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9.3: Nusselt's Technique

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The Nusselt's method is a bit more labor intensive, in that the governing equations with the boundary and initial conditions are used to determine the dimensionless parameters. In this method, the boundary conditions together with the governing equations are taken into account as opposed to Buckingham's method. A common mistake is to ignore the boundary conditions or initial conditions. The parameters that results from this process are the dimensional parameters which control the problems. An example comparing the Buckingham's method with Nusselt's method is presented. In this method, the governing equations, initial condition and boundary conditions are normalized resulting in a creation of dimensionless parameters which govern the solution. It is recommended, when the reader is out in the real world to simply abandon Buckingham's method all together. This point can be illustrated by example of flow over inclined plane. For comparison reasons Buckingham's method presented and later the results are compared with the results from Nusselt's method.

Example 9.10

Utilize the Buckingham's method to analyze a two dimensional flow in incline plane. Assume that the flow infinitely long and thus flow can be analyzed per width which is a function of several parameters. The potential parameters are the angle of inclination, θ, liquid viscosity, ν, gravity, g, the height of the liquid, h, the density, ρ, and liquid velocity, U. Assume that the flow is not affected by the surface tension (liquid), σ. You furthermore are to assume that the flow is stable. Develop the relationship between the flow to the other parameters.

Solution 9.10

Under the assumptions in the example presentation leads to following

˙m=f(θ,ν,g,ρ,U)
The number of basic units is three while the number of the parameters is six thus the difference is 63=3. Those groups (or the work on the groups creation) further can be reduced the because angle θ is dimensionless. The units of parameters can be obtained in Table 9.3 the following table.

Parameter Units Parameter Units

ν

L2t1

g

L1t2

˙m

Mt1L1

θ

none

U

L1t1

ρ

ML3

The basic units are chosen as for the time, U, for the mass, ρ, and for the length g. Utilizing the building blocks technique provides

˙mMtL=(ρML3)a(gLt2)b(ULt)c

The equations obtained from equation (2) are

Mass,Ma=1Length,L3a+b+c=1time,t2bc=1}π1=˙mgρU3

νL2t=(ρML3)a(gLt2)b(ULt)c The equations obtained from equation (2) are

Mass,Ma=0Length,L3a+b+c=2time,t2bc=1}π2=νgU3 Thus governing equation and adding the angle can be written as

0=f(˙mgρU3,νgU3,θ) The conclusion from this analysis are that the number of controlling parameters totaled in three and that the initial conditions and boundaries are irrelevant.

A small note, it is well established that the combination of angle gravity or effective body force is significant to the results. Hence, this analysis misses, at the very least, the issue of the combination of the angle gravity. Nusselt's analysis requires that the governing equations along with the boundary and initial conditions to be written. While the analytical solution for this situation exist, the parameters that effect the problem are the focus of this discussion. In Chapter 8, the Navier–Stokes equations were developed. These equations along with the energy, mass or the chemical species of the system, and second laws governed almost all cases in thermo–fluid mechanics. This author is not aware of a compelling reason that this fact should be used in this chapter. The two dimensional NS equation can obtained from equation (??) as

ρ(Uxt+UxUxx+UyUxy+UzUxz)=Px+μ(2Uxx2+2Uxy2+2Uxz2)+ρgsinθ

and

ρ(Uyt+UxUyx+UyUyy+UzUyz)=Px+μ(2Uyx2+2Uyy2+2Uyz2)+ρgsinθ With boundary conditions

Ux(y=0)=U0xf(x)Uxx(y=h)=τ0f(x) The value U0x and τ0 are the characteristic and maximum values of the velocity or the shear stress, respectively. and the initial condition of

Ux(x=0)=U0yf(y) where U0y is characteristic initial velocity. These sets of equations (7-??) need to be converted to dimensionless equations. It can be noticed that the boundary and initial conditions are provided in a special form were the representative velocity multiply a function. Any function can be presented by this form.
In the process of transforming the equations into a dimensionless form associated with
some intelligent guess work. However, no assumption is made or required about whether or not the velocity, in the y direction. The only exception is that the y component of the velocity vanished on the boundary. No assumption is required about the acceleration or the pressure gradient etc.
The boundary conditions have typical velocities which can be used. The velocity is selected according to the situation or the needed velocity. For example, if the effect of the initial condition is under investigation than the characteristic of that velocity should be used. Otherwise the velocity at the bottom should be used. In that case, the boundary conditions are

Ux(y=0)U0x=f(x)μUxx(y=h)=τ0g(x) Now it is very convenient to define several new variables:

¯U=Ux(¯x)U0xwhere: ¯x=xh¯y=yh The length h is chosen as the characteristic length since no other length is provided. It can be noticed that because the units consistency, the characteristic length can be used for "normalization'' (see Example 9.11). Using these definitions the boundary and initial conditions becomes

¯Ux(¯y=0)U0x=f(¯x)hμU0x¯Ux¯x(¯y=1)=τ0g(¯x) It commonly suggested to arrange the second part of equation (13) as

¯Ux¯x(¯y=1)=τ0U0xhμg(¯x) Where new dimensionless parameter, the shear stress number is defined as

¯τ0=τ0U0xhμ With the new definition equation (14) transformed into

¯Ux¯x(¯y=1)=¯τ0g(¯x)

Example 9.11

Non–dimensionalize the following boundary condition. What are the units of the coefficient in front of the variables, x. What are relationship of the typical velocity, U0 to Umax?

