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Hello,
I have a vapor production device that provides me with air+steam flow of 80% or 40% w/w (volume ratio) water vapor quality at 100 degC. The flow passes through a series of valves and tubes where it cools down because of heat transfer. There, the temperature is measured to be something around 50deC. I know that at 50degC, this amount of water vapor corresponds to super-saturated flow. So, I assume that the extra water vapor has been condensed inside the tube, and the resulting flow is a saturated vapor (I don't have access to see how much water has been condensed). Do you find this assumption realistic? Or I may be dealing with a super-saturated flow?
On the other hand, I am a bit uncertain about the measurements of the temperature sensor. To what extent, do you think that latent heat of condensation on the sensor can affect the temperature reading?
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I don't understand your problem. Do you generate steam mixed with air at 100 C at above ambient pressure, or is it ambient pressure and you are sucking vacuum (since you have pressure drop in valves)? Anyway, there is no straight forward answer to the temp. reading. You may have measurement errors if the sensor is not properly insulated or calibrated, it may also be due to heat loss, since you have a significant amount of inert gas present. You should carefully check your measurement system before diving into speculations.
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The complete flow equations for a third grade flow can be derived from the differential representation of the stress tensor. Has anyone ever obtained any results, experimentally or otherwise, that indicate the space-invariance (constancy) of the velocity gradient, especially for 1D shear flow in the presence of constant wall-suction velocity? Under what conditions were the results obtained?
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Academic resources on Fluid Mechanics are provided on
SINGLE PHASE AND MULTIPHASE TURBULENT FLOWS (SMTF) IN NATURE AND ENGINEERING APPLICATIONS | Jamel Chahed | 3 publications | Research Project (researchgate.net)
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Multiphase and multi-layer flows such as flash floods, shock waves, tsunamis, dam-breaks, etc. which are more than 3 phases including air, water, and saturated sediment layer, considering different forms (fluid-fluid or fluid-solid mode) with which of the software, Ansys Fluent, OpenFOAM, Flow 3D, HEC-RAS, etc. is better simulated and also which software is easier to understand and more user-friendly?
In addition, please introduce helpful sources and links for self-studying and -learning to run a model with such software.
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Foad, if you are a student you can use the ANSYS Workbench for free (use the correct license). This software aims to use the Fluent and CFX. In my opinion consist of the two main CFD solvers/tools on the Simulation Engineering.
With kind regards,
Chris
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Hi!
Has any study used magnetic bead to be dripped into drop in microfluidics, just as other micro particle? The density (~27g/cm3 for magnetic bead) could be a major problem. How to keep the magnetic bead evenly float in syringe onto pump? Can magnetic bead be encapsulated in drop during dripping? in collection, is the fluorosurfactant strong enough to hold magnetic bead inside, instead of drop cleavage by sedimentation of magnetic bead?
Thanks!
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definitely you can. Please follow the article attached to get more information
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I want to use standard sandpapers of different grit sizes to impart flow resistance to a surface. I am wondering how to convert the roughness of sandpaper to an equivalent sand-grain roughness. Is there any established correlation between grit size and equivalent sand-grain roughness?
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I am currently designing a set-up for PIV measurements of turbulent flow statistics in a circular pipe. Different approaches can be found in literature. In my view, the best way is the use of refractive index matching technique to reduce a distortion effect and to get closer to the wall. However, I could not find any paper where flow statistics, such as mean velocity, turbulent fluctuations, dissipation rate etc., are accurately determined in viscous sublayer and buffer zone.
So, can anyone recommend any research paper dedicated to accurate estimation of the flow statistics in the circular pipe for y+<30? I would also appreciate if someone shared some papers dedicated to the dependence of the accuracy of data determined by PIV on pipe radius and refractive index difference between wall material and fluid.
Thank you very much in advance and take care of yourself
Dinar
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Hi Dinar,
what is a CIRCULAR pipe? Is it a pipe with circular cross section? If yes, then surely the radius and the lenght scale of energy-containing eddies near boundary matter. I am just thinking exactly about this topic and a nice application would be the Princeton superpipe, http://www.princeton.edu/~gasdyn/Facilities/Facilities.html ,
and/or the COLA pipe,
Note also this theory-oriented paper:
If you see a chance for collaborative work, it could be nice. Only some research coins are needed: not for profit. For the honor only ...
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Previously, I have used Reynolds number for dimensionless analysis for optimizing orifice shape ( ). I was hoping to do a similar analysis for more complex geometries. In that, the calculation of Re at the exit surface is tricky due to the curved exit surface (see attached figure).
Re= (ρ∗v∗Dh)/μ = (4∗ρ∗Q)/(μ∗p)
Where, Dh = Hydraulic diameter, v = Mean exit velocity, Q = Flow rate, p = Wetted perimeter.
Would it be more accurate to consider the wetted perimeter of the projected surface rather than a curved surface? (see figure)
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In such computations I always compute the Re number based on the projected area.
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We are gonna built an experimental setup about the flow of supercritical CO2 under constant heat flux in stainless steel circular microtube (0.5 mm diameter and 300 mm length). It will be placed lots of T-type thermocouple along the tube to measure the wall and hence the bulk temperature. I would like to take your advice about the correct inserting types of thermocouples on the tube to take the better results. Regards,
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You could use a pure copper pipe and spot weld to it small constantan (0.1mm diameter) single wires so making a series of T thermocouples. Be sure they are not electrically shorted on the cold side.
The copper tube will also ensure a temperature very close to the pipe internal one.
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Particle Image Velocimetry, or PIV, is a technique for flow visualization and velocity measurement. Also frequently encountered in the literature is 'Large-Scale Particle Image Velocimetry', or LSPIV. What are the conceptual and technical differences between PIV and LSPIV? What kinds of applications are they most suitable for? Do they require the same equipment and procedure?
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Thank you for your answer Alex. The problem is that the cc map is not at all Gaussian and often has fancy shapes that are not well fitted by a Gaussian. Hence the peak location becomes very ambiguous. I‘ll try to look for an example when I come back from holidays !
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Trying to find any benchmark on the drag coefficient for a cylinder in axial flow. There are many on the flow from the side, also several on axial flow for Re over 10000. What about the near-Stokesian regime?
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Dear researchers,
The following questions are the issues we had when using PIV. We would truly appreciate it if you could help us solve them.
1- Why are velocity vectors highly disorderly shown and recorded with reversed directions or mostly upward directions, and very low values during data collection by rtcontrol software?
