Fluid Dynamics

Fluid Dynamics

  • Gerro Prinsloo added an answer:
    Mechanical or mechatronic equavalent to reactive power and power factor in electrical systems ?

    The concept of power factor or power factor correction as relationship between the apparent power and active or real power is interesting (reactive power associated with inductor/inductive or capacitor/capacitive components characterized by imaginary numbers in an electrical circuit or distribution system).

    The concept of dissipating "imaginary" energy or reactive energy is slightly complex for mechanical engineers.

    Are you aware of analogous mechanical, Mechatronic, thermodynamic, heat transfer, fluid flow or thermodynamic principles of the same kind, conceptually or mathematically ?

    Gerro Prinsloo

    A friend of mine also referred me to these sites:

    The power factor in mechanical system


    Power factor analyses in mechanical system focusing on trajectory and environment


    • [Show abstract] [Hide abstract]
      ABSTRACT: This paper represents a analytical, simulation and experimental study of power factor in mechanical system. In electrical system, power factor is often used to evaluate how intense we can use the electrical power as a active electrical power. It is useful when considering the power consumption of the electrical system. However, this kind of approach did not exist in mechanical system. In mechanical system, we consider mechanical loss of energy, however we did not consider the amount of power used in the phase change. Thus this study introduces the power factor in the mechanical system and investigates how much power is actually used in the motion. Power factor of certain frequency of the motion was analyzed and compared with the simulation results. Experiments were conducted to further study the behavior of power factor in many situations. Experimental results signify power factor changes due to the speed of the motion and also path of the motion.
      No preview · Conference Paper · Jan 2013

    + 1 more attachment

  • Salim salim Hamieh added an answer:
    I have included the details below, anyone could give a hint on how to improve the cl achieved through simulation?

    Dear All, I have been trying to conduct numerical simulation on NACA43012 airfoil, but unfortunately i was not able to achieve verified results for cl, while the answers i get for cd is correct. I am using C-mesh topology with 7 million cells, i prepared the mesh Using POINTWISE software and imported into FLUENT, I achieved cl of 0.04 while the published data is 0.2, and for Cd i achieved 0.0069 which is the same as the published data, i would appreciate if anyone could guide me or give me some hints regarding the uncorrect cl.

    Salim salim Hamieh

    Very interesting idea and very helpful data mentioned above , I'm simulating a Naca 0012 , hope this data help me , and ill share , my data once been finished .


  • Mahmoud Zayed added an answer:
    Is manning equation the best method to determine the discharges in open channels?

    Is manning equations the best method to determine the discharges in open channels?

    Mahmoud Zayed

    dr Steven J. Wright

    thank you

  • Mohan Kumaraswamy added an answer:
    How to Calculate the Absorb Kinetic energy and dissipated kinetic energy on to the fabric to evaluate the Ballistic Performance ?

    I have created the Sandwich of Fabric and it was subjected with high velocity projectile and now i want to calculate the following parameter.

    Mohan Kumaraswamy

    If it has not completely penetrated into the fabric, it has lost all its energy. Hence the entire initial energy is absorbed by the fabric and the final velocity is zero.

  • Sandro Longo added an answer:
    How is possible to use PIV for a fluid flow initially perfectly at rest?

    I need to use planar PIV for detecting the instability cells in a Hele-Shaw cell triggered by a gradient of temperature, with a window size of 40x40 mm^2. The fluid in the cell (1 or 2-mm gap) is initially perfectly at rest, then increasing the vertical gradient of temperature instabilities develop at a given critical Rayleigh number. I need small particles (tens of micrometer in diameter) which do not settle or float for, say at least 10 minutes, to act as tracer. Probably some natural substance, able to absorb water and to equalize its density with the density of the ambient fluid, could represent a good solution. Does anybody have experience on the topic, in order to give me hints and suggestion?

    Sandro Longo

    Ciao Hubert,

    I thought the same thing. However we are working with a macro lens and the field of view is 40x40 mm2 at 1024x1024 pixels. The overall resolution is nearly 40 micron/pixel. Hopefully the pollen particles will lump in order to occupy at least one pixel. I will let you know, thank you.


  • Kamal Mohammedi added an answer:
    I want a way for measuring small amounts of mass flow rate of gas ?

    i designed an orifice with vena contra 1/27 this design isn't following standard designs i ask about how can i calibrate this orifice ... orifice hole is 1 mm and pipe is 27 mm .. 

