers!

  • Abdallah Daddi Moussa Ider asked a question:
    How the traction over the surface of a particle is computed from the mobility of a point force in Fourier space?

    Let us consider a spherical particle of radius a, moving inside a viscous fluid in the low Reynolds number regime. After solving the Stokes equations in Fourier space, for a time dependent point force Fz, acting perpendicularly on the particle, I came out with the attached equation. Here q is the Fourier transform variable and z0 is the vertical position of the point force. How the well-known bulk mobility can be obtained from this equation?

  • Zijing Ding added an answer:
    Can anyone explain me the problem in CFD?

    For a flow in cavity, if we use the primitive variables to solve the problem by a fully implicit method, why does the system is singular if we use the mass continuity equation rather than deriving the pressure Poisson equation? However, when I applied the pressure Poisson equation and set div u=0 at the boundaries, it does not give div u=0 in the interior of the flow.

    Zijing Ding · Nanyang Technological University

    But why do the original governing equations are singular, i.e the mass continuity equation+the momentum equation.

  • Marković G. Đoko added an answer:
    Can we utilise the relative wind energy of a moving train?
    While a high speed train is moving, the wind is blowing in the opposite direction. Can that wind energy be utilized to run the fans of the train?

    or if that wind can be utilized to generate power? like installing dynamo in the poles near the tracks
    Marković G. Đoko · University of Montenegro

    I think if it possible and we make a device that uses that energy, that would be a kind of perpetummobile, and we know that it is not until now been resolved.

  • Mark A. Levenstein added an answer:
    How can I calculate the airway resistance of a tube?

    Hi to all,

    I construct a plant including a way for passing of air. I can measure pressure at the start and end of tube and flow rate and maybe temperature. I add an extra artificial orifice as resistance Intentionally. How can i calculate airway resistance of this combination with these measures ?

    As you know, the relation between Pressure, flow and airway resistance are obtained as below : R = (P_end - P_start) / (Flow_av) where  Flow_av=(Flow_end + Flow_start)/2.

    1- I have done numerous experiments with different flow rate. In every flow rate, resistance is different but i expect the airway resistance (based on electrical analogous) is constant ! isn't it ?! 

    2- Also i have done numerous experiments with constant flow rate and different artificial orifice. When i add a orifice with R=5 cmH2O/lit/s, the airway resistance (obtained from above eq.) was 6 cmH2O/lit/s. Then I add another orifice with R=5 cmH2O/lit/s and repeated the experiment again, but this time the value of airway resistance was 7 cmH2O/lit/s. What is wrong ?!!

    Mark A. Levenstein · University of Leeds

    What is your length scale? And is the flow laminar? If so, I would suggest you check your pressure drop using the Hagen-Poiseuille equation: delta P = (8uLQ)/(pi*r^4) . This would be how air flow in the lungs can be modeled. Similarly you can modify the equation to estimate the resistance directly: R =  (8uL)/(pi*r^4) . This may be a good check during a control experiment without the additional orifices to see if your pressure meters give an accurate answer. Also to address your first point, I don't think your resistance will necessarily be constant, as by changing your flowrate you could be moving into different flow regimes where wall effects play a greater or lesser role. For your second point, I think doing a control experiment as I mentioned above would help determine what the problem is, for instance, whether it is a meter problem, or the orifice resistances are not correct.

  • Hannes Grimm-Strele added an answer:
    Anyone familiar with Weno algorithm on characteristic variables?


    I work in the area of multiphase modeling and encountered the following problem.

    I want to use weno algorithm on the projected characteristic variables of the problem, as follows

    W(i)=inverse(P(i+1/2)) V(i)

    The estimation of P is done via arithmetic average or roe's averaging of V(i) and V(i+1) where V is the primitive vector, W is the characteristic vector and P is the diagonalization matrix of the corresponding jacobian matrix of the system.

    However, the cell value of the characteristic have the following structure: the same value for all the cells except on cell which has a different value. This structure of the data is obtained for the shock tube problem (and some other problems as well). If I try to implement weno on this kind of data pattern, eventually spurious oscillation would be emerged.

    Summarizing s the problem: W(i)=1 for all “i” except for one cell which has value other than one.

    For the one-phase problem, the data pattern of the characteristic values is different and therefore this problem does not exist. Trying implementing weno on conservative or primitive variables, results in oscillations as well

    Is it possible to circumvent this problem somehow? should I think of other ways to obtain high order spacial accuracy without weno knowing this data structure of characteristic variavles?

    Hannes Grimm-Strele · Numeca International

    There is some information missing which, I think, could be important. First, which equations are you solving? Multi-component Euler, I suppose, but with viscosity? And finally, which kind of flux splitting do you use?