Ux(y=h)=U0(ax2+bxp(x))

Solution 9.11

The coefficients a and b multiply different terms and therefore must have different units. The results must be unitless thus a

L0=ax2L2a=[1L2]

From equation (18) it clear the conversion of the first term is Ux=ah2¯x. The exponent appears a bit more complicated as

L0=bxp(hxh)=bxp(h)xp(xh)=bxp(h)xp(¯x) Hence defining

¯b=1xph With the new coefficients for both terms and noticing that y=h¯y=1 now can be written as

Ux(¯y=1)U0=¯aah2x2+¯bbxp(h)xp(¯x)=¯a¯x2+¯bxp¯x Where ¯a and ¯b are the transformed coefficients in the dimensionless presentation.

After the boundary conditions the initial condition can undergo the non–dimensional process. The initial condition (10) utilizing the previous definitions transformed into

Ux(¯x=0)U0x=U0yU0xf(¯y)

Notice the new dimensionless group of the velocity ratio as results of the boundary condition. This dimensionless number was and cannot be obtained using the Buckingham's technique. The physical significance of this number is an indication to the "penetration'' of the initial (condition) velocity. The main part of the analysis if conversion of the governing equation into a dimensionless form uses previous definition with additional definitions. The dimensionless time is defined as ¯t=tU0x/h. This definition based on the characteristic time of h/U0x. Thus, the derivative with respect to time is

\[ \label{dim:eq:twoDNSdefDevT}
\dfrac{\partial U_x}{\partial t} =
\dfrac{\partial \overbrace{\overline{U_x}}^{\dfrac{U_x}{U_{0x}} } U_{0x}}
{\partial \underbrace{\overline{t}}_{ \dfrac{t\,U_{0x}}{ h } }\dfrac{h}{U_{0x}} }
=
\dfrac

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U_{0x}
\dfrac{\partial \overbrace{\overline{U_x} }^{\dfrac{U_x}{U_{0x}}} U_{0x}}
{\partial \underbrace{\overline{x} }_{\dfrac{x}{h}} h }
=
\dfrac
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{\partial \overline{x} }
\] The second derivative of velocity looks like

2Uxx2=(¯xh)(¯UxU0x)(¯xh)=U0xh22¯Ux¯x2 The last term is the gravity g which is left for the later stage. Substituting all terms and dividing by density, ρ result in

\[ \label{dim:eq:twoDNSxx}
\begin{array} {rll}
\dfrac

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+
\overline{U_y} \dfrac{\partial \overline{U_x}}{\partial \overline{y}} +
\left. \overline{U_z} \dfrac{\partial \overline{U_x}}{\partial \overline{z}}\right) = &\\
&- \dfrac{P_0-P_\infty}{h\,\rho}
\dfrac{\partial \overline{P}}{\partial \overline{x}} +
\dfrac
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\,\sin\theta
\end{array}
\] Dividing equation (27) by U0x2/h yields

\[ \label{dim:eq:twoDNSx1i}
\begin{array} {rll}
\left(\dfrac{\partial \overline{U_x} }{\partial \overline{t} } +
\right. & \overline{U_x}\dfrac{\partial \overline{U_x} }{\partial \overline{x}} +
\overline{U_y} \dfrac{\partial \overline{U_x}}{\partial \overline{y}} +
\left. \overline{U_z} \dfrac{\partial \overline{U_x}}{\partial \overline{z}}\right) = &\\
&- \dfrac{P_0-P_\infty}

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{\partial \overline{x}} +
\dfrac{\mu}{U_{0x}\,h\,\rho} \left(\dfrac{\partial^2 U_x}{\partial x^2} +
\dfrac{\partial^2 U_x}{\partial y^2} + \dfrac{\partial^2 U_x}{\partial z^2}\right) +
\dfrac{ g \,h }
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{\partial \overline{x}^n}
\] The third kind of boundary condition is the mix condition. This category includes combination of the function with its derivative. For example a typical heat balance at liquid solid interface reads

h(T0T)=kTx This kind of boundary condition, since derivative of constant is zero, translated to

h(T0Tmax)(T0TT0Tmax)=k(T0Tmax)(TT0T0Tmax)(x) or

(T0TT0Tmax)=kh(TT0T0Tmax)(x)Θ=1NuΘ¯x temperature are defined as

\boldsymbol{\label{dim:eq:NuDef} 
    Nu = \dfrac{h\,\ell}{k} && \Theta = \dfrac{ T - T_0}   { T_0 -T_{max} }}
and Tmax is the maximum or reference temperature of the system. The last category is dealing with some non–linear conditions of the function with its derivative. For example,