    Kamal Mohammedi

    How about the soap bubble gas massflowrate measurement.

  • Mahmoud M. Abdelrahman added an answer:
    What is the best way to calculate airflow rate in a building interior whether using CFD or any other measurement tools?
    What is the best way to calculate airflow rate in a building interior whether using CFD or any other measurement tools?
    Mahmoud M. Abdelrahman

    I have found this paper and I hope it is useful :

  • Siamak N Doost added an answer:
    Why can a problem with FSI occur in atherosclerosis modeling (Nonlinear solver did not converge error)?

    Hi everybody
    Im trying to solve model of fluid dinamic in streight cilinder as blood vessel, with asimetric stenosis.

    vessel model contain plaque with thin cap and lipid core (with heperelastic material behavior ). Height of the vessel 10mm. Model based on fsi interface of Comsol

    boundary conditions:
    - laminar inflow with pulsative velocity defined by U*(t^2)/sqrt((0.0016-(t^4))+(0.01*t^2))
    - outflow pressure define as 0
    - exterior vessel wall defined with no-slip conditions
    Study step is Time Dependent, Fully Cpoupled solver

    When i try to solve, its failed with following error:
    Nonlinear solver did not converge.
    Maximum number of Newton iterations reached.
    There was an error message from the linear solver.
    The relative residual (0.51) is greater than the relative tolerance.
    Time : 0.005641878809722879
    Last time step is not converged.

    and two warnings:
    - Inverted mesh element near coordinates (0.1, 0.00973765).
    - There was an error message from the linear solver.
    The relative residual (0.51) is greater than the relative tolerance.

    Where could i be mistaken?

    Please help me

    Siamak N Doost

    Hi Alex, 2-way coupling is really unstable especially for hyperelastic material such as artery. But, you can overcome to this instability by improving the mesh quality and/or doing some other tricks. I recommend reviewing the workshop notes available in the ANSYS portal and practice all the tutorial models. They really could help you.  Regards, Siamak

  • Billa Narasimha Rao added an answer:
    How much rpm for spinning coat CORIN® XLS Polyimide resin(viscosity 9400) to 15um thickness?

    I want to know what rpm to spin coat 15 um Polyimide thickness

    Billa Narasimha Rao

    It depends on 

    both concentration of solution and rps.

    So one has to adjust adjust for their requirement(thickness)


  • Krishna Boddupalli asked a question:
    I am facing a problem in doing FSI in ANSYS, but I am not able to apply boundary layer(prism). Can any one help me ?

    I am simulatin gFSI problem but the initial mesh is fine and I want to know how to apply inflation layer since it is working only for hexa mesh not for the TET mesh can any one sugges the dynamic mesh conditonss?

  • Lakshmi Nn added an answer:
    How does friction factor depend on roughness?

    Hello all,
    I would like to seek some clarification regarding smooth and rough pipes and subsequently friction factor relations.

    As per the moody diagram we have, that in laminar regime friction factor is only affected by Reynolds number while in turbulent regime, at very high Reynolds number it depends only on relative roughness.

    If I am correct till here, then I have confusion regarding smooth and rough pipes. As per one of the criteria where smooth pipes are defined as 0 ≤ u*ε/ν ≤ 5, and rough pipes as u*ε/ν > 70, I would like to clarify the following points.
    Say for a given relative roughness, ε, depending on friction velocity u*, the pipe may categorized as rough or smooth pipe. In case I have a laminar flow through rough pipe (i.e. velocity is such that Reynolds number is less but criteria of rough pipes u*ε/ν > 70 is met) , how do I decide the friction factor in such case. Will it be still governed linear relation between f.Re and Re??

    In other words, is there any effect of roughness in determining the friction factor in laminar regime? As per my understanding in turbulent regime, the friction factor in both is governed by log law, the difference being term used in log in case of smooth and rough pipes)

  • Francisco Angel added an answer:
    How to calculate consistency index and flow behavior index used in the Herschel Bulky model from experimental data to elaborate flow type of fluids ?

    nanofluids, rheology, mechanical engineering, chemical engineering, chemistry

    Francisco Angel

    Alan is right, once you are decided about the model, the fit procedure is simple. But from the question is not clear what you actually know and have: Do you know the model well?, Do you already have the experimental data?, What kind of fitting procedure are you looking for? A simple Excel Solver fit? Or are you required to rigorously justify the fitting method, confidence interval, etc?

    Best regards.