2- Although the obstacle on flow path cuts the laser emission coordinates, why are velocity vectors coordinates illustrated continuously rendering the flow crossing path not distinguishable?
3- Why is velocity vector illustrated in the empty space above the flow (with no water), too?
4- What is the best settings for initial pass, overlap, and final window size options?
Best regards,
Maryam
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I should make a more specific statement on the ambient light that I was thinking of. It comes from any objects in the lab with their surfaces being able to reflect lights. In this case light source is most likely from the light scattering of laser beam or laser light sheet. The energy of such ambient light is undoubtedly much lower than that of light scattering. If there are objects with excellent reflective surface in the lab, chances are that such energy level may still be high enough to illuminate dark or even low-light area in the filed of view and sensed by the camera sensor with extremely high sensitivity. However, this situation is much less likely to happen because it causes safety issues and should always be avoided.
For open source PIV analysis tool, you can consider PIVlab, a MATLAB-based software package. I have been using DaVis for many years, and PIVlab for about a year. For now I don't see PIVlab is any different from DaVis in terms of velocity determination.
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We are looking at some interesting behavior by suspensions in confined thin films. But we're unaware of the basic fluid fundamentals that govern the phenomena of wetting and dewetting of solid substrates. For example, we're searching for simple physical models which can conclude about the role of factors like surface tension, viscosity, and surface wettability. Can anyone suggest some good resources, beyond basic fluid mechanics textbooks, to get us started? Or recent advances in this field?
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Soroush Abolfathi Many thanks for your response Sir
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Hello Everyone !
I am doing validation of a research article . The wind tunnel used in experiment is an open circuit , low turbulence wind tunnel .
I am not sure what boundary conditions should I use in the numerical simulations to accurately validate the wind tunnel experiments .
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Not sure what kind of results/research you are trying to validate because that might influence how to answer the question. Is it surface pressures or integrated forces? What's the target Re range to know the susceptibility to flow separation across the range of angle of attack? Or is it a bluff body? Or is it simply flowfield data acquired through diagnostics (PIV, Hot-Wire, etc)?
All of these questions point towards the relative importance of reproducing the inlet geometry. Most tunnel inlets maintain some level of flow angularity in the entry to the test section. This is a good reason (along with boundary layer growth mentioned by above) to include the inlet geometry. Since it's a low turbulence tunnel, presumably tunnel turbulence intensity is relevant and should thus be correctly modeled.
Freestream component turbulence intensity reduction as a function of inlet to test section area ratio is given by 1/C^2 where C is the contraction ratio. So if you know the test section turbulence intensity you could back out what it would be post-screen and post-honeycomb or the equivalent of your inflow boundary.
Good luck indeed!
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I'm looking for experimental data about the free surface elevation downstream of a horizontal cylinder in shallow water flow. Ideally for different Froude- and Reynolds numbers in a 2D setting. I'm not interested in the detailed flow structure, only the free surface shape is relevant.
Does anybody know some literature/papers?
For a submerged hydrofoil (Duncan 1981, 1983) such data is available.
Thank you
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We have carried out some experiments in the past. They are described in the publication that I send to you. If you are interested in some, I can try to find the data
Alessandra
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The speed of the wind is in the range of 1-9m/s and we are targeting an area of 3 by 3 cm or even smaller, say 1cm by 1cm. I was thinking about using a continuous 532nm laser with a high speed camera for that. People employed pulsed laser up to 5kHz as the light source, but the frame rate when capturing the image is set as 1kHz. I'm thinking if it gonna work if I use a 1kHz frame camera with a continuous laser. The seeding particles I'm thinking are atomized olive oil.
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As the previous responses pointed out correctly, PIV might not be the right way for this case of the CW laser and 1000 Hz recording. You could consider using particle tracking and especially streaks particle tracking velocimetry. For that case I can recommend (disclosure - I'm involved in its development) the open source particle tracking velocimetry project, OpenPTV (www.openptv.net) or an excellent trackpy project.
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Hello all, currently I am doing experiments, in these experiments I am using a nozzle in order to cool a hot surface, meanwhile I am changing the water flow rate, and what I noticed is, when I change the water flow rate, the cone angle changes, and I am wondering whether the quality of the nozzle is low, or the water flow rate may affect the cone angle of the nozzle! Any answer will be very appreciated. thanks in advance for you all...  :) 
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there is some papers that worked on reynolds number (lower and upper limit) for which darcy law can work. is there any upper limit reynolds number in forchheimer equation?
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Dear researchers
usually water flow in porous media is too low especially for fine grained formations which may be regarded as laminar flow category. So what is the importance of scraping through this?
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Hi everyone. I would like to ask fro suggestion about transferring cells out from a microfluidic device.
My work is to form cell spheroid inside the microcavity channel. Then, I will transfer them out from the device, but we get less cell spheroids as we did in the device.
Conditions in the experiment:
1. Cavity channel - 3 x 3 mm, channel height 100 micron
Note: We cannot change the height of the channel due to some limitations in forming spheroid.
2. Inlet and outlet size is 1.0 mm outer diameter
3. Coat the device with 10%BSA before loading cells for preventing cell adhesion
4. Inlet is connected with syringe + PBS / Outlet is connected with pipette tips
In our conclusion about the cause of this problem,
1. Cells may be exposed to high shear stress in the abrupt direction change from horizontal to vertical channel which causes spheroid to be broken. So we get less spheroid resulting from cell breakage
2. Or I see some cells get stuck to inside the channel near the outlet via a microscope even though we try to flush them out. so we get less spheroid originating from cell stuck in a device.
Does anyone deal with this problem before? 
Thank you for your comment in advance.
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Update to readers.
Right now, we edit some points in the protocol then we get satisfied result as described
1. We reduce the concentration of BSA down to 2% w/v and we can see less cells get stuck while flushing them out.
2. Thinner PDMS. Maybe the thicker PDMS may accumulate more cells at the inlet and outlet. Gravitational force will pull cells into the ground and aggregate at the bottom of inlet and outlet.
3. Flushing cell out controlled by using syringe pumps is preferred.
I hope that the one who may encounter this problem can adapt some techniques posted by others and me to your work.
Best Regards,
Surasak
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I work on the flow of a gas in a micro channel, the goal is to compare between the analytical solution and the digital one for the temperature fields, the problem is that analytically I have that the difference in temperature Between the wall and the gas
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Please see the file here:
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CD is the ratio of actual flow rate to the theoretical flow rate 
So, how can we calculate the theoretical flow rate?
