Showing papers on "Vortex shedding published in 1998"
TL;DR: In this paper, an incompressible SIMPLEC code is used with a non-staggered grid arrangement for the convective terms and the time discretization is implicit and a second-order Crank-Nicolson scheme is employed.
Abstract: Calculations of unsteady 2D flow around a square cylinder at incidence (α=0°−45°) are presented. The Reynolds numbers are low (Re=45–200) so that the flow is presumably laminar. A von Karman vortex sheet is predicted behind the cylinders with a periodicity which agrees well with experiments. An incompressible SIMPLEC code is used with a non-staggered grid arrangement. A third-order QUICK scheme is used for the convective terms. The time discretization is implicit and a second-order Crank–Nicolson scheme is employed. At the outlet of the computational domain a convective Sommerfeld boundary condition is compared with a traditional Neumann condition. The convective boundary condition is shown to be more effective in reducing the CPU time, reducing the upstream influence of the outlet and thus reducing the necessary downstream extent of the domain. A study of the effects of spatial resolution and blockage is also provided. The onset of vortex shedding is investigated by using the Stuart–Landau equation at various angles of incidence and for a solid blockage of 5%. A number of quantities such as Strouhal number and drag, lift and moment coefficients are calculated. © 1998 John Wiley & Sons, Ltd.
540 citations
TL;DR: In this article, the detailed information of flow quantities on the cylinder surface at low Reynolds numbers was reported and compared with those from the existing experimental and numerical studies, and a high resolution simulation of flow past a circular cylinder at Reynolds numbers up to 160 was performed.
Abstract: Flow past a circular cylinder at Reynolds numbers up to 160 is simulated using high resolution calculations. Flow quantities at the cylinder surface are obtained and compared with those from the existing experimental and numerical studies. The present study reports the detailed information of flow quantities on the cylinder surface at low Reynolds numbers.
365 citations
TL;DR: In this article, the authors performed time-averaged LDA measurements and time-resolved numerical flow predictions to investigate the laminar flow induced by the harmonic in-line oscillation of a circular cylinder in water at rest.
Abstract: Time-averaged LDA measurements and time-resolved numerical flow predictions were performed to investigate the laminar flow induced by the harmonic in-line oscillation of a circular cylinder in water at rest. The key parameters, Reynolds number Re and Keulegan–Carpenter number KC, were varied to study three parameter combinations in detail. Good agreement was observed for Re=100 and KC=5 between measurements and predictions comparing phase-averaged velocity vectors. For Re=200 and KC=10 weakly stable and non-periodic flow patterns occurred, which made repeatable time-averaged measurements impossible. Nevertheless, the experimentally visualized vortex dynamics was reproduced by the two-dimensional computations. For the third combination, Re=210 and KC=6, which refers to a totally different flow regime, the computations again resulted in the correct fluid behaviour. Applying the widely used model of Morison et al. (1950) to the computed in-line force history, the drag and the added-mass coefficients were calculated and compared for different grid levels and time steps. Using these to reproduce the force functions revealed deviations from those originally computed as already noted in previous studies. They were found to be much higher than the deviations for the coarsest computational grid or the largest time step. The comparison of several in-line force coefficients with results obtained experimentally by Kuhtz (1996) for β=35 confirmed that force predictions could also be reliably obtained by the computations.
340 citations
TL;DR: In this article, the authors used a Reynolds-averaged two-dimensional computation of a turbulent flow over an airfoil at post-stall angles of attack, and showed that the massively separated and disordered unsteady flow can be effectively controlled by periodic blowing-suction near the leading edge with low-level power input.
