Boundary Layer - Science topic

A little outside my expertise, but I did find this...

Let us know if this helps!

Dear Armin Hajighasem Kashani

trying to achieve a match with experimental measurements is not a simple task. In your formulation you should match the statistics of the experimental inflow and that is not straightforward also for a laminar flow problem as detailed in Sec.4 here

A further issue could be also due to the fact your grid resolution is not sufficient to resolve the BL. You should be able to set at least 3-4 nodes at y+<1 along the surface of the body.

Again, I strongly suggest to try your setting for the well studied problem of the backward facing step turbulent flow. You can find a lot of results in literature.

The following references shows how one can use various transforms to aid in solving specialized PDE's by transforming into problems that can be solved by ODE's. The Fourier transform is one.

Of course separation of variables is a well known technique when the variables can be decoupled. In the analysis of the Hydrogen atom, one can use the fact that the potential is spherically symmetric (only depends on the distance from the proton) and the problem can be transformed into the the analysis of the equivalent Strum-Louisville problem in ODE.

For first order PDE, the method of characteristics can be used to generate an equivalent set of PDE's. For a rigid body in three space, the dynamics is described by ODE instead of PDE since the dynamics is invariant under SO(3), the Lie Group of rotations in R^3.

In general symmetry that results by the invariance of a Lie Group action the PDE will lead to the decoupling and/or reduction of variables. This can often head to solution by ODE. However, it is very equation dependent.

Sorry to say that the figures have no sense, why do you have different range of resolved frequencies?

And if you are in the BL, the flow is not homogeneos, the spectra will depends on y+.

You should clarify the way you are measuring the velocity.

try these.. geometry looks simple enough to have adequate meshing.

A tough question in any kind of complex terrain. The classic definitions do not work

From a theorethical point of view, radiation does not require a material to propagate, in fact, light propagates better in vacuum.

In a medium light is partially absorbed and the rest is transmitted. The part of light which is absorbed, depends of the interaction of light with the molecules and after it this reemited as Rutherford Scattering. Then Roseland does not apply.

If you have experimental data of absorbance and transmitance spectra, you could introduce the information through an external routine in fortran, but would need to create the model.

Perhaps is better to quantify if convection and difussion are more important than radiation

Of course, the ANSYS Fluent can be used to simulate the shock wave/boundary layer interaction. Firstly, the SBLI is a compressible flow problem, we can choose the density-based solver for simulation when the flow Mach number is high or the pressure-based solver for simulation when the flow Mach number is low. Secondly, the energy equation should be activated. Thirdly, the turbulence model is significant for the simulation results: if the inflow Reynolds number is low so that the boundary layer is in a laminar state, the laminar flow should be chosen, or the RANS model such as SA, SST, etc., which can predict the flow separation induced by an adverse pressure gradient, should be selected. Moreover, we can also try the LES or RANS/LES simulation method. That's a little self-experience for simulating SBLI. Hope that can help you. :)

This is a classical problem of wall-bounded parallel flow. Let D be the pipe diameter, Re the Reynolds Number, nu the kinematic viscosity, Ro the fluid density and y is the distance to the wall. In the logarithmic zone, the turbulent shear stress Roxu*^2 is constant where u* is the friction velocity. We should have:

Ro*u_*²*Pi*D*dx=dp*pi*D²/4 so that we have:

(dp/dx)=4*Ro*u_*²/D

So if we may calculate the pressure loss (dp/dx) using for example Blasius Formula for smooth pipe : (dp/dx)=Lamda*Ro*V²/2 with Lamda=0.316/Re^0.25 (or any other head loss formula for rough pipes) we will thus be in a position to calculate the friction velocity u_*. Let's define the Reynolds number y_⁺(adimensional distance to the wall) y_⁺=u_*y/nu The thickness for internal flows (y) is so that y⁺=11

Intraparticle diffusion kinetic model is explained herein: https://youtu.be/iwi1c_-_qyQ

Please send me the case file

Ahmed Gharib Yosry Thanks for your assistance. i tried to reverse the boundary conditions, replace the inlet and outlet with each other. but the torque doesn't goes down the optimum value.