  • Darlena Pagan added an answer:
    How can we extract the rotation tensor from the Jeffery equation for a rotating rod in turbulence?
    The motion of a single small ellipsoidal body immersed in a Newtonian fluid is governed by the equation given by Jeffery (1922), where the inertia and Brownian rotation are neglected. This equation was derived using a no-slip boundary condition at the body surface and matching the velocity field in the inner region near the body to the outer region in the surrounding flow.

    In terms of the temporal change of the orientation vector in the major semi-axis direction, the Jeffery equation reads using the tensorial notation for a rotating rod (please see the formula below).

    My question is:
    How can we express the rotation tensor (the anti-symmetric part of the velocity gradient tensor) knowing instantaneously the positions and velocities of the rod two extremities? Thanks.
    Darlena Pagan · Southern New Hampshire University

    Dear Abdallah:

    Rotation tensor and velocities of rod two extremities, see the below links:

    Rotations by Berkeley at




    The Motion of Rigid Bodies


    I hope this help.

  • Ergin Bayrak added an answer:
    How to do optimum refrigerant distributor design?

    I have some problem at evaporator such as flow side non-uniform distribution. I want to design a new distributor in detail. Normally I have been using nozzle type distributor but I don't ensure desirable pressure drop and thereby distribution. ( It is clear from thermocouple result which is located circuit outlet and capacity reduction both). As a result, this design isn't easy due to two phase flow in distributor. I want to learn your advices and opinions. Thanks.

    Ergin Bayrak · Gebze Institute of Technology

    I want to approach analytical and numerical both. I have added mono-phase flow at Autodesk CFD. I have been dealing to solve two phase problem at this ptogram. But more important thing is analytical approach for me. Thanks

  • Bojan Niceno added an answer:
    What are the applications of Ventilation in computational fluid dynamics?

    how the ventilation is applicaable? give some applications

    Bojan Niceno · Paul Scherrer Institut

    You can vent the room in which you are setting up your CFD simulations, as the most direct example. 

    Another example, although not quite ventilation but more cooling, is applied through fans in the computers on which you run CFD.

  • A.O. Borisyuk added an answer:
    Does someone know where could I find material on the multiphase SIMPLE algorithm?

    I am looking for a very specific paper "A phase coupled method for solving multiphase problems on unstructured meshes" from ASME 2000 meeting. 

    Or what ever information about the PC-SIMPLE algorithm for multiphase solution.


    A.O. Borisyuk · National Academy of Sciences of Ukraine

    Try to find it here


    Best wishes

  • Vasuki Shankar added an answer:
    Is anyone familiar with Ultrasonic flow measurements?

    Hey, I have 2 pairs of PZT crystals (Piezoelectric) fixed to a tube in which water flows. The Transit time, or time from input transmitter (>500 KHz) to output receiver is to be measured. The time Along the flow is smaller and time Against the flow is larger. This time difference gives Velocity.

    1. When I give a Sine input to both the Tx, I am getting the same phased output at Rx, Without any delay, or sometimes a sudden unmeasurable change of phase.

    2. This happens for a pulse also

    (The tubes are of resin and steel and have thick walls)

    Vasuki Shankar · Visvesvaraya Technological University

    Thanks  Gregg Pulley for that brilliant suggestions!!

  • Mohamad Afifi added an answer:
    Can a floating body exhibit negative damping in a real fluid?

    Dear all,

    I am studying the damping characteristics of a floating body.

    At low frequency it exhibits negative damping.

    How can negative damping ratio of a floating body be explained in a real fluid?

    What is causing it to exhibit such phenomenon?

    Mohamad Afifi · Universiti Teknologi Malaysia

    Use the Strouhal Number to check your vortex shedding frequency...if it it matches the resonance frequency  of the body well tou have it

  • Galih Bangga added an answer:
    How do I calculate the total rotation angle of a 3D wind turbine blade depending on the free flow direction for CFD flow simulations?

    I am trying to calculate the power output from a rotating wind turbine blade (3D) using ANSYS Fluent. I am confused about the angle of attack for a rotating blade.

    I want the angle of attack to be 10 degrees, for instance. How much do I need to rotate the wind turbine blade along its axis considering the airflow direction (for example you can assume that the airflow is from down to up)?

    Second question: Fluent calculates the torque value and the power is the torque multiplied by RPM. Do I need to select a specific RPM considering a specific tip speed ratio to calculate the power output for various wind speeds (same RPM for every wind speed)?

