# What is Boundary layer: Definition and 59 Discussions

In physics and fluid mechanics, a boundary layer is the layer of fluid in the immediate vicinity of a bounding surface where the effects of viscosity are significant. The liquid or gas in the boundary layer tends to cling to the surface.
The boundary layer around a human is heated by the human, so it is warmer than the surrounding air. A breeze disrupts the boundary layer, and hair and clothing protect it, making the human feel cooler or warmer. On an aircraft wing, the boundary layer is the part of the flow close to the wing, where viscous forces distort the surrounding non-viscous flow. In the Earth's atmosphere, the atmospheric boundary layer is the air layer (~ 1 km) near the ground. It is affected by the surface; day-night heat flows caused by the sun heating the ground, moisture, or momentum transfer to or from the surface.

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1. ### Aerodynamics: flow separation's effect on lift and drag

In aerodynamics, how does flow separation reduces the lift and drag of a wing at the same time?
2. ### Engineering Fluid dynamics problem involving the Prandtl boundary layer equation for a two dimensional steady laminar flow

I have tried to approach in the following way I am stuck. How should I approach this next.please help
3. ### Perturbation Methods: Guarantee a boundary layer?

Hi, I was working on the following problem: Question: A small parameter multiplying the highest derivative does not guarantee that the solution will have a boundary layer for small values of ##\epsilon##. This may be due to the form of the differential equation, or the particular boundary...
4. ### I Help Regarding Application of Bernoulli in a Boundary Layer

Hey all, I recently took an aerodynamics exam that included the question "Please Explain how the Bernoulli Equation can be Applied Inside a Boundary Layer". Now, it is my belief that the Bernoulli equation, defined by my textbook as P+0.5ρV2=ℂ, requires inviscid flow to be properly applied...
5. ### Frequency of boundary layer instability

I've been searching the net but can not find any freely available literature. Can someone give me a quick lesson on boundary layer instability and its frequencies? I have an equation that claims to scale the instability frequency by: F = U/2*delta U is leading edge velocity delta is layer...
6. ### Question about boundary layer growth in convergent and divergent ducts

Hi, I was recently reading about convergent-divergent nozzles and was wondering about how boundary layers grow in them. Question: How does a boundary layer grow in a convergent duct in subsonic flow? How does this compare to the growth of a boundary layer in a divergent duct in subsonic flow...
7. ### Fully Developed Flow: Laminar vs Turbulent

Does the definition of fully developed flow is different for laminar and turbulent? I understand the fact that the entrance length are different in laminar and turbulent flows, but I believe the definition of fully hydrodynamically developed flow means that the velocity profile (hence momentum)...
8. ### Aerofoil Reynolds number, meaning of a statement in my homework

Homework Statement hello, i have been given a piece of homework which is determining the characteristics on an aerofoils boundary layer. In the brief the following statement is made with no context “Aerofoil is flying in chord Reynolds number 0.2x10^6” am i correct in saying that is the point...
9. ### Would there be turbulence around objects w/o boundary layers

According to wikipedia "The onset of turbulence can be predicted by the Reynolds number, which is the ratio of inertial forces to viscous forces within a fluid which is subject to relative internal movement due to different fluid velocities, in what is known as a boundary layer in the case of a...
10. ### A What is the difference between Flow and Boundary Separation?

Could someone please go here... https://en.wikipedia.org/wiki/Flow_separation And explain to me the difference between Flow and Boundary Separation discussed in the opening. Because this explanation makes no sense to me. I have the problem. I just don't understand the difference.
11. ### I Fluid separation in boundary layer

Hi PF, I'm trying to derive the Prandlt condition (not sure if it is Prandlt's work tough) for a fluid to separate from a surface in the boundary layer. The equation should be: ##\frac {\partial^2 v_x} {\partial y^2} = 0## which is quite "intuitive" to me. To derive it let's start from the...
12. ### Boundary layer thickness in partial air vacuum