In this case you can find boundary layer thickness less than tube radius between zero and tube radius X. The approximate experimental expression for the tube inlet length (L) calculation is: L/D=0.0288ReWhere, d  is tube diameter, Re  is Reynolds number.

For more information please read through the article herewith.

All the best Guo Li

I think there is a file called "namelist" which you can edit it by changing the input characteristics such as vertical level number.

Hi,

In this case, you can draw the temperature profile between the upper and the lower walls (vertical on the walls).

Thank you .A new way might be to see by converting it to and from quadratic and check Y+ value.Good strategy.

Twist is an aerodynamic feature added to aircraft wings to adjust lift distribution along the wing. Now by using twistable flaps could augment aerodynamic efficiency during critical flight paths i.e. take off and landing where flaps play important role.

I would add that the meaning of y+ is very very simple. It is nothing but that the local Reynolds number measured along a normal-to-wall direction. That is, y+=0 is the wall position and then the value increases according to y+=(u_tau/vi)*y.

A related issue is the consistency between vertical & horizontal spacings. A paper was written by Mahrer in the 90s and another by by Fox-Rabinovich & Lindzen

All the best

Hi Amir Hossein Zakeri ,

In unstructured mesh, you can set the first layer height using the inflation option.

Also, in a structured mesh, you can set that using edge sizing and proper "number of divisions" and "bias factor."

Thanks a lot Sven. This is good to hear about.

I believe it would still be the same. Laminar boundary-layer flows occur when a moving viscous fluid comes in contact with a solid surface and a layer of rotational fluid, the boundary layer, forms in response to the action of viscosity and the no-slip boundary condition on the surface.

My reference is: Pijush K. Kundu, Ira M. Cohen, David R. Dowling,

Chapter 10 - Boundary Layers and Related Topics,

Editor(s): Pijush K. Kundu, Ira M. Cohen, David R. Dowling,

Fluid Mechanics (Sixth Edition),

Academic Press,

2016,

ISBN 9780124059351,

https://doi.org/10.1016/B978-0-12-405935-1.00010-1.

The thickness of a diffusion boundary layer around a cylinder can be estimated for short times, by considering the growth of a thin unsteady boundary layer on a flat plate. The thickness d will scale as d=1.7 sqrt(D t) with D the diffusion coefficient of the polymer in the solvent. Assuming the concentration of the polymer on the surface to be unity, one can calculate the flux of mass towards the cylinder, which decreases as 1/sqt(T). The integral of this mass flux provides an indication for the thickness of the polymer layer (taking into acount the density of the polymer). This should yield some order of magnitude estimate and insight into relevant control parameters. This is analogous with heat transfer. This very crude model has been used for a viscous boundary layer on a rotating cylinder in order to estimate the onset of centrifugal instability. Onset of centrifugal instability at a rotating cylinder in a stratified fluid . The model does not assumes any effect of the rotation of the cylinder on the boundary layer thickness and is still quite succesful.

When calculating the heat transfer Colburn factors were used.

You will have to calculate the diffusivity, yes, and be able understand the boundary layer theory. The diffusivity in this case solid-gas interface. There are alots of analogies that are made available to this extreme. The concentrations and pressure data and mass transfer coefficients must be made available to clearly understand this concept. I recommended you consult materials Transport Phenomenon. You may also check review material on Mass transfer coefficients, and boundary layer theories.

Dear Obayes

The pdf attached by you is a master thesis which is just a preview of the work, not the entire thesis. If possible kindly attached the full thesis.

Thanking You

The Reynolds number (Re) helps predict flow patterns in different fluid flow situations.

