Experimental investigation of freely falling thin disks. Part 1. The flow structures and Reynolds number effects on the zigzag motion

2013 ◽  
Vol 716 ◽  
pp. 228-250 ◽  
Author(s):  
Hongjie Zhong ◽  
Cunbiao Lee ◽  
Zhuang Su ◽  
Shiyi Chen ◽  
Mingde Zhou ◽  
...  
Keyword(s):  
Experimental Investigation ◽  
Reynolds Number ◽  
Degrees Of Freedom ◽  
Three Dimensional ◽  
Vortex Pair ◽  
Leading Edge ◽  
Circular Disk ◽  
Stereoscopic Vision ◽  
Time Resolved ◽  
Dipole Vortex

AbstractThis paper describes an experimental investigation of the dynamics of a freely falling thin circular disk in still water. The flow patterns of the disk zigzag motion are studied using dye visualization and particle image velocimetry. Time-resolved disk motions with six degrees of freedom are obtained with a stereoscopic vision method. The flow separation and vortex shedding are found to change with the Reynolds number, $\mathit{Re}$. At high Reynolds numbers a new dipole vortex is shed that is significantly different from Kármán-type vortices. The vortical structures are mainly composed of leading-edge vortices, a counter-rotating vortex pair and secondary trailing-edge vortices. The amplitude of the horizontal oscillation is also dependent on the Reynolds number with a critical Reynolds number ${\mathit{Re}}_{cr} \approx 2000$, where the oscillatory amplitude is proportional to $\mathit{Re}$ for $\mathit{Re}\lt {\mathit{Re}}_{cr} $, but becomes invariant for $\mathit{Re}\gt {\mathit{Re}}_{cr} $. Three-dimensional dipolar vortices were also observed experimentally.

Base-Vented Hydrofoils of Finite Span Under a Free Surface: An Experimental Investigation

1984 ◽  
Vol 28 (02) ◽  
pp. 90-106
Author(s):  
Jacques Verron ◽  
Jean-Marie Michel
Keyword(s):  
Experimental Investigation ◽  
Free Surface ◽  
Flow Rate ◽  
Cavity Length ◽  
Three Dimensional ◽  
Leading Edge ◽  
Pulsation Frequency ◽  
Cavity Pressure ◽  
Air Flow Rate ◽  
Cavity Configuration

Experimental results are given concerning the behavior of the flow around three-dimensional base-vented hydrofoils with wetted upper side. The influence of planform is given particular consideration so that the sections of the foils are simple wedges with rounded noses. Results concern cavity configuration, the relation between the air flow rate and cavity pressure, leading-edge cavitation, cavity length, pulsation frequency, and force coefficients.

Secondary instability and subcritical transition of the leading-edge boundary layer

2016 ◽  
Vol 792 ◽  
pp. 682-711 ◽  
Author(s):  
Michael O. John ◽  
Dominik Obrist ◽  
Leonhard Kleiser
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
High Speed ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Secondary Instability ◽  
Bypass Transition ◽  
Wall Suction ◽  
Transition Mechanism

The leading-edge boundary layer (LEBL) in the front part of swept airplane wings is prone to three-dimensional subcritical instability, which may lead to bypass transition. The resulting increase of airplane drag and fuel consumption implies a negative environmental impact. In the present paper, we present a temporal biglobal secondary stability analysis (SSA) and direct numerical simulations (DNS) of this flow to investigate a subcritical transition mechanism. The LEBL is modelled by the swept Hiemenz boundary layer (SHBL), with and without wall suction. We introduce a pair of steady, counter-rotating, streamwise vortices next to the attachment line as a generic primary disturbance. This generates a high-speed streak, which evolves slowly in the streamwise direction. The SSA predicts that this flow is unstable to secondary, time-dependent perturbations. We report the upper branch of the secondary neutral curve and describe numerous eigenmodes located inside the shear layers surrounding the primary high-speed streak and the vortices. We find secondary flow instability at Reynolds numbers as low as$Re\approx 175$, i.e. far below the linear critical Reynolds number$Re_{crit}\approx 583$of the SHBL. This secondary modal instability is confirmed by our three-dimensional DNS. Furthermore, these simulations show that the modes may grow until nonlinear processes lead to breakdown to turbulent flow for Reynolds numbers above$Re_{tr}\approx 250$. The three-dimensional mode shapes, growth rates, and the frequency dependence of the secondary eigenmodes found by SSA and the DNS results are in close agreement with each other. The transition Reynolds number$Re_{tr}\approx 250$at zero suction and its increase with wall suction closely coincide with experimental and numerical results from the literature. We conclude that the secondary instability and the transition scenario presented in this paper may serve as a possible explanation for the well-known subcritical transition observed in the leading-edge boundary layer.