Abstract: By using a Reynolds-averaged two-dimensional computation of a turbulent flow over an airfoil at post-stall angles of attack, we show that the massively separated and disordered unsteady flow can be effectively controlled by periodic blowing–suction near the leading edge with low-level power input. This unsteady forcing can modulate the evolution of the separated shear layer to promote the formation of concentrated lifting vortices, which in turn interact with trailing-edge vortices in a favourable manner and thereby alter the global deep-stall flow field. In a certain range of post-stall angles of attack and forcing frequencies, the unforced random separated flow can become periodic or quasi-periodic, associated with a significant lift enhancement. This opens a promising possibility for flight beyond the static stall to a much higher angle of attack. The same local control also leads, in some situations, to a reduction of drag. On a part of the airfoil the pressure fluctuation is suppressed as well, which would be beneficial for high-α buffet control. The computations are in qualitative agreement with several recent post-stall flow control experiments. The physical mechanisms responsible for post-stall flow control, as observed from the numerical data, are explored in terms of nonlinear mode competition and resonance, as well as vortex dynamics. The leading-edge shear layer and vortex shedding from the trailing edge are two basic constituents of unsteady post-stall flow and its control. Since the former has a rich spectrum of response to various disturbances, in a quite wide range the natural frequency of both constituents can shift and lock-in to the forcing frequency or its harmonics. Thus, most of the separated flow becomes resonant, associated with much more organized flow patterns. During this nonlinear process the coalescence of small vortices from the disturbed leading-edge shear layer is enhanced, causing a stronger entrainment and hence a stronger lifting vortex. Meanwhile, the unfavourable trailing-edge vortex is pushed downstream. The wake pattern also has a corresponding change: the shed vortices of alternate signs tend to be aligned, forming a train of close vortex couples with stronger downwash, rather than a Karman street.
327 citations
TL;DR: In this paper, a new law is proposed for the vortex shedding from a circular cylinder, which describes in a consistent way the Strouhal-Reynolds-number dependency as Sr=Sr*+m/Re from the beginning of vortex shedding at Re=47 up to the laminar-turbulent transition of the cylinder boundary layer at Re>2×105.
Abstract: Based on experiments a new law is proposed for the vortex shedding from a circular cylinder which describes in a consistent way the Strouhal–Reynolds-number dependency as Sr=Sr*+m/Re from the beginning of the vortex shedding at Re=47 up to the laminar–turbulent transition of the cylinder boundary layer at Re>2×105. The various vortex shedding processes, occurring with increasing Reynolds number, are described by different coefficients Sr* and m.
308 citations
TL;DR: In this paper, the authors focus on the underlying flow physics, with the aim of clarifying the origin of the induced loading of vortex interactions and developing creative strategies, distinct from those traditionally employed for control of unstable shear flows.
Abstract: ▪ Abstract Interaction of a vortex, or combinations of them, with a cylinder, blade, or foil may involve both rapid distortion of the incident vorticity field and shedding of vorticity from the surface of the body. This review focuses on the underlying flow physics, with the aim of clarifying the origin of the induced loading. In the case of near or direct encounter of the incident vortex, the relation between three-dimensional features of the flow structure and the local loading poses interesting challenges for further research. With recently developed simulation and laboratory techniques, opportunities now exist to determine the instantaneous quantitative structure of these complex distortions and to interpret them within a theoretical, vorticity-based framework. Effective control of vortex interactions appears to be attainable. It will be necessary, however, to develop creative strategies, distinct from those traditionally employed for control of unstable shear flows.
264 citations
TL;DR: In this article, a Strouhal number that depends upon an effective wake width, which includes not only the physical body diameter, but also a characteristic width of the separating shear layers, was used for the cylinder wake.
222 citations
TL;DR: In this article, a summary of results of numerical investigations of the two-dimensional flow around a heated circular cylinder located in a laminar crossflow is presented, which yields information on Nu and C D variation with Reynolds number.
207 citations
TL;DR: In this article, the influence of spanwise waviness of separation lines on the flow around common bluff forms is investigated in wind-tunnel measurements on thin plates normal to the flow and on rectangular cross-section bodies at Reynolds numbers of about 40 000.
192 citations
TL;DR: In this article, the results of numerical simulations of vortex shedding past a free-standing square cylinder at ReD22000, obtained with different turbulence models, were compared with a modification suggested by Kato and Launder (Proc. 9th Symp. 10-4-1 (1993)).
