Three-cylinder rotating system flows and their effects on a downstream dimpled airfoil

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Abstract

Effects from turbulence levels created by a three-cylinder rotating system on the flow over a dimpled surface of an airfoil are reported. The three-cylinder rotating system effect is tested on the flow over a NACA0015 with leading-edge dimples at various angles of attack at Reynolds numbers of 0.5 × 105 and 1.0 × 105 in a low-speed wind tunnel. To the effect of mimicking free-stream flow perturbations, the three-cylinder system had small dimensional characteristics compared to those of the airfoil. Two-dimensional (2D) Particle Image Velocimetry (PIV) was used to characterize the flow with illumination from a dual-head Nd:YAG laser and imaging from two high-resolution PIV cameras setup to capture details of the three-cylinder system and the airfoil flow simultaneously. Results show wake characteristics that create complex unsteady and turbulent flows of various intensities as they rotate at various speeds in the free-stream. These free-stream flow disturbances establish the unsteady and turbulent flows that interact with the airfoil flow. The study finds substantial effects occurring for certain cylinder-system settings, airfoil angle-of-attack, and Re number, notably detecting a diminishing separation region and a decrease in its turbulence levels when compared to those for the flow without turbulence generated in the free stream.

Introduction

The study of effects of perturbations on the flow over obstacles is of great interest for engineering applications that contain boundary layer transition, flow instabilities, and flow separation [1]. Perturbations can have both detrimental and beneficial effects on the aerodynamic and structural integrity of bodies such as turbomachinery components [2], [3], [4], wind turbines, and aircraft wings [5], [6], [7]. From small turbulence to high gusty winds, airfoils face challenges that affect the efficiency and performance of aircraft flight and rotorcraft maneuvering missions [5], [6], [7]. To study the effects by mimicking actual conditions of turbulence on those environments, numerical simulations and experiments attempt to generate the turbulence conditions that represent the actual conditions encountered in internal and external aerodynamics situations [2], [3], [4], [5], [6], [7]. A range of methodologies have been used for generation of distinct levels and scales of turbulence in wake phenomena occurring in rotating turbomachinery (blade-row and stator-blade interactions) and high free-stream turbulence in turbine engine flows [2], [3], [4]. Studies of turbulence generation for studies of flow interactions in rotorcraft, wind turbine, and heat exchanger flows have also been of paramount importance [8], [9], [10], [11].

In this study, turbulence generated from a three-cylinder rotating system is used to generate free-stream turbulence of various intensities and to investigate its effects on airfoil flow separation under various angles of attack for two representative Reynolds numbers. It is found that a three-cylinder rotating system generates various degrees of “free-stream” turbulence that has significant consequences in separation and turbulence levels on airfoil flow. Among multiple cylinder configurations that can be used to generate turbulence, the particular cylinder configuration chosen in the study is based on the idea of creating a new method for generation of small scale turbulence and its practical effects on airfoil separation region. Its focus is on producing free-stream flow perturbations by use of a three-cylinder system having small dimensional characteristics compared to those of the airfoil. The dimples in the baseline airfoil had similar dimensions to the cylinder diameters and the cylinder circumference similar to the airfoil thickness. Choices are also consistent with those from studies in the literature as it is described in the following summary on flow cylinder configurations research. It also puts in perspective the contribution of this investigation that introduces the rotation degree-of-freedom in the wake system.