Dear Prashant, thank you so much for your help

The pressure-based approach was developed for low-speed incompressible flows, while the density-based approach was mainly used for high-speed compressible flows. However, recently both methods have been extended and reformulated to solve and operate for a wide range of flow conditions beyond their traditional or original intent." "In both methods, the velocity field is obtained from the momentum equations. In the density-based approach, the continuity equation is used to obtain the density field while the pressure field is determined from the equation of state." "On the other hand, in the pressure-based approach, the pressure field is extracted by solving a pressure or pressure correction equation which is obtained by manipulating continuity and momentum equations." The pressure-based solver traditionally has been used for incompressible and mildly compressible flows. The density-based approach, on the other hand, was originally designed for high-speed compressible flows. Both approaches are now applicable to a broad range of flows (from incompressible to highly compressible), but the origins of the density-based formulation may give it an accuracy (i.e. shock resolution) advantage over the pressure-based solver for high-speed compressible flows."

I hope this answer helps you

Following

The pressure across the boundary layer does not change. The pressure is impressed on the boundary layer it value can be calculated by hydrodynamic consideration.

Dear Zein

I hope you are doing well.

what you suggested is fine and Dr Han approach is very typical

Good luck

Osama

One can also use the description based on the analogy to catalytic non-stationary processes. However, solving the inverse problem for partial differential equation/s is difficult.

Kanishka Ganesh

There are a few parameters missing from your udf,

You can get it from the udf attached below.

This worked with the test condition I have used.

Cheers

Dear Prof.Kim

Different materials heat up at different rates, and calculating how long it will take to raise an object’s temperature by a specified amount is a common problem for physics students. To calculate it, you need to know the specific heat capacity of the object, the mass of the object, the change in temperature you’re looking for and the rate at which heat energy is supplied to it. See this calculation performed for water and lead to understand the process and how it’s calculated in general.

Q = mc∆T

Where m means the mass of the object, c stands for the specific heat capacity and ∆T is the change in temperature. The time taken (t) to heat the object when energy is supplied at power P is given by:

t = Q ÷ P

Q = mc∆T

Where m means the mass of the object, c is the specific heat capacity of the material it’s made from and ∆T is the change in temperature. First, calculate the change in temperature using the formula:

∆T = final temperature – starting temperature

∆T = 50° – 10°

This might be useful:

Thanks, N. Bennini for your reply,

There are narrow channels within the geometry, then it is not good to create coarse mesh.

Dear all,

Thank you for the comments.

Ahmad K. Sleiti and Ahmed Ramadhan Al-Obaidi, I could not find the coordinates of RAE 5225 or DRA 2303 airfoils on the website. Please correct me if I am wrong.

Faraz Ahmad, As I mentioned, I could not find the coordinates anywhere given explicitly.

You can begin by trying different k-epsilon models (standard, RNG, realisable ) and compare it with experimental data.

Several researchers in this field: Zheltovodov, Bushnell, Borovoy.

Hi Nakul,

In most of the ceilometer, aerosol topmost layer (seibert et al., 2000, Munkel et al., 2007) is consider as PBL (most of the time it referred as mixing layer height during windy day time period/residual layer during stable night period).

As you mentioned that you want to retrieve PBL directly from a software. So, I would like to suggest you that based on manufacturer of ceilometer, software available.

CHM viewer -Lufft CHM Ceilometer

Lab View - Vaisala ceilometer.

Hope so this information is useful for you.

Dear Rochak Badyal ,

The boundary layer appears when there is a no-slip condition at the interface between the fluid and its wall. You may assume the no-slip condition just for the real fluid, not for ideal fluid. As you know, when you assume the fluid is ideal, then it means that the fluid has no viscous effect or inviscid (non-viscous). In other words, the no-slip assumption is valid when you consider that the fluid has its viscosity effect. With this assumption then you may have the boundary-layer flow.

Thank you and have a nice Saturday night at home.

Best regards.

Nazar

@Lifeng Tian,

I did it for turbulent BL. Anyways, I don't know whether it will work in your case or not. However, if it doesn't work, then PIV is only option, I think!

Thanks.

--Arnab.