Thermogasodynamic Analysis of the Segmented Annular Seal

2020 ◽  
Vol 143 (7) ◽  
Author(s):  
Samia Dahite ◽  
Mihai Arghir
Keyword(s):  
Parametric Study ◽  
Thermal Effects ◽  
Degrees Of Freedom ◽  
Three Dimensional ◽  
Leading Edge ◽  
Test Case ◽  
Isothermal Model ◽  
Pocket Depth ◽  
Annular Seal ◽  
Three Degrees Of Freedom

Abstract The present work deals with the thermogasodynamic analysis of the segmented annular seal provided with Rayleigh pockets. The paper is a continuation of the work presented Arghir, M., and Mariot, A. (2017, “Theoretical Analysis of the Static Characteristics of the Carbon Segmented Seal,” ASME J. Tribol., 139(6), p. 062202.) where an isothermal model of the segmented annular seal was first presented. Each segment had three degrees-of-freedom, and its static position was obtained by solving the nonlinear equations of equilibrium. Thermal effects are now introduced by considering a simplified form of the energy equation in the thin gas film coupled with the three dimensional heat transfer in a segment of the seal and in the rotor. An efficient numerical algorithm is developed. A parametric study was performed for a segmented annular seal with pockets taken from the literature and operating with air. First, a test case proved the necessity of considering three degrees-of-freedom for the segment and not only its radial displacement. The parametric study was then performed for two different pocket depths, two pressure differences, and different rotation speeds. The results showed a non-uniform heating with larger temperatures at the leading edge of the segment where the minimal film thickness occurs. Heating is proportional to the pocket depth that lowers the lift force of the segment and to the pressure difference that closes the seal.

scholarly journals The leading-edge vortex on a rotating wing changes markedly beyond a certain central body size

2018 ◽  
Vol 5 (7) ◽  
pp. 172197 ◽  
Author(s):  
Shantanu S. Bhat ◽  
Jisheng Zhao ◽  
John Sheridan ◽  
Kerry Hourigan ◽  
Mark C. Thompson
Keyword(s):  
Reynolds Number ◽  
Body Size ◽  
Central Body ◽  
Three Dimensional ◽  
Leading Edge ◽  
Particle Image Velocimetry Technique ◽  
Leading Edge Vortex ◽  
Rapid Acquisition ◽  
Edge Vortex ◽  
Rotating Wing

Stable attachment of a leading-edge vortex (LEV) plays a key role in generating the high lift on rotating wings with a central body. The central body size can affect the LEV structure broadly in two ways. First, an overall change in the size changes the Reynolds number, which is known to have an influence on the LEV structure. Second, it may affect the Coriolis acceleration acting across the wing, depending on the wing-offset from the axis of rotation. To investigate this, the effects of Reynolds number and the wing-offset are independently studied for a rotating wing. The three-dimensional LEV structure is mapped using a scanning particle image velocimetry technique. The rapid acquisition of images and their correlation are carefully validated. The results presented in this paper show that the LEV structure changes mainly with the Reynolds number. The LEV-split is found to be only minimally affected by changing the central body radius in the range of small offsets, which interestingly includes the range for most insects. However, beyond this small offset range, the LEV-split is found to change dramatically.