  • Mahdi Khademishamami added an answer:
    How to simulate the runout of the three-dimensional debris flow using FLUENT?

    how to simulate the runout of the three-dimensional debris flow using FLUENT?

    Mahdi Khademishamami

    If we suppose that debris flows can be formulated in context of kinetic theory, then Eulerian-Eulerian treatment of multiphase flows in Fluent software is a good option.

  • Amir Malvandi added an answer:
    Mixed convection of nanofluid: theoretical modeling

    I recently published a paper on mixed convection of nanofluids in a vertical annulus.It is a theoretical investigation which considers the effects of thermophoresis and Brownian diffusion. You can download the paper easily at (Till 50 days from now)


    I would be most grateful to check your idea about the model and your suggestions on improving the subject on my future studies.

    Thank you

    Amir Malvandi

    Dear Professor Mohamad and Dear Ali akbar

    I use the word abnormally to describe that the heat transfer rate is exceeds more than one expected from the mixture theory (thermal conductivity enhancement). This can be explained by the non-uniform distribution of nanoparticles which can change the viscosity (so the velocity profile changes) and the thermal conductivity. In some cases, I observe that due to a reduction of nanoparticle volume fraction at the heated walls, the viscosity of nanofluid decreases there (so the velocity increases). Then, the momentum shifts toward the heated walls which leads to an enhancement in the convective heat transfer rate.  All of my observations is theoretical and I think suitable experimental verification is required.

    Do you have any criticism about my idea?

    I think, at least in theoretical nature, my results can be accurate. But, I really needs your opinions and contributions to develop my idea and knowledge.

    Thank you again.

  • Chelakara Subramanian added an answer:
    What is difference between numerical diffusion and numerical dispersion?

    Hello all,

    May some one explain in simple way the difference between numerical diffusion and numerical dispersion?

    Most of the literature I could find used Finite Difference Method to describe. However I am looking for explanation from Finite Volume aspect.(just in case type of method matters)



    Chelakara Subramanian

    I guess you question pertains to physical processes.  In that context diffusion is changes of physical property caused by changes in molecular motions whereas, dispersion is change of physical property caused by bulk particle motion.

  • Фык Михаил added an answer:
    What is the physical explanation for centeral dip and annulus peaks in radial pitot measurements after the Mach disk in supersonic gas expansion?

    In the first attached article: "Behavior of free and impinging supersonic micro-jets"  in Figure 15 the radial measurement, of a PITOT tube after the Mach disk in a supersonic free gas expansion, is presented.  Similar measurement  is also presented in Figure 4 of the second attached article: "Flow and acoustic properties of... under-expanded supersonic jets". Regarding these distributions- the authors only state that the radial profile obtained is due to lower-velocity core and a higher velocity annulus downstream from the end of the first shock cell.

    My main question is:

    1) What is the physical explanation for having lower core velocity and higher velocity annulus after the Mach disk?

    Two more related questions are:

    2) The radial profiles presented in the mentioned figures are of the total pressure and the Mach number. Is it safe to assume that also the jet radial density has the same profile ?  and why ?

    3)  The above mentioned measurements were obtained in conditions of NPR= 6 to 9 (Where NPR is the ratio between the nozzle exit pressure and the ambient pressure ). In our experiment we got similar profiles of density, but our NPR is of the order of 10^7. This is so since in our case the expansion of the jet is from several bars (through a converging nozzle) to a vacuum of 10^(-4) torr (it is a pulsed jet). Under these conditions the location of the Mach disk, according to the formula:  X_mach=d*0.67*(P0/P_ambient)^0.5 (~2 meter),   is well beyond the length of our test chamber. So the explanation of having a central dip after the Mach disk location does not seem to apply in our case.

    My question is if someone encountered articles about free expanding jets into vacuum with similar NPR as ours (maybe pulsed jets) , which do have shocks/Mach disks in their test chamber ? - If so please attach those articles, and if not- Does someone have a reasonable explanation to having central dip in our case ?   

    + 1 more attachment

    Фык Михаил

    In the gas dynamics of an ideal gas theory is far from practice. If you could understand the outline of your experiment, I found you try to explain the phenomenon.

  • Фык Михаил added an answer:
    How is the study of flow stabilty problem is important on the basis of Rayleigh's discriminant ? Especially in the case of pulsed flows ?