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Go to the definition of discharge coefficient in https://en.wikipedia.org/wiki/Discharge_coefficient
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To work out the aerodynamics of vertical axis wind turbine (VAWT) and probably the HAWT, we need to know Reynold number (Re) to use it along with angle of attach to find the drag and lift coefficients (Cd & Cl) from the airfoil characteristics, I would like to know which velocity we use to find Reynold number, Re=v*L/neu, is it the induced (axial flow) velocity (assuming that MST or DMST is considered), or the relative velocity, or the tangential velocity Vt=Omega*R?
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Amin- You are right. You may use free wind speed if you are working on large Reynolds number. However, technically, you should be using relative wind speed, especially for small wind turbines. Thanks and best
Thanks and best wishes
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I have met a problem in measuring boundary layer by using hot-wire and sincerely ask for advice: There always exist low frequency disturbacnes ( around 10hz~20hz) when the probe intrude into the boundary layer (the frequency varies a little with change of traverse system ). When the probe is fixed on the plate, there is no this low frequency disturbance. (with Freestream velocity of 8m/s, its turbulent intensity is below 0.15%. In the experiment set-up, the probe used is TSI 1218-T1.5, and traverse rod with elliptic section (a=4mm, b=8mm) is inserted through wind tunnel side wall ) .
       What is the most probably factor that  causing this low frequency disturbance and how to avoid it? It is suspected that traverse system may be the cause of this problem, then what is the best set-up for traverse system? Thank you very much!
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Thanks for all  answers above!  It is finally found that the low frequency disturbance comes from the vibration of cantilever beam which is mounting my hot-wire probe ( I changed the weight of cantilever beam , the vibration frequency indeed changed as well.).  The microscopic vibrations of the hot-wire probe in the wall-normal direction in the boundary layer will cause this false signal ("a strong wall-normal mean-velocity gradient" is the source ). By using the traversing system which is  provided a prescribed pressing on the surface like “Komarik”  used by Borodulin et al. 2014 (https://www.researchgate.net/publication/283265027), this problem could be acceptably resolved. I am particularly grateful to Y.S Kachanov for his kind proposal of this question.
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Normally, we will use pressure drop to calculate the pressure in a pipeline.
Can Bernoulli's principle combine with pressure drop along the pipe in calculating the pressure of the fluid?
What is the condition needed to use Bernoulli's principle? Instead of incompressible fluid, inviscid region of flow* steady flow?
Is the pressure drop formula, like Darcy–Weisbach equation derived from Bernoulli's principle?
Appreciate if someone can answer my question. Thank you.
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Dear Loo,
I just sent you the document (55 Mo) in your email box.
With my best regards
Prof. Bachir ACHOUR
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I am trying to determine surface roughness of a curved surface of 100 micron length. I wish to segregate it into five equal parts and then determine the surface roughness of each part. The surface roughness should be of the order of <=1 um. As per theory, the cut-off length should be minimum 0.08 mm. However, the sample is not that large enough. What should be the proper way for measuring surface roughness in such case?
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You can also take a look at ISO 4287 and 4288.
I remember there was a table referring to the required minimum profile length.
Good luck
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I am trying to build several cases for solver and CAA-library code validation. There are breathing, trembling sphere, baffled piston.
In all cases we can see phase difference between pressure signals. I'm explaining it by inertia like in the mass-spring system. Is it correct?
How can i modify wave equation results or my CFD solution to clear it from such kind of discrepancy? I need to clear it because i'm trying to validate very different solvers (for example with mesh motion) and already observe reflections from boundaries (so then i will need to test some types of non-reflecting) and i suppose that this phase difference could make further analysis harder.
Thank you for any help ("to read" recommendations are also appreciated).
Last figure just for reference (baffled piston, 2d-axissymmetric case).
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You can get dispersion error from your numerical scheme. Try decreasing your mesh spacing to see if the dispersion reduces. You could try using dispersion-relation-preserving (DRP) schemes. Another reason for inaccuracy is your boundary condition. When comparing against the analytical solution, do it at an instant when the wave has not reached the boundary, so you can isolate the effect of boundary conditions. 
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Due to the alternating vortex wake (“Karman street”) the
oscillations in lift force occur at the vortex shedding frequency
and oscillations in drag force occur at twice the vortex
shedding frequency. Why is that?
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Two vortices of equal strength and opposite sign are shed each oscillation period in the wake of a cylinder. The drag of the cylinder is however not sensitive to the sign of the vortex. This results in a period doubling compared to the oscillation of the lift (proportional to the circulation of the vortices).
For an airfoil, one usually consider an airfoil with an average lift. Hence there is symmetry breaking. An oscillating drag for an airfoil is as mentioned by Yuichi Kuya usually not dominated by vortex shedding, but by skin-friction, unless there is massive flow separation. If there is massive flow separation the vortex shedding is dominated by vortices with the same sign. Basically in both cases the oscillations in drag have the same cause as the lift oscillation and the same frequency (because there is no symmetry).
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I have read of lasers used to measure flex in a kayak paddle, but wonder if water displacement is more important than an indirect measure. But how can it be done on a lab and in the ocean?
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I came across this instrument which is a clip-on, so easy to use with existing gear. https://www.motionizeme.com/
It doesn't measure force, but if you integrate the data (and video) with good real-time speed measurement (so you get the change of velocity during the stroke), the 'skeptical young guys' will be able to observe the impact of changes to the stroke.
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As it is known if pressure drop between inlet and outlet of a geometry is known friction factor can be calculated from equation below;
Dp = f (L\D) (ro U2\2)
But use of this equation is only possible for fully developed flow. There are some other correlations to for pipes and square ducts but I need friction factor for wavy channel. Is it impossible to find it by measuring only two pressure value?
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I am sorry I forgot to emphasis I need to calculate it for developing flow. Therefore I am suspicious about use of formula given in my previous post.
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Please suggest any books or materials for reference.
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Dear Jyoti Prakash Panda,
I would suggest the following books:
(1) Turbulent Flows by Stephen Pope
(2) Advanced Engineering Fluid Mechanics by K. Muralidhar and G. Biswas
Personal favorites...
(3) Introductory Lectures on Turbulence (Physics, Mathematics and Modeling) by J. M. McDonough
(4) Lectures in Turbulence for the 21st Century by William K. George
Of these, (3) and (4) are available for free...just Google them.
Hope this helps.....best regards.