Three-Dimensional Vortex Structure in a Leading-Edge Separation Bubble at Moderate Reynolds Numbers

1991 ◽  
Vol 113 (3) ◽  
pp. 405-410 ◽  
Author(s):  
Kyuro Sasaki ◽  
Masaru Kiya
Keyword(s):  
Reynolds Number ◽  
Separation Bubble ◽  
Plate Thickness ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Vortex Filaments ◽  
Separated Shear Layer ◽  
Hydrogen Bubbles ◽  
Edge Separation

This paper describes the results of a flow visualization study which concerns three-dimensional vortex structures in a leading-edge separation bubble formed along the sides of a blunt flat plate. Dye and hydrogen bubbles were used as tracers. Reynolds number (Re), based on the plate thickness, was varied from 80 to 800. For 80 < Re < 320, the separated shear layer remains laminar up to the reattachment line without significant spanwise distortion of vortex filaments. For 320 < Re < 380, a Λ-shaped deformation of vortex filaments appears shortly downstream of the reattachment and is arranged in-phase in the downstream direction. For Re > 380, hairpin-like structures are formed and arranged in a staggered manner. The longitudinal and spanwise distances of the vortex arrangement are presented as functions of the Reynolds number.

Time-Resolved Numerical Investigation of the Interaction of Labyrinth Seal Leakage and Main-Flow in a 1.5-Stage LP Turbine

2011 ◽  
Author(s):  
Marc H.-O. Biester ◽  
Lasse Mueller ◽  
Joerg R. Seume ◽  
Yavuz Guendogdu
Keyword(s):  
Turbine Blades ◽  
Three Dimensional ◽  
Vortex Pair ◽  
Labyrinth Seal ◽  
Main Flow ◽  
Leakage Flow ◽  
Labyrinth Seals ◽  
Time Resolved ◽  
Lp Turbine ◽  
German Aerospace

In axial turbomachinery such as low pressure turbines, shrouded airfoils with labyrinth seals are commonly used. Among different sealing options, labyrinth seals in particular are characterized by long term durability and high sealing efficiency. Since a leakage flow is inevitable, a thorough understanding of how the leakage flow exits the cavities, its interaction with the main flow, and the induction of losses is necessary. In order to take into account unsteady effects, three-dimensional time resolved RANS computations of a 1.5 stage LPT rig in its design operating point are conducted. To capture effects in the boundary layer, a low Reynolds approach is used at the blade surface as well as on the hub and tip surfaces. To match the real geometry of the turbine blades, fillets have been modeled. Simulations were performed using the TRACE solver developed by the German Aerospace Center (DLR). The investigation shows how cavity flows have a significant influence on the main-flow aerodynamics and the loss generation. Steady and unsteady results with full spatially discretized cavities show a significant decrease of isentropic efficiency compared to simulations without cavities. The efficiency drop for the steady and time-averaged cavity computations can be explained with intensified secondary flow. The time resolved calculation shows a strong non-uniformity of the leakage flux depending on the instantaneous circumferential position of the up- and downstream blades. The time dependent ingress of cavity leakage results in the formation of a counter-rotating vortex pair. In terms of the influence on the main flow, it is shown that the interaction is limited to the end walls with almost no influence on the midspan flow.

Characterization of a Three-Dimensional Leading-Edge Separation Bubble on Swept, Low Aspect-Ratio Propeller Blades

2017 ◽  
Author(s):  
Ye-Bonne Koyama Maldonado ◽  
Gregory Delattre ◽  
Cedric Illoul ◽  
Clement Dejeu ◽  
Laurent Jacquin
Keyword(s):  
Delta Wing ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Leading Edge ◽  
Time Resolved ◽  
Leading Edge Vortex ◽  
Wall Structures ◽  
Edge Vortex ◽  
Propeller Blades ◽  
Leading Edge Vortices

Leading-edge vortex flows are often present on propeller blades at take-off, however, their characteristics and aerodynamic impact are still not fully understood. An experimental investigation using Time Resolved Particle Image Velocimetry (TR-PIV) has been performed on a model blade in order to classify this flow with respect to both delta wing leading-edge vortices and the low Reynolds number studies regarding leading-edge vortices on rotating blades. A numerical calculation of the experimental setup has been performed in order to assess usual numerical methods for propeller performance prediction against TR-PIV results. Similar characteristics were found with non slender delta wing vortices at low incidence, which hints that the leading-edge vortex flow may generate vortex lift. The influence of rotation on the characteristics of the leading-edge vortex is compared to that of the pressure gradient caused by the circulation distribution. A discussion on the quality of the PIV reconstruction for close-wall structures is provided.