Abstract: SUMMARY This paper presents the results of numerical simulations of vortex shedding past a free-standing square cylinder at ReD22000, obtained with different turbulence models. Using wall functions, the standard k‐o model is compared with a modification suggested by Kato and Launder (Proc. 9th Symp. Turbulent Shear Flows, Kyoto, 10-4-1 (1993)). In addition, both versions are used in a two-layer approach, in which the flow close to the cylinder is computed with a locally more suitable one-equation turbulence model and only outside the viscous near-wall layer with the two mentioned high-Re model versions. To allow a comparison, the simulations are performed first using the same computational domain and boundary conditions as in previous investigations. Then results are presented that were obtained on a computational domain and with boundary conditions more suitable for a comparison with the experiments. © 1998 John Wiley & Sons, Ltd.
187 citations
TL;DR: A multiple-sensor control strategy for the cylinder wake which succeeds in controlling a simplified wake model at a Reynolds number above that at which single-s sensor schemes fail is described.
Abstract: Many wake flows exhibit self-excited flow oscillations which are sustained by the flow itself and are not caused by amplification of external noise. The archetypal example of a self-excited wake flow is the low Reynolds number flow past a circular cylinder. This flow exhibits self-sustained periodic vortex shedding above a critical Reynolds number. In general, control of such flows requires stabilization of many globally unstable modes; the present work describes a multiple-sensor control strategy for the cylinder wake which succeeds in controlling a simplified wake model at a Reynolds number above that at which single-sensor schemes fail.Representation of the flow field by a finite set of coherent structures or modes, which are extracted by proper orthogonal decomposition and correspond to the large-scale wake components, allows the efficient design of a closed-loop control algorithm. A neural network is used to furnish an empirical prediction of the modal response of the wake to external control forcing. This model avoids the need for explicit representation of the control actuator–wake interaction. Additionally, the neural network structure of the model allows the design of a robust nonlinear control algorithm. Furthermore the controller does not necessarily require velocity field information, but can control the wake using other quantities (for example flow visualization pictures) which characterize the structure of the velocity field. Successful control of a simplified cylinder wake model is used to demonstrate the feasibility of the low-dimensional control strategy.
TL;DR: In this paper, measurements of velocity taken at a high spatial and temporal resolution in a turbulent flow upstream and downstream from a pebble cluster are presented. But the results are limited to a 5m long section.
TL;DR: In this paper, the axial and radial velocities in a meridional plane for non-wavy and wavy Taylor-Couette flow in the annulus between a rotating inner cylinder and a fixed outer cylinder with fixed end conditions were measured.
Abstract: The stability of supercritical Couette flow has been studied extensively, but few measurements of the velocity field of flow have been made. Particle image velocimetry (PIV) was used to measure the axial and radial velocities in a meridional plane for non-wavy and wavy Taylor–Couette flow in the annulus between a rotating inner cylinder and a fixed outer cylinder with fixed end conditions. The experimental results for the Taylor vortex flow indicate that as the inner cylinder Reynolds number increases, the vortices become stronger and the outflow between pairs of vortices becomes increasingly jet-like. Wavy vortex flow is characterized by azimuthally wavy deformation of the vortices both axially and radially. The axial motion of the vortex centres decreases monotonically with increasing Reynolds number, but the radial motion of the vortex centres has a maximum at a moderate Reynolds number above that required for transition. Significant transfer of fluid between neighbouring vortices occurs in a cyclic fashion at certain points along an azimuthal wave, so that while one vortex grows in size, the two adjacent vortices become smaller, and vice versa. At other points in the azimuthal wave, there is an azimuthally local net axial flow in which fluid winds around the vortices with a sense corresponding to the axial deformation of the wavy vortex tube. These measurements also confirm that the shift-and-reflect symmetry used in computational studies of wavy vortex flow is a valid approach.
TL;DR: In this article, the separation of flow past a zero-thickness flat plate held normal to a free stream at Re=250 has been investigated through numerical experiments, and it was observed that the flow gradually varies between two different regimes: a regime H of high mean drag and a regime L of low mean drag.
