# Fluid mechanics boundary layer solution

• uby
In summary, the conversation discusses the request for a reference with solutions for the laminar flow boundary layer for a circular cylinder with a flat circular cap and uniform laminar flow. The conversation also mentions the desire for an analog to the classical solution of an infinite flat plate and the difficulty in finding a solution in literature. The expert suggests using the flat plate solution for the given parameters.

#### uby

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 analog to the classical solution of an infinite flat plate (which has a parabolic dependence of the thickness of the boundary layer vs. position from the edge) for an infinite cylinder in which the flow is moving along the length of the cylinder (not around the cross-section!).

I've looked through a number of reference texts and have not found a solution.

I'm not well versed enough in the mathematics to derive my own numerical solution.

--Dave

Using the wall coordinate y=r-a; dy=dr, the axisymmetric boundary-layer equations become
$$\frac{\partial u}{\partial x} + \frac{1}{a+y} \frac{\partial}{\partial y} [(a+y)v]=0$$

$$u\frac{\partial u}{\partial x} + v\frac{\partial u}{\partial y} = \frac{v}{a+y}\frac{\partial}{\partial y}\left[ (a+y)\frac{\partial u}{\partial y}\right]$$
Note that if y<<a, these equations reduce to the flat plate equations. The boundary layer on a short cylinder is the Blasius solution.

Glauert and Lighthill pointed out that for $$\delta >> a$$ the convective acceleration is negligible and the momentum equation may be solved for
$$u(x,y) \approx \frac{a}{u}\tau_w (x) \ln\left(1+\frac{y}{a}\right)\,\,\,\,\delta>> a$$
That formula was then extended with the parameter $$\alpha(x) = \mu U_0 / (a\tau_w)$$ to define the profiles

For $$y \le \delta = a(e^{\alpha}-1)$$

$$u = \frac{U_0}{\alpha}\ln\left(1+\frac{y}{a}\right)$$
And for $$y\ge\delta$$
$$u=U_0$$

An algebraic expression for \alpha can be given from the momentum integral which requires some numerical technique. I can provide more information as needed, however you may be fine just using the flat plate solution.

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though the LaTeX equations did not properly show, i gather that the form of the equations has the same power dependence as the flat plate solution.

having spent some time in the library, i am sorry to report that i have only found 1 reference in the literature directly addressing this situation (LAMINAR BOUNDARY LAYER ON A YAWED
INFINITE CYLINDER ... J.C. BHATIA, Appl. Sci. Res. 30(6) 1975, p.469-75). no texts dealt with any boundary layer solutions for laminar flow other than for an infinite flat plate.

are you aware of a text the explicitly derives the formulai that you have derived here? i will need such as a reference if i were to further invoke this relationship.

(fyi, the purpose of this is so that i may model the kinetics of a gas-phase transport limited reaction while passing a reactant gas over a cylindrical substrate. my expertise is in chemistry, not fluid mechanics. my apparatus does not permit the use of anything approximating a flat plate due to proximity to the reactor walls and the drag they induce seems substantial.)

thanks!

OK, I have "fixed" the latex. Apparently we're not allowed...(ooo, forgot the being equation environment before split)...I digress. Either way, for short cylinders, the flat plate solution is the cylinder solution. As the cylinder gets longer, an additional factor becomes involved.

What kind of scales are looking at?

p.s. Yes I seen the yawed cylinder too, it is for delta wing approximations.

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realistically, i am looking at a circular cylinder of radius 0.25 inch, length of about 0.75 inch, so a 3:2 L:d ratio with Re < 1.

That would seem to indicate to me that you can safely use the Blassius solution. The long cylinder mentioned in the book references towing cables through the oceans; very long cylinders.

Go ahead and simply use flat plate, you should be fine.

## 1. What is a boundary layer in fluid mechanics?

A boundary layer in fluid mechanics is a thin layer of fluid that forms near a solid surface, where the fluid velocity is significantly affected by the presence of the surface. The thickness of the boundary layer typically increases with distance from the surface and is important for understanding the behavior of fluids in contact with solid surfaces.

## 2. What is the difference between laminar and turbulent boundary layers?

Laminar boundary layers are characterized by smooth, orderly flow of the fluid near the surface, while turbulent boundary layers have chaotic, irregular flow with fluctuations in velocity and pressure. Turbulent boundary layers are typically thicker and have a higher drag force compared to laminar boundary layers.

## 3. How is the boundary layer thickness determined?

The boundary layer thickness is typically determined by the Reynolds number, which is a dimensionless parameter based on the fluid velocity, density, viscosity, and length scale. Other factors that can influence the boundary layer thickness include the shape and roughness of the surface, as well as the fluid properties.

## 4. What is the purpose of a boundary layer solution in fluid mechanics?

A boundary layer solution is used to mathematically model and analyze the behavior of fluid flow near a solid surface. It allows scientists and engineers to predict the effects of viscosity, turbulence, and other factors on the flow of fluids, which is essential for designing efficient and effective systems for various applications such as aircraft design and hydraulic engineering.

## 5. What are some common methods for solving boundary layer equations?

Some common methods for solving boundary layer equations include analytical methods, such as the Blasius or Falkner-Skan solutions, which provide exact solutions for simplified boundary layer problems. Numerical methods, such as finite difference or finite element methods, are also commonly used to solve more complex boundary layer problems. Additionally, empirical or semi-empirical methods, based on experimental data, are often used to estimate boundary layer behavior in practical applications.