Both the near and far wake structures behind a bluff body in fluid flow have been topics of extensive research [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. It is noted that there is no prescription for the terms ‘near’ and ‘far’, however, it has been defined that the near region covers up to ten diameters downstream from the cylinder [13], [14]. Using numerical approaches, computational fluid dynamics (CFD), and experimental techniques such as particle image velocimetry (PIV), researchers have attempted to characterize and control the flow and separation points behind various bluff body geometries. One primary goal of existing research has been to delay the flow separation from the surface of the body. Many studies have been conducted experimentally and numerically that have characterized cylinder wakes and vortex shedding at low and high Reynolds numbers [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Extensive research in flow past a single circular cylinder has demonstrated the existence of several flow regimes with increasing Re number that yield a variety of turbulence levels downstream. A characteristic feature of the cylinder flow is the formation of a repeating pattern of alternating swirling vortices known as the von Karman vortex street resulting from flow instability mode growth [12], [13], [20], [22]. The vortices become staggered when the initial symmetrical arrangement is no longer hydro-dynamically stable from two interacting counter-rotating shear layers. This phenomenon is typically observed at Re > 90 where it is two-dimensional until Re > 170 [13], [20]. Three-dimensional characteristics appear as a result of various transverse and other instability modes [12], [13], [14], [15], [16], [17], [18], [19], [20], [21] that indicate onset of turbulence. Beyond transition stages, the flow exhibits the time-averaged, statistical, and coherent characteristics of turbulent wake phenomena [23], [24], [25].

Numerical studies have provided important insight in cylinder wake study. For instance, Hourigan et al. [20] simulated the unsteady turbulent vortex structure downstream of a static cylinder using a Reynolds-Averaged Navier-Stokes CFD simulation. For a Reynolds number of 20,000 used in the investigation, it was found that the turbulence intensity as a function of distance from the trailing edge of the cylinder was in line with experimental PIV results obtained by Aradag [21]. Lim and Lee [26], using systematic spectral direct numerical simulation (DNS) and large-eddy simulation (LES) at Reynolds numbers between 500 and 5,000, found that the very near wake, up to 3 cylinder diameters, is dominated by shear layer dynamics and is very sensitive to disturbances in the cylinder aspect ratio. Dong et al. [27] investigated the near wake of a cylinder at Reynolds numbers corresponding to the onset and development of shear-layer instabilities. That investigation compared a numerical simulation to experimental data and found that for varying Reynolds numbers the mean velocity, vorticity, Reynolds stress, and r.m.s. fluctuations in the wake structure are notably altered. The variation of Reynolds number also influenced the shear-layer instability. The investigation revealed that at a lower turbulent Reynolds numbers the shear-layer velocity was characterized by high-frequency low-amplitude fluctuations caused by shear-layer vortices superimposed on top of large-scale fluctuations caused by the Karman vortex formation. At higher Reynolds numbers the fluctuations dominated the shear-layer motions and overwhelmed the fluctuations caused by Karman vortex formation.

Cylinder flow research expanded by exploring the flow generated by non-smooth cylinders [26], cylinders with synthetic jets [28], multiple-cylinder combinations [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], and slotted-cylinder configurations [42] to examine and categorize different flow regimes and complicated wake structures generated by them. Multiple cylinder arrangements studied mostly include cylinders in tandem, side by side, staggered, and slotted [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]. The most widely studied of the aforementioned arrangements are those of two cylinders in tandem. Reynolds numbers from 8.7 × 103 to 5.2 × 104 and center-to-center distance/diameter ratios from 1.03 to 5.0 have been used to examine the wake structures experimentally. In this arrangement, eight different flow regimes were discovered greatly dependent on Reynolds number [33]. A numerical study conducted at low Reynolds number (Re = 100) for varying cylinder spacing [34] suggested that there exists a critical cylinder separation distance between 3.75 and 4.0 cylinder diameters where significant jumps in the alternating forces and Strouhal numbers occur. This suggests that in applications where cyclic loading forces are to be minimized, the primary consideration should be to avoid this critical spacing zone. Those investigations of side-by-side arrangements of circular cylinders studied experimentally have led to the general observations that vortex shedding occurs either in- or out-of-phase depending on Reynolds number and cylinder spacing. In a numerical simulation at larger gap to diameter ratios, even at low Reynolds numbers (1 ≤ Re ≤ 20) [35] it was shown that the flow patterns are significantly different from the case of a single circular cylinder. It was found that variations in lift coefficient caused by wake interference was a function of cylinder spacing.

