Chapter II: Leakage Flow Model
2.2 Fluid Equations of Motion in Two Dimensions
2.2.1 Lubrication Like Closure for N x and F visc,x
Thus far, equations 2.3 and 2.5 only have restrictions on compressibility and deriva- tives of the channel geometry, which encompass a fairly broad range of problems.
Here, we consider further restrictions to the channel geometry associated with the flow-energy harvester designs discussed in chapter 1. Assuming incompressibil- ity, we begin with continuity and the infinitesimal Navier-Stokes equations in two dimensions for a Newtonian fluid, where∇ ·σ =−∇P+ µf∇2u,
∂u
∂x + ∂v
∂y = 0 (2.12)
ρfDu
Dt = −∂P
∂x + µf ∂2u
∂x2 + ∂2u
∂y2
ρfDv
Dt = −∂P
∂y + µf ∂2v
∂x2 + ∂2v
∂y2
. (2.13)
Considering the relevant dimensional scales from figure 2.1 and those listed on table 2.1, we would like to non-dimensionalize equations 2.12 and 2.13 in a way to take limits over relevant parameters to yield a useful expression for the velocity
profileu. We choose the length scale associated withxasL, and velocity associated withu as Uc. Those are rather clear choices. A less obvious choice is the length scale associated with y, hence we define h¯ ∼ h0− δ as a representative channel gap length scale that will not necessary increase significantly over L, given the restrictions from 2.8 onh020 andδ02. The value of h¯ will depend on the h0function and initial value for the beam shape δ|t=0, if other than 0. The relation between dimensional and non-dimensional values, the latter with(·)∗notation, are
x∗ = x
L, y∗ = y
h¯, u∗ = u
Uc, t∗ = Uc
L t. (2.14)
The scaling for v is also not obvious. In the absence of an initial beam velocity δÛ|t=0, we consider continuity from equation 2.12; substituting the non-dimensional quantities in 2.14 and normalizing,
∂u∗
∂x∗ + L
hU¯ c ∂v
∂y∗ =0. (2.15)
In order to satisfy continuity, either the two terms are identically 0 or they are equal.
Taking the latter hypothesis, the appropriate scaling ofvis the factor in front of the
∂y∂∗(·). Lastly, P is of the order of the stagnation pressure Pin driving Uc. The relevant non-dimensional parameters are
x∗= x
L, y∗ = y
h¯, u∗ = u
Uc, v∗ = L
hU¯ cv, t∗ = Uc
L t, P∗ = P
Pin. (2.16) Substituting 2.16 into 2.13,
ε2hReL
Du∗ Dt∗ =−1
Λ
∂P∗
∂x∗ +ε2h∂2u∗
∂x∗2 + ∂2u∗
∂y∗2 ε4hReLDv∗
Dt∗ =−1 Λ
∂P∗
∂y∗ +ε4h∂2v∗
∂x∗2 +ε2h∂2v∗
∂y∗2
, (2.17)
where we have defined εh = h¯
L, ReL = ρfUcL
µf , Λ= µfLUc
Pinh¯2 , (2.18) as the gap ratio, Reynolds number, and Bearing number, respectively. These pa- rameters are those expected from lubrication theory ([53, p. 319]). We now take
the limit as εh → 0, where the channel gap becomes narrow relative to the beam length, and equation 2.17 becomes
0= −1 Λ
∂P∗
∂x∗ + ∂2u∗
∂y∗2 0= −1
Λ
∂P∗
∂y∗.
(2.19)
In this limit we can easily solve for u∗. From the ˆy momentum, P∗ = P∗(x,t), independent ofy∗, yieldsv∗ ≈ 0. Theˆxmomentum gives, after applying the no-slip boundary condition at y∗ = δh¯ andy∗ = hh¯0, a parabolicu∗profile in y∗,
u∗ = 1 2Λ
∂P∗
∂x∗ δ
h¯ −y∗ h0
h¯ − y∗
. (2.20)
The first task is to define Nx in terms of Qx. We begin by evaluating Nx from equation 2.9,
Nx = h¯ 4
Uc Λ
∂P∗
∂x∗
2∫ h0/h¯ δ/h¯
δ
h¯ −y∗ h0 h¯ −y∗
2
dy∗ =− 1 120
Uc Λ
∂P∗
∂x∗
2(δ−h0)5 h¯4 . (2.21) Using the definition in equation 2.6 and integratingu∗in equation 2.20,
Qx = hU¯ c 12Λ
∂P∗
∂x∗
∫ h0/h¯ δ/h¯
δ
h¯ − y∗ h0 h¯ −y∗
dy∗ = 1 12
Uc Λ
∂P∗
∂x∗
(δ−h0)3
h¯2 . (2.22) Looking closely at equations 2.21 and 2.22, we can defineNxas
Nx =
∫ h0
δ u2dy =ξx Q2x
h0−δ, (2.23)
where ξx can be considered a “profile factor” that characterizes the velocity pro- file dependence on y. For the parabolic profile of u∗ in equation 2.20, ξx = 6/5 considering equations 2.21 and 2.22, while for an uniformu∗iny∗,ξx =1.