ΔPσ(1r1+1r2)=σr1r1+r2r2 Where r1 and r2 are the typical principal radii of the free surface curvature, and, σ, is the surface tension between the gas (or liquid) and the other phase. The surface geometry (or the radii) is determined by several factors which include the liquid movement instabilities etc chapters of the problem at hand. This boundary condition (38) can be rearranged to be

ΔPr1σr1+r2r2Avr1+r2r2 The Avi number represents the geometrical characteristics combined with the material properties. The boundary condition (39) can be transferred into

ΔPr1σ=Av Where ΔP is the pressure difference between the two phases (normally between the liquid and gas phase). One of advantage of Nusselt's method is the object-Oriented nature which allows one to add additional dimensionless parameters for addition "degree of freedom.'' It is common assumption, to initially assume, that liquid is incompressible. If greater accuracy is needed than this assumption is removed. In that case, a new dimensionless parameters is introduced as the ratio of the density to a reference density as

¯ρ=ρρ0 In case of ideal gas model with isentropic flow this assumption becomes

ˉρ=ρρ0=(P0P)1n The power n depends on the gas properties.

Characteristics Values

Normally, the characteristics values are determined by physical values e.g. The diameter of cylinder as a typical length. There are several situations where the characteristic length, velocity, for example, are determined by the physical properties of the fluid(s). The characteristic velocity can determined from U0=2P0/ρ. The characteristic length can be determined from ratio of =ΔP/σ.

Example 9.12

One idea of renewable energy is to use and to utilize the high concentration of brine water such as in the Salt Lake and the Salt Sea (in Israel). This process requires analysis the mass transfer process. The governing equation is non-linear and this example provides opportunity to study nondimensionalizing of this kind of equation. The conversion of the species yields a governing nonlinear equation (??) for such process is

U0CAx=yDAB(1XA)CAy

Where the concentration, CA is defended as the molar density i.e. the number of moles per volume. The molar fraction, XA is defined as the molar fraction of species A divide by the total amount of material (in moles). The diffusivity coefficient, DAB is defined as penetration of species A into the material. What are the units of the diffusivity coefficient? The boundary conditions of this partial differential equation are given by

CAy(y=)=0

CA(y=0)=Ce Where Ce is the equilibrium concentration. The initial condition is

CA(x=0)=C0 Select dimensionless parameters so that the governing equation and boundary and initial condition can be presented in a dimensionless form. There is no need to discuss the physical significance of the problem.

Solution 9.12

This governing equation requires to work with dimension associated with mass transfer and chemical reactions, the "mole.'' However, the units should not cause confusion or fear since it appear on both sides of the governing equation. Hence, this unit will be canceled. Now the units are compared to make sure that diffusion coefficient is kept the units on both sides the same. From units point of view, equation (43) can be written (when the concentration is simply ignored) as

ULtCxCL=y1LDAB(1X)DAB1CyCL

It can be noticed that X is unitless parameter because two same quantities are divided.

1t=1L2DABDAB=L2t Hence the units of diffusion coefficient are typically given by [m2/sec] (it also can be observed that based on Fick's laws of diffusion it has the same units). The potential of possibilities of dimensionless parameter is large. Typically, dimensionless parameters are presented as ratio of two quantities. In addition to that, in heat and mass transfer (also in pressure driven flow etc.) the relative or reference to certain point has to accounted for. The boundary and initial conditions here provides the potential of the "driving force'' for the mass flow or mass transfer. Hence, the potential definition is

Φ=CAC0CeC0 With almost "standard'' transformation

\boldsymbol{\label{highMass:coordinates} 
    \overline{x} = \dfrac{x}{\ell} &\qquad & \overline{y} = \dfrac{y}{\ell}}
Hence the derivative of Φ with respect to time is

Φ¯x=CAC0CeC0x=CeC0(CAC00)x=CeC0CAx
In general a derivative with respect to ¯x or ¯y leave yields multiplication of . Hence, equation (43) transformed into

U0(CeC0)Φ¯x=1¯yDAB(1XA)(CeC0)Φ¯y \Huge U0Φ¯x=12¯yDAB(1XA)Φ¯y Equation (52) like non–dimensionalized and proper version. However, the term XA, while is dimensionless, is not proper. Yet, XA is a function of Φ because it contains CA. Hence, this term, XA has to be converted or presented by Φ. Using the definition of XA it can be written as

XA=CAC=(CeC0)CAC0CeC01C Thus the transformation in equation (53) another unexpected dimensionless parameter as

XA=ΦCeC0C Thus number, CeC0C was not expected and it represent ratio of the driving force to the height of the concentration which was not possible to attend by Buckingham's method.

Contributors and Attributions

  • Dr. Genick Bar-Meir. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or later or Potto license.


This page titled 9.3: Nusselt's Technique is shared under a GNU Free Documentation License 1.3 license and was authored, remixed, and/or curated by via source content that was edited to the style and standards of the LibreTexts platform.


This page titled 9.3: Nusselt's Technique is shared under a GNU Free Documentation License 1.3 license and was authored, remixed, and/or curated by Genick Bar-Meir via source content that was edited to the style and standards of the LibreTexts platform.

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