Abstract: The separated flow past a zero-thickness flat plate held normal to a free stream at Re=250 has been investigated through numerical experiments. The long-time signatures of the drag and lift coefficients clearly capture a low-frequency unsteadiness with a period of approximately 10 times the primary shedding period. The amplitude and frequency of drag and lift variations during the shedding process are strongly modulated by the low frequency. A physical interpretation of the low-frequency behaviour is that the flow gradually varies between two different regimes: a regime H of high mean drag and a regime L of low mean drag. It is observed that in regime H the shear layer rolls up closer to the plate to form coherent spanwise vortices, while in regime L the shear layer extends farther downstream and the rolled-up Karman vortices are less coherent. In the high-drag regime three-dimensionality is characterized by coherent Karman vortices and reasonably well-organized streamwise vortices connecting the Karman vortices. With a non-dimensional spanwise wavelength of about 1.2, the three-dimensionality in this regime is reminiscent of mode-B three-dimensionality. It is observed that the high degree of spanwise coherence that exists in regime H breaks down in regime L. Based on detailed numerical flow visualization we conjecture that the formation of streamwise and spanwise vortices is not in perfect synchronization and that the low-frequency unsteadiness is the result of this imbalance (or phase mismatch).
TL;DR: In this paper, the effects of corner cut, recession and roundness on aeroelastic instabilities such as vortex induced excitation and galloping oscillation were investigated by wind tunnel tests for square and rectangular prisms.
TL;DR: In this paper, it is shown that the wake transition regime for a circular cylinder involves two modes of small-scale three-dimensional instability, modes “A” and “B”, occurring in different Reynolds number ranges.
Abstract: It is now well-known that the wake transition regime for a circular cylinder involves two modes of small-scale three-dimensional instability, modes “A” and “B”, occurring in different Reynolds number ranges. These modes are quite distinct in spanwise lengthscale and in symmetry, and they are found to scale on different physical features of the flow. Mode A has a large spanwise wavelength of around 3–4 cylinder diameters, and scales on the larger physical structure in the flow, namely the core of the primary Karman vortices. The feedback from one vortex to the next gives an out-of-phase streamwise vortex pattern for this mode. In contrast, the mode B instability has a distinctly smaller spanwise wevelength (1 diameter) which scales on the smaller physical structure in the flow, namely the braid shear layer. The symmetry of mode B is determined by the reverse flow behind the bluff cylinder, leading to a system of streamwise vortices which are in phase between successive half cycles. The symmetries of both modes are the same as the ones found in the vortex system evolving from perturbed plane wakes studied by Meiburg and Lasheras (1988) and Lasheras and Meiburg (1990). Furthermore, the question of the physical origin of these three-dimensional instabilities is addressed. We present evidence that they are linked to general instability mechanisms found in two-dimensional linear flows. In particular, mode A seems to be a result of an elliptic instability of the near-wake vortex cores; predictions based on elliptic instability theory concerning the initial perturbation shape and the spanwise wevelength are in good agreement with experimental observations. For the mode B instability, it is suggested that it is a manifestation of a hyperbolic instability of the stagnation point flow found in the braid shear layer linking the primary vortices.
TL;DR: In this paper, a two-dimensional viscous incompressible flow past five generic bridge deck cross sections is investigated by means of the discrete vortex method, yielding root mean square lift coefficients and Strouhal numbers for fixed cross sections and aerodynamic derivatives for the cross sections undergoing forced oscillatory cross wind and twisting motion.
TL;DR: In this article, the Lighthill acoustic analogy combined with Reynolds-averaged Navier-Stokes flow computations are used to investigate the ability of existing technology to predict the tonal noise generated by vortex shedding from a circular cylinder for a range of Reynolds numbers (100 < Re < 5 million).
