Reynolds number can be interpreted as the ratio between the viscous force and inertia force. If the characterization of the flow regime is sufficiently described only with this number, then the explanation of the phenomenon of flow transition becomes very simple, that is, only by explaining the relationship between these two forces in the transition flow.
The Reynolds number depends on the choice of the characteristic lenght and velocity scales. For the same physical flow problem, you can detect high or low Re number depending on the choice. The range of turbulent scales is very wide.
The flow problems are very different, in a pipe turbulence appears at O(10^3) while on an airfoil you can have laminar flow also at value greater than O(10^5).
Yes, Reynolds number is sufficient condition to judge laminar flow or turbulent flow when subjected to forced convection. This condition is applied to flow on flat plates, inside tubes, conduits, external flows, annular flows, channel flows etc. Reynolds number is defined as the ratio of inertia force to viscous force.
Does it mean that for some cases the Reynolds number is a sufficient condition to distinguish laminar and turbulent flow?
Dear Nazaruddin Sinaga
the hystorical experiment of Reynolds showed that a critical value for the flow in pipe is responsible for the onset of transition. However, there is still not a well defined value for the Re in this case and the researches about stability constraint continue to be presented.
Other flow problems can show different characteristics, just think about the flow over a flat plate that shows simultaneously laminar, transitional and turbulent regions depending on the local Re number.
For all conditions, Reynolds number is the deciding factor to decide laminar flow or turbulent flow.
Dear all,
We all know that when solving flow problems, for example with CFD, we usually need to specify the flow types/regimes, whether the flow is laminar or turbulent. Likewise, the correlation equation for the experimental results, for instance, the relation between friction losses or heat transfer coefficients to Reynolds numbers, mostly distinguishes between laminar and turbulent flows. Therefore, determining exactly the flow type/regime is very important. The question is: how do we determine a flow regime? If the Reynolds number cannot be used as an indicator, then what parameters should be used as the right indicator?
Nazaruddin Sinaga
If you are doing a CFD experiment, no matter of laminar, transitional or turbulent regime, the Navier-Stokes equations are valid. You do not specify nothing at this level. Only if you need to use a statistical formulation, such as RANS/URANS, you need to know before of the simulation that your flow problem is involving fully developed turbulence. Usually, this is not linked at some specific Re value but to the physics of the problem and to experimental evidence.
As general observation, the nature of almost all flow problem is turbulent, the laminar regime is rarely encountered in practical problems.
Yes, that's right but it depends on the problem, natural laminar flow wing exists at low AOA but just increasing this latter you have a different framework (at the same nominal Re number) with the appearence of separated turbulent region. Apart some ad hoc model for transition, RANS formulations assume by definition that the turbulence is fully developed. Is it that really correct? Maybe not or can be questionable for many flow problems but it is an intrisic assumption of the statistical formulation. I agree that the assumption of a fully developed turbulence is very specific. But that is done largely in engineering applications.
However, the key in the answer to the original question is that a value of the Reynolds number cannot be used a general rule for the turbulence regime.
Abbas J. Jubear,
Would you inform us the other factors that have to be considered to indicate or to make sure that the flow is turbulence, please? For example, the flow across the cylinder pipes......
Thank you
Dear Nazaruddin Sinaga ,
In order to describe a flow to be laminar flow or turbulent flow, according to your question, it may be said that Reynolds number (ratio of inertia force to viscous force) be the condition, which is also dependent on absolute viscosity as per one reply. Transition flow is said to happen in this range of Reynolds number. However, if the transition flow is found out, it was also observed that there are almost no calculations as per literature. As you mentioned, only taking into account Reynolds number and as I understand, transition flow is not simple as laminar flow or turbulent flow. As pointed out in the other reply, Reynolds number is not the only parameter to say whether laminar or turbulent and as per yours, transition is not discussed. I agree that Reynolds number for laminar and turbulent flows in general is given but changes for different conditions and different flow conditions and your aspect is not discussed. Here, each one has a certain point of view and the question will be answered as per the knowledge. If this is good, the answer be taken, If at all the direction is not good, one may suggest the right one and addressed, instead of putting some odd comments. In this aspect, I am getting clear from your side, that your interest is in transition region.
