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Experiments were performed for individual realizations of the vortex shedding process behind a circular disk at Reynolds
numbers of 103–105, at which periodic vortex shedding prevails in the wake. The phase differences regarding the individual vortex shedding structures
detected at multiple circumferential locations in the wake were obtained by an...
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... the presence of the circular disk in the test section normal to the mean flow gives an area blockage ratio of about 4%. The disk was held by six stainless steel wires of 0.2 mm in diameter, see figure 1 for details. Experiments of flow visualization were made at a Reynolds number, Re d , about 103. ...
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... mentioned data reduction procedure provided the statistics of !\ with reference to a fixed value of \ . Figure 10 presents five histograms of !\ corresponding to the situations of \ :0, T/4, 9T/4, T/8 and 9T/8, respectively. In Fig. 10a, i.e. the histogram of the !\ values for \ :0, the events of positive and negative !\ values are quite even, implying no preference in the clockwise or counter-clockwise shedding. ...
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... to be situated at the circumferential positions :90°, 0° and 990°, denoted as points A, B and C, respectively. The mentioned data reduction procedure provided the statistics of !\ with reference to a fixed value of \ . Figure 10 presents five histograms of !\ corresponding to the situations of \ :0, T/4, 9T/4, T/8 and 9T/8, respectively. In Fig. 10a, i.e. the histogram of the !\ values for \ :0, the events of positive and negative !\ values are quite even, implying no preference in the clockwise or counter-clockwise shedding. This is reasonable because as far as these events are concerned an interchange of positions A and B should not affect the phase relation of vortex shedding. ...
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... detected at :T/8, indicates that a large portion of the sampled events have negative values. This figure shows no predominant peak. Nevertheless, in terms of time averaging one can obtain the idea that the character of anti-phase vortex shedding is largely preserved at the circumferential locations A and C. A similar appearance can be seen in Fig. ...
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... histograms of Fig. 10 confirm the observations obtained from flow visualizations that the phase differences measured at the circumferential positions 180° apart scatter, namely varying from one vortex shedding event to the other, unlike the process of vortex shedding behind a two-dimensional bluff body. Nevertheless, the anti-phase vortex shedding is ...
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Citations
... Using the spanwise extent of the St ¼ 0.6 core region, r=D ¼ 0:28, as the characteristic length, l core , a different Strouhal number can be defined for this peak, St core ¼ 0.17. In classical solid disk wake experiments, shedding has been characterized by St ¼ 0.135, [72][73][74] and in another study, (Ref. 75), the shedding was classified by St ¼ 0.15. ...
Dominant flow features in the near and intermediate wake of a horizontal-axis wind turbine are studied at near field-scale Reynolds numbers. Measurements of the axial velocity component were performed using a nano-scale hot-wire anemometer and analyzed using spectral methods to reveal the extent and evolution of the flow features. Experiments were conducted at a range of Reynolds numbers, of [Formula: see text], based on the rotor diameter and freestream velocity. Five different downstream locations were surveyed, between [Formula: see text], including the near wake, transition to the intermediate wake, and the intermediate wake. Three dominant wake features are identified and studied: the tip vortices, an annular shear layer in the wake core, and wake meandering. The tip vortices are shown to have a broadband influence in the flow in their vicinity, which locally alters the turbulence in that area. It is shown that shedding in the wake core and wake meandering are two distinct and independent low frequency features, and the wake meandering persists into the intermediate wake, whereas the signatures of the core shedding vanish early in the near wake.
... Such concepts have been under intensive investigation because they offer an attractive and flexible alternative that improves and promotes the lean premixed combustion concept, which is more frequently used in practical combustion systems. The majority of the previously reported studies on bluff body flow and flame stabilization has focused on examining stand-alone bluff geometries placed against a cross flow [6,27,[31][32][33][34][35]. ...
This work investigates the non-reacting time averaged and fluctuating flow field characteristics downstream of a variety of axisymmetric baffles, operating in combination with an upstream double-cavity premixer arrangement. The study aims to broaden knowledge with respect to the impact of different bluff body shapes, leading and trailing edge flow contours, blockage ratios and incoming flow profiles impinging on the bluff body, on the development and properties of the downstream recirculating wake. Particle Image Velocimetry (PIV) measurements have been employed to obtain the mean and turbulent velocity fields throughout the centrally located recirculation zone and the adjacent developing toroidal shear layer. The results are helpful in demarcating the cold flow structure variations in the near wake of the examined baffles which support and, to some extent, determine the flame anchoring performance and heat release disposition in counterpart reacting configurations. Additionally, such results could also assist in the selection of the most suitable flame stabilization configuration for fuels possessing challenging combustion behavior such as multi-component heavier hydrocarbons, biofuels, or hydrogen blends.