How does low ambient pressure effect on the thickness of the boundary layer for given flow conditions of gas? In absolute vacuum the thickness of the boundary layer is 0 of course, but it seems that all the boundary layer thickness correlations use the Reynolds number, which doesn’t change much...
13. M

### A Fluid Boundary Layer Mathy Question

Hi PF! So after scaling Navier-Stokes for a flow over a flat plate we ultimately arrive at ##f f'' + f''' = 0## subject to ##f(0)=0##, ##f'(0)=0##, and ##f'(\infty) = 1## where independent variable is ##\eta##. The source I was reading is trying to reduce this BVP to an IVP. Thus they suggest...
14. ### Fluid Boundary layer in presence of a postive heat gradient

I was wondering how a boundary layer would be dissipative of momentum if it was under the influence of a positive heat gradient. I understand that the reason that we don't see the boundary pressure equal the stagnation pressure is that the boundary is dissipative (so excess pressure above...
15. ### Boundary Layer Separation in Laminar and Turbulent Flows

When compared to laminar flows, the fluid "sticks" with the solid surface longer in case of turbulent flows. For example, the angle of separation for flow over a circular cylinder is 80 degrees for laminar flows, and 140 degrees for turbulent flows. What is the reason?
16. ### I How Can Boundary Layer Suction Reduce Flow Separation in Pipe Expansion?

I have a question about the following scenario involving a flow separation issue in a pipe expansion The angle of the expansion is 30* - doubling the diameter from 1D to 2D We can consider this flow fully developed with a Reynolds of 5000+ Associated with this expansion is a head loss...
17. ### Temperature gradient vs thermal boundary layer thickness

what does the relation between the temperature gradient inside the thermal boundary and thermal boundary layer thickness i mean what will be the temperature gradient ( high or low) when the thermal boundary layer is thick relative to the thin one? Kindly explain mathematically and physically as...
18. ### Boundary layer thickness confusion

Hi, PF! Recently, while reading chapter 6 of Incropera's Fundamentals of Heat and Mass Transfer I got into a confusion regarding the velocity boundary layer. The book first states that, as the flow becomes more turbulent, the boundary layer gets thicker, as indicated by both figures attached at...
19. ### Blasius Solution for Boundary Layer

Is the Blasius solution valid for internal flow in an underdeveloped pipe? Can the function for the boundary layer thickness be used to calculate when the boundary layers will meet in the center of the pipe? I don't see how it could be valid since the assumption in its derivation is that...
20. ### Boundary layer equations for incompressible flow

Hello everyone. I posted this question in another forum and got no answers so I'll try and re-post it question here. I need to deliver a correct answer to the cited question below to my course responsible teacher. Below is also my own solution and thoughts on the problem but I don't know if I am...
21. ### Boundary layer equations for incompressible flow

Hi. I am new here so I please let me know if I should post this in another forum. I have been struggling for a while with the following homework problem: "State the boundary layer equations for incompressible flow over a solid, weakly curved boundary of a Newtonian fluid. What approximations...
22. ### Boundary layer at airfoil stagnation points

I was reading this: http://www.creatis.insa-lyon.fr/~dsarrut/bib/Archive/others/phys/www.mas.ncl.ac.uk/%257Esbrooks/book/nish.mit.edu/2006/Textbook/Nodes/chap06/node29.html Under the first figure it states "Figure 6.20: The boundary layer at a stagnation point on an airfoil has a constant...
23. ### Concentration Boundary layer thickness

Hello, I am simulating an experiment I did in the lab where we had air flow over a tray of water to determine the mass transfer coefficient scaling with velocity, as well as boundary layer thickness scaling with velocity. Now I am using COMSOL to simulate the experiment, and here is the...
24. ### Concentration boundary layer thickness

Hello, I'm doing an experiment where I will be blowing warm air parallel to a stagnant water surface, and I will investigate the scaling of air velocity with mass transfer coefficient. I am trying to find some kind of scaling of the concentration boundary layer thickness with air velocity, and...
25. ### Thermal Boundary Layer vs. Hydrodynamic Boundary Layer