With regards,
Dear Pisipaty Srinivas Kishore,
Thanks for your reply. Actually I need more responses from ones that interested with the flow regime problems. There will be many consequences with my question. For example, do you know the flow regime of the flow inside the pipe when the Reynolds number Re = is 10,000? Most common scientist will answer that the flow regime is turbulence, but for me, the flow is laminar (according to my real experiment). And when I try make a calculation, for instance with RANS, then I have to define firstly the flow model is laminar. On the other side, most people will use a turbulence model. And what about the results? It is easily say that the results would be different. And which results is true? It still a question......
I hope you understand what I mention.
P.S. Please give your comment, Mr. Tapan K. Sengupta, Mr. Filippo Mario Denaro, Mr. Abbas J. Jubear, and all the professor. Thank you very much.
Dear Nazaruddin Sinaga,
Thanks for your reply. First of all, you are not clear in discussion, whether you want laminar flow or turbulent flow. You are discussing laminar of 10000, where as inclining towards turbulence. In the question you are speaking about transition. Assumptions in software is different. You need to act accordingly. I have given my information and rest you can have from others.
With regards,
Dear Pisipaty Srinivas Kishore,
Please have a look to my original question. The question is: " Is Reynolds number a sufficient condition to indicate a laminar and turbulent flow?" When I made my experiment, the flow is laminar even the Reynolds number is 10,000. And then I made a numerical calculation (with laminar model) to compare the result of the wall shear stress. The result is not agree well with the experiment. So, I change the calculation with turbulent flow model. The result was agree well with the experiment. So, what do you think? The real flow regime is laminar, but we should use the turbulent flow model in calculation. What's wrong with this?
Regards...
Nazaruddin Sinaga
what you are addressing now is a quite different issue.
- what about the details of you experiment? How did you assess that the experiment produces a laminar flow everywhere?
- to compare the numerical results in terms of wall stress, you need a correct resolution of the grid at Re=10^4. How do you ensure that?
- what kind of formulation do you adopt in the turbulent model? have you used the same BCs of the laminar formulation?
Dear Filippo Maria Denaro
- What I am addressing now is not a different issue. Before discussing about the transition I'd like you all to help me first to answer the question: " Is Reynolds number a sufficient condition to indicate a laminar and turbulent flow?"
- I made the experiment in the laboratory in Valenciennes, France by using a water channel and using LDV as the main instrument to measure velocities, and measure the wall shear stress with Polarographic methods. The laminar flow is indicated by flow visualization and the observation of the fluctuated velocities in all main directions. Then, I calculated the turbulence intensity. If the TI is less then 0.1% then we assume that the flow is laminar. The calculation was conducted by using ANSYS FLUENT.
- In the calculation I used around 2 million cell number, both for laminar and turbulent flow models, with convergence criteria of 1E-6, and I also made a grid independence tests.
- I used k-e standard turbulence model with standard wall treatment.
- The BCs were the same for both laminar and turbulent model
Regards...
Nazaruddin Sinaga
Once assumed that the experimental device produced actually a laminar flow, the problem is in the numerical simulation you did using the laminar formulation. As a rude estimation, from your integral lenght-based Reynolds number of 10^4 I can think of a Re_tau of about 500, which is a value we usually use in turbulent channel flow simulations. If you want to reproduce the laminar solution at such Re number, two million of nodes in your 3D simulations is quite a poor resolution. The description of the BL could be quite poor. The grid should be fine enough that a cell Reynolds number should not be greater than 2.
Be aware that no matter about the laminar, transitional or turbulent regime of your flow experiment, the 3D time-dependent solution of the NSE must provide the correct answer. If the experiment has only laminar flow that MUST be reproduced in the calculation. If the adoption of wall model formulation accords better that must be a warning, the RANS formulation does not reproduce a laminar solution.