Next we quantify the viscous termFvisc based onu∗. Taking the limit as x2 → x1 of 2.11 and substituting the non-dimensional parameters in equation 2.16 (Fvisc,xf
remains dimensional),
Fvisc,x=
−2εhµfUc L
∂u∗
∂x∗
∂y∗
∂x∗ + µfUc h¯
∂u∗
∂y∗ +εhµfUc L
∂v
∂x
y∗=h0/¯h y∗=δ/¯h
, (2.24) and by taking the limit asεh→0and substitutingu∗from equation 2.20
Fvisc,x ≈ µfUc h¯
∂u∗
∂y∗
y∗=h0/h¯ y∗=δ/¯h
= µf Uc
Λ
∂P∗
∂x∗
δ−h0
h¯2 . (2.25) We can solve equation 2.22 for the quantity in the parenthesis above, and rewrite 2.25 in terms ofQx,
Fvisc,x ≈ −12µf Qx
(h0−δ)2. (2.26)
Equations 2.21 (withξx =6/5) and 2.26, along with the conclusion thatP= P(x,t) allow us to close equation 2.8,
∂Qx
∂t + ∂
∂x
ξx
Q2x h0−δ
=− 1 ρf
∂P
∂x (h0−δ) − 12µf ρf
Qx
(h0−δ)2, (2.27) without much regard to the ˆycomponent of equation 2.5, sincev∗ ≈ 0.
In addition to the two-dimensionality of the flow, equation 2.27 is valid under the following conditions:
1. Axial variations in channel gap are not large:h002 1andδ02 1;
2. The characteristic length of the channel gap is small:εh 1;
3. Inertial effects associated with velocity profile are small: ε2hReL 1.
Condition 3 enforces the inertial term in the infinitesimal equation (left-hand-side of equation 2.17) to be small relative to pressure and viscous stresses. The inertia associated with the motion of the channel walls, in particular δ, is captured by Qx andQÛx in equation 2.27. The normalized profileu∗ is a function of δ and h0, and that ε2hReL 1 implies that the u∗ profile behaves quasi-statically, and that its shape adapts almost instantaneously when compared with δ. This may pose aÛ problem if the beam has a large initial velocity δ|Ût=0 ∼ Uc, so that the scaling of equations 2.16 is no longer appropriate. We will, however, focus on understanding
the dynamics around δÛ|t=0 = 0 for the analysis at hand (although δ|t=0 , 0 is allowed). In essence, equation 2.27 allows us to capture the inertial effects of the moving channel walls, while the lubrication theory closure allows us to ignore the inertial effects associated with changing the shape ofu∗.
Condition 3 also indicates that for a fixed channel geometry, there is an upper limit on the Reynolds number for which the model is valid. That upper bound, however, may be quite large for very narrow channels. Thus, possibility of turbulent flow at high Reynolds number can be accounted for, however crudely, by adjusting the profile shape factor and the equation for Fvisc,x. It is reasonable to supposed that turbulence “flattens” profile such thatξx ≈ 1and thatFvisc,xcan take the form of a turbulent correlation. Following [31, 33], we write
Fvisc,x=− f(Qx) 4
Q2x
(h0−δ)2 (2.28)
where f is the Fanning friction factor and takes the form [54],
f =
48Re−1h if Reh <1000 0.26Re−0.24h if Reh ≥ 1000
. (2.29)
Rehis the local Reynolds number based on the channel gaph0−δand is simply
Reh = ρfQx
µf , (2.30)
with the onset of turbulence at Reh = 1000. We note here that Fvisc,x term for f < 1000in equations 2.28 and 2.29 is identical to equation 2.26. We can include in the model the profile factors that we derived
ξx =
6/5 if Reh < 1000 1 if Reh ≥1000
. (2.31)
Substituting equation 2.28 into equation 2.27,
∂Qx
∂t +ξx ∂
∂x Q2x
h0−δ
=− 1 ρf
∂P
∂x (h0−δ) − f 4
Q2x
(h0−δ)2. (2.32)