An analytical solution of the local balance equations for a Newtonian fluid flowing in a horizontal parallel channel bounded by adiabatic walls has been obtained. The characteristic feature of this stationary and two–dimensional solution is that it predicts a duality of flows for a prescribed assignment of the pair
. It is important to keep in mind that the Gebhart number, Ge, is the only parameter governing the flow system, as is shown by Equations (
1)–(
3) and (
5), while the Péclet number,
, serves to fix the mass flow rate associated with the solution. Strictly speaking, the Péclet number plays the role of a scaling parameter for the dual solutions, so that such dual flows degenerate into a single trivial solution when
. Indeed, on account of Equations (
9), (
26) and (
34), we find
,
and
when
. This is what is to be expected physically as the mass flow rate through the channel causes the frictional heating which, in turn, activates the temperature gradient and the buoyancy force. This chain of physical effects breaks down in the absence of a forced flow rate across the channel.
Beyond the scaling effect of the Péclet number, the phenomenon of flow duality is a consequence of a third parameter,
, which depends only on Ge in a double determination given by Equation (
32). In other words, the obtained stationary solution features two branches which, hereafter, will be denoted as
–branch and
–branch.
4.1. Characteristics of the Velocity Profiles
The deep diversity of features between the
–branch and
–branch emerges with the analysis of the velocity profiles. As
represents an overall scaling factor for the
x–component of velocity, the velocity distribution in a
cross–section can be displayed by the function
and by its parametric dependence on the Gebhart number. This is made evident by
Figure 2 where the dual branches
and
are illustrated for different values of Ge. A very wide range of Gebhart numbers is accounted for in
Figure 2, from the limiting case
to
. The latter value of Ge is really huge for most real-world applications except for geophysical applications where the channel width can be sufficiently large to achieve these Gebhart numbers. We bear in mind that Turcotte et al. [
13] devised a range of Gebhart numbers up to
.
Figure 2 features a left–hand frame with the
–branch velocity profiles and a right-hand frame with the
–branch velocity profiles. The comparison reveals that the two branches yield quite dissimilar profiles for a given Gebhart number. The
–branch shows slight departures from the Poiseuille velocity profile, which is exactly approached in the limit
. This is rigorously proved by considering the lowest orders of a power series expansion of
with respect to Ge. In fact, from Equations (
26) and (
32) with
, one has
where the zero–order term, and hence the limit of
for
, is the Poiseuille profile. On account of
Figure 2, the velocity profiles for the
–branch display a gradual, yet slight, loss of symmetry as Ge increases above zero. The cause of the asymmetry is the buoyancy effect produced by the viscous dissipation. Even when Ge moves close to its largest value compatible with the existence of the dual solutions,
, the departure from the Poiseuille profile becomes marked but not extremely large, as illustrated in
Figure 3. The black line in
Figure 3 displays the merging
and
profiles for
. We note that the merging velocity profiles display a bidirectional character with a flow reversal region close to
.
Flow reversal is a permanent property of the
–branch, as shown by both
Figure 2 and
Figure 3. On the other hand, the
–branch displays flow reversal close to the upper boundary only in the narrow range
, as it can be easily inferred from Equations (
26) and (
32) with
.
Figure 2 shows that the velocity profiles of the
–branch are bidirectional. The choice of plotting
in this case is a consequence of the singular behaviour of the
profile
when
. This feature is immediately gathered on evaluating, from Equations (
26) and (
32), the lowest orders of a power series expansion of
with respect to Ge with
,
By employing Equation (
36), one obtains
The right–hand side of Equation (
37) is a cubic polynomial profile describing an odd function of the shifted coordinate
.
4.2. Characteristics of the Temperature Profiles
If function
represents the velocity distribution in a
cross–section, the temperature profile at
can be displayed by plotting function
as given by Equations (
32) and (
33).