Shaswat Saincher
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Hi,
I'm studying the turbulent flow past an elliptic cylinder at different angle of attacks.
What reference area should be given to FLUENT for lift and drag coefficients?
Should it be varying with the angle of attack?
What should be the characteristic length for calculating Reynolds number? Should this also vary with the angle of attack?
Thanks.
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Making a parallel to what one usually does for a wing at incidence, the reference area for the calculation of forces should remain constant. For the cylinder it looks reasonable to employ the cross-sectional area across the axis of the cylinder (a rectangle). It should not change as the angle of attack is changed. However, it is also usual, for the calculation of other non-dimensional quantities, such as the Strouhal number for vortex shedding (which eventually will occur in this case) to consider the projected area in a section perpendicular to the flow direction (i.e. taking the sinus of the angle of attack) because, in this case, the physical phenomenon is actually directly depending on that scale. When accounting for aerodynamic forces it is a different situation and one definitely needs a fixed reference area (though different choices can be made) to be able to make a sensible analysis of the results.
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Oops, thanks Xiang Lei. Of course - the higher viscosity phase displaces the lower viscosity phase if you want to see fingering. My mistake.
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          In flow analysis for rough pipe using Large Eddy Simulation, how the roughness height and roughness constant value can be incorporated. Is it possible?....
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Happy to see your reply sir, but when i choose LES model in viscous , those options are locked. I couldn't able to enter the values.... 
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Hi, 
I am trying to employ stereoscopic PIV in a large flume which is 15-m long, 1.2-m wide and 0.6-m wide. I am looking for a local seeding technique in order to control the concentration of tracer particles in the object plane. Currently, I am using two spray pump installed about 3-m upstream.  Are you aware of any other techniques?
Thanks,
Nasser
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Just mix some particle like hollow glass bead or TiO2 powder with some water. Add the mixture into the flume. Use the PIV image itself to adjust the density. Usually, several grams of powder was enough for a flume.
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Hi,
i am simulating flow past a rotaionally oscillating cylinder. i've read some papers related to this and found that some authors were taking Re based on the perimeter and some based on the sqrt(a.b) (a,b- half of major and minor axes).
Mean position of the cylinder is going to be the major axis which is perpendicular to the flow.
Thanks.
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In an external flow or Internal flow problems
In General D (Diameter of cylinder) is proffered.
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I want to use vortex mixer that has temperature control when I am coating iron oxide nanoparticle with DMSA(dimercapto succinic acid). 
But it is difficult for me to find vortex mixer "with temperature control".
Is there anyone who can recommand this vortex mixer model?
reference is here:
Colloids and Surfaces A: Physicochem. Eng. Aspects 316 (2008) 210–216
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You might relate temperature to motor power and speed with a feed back controller, tuned to the character of the equipment.
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A flow between concentric cylinders is called Taylor Couette flow. A matrial having pores (void space) is called porous material.
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Dear Arshan,
as far as I know, the Couette flow is used by NASA in rotating bioreactor for cartilage tissue engineering applications (a good resource for a start could be this https://smartech.gatech.edu/handle/1853/18174 or this http://ajprenal.physiology.org/content/281/1/F12). Also, the Magnus effect on a cylinder has been used for an exotic type propulsion, called "Flettner rotors" (http://en.wikipedia.org/wiki/Rotor_ship). 
Finally, I believe page 1 of this (ftp://ftp.nist.gov/pub/itl/div891/thesis-final.pdf) thesis contains all the information you need. I quote: "applications include rotary fractionation columns (Willingham et al. 1947; Macleod & Matterson 1959), heat transfer (Gazley 1958; Kaye & Elgar 1958; Becker & Kaye 1962; Kataoka, Doi & Komai 1977), electrochemical reactors using rotating cylinder electrodes (Gabe 1974; Legrand, Dumargue & Coeuret 1980; Coeuret & Legrand 1981; Gu & Fahidy 1982; Gabe & Walsh 1983; Eklund & Simonsson 1988; Gabe et al. 1998; Gao, Scheeline & Pearlstein 2002), membrane oxygenation and filtration devices (Strong & Carlucci 1976; Moore & Cooney 1995), and so-called "vortex flow reactors" (Giordano et al. 1998, 2000; Resende et al. 2001).
Other applications include continuous-flow photochemical reactors (Haim & Pismen 1994; Sczechowski, Koval & Noble 1995), and reactors for certain classes of chemical reactions in which a nearly uniform residence-time distribution is desired (Kataoka et al. 1975; Cohen & Marom 1983; Haim & Pismen 1994; Giordano et al. 1998). When the inner shaft has either an axisymmetric axially-periodic radius variation or has helical symmetry (e.g., a screw), applications include screw extruders (Griffith 1962; Kroesser & Middleman 1965; Tung & Laurence 1975; Choo, Neelakantan & Pittman 1980; Booy 1981; Choo, Hami & Pittman 1981; Elbirli & Lindt 1984; Bruker, Miawl, Hasson & Balch 1987; B¨ohme & Broszeit 1997; Broszeit 1997) used in polymer processing, and viscoseals or labyrinth pumps and seals (Stoff 1980; Rhode et al. 1986). Other applications include "through-hole" Schwartz et al. (1992) and "blind-hole" plating processes, and microfluidic applications of static mixers with helical elements (cf. Bertsch et al. 2001)."
Regards,
Andrea Antonello
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I've found some references that use 2200 to 2300 xg for 30 minutes? But then I've found some people on here asking about cell fractionation that have been recommended that 500xg is the max they should use. Is 2200xg overkill? Does 500xg just apply to separation and not packing of RBC?
Any help would be greatly appreciated.
Alan
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Thank you  for your advice Wolfgang.
I've since come across several sources that use 2300 xg for 30 minutes to compact erythrocytes. This method has a strong positive correlation with methods such as cell counts so I have opted to use this in my study. I have tried methods similar to yours but have found the outcome to be inflated when compared to what would be expected from literature. If I find any other information I will be sure to re-post.
Alan
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Hello! Does anyone know about speed flying from a Taylor cone of droplets? How to calculate it? We use horizontal electrospinning. 
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It is not trivial to calculate the droplet velocity emanating from the Taylor cone breakup in a typical electrospray application.
There are different scaling available which take into consideration balance between the applied electrical field, surface tension force and viscous force that determines the Taylor cone-jet stability hence formation of the droplets from jet breakup. The minimum flow rate scaling could be found in:
Scheideler and Chen, 2014, The minimum flow rate scaling of Taylor cone-jets issued from a nozzle. Applied Physics Letters 104, 024103, pp.1-4.