Your notation is questionable, I see explicitly the independent variable "csi" but then you use the prime for the derivatives of f. What about the other independent variable, is that the vertical direction "eta"?

Generally, other than using the FD method, a third order PDE is converted in three first order ODE, as illustrated here

You can find a lot of literature about the numerical methods for such equations.

For compressible flows because the airfoil is curved there is a transverse pressure gradient near the surface. In such cases it is necessery to compute the theoretical (ideal) velocity profile first (by taking total pressure equal to stagnation pressure of the inflow and using isentropic flow equations to get velocity which is not a constant value as in case of a flat plate flow). In the next step you need to find a point at which the velocity in your flow is reaching e.g. 99.5% of this theoretical (ideal) profile. This way you will find your boundary layer edge on a convex surface and be able to compute for example integral parameters, such as displacement or momentum thickness.

Check any Myring's publications for details...

Regards

Oskar

Thanks a lot Ravindra!

Dear Yuna,

much depends on the Reynolds number and on the type of fluid we are talking about (gas, Newtonian or non-Newtonian fluid ...). In general, these topics are not of binary character, they are continuous. Although I did not read Leonardi and Bhaganagar, I am not surprised from their result as for me it is absolutely clear that Townsend's ideas stem from a futher past and cannot simply applied today, when a simple cook-book like approach is no longer applicable.

I attach couple of papers which underline that you touched a serious problem: the task to overcome simple empiricism which today is still very common. We measure "turbulent things" mostly very nicely, but we do not UNDERSTAND them.

Here is a paper which makes a first attempt to change that:

For me it would be a great joy to cooperate with people who are intersted in wall turbulence, e.g. around the superpipe/Princeton or CICLOPE and other facilities. Then one could improve or extend existing approaches. E.g., if you come from the oil world, then please are aware that oil is no standard fluid like e.g. water, it is of non-Newtonian character so that the turbulent boudnary layer with non-zero roughness challenges the brain.

This question is somehow often not well formulated by people.

1) First of all, the definition of y+=y*u_tau/ni says that it is a local Reynolds number evaluated from y=0 at the wall. That variable is similar to the Re_x along a flat plate. Therefore, the wall law U+(y+) is defined in terms of a continuous variable in the physical space, that is theory not a numerical results. There is not discretization in the physical meaning of the log-law.

2) When you talk about a numerical simulation, the grid in normal-to-wall direction defines a set of discrete values y+, starting from y+= for the node on the wall, y+=hy*u_tau/ni is the first discrete value corresponding to the node at the space h+ from the wall. So on for the other nodes in the vertical direction.

3) The presence of u_tau means that this function depends on the averaged stress at the wall, that is you should evaluate y+ after that the solution is known.

4) The log law is in a physical range of y+, the numerical simulation can be twofold, a) resolving the BL that is you have at least 3-4 nodes distributed at y+<1 in such a way to resolve the viscous laminar sublayer b) wall model condition, you do not solve the BL but you prescribe the stress you have at y+ in the log law.

In conclusion, the evaluation of the grid distribution of y+ requires to use the solution (that is to compute tau_wall) and, eventually, refine the grid and repeat the simulation.

Define boundary layer: Mesh->Define->Size fields->Boundarylayer

Refine mesh: Mesh->Transfinite

Hi,

from my perspective this is rather logical. The estimates calculated for y+ in the meshing process for specifying the inflation layer parameters are based on correlations for a fully developed turbulent boundary layer. For a long pipe this is a pretty valid assumption, since the flow has enough length along the wall to develop from the inlet boundary conditions to fully turbulent flow conditions, so that after some pipe length the postprocessing y+ is in pretty good agreement with what was used during the generation of the inflation layers in the mesh.