Abstract: The Lighthill acoustic analogy combined with Reynolds-averaged Navier–Stokes flow computations are used to investigate the ability of existing technology to predict the tonal noise generated by vortex shedding from a circular cylinder for a range of Reynolds numbers (100 < Re < 5 million). Computed mean drag, mean coefficient of pressure, Strouhal number, and fluctuating lift are compared with experiment. Two-dimensional calculations produce a Reynolds number trend similar to experiment but incorrectly predict many of the flow quantities. Different turbulence models give inconsistent results in the critical Reynolds number range (Re≈ 100000). The computed flow field is used as input for noise prediction. Two-dimensional inputs overpredict both noise amplitude and frequency; however, if an appropriate correlation length is used, predicted noise amplitudes agree with experiment. Noise levels and frequency content agree much better with experiment when three-dimensional flow computations are used as input data.
TL;DR: Moreno-Insertis and Emonet as discussed by the authors performed two-dimensional simulations of the buoyant rise of twisted horizontal magnetic flux tubes through an adiabatically stratified layer representing the solar convection zone or other marginally stable atmosphere.
Abstract: We perform two-dimensional simulations of the buoyant rise of twisted horizontal magnetic flux tubes through an adiabatically stratified layer representing the solar convection zone or other marginally stable atmosphere. The numerical calculations employ the anelastic approximation to the basic MHD equations. We confirm the results of recent compressible simulations by Moreno-Insertis & Emonet that the azimuthal component of the tube magnetic field can prevent the splitting of the tube into a vortex pair, and that most of the flux in the initial tube cross section rises in the form of a rigid body that reaches a terminal speed similar to the prediction of the often-employed thin-flux-tube model. We also study the interaction between a pair of buoyant flux tubes as they rise in proximity. In the case of two identical flux tubes that start from the same level, we find that the wake behind each tube interacts with the wake of the other, prompting mirror-symmetric vortex shedding in each wake. As a result, each tube gains around it a net circulation of the opposite sign of the most recently shed eddy; this causes a periodic, horizontal lift force that makes the tubes oscillate horizontally as they rise. The tube interactions in this case differ substantially from the inviscid limit studied previously. For two identical flux tubes that start at different levels, the resulting interactions depend upon the details of the initial configuration of the two tubes and can be very different from the interactions seen in the symmetrical case. In the asymmetric case, it becomes possible for one flux tube to be drawn into the wake of the other, leading eventually to a merger of the tubes.
TL;DR: The ability of the unsteady bleed technique to control the asymmetry of the steady tip vortices separating from a forebody model is demonstrated in this paper, where mean velocity profiles measured behind the model at α = 45 deg and Re = 6.3 x 10 3 clearly show the exponential spatial growth of the disturbance in the wake.
Abstract: The ability of the unsteady bleed technique to control the asymmetry of the steady tip vortices separating from a forebody model is demonstrated. Mean velocity profiles measured behind the forebody model at α = 45 deg and Re = 6.3 x 10 3 clearly show the exponential spatial growth of the disturbance in the wake. This exponential growth is consistent with a spatial type of flow instability. The type of spatial instability governing the flow determines the behavior of the vortex system. The continuous variation of vortex position with control input found at α = 45 deg is consistent with a convective type of instability and allows proportional control of the forebody vortices with very low forcing amplitudes and input power levels. The forebody wake shows characteristics similar to a global type of instability for the bistable behavior found at α=55 deg. For the global type of flow instability, the vortex system is locked into one of two stable configurations, and proportional control does not seem feasible under these conditions
TL;DR: In this paper, the time-averaged kinematical and dynamical characteristics of the junction vortex system in front of a symmetrical obstacle are systematically analyzed for both laminar and turbulent flows.
Abstract: The time-averaged kinematical and dynamical characteristics of the junction vortex system in front of a symmetrical obstacle are systematically analyzed for both laminar and turbulent flows. A wide set of experimental and numerical results from the literature is coordinated in nondimensional form together with some new computational data. In turbulent flows the dimensions of the vortex system in the symmetry plane depend only on the obstacle geometry; in laminar systems they are also correlated with the Reynolds number and the thickness of the incoming boundary layer. The horseshoe vortices induce shear stresses on the bottom several times higher than those of the undisturbed boundary layer
TL;DR: In this paper, a supercritical turbulent boundary layer flow over a hemisphere with a rough surface has been simulated using Large Eddy Simulation (LES) and analyzed using the Karhunen-Loeve expansion (Proper Orthogonal Decomposition, POD).
