# Fluid Dynamics

Do I need to perform my microscale mixing analysis using turbulence or Laminar flow?
I am doing analysis using FLUENT wherein two fluid streams mix together as follows. In a straight channel there is another inlet at the midway where the second stream of fluid enters (same fluid) and then both the streams pass through as one. My concern is whether the analysis can be performed using laminar assumption or do I need to consider any turbulence also? The channel cross section is of the order of 200 microns and the Re number at both the inlets is well within the laminar regime (around 200-500). P.S: The fluid is liquid under consideration.
Aria Tsam · Aristotle University of Thessaloniki
Good evening.please refer on site: http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=3579&context=etd
How can I compute the angular velocity vector for a pair of particles in a 3D turbulent flow?
Given a pair of particles released at time t = 0 from a localized source. I would like to study the evolution of the mean-squared-angular velocity with respect to time for a given separation (for example r = 100 spatial units). Data obtained from fully resolved numerical simulations give at every time step the particle pair position and the three velocity components. My question is to know whether the following formula could be applied (please see below) in order to compute the angular velocity vector of the “rod” formed by these two particles. I don’t know if the dilatation / strain effects are included or not since the distance between the two particles is allowed to change with time. Any help would be highly appreciated! Thank you!
Helmut Baumert · IAMARIS - Research and Consulting, Ludwigslust, Germany
Hi Abdallah, I assume you mean rms values so that a statistical theory could do the job. I attach a paper which may give you some basic ingredients. At least I know what I would have to do to answer your question and I would be happy to do it en detail but I have too many other obligations for the next two months. If you have specific questions about my text then send a direct mail to baumert@iamaris.org Cheers, Helmut
Can't we utilise the relative wind energy of of a moving train?
While a high speed train is moving , the wind is blowing in the opposite direction. Can't that wind energy be utilized to run the fans of the train?
Nageswara Posinasetti · University of Northern Iowa
I agree with Daniel. It will increase the drag on the train and will add to the fuel consumption.
How can there be flow if there is no energy differential in the Bernoulli equation?
Bernoulli equation: for steady flow of a frictionless and incompressible fluid the energy per unit mass of the fluid at any point along a streamline is constant v^2/2 + gz + P/rho = constant Divide by g and you have (total head along a streamline) = constant. How can there be flow if there is no energy differential along a streamline?
Raymundo Reyes · Metropolitan Autonomous University
Dear Wes Christensen. I want to let know you that Bernoulli's equation as you write it, represents a potential energy, i.e. defines the hydraulic head total in an aquifer, each component is expressed in units of length (ft or m). This means raising the groundwater level under a datum, in steady state.
• Jignesh Thaker asked a question:
Is K-H instability responsible for the slug generation in two phase horizontal pipe flow?
For the onset of slugging in horizontal two-phase pipe flow many researchers told that K-H instability was responsible. Is it true? if true then how?
Does anyone know the values of the inertia resistance and viscous resistance?
I would appreciate it if anyone can help me to get the values of the inertia resistance and viscous resistance for air flow through fiberglass on mineral wool used in thermal insulation.
Nawaf Saeid · Brunei Institute of Technology
Many thanks for all the answers.However my question is is about the air flow through fiberglass or mineral wool specifically. I think I should insert a panel of fiberglass in a duct and force the air to flow through the panel. Then I can measure the pressure drop across the panel and the velocity of the air penetrating it. Just to save some time I prefer to take it from any publication rather than repeating the experiment.
Is there anyone who has taken a course in Microfluidics?
If so what were the materials in the class? I mean any sources including books, notes and so on.
Benedetta Marmiroli · Graz University of Technology
I also advise "Fundamentals and Applications of Microfluidics" by Nam-Trung Nguyen, Steven T. Wereley Very nice hints are also given in the lecture of Jean Baptiste Salmon, of LOF Pessac in France, available on the web at the following address: http://www.esonn.fr/esonn2012/cours/salmon.pdf
What are the best locations for the observations, if you want to do flow control?
Suppose that you can chose where to observe the velocity of a fluid, inside a pipe, so that you can control the inflow boundary conditions in such a way that the solution of your model will match the observed velocities. Should we take the observations closer to the inflow boundary or not? Or should they be equally distributed? And if, instead of the inflow boundary, we control the velocity profile on the wall? I've seen some work on this, but nothing too general. Any suggestions?