I suppose the problem is in the setting you are using in Fluent. I suggest to use in the laminar model a second order explicit time integration with centered second order discretization. That can be easily prescribed. Be careful that the BCs. reproduce correctly the experimental device. Run the code until a steady state solution is achived.
Filippo Maria Denaro
Thanks for your reply. I understood what you meant.
Would you tell me why did you suggest me to make calculation in time-dependent flow? Cause in my experiment, the situation was not unsteady....
Thank you.
Nazaruddin Sinaga
you are mixing a lot of issues, about physical transition, numerical accuracy, turbulence modelling ....
First of all, differently from what was assumed in the past, the Re number at which transition to turbulence occurs in a pipe is now debated, see for example
https://www.annualreviews.org/doi/abs/10.1146/annurev-fluid-122109-160652
But let us assume you have really a laminar steady flow in your experiment. Then, you have also an analytical solution, the Hagen-Poiseulle quadratic velocity profile. Just compute analitycally the first derivative and evaluate it at the wall.
Now check what about happens on a numerical derivative. Take your numerical solution in the laminar formulation and check that.
I suggested to run a time-dependent case to check if the time-derivatives of your solver vanish naturally.
If you use a RANS formulation you are assuming by definition that the solution is turbulent. You can not match the laminar solution in a general case.
My last comment is about the realization of the experiment. I suppose that the inflow in the pipe is not an exact quadratic velocity profile so that you have need of some lenght to stabilize the solution towards the analytical one. On the other hand, that introduce also the problem of what inflow velocity you are prescribing in the simulation.
Dear Tapan K. Sengupta,
I have simulated the flow already in a water channel with 15 cm x 15 cm cross section as shown in the attached picture, for which the Reynolds number is 10,000 based on diameter and average velocity.
Thank you very much for your kindness.
Tapan K. Sengupta
is right, are we talking about a pipe or a squared channel flow?
Dear Filippo Maria Denaro
- Thank you very much for your reply
- Please think that my first question/issue was about whether the Reynolds number is a sufficient condition to determine the flow regime: laminar and turbulent. Since this issue has many consequences, then it extends/mixes with other relevant one. And will give us more issues, for example when we talk about convection heat transfer, mass diffussion, and so forth.
- I have read your suggested paper and many other relevant papers related to my main issue. I am sure that the Reynolds number is not a sufficient condition to determine the flow regimes, i.e. laminar or turbulent.
- I have also checked the numerical calculation for both the laminar and turbulent flow, for Re = 10,000 (based on the hydraulic diameter. I did not try to use the Hagen-Poiseuille velocity profile, cause the assumptions of the equation are that the fluid is incompressible and Newtonian; the flow is laminar through a pipe of constant circular cross-section that is substantially longer than its diameter.
- Please mention my first question. As you, Sengupta, other researchers, and me, agree that the Reynolds number is not a sufficient condition to determine the flow regime, so how to determine it more accurate? I think we need more parameter/s, beside the Re number, to determine the flow regime. What is/are the parameter/s? That's why I asked other fluid mechanics to discuss it.
- Thank you very much for your sharing.
Dear Filippo Maria Denaro Tapan K. Sengupta
Sorry, it was the hydraulic diameter (4A/P) of the square water channel.
Thank you.
Dear Tapan K. Sengupta
The calculation had already been made for the Test Section only. So, the calculation domain was only in this section.
Thank you...
Nazaruddin Sinaga
Your experiment is about a flow entering in a smoothed convergent squared channel and is really different from any pipe flow. Assuming that the lateral walls are sufficiently far each other so that 3D effects are confined in the extreme regions (but that can be a very rude assumption!) you can have the central part of the flow resembling the double-flat plate flow problem. In this case you need to consider the local Reynolds number along x. If this model is enough realistic compared to your experimental device, you can observe a laminar region in the central part of the channel for Re_x
Dear Tapan K. Sengupta
I just have made the calculation, but I am not satisfied with the results. What is my concern is about the flow model. In my calculation with turbulent flow model, the results was not agree with the experiment one. When I calculate with laminar flow model, the results is better. Many researchers used turbulent flow model for Re = 10,000 for internal flow. But my calculation gave a better solution with laminar flow. That's my concern....