The dynamic reactions of wake interference have also been analyzed from the point of view of dynamical systems and theoretical models have been developed using the Landau equations combined with experimentally determined coefficients [36] to describe phase-locked, phase-opposition, and asymmetric bi-stable locked states. Sunmer at al. [31] reported in an experimental investigation for the general case of two cylinders in a staggered arrangement that nine distinct flow patterns for 850 < Re < 1900 exist. Research using numerical simulations has found ten different patterns for flow in the laminar region Re < 160 [32]. Furthermore, experimental data taken at varying stagger angles (0 to π) and gap ratios (0.1–5.0) and using the variables of Strouhal number, fluid forces, and flow structures, other distinct flow regimes have appeared [33], [34], [35], [36], [38]. The next level of complexity in the study of cylinder arrangements is the addition of a third cylinder. Most studies of a three-cylinder system have used an equilateral arrangement where the cylinders are placed at the vertexes of an equilateral triangle. Zdrarkovich [37] used a formation such as this with 60 < Re < 300 to visually inspect the phenomena using smoke seeding. Other studies have focused on varying angle of incidence and spacing between various cylinder configurations (equilateral, four-in-row, 3x3 grid, etc.) and described a variety of the resulting flow patterns [38], [39], [40], [41], [42]. Gao et al. [42] experimentally investigated the flow characteristics around a circular cylinder with a slot at different angles of attack at Re = 2.67 × 104 and observed peculiar features of wake-jet mixing phenomena. In addition to the study of wake structure generated by different combination of blunt bodies the blunt body has also been used to create free-stream turbulence to simulate airfoil performance under high turbulent flow. Lange et al. [43] identified the effect of freestream turbulence on a blunt body. This study showed how the obstacle in the flow created changes on the turbulence intensity and created horseshow vortex core with varying Reynolds numbers. Moon et al. [44] investigated an airfoil experiencing periodic turbulent flows. In that study, wake flows were created by placing a cylinder upstream of NACA 63215b airfoil, PIV measurements were performed in different planes, and by using phase-averaging the boundary layer profiles were reconstructed.

In order to simulate high-intensity freestream conditions, researchers thus have used blunt bodies like circular cylinder systems to create high-intensity turbulent flow. The present investigation introduces a new degree of freedom in the turbulence generation scheme by adding rotation and focuses on studying flow effects of its complex wake into a downstream airfoil flow. The rotating three-cylinder system is capable of providing new levels of turbulence and unsteadiness representative of free-stream turbulence that can be applied to airfoil flows. This study finds new effects of this free-stream turbulence on the performance of a downstream dimpled airfoil at various Reynolds numbers and AoA. Results that compare the airfoil flow with and without the effect of the high intensity free stream turbulence generated from the rotating cylinder system are provided. The turbulent intensity is varied by adjusting the rotation speed of the cylinder system and its effect on the downstream airfoil is quantified using Particle Image Velocimetry [45], [46], [47]. The upstream conditions influence on separation and turbulence level on airfoils is outlined to stress its profound implications in practical applications such as those found in wake/blade or wind/airfoil interactions occurring in aircraft and rotorcraft wings, turbomachinery blade-rows, and wind turbines, among others.

Section snippets

Experimental setup

The experiments in this investigation were conducted in a low speed open-circuit wind tunnel having a one-meter square intake and a test section measuring 0.3 × 0.3 × 0.9 m in height, width, and length, respectively. The wind tunnel is powered by a single axial fan at the end of a diffuser with radius of 0.7 m and uses a system of honeycomb and wire mesh at the intake for flow conditioning. The main components of the study comprise the upstream cylinder system and the downstream airfoil. The

Results and discussion

Results and discussion pertinent to the objectives aforementioned are presented in this section. The section is organized to first show distinct and peculiar characteristics of the cylinder system wake phenomena related to the study and second to show the effects of the three-cylinder system turbulence on representative airfoil conditions. Unlike wakes produced from rotating solid or sliced cylinders [42], [50], that generate large levels of turbulent scales, the three-cylinder system mimics

