Length Of Recirculation Bubble Research Articles

Effect of Synthetic Jet on Flow Field over Different Shaped Ribs and Its Impact on Heat Transfer

The present work reports the manipulation of the flow field over different shapes of surface-mounted obstacles using a synthetic jet. A round synthetic jet is placed upstream and downstream of surface-mounted ribs on a flat surface. Particle image velocimetry and hotwire anemometry are used to study the flow field characteristics and features. Heat transfer rate was obtained using transient liquid crystal thermography technique. Experiments are carried out at Reynolds number of 32,000 in a low-speed wind tunnel. Flow field features behind ribs are analyzed using time-averaged velocity, streamline, vorticity Reynolds stress and turbulent intensity. The spanwise average Nusselt number distribution for the downstream rib location is compared for two optimum locations of the synthetic jet relative to the rib. The recirculation bubble length formed downstream of a rib varies with the frequency of the synthetic jet, its location, and the shape of the rib. A drastic reduction in reattachment length is observed at 50 and 80 Hz frequencies for square rib. The highest heat transfer rate is obtained for the square rib at 50 Hz and 4 voltage peak-to-peak excitation amplitude for the upstream location of the synthetic jet relative to the rib.

Supersonic flow unsteadiness induced by control surface deflections

Control surface deployment in a supersonic flow has many applications, including flow control, mixing, and body-force regulation. The extent of control surface deflections introduces varying flow unsteadiness. The resulting fluid dynamics influence the downstream flow characteristics and fluid–structure interactions significantly. In order to understand the gas dynamics, an axisymmetric cylindrical body with a sharp-tip cone at zero angles of attack (α=0°) is examined in a free stream Mach number of M∞=2.0 and Reynolds number of ReD=2.16×106 (D = 50 mm). Four static control surface deflection angles (θ=π/36,π/6,π/3, and π/2 rad) are considered around the base body. The cases are computationally investigated through a commercial flow solver adopting a two-dimensional detached eddy simulation strategy. Recirculation bubble length, drag coefficient's variation, wall-static pressure statistics, acoustic loading on the model and the surroundings, x − t trajectory and x − f spectral analysis, pressure fluctuation's correlation coefficient on the model, and modal analysis are obtained to understand the flow unsteadiness. At θ=[π/36], the wall-static pressure fluctuations behind the control surface are minimal and periodic, with a mere acoustic load of about 50 dB. At θ=[π/2], a violent periodic fluctuation erupted everywhere around the control surface, leading to a higher acoustic load of about 150 dB (three times higher than the previous). For θ=[π/6] and [π/3], high-frequency fluctuations with small- and large-scale structures continuously shed along the reattaching shear layer, thereby causing a broadened spectra in the control surface wake.

Gradient Heatmetry and PIV Investigation of Heat Transfer and Flow near Circular Cylinders

This paper presents an experimental investigation of convective heat transfer around circular cylinders installed one after another. The experimental approach based on the combined application of gradient heatmetry and particle image velocimetry. The method made it possible to measure velocity fields and heat flux simultaneously. Investigation of the flow characteristics and the heat transfer coefficient distribution over a system of cylinders was carried out for various Reynolds numbers in the range of Re = (4…40) × 103. In addition, the distance between the cylinders varied in the experiment. The results showed the the influence of the re-circulation bubble length and the features of vortex formation on the flow pattern and characteristics for the configuration under study in the entire range of regimes. The results were compared with experimental and numerical data from previous literature and found to be in good agreement. Our innovative method based on gradient heatmetry showed high information content and a fairly high accuracy of measurement.

Non-isothermal flow past a heated circular cylinder in subcritical regime: a numerical investigation based on large-eddy simulation

Compressible flow past a heated circular cylinder at subcritical Reynolds number of 3900 is numerically investigated by using the large-eddy simulation method. Rigorous validations of the numerical model are carefully performed under isothermal conditions on the basis of available experimental data. The calculated mean flow and Reynolds stresses show good agreement with the published experimental data. The effects of temperature difference between the cylinder surface and the freestream on the flow statistics and thermal characteristics are further studied in detail by setting two kinds of wall temperature boundary conditions. It is manifested that increasing the wall temperature leads to the augmentation of skin friction drag, suppression of turbulent intensity, enhancement of flow mixing and extension of recirculation zone. In addition, it is found that the variations of thermo-physical properties pose a slight effect on the wall heat flux before the boundary layer separates from the cylinder surface. It is worth noting that the recirculation bubble length can be used as a distance scaling parameter to weaken the temperature dependence of the flow and thermal statistics. These results provide a more detailed insight into the statistical difference in the wake region of cylinder when the temperature effect is taken into account.