For a more complicated geometry like your centrifugal pump the cord length of the blades of the centrifugal pumps is rather much too short to develop a fully developed turb. boundary layer. Furthermore flow development over the blade surfaces is disturbed by flow separation and so forth. Consequently the underlying assumption of a fully developed turb. boundary layer is violated (for this assumption being taken into account for mesh property estimates) and you end up with differing y+ values in the postprocessing. But the values in the postprocessing are the physically valid values. Consequently you need to carry out a few iterations between mesh generation --> flow simulation --> postprocessing --> mesh generation in order to find good mesh properties and consequently tolerable y+ values. If for example you encounter a wide-spread y+~10 in the first results, than you might to reduce the 1st layer height in the inflation layers by approx. the same factor (10) in order to end up with y+~1 in your next flow simulation in the same place.

And regarding your last question: over wide parts of the blade surfaces it is possible to have reasonable low y+ in the order of 1. But in front stagnation points, where the flow hits the front of a blade or other parts of the geometry it is usually not possible with reasonable efforts to reduce y+ to 1, but this concerns usually only very minor part of the overall surface areas.

Best regards,

Dr. Th. Frank.

Have a look here http://www.fedoa.unina.it/3828/

Why do you get a boundary layer in case of Omega=0 ? In your case, if I understood it right, there should be no flow under these circumstances, and therefore no boundary layer as well. So why do you see an almost constant U of approx. 2.2m/s in the free stream with no respect to the rotational velocity of the cylinder?

Best regards,

Dr. Th. Frank.

I'm not sure exactly what your model describes, but I can tell you the story of a huge expensive blunder that arose from not considering the difference between inter- and intra-particle diffusion. I came along afterward and fixed the modeling error. I am intentionally vague about who did what and where, but the mathematics are described: http://dudleybenton.altervista.org/publications/Contaminant%20Transport%20in%20Granular%20Media.pdf If I knew more about what you were investigating, I might be able to locate some free software.

It's a good question! You may think of another measuring device, hot wire! or to use this tube in measuring known flow data. Good luck,,,

If you use KBr pellet you might have scattering problem using IR if you have microparticles but not if you use a micro-Raman. You can always use ATR-FTIR to avoid the problem. This would tell you if you have polymer in the sample (not necessary if that polymer is where you want it to be).

Maybe you could use backscattering SEM to check if you have organic on the Mn particle surface or use a transmission technique (TEM or optic microscopy if both particles and shell are tiny or big enough).

Also an Mn quantification might help to know how mich of your final mass is MnCO3 and how migh is possibly polymer.

I do go with Dear François Morency for his answer.

Well, you could perform several analyses. For example you can compute the TKE over several planes normal to the streamwise direction and associate the evaluation of the energy spectra.

A qualitative approach could be the identification of the vortical structures.

I don't think it exists a unique approach.

This is a quite interesting discussion that reminds me of Professor Moin Lecture at DOE CSGF 2011: Turbulence: V&V and UQ Analysis of a Multi-scale complex system.

The following questions may arise. Are you comparing two numerical computations? Are you using experiments to assess which computation is more accurate? Are you refining in the spanwise direction as well?

Professor answer is right on the money, I have seen that myself on some numerical experiments. That makes perfect sense based on numerical methods discussions.

Are you using LES? if so, are you "updating your BC?" something I have always had is there is a temporal-space relation for LES. indeed, for any numerical methods where you are really interested in temporal-spatial high accuracy. Professor Tapan also have great paper in that field too.

Don't cut the trailing edge like that. Make it a smooth half circle (or just slightly less, to get the tangents right). Put about 10 mesh points on that small half circle.

You can use the software of xy extract

Thanks dear Saad Najeeb Shehab

Hi Indu,

You can identify turbulence according to the fluctuation of velocity of the history of a sampling point. If you see random fluctuations, then it is turbulence. You can also judge according the energy spectral.

For classical boundary layers it's no use setting such low values for y+, usually going down to y+#1 is enough, so your max value of 0,2 seems already small. In the boundary layer, we expect a behavior of the flow such as u+=y+, which means a linear growing rate in y+ for the scaled velocity u+, so you don't need so many mesh layers to reproduce a linear behavior. Nevertheless, layers in your mesh shouldn't grow too fast in size : a factor 5 between adjacent cells seems too high because you will get strong numerical errors with such ratios, a more acceptable value could be a ratio of 1,2 for two contiguous cells.

