Bluff Body Aerodynamics

A combined experimental, computational and theoretical study is presented on the dynamics and stability characteristics of turbulent flow past a circular cylinder placed near and parallel to a moving ground. The study consists of four main parts: (i) wind tunnel experiment, (ii) numerical simulation, (iii) linear stability analysis, and (iv) proper orthogonal decomposition (POD) analysis. The main focus of the study is on the cessation of large-scale, von Karman-type vortex shedding in 'ground effect,' i.e., the cessation observed when the cylinder comes close to the ground. The experiments, performed at upper-subcritical Reynolds numbers of 0.4 and 1.0x10^5, show that the cessation of von Karman-type vortex shedding and an attendant critical drag reduction of the cylinder (equipped with end-plates) occurs at the gap-to-diameter ratio h/d of around 0.35, at which point the flow through the gap between the cylinder and the ground is still not blocked at all due to the ground moving at the same speed as the free stream. It is subsequently shown that detached-eddy simulations (DES) can correctly reproduce these critical phenomena, whereas unsteady RANS simulations predict them at much smaller h/d of between 0.1 and 0.2, despite the fact that the unsteady RANS simulations are 'overly dissipative' compared with the DES. The linear stability analysis of analytical wake profiles then provides a possible explanation for the above experimental and computational results; that is, the cessation of the von Karman-type vortex shedding in ground effect may also be largely explained by the change of inviscid instability characteristics in the near wake region from 'absolutely unstable' to 'convectively unstable,' in analogy with the case for a cylinder equipped with a backward splitter plate in a free stream. Finally, the near wake structure of the cylinder in ground effect is further investigated with the POD analysis. The results show that about 60% of the total kinetic energy in the near wake region (in the time-averaged sense) is contained only in the first three POD modes even when the energetically dominant, von Karman-type vortex shedding becomes intermittent at h/d = 0.4. It is also shown that both shedding and non-shedding states at this gap ratio can roughly be reproduced from the combination of these three POD modes.

Flow separation over a surface-mounted obstacle is prevalent in numerous applications. Previous studies of 3D sepa- ration around protuberances have been limited to steady  ow. In biological and geophysical  ows, pulsatile conditions are fre- quently encountered, yet this situation has not been extensively studied. Primarily motivated by our previous studies of the  ow patterns observed in various human vocal fold pathologies such as polyps, our research aimed to  ll this gap in the knowledge concerning unsteady 3D  ow separation. This is achieved by characterizing velocity  elds surrounding the obstacle, focused primarily on the vortical  ow structures and dynamics that occur around a hemispheroid in pulsatile  ow. As part of this study, two- dimensional, instantaneous and phase-averaged particle image velocimetry data in both steady and pulsatile  ows are presented and compared. Coherent vortical  ow structures have been identi-  ed by their swirling strength. This analysis revealed  ow struc- tures with dynamics dependent on the pulsatile forcing function. A mechanism to explain the formation and observed dynamics of these  ow structures based on the self-induced velocity of vortex rings interacting with the unsteady  ow is proposed.

A simple way to decrease the drag and oscillating lift forces in the flow around a circular cylinder consists of positioning a splitter plate in the wake, parallel to the flow. In this paper, the effect of the splitter plate on the wake dynamics, more specifically on the wake transition, is described in detail. First, two-dimensional and three-dimensional direct numerical simulations (DNS) using the spectral element method were used to observe the behaviour of the wake in the presence of the splitter plate. Then, a linear stability analysis based on the Floquet theory was performed in order to obtain information on how the splitter plate changes the instabilities that lead to wake transition. Simulations were carried out for several gaps between the splitter plate and the cylinder, with the Reynolds number varying in the range between 100 and 350, which corresponds to the wake transition in the flow around a circular cylinder. The results of the simulations showed a discontinuity in the Strouhal number curve that is consistent with the results available in the literature. The stability analysis showed how the splitter plate modifies the transition of the flow to a three-dimensional configuration. The splitter plate has a stabilizing effect on the flow for small gaps, delaying the appearance of three-dimensional structures to higher Reynolds numbers. Mode A and a quasi-periodic (QP) mode are observed for such small gaps. As the gap is increased the discontinuity in the Strouhal number curve also caused a clear change in the characteristics of the neutral stability curve, and the existence of an unstable period-doubling mode was observed. The onset characteristics of the unstable modes are analysed and discussed in depth.