Dear Tapan K. Sengupta
This is what you said: Starting from Osborne Reynolds, it has been noted that there is no fixed Reynolds number for pipe flow transition. While Reynolds could increase it to 12880 by careful design of the experiment, subsequently other researchers have raised the critical Reynolds number to more than 100,000! Same is the truth for flow instability past a circular cylinder.
Could you tell me what do you exactly mean about the "careful design of the experiment" ? Could it be described in technical quantity?
Thank you very much.
Dear all,
Please correct my statement before. My calculation with turbulent flow model gave me a better results than the laminar one. The real condition of the flow is laminar (by visualization and Turbulence Intensity is < 0.1% in the test section) with Re = 10,000. I am not satisfied because the real condition is laminar but it should be calculated by turbulent flow model....
You cannot assess that the solution along the channel is laminar only by considering the TI
Dear Filippo Maria Denaro
I decided the flow is laminar by measure the TI
Could you show the details of your flow visualization in the channel?
Dear Filippo Maria Denaro
I am sorry I could not find the visualization picture of the channel flow. It was long time ago when I did the measurements for my Pd.D research. The calculation has been made just three months ago when I was interested with the Reynolds number issue.....
Hi Tapan K. Sengupta
I appreciate your concerned with the issue. There still many other problems to be thought. I think it is not only my problem, but for many researchers and practicioners too. Regarding my observation, there was many problems in fluid dynamics and its applications that should be evaluate and rethink. Please remember that the fluids consist of micro particles, and we oftenly think them as a bulk macro materials. There are also many assumptions regarding the fluid flow such as continuum hypothesis, slip and no slip condition, compressibility, steadyness, dimensionality, wall and blockage effects, disturbances, and so forth.
I hope you and other professor would like to think about it.
Thank you very much for your kindness attention and...have a nice weekend with your family and friends....
Kind regards
Nazar
Re is - generally - not enough, only in trivial cases. E.g. look at the way mol. viscosity is established, in simple molecular form or as long chains of connected molucules etc. etc. --- a VERY long story. We learn: only trivial things are trivial ... :-)
Helmut Ziegfeld Baumert
- Thanks for you reply. I agree with you.
- Could you tell us some examples of the trivial cases?
- Could you give your opinion about the enough condition to classified the flow regime, please?
- Thank you
Regards
Dear all,
After evaluating the results of the discussion from some of us, and also based on a review of some literature, it can be concluded that the Reynolds number is not a sufficient condition to determine the flow regime, namely laminar, transition, and turbulent.
Next I want to present the following discussion material, i.e. if a flow has an Intensity Turbulence in free-stream flow, can we determine it as turbulent flow?
Turbulence is not simply a property of a flow, it is a property of a flow and of a length scale. At small scales (e.g., smaller than the Kolmogorov scale) every flow may look smooth. Similarly, the Reynolds number is vaguely defined in terms of a "length scale of interest." A more interesting question is how the governing equations of a flow depend on the scale at which it is observed. Most subgrid scale models used for simulating turbulence on the computer depend on the square of the discretization parameter.
What I found in Wikipedia is like this:
Kolmogorov microscales are the smallest scales in turbulent flow. At the Kolmogorov scale, viscosity dominates and the turbulent kinetic energy is dissipated into heat.
The Kolmogorov 1941 theory is a mean field theory since it assumes that the relevant dynamical parameter is the mean energy dissipation rate. In fluid turbulence, the energy dissipation rate fluctuates in space and time, so it is possible to think of the microscales as quantities that also vary in space and time. However, standard practice is to use mean field values since they represent the typical values of the smallest scales in a given flow.
No,,,,for example, the roughness of the wall could convert the flow to the turbulent although Re is laminar
Dear all,
As a result of our discussion, so far, it can be concluded that the Reynolds
number is not a sufficient condition to indicate the flow regimes, i.e. laminar, transition and turbulent.
Please give your comment.
Thank you
Thank Mohamed H. Shedid
So, we still can use Reynolds number for numerical calculations, right?