Hello Guys, Could someone explain to me the meaning of greater thermal boundary layer over hydrodynamic boundary layer over a flat plate surface? I know how to calculate both streams, but I don't understand the meaning of smaller thermal boundary vs. hydrodynamic boundary and vice-versa. What...
26. ### Calculating Reynolds number for a wind tunnel

Hi All! as you can see I'm new here! i signed up in order to ask for a wee bit of help with my thesis! (now don't get me wrong I am not exactly asking anyone to write it for me!) I've found myself at a small bit of a loose end here, as a mechanical engineering student i am perfectly capable of...
27. ### Boundary Layer Theory: Displacement & Momentum Thickness Explained

Hi, could you tell the physical meaning of the displacement and momentum thickness of a boundary layer.And why the stream line diverges away from the body in the boundary layer to conserve mass?
28. ### Stability theory (Boundary Layer)

Hi all. I am new to boundary layers. Do the stability region of viscous Laminar and inviscid flow overlap? Why? Thanks :)
29. ### Thermal boundary layer and hydrodynamic boundary layer

So I know individually how these form. Unfortunately I haven't found any sources that describe more detailed questions that pop up in my mind. Could someone help me answer a couple of questions? 1. So if a thermal boundary layer forms in a 'plug flow' model i.e. when there is no momentum...
30. ### Development thermal boundary layer

Can anyone explain me why the thermal boundary layer develops faster for viscous fluids? I would just say it would develop more slowly because due to high viscosities there are low reynoldsnumbers and thus less turbulence or mixing. This causes a slow homogenization of temperature (assume a...
31. ### Did I calculate the boundary layer thickness correctly?

Homework Statement A flat plate moves in water (20°C) in the direction of the plate at a speed of 1 m/s. What is the boundary layer thickness 0.1 meter downstream of the plate?Homework Equations Reynolds number: ##Re_x=\frac{xU_∞}{\nu}## Boundary layer thickness for laminar flow...
32. ### What is the boundary layer (aerofoils)?

Can someone please explain to me the nature of the boundary layer and its effects on fluid flow over an aerofoil? Thanks,
33. ### Incompressibility in boundary layer (Fluid Dynamics)

I have started studying fluid mechanics recently and seems to be a very basic conceptual question that is bugging me and unfortunately I am unable to find a reasonable explanation for it. Your help would be more than appreciated. The mathematical definition for incompressiblility in fluid...
34. ### Boundary Layer Equations: Neglecting the term for x-axis momentum

I'm having a little difficulty in the topic 'Boundary layer Equations' in Fluid Mechanics due to my weak math skills. With reference to the figure in attachment, if we say that "we neglect \frac{∂^{2}u}{∂x^{2}}", does it mean that we will only consider the portion where \delta(x) is almost...
35. ### What Equation Models Boundary Layer Thickness in Early Stage Pipe Flow?

Hi I cannot find an equation for a boundary layer in a pipe flow (laminar). I am looking for an equivalent of the equation δ(x)=4.91x/(√Re) that works for a flow between plates (x is the distance downstream). The thing is- I am looking for BL thickness for still undeveloped flow. I would be...
36. ### Fluid Dynamics - Spanwise Vorticity of Turbulent Boundary Layer

Hi all, Am reading a few papers for a Uni case study about structures in Turbulent Boundary Layers over a Flat Plate (water), particularly low-speed streaks. I'm confused over what mechanism causes the span wise variations in velocity that seems to cause low speed streaks. Would...
37. ### Effect of Prandtl Number on Thermal Boundary Layer

I'm currently having a bit of an intuitive problem understanding the Prandtl number effect on boundary layers and I'm hoping that someone can explain it better than what I've read in some heat transfer books. According to various HT books, a low Prandtl number means that heat diffuses quickly...
38. ### Delaying boundary layer transition and flow separation

Hi all, I am looking to learn more about boundary layer control devices that can delay boundary layer transition (Laminar to Turbulent) and also BL devices that can delay flow separation. I have already found loads of info on Vortex Generators but i am having trouble finding info on things...
39. ### Boundary Layer Thickness of Blunt Body

I am working on a hypersonic vehicle project and would like to calculate the boundary layer. The vehicle that I am studying is the Apollo re-entry capsule. I'm assuming a 2-D flow. Here's what I have done so far. Calculate the local surface inclination angle at any point on the blunt body...
40. ### How can I calculate the displacement thickness of a turbulent boundary layer?