This is however only the volume flow rate and needs to be divided by the jet diameter to obtain the jet velocity. It should be noted that the actual nozzle diameter is much larger than the jet diameter hence should not be used to determine the cross sectional area.
A time-of-flight and energy analysis technique could be found in the below reference where the droplet velocities were measured for different electrical conductivity of the fluid. Please see Table 2 of this reference.
Gamero-Castano and Hruby, 2002, Electric measurements of charged sprays emitted by cone-jets, J. Fluid Mech., vol. 459, pp.245-276.
Hope it helps,
SM
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In solving the momentum equation which is more realistic BC's, mass flow rate or pressure drop. How can we justify mass flow boundary condition for BWR and SCWR? Is there any reactor practically working on controlled mass flow rate or something like that?
Any help is greatly appreciated. Thanks  
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Good day !
In my opinion BC with velocity distribution (= mass flow rate) at the inlet are more stable.  If you want use pressure at inlet nozzle then you need to prepare long input branch (5-7) diameters.
Best regards,
Serge
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Dear,
I am trying to use Salome recently, first with a simple geometry, as attached.
But after I used the partition tool, there are some internal faces, which are not expected at all, as highlighted in the attachment.
Could any body show me how to remove the internal faces please?
Thank you in advance.
Yours Sincerely,
Bill
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Dear Zoubair,
Thank you for the reference. The same one as Babatunde has recommended.
Still many thanks. I have solved the internal problems.
Happy new year.
Yours Sincerely,
Bill
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Well, I feel so confused about calculate camber on glulam.
Is it determined by radius or span or else?
Does anyone here have some formula to calculate it?
Thank you anyway.
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Dear Zoubair Boulahia,
Thank you for your answer and the document you sent. I'll try to learn about Camber from it first
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There appears to be an aerodynamic instability of 3-wire conductors in overhead power lines.
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Although, I have not worked on this specific problem, I am inclined to agree with your anticipation of the occurrence of aerodynamic instability. I have however worked on flow-induced vibrations of a tube bundle in cross flow, which to a good extent resembles the case of 3-wire conductor subjected to wind cross flow. In our study, we pointed out the onset of instability, and how to determine the conditions at which it occurs by solving the generalized nonlinear eigenvalue problem. You may wish to check our related publications that may be of benefit.  
Regards
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Hi, I want to plot dean vortices inside a 90 degree pipe bend at different section (30 degree, 60 degree etc.). The pipe bend axis rotates from x-axis to y-axis from 0 to 90 degree. I have already plotted vortices at bend outlet (90 degree) and in the downstream region by using vector plot and streamtraces option available in tecplot of x-velocity and z-velocity. But what will be the vector variables for plotting that inside 30 degree of bend section as the bend rotates about z-axis, i.e from x-axis to y-axis? Should I have to create new variables of velocity components? Hope my problem is clear to all.
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For post processing, currently I am using tecplot 360
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Water is flowing over a flat inclined plate and I have to cool the plate to freeze the water. How should I decide the geometry of the plate ?
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The Geometry has to provide the maximum contact area allowed between the water and the surface, so, small thickness of the water-stream in the perpendicular direction to the plate, and wide span of the plate's cross section, the perpendicular one to the stream velocity vector.
If the velocity of the stream is not high, and you have no problems with skin friction, I would suggest to introduce so small spike, or irregularities on the surface, where the stream flows, to create a more energetic boundary layer, that would advance the termal exchange between plate and stream.
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Dears
I have a question regarding the time derivative of pressure in low-Mach number approximation of NS equations. Referring to the work " Numerical study of a channel
flow with variable properties" done by Nicoud, depending on the whether the simulation system is closed or open, the time derivative of pressure can be defined as ... (see the equation 8 of his paper) for the closed system or zero for the open system.
Could somebody tell me why for an open system this is zero?
Thank you all
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The thermodynamic pressure (or total pressure) is a global quantity and does not depend on space variables, but only on time. This is noted just before eq. 4 in the paper. If the system is open, equilibrium conditions at the boundary dictate the value of the total pressure, which is however not  a priori constant. This is remarked just after eq. 7. If the fluid is assumed inviscid, the friction coefficient and the conduction coefficient are implicitly null, thus leading the right-hand side on eq. 6 to be zero. Under this condition, the time derivative of the thermodynamic pressure is zero.
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I want to design steam jet refrigeration system and i am facing deficity in design super sonic CD nozzle. can anyone give some guidance about how to start design designing for particular pressure difference?  
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thank you for your help
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One way for this that I came across in literature was to represent the data in terms of irrotational and divergence free component and subsequently solving a poisson equation for the divergence-free part. Numerical solution for which is not coming correct maybe due to smaller window. 
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Please refer to the following paper:
P. Vedula and R. J. Adrian, “Optimal solenoidal interpolation of turbulent vector fields: Application to PTV and super-resolution PIV,” invited paper, Experiments in Fluids (Special issue), 39, pp. 213-221 (2005)
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To be used inside motor.
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There are so many ferrofluids available. However, suspension stabilty did not depend on the type of ferrofluids its mostly depend on various parameter like size of particles and so many other factors.
Please can you explain details of application 
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Microchannel experiment, Polymer flow
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What is your application? If your fluid is liquid and the regime is turbulent you can sometimes use the inhomogeneities (and also air bubbles) and follow them
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For external flow over a flat plate, what is the reason that boundary layer thickness is continuously increasing. What physics is responsible for this ?
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The above answers given are very good. But I will try to answer in a more bascic level. The momentum of the flat plate is zero and the momentum of the uniform flow has a finite value. When the incoming uniform flow flows over a flat plate, the fluid particles near the plate will stick to the plate (no-slip condition). That means that the momentum of the flat plate is diffused to the fluid. The reason for this no-slip condition is adhesion between the flat plate and the fluid particles. The fluid particles have a force between them that holds them together, which is cohesion. When the fluid flows this cohesive force along with adhesion shows itself in the macroscopic scale as viscosity. As the flow proceeds downstream of the flat plate the viscosity is able to slow down more and more fluid layers above the flat plate. This is what is called momentum transfer. And hence the boundary layer thickness increases as the fluid moves downstream.