    Thank you

    Galih Bangga · Universität Stuttgart

    It can't be directly determined, since there would be an induction velocity existed on the rotor plane. What you can do is approximate the angle using
    AoA = AoAgeom - Twist - Pitch - AoA*

    AoAgeom = atan(Uinf / (Omega*r))
    Twist = twist angle
    Pitch = pitch angle
    AoA* = induced angle (which can't directly be meassured)

    Assuming AoA* to be zero would make the calculation easier, however you would get a wrong value, I suggest to use higher angle of attacks (e.g. set AoA > 10°). Good luck!

    Galih Bangga

  • Miguel Coussirat added an answer:
    What is the relationship between average Nusselt number and Reynolds number?

    avg nusselt number              reynoldsno


    Miguel Coussirat · National University of Technology, Mendoza, Argentina

    In my experience, many times you use these correlations (between Nu and Re number) in order to calibrate/evaluate or check different numerical models in problems involving turbulent flows and natural/forced heat transfer. For complex problems, in general and depending on the problem to study, these correlations are obtained from experiments (e.g., heat transfer in impinging jets or double glazed facanes buildings).....These correlations help to put in order a lot of experimental results, in a compact way and then using those data to compare the obtained numerical results. You can check the theory to obtain these non-dimensional number in White's book, 'viscous flow', for example.

  • Alexander Schesny added an answer:
    Which one offers biggest shear stress value than other, laminar or turbulent?

    If I compare between the flow conditions in laminar with turbulent, there is difference between the viscosity generated resistance for both laminar and turbulent which one have the biggest value of shear stresses?

    Alexander Schesny · Mississippi State University

    The stronger dependence of pressure drop on mass flow rate for turbulent flow results from the fact that more energy has to be supplied to maintain the violent eddy motion in the fluid. 

  • Alexander J. Babchin added an answer:
    Is there any correlation to predict capillary pressure in soil or other porous media?

    I want to generate a code for simulating three phase (water, oil, gas) flow in soil and it needs to calculate capillary pressure between each two phase as a function of saturation, soil properties, and fluid surface tension

    Alexander J. Babchin · Tel Aviv University

     In the case the gas is steam, only condensate contacts the solid surface, along with oil. Then, you have only one Pc. 

  • Ali Heydari added an answer:
    Is it the flow in a tube same with a differential measurement of the fluid reservoir ?

    with a number of assumptions,Is it the flow in a tube similar with a differential measurement of the fluid reservoir ?
    How far are we from the fact that if these assumptions can be made? I think that might be reviewed earthquake on the pipeline same with a tank. of course in the small time.
    thank you.

    Ali Heydari · Semnan University

    Thanks for your consideration john:
    But I think the question is clear.

    might be reviewed earthquake on the pipeline similar with a tank. of course in the small time and with a differential measurement of the pipeline?

  • Carlos E Losada added an answer:
    There any previous experimental experience on Impeller designs for Inline Mixers?

    There any previous experimental experience on Impeller designs for inline Mixers?

    Carlos E Losada · GL&V Pulp and Paper

    Thanks you Professor Maddali V S.

    The difficulty has been selecting the correct Viscosities (cSt) for a variety of Pulps with different consistency (Stock).

    There some data from Gullichen but none seems to have a appropriate update



  • I. Mohammed Rizwan Sadiq added an answer:
    How can I find the velocity profile of thin liquid films in cylinder?

    fluid mechanics

    I. Mohammed Rizwan Sadiq · Frankfurt School of Finance & Management

    For a theoretical approach, people use long-wave assumptions to find the expressions of velocity, pressure and temperature and derive a long-wave model using it. The literature below  will give you a deep insight

    1) http://www.utwente.nl/tnw/pcf/education/j.m.burgerscentrum_research_school_for_fluid_mechanics/jmbc_course_on_capillarity_driv/Articles/anton_darhuber/surface_tension_etc/Oron_RevModPhys97_thinFilms.pdf

    2) http://www.coating.udel.edu/pdfs/obsch.pdf

    3) http://link.springer.com/article/10.1007%2Fs10665-004-1772-7#page-1

    4) http://www.aps.org/units/dfd/meetings/upload/Ashmore_DFD04.pdf

    5) http://www.iscst.org/pages/ECSConf/ECS2007/Posters/Duffy_07.pdf

    6) http://www.staff.ul.ie/eugenebenilov/hpage/pubs/jfm13b.pdf

    You can also refer to work done as proposed by Joanne.

    If you are doing experiments, I would recommend what Marcin and Alexander have proposed.

  • Abhinav Dasari added an answer:
    Anyone have study materials for non-equilibrium in hypersonic flows? Can anyone suggest some books?

    I want to study non-equilibrium phenomena in hypersonic flow. Please provide me the materials related with the same.

