1. Introduction
The turbulence intensity (TI) is of great importance in, e.g., industrial fluid mechanics, where it can be used for computational fluid dynamics (CFD) simulations as a boundary condition [
1]. The TI is at the center of the fruitful junction between fundamental and industrial fluid mechanics.
This paper contains an extension of the TI scaling research in [
2] (smooth-wall pipe flow) and [
3] (smooth- and rough-wall pipe flow). As in those papers, we treat streamwise velocity measurements from the Princeton Superpipe [
4,
5]. The measurements were done at low speed in compressed air with a pipe radius of about 65 mm. Details on, e.g., the bulk Reynolds number range and uncertainty estimates can be found in [
4]. Other published measurements including additional velocity components can be found in [
6] (smooth-wall pipe flow) and [
7,
8] (smooth- and rough-wall pipe flow).
Our approach to streamwise TI scaling is global averaging; physical mechanisms include separate inner- and outer-region phenomena and interactions between those [
9]. Here, the inner (outer) region is close to the pipe wall (axis), respectively.
The local TI definition (see, e.g., Figure 9 in [
10]) is:
where
r is the radius (
is the pipe axis and
is the pipe wall),
is the local mean streamwise flow velocity, and
is the local root-mean-square (RMS) of the turbulent streamwise velocity fluctuations. Using the radial coordinate
r means that outer scaling is employed for the position. As was done in [
2,
3], we use
v as the streamwise velocity (in much of the literature,
u is used for the streamwise velocity).
The measurements in [
10] were on turbulent flow in a two-dimensional channel and similar work for pipe flow was published in [
11].
In this paper, we study TI defined using a global (radial) averaging of the streamwise velocity fluctuations. The mean flow is either included in the global averaging or as a reference velocity. This covers the majority of the standard TI definitions.
There is a plethora of TI definitions, which is why we use the term fugue in the title. This is inspired by [
12], where Frank Herbert’s
Dune novels [
13] are interpreted as “an ecological fugue”.
The ultimate purpose of our work is to be able to present a robust and well-researched formulation of the TI; an equivalent TI in the presence of shear flow. Our work is not adding significant knowledge of the fundamental processes [
14,
15,
16], but we need to understand them in order to use them as a foundation for the scaling expressions.
The main contributions of this paper compared to [
3] are:
The introduction of additional definitions of the TI
Log-law fits in addition to power-law fits
New findings on the rough pipe friction factor behaviour of the Princeton Superpipe measurements.
Furthermore, we include a discussion on the link between the TI and the friction factor [
3,
17] in the light of the Fukagata–Iwamoto–Kasagi (FIK) identity [
18].
Our paper is organised as follows: In
Section 2, we introduce the velocity definitions. These are used in
Section 3 to define various TI expressions. In
Section 4 we present scaling laws using the presented definitions. We discuss our findings in
Section 5 and conclude in
Section 6.
4. Turbulence Intensity Scaling Laws
The Princeton Superpipe measurements and the TI definitions in
Section 3 are used to create the TI data points. Thereafter we fit the points using the power-law fit:
and the log-law fit:
Here, a, b, c, and d are constants. Q is the quantity to fit and x is a corresponding variable. We first apply the two fits using and .
The log-law fit is obtained by taking the (natural) logarithm of the power-law fit. The reason we use these two fits is that they have been discussed in the literature [
21,
22] as likely scaling candidates. We apply the two fits to the measurements and calculate the resulting root-mean-square deviations (RMSD) between the fits and the measurements. A small RMSD means that the fit is closer to the measurements.
Note that the smooth pipe measurement range is much larger than the rough pipe measurement range: a factor of 74 (9 points) compared to a factor of 6 (4 points). The consequence is a major uncertainty in the rough pipe results, e.g., (i) fits and (ii) extrapolation.
It is also important to be aware that we only have two sets of measurements with the following sand-grain roughnesses :
Smooth pipe:
m [
23]
Rough pipe:
m [
24] (see the related discussion in
Section 5.1)
We do not discuss the quality of fits to the rough pipe, since there are only 4 measurements for a single . Thus, these values are provided as a reference.
For the smooth pipe, the power-law fits perform slightly better than the log-law fits, except for the CL definition. The best fit is using the power-law fit to the AA definition of the TI:
which is the same as Equation (5) in [
3].
Instead of
, one can also express the TI fits using the friction Reynolds number [
4]:
The relationship between
and
can fitted using Equation (
15) where
and
, see
Figure 6 and
Table 5. A log-law fit was also performed but resulted in a bad fit, i.e., a RMSD which was between one and two orders of magnitude larger than for the power-law fit. As mentioned above, the rough pipe fit is only provided as a reference. For channel flow, it has been found that
and
, see Figure 7.11 in [
25] and associated text.
We choose to focus on the bulk Reynolds number since it is possible to determine for applications where the friction velocity is unknown.
6. Conclusions
We have used Princeton Superpipe measurements of smooth- and rough-wall pipe flow [
4,
5] to study the properties of various TI definitions. The scaling of TI with
is provided for the definitions. For scaling purposes, we recommend the AA definition and a power-law fit: Equation (
17). The TI also scales with
, where the best result is obtained with a power-law fit and the AM definition. This fit implies that the turbulence level scales with the friction factor: Equation (
23).
Scaling of TI with and was done using both power-law and log-law fits.
A proposed procedure to calculate the TI, e.g., CFD is provided and exemplified in
Section 5.2.4.