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I found the vortex type generated by pitched blade or hydrofoils agitators but all the times there are oriented to push the fluid down and in clockwise flow. But if i have a blade that is oriented to push the fluid up i am not able to guess if the vortex will be oriented upwards or how. I attach a view of my complex agitator and i am talking about the vortex created by the outer blades. Thanks in advance!
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Dear Dr
Some of my papers
1- A. R. Seadawy, Fractional solitary wave solutions of the
nonlinear higher-order extended KdV equation in a stratified
 shear flow: Part I, Comp. and Math. Appl. \textbf{70} (2015)
 345–352.
2- Khater, A. H., Callebaut D. K., Malfliet, W. and Seadawy A. R., Nonlinear Dispersive
Rayleigh-Taylor Instabilities in Magnetohydro-dynamic Flows,
Physica Scripta, 64 (2001) 533-547.
3- Khater, A. H., Callebaut D.
K. and Seadawy A. R., "Nonlinear Dispersive Kelvin-Helmholtz
Instabilities in Magnetohydrodynamic Flows" Physica Scripta, 67
(2003) 340-349.
4- A.R. Seadawy, Exact solutions of a two-dimensional
nonlinear Schrodinger equation, Appl. Math. Lett. 25 (2012) 687.
5- A.R. Seadawy, Stability analysis for Zakharov-Kuznetsov equation of weakly
nonlinear ion-acoustic waves in a plasma, Computers and
Mathematics with Applications 67 (2014) 172-180.
6- A.R. Seadawy, Stability analysis for two-dimensional ion-acoustic waves in
quantum plasmas, PHYSICS OF PLASMAS 21 (2014) 052107.
7- Helal, M. A. and  Seadawy A. R., Variational method for the
derivative nonlinear Schrodinger equation with computational
applications, Physica Scripta, 80, (2009) 350-360.
8- Helal, M. A. and  Seadawy A. R., Exact
soliton solutions of an D-dimensional nonlinear Schrodinger
equation with damping and diffusive terms, Z. Angew. Math. Phys.
(ZAMP) 62 (2011), 839-847.
9-  Khater, A. H., Callebaut D. K. and Seadawy A. R., General
soliton solutions of an n-dimensional Complex Ginzburg-Landau
equation, Physica Scripta, Vol. 62 (2000) 353-357.
10- Khater, A. H., Helal M. A. and Seadawy A. R.,
General soliton solutions of n-dimensional nonlinear Schrodinger
equation" IL Nuovo Cimento 115B, (2000) 1303-1312.
11-  Khater, A. H., Callebaut D. K., Helal, M. A. and  Seadawy A.
R., Variational Method for the Nonlinear Dynamics of an Elliptic
Magnetic Stagnation Line, The European Physical Journal  D, 39,
(2006) 237-245.
12-  Khater, A. H., Callebaut D. K.,
Helal, M. A. and  Seadawy A. R., General Soliton Solutions for
Nonlinear Dispersive Waves in Convective Type Instabilities,
Physica Scripta, 74, (2006) 384.
13- Seadawy A. R., Three-dimensional nonlinear modified Zakharov–Kuznetsov
equation of ion-acoustic waves in a magnetized plasma, Computers
and Mathematics with Applications 71 (2016) 201-212.
14- M.A. Helal and A.R. Seadawy, Benjamin-Feir-instability in
nonlinear dispersive waves, Computers and Mathematics with
Applications 64 (2012) 3557-3568.
15- Seadawy, A.R., and El-Rashidy, K., Traveling wave solutions for some coupled nonlinear
evolution equations by using the direct algebraic method, Math.
and Comp. model. \textbf{57} (2013) 1371.
16- A. R. Seadawy, New exact solutions for the KdV equation with higher  order nonlinearity
by using the variational method, Comp. and Math. Appl. 62 (2011)
3741-3755.
17- Seadawy A. R, Stability analysis solutions for nonlinear
three-dimensional modified Korteweg-de Vries-Zakharov-Kuznetsov
equation in a magnetized electron-positron plasma" Physica A:
Statistical Mechanics and its Applications  Physica A 455 (2016)
44-51.
18- A.R. Seadawy, Nonlinear wave solutions of the three-dimensional
Zakharov–Kuznetsov–Burgers equation in dusty plasma, Physica A
439 (2015) 124–131.
19- A.R. Seadawy, Stability analysis of traveling wave
solutions for generalized coupled nonlinear KdV equations, Appl.
Math. Inf. Sci. 10 (1) (2016) 209–214.
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Dear Researchers and Professors,
I dispersed 0.5 gms of MgO in a litre of DI water and sonicated it for 3 hours and found that, it did not disperse well. After keeping it idle for a day, found that it got agglomerated.
I did the same procedure for CuO in DI water. Got dispersed partially, but settled after 8 hrs.
Can anyone please suggest the optimized volume or weight fraction and surfactants and/or dispersants to have stable Mgo and CuO based nanofluids with DI water as base fluid?
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Thank you Prof.Hassan
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Dear All,
I am calculating lift (Cl) and drag coefficients (Cd) for a rotating airfoil. Let's say the tip speed ratio (TSR) is equal to 2 and the wind speed is 8m/s. I noticed that due to rotational motion, the airfoil will see an additional velocity tangential to the airfoil body, therefore Cl and Cd should be calculated based on the relative velocity (Vr).
However, when I did this, Cl and Cd were not comparable with the aerodynamic polar in static conditions. Since the dynamic stall (DS) should occur, the lift is expected to increase, and this was not observed when I used (Vr) as reference. Cl was significantly reduced compared to the static polar.  Nevertheless, this was not the case when the wind speed was used as the reference. Cl was increased and behaved as usual DS characteristics. Please see the attached picture.
As a note, DS characteristics such as the leading edge vortex creation and convection were clearly observed. So, I am sure that DS is occurring.
Can somebody suggest me an idea? Thanks in advance for your response!
Galih
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It maybe
1. Solution is not correct when you use relative velocity
or 2. the non-dimensionalization is not consistent.
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When a pump is tested in air, fluid leakages are entering inside the pump through the gap betwen the impeller and the diffuser for some flowrates. These fluid leakages can significally modify the flow inside the diffuser.
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I think it is hard to say. If add leakages, it means more engry. It is a mix process between leakage flow and origin area with energy exchange.
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In order to understand the behaviour of 3D compressible unsteady flow we use URANS. Output processing of particle paths is very valuable, particularly if we include the thermodynamic variables for the particles at each time step. But I know the energy does not simply convect with the mass particles. It can travel 'sideways' by diffusion of heat and also by frictional transfer of work between particles.
