Wakes Behind Blunt Bodies

Oertel, Herbert

doi:10.1146/annurev.fl.22.010190.002543

Cited by 231 publications

(78 citation statements)

References 25 publications

(25 reference statements)

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“…Examples are given by Huerre & Monkewitz (1990). These include countercurrent mixing layers in circular jets (Strykowski & Niccum 1991), and the wakes behind circular cylinders (Mathis, Provansal & Boyer 1984;Koch 1985;Triantafyllou, Triantafyllou & Chryssostomidis 1986;Monkewitz 1988;Strykowski & Sreenivasan 1990), blunt bodies (Hannemann & Oertel 1989;Oertel 1990) and a floating cylinder (Triantafyllou & Dimas 1989). But reverse flow certainly does not guarantee absolute instability, nor is it always necessary.…”

Section: Referred To As I)mentioning

confidence: 99%

“…The global instability of other wake flows has also been investigated by numerical simulation, e.g. the wakes behind blunt bodies (Hannemann & Oertel 1989;Oertel 1990), shallow-water flow past bottom topography in the form of a circular bump (Schär & Smith 1993), and the wake of a triangular cylinder (Zielinska & Westfreid 1995). Certain jet and plume flows have also been found to exhibit global instability, e.g.…”

Section: Referred To As I)mentioning

confidence: 99%

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Global behaviour corresponding to the absolute instability of the rotating-disc boundary layer

2003

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A study is carried out of the linear global behaviour corresponding to the absolute instability of the rotating-disc boundary layer. It is based on direct numerical simulations of the complete linearized Navier–Stokes equations obtained with the novel velocity–vorticity method described in Davies & Carpenter (2001). As the equations are linear, they become separable with respect to the azimuthal coordinate, $\theta$. This permits us to simulate a single azimuthal mode. Impulse-like excitation is used throughout. This creates disturbances that take the form of wavepackets, initially containing a wide range of frequencies. When the real spatially inhomogeneous flow is approximated by a spatially homogeneous flow (the so-called parallel-flow approximation) the results ofthe simulations are fully in accordance with the theory of Lingwood (1995). If the flow parameters are such that her theory indicates convective behaviour the simulations clearly exhibit the same behaviour. And behaviour fully consistent with absolute instability is always found when the flow parameters lie within the theoretical absolutely unstable region. The numerical simulations of the actual inhomogeneous flow reproduce the behaviour seen in the experimental study of Lingwood (1996). In particular, there is close agreement between simulation and experiment for the ray paths traced out by the leading and trailing edges of the wavepackets. In absolutely unstable regions the short-term behaviour of the simulated disturbances exhibits strong temporal growth and upstream propagation. This is not sustained for longer times, however. The study suggests that convective behaviour eventually dominates at all the Reynolds numbers investigated, even for strongly absolutely unstable regions. Thus the absolute instability of the rotating-disc boundary layer does not produce a linear amplified global mode as observed in many other flows. Instead the absolute instability seems to be associated with transient temporal growth, much like an algebraically growing disturbance. There is no evidence of the absolute instability giving rise to a global oscillator. The maximum growth rates found for the simulated disturbances in the spatially inhomogeneous flow are determined by the convective components and are little different in the absolutely unstable cases from the purely convectively unstable ones. In addition to the study of the global behaviour for the usual rigid-walled rotating disc, we also investigated the effect of replacing an annular region of the disc surface with a compliant wall. It was found that the compliant annulus had the effect of suppressing the transient temporal growth in the inboard (i.e. upstream) absolutely unstable region. As time progressed the upstream influence of the compliant region became more extensive.