Thanks Mohamed H. Shedid
Anybody disagree with the conclusion that Reynolds number is not a sufficient condition to indicate the flow regimes, i.e. laminar, transition and turbulent?
Please give your comment.
Thank you.
Thanks Taha Ahmed Abdullah
Sufficient not for all cases. So it is not a sufficient condition...
Yes Tapan K. Sengupta
I believe many researchers are interested in your outcome. Please post it here.
Thank you.
Dear,
At low speeds of fluid flow there may be swirling and turbulent flow in the case of a heat exchange ...
It also depends on the fluid flow profile at the inlet of the tube
See Matais, Eckert (1964) Forced, free and mixed convection regimes ...
It depends. In engineering field, Re number is used to calculate pressure loss or some terms, which could roughly use to evaluate some macro-parameters. While in fundamental science field or say in mechanism research, the ratio of inertia force to viscous force (Re) is not enough to evaluate the flow state (laminar, transition or turbulence). For example, inlet turbulence intensity, channel shape of inlet (like bell mouth shape inlet, or sharply inlet), disturb values, and also Re, affect the onset of turbulence (laminar-turbulence transition process).
Dear Professor Sinaga,
In engineering, Re number can provide limits for laminar or turbulent flow.
The lower limit is the one below which the flow is almost certainly laminar. The upper is the one above which the flow is almost certainly turbulent.
The more the flow is specified, for example water flow in circular pipes, air flow over glider wings, the closer are the two limits.
Yours sincerely,
N. Kehayas
Thanks Nikolaos Kehayas
We have arrived to the conclusion that Reynolds number is not enough to indicate the flow regimes. Please look to our discussion above....
Is there any other comments about this discussion? Please give your reply.
Thank you very much.
Dear all distinguished scholars,
Thank you very much for your participation in the discussion. Hope we can make a good friendships and collaboration.
Best wishes
Nazaruddin Sinaga
Dear respected scholars, please join our important and interesting discussion at:
https://www.researchgate.net/post/Do_you_agree_if_we_create_an_international_organization_of_university_lecturers#view=5e3cbc1e2ba3a1a3a972a2d2
We are discussing about creating an international organization of lecturers and researchers for us. I think you would like to be a member or organizer.
Thank you very much.
Best regards.
Dear Naseem Ahmad
,Thank for your reply.
We have arrived at the conclusion that the Reynolds number is not enough to indicate the flow regimes. Please look at our discussion above....
Yes Naseem Ahmad
, there is other requirements except Reynolds number to indicate exactly the flow regimes.Thank you for your reply.
Have a nice time.
The dimensionless number (Reynolds Number) is important in analyzing any type of flow when there is a substantial velocity gradient. It indicates the relative significance of the viscous effect compared to the inertia effect.
Laminar flow
Laminar flow generally happens when dealing with small pipes and low flow velocities. Laminar flow can be regarded as a series of liquid cylinders in the pipe, where the innermost parts flow the fastest, and the cylinder touching the pipe isn't moving at all.
Turbulent flow
In turbulent flow vortices, eddies and wakes make the flow unpredictable. Turbulent flow happens in general at high flow rates and with larger pipes.
The flow is: laminar when Re < 2300
transient when 2300 < Re < 4000
turbulent when 4000 < Re
* Engineering ToolBox, (2004). Laminar, Transitional or Turbulent Flow. [online] Available at: https://www.engineeringtoolbox.com/laminar-transitional-turbulent-flow-d_577.html [Accessed Day Mo. Year].
Reynolds number (Re) is one of the most important dimensionless quantities in microfluidics and it is defined as Re=(ρVL)/μ=(VL)/ν, where V is the fluid velocity relative to the body and L is a typical length.
The Reynolds number is used to determine whether a fluid is in laminar or turbulent flow. Based on the API 13D recommendations, it is assumed that a Reynolds number less than or equal to 2100 indicates laminar flow, and a Reynolds number greater than 2100 indicates turbulent flow.
https://www.sciencedirect.com/topics/engineering/reynolds-number
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