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I have found a partial answer to my own question its in the text book A. Bejan "Convection Heat Transfer" defines "Heat lines" to allow the visualisation of convection. All I have to do now is to extend his mathematical definition to include the unsteady work transfer process (dp/dt). Anyone know how to do that?
Martin
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I would be obliged if someone knows good lecture to this topic. In my case, I'have to focus on a compression shock in shock tube from round to square geometry.
I'm glad for every source of information.
Sincerely yours
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Better to read first the book :Gas Dynamics by Liepmann and Roshko. 
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Hello everyone,
I am recently working on experimental analysis of intermittent type two-phase air-water horizontal pipe flow. In this regards, i want to eagerly know the dynamics of intermittent flow structures (mainly governing forces or force balance inside the slug or plug type intermittent flow). Can anybody suggest me the literature which includes the free body diagram with considering all forces inside the slug or plug body?
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I am amazed so much as been done on the slug and plug flows. Surely, nice to learn that so much has been accomplished. Perhaps, there is another way of looking at the structures of the two-phase flows you are interested in. When I needed to know more about how such as a flow "moves", but knew only the approximate value of the ratio of void fractions to the fluid fractions volumes' and desperately wanted my device to work for a wide range of surface tension and liquid-solid adhesion values, I have resorted to a trick. It may be a shortcut that may help you out.
I have started at the inlet, took gas as "massles" and computed accelleration of liquid plugs under an action of the motive pressure, being common for gas and liquid masses reamaining in contact. SUrely I have included viscous friction of liquid plugs, taking this to corrrspond to liquid cylinder in laminar flow. When you have more than a dozen of such composite pairs (bubble and plug) you get a flow that is averaged out over a couple of impulses. Moreover, a bubble, say, number n, was bubble the n-1, at a previous impulse juncture. Having this averaged over several impulses, gave me the pipe exit pressure value. Not trusting these calulations too much, I have proceded with my design work and results were very, very  good. What is funny about that is that values of the pressure and the throughput corresponded very well with expierimental values for plug and and for slug flows,all the same, althouh for the slug flow gas slugs may overtake liquid blobs. Apparently, for the pressure distribution occasional situation when the liquid plug stretches accross the entire pipe span (cross section) have a lot more to do with the overall behavior of the (averaged) flow than the precise structure of the very flow. Mathrmatically speaking, such flows are chaotic, but the outlayers (cross spans, near-pugs ) may be the controlling factor, but we stilll keep on segregating the pheonomenon based on our failry superficial observations.    
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how to reduce drag on ships by lubrication of air?
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I think you could find some advices in superhydrophobic surface. Paper:
PRL 114, 014501 (2015)
Experimental Thermal and Fluid Science 51 (2013) 239–243
Best wishes
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I have been researching the air flow effects on frost formation at fin and tube heat exchanger and I need uniform flow media in front of heat exchanger. However, I haven't ensured at my tunnel illustrated attached image due the fact that it doesn't a contaction section including honeycomb or screen. I would like to thankful if you share documentation regarding contraction design at low-speed wind tunnel (my velocities are max. 5 m/s) and your recommendations. Also, Is there another method to form more uniform flow at entrance? Thanks and regards.
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Hi Ergin,
The contraction follows the flow conditioner unit that houses honeycomb section, the screens etc. Honeycomb section first breaks the large whirl into smaller ones at its entrance and then makes the flow align along its length. The screens attenuate the turbulent fluctuations. Large-scale turbulent eddies are broken into smaller eddies and finally made to decay. Then follows the important contraction which accelerates and aligns the flow and feeds the test section with flow with reduced turbulence.
Useful References:
1.       A very digested publication on wind tunnel testing can be found on Researchgate profile of Louis Cattafesta:
2.       Barlow, Rae and Pope.  Low-Speed Wind Tunnel Testing, John Wiley and Sons, NY 1999.
3.       Bradshaw &  Mehta.  Wind Tunnel Design, http://www-htgl.stanford.edu/bradshaw/tunnel
Hope this helps.
Rajeev
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Hi Dear Colleagues,
As you know, refrigerant two phase distribution at evaporator is big problem because of the fact that it can cause significant capacity reduction unless used optimum distributor. Thus, I am planning a experimental test rig but I'll use water- vapor mixture as refrigerant, merging this mixture at determined ratios and measuring the flow rate at outlet of each circuit. Does using water instead of refrigerant such as R404A give certain result? I would like to take your value opinions. Thanks    
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I am wondering if velocity in formula means average velocity of fluid passing through cavitator, or velocity of fluid within the cavitator, between points with different pressure?
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There are many cavitation numbers depending on the purpose of research. If you want to define the incipient or end of cavitation, or cavitation threshold, or supercavitation, initial cavitation erosion or etc. and want to use data from previous studies, it is important to use the same cavitation number and to read the definition where the pressure difference and the speed is taken. In most cases you have to trust the selected cavitation number. Sometimes based on the experimental data (if published), it can make interesting conclusions or change cavitation number and propose new criteria.
If you're doing research perhaps for ultrasonic cavitator you should know the pressure distribution and choose correct place for measurements of the pressure difference and the velocity.
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I'm trying to reconstruct the density distribution of an atomic gas jet from measured projections. I want to compare my results with what is expected from jets' distributions. I found an article which shows jet density profiles for Laval and conical nozzles, but this article discusses jets with clustering. I wanted to know if I can rely of these profiles even in the case with no clustering ?
I'd appreciate if you can refer me to a scientific source backing the answer.
By the way, if you also know of good articles which discuss the density profiles of gas jets (with and without perturbations in the nozzle), I'd also appreciate if you can refer me to them. 
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Thanks for the paper. I'll try to implement the equations. If I may, I wanted to ask you some other question, as an expert in the field : I need to Abel reconstruct the measured projections of the gas jet. However, due to noise amplification of the reconstruction algorithm I want to filter first the measured data. I wonder if  the frequency spectrum of the density profile function has some kind of a cut-off frequency, so that I can be assured that filtering the projection data with a wider filter won't result in any loss of information ? 
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As stated in the literature, in species transport model mixture formulation, the components are mixed at molecular level and velocity and temperature are the same for all components.
Given the case for non-reactive mixture and the simulations I carried out, the mass fraction data is available for different components at various axial locations inside a cylindrical channel.
However, I intend to know how the species transport model can be used to know the extent of mixing between the two gases.