Lets go straight to the point. I need to find a way to calculate the displacement thickness of a turbulent boyndary layer. The laminar part has been simulated with Thwaite's method but I need to go from there. I'v heard of "Head's-model" but can't find the solution for it. Anyone that can...
41. ### How do you I calculate the Laminar Boundary Layer Y in meters

Homework Statement How do you I calculate the Laminar Boundary Layer Y in meters with Blasius Equation? I have an expression for U (m/s) and u/U and u(m/s) and also eta=0.1, 0.2...5.2. I am wondering how one could calculate the y laminar boundary layer (m), the y buffer boundary layer (m)...
42. ### EM Wave Boundary Layer Question

I understand application of Snell's law for transition from one medium to another but I have a question regarding this model. When an electromagnetic wave transitions from air into a conductive medium does the wavelength change instantaneously as the theory seems to imply or is there a boundary...
43. ### Boundary layer thickness over square cylinder

Hello all, I am looking for a reference (text or peer-reviewed journal article) for the derivation of the boundary layer thickness over a short square cylinder as a function of distance from the leading edge for the simplest case of steady, uniform and laminar axial flow of an inviscid...
44. J

### Simplifying a Boundary Layer Theory Equation

Homework Statement The problem is write this d\Psi/dy(d^2\Psi/dxdy)-d\Psi/dx(d^2\Psi/dy^2=-\nu(d^3\Psi/dy^3) in the form of -ff''=f''' where \Psi(x,y)=-sqrt(V*\nu*x)f(\eta) f(\eta)=integral(from 0 to \eta)(\Pi')*(\overline{\eta})*d(\overline{\eta}) where \overline{\eta} is a dummy...
45. ### Fluid mechanics boundary layer solution

Hello, I am looking for a reference which has solutions for the laminar flow boundary layer for the following scenario: circular cylinder, L>>d, length in direction of flow, with flat circular cap uniform laminar flow inviscid, incompressible fluid In other words, I would like the...
46. ### Calculating Boundary Layer Thickness

I am trying to calculate the thickness of the boundary layer for a NACA 2412 airfoil. I am working on constructing some wind tunnel tests that simulate a Cessna 172 (NACA 2412 airfoil). I am trying to determine the thickness of the boundary layer (distance from surface to 99% of free-stream...
47. ### Boundary layer thickness, accelerating flow.

Hello, You've all probably seen the classic Blasius solution concerning the thickness of a boundary over a flat plate. This problem though assumes that the free stream velocity is constant. http://www.see.ed.ac.uk/~johnc/teaching/fluidmechanics4/2003-04/fluids9/2-dboundary.html I am...
48. ### Need help on boundary layer meteorology

Hi, i am mech undergrad student, currently i am doing my thesis for CFD simulation of wind flow in urban area by using FLUENT. I am trying to apply logarithmic velocity profiles for my inlet boundary condition. My question is that from the formula of logarithmic wind profile, the velocity will...
49. ### Tripping the boundary layer why?

Tripping the boundary layer...why?? I don't understand why we have these "vortex generators" to trip the boundary layer into becoming turbulent...i've seen this a lot on F1 cars. On a regular car you can trip the boundary layer in front of your windshield...does the boundary layer somehow...
50. ### Boundary Layer displacement thickness

Hey guys, The streamlines just outside a boundary layer are pushed away from the wall by the displacement thickness \delta* and I understand that; \delta*=\int^{\infty}_{0}(1-\frac{u}{U})dy Now this is for flow over a plate with length x=4m. At x=0 is the leading edge and at x=4...