... The structures shared the same geometry but had different combined specific gravity SG ∈ [1. 2,8]. Results showed four distinct free-fall patterns, which were modulated by the degree of heterogeneity and SG. ...
... Over a century ago, Maxwell [4] discussed the falling of flexible plates. Early experimental studies using dye visualization focused on the wake of falling bodies, including liquid droplets [5], spheres [6], cylinders [7], and circular disks [8]. They noted the formation of distinct vortex rings under sufficiently large Reynolds numbers, and the possibility of oscillatory motions. ...
The late stage of the free fall of homogeneous and heterogeneous cones in a quiescent water medium and the induced flow are experimentally inspected using three-dimensional particle tracking velocimetry and volumetric particle image velocimetry. The structures shared the same geometry but had different combined specific gravity SG∈[1.2,8]. Results showed four distinct free-fall patterns, which were modulated by the degree of heterogeneity and SG. They included (i) straight fall, (ii) regular sinusoidal-like trajectories with a maximum angle of oscillation with respect to the vertical, θmax<π/2, (iii) motions characterized by inclined translations and large rotations with θmax>π/2, and (iv) tumbling followed by an irregular motion. The straight fall occurred in cones with SG≳6 regardless of the mass distribution. The sinusoidal motions arose in the homogeneous cones with SG<3. The large rotations occurred with heterogeneous cones of SG<3, whereas the tumbling with irregular motions occurred in the heterogeneous cone with the highest ratio between the center of mass and geometrical centroid. Inspection of the linear and angular components of the velocities and accelerations reveal the distinct patterns and the linkage with the surrounding flow.
... Strouhal numbers (St) based on the diameter associated with this instability are high and are measured to be 1.62 (Berger, Scholz, Schumm 1990). Apart from the shear layer instability, the wake behind the bluff body becomes unstable ending in Benard-Von Karman (BVK) type vortex shedding (Berger, Scholz, Schumm 1990;Miau et al. 1997). Vortex shedding occurs at a frequency much lower than the frequency of shear layer instability. ...
... The associated Strouhal number lies in the range of 0.13-0.15 (Kiya, Ishikawa, Sakamoto 2001;Miau et al. 1997). Investigations by Berger, Scholz, and Schumm (1990); Miau et al. (1997) indicate that vortex shedding occurs in pairs, where successive vortices are shed from diametrically opposite ends in an anti phased manner (refer Figure 1a). ...
... (Kiya, Ishikawa, Sakamoto 2001;Miau et al. 1997). Investigations by Berger, Scholz, and Schumm (1990); Miau et al. (1997) indicate that vortex shedding occurs in pairs, where successive vortices are shed from diametrically opposite ends in an anti phased manner (refer Figure 1a). However, the azimuthal location where this vortex shedding pair occurs is observed to be random in time. ...
We investigate the phenomenon of vortex-acoustic lock-in in a bluff body stabilized Rijke type combustor, which experiences self-excited thermoacoustic oscillations. The focus is to identify and understand the various regions (in airflow rate) occurring during the transition to lock-in. In this regard, an axisymmetric bluff body stabilized burner is used, which sheds Benard–Von Karman vortices. The dynamics of the combustor is monitored through the measurement of unsteady pressure and heat release rate fluctuations. At low airflow rates, peaks associated with both vortex shedding and acoustic modes are observed in the measured spectra. These peaks are far apart. As the airflow rate is increased, vortex shedding frequency increases. At a certain flow rate, it reaches the acoustic frequency. Beyond this flow rate, large amplitude oscillations occur and only one common peak is observed in the spectra. This common peak has the frequency close to the acoustic mode of the duct. Vortex shedding process locks in to this common frequency and the phenomenon is termed as vortex-acoustic lock-in. We observe a series of well-defined regions that appear as the system progresses toward lock-in. In this paper, we made an attempt to understand the characteristics of these regions in a systematic manner. The study will help in developing lower-order models, which capture the essential dynamics of lock-in observed in vortex shedding combustors.