Secondly, since there is no reaction, what is the factor that is causing the mass fraction change?
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Salman Bin Kashif,
I did not understand your question fully. What do you mean by "extent of mixing between the two gases" ? Don't you think, mass fraction contour presents the extent of mixing of gases? 
Change in mass fraction is not caused by reactions. It is caused by diffusion occurring due to species concentration gradient. 
Palani.
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I have simulation results for a particular problem (say 1D advection equation) for two grids. One is a coarse grid while the other is refined. I wish to determine the error scaling with time between the two grids theoretically (using pen and paper). Can I do this?
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Respected Contributors,
My intent to put forth this question was to check if I am updated enough about the theoretical developments in studying space-time discretizations. My question being naive, yet it was solely meant to see if there was an answer to the question. I guess, I have benefitted a lot from the answers here, and also refreshed some of the concepts that were taught to me during my Master's. I also agree now that the answer to my question is "NO", and would like to again say that, I was looking for a theoretical (pen and paper) solution, without having to do any simulation, and that the error scaling would depend upon numerical parameters and not time explicitly. Hence, I was not looking for an answer wherein, I have to do spatial/temporal evaluations via simulations, which I again state was a very naive question.
I would sincerely thank all the respected contributors above.
Regards
Shakti
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I'm considering heating and vacuuming a solution to decrease dissolved gases as well as coating with a superhydrophobic material. Any ideas?
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DMEM = Dulbecco's Modified Eagle Medium ? first time i hear about it, it is out from  my field of expertise, I don't know if standard degasing methods used for physical research (boiling, vacuum , ultrasonic,) may destroy  your medium.
if your problem is the oxigen, then work with a different gas (Nitroge, elium..)
If you don't want any gas, you could connect your device to a vaccum pump and to the bottlle of your medium. all connections controlled by valves. First vacuum youir device, may be also yout bottle, then let your fluid flow from the bottel to the device. Take care your fluid do not flow into the vaccum pump (it could destroy it). 
Apart these general ideas, the details pof your necessity remain still quite unknown to me.
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Hi,
    I want to carry out DNS of a high-speed turbulent reacting flow. How can I properly determine the Kolmogorov length scale according to the basic flow parameters such as velocity, temperature, pressure, etc. Or how can I determine the required grid resolution?
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In addition to Prof. Qin's answer about the resolution requirements in the near wall region, the mesh refinement level is dictated by the resolution of the species mass fraction profiles in the reaction zone. I assume the reacting flow you are simulating is a propagating flame. If so, then the reaction zone within the flame front should be discretized with at least 10-15 grid points. On top of this, under the influence of high pressure the flame thickness becomes much lower, hence the spatial discretization step needs to be smaller.
A way to extract the length scale of the reaction zone is to solve the 1D profile of the reacting mixture using software like chemkin or cantera.
The Kolmogorov scales are: n/L ~ Re^(-3/4), where n/L is the ratio of the smallest to the largest length scale. 
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Hi,
 I want to actually model a vortex tube in CFD. To do the meshing part I am using ICEM CFD. So I actually need to block the inlet region in it. I have trying various to achieve this but my attempts to do were a complete waste as i had a very bad mesh. I want to know how to actually block that in order to have a good mesh. Please reply as soon as possible.
attached are the model files i have made in ICEM
Thank you,
Regards,
Sankeerth
Thank you
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 Hi Bernard,
Thanks for the reply.  What do you mean by additional O-grids? I mean should i go to create block and using entities create a block and merge it with the existing one? Iam actually a beginner in ICEM CFD. I actually included a block and tried meshing but it din't pan out well. 
Thanks
regards
Sankeerth
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How can we take into account seagrass (posidonia) and bed rocks rougness in the MIKE 21/3 coupled Model FM?
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Starting from your mesh you can create a dfsu file in which you can specify the desired value of  roughness (z0) for each node, obviously different for seagrass and rock. In this way you can load a space-varying roughness as input. 
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I applied FFT to my hotwire data downstream of a circular cylinder to find the vortex shedding frequency, however two clear peaks appear in my final results. How can I say which one is the actual vortex shedding? what could be the other one for? 
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Could you please post the figure of the spectrum? Previous answers addressed problems in doing FFT but if you are solving high Re number flows, many characteristic frequencies can also appear..
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Hi, recently I am focusing on developing a new way to measure the size of drops adhered to solid. So I need to create different sizes of drops at first, and the more uniform the drops, the better.
Is there anyone that has some knowledge about this? 
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If I understand your question correctly, a straightforward way to create an array of water drops with different sizes would be to pattern a surface with alternating circular hydrophobic/hydrophillic regions of different diameters (hydrophillic where you want the drops). SiO2 on a silicon wafer would work fine.Pooling water on the top and then slowly tilting to remove excess water would leave the drop pattern you need (on the hydrophillic regions).
Though the diameter of the drops would be a known quantity (based on the size of the patterned features), the volume for a given diameter would have to be characterized (though probably just once) or inferred from knowledge of the different surface energies.
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Is there any relation for calculation of x(in the picture)?
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Mostly  50 times the diameter gives good results in most applications where flow geometries change. Although different researchers take any value from 10 D to 100 D, depending how flow development length is defined.
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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.
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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. 
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I am using a plunger pump to generate a 20 l/min of high pressure water flow rate to be injected through a nozzle. In order to avoid propagation of disturbances and provide smoother operation, a buffer tank is installed between the pump and the nozzle. The problem I'm facing now is that the flow rate after the buffer tank is reduced to approximately 8 l/min. Is there any explana for this? And is it possible to keep the flow rate constant (20 l/min)? If yes, how?
Thank you in advance.
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Probably you require a pulsation dampener which works well to reduce pressure fluctuations at discharge of any metering/positive displacement pump.
Have a look at this supplier:
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I want to know whether we can use Bernoulli's equation in the case of pressurized (above 5 bar) pipelines of gasoline.
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Thank you sir
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I work on polymer gels. The yield stress is ~100 Pa. Below 100 Pa, both G' and G'' show plateau wrt stress. But the G'' plateau fluctuates from run to run while G' does not. Can anyone explain this? Thanks in advance :)
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Answer
Agreed. The G' and G'' values are in fact results of Fourier transform of the raw signal (stress or displacement). When one of the values dominate, the accuracy of determination of another one gets poorer. As a rule of thumb, when the lower modulus is below1% of the higher one (this gives two order of magnitude difference), the lower modulus cannot be measured accurately.