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Section: Referred To As I)mentioning

confidence: 99%

Section: Referred To As I)mentioning

confidence: 99%

Global behaviour corresponding to the absolute instability of the rotating-disc boundary layer

2003

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“…Gaster, Kit & Wygnanski 1985;Oertel 1990;Theofilis 2003). In fact, certain aspects of the development can be captured with linear stability theory, although intensities (amplitudes) of the structures are often too large for the linear stability theory to be valid.…”

Section: Introductionmentioning

confidence: 99%

Numerical investigation of transitional supersonic axisymmetric wakes

Sandberg

Fasel

2006

J. Fluid Mech.

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Transitional supersonic axisymmetric wakes are investigated by conducting various numerical experiments. The main objective is to identify hydrodynamic instability mechanisms in the flow at M = 2.46 for several Reynolds numbers, and to relate these to coherent structures that are found from various visualization techniques. The premise for this approach is the assumption that flow instabilities lead to the formation of coherent structures. Three high-order accurate compressible codes were developed in cylindrical coordinates for this work: a spatial Navier-Stokes (N-S) code to conduct direct numerical simulations (DNS), a linearized N-S code for linear stability investigations using axisymmetric basic states, and a temporal N-S code for performing local stability analyses. The ability of numerical simulations to exclude physical effects deliberately is exploited. This includes intentionally eliminating certain azimuthal/helical modes by employing DNS for various circumferential domain sizes. With this approach, the impact of structures associated with certain modes on the global wake-behaviour can be scrutinized. Complementary spatial and temporal calculations are carried out to investigate whether instabilities are of local or global nature. Circumstantial evidence is presented that absolutely unstable global modes within the recirculation region co-exist with convectively unstable shear-layer modes. The flow is found to be absolutely unstable with respect to modes k > 0 for Re D > 5000 and with respect to the axisymmetric mode k = 0 for Re D > 100 000. It is concluded that azimuthal modes k = 2 and k = 4 are the dominant modes in the trailing wake, producing a 'four-lobe' wake pattern. Two possible mechanisms responsible for the generation of longitudinal structures within the recirculation region are suggested.

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“…The shear layer behind the tabbed cylinder is divided into two parts, one part upstream the tab and the other part downstream the tab, as shown in Figure 5(a). In general, the shear layer is convectively unstable [26]. Perturbations are introduced at the tip of the tabs in the shear layers, producing compression waves that travel upstream to the separation point, so that self-sustained oscillations can be achieved.…”

Section: Self-sustained Oscillations Along the Shear Layermentioning

confidence: 99%

Large-eddy simulation of the compressible flow past a tabbed cylinder

Zhao

Sun

2012

Chin. Sci. Bull.

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The compressible flow past a tabbed cylinder has been studied numerically using large-eddy simulation for a free-stream Mach number M∞=0.75 and a Reynolds number based on the diameter Re=2×10 5 . Because of the passive control of the flow past a tabbed cylinder, the mean drag coefficient of tabbed cylinder is less than that of a corresponding circular cylinder with a drag reduction up to 33%. The fluctuating lift coefficient is greatly suppressed to be nearly zero. Drag reduction due to the shearing process prevails over that due to the compressing process in this flow. Through investigating the mechanisms relevant to passive control of the tab, it is found that suppression of the shear layer instability can lead to a higher-base-pressure distribution, which can reasonably be associated with drag reduction and suppression of the lift fluctuations. The analysis of convective Mach number and self-sustained oscillations phenomenon inside the shear layers indicates that both the compressible effect and high-frequency forcing result in suppression of the shear layer instability of tabbed cylinder. circular cylinder, shock wave, compressible turbulence, large-eddy simulation Citation:Xu C Y, Zhao L Q, Sun J H. Large-eddy simulation of the compressible flow past a tabbed cylinder.

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Wakes Behind Blunt Bodies

Cited by 231 publications

References 25 publications

Global behaviour corresponding to the absolute instability of the rotating-disc boundary layer

Global behaviour corresponding to the absolute instability of the rotating-disc boundary layer

Numerical investigation of transitional supersonic axisymmetric wakes

Large-eddy simulation of the compressible flow past a tabbed cylinder

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