Abstract: Supercritical turbulent boundary layer flow over a hemisphere with a rough surface (Re= 150000) has been simulated using Large Eddy Simulation (LES) and analyzed using the Karhunen--Loeve expansion (“Proper Orthogonal Decomposition,” POD). The time-dependent inflow condition is provided from a separate LES of a boundary layer developing behind a barrier fence and a set of vorticity generators. LES results using significantly different grid resolutions are compared with a corresponding wind tunnel experiment to demonstrate the reliability of the simulation. The separation processes are analyzed by inspecting second-order moments, time spectra, and instantaneous velocity distributions. Applying POD, a detailed study of the spatiotemporal structure of the separation processes has been carried out. From this analysis it can be concluded that the major event in the separated flow behind the obstacle is the shedding of “von Karman”-type vortices, which can be represented by the first three energetically dominant modes.
TL;DR: In this paper, three normal triangle tube arrays have been tested to investigate the phenomenon of vortex shedding, its relation to the occurrence of acoustic resonance and the dynamic fluid forces exerted on the tubes.
TL;DR: In this article, a numerical pressure correction method is extended through the use of deformable hybrid grids for solution of the incompressible Navier?Stokes equations, loosely coupled with an elastic body structural response to obtain unsteady flow structure solutions.
TL;DR: In this paper, it is shown that within the lock-in region and above a threshold amplitude, no cells form and, instead, a single frequency of response dominates the entire span.
Abstract: Visualization studies of the flow behind an oscillating tapered cylinder are performed at Reynolds numbers from 400 to 1500. The cylinder has taper ratio 40[ratio ]1 and is moving at constant forward speed U while being forced to oscillate harmonically in the transverse direction. It is shown that within the lock-in region and above a threshold amplitude, no cells form and, instead, a single frequency of response dominates the entire span. Within certain frequency ranges a single mode dominates in the wake, consisting of shedding along the entire span of either two vortices per cycle (‘2S’ mode), or four vortices per cycle (‘2P’ mode); but within specific parametric ranges a hybrid mode is observed, consisting of a ‘2S’ pattern along the part of the span with the larger diameter and a ‘2P’ pattern along the part of the span with the smaller diameter. A distinct vortex split connects the two patterns which are phase-locked and have the same frequency. The hybrid mode is periodic, unlike vortex dislocations, and the location of the vortex split remains stable and repeatable, within one to two diameters, depending on the amplitude and frequency of oscillation and the Reynolds number.
01 Jan 1998
TL;DR: In this paper, an experimental study on the suppressing of vortex shedding from a circular cylinder by feedback sound was performed in a wind tunnel, and the feedback sound is generated inside the cylinder and locally introduced into the flow through a thin slit on the cylinder surface.
Abstract: This paper presents an experimental study on the suppressing of vortex shedding from a circular cylinder by feedback sound. Experiments were performed in a wind tunnel, and the feedback sound was generated inside the cylinder and locally introduced into the flow through a thin slit on the cylinder surface. In this way, the shear flow nearest to the slit was directly manipulated during the control. The results show that the suppression of vortex shedding can be achieved at Reynolds numbers ranging from 4×103 to 1.3× 104, according to signals from a hot-wire checking throughout the wake and signals from a remote microphone. This local and one-sided feedback, being different from other control techniques, allows a better understanding of the control mechanism, which in this case probably causes a destructive interaction between two shear flows separated from both sides of the cylinder. The technique has been useful to deepen our understanding to the wake instabilities behind the cylinder.
TL;DR: In this paper, the flow field and temperature distribution around a heated/cooled circular cylinder placed in an insulated vertical channel are determined using a novel finite volume algorithm that combines some important features of the finite element methods with the finite difference based techniques.