    On the basis of Rayleigh's discriminant I have studied the stabilty if pulsed flow in Maxwell fluid. Results obtained in this framework represent a good agreement to the numerical one. Furthermore, potentially unstable regions inside the gap can be shown.

    Фык Михаил

    Explore the invention of scientists link ... and you will understand the way your task http://www.findpatent.ru/patent/164/1642402.html...так also advised to refer to British Patent 2,140,160 ..., cl . G 01 P 5/00, 1984. (54) METHOD OF MEASUREMENT PULSATIONS

  • Alessandro Ruggiero added an answer:
    What is the recommended limit for the eccentricity of fluid-film bearings?
    Is there any relation between this limit and the accuracy of the numerical prediction of fluid-film bearing dynamic coefficients?
    Alessandro Ruggiero

    I agree with Michel Fillon. In the case of nonlinear modeling it could be convenient to use analytical approximate equations for the fluid film force description; this approach shows the benefit of minimising the calculation time required for the non-linear dynamic analysis of rotors on  journal bearings without any significant loss of accuracy, giving a better readability of the parameter involved on the system behaviour. 

  • Shiuh-Hwa Shyu added an answer:
    What is Skin Friction coefficient?

    Regarding Skin Friction coefficient (C_f), I have gaon through several papers. Almost all papers calculate C_f*Re instead of C_f. Is this correct? I mean C_f*Re gives skin friction coefficient? Also C_f involves Tao_w, shear stress. Some papers use negative velocity and some papers use positive velocity gradients. i.e, Tao=-mu(patial u/partial y) or Tao= mu(patial u/partial y). Which one of this is correct? Please give your suggestions.

    Shiuh-Hwa Shyu

    In lubrication flow, we use friction coefficient, which is defined as tauw/(mu U / h) instead of tauw/ (rho U^2). The conversion between these two will be the Reynolds number. tauw/(rho U^2) is normally used in aerodynamics

  • Shrish Shukla added an answer:
    Can anyone explain the mixing plane approach and frozen rotor approach for computational fluid dynamics (CFD) analysis of turbomachines?
    For computational analysis of turbomachines
    Shrish Shukla

    MRF method is an approximation of the instantaneous (unsteady) flow solution when the rotor is in the position defined by the (fixed) mesh i.e. Meshes do not move with time. At the interfaces, appropriate transformations of the velocity vectors and velocity gradients are performed, and local fluxes of mass, momentum, energy, and other scalars are determined.

    Limitations:-  It ignore the relative motions of the fluid zones with respect to each other, and thus do not account for fluid dynamic interaction at the interfaces.
     No account is taken for the relative motion of one domain with respect to the other.
     MRF in can only be used under equal periodic angle conditions.
     Limited up to steady-state numerical solutions.

    As it's name suggests, the mixing plan model simply convert the interface boundary into Mixing plan. The mixing plane model uses a circumferential averaging technique which takes a general, non-uniform distribution of a flow variable on boundary and approximate through number of averaging points. As for multistage turbomachinery problems, we often know the stage boundary conditions (e.g. inlet total pressure and temperature and stage outlet static pressure) but not the inter-stage conditions. In addition, the blade counts will generally not be the same from one blade row to the next. The limitations of MRF model are overcome through the implementation of MPM model.
    Limitations:-  Interpolation process can introduce errors in the mixing plane interface, if sufficient averaging points not considered.
     Wake effects, shock wave interactions will not be predicted.
     Errors in the mixing plane model increase as the interaction will be increased at the interface boundary i.e. decreasing the spacing between the consecutive stages.
     Note the following models may not be used with mixing planes:
    – The LES turbulence model
    – The models for species transport and combustion
    – The VOF multiphase model.
    – The discrete phase model for coupled flows.

  • Sa Ja added an answer:
    How can I calculate the Liquid/Solid friction coefficient in lammps?

    Dear all,

    I want to calculate friction coefficient of water in cnt using Green-Kubo relation. it uses the friction force autocorrelation function, but i don't know how to define force for it. which compute or fix should i use to calculate force?

    after that i could use fix ave/correlate to calculate friction coefficient, but my problem is about calculating of Force?

    any idea is appreciated.


    Sa Ja

    Dear Shuai,

    for non-equilibrium simulation, could i use this command?

    for example atfer applying a force on water molecules using fix addforce. how can i compute force between water and cnt in this case?

    thanks in advance

  • Hengbin wu added an answer:
    What is UDF and how can I use it in fluent for EHD (electro hydro dynamic)?

