skip to main content

Explore Research Products in the NSF-PAR

Title: Role of flow reversals in transition to turbulence and relaminarization of pulsatile flows
The instability and transition to turbulence and its evolution in pulsatile flows, which involve reverse flows and unsteady flow separations, is the primary focus of this experimental work. A piston driven by a programmable DC servo motor was used to set-up a water flow system and provide the pulsation characteristics. Time-resolved particle image velocimetry data were acquired in a refractive index matching set-up by using a continuous wave laser and a high-frame-rate digital camera. The position of the piston was continuously recorded by a laser proximity sensor. Five different experiments were carried out with Reynolds numbers in the range of 535–4825 and Womersley numbers from 11.91 to 23.82. The non-stationarity of the data was addressed by incorporating trend removal methods involving low- and high-pass filtering of the data, and using empirical mode decomposition together with the relevant Hilbert–Huang transform to determine the intrinsic mode functions. This latter method is more appropriate for nonlinear and non-stationary cases, for which traditional analysis involving classical Fourier decomposition is not directly applicable. It was found that transition to turbulence is a spontaneous event covering the whole near-wall region. The instantaneous vorticity profiles show the development of a large-scale ring-like attached wall vortical layer (WVL) with smaller vortices of higher frequencies than the pulsation frequency superimposed, which point to a shear layer Kelvin–Helmholtz (K–H) type of instability. Inflectional instability leads to flow separation and the formation of a major roll-up structure with the K–H vortices superimposed. This structure breaks down in the azimuthal direction into smaller turbulence patches with vortical content, which appears to be the prevailing structural content of the flow at each investigated Reynolds number ( Re ). At higher Re numbers, the strength and extent of the vortices are larger and substantial disturbances appear in the free stream region of the flow, which are typical of pipe flows at transitional Re numbers. Turbulence appears to be produced at the locations of maximum or minimum vorticity within the attached WVL, in the ridges between the K–H vortices around the separated WVL and the upstream side of the secondary vortex where the flow impinges on the wall. This wall turbulence breaks away into the middle section of the pipe, at approximately $Re \ge 2200$ , by strong eruptions of the K–H vortices.  more » « less
Award ID(s):
1803845
NSF-PAR ID:
10278476
Author(s) / Creator(s):
; ;
Date Published:
Journal Name:
Journal of Fluid Mechanics
Volume:
917
ISSN:
0022-1120
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ( , 11th International Symposium on Turbulence and Shear Flow Phenomena (TSFP11))
    In past experiments, simulations and theoretical analysis, rotation has been shown to dramatically effect the characteristics of turbulent flows, such as causing the mean velocity profile to appear laminar, leading to an overall drag reduction, as well as affecting the Reynolds stress tensor. The axially rotating pipe is an exemplary prototypical model problem that exhibits these complex turbulent flow physics. For this flow, the rotation of the pipe causes a region of turbulence suppression which is particularly sensitive to the rotation rate and Reynolds number. The physical mechanisms causing turbulence suppression are currently not well-understood, and a deeper understanding of these mechanisms is of great value for many practical examples involving swirling or rotating flows, such as swirl generators, wing-tip vortices, axial compressors, hurricanes, etc. In this work, Direct Numerical Simulations (DNS) of rotating turbulent pipe flows are conducted at moderate Reynolds numbers (Re=5300, 11,700, and 19,000) and rotation numbers of N=0 to 3. The main objectives of this work are to firstly quantify turbulence suppression for rotating turbulent pipe flows at different Reynolds numbers as well as study the effects of rotation on turbulence by analyzing the characteristics of the Reynolds stress tensor and the production and dissipation terms of the turbulence budgets. 
    more » « less
  2. ; ; ; ( , Journal of Fluid Mechanics)

    Well-resolved direct numerical simulations (DNS) have been performed of the flow in a smooth circular pipe of radius$R$and axial length$10{\rm \pi} R$at friction Reynolds numbers up to$Re_\tau =5200$using the pseudo-spectral code OPENPIPEFLOW. Various turbulence statistics are documented and compared with other DNS and experimental data in pipes as well as channels. Small but distinct differences between various datasets are identified. The friction factor$\lambda$overshoots by$2\,\%$and undershoots by$0.6\,\%$the Prandtl friction law at low and high$Re$ranges, respectively. In addition,$\lambda$in our results is slightly higher than in Pirozzoliet al.(J. Fluid Mech., vol. 926, 2021, A28), but matches well the experiments in Furuichiet al.(Phys. Fluids, vol. 27, issue 9, 2015, 095108). The log-law indicator function, which is nearly indistinguishable between pipe and channel up to$y^+=250$, has not yet developed a plateau farther away from the wall in the pipes even for the$Re_\tau =5200$cases. The wall shear stress fluctuations and the inner peak of the axial turbulence intensity – which grow monotonically with$Re_\tau$– are lower in the pipe than in the channel, but the difference decreases with increasing$Re_\tau$. While the wall value is slightly lower in the channel than in the pipe at the same$Re_\tau$, the inner peak of the pressure fluctuation shows negligible differences between them. The Reynolds number scaling of all these quantities agrees with both the logarithmic and defect-power laws if the coefficients are properly chosen. The one-dimensional spectrum of the axial velocity fluctuation exhibits a$k^{-1}$dependence at an intermediate distance from the wall – also seen in the channel. In summary, these high-fidelity data enable us to provide better insights into the flow physics in the pipes as well as the similarity/difference among different types of wall turbulence.