Anthony Gardner · German Aerospace Center (DLR)
Hi Jorge, you probably won't get the answer you want by placing your sensor in the recirculation region, although it depends on the flow. I don't know your exact situation, but mostly you want a sensor for which (Sensor pressure too low)-->(increase inflow pressure)-->(sensor pressure increases), and its converse will be a valid control strategy. This is nearly always true for points where the pressure is controlled by the potential flow. For points in recircultion regions, or which can move in and out of a recirculation region this is not always true, and can lead to a failure of your feedback module. I would suggest putting your sensor for control outside the recircultion region and then varying other things (turbulence model, wall temperature, fluid mixture) so that you get the size of recirculation region which you expect. Since you mention wall profiles: it may be necessary to add a (non-physical) lead-in to your pipe if you're not using periodic walls to get fully developed pipe flow.
What is the difference between wall distance and Y+?
Apart from the physical formula to calculate Y+ , what is the difference between them explanation-wise?
Maziar Dehghan · Semnan University
y+ is similar to a local Reynolds number, so its magnitude can be expected to determine the relative importance of viscous and turbulent processes. Different regions, or layers, in the near-wall flow are defined on the basis of y+. In the viscous wall region y+ < 50, there is a direct effect of molecular viscosity on the shear stress; whereas, conversely, in the outer layer y+>50 the direct effect of viscosity is negligible. Within the viscous wall region, in the viscous sublayer y+<5, the Reynolds shear stress is negligible compared with the viscous stress. As the Reynolds number of the flow increases, the fraction of the channel occupied by the viscous wall region decreases [*]. [*]. Stephen B. Pope, Turbulent flows, CAMBRIDGE UNTVERSITY PRESS, 2000.
What is the difference between hydraulic diameter and equivalent diameter?
When to use diameter and when to use hydraulic diameter?
Kalyana Raman · Indian Institute of Technology Madras
In a internal flows, mass flow rate should be maintained constant when you change the shape. If you use equivalent diameter there is a possibility of change in cross sectional area which in turn can vary the mass flow rate if you give velocity as input. I think that may be the reason for using hydraulic diameter which maintains the same area.
What is the best source for learning PDE (partial differential equations)?
Videos prefered.
Amro Elfeki · King Abdulaziz University
I think one of the effective way to learn is through applications you make yourself some simple codes once you know the theory. Below are some links (spreadsheet and ppt) among others available on the website that can help you do learn. Start with simple until you master it. https://www.researchgate.net/publication/256038858_One-Dimensional_Groundwater_Model_Details https://www.researchgate.net/publication/235955596_A_spreadsheet_model_to_solve_steady_state_groundwater_flow_equation_in_a_non-homogeneous_soil_column https://www.researchgate.net/publication/236949622_Simple_Groundwater_Model_on_Excel_Spreadsheet?ev=prf_pub https://www.researchgate.net/publication/236896993_Numerical_Solution_of_2D_Diffusion_Using_Explicit_Finite_Difference_Method?ev=prf_pub
Why do some researchers prefer to work on the vorticity equation rather than on the original Navier Stokes equation in the study of 2D fluid flows?
Are there any numerical advantages of the vorticity equation over the original momentum equation in the study of 2D incompressible flows?
David Burton · Lancaster University
Vorticity-based algorithms are computationally powerful because they lend themselves to grid-free calculations. They are very useful if you can assume that the vorticity is concentrated in space. However, as suggested above, boundaries present a particular challenge to this type of approach.
What kind of evolution does a fluid particle triplet have in 2D incompressible homogeneous isotropic turbulence?
Recently the focus in turbulence is shifted to the study of shape evolution. Staring from regular shapes at the Kolmogorov dissipative scale, one wants to characterize the geometrical evolution (size, area, eigenvalues of the moment of inertia tensor, etc.) with respect to time. The evolution of higher-order correlations such as tetrahedral shapes in turbulence provides an important information on velocity gradient. I would like to know whether there are analytical / numerical predictions regarding the evolution law of a triangle area formed by three neighboring fluid particles in 2D incompressible HIT. Here what I obtained from a numerical simulation performed during 1000 times steps in the tracking algorithm averaged over 1000 particles. The mean area is plotted versus time. Thank you!