... For comparison, Fig. 13b presents a photograph of dye visualization unveiling the shedding flow structures from the upper left leg, which was obtained in the water channel experiment. While such loop structures are commonly seen in the wake behind a three-dimensional bluff body, such as flow (Miau et al. 1997) or a sphere (Taneda 1978), the present ones in Fig. 13a appear to be much more complex. This is due to that multiple streamwise coherent vortices were originated from different parts of the contoured body. ...
Aerodynamic flow around an 1/5 scale cyclist model was studied experimentally and numerically. First, measurements of drag force were performed for the model in a low-speed wind tunnel at Reynolds numbers from \(5.5 \times 10^{4}\) to \(1.8 \times 10^{5}\). Meanwhile, numerical computation using a large eddy simulation method was performed at three Reynolds numbers of \(1.1 \times 10^{4}\), \(6.5 \times 10^{4}\) and \(1.5 \times 10^{5}\) to obtain the drag coefficients for comparison. Second, flow visualization was made in a water channel and the wind tunnel mentioned to examine the three-dimensional flow separation pattern on the model surface, which could also be realized from the numerical results. Finally, a wake flow survey based on the hot-wire measurements in the wind tunnel showed that in the near-wake region, the flow was featured with the formation of multiple streamwise vortices. The numerical results further indicated that these vortices were evolved from the separated flows occurred on the model surface.
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... The turbulent wakes of bluff bodies such as circular cylinders, 1-6 rectangular cylinders, 7-9 thin flat plates [10][11][12][13][14][15][16][17] and flat disks [18][19][20][21][22][23][24][25] have been studied to understand the vortex formation and interaction processes. However, there have been few investigations of the surface pressure behaviour during vortex shedding in these wakes. ...
The relationship between vortex dynamics and surface pressure fluctuations on the leeward face of a two-dimensional normal thin flat plate was studied using Direct Numerical Simulations for incompressible flow at a Reynolds number of 1200. The vortex shedding frequency was observed in the spectra of pressure fluctuations on both faces of the plate, while a lower frequency spectral peak was only evident on the leeward pressure fluctuations. Local sharp peaks of low pressure in the separated shear layers coincide with increases in the leeward face pressure fluctuations. These observations are tied to the vortex dynamics using the invariant Q, commonly used for vortex identification. Q is also proportional to the source term for the Poisson equation for the instantaneous pressure. The pressure, which is the only contributor to the plate drag, varies significantly in response to alterations to the vortex shedding, which can be observed in differences of vortex trajectories. At minimum drag, the pressure fluctuations are small, which is attributed to lower values of Q associated with weaker vortices.
... In addition tothe wake structures, the vortex shedding regime in the turbulent wake has also been studied by a few researchers. Fuchs et al (1979) found that vortex shedding occurred in random phase circumferentially behind an axisymmetric body, but with an anti-phase characteristic for vortex shedding at azimuthal locations180°apart. Miau et al (1997) investigated the individual vortex shedding process behind a circular disk at Reynolds numbers of 10 3 to 10 5 , and found that theanti-phase characteristic was largely preserved. However, even with the experimental findings above, the wake structures are still not clear and the vortex shedding regime needs to be further examined, since those experiments were performed mainly using flow visualization based on hot-wire measurements, which were limited to only a small region of the whole flow field. ...
In the present work, the wakes behind a circular disk at various transitional regimes are numerically explored using fully 3D proper orthogonal decomposition (POD). The Reynolds numbers considered in this study (Re = 152, 170, 300 and 3000) cover four transitional states, i.e. the reflectional-symmetry-breaking (RSB) mode, the standing wave (SW) mode, a weakly chaotic state, and a higher-Reynolds-number state. Through analysis of the spatial POD modes at different wake states, it is found that a planar-symmetric vortex shedding mode characterized by the first mode pair is persistent in all the states. When the wake develops into a weakly chaotic state, a new vortex shedding mode characterized by the second mode pair begins to appear and completely forms at the higher-Reynolds-number state of Re = 3000, i.e. planar-symmetry-breaking vortex shedding mode. On the other hand, the coherent structure at Re = 3000 extracted from the first two POD modes shows a good resemblance to the wake configuration in the SW mode, while the coherent structure reconstructed from the first four POD modes shows a good resemblance to the wake configuration in the RSB mode. The present results indicate that the dynamics or flow instabilities observed at transitional RSB and SW modes are still preserved in a higher-Reynolds-number regime.