Bluff bodies may assume arbitrary attitudes in a flow, causing aerodynamic loads that are sensitive to attitude. The Continuous Rotation technique obtains 6-component loads on bluff bodies with 1-degree azimuth resolution about selected axes at a rate of 1 revolution per minute. The load coefficient variation is Fourier transformed and the resulting complex series is truncated in order to obtain rapidly computable, analytical formulae. The method is applied to bluff body shapes including cylinders, a cuboid, a flat plate and a porous box. A cylinder whose length is equal to its diameter, is used to show that rate effects, hysteresis, vortex shedding and other unsteady aerodynamic phenomena are negligible below 10 revolutions per minute. Approaches to generalize the aerodynamic loads on yawed finite cylinders of various aspect ratio are studied. The reasons for differences in aerodynamic load behavior between 2 cylinder models, are analyzed. To complement experiments, the ROTCFD unsteady Navier Stokes code is used to perform diagnostic computations. Methods to generalize the predictions are explored. Maps of the leading coefficients of the Fourier series of each load component over the aspect ratios space, are interpolated. The interpolation varies sharply for aspect ratios between 0.5 to 1. The variation is more gradual beyond aspect ratio 2. By aspect ratio 4, a 'high aspect ratio' limit appears to be reached.

The turbulent flow past a wall-mounted square cylinder with an aspect ratio of 4 was investigated with the aid of Spalart-Allmaras improved delayed detached-eddy simulation and proper orthogonal decomposition (POD) analysis. The Reynolds number was equal to 12 000 (based on the free-stream velocity and obstacle width). The boundary layer thickness was ∼0.18 of the obstacle height. This study focused on analyzing the vortical structure of the wake and vortex shedding process along the obstacle height. A quantitative comparison of the first-and second-order flow statistics with the available experimental and direct numerical simulation data was used to validate the numerical results. The numerical model coupled with the vortex method of generating the turbulent inflow conditions could successfully reproduce the flow field around and behind the obstacle with commendable accuracy. The flow structure and vortex shedding characteristics near the wake formation region were discussed in detail using time-averaged and instantaneous flow parameters obtained from the simulation. Dipole type mean streamwise vortices and half-loop hairpin instantaneous vortices with energetic motions were identified. A coherent shedding structure was reported along the obstacle using two-point correlations. Two types of vortex shedding intervals were identified, namely, low amplitude fluctuations (LAFs) and high amplitude fluctuations (HAFs) [P. Sattari et al., Exp. Fluids 52, 1149-1167 (2012)]. The HAFs' interval exhibits von Kármán-like behavior with a phase difference of ∼180 ○ , while the LAFs' interval shows less periodic behavior. It was observed that the effect of the LAFs' interval tends to weaken the alternating shedding along the obstacle height. The POD analysis of the wake showed that for the elevations between 0.25 and 0.5 of the obstacle height, the first two POD modes represent the alternating shedding and contribute to 66.6%-57.6% of the total turbulent kinetic energy. However, at the free end of the obstacle, the first two modes have a symmetrical shedding nature and share 36.5% of the kinetic energy, while the rest of the energy is distributed between the alternating and the random shedding processes. A simple low-order model based on the vortex-shedding phase angle and the spectrum of the time coefficients obtained from POD was developed to predict the wake dynamics at the range of elevations where the alternating shedding is dominated.

Large-eddy simulations (LES) of the flow past a circular cylinder are used to investigate the flow topology and the vortex shedding process at Reynolds numbers Re = 2.5×10 5 −8.5×10 5. This range encompasses both the critical and super-critical regimes. As the flow enters the critical regime, major changes occur which affect the flow configuration. Asymmetries in the flow are found in the critical regime, whereas the wake recovers its symmetry and stabilises in the super-critical regime. Wake characteristic lengths are measured and compared between the different Reynolds numbers. It is shown that the super-critical regime is characterised by a plateau in the drag coefficient at about C D ≈ 0.22, and a quasi-stable wake which has a non-dimensional width of d w /D ≈ 0.4. The periodic nature of the flow is analysed by means of measurements of the unsteady drag and lift coefficients. Power spectra of the lift fluctuations are computed. Wake vortex shedding is found to occur for both regimes investigated, although a jump in frequencies is observed when the flow enters the super-critical regime. In this regime, non-dimensional vortex-shedding frequency is almost constant and equal to St = f vs D/U ref ≈ 0.44. The analysis also shows a steep decrease in the fluctuating lift when entering the super-critical regime. The combined analysis of both wake topology and vortex shedding complements the physical picture of a stable and highly coherent flow in the super-critical regime.

An experimental study was performed on a 2D square prism with one rounded front edge with the purpose of reducing the aerodynamic drag by means of active flow control of boundary layer separation. Highly efficient suction and oscillatory blowing (SaOB) actuators were installed at the front curved edge, aimed to inhibit the separation phenomenon. The experimental results show significant decrease in the form-drag, narrowing of the wake and correspondingly an increase in the base pressure in the presence of the SaOB actuation. The mechanism of drag reduction is elimination of the separation from the front-cured edge of the model.

Comparison of the experimental results of turbulent flow structures between a smooth sphere and a sphere with a vent hole, roughened, and o-ring is presented in the presence of a free-surface. Dye visualization and particle image velocimetry (PIV) techniques were performed to examine effects of passive control methods on the sphere wake for Reynolds number Re = 5000 based on the sphere diameter with a 42.5mm in an open water channel. Instantaneous and time-averaged flow patterns in the wake region of the sphere were examined from point of flow physics for the different sphere locations in the range of 0≤h/D ≤2.0 where h was the space between the top point of the sphere and the free surface. The ratio of ventilation hole to sphere diameter was 0.15, o-ring was located at 55 o with a 2 mm from front stagnation point of the sphere and roughened surface was formed by means of totally 410 circular holes with a 3 mm diameter and around 2 mm depth in an equilateral triangle arrangement. The flow characteristics of instantaneous velocity vectors, vorticity contours, time-averaged streamline patterns, Reynolds stress correlations and streamwise and cross-stream velocity fluctuations for both the smooth and passively controlled sphere were interpreted.

Drag reduction is an interesting and important practical problem with wide range of engineering applications. D-Shape is a representative model for various bluff bodies and its drag influences the performance of the particular body. Drag reduction on D-Shape model with various roughness and forebody effects is the subject of this research. A standard D-Shape model with and without Roughness is compared with modified forebody D-Shape configuration for different dimensions. The study has been done in computational fluid dynamics by modeling standard D-Shape model with and without roughness and modified forebody D-Shape configuration. The models are analyzed in CFD using k – ε turbulence and LES model. Drag coefficient, Pressure coefficient around the model are the parameters considered. Results show that drag has been reduced more than 10% by infusing the roughness or by modifying the forebody. The main reason behind this is the flow field gets energized by the influence of surface roughness and the flow separation has been delayed due to the modified forebody. This leads to the drag reduction and it is evident from the pressure distribution over the investigated models.

The aerodynamic response of a circular cylinder to nonharmonic forcing of the inflow velocity is studied by numerically solving the equations of two-dimensional fluid motion on an orthogonal curvilinear mesh. The effect of varying the inflow velocity waveform while maintaining other forcing parameters constant at a Reynolds number of 180 is considered in this study. The forcing frequency is 84% of the natural vortex shedding frequency in the unforced wake while the peak-to-peak amplitude of velocity oscillation is 65% of the reference velocity. Results are reported for the drag and lift coefficients and the flow field in terms of streamline patterns and vorticity distributions. It is shown that the wake is locked-on to the forcing frequency for all cases tested but the aerodynamic response is systematically modified by the imposed changes in the velocity waveform. The magnitude and the phase of the fluctuating drag and lift forces and the mean drag force are affected. These effects are associated with changes in the mechanism of vortex formation and shedding in the wake of the cylinder; it is found that the rolling up of the individual shear layers on both sides can be manipulated to promote shedding of single vortices or vortex pairs.