Conjugate heat transfer from surface-mounted finite blunt plates

2006 ◽  
Vol 220 (1) ◽  
pp. 83-94 ◽  
Author(s):  
M Yaghoubi ◽  
E Velayati
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Three Dimensional ◽  
Leading Edge ◽  
Fin Efficiency ◽  
Flow And Heat Transfer ◽  
Flow Reynolds Number ◽  
The Heat Transfer Coefficient

Numerical studies of fluid flow and heat transfer are made in the separated, reattached, and redeveloped regions of the three-dimensional air flow on an array of finite plates with blunt leading edge. The flow reattachment occurs at a place downstream from the leading edge and the heat transfer coefficient becomes maximum around this region. The heat transfer coefficient is found to increase sharply near the leading edge and reduces in the wake. For the range of the parameters investigated in this study, some correlations have been developed for the length of reattachment region and variation of overall heat transfer coefficient for the considered bluff obstacles with various geometry and flow Reynolds number. For such blunt plates, when they are acting like fins, fin efficiency is determined and a relation based on flow Reynolds number and geometric parameters is developed to predict variation of the overall fin efficiency.

Dynamics of laminar separation bubbles at low-Reynolds-number aerofoils

2009 ◽  
Vol 630 ◽  
pp. 129-153 ◽  
Author(s):  
R. HAIN ◽  
C. J. KÄHLER ◽  
R. RADESPIEL
Keyword(s):  
Reynolds Number ◽  
Particle Image ◽  
Separation Bubble ◽  
Three Dimensional ◽  
Transition Process ◽  
Spanwise Direction ◽  
Time Resolved ◽  
Laminar Separation ◽  
Image Velocimetry ◽  
Dominant Frequencies

The laminar separation bubble on an SD7003 aerofoil at a Reynolds numberRe= 66000 was investigated to determine the dominant frequencies of the transition process and the flapping of the bubble. The measurements were performed with a high-resolution time-resolved particle image velocimetry (TR-PIV) system. Contrary to typical measurements performed through conventional PIV, the different modes can be identified by applying TR-PIV. The interaction between the shed vortices is analysed, and their significance for the production of turbulence is presented. In the shear layer above the bubble the generation and amplification of vortices due to Kelvin–Helmholtz instabilities is observed. It is found that these instabilities have a weak coherence in the spanwise direction. In a later stage of transition these vortices lead to a three-dimensional breakdown to turbulence.

scholarly journals Estimating intermittency in three-dimensional Navier–Stokes turbulence

2009 ◽  
Vol 625 ◽  
pp. 125-133 ◽  
Author(s):  
J. D. GIBBON
Keyword(s):  
Reynolds Number ◽  
Degrees Of Freedom ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Space Time ◽  
Navier Stokes ◽  
Local Velocity ◽  
Vortex Sheets ◽  
Navier Stokes Equations ◽  
Local Number

The issue of why computational resolution in Navier–Stokes turbulence is hard to achieve is addressed. Under the assumption that the three-dimensional Navier–Stokes equations have a global attractor it is nevertheless shown that solutions can potentially behave differently in two distinct regions of space–time $\mathbb{S}$± where $\mathbb{S}$− is comprised of a union of disjoint space–time ‘anomalies’. If $\mathbb{S}$− is non-empty it is dominated by large values of |∇ω|, which is consistent with the formation of vortex sheets or tightly coiled filaments. The local number of degrees of freedom ± needed to resolve the regions in $\mathbb{S}$± satisfies $\mathcal{N}^{\pm}(\bx,\,t)\lessgtr 3\sqrt{2}\,\mathcal{R}_{u}^{3},$, where u = uL/ν is a Reynolds number dependent on the local velocity field u(x, t).

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