Hope it's useful...

Regards

J. Collinet

T= 2 mu S -p I

This expression is always valid and account for the time-dependent velocity.

That's my question.

It's not clear if you are seeking steady RANS solutions or whether you will be using the URANS (unsteady RANS) approach that helps capture large scale unsteady structures within the flow (e.g., vortex shedding behind the bluff body).  There is evidence (see Ong et al., Marine Structures 22 (2009) 142–153) that the URANS method may provide reasonable engineering estimates in the context of bluff body flows.  URANS computations would be more expensive than the RANS computations because of the need to perform a time accurate simulation and to accumulate a long enough time series to compute flow statistics.  However, keep in mind that the RANS computations may or may not converge to a stationary flow for bluff body flows. 

Back to your query, the grid resolution requirements would vary significantly from the front of the bluff body (where the flow is attached) from those in the rear (i.e., the "base" flow, where the flow is separated).  Even in the attached flow region, the flow in the vicinity of the stagnation point is bound to be laminar, although a quick transition to turbulent flow could ensue depending on the surface roughness, freestream turbulent intensity, and the flow Reynolds number. If the laminar region is small, and therefore, can be ignored for the purpose of your computation, then you may use a skin friction estimate based on van Driest (Van Driest, E. R., “On the Aerodynamic Heating of Blunt Bodies,” ZAMP, 17, Vol. 9, issue 5-6, pp. 233-248. 1958) to ensure that your near wall spacing (i.e., first cell height) is consistent with the requirements of the turbulence model.

However, assuming that you are interested in relatively high Reynolds numbers, I would suggest an iterative approach based on URANS that begins with a grid that is based on a preliminary knowledge of the flowfield.  After scrutinizing the computed flowfield on the preliminary mesh, you can refine the mesh to ensure that all "relevant" flow features are adequately resolved and that the flow metrics of interest have become insensitive (to the order of the desired accuracy) to further refinements and/or some degree of grid coarsening. This is also the most convenient approach for any new class of fluid flow simulations.

Hallo again

I do agree that drawing an isoline would be helpful. However, the boudary layer thickness for airfoil cannot be defined as U_loc = 0.99*U_inf like in a flat plate. The main reason is, that flow accelerates significantly near the airfoil contour. This holds for inviscid and viscous flows especially when the airfoil thickness is large or when the angle of attack is not zero. In contrast, the inviscid velocity for a flat plate remains U_inf along the streamwise direction. Therefore, the definition of the BL thickness shall be V_total_loc = 0.99*V_total_loc_inviscid. In this sense, one should obtain the inviscid distribution outside the boundary layer but still in the vicinity of the wall. This is not a straightforward task. Therefore, plotting an isoline directly for U_loc = 0.99*U_inf will introduce some errors.

The thickness of the first prism layer Y can be estimated by Y=(Y+)*L/0.172/power(Re,0.9), where Y+ is what you want to set, for example 50; L is the ship length; Re is Reynolds number. The number of layers and strech factor can be chose according to the best practice provided by STAR-CCM+ or by ITTC CFD guide, such as 6~10 layers and 1.2 respectively for high Y+ model. Generally, some difference still exists between Y+ what you want and Y+ what you read. However, their ratio seems to be constant in my experience. In other words, if you want to set Y+=50, what you read will be 25. So you can use Y+=100 in the formula above to get Y+=50 in the software. You can do some tests to find similar relation in your settings. I hope this can help you.

Thanks for the earliest reply. I will look into it.

As your question is incomplete so I can't explain it in detail but below mention thesis is might be helpful in your case.

"DRAG FORCES ON PILE GROUPS By RAPHAEL CROWLEY A THESIS"

thank you so much. I already tired using an airfoil with very small thickness (about 1 to 2 % of chord) and it did work somehow. Do you have any idea how to analyze the BL over a Cylinder ?

thanks