    I am working on EHD (electro hydro dymanic) and i understand i have to use UDF in fluent for EHD, but I dont khow what udf is and how to use it for fluent.

    Hengbin wu

    u can fall back on the help files

  • Abdelkader BOUAZIZ added an answer:
    How can I justify a drop in viscosity of alginate solutions by addition of monovalent salts such as sodium chloride?

    I measured the viscosity of both pure alginate at 3% (w v-1) and a mixed solution of alginate at 3% (w v-1) +1% (w v-1) sodium chloride. Interestingly, instead of ending up with an enhancement in viscosity, I could see that  the viscosity dropped slightly at low shear rates. This trend was then followed with a slower rate until both alginate and alginate-NaCl solutions reached to the similar final viscosity at higher shear rates about 300 S-1.

    Abdelkader BOUAZIZ

    Dear all,

    besides the fruitful aforementioned contributions, this is a typical shear-thinning behavior. In addition to that, ion specific binding and power are to be considered. At high shear it is the main chain that contributes to the viscosity values due to alignment under the effect of shear. Regards

  • Kamil Urbanowicz added an answer:
    Which is the first paper for water hammer relationship (P=rho V C)?

    Hello all,

    I have come across the following relation at many places  reg. water hammer and acoustic in compressible flow

    i.e Pressure = density x fluid velocity x sound speed in fluid.

    I am looking for the research paper wherein this proof may have been presented. I could find in standard textbooks but I am looking for research article regarding the same.



    Kamil Urbanowicz

    Please do not forget that this relation showes only pressures on first amplitude after a pump failure or quick instanteneus valve closure. But if cavitation occurs in the pipe the pressure on the second amplitude can be up to twice that big as calculated from this equation! S have it in mind and check the articles about water hammer with cavitation. Cheers

  • Prajakta Nadgir added an answer:
    Which approach is better to avoid pressure surges occuring in case of extreme valve position changes?

    The existing control system for the fluid flow control consists of a cascaded PI controller with inner (faster) PI controller for controlling valve position change and outer (slower) PI controller for volume flow rate control. In case of extreme set point changes, for example in case of emergency shut down, it produces pressure surges. To avoid this from happening the approaches considered so far -

    1) To add optimal feedforward controller in parallel to volume flow rate controller(outer loop) which will optimize the trajectory to be followed by valve for a given set point change with the pressure inside pipe maintained below given value(boundary constraint), using transient pipe flow model.

    2) To add optimizer or a clever signal processor before sending the reference input to the control system

    Can you suggest which can be the better approach to handle such scenario? Any other suggestions are also welcome.
    Thank you!

    Prajakta Nadgir

    Thank you all for the answers. @Utkal Mehta, There is no D part in existing control system and still oscillations are seen due to reflection of pressure waves. So by clever signal processor what I mean is, it should also consider the current dynamic state of whole pipe and the boundary conditions at its other end. So a pressure surge with peak beyond the limits has to be avoided at any cost. I am doubtful if with a low pass filter for reference signal or with a setpoint weighting we can assure strict prevention. Thank you!

  • Thomas Mazarakos added an answer:
    How the hydrodynamic mobility of an oblate ellipsoid is determined from the primary fundamental solution?

    Using the singularity method, the primary fundamental solution (stokeslet) associated with a point force is obtained. The particle mobility for a spherical particle can be found after integrating the stokeslet over the surface of the particle. I would like to know please whether integrating the stokeslet over the surface of an anisotropic particle e.g. an oblate ellipsoid would also lead to the desired mobility. In fact, I tried this approach and compared with the analytically known solutions from Kim and Karrila textbook [Microhydrodynamics: Principles and Selected Applications] but it turns out that they are different. Does actually this integration procedure holds as well for anisotropic particles? Thank you!

    Thomas Mazarakos

    You can also read the following paper for spheroidal bodies:


  • Yuri Yegorov added an answer:
    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?

    Yuri Yegorov

    I think that in viscous fluid the velocity can be proportional to the force. It is work against the friction of viscous medium. The 2nd law on Newton works on micro level, for each pairwise interaction, but the global motion does not obey it. So I see no contradiction. See for example this text: http://cnx.org/contents/Ax2o07Ul@8.8:E1HJQcnG@5/Motion-of-an-Object-in-a-Visco

  • Mohammed Kamel added an answer:
    Which is the best numerical method to solve nonlinear PDE which arise in Fluid dynamics?


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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