     
    more » « less
  3. ; ; ; ; ; ( , AIAA Scitech 2020 Forum)
    Rotating and swirling turbulence comprises an important class of flows, not only due to the complex physics that occur, but also due to their relevance to many engineering applications, such as combustion, cyclone separation, mixing, etc. In these types of flows, rotation strongly affects the characteristics and structure of turbulence. However, the underlying turbulent flow phenomena are complex and currently not well understood. The axially rotating pipe is an exemplary prototypical model problem that exhibits these complex turbulent flow physics. By examining the complex interaction of turbulent structures within rotating turbulent pipe flow, insight can be gained into the behavior of rotating flows relevant to engineering applications. Direct numerical simulations are conducted at a bulk Reynolds number up to Re_D = 19,000 with rotation numbers ranging from N = 0 to 3. Coherence analysis, including Proper Orthogonal Decomposition and Dynamic Mode Decomposition, are used to identify the relevant (highest energy) modes of the flow. Studying the influence of these modes on turbulent statistics (i.e. mean statistics, Reynolds stresses, turbulent kinetic energy, and turbulent kinetic energy budgets) allow for a deeper understanding of the effects of coherent turbulent flow structures in rotating flows. 
    more » « less
  4. ( , Fluids)
    One of the key factors in simulating realistic wall-bounded flows at high Reynolds numbers is the selection of an appropriate turbulence model for the steady Reynolds Averaged Navier–Stokes equations (RANS) equations. In this investigation, the performance of several turbulence models was explored for the simulation of steady, compressible, turbulent flow on complex geometries (concave and convex surface curvatures) and unstructured grids. The turbulence models considered were the Spalart–Allmaras model, the Wilcox k- ω model and the Menter shear stress transport (SST) model. The FLITE3D flow solver was employed, which utilizes a stabilized finite volume method with discontinuity capturing. A numerical benchmarking of the different models was performed for classical Computational Fluid Dynamic (CFD) cases, such as supersonic flow over an isothermal flat plate, transonic flow over the RAE2822 airfoil, the ONERA M6 wing and a generic F15 aircraft configuration. Validation was performed by means of available experimental data from the literature as well as high spatial/temporal resolution Direct Numerical Simulation (DNS). For attached or mildly separated flows, the performance of all turbulence models was consistent. However, the contrary was observed in separated flows with recirculation zones. Particularly, the Menter SST model showed the best compromise between accurately describing the physics of the flow and numerical stability. 
    more » « less
  5. ; ; ; ; ( , Physical Review Fluids)
    At high Reynolds number, the interaction between two vortex tubes leads to intense velocity gradients, which are at the heart of fluid turbulence. This vorticity amplification comes about through two different instability mechanisms of the initial vortex tubes, assumed anti-parallel and with a mirror plane of symmetry. At moderate Reynolds number, the tubes destabilize via a Crow instability, with the nonlinear development leading to strong flattening of the cores into thin sheets. These sheets then break down into filaments which can repeat the process. At higher Reynolds number, the instability proceeds via the elliptical instability, producing vortex tubes that are perpendicular to the original tube directions. In this work, we demonstrate that these same transition between Crow and Elliptical instability occurs at moderate Reynolds number when we vary the initial angle   between two straight vortex tubes. We demonstrate that when the angle between the two tubes is close to  =2, the interaction between tubes leads to the formation of thin vortex sheets. The subsequent breakdown of these sheets involves a twisting of the paired sheets, followed by the appearance of a localized cloud of small scale vortex structures. At smaller values of the angle   between the two tubes, the breakdown mechanism changes to an elliptic cascade-like mechanism. Whereas the interaction of two vortices depends on the initial condition, the rapid formation of fine-scales vortex structures appears to be a robust feature, possibly universal at very high Reynolds numbers. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email