... The shedding was very clear for the solid disc and the 85% case. Miau et al. [14] measured the shedding frequency behind discs at Reynolds numbers between 10 3 and 10 5 . The Strouhal number was found to be around 0.14 and slightly increasing with the Reynolds number. ...
... The nature of this kind of flow is such that an oscillatory coherent flow structure tends to develop from a relatively steady boundary condition, even in a symmetric geometry. Also present is the well-known phenomenon of helical vortex shedding behind bluff-bodies occurring at distinct Strouhal number, which has been extensively investigated both experimentally [8,9] and numerically [10]. The challenge in the prediction or such flows arise at higher Reynolds number when the flow starts transitioning to turbulence through multiple bifurcations and eventually various instabilities set in [11,12]. ...
This work is concerned with the investigation of fluid-mechanical behaviour and the performance of different subgrid-scale models for LES in the numerical prediction of a confined axisymmetrical bluff-body flow. Four subgrid-scale turbulence models comprising the Smagorinsky model, Dynamic Smagorinsky model, WALE model and subgrid turbulent kinetic energy model, are validated and compared directly against the experimental data. Two different mesh counts are used for the LES studies, one with a higher mesh resolution in the shear layer than the other. It is found that increasing the mesh resolution improves the time-averaged fluctuating velocity profiles, but has less effect on the time-averaged filtered velocity profiles. A comparison against experiment shows that the recirculation zone length is well predicted using LES. The accuracy of the four different subgrid scale models is then assessed by comparing the LES results using the dense mesh with the experiment. Comparisons with the time-averaged axial and radial velocity profiles demonstrate that LES displays good agreement with the experimental data, with the essential flow features captured both qualitative and quantitatively. The subgrid velocity also matches well with the experimental results, but a slight underprediction of the inner shear layer is observed for all subgrid models. In general, it is found that the Smagorinsky and WALE models are more dissipative than the Dynamic Smagorinsky model and subgrid TKE model. Comparison of the spectra against the experiment shows that LES can capture dominant features of the turbulent flow with reasonable accuracy, and weak spectral peaks related to the Kevin-Helmholtz instability and helical vortex shedding are present.
... The Proper Orthogonal Decomposition (POD) technique has been extensively utilized in studies pertaining to non-reacting flow over bluff bodies in order to gain insight into the underlying physics, see for example, [29][30][31][32][33][34][35][36]. However, to the best knowledge of the authors, application of this technique for analysis of the flame front corrugations is yet be investigated. ...
... The POD technique has been utilized for analysis of the results presented in this section as well as those in section 3.3. This technique has been applied mainly for analysis of non-reacting flow, see, for example, [29][30][31][32][33][34][35][36]. This technique can be used for analysis of the processes that are statistically stationary, see for example the results presented in [54,55]. ...
Periodicity in evolution of premixed methane–air V-shaped flames in the space domain is investigated experimentally. The experiments were performed using the Mie scattering and Particle Image Velocimetry techniques. Three Reynolds numbers of 510, 790, and 1057 along with two fuel–air equivalence ratios of 0.6 and 0.7 were tested in the experiments. The analyses were performed using the Proper Orthogonal Decomposition (POD) technique for the flame front position as well as the velocity data pertaining to non-reacting flow condition. The POD analysis shows that the spectral characteristics of the mode shapes associated with the velocity and the flame front position data feature similarities; however, the corresponding temporal coefficients are significantly different. Specifically, the POD mode shapes pertaining to both velocity and flame front position data feature dominant instabilities. It was shown that the normalized wave number pertaining to these instabilities are similar and equal to the Strouhal number corresponding to non-reacting flow over a circular cylinder. Comparison of the normalized temporal coefficients show that, for the flame front position data, the normalized first and second coefficients are mainly centered close to the origin; however, those associated with the velocity data are positioned around a unity radius circle. This was argued to be linked to the ratio of the corresponding first and second eigenvalues. Specifically, it was shown that, as this ratio approached to unity, the signal energy becomes distributed between the first and the second POD modes. As a result, the normalized temporal coefficients follow a circular pattern.