Jaan Kalda · Institute of Cybernetics
His graph also starts with a plateau and starts growing rapidly only later (when the triangle is larger)
How can we derive (in a formal way) the diffuse reflection boundary condition for the Boltzmann equation?
Diffuse reflection boundary condition (bc) for the Boltzmann equation is widely used. However, it is not easy to find results on the derivation of this bc and more generally on bc for the Boltzmann equation. I should be interested in any references you may have pertaining to this question.
P. Charrier · Institut Polytechnique de Bordeaux
Dear Lynn, thank you for answer. I know the book by C. Villani. Thank you for the reference of the paper by Gressman and Strain. However, my question was not on existence of the Boltzmann equation, but on formal derivation (modelling) of usual or unusual boundary conditions.
Tight and durable connection between glass and teflon?
I want to connect a teflon hose to a glass pipe (3 mm inner diameter, 5 mm outer diameter). The connection should be tight and durable. Inside, fluids will be transported (not pressurized, temperature range 5-40°C). * Do you have experience how to obtain such a connection, e.g. is there a special glue or clamp? * Where can I source flexible Teflon(tm) hoses for this purpose?
Yingfeng Shen · VTT Technical Research Centre of Finland
The mechanical fitting method surely is simple and good. I would go for that. But if you want to use glue, maybe you can first roughen the TEFLON surface with a sand paper, then apply epoxy glue and just glue it. I am assuming that the roughness will create interlocking enough for holding them together. I have never tried this with TEFLON though.
Any suggestions for the boundary conditions of the magnetic field equation in MHD?
There is an equation: dB/dt - curl( v x B) = - curl ( L curl B) for the magnetic field B in MagnetoHydroDynamic domains. For the boundary conditions we have curl B = -mu j (j is the electric current density)
Abdellah Kharicha · Montanuniversität Leoben
no references no text only experience. Well you can find plenty of references in the field of Astrophysical MHD.
Turbulence Spectral DNS code needed
I am urgently in need of a spectral DNS code for homogeneous statistically stationary turbulence, 2D and/or 3D. I have a new theory for turbulent particle pair diffusion that replaces Richardson's 1926 famous theory which was based upon locality. I have submitted 2 papers recently to J. Fluid Mechanics (under review - available on my ResearchGate site, here). Anyway, I have some ideas on how to test some of the critical assumptions in my new theory, but I need spectral codes in order to generate large inertial ranges. I just need simple periodic boundary conditions; the code obviously needs to be suitable to mount on a parallel super-computer. Are there any off the web or private codes available?
Nadeem Malik · King Fahd University of Petroleum and Minerals
Thank you Abdallah. This is very helpful, and I am sure that I will make use of the database in due course. There is a good deal of interest in this field. Nadeem
What are the specific difficulties in handling free surface flows?
What are the specific difficulties in handling free surface flows?
Chinedu Nwaigwe · The University of Warwick
The boundaries of the domain of Free surface flows are not known a priori, so modelling the evolution of the boundary is a severe challenge. Usually, the equations of fluid dynamics used in these flows are only applicable in the regions where there is fluid, so in the regions where there is no fluid, you have to worry how to handle such areas. This a kind of wetting and drying problem. So, correctly computing interfaces, propagating shocks and other discontinuities at the correct speeds, and satisfying other physical conditions such as steady state conditions are some eal challenges in this area.
• Mahesh K. Varpe asked a question:
In a low speed, single stage (a pair of rotors) counter rotating fan or compressor, can flow coefficient be equal to or greater than 1?
For inlet axial velocity / tangential velocity of the rotor. Such high values does not occur in a conventional low speed single stage compressor (rotor and stator).
What is the correct boundary condition at an impermeable wall for porous media flow?
The most common condition I have seen is vanishing of normal component of fluid velocity at the wall. It makes sense, because Darcy's law describes pore-averaged flow, and the fluid moves even within pores contacting the wall. But, I have found a series of publications where the adhesion condition is set as if it is a pure fluid flow. Is this a possible/correct method? Will the difference cause rapid changes of flow characteristics? N.B. The articles use a Darcy-Brinkman flow model.