    Abhinav Dasari · University of Michigan

    I would suggest Hypersonic Aerothermodynamics by J J Bertin and Hypersonic and High-Temperature Gas Dynamics by J D Anderson. I used these two from my Hypersonic Aerodynamics at the University of Michigan. Vincenti and Kruger's book mentioned by Avraham is also an excellent resource for molecular dynamics and kinetic theory

  • Is there any FLUENT UDF macro to replace the new mesh in each time step?

    I am trying to simulate a model in FLUENT that the geometry shape is varying by time. I was wondering can I generate different geometry/mesh each time step and subsequently replace the new mesh each time steps?  In fact, in the dynamic mesh events, there is option can manually define to replace the mesh in given time but I want to know can I use a UDF to do it automatically (i.e, load the new prepared geometry with new mesh each time step)?

    Siamak Nezami Doost Alamdari · Swinburne University of Technology

    Yes Soroush. Reconstructing the geometries one by one and then meshing is time consuming. I have not reconstructed geometries/meshes yet. I just generate points in each time steps and looking do that automatically (reconstructing the geometry by loading the node position each time-steps and then using the remeshing features of the FLUENT).

  • Hossein Khalili Shayan added an answer:
    What distance occurs after sudden expansion flow is fully developed?

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

    Hossein Khalili Shayan · University of Tehran

    In the scientific references, a distance of 20-50 diameter is recommended for fully developing of flow after sudden expansions. For the open channels, the experimental data shows that using a hump decrease the flow separation and energy loss. In this case, hump with a crest height of 5-9% of the approach depth are recommended. for more information, you can refer to this valuable paper:


  • Edson Del Rio Vieira added an answer:
    Where is the best place to locate the hotwire for vortex shedding measurement of a circular cylinder?

    How far it should be from the cylinder, also has it to be located in the y=0 position or it needs to be relocated in y direction?

    Edson Del Rio Vieira · São Paulo State University

    I use flow visualization to place the hot film/wire probes in the vortex street in order to determine the vortex shedding frequency of cylinders.  In several flows, the velocity Fourier Transform shows very difficulties in order to determine the fundamental vortex frequency, principaly to very low frequencies. Flow visualization is a useful tool in order to place the probes and to obtain a good relation signal to noise in the FFT. Please, see my papers to more details.

  • Rajesh Alayil added an answer:
    Are static pressure and absolute pressure the same in magnitude?

    I am aware of the individual definitions of both static and absolute pressure.

    Will the magnitude of static and absolute pressure be the same, even though  definition wise they represent different concepts?

    Rajesh Alayil · Indian Institute of Technology Madras

    @ krishna, thanks for the info.

  • Mohamad Afifi 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

    Mohamad Afifi · Universiti Teknologi Malaysia

    Include the losses due to friction in your Bernoulli equation 

  • Jignesh Thaker added an answer:
    How to measure velocity near pipe wall using LDV?

    I am using Laser Doppler Velocity (LDV) meter for measuring the velocity profile along the radial direction in pipe flow (ID 25mm). I am finding difficulty near the wall region. Near the wall, i am getting abrupt value of velocity and i also observed that the turbulent intensity (%) reaches up to 100 or more than that. Please anybody suggest me how to measure correct value of velocity near the pipe wall.

    Jignesh Thaker · Sardar Vallabhbhai National Institute of Technology

    Thank you for reply. I am measuring the velocity for single phase flow (medium is water) using LDV. There is one doubt, at low Reynolds number (in the range of laminar flow) can LDV measure the velocity profile which is exactly match with the analytical or empirical study? If yes than in what accuracy it measures? Suppose at the center point of the pipe can i get maximum value which is equal to twice to the average value of velocity by using LDV? I am using LDV which has a lens of 250 mm and it covers 2 to 3 mm measuring volume (fringe volume).   

  • Abhilash Kumar Tilak added an answer:
    Can anybody suggest how I can purchase Nanofluid (Al2 O3)?


    I'm doing my project in B.Tech program.and i need a nano fluid(Al2 O3)(we are mixed Al2o3 with water) to find out a friction factor in pipe and pipe fittings.

    So i would like to mention below your idea of my project and any suggestion and also must post how can I get the Nano fluid(Al2o3) or (*other Nanofluids)

    Thanks in Advance,,,,,please help me out

    Abhilash Kumar Tilak · Rajiv Gandhi University of Knowledge Technologies

    As Bayram Sahin suggested you can purchase the nanoparticle from Sigma Aldrich but you should be specific about the type whether it is gamma alumina or some other.

    There are various methods available to prepare the nanofluid. Either you can follow one step method or two-step method after this important part is the stabilization of the nanofluid.

    A Review on Nanofluids: Preparation, Stability Mechanisms, and Applications by Wei Yu and Huaqing Xie  gives a good explanation about the preparation of nanofluids. 

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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