Abstract: The flow field and the temperature distribution around a heated / cooled circular cylinder placed in an insulated vertical channel are determined using a novel finite volume algorithm that combines some important features of the finite element methods with the finite difference based techniques. Elementwise interpolation ( isoparametric) and transformation of nonorthogonal element geometry into a square computational element have been made use of while solving for the integral conservation equations. An unsteady mixed convection situation has been considered for the study, and the effect of buoyancy on vortex shedding in the wake of a cylinder has been observed for a fixed Reynolds number of 100. Vortex shedding is found to stop completely at a critical Richardson number of 0.15. Below this critical value of the Richardson number, shedding of vortices into the stream is quite prominent. Heat transfer characteristics of the heated / cooled cylinder are studied as a function of Richardson number. The result...
TL;DR: In this paper, the authors used a laser Doppler anemometer to characterize the flow field over a delta wing, with an 85° sweep angle, at 12.5° incidence.
Abstract: Experiments have been undertaken to characterize the flow field over a delta wing, with an 85° sweep angle, at 12.5° incidence. Application of a laser Doppler anemometer has enabled detailed three-dimensional velocity data to be obtained within the free shear layer, revealing a system of steady co-rotating vortical structures. These sub-vortex structures are associated with low-momentum flow pockets in the separated vortex flow. The structures are found to be dependent on local Reynolds number, and undergo transition to turbulence. The structural features disappear as the sub-vortices are wrapped into the main vortex core. A local three-dimensional Kelvin–Helmholtz-type instability is suggested for the formation of these vortical structures in the free shear layer. This instability has parallels with the cross-flow instability that occurs in three-dimensional boundary layers. Velocity data at high Reynolds numbers have shown that the sub-vortical structures continue to form, consistent with flow visualization results over fighter aircraft at flight Reynolds numbers.
TL;DR: In this paper, a numerical investigation performed to understand better the entrainment and residence-time characteristics of cavity flows for different cavity and spindle sizes was performed, and it was found from the nonreacting flow simulations that the drag coefficient decreases with cavity length and that an optimum size exists for achieving a minimum value.
Abstract: Spatially locked vortices in the cavities of combustor aid in stabilizing the flames. On the other hand, these stationary vortices also restrict the entrainment of the main air into the cavity. For obtaining good performance characteristics in a trapped-vortex combustor, a sufficient amount of fuel and air must be injected directly into the cavity. This paper describes a numerical investigation performed to understand better the entrainment and residence-time characteristics of cavity flows for different cavity and spindle sizes. A third-order-accurate time-dependent Computational Fluid Dynamics with Chemistry (CFDC) code was used for simulating the dynamic flows associated with forebody-spindle-disk geometry. It was found from the nonreacting flow simulations that the drag coefficient decreases with cavity length and that an optimum size exists for achieving a minimum value. These observations support the earlier experimental findings of Little and Whipkey (1979). At the optimum disk location, the vortices inside the cavity and behind the disk are spatially locked. It was also found that for cavity sizes slightly larger than the optimum, even though the vortices are spatially locked, the drag coefficient increases significantly. Entrainment of the main flow was observed to be greater into the smaller-than-optimum cavities. The reacting-flow calculations indicate that the dynamic vortices developed inside the cavity with the injection offuel and air do not shed, even though the cavity size was determined based on cold-flow conditions.
TL;DR: In this article, the formation and downstream transport of the Strouhal vortices in the near wake of a circular cylinder are investigated using the large-eddy simulation (LES) method.
Abstract: The formation and the downstream transport of the Strouhal vortices in the near wake of a circular cylinder are investigated using the large-eddy simulation (LES) method. The governing equations are formulated in curvilinear coordinates to accommodate a nonorthogonal grid with formal development of a dynamic model to account for the subgrid turbulent scales. Results were produced with and without use of the model. The focus of the investigation is at a subcritical Reynolds number of 5600. Using the dynamic model, the LES results compared best to the published experimental data in terms of both the global and local wake characteristics such as the drag and base pressure coefficients, shedding and detection frequencies, peak vorticity, and the downstream mean velocity-defect and Reynolds stresses. The results further showed streamwise filaments that connect subsequent Strouhal vortices. Qualitatively, the time-averaged Reynolds stresses of the formation region revealed similar symmetric characteristics over the range 525 ≤ Re ≤ 140,000