Chinedu Nwaigwe · The University of Warwick
Dear Kirill, though I don't know much in this area, but if velocity is zero at the solid boundary, then it should be as a result of no-slip condition, which I suppose makes physical sense. This is so since the velocity of fluid adjacent to the solid has to be zero.
Does anyone know of an open source 1D finite volume shallow water solver for FORTRAN, that is openly available?
I am currently having to integrate an actuator disc into a 1D shallow water code as part of my thesis. I have attempted this by putting a line discontinuity into a finite difference scheme, which simply does not work. Thanks.
Chinedu Nwaigwe · The University of Warwick
Since Matlab and fortran are very much alike. In fact, since you know fortran, matlab should be easier for you to read.
How to solve a multivariable transcendental heat equation by Newton–Raphson method?
I am trying to solve finite difference heat equation by Newton–Raphson method
Mohammed Lamine Moussaoui · University of Science and Technology Houari Boumediene
It is not a good idea to use the NR method because it has the problem of starting point to converge. There several finite difference schemes which are able to solve your popular equation.
How can I calculate the fluid velocity in fusion fuel?
In inertial confinement fusion, ion beams have a initial velocity when they collide on the target . How can I calculate the velocity of layers later? Could you please introduce a reference for it to understand how to calculate it .
Is there any equation to calculate the fluidity of Zinc Aluminium alloys?
I want to know the fluidity of ZA8 ZA12 and ZA27 alloys at different temperature.
Manohar Sehgal · DAV College Jalandhar
In general: Lf=V.ts For calculating fluidity , either use a spiral mould or equivalent die. So we apply two equations: For sand mould: Lf =V. km.S^2/ 4 For equivalent Die: Lf =V. ki.S/ 2 Lf=fluidity distance V=flow velocity S=thickness ts= solidification time km and ki are constants whose values(if not given) can be calculated from the solidification time[ts] by applying the relations: ts= km.S^2/4( for mould) and ts= ki.S/2( for equivalent die casting) . Reference: TALAT Lecture 3205 The Fluidity of Molten Metals Advanced Level prepared by John Campbell and Richard A. Harding, IRC in Materials, The University of Birmingham
On the particle dispersion in turbulent fluid flows - any thoughts?
Is it possible to produce good trajectories of advected fluid particles and then make accurate statistical analysis of the Lagrangian dynamics without basing on the Eulerian field of a given diagnostic function?
Martin Hoecker-Martinez · Oregon State University
Yes this is the basic premise of Particle Image Velocimetry, but for similar statistical significance as you get from Eulerian measures you need as many particles, and probably much greater because the particle density may not follow variations in your quantity of interest, as you would Eulerian measurements. This need for oversampling is true for numerical as well as practical experiments
How does turbulence look like in a non-Newtonian fluid?
Do we see strong deviations from Kolmogorov spectra?
Helmut Baumert · IAMARIS - Research and Consulting, Ludwigslust, Germany
Hi Rogerio, thanks for your interesting advice. Your studies are extremely interesting although non-Newtonicity and stratification are possibly too much of unknowns in one question, I fear. I studied neutrally stratified turbulence for a while and found a purely geometric solution, see http://arxiv.org/abs/1203.5042 (appeared in 2013 in Physica Scripta, may send you a copy if you are interested). I also studied stratified shear flows and found also here a solution - as far as we talk about the laboratory because under geophysical conditions other forces come into the game: the (internal) tides, i.e. the solar system. We can't switch these forces off so that we have a permanent and uncontrollable background mixing. A solution is given here: http://arxiv.org/abs/1207.1633 I am now happy to tell that also non-Newtonian turbulence is no longer a miracle. However, at present I have not the right to publish details. Bernhard knows why. But your results sound extremely interesting and I would love to read more about them. If you have published material than please send me copies. All my data you also may find here: www.iamaris.org Thank you! Helmut.
Does pressure sensitive paint (PSP) work in water?
I am wondering if pressure sensitive paint will work under water or if it will will work with a limited amount of oxygen available. Or more specific: does oxygen quenching 'consume' the oxygen?
Timo Eppig · Universität des Saarlandes