Conclusions

A rotating three-cylinder system studied with PIV showed its distinctive wake turbulence levels. The system generates various levels of small scale turbulence that mimic high-free-stream turbulence with levels that depend on the rotational speed. The wake properties complexity varies and can be adjustable based on the rotational speeds for a given free-stream velocity. Utilizing the small three-cylinder rotating system to introduce a ‘free-stream-like’ turbulence perturbation on an airfoil

CRediT authorship contribution statement

Al Habib Ullah: Data curation, Formal analysis, Methodology, Visualization. Braden L. Rostad: Data curation, Formal analysis, Methodology, Visualization. Jordi Estevadeordal: Conceptualization, Data curation, Formal analysis, Methodology.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors acknowledge the financial and shop support of the Mechanical Engineering Department of NDSU.

References (55)

  • M. Langford, C. Minton, Wing Ng, J. Estevadeordal, R. Burdisso, Fan flow control for noise reduction Part 2:...
  • J. Estevadeordal et al.

    Study of wake-blade interactions in a transonic compressor using flow visualization and DPIV

    ASME J. Fluids Eng.

    (2002)
  • R.E. Gordnier, M.R. Visbal, Impact of vortical gust on the aerodynamics of a finite aspect-ratio wing, AIAA paper...
  • V.V. Golubev, L. Nguyen, M.R. Visbal, High fidelity simulations of a transitional airfoil interacting with upstream...
  • M.R. Visbal et al.

    Effect of sweep on dynamic stall of a pitching finite-aspect-ratio wing

    AIAA J.

    (2019)
  • D.-G. Capracea et al.

    Wakes of rotorcraft in advancing flight: A large-eddy simulation study

    Phys. Fluids

    (2020)
  • H. Sarlaka et al.

    Role of subgrid-scale modeling in large eddy simulation of wind turbine wake interactions

    Renew. Energy

    (May 2015)
  • A. Sanyal et al.

    Wake interactions in a fluid flow past a pair of side-by-side square cylinders in presence of mixed convection

    Phys. Fluids

    (2017)
  • F. Durand

    Aerodynamic Theory

    (1963)
  • C.H. Williamson

    Vortex dynamics in the cylinder wake

    Ann. Rev. Fluid Mech.

    (1996)
  • X. Ma et al.

    Dynamics and low-dimensionality of a turbulent near wake

    J. Fluid Mech.

    (2000)
  • S. Dennis et al.

    Numerical solutions for steady flow past a circular cylinder at Reynolds numbers up to 100

    J. Fluid Mech.

    (1970)
  • S. Sen et al.

    Steady separated flow past a circular cylinder at low Reynolds numbers

    J. Fluid Mech.

    (2009)
  • B. Kumar et al.

    Effect of blockage on critical parameters for flow past a circular cylinder

    Int. J. Numer. Method Fluids

    (2005)
  • C.D. Cantwell et al.

    Computational study of subcritical response in flow past a circular cylinder

    Phys. Rev. E

    (2010)
  • K. Hourigan, M.C. Thompson, G.J. Sheard, K. Ryan, J.S. Leontini, S.A. Johnson, Low Reynolds number instabilities and...
  • S. Aradag

    Unsteady turbulent vortex structure downstream of a three dimensional cylinder

    J. Therm. Sci. Technol.

    (2009)
  • Cited by (3)

    • Experimental investigation of flow past three-cylinder rotating system

      2023, Experimental Thermal and Fluid Science
      Citation Excerpt :

      The equidistant triangular cylinder system here rotates around its center relative to the axis perpendicular to the main flow direction and changes in RPM generate a variety of complex wake structures in the flow field. The use of three-cylinder rotation and also its effects in a downstream dimpled airfoil has been reported in our previous studies [25–27] . However, no comprehensive study has been found that shows the alteration of flow structure with various rotational speeds of the cylinder and flow velocity using high-resolution PIV to reveal details of the complex wake interactions and its productions.

    1

    Current: USCG Office of Design and Engineering Standards, Portsmouth, VA 23704, United States.

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