Figure 4 shows that the
–branch yields an asymptotic temperature profile in the limit
. This temperature profile is that corresponding to the forced convection regime where the Poiseuille profile describes velocity. In fact, this conclusion is supported by Equations (
32) and (
33) when we evaluate the lowest orders of a power series expansion of
with respect to Ge with
,
The black line in the left–hand frame of
Figure 4 displays the fourth–order polynomial that appears in Equation (
38) multiplied by Ge. Hence, the choice of plotting the scaled function
for the
–branch profiles in
Figure 4 is meant to capture the forced convection asymptotic case, where the buoyancy force is neglected with respect to the dynamic pressure gradient contribution in the local momentum balance. The
temperature profiles displayed in
Figure 4 for nonzero Ge display a departure from the symmetric temperature distribution in the forced convection limit,
. As for the velocity profiles, the asymmetry is induced by the effects of the buoyancy force.
A power series expansion of
, analogous to Equation (
38), can be written for the
–branch,
The singular behaviour of the
–branch for
is highlighted by Equation (
39). In the limit
, the velocity diverges with
, as shown by Equation (
36), while the temperature diverges with
. This limiting behaviour is the reason for the scaling,
, used in the right–hand frame of
Figure 4.
Figure 5 serves to complete the analysis of the dual temperature profiles in a regime of extremely large Gebhart numbers close to the upper threshold,
, above which no stationary parallel solutions are allowed. The merging of the two branches,
and
, is illustrated by this figure.
Figure 4 and
Figure 5, together with Equation (
26), show that
is negative in the lower part of the channel for the
–branch. Physically, this means a possibly unstable thermal stratification of the fluid that may give rise to a thermal instability of the flow. As already pointed out in
Section 3.3, a stability analysis is beyond the scope of this paper, but the possible onset of the instability may be an interesting chance for future developments of this research. If the possibility of a thermally unstable behaviour of the
–branch is realistic, this phenomenon seems much less conceivable for the
–branch. In fact, for the
–branch, an unstable thermal stratification in the lower part of the channel is present, but it is counterbalanced, to a large extent, by a markedly stable thermal stratification in the largest part of the channel. If the
–branch is the physically unexpected feature of the solution obtained in
Section 3, such an unexpected result is unlikely to be ruled out by a stability analysis. A similar conclusion is drawn in Barletta and Rees [
10] with reference to the fluid flow in a saturated porous channel.
4.3. Are the Dual Flows Compatible with the Boussinesq Approximation?
The governing Equation (
2) represents the widely employed Boussinesq approximation of the local mass, momentum and energy balance equations. As pointed out by several authors [
14,
15,
16,
17], the Boussinesq approximation is a correction to a fully–incompressible, constant properties flow model where the small effect taken into account is the buoyancy force. As such, the Boussinesq approximation corresponds to an asymptotic regime where a dimensionless parameter,
tends to zero. Here,
is the actual maximum dimensional temperature difference occurring in the flow domain. We mention that the conceptualisation of buoyancy–induced flows where the approximate scheme is obtained in the asymptotic case
is already quite clearly laid out in the pioneering paper by Boussinesq [
18].
The often overlooked situation is when the solution of the Boussinesq approximate equations is incompatible with the smallness of
. In rare cases, such a situation may arise even for perfectly reasonable values of the dimensionless parameter(s) governing the flow. In our case, the governing parameter is the Gebhart number which is assumed to be finite, though small enough. By considering the small–Ge expansion of the dimensionless temperature for the
–branch, Equation (
39), we can approximate the maximum dimensionless temperature difference on a
cross–section as
where Equation (
26) has been also employed. Thus, Equations (
6), (
11), (
40) and (
41) lead to an expression of
for the
–branch at small Gebhart numbers,
where
is the Froude number based on the dimensional average velocity across the channel,
Equation (
42) discloses the reason why the
–branch is incompatible with the asymptotic regime
characteristic of the Boussinesq approximation. In fact, the Gebhart number is usually small and the smaller is Ge the larger
is according to Equation (
42). Therefore, the only chance to obtain a compatibility between the
–branch and the core assumption of the Boussinesq approximation, i.e., the limit
, is considering large Gebhart numbers which, as suggested by
Figure 4 and
Figure 5, yield definitely smaller differences
. However, such large Gebhart numbers can be observed only with extremely wide flow systems such as those typical of geophysical or astrophysical applications.