Flow and spreading behaviors of coaxial jets wake

Flow and spreading behaviors of swirling jets using a dual-blockage disk are studied experimentally. The control and blockage disks are placed concentrically in tandem. The smoke flow patterns are obtained using the flow visualization technique. The axial velocity and turbulence intensity are detected using a 1-D hot-wire sensor. The jet spreading characteristics are illustrated by using an Edge Detection Method. Two pairs of lung-formed vortices and triangle-formed vortices are induced in downstream wake at Rec ≤ 200. Two vortices are found near the field at 200 < Rec < 700, while no toroidal structure was found above the reflected jet at Rec ≥ 700. The recirculation bubble length was increased with increasing Rec until Rec < 700. The axial velocity and turbulence intensity at 200 < Rec < 700 are significantly greater than those in other modes. At Rec ≥ 700, the shear-layer vortices are found far away from the control disk.

Direct numerical simulation of the wake adjustment and hydrodynamic characteristics of a circular cylinder symmetrically attached with fin-shaped strips

This work has numerically studied the effects of the coverage ratio and angular position of a pair of fin-shaped strips on the hydrodynamic forces and wake structures of a circular cylinder at both low and medium Reynolds numbers (Re = 60–500) using direct numerical simulation (DNS). The coverage ratio varies from 0.11 to 0.22, corresponding to the coverage angle (β) of a strip from 20° to 40°. The angular position (θ), measured from the forward stagnation point to the front edge of strips, ranges from 30° to 120° with increment of 30°. The numerical results indicate that the hydrodynamic coefficients are sensitive to the coverage and placement angles of the strips. The fluid forces are reduced with the introduction of a couple of 20°-fin-shaped strips, while they are amplified by placing 30°-fin-shaped strips or 40°-fin-shaped strips in the front surface of cylinder. The amplification/suppression is associated with the development of boundary layer and the wake structure, embodied in the length of recirculation bubble, the wake width, the tripping and reattachment of boundary layer as well as the separation point. By placing a pair of 40°-fin-shaped strips at θ = 60°, the time-mean drag and the lift fluctuation at Re = 180 are increased by 38.7% and 97.7%, respectively. For the cylinder equipped with 30°-fin-shaped strips at θ = 30°, the three-dimensional mode A and mode B wake structures are observed at medium Re = 200–500.

Effect of excitation frequency on flow characteristics around a square cylinder with a synthetic jet positioned at front surface

The flow over a square cylinder controlled by a slot synthetic jet positioned at the front surface is investigated experimentally at different excitation frequencies. The Reynolds number based on the free-stream velocity and the side length of the square cylinder is 1000. The flow visualization was conducted using the laser-induced fluorescence technique. The velocity fields upstream and downstream of the square cylinder were measured synchronously with the two-dimensional time-resolved particle image velocimetry technique. Both the evolution of vortex structures and the characteristic frequencies of upstream and downstream flow fields are presented. The flow dynamics vary significantly with the excitation frequency at a fixed stroke length. During one excitation cycle, the synthetic jet vortex pair deflects to one side and later swings to the other side at a quite small excitation frequency of $f_{e}/f_{0}=0.6$, while it only deflects toward one side and does not turn to the other side at $f_{e}/f_{0}=1.0$. Compared with the natural case, the wake characteristics for the above two cases are not changed much by the synthetic jet adopted. At a moderate excitation frequency of $f_{e}/f_{0}=2.0$, the synthetic jet deflects upwards and downwards alternatively. The upstream flow field has a dominant frequency identical to half of the excitation frequency. Under the perturbations of the synthetic jet, two wake vortex pairs are formed per shedding cycle with a shedding frequency equal to that of the square cylinder without control. At a higher excitation frequency of $f_{e}/f_{0}=3.4$, the synthetic jet keeps deflecting to one side, and the upstream flow field is governed by the excitation frequency. The flow separation on the deflected side is suppressed effectively, and no periodic vortex shedding can be observed in the wake. Statistically, the velocity profiles also change with control. The recirculation bubble length in the wake is shortened, and the time-averaged velocity fluctuation is weakened remarkably. The control effects of the synthetic jet and the continuous jet are compared in this paper when placed at the front surface of a square cylinder.

Numerical studies of the flow structure and aerodynamic forces on two tandem square cylinders with different chamfered-corner ratios

Three-dimensional large eddy simulations were carried out to investigate the flow over two tandem square cylinders for a fixed spacing ratio of L/D = 4 and a Reynolds number of 5.3 × 103, where L is the cylinder center-to-center distance between the cylinders and D is the cylinder width. The corners of each cylinder were chamfered with a ratio of ξ = B/D = 0%, 5%, 10%, and 15%, where B is the chamfered corner dimension. The focus is given on how ξ influences the flow structure, wake recirculation bubble, flow separation bubble, Strouhal number (St), aerodynamic force, and phase lag (ϕ) between vortex sheddings from the cylinders. With increasing ξ, the recirculation bubble length and minimum velocity in the wake of the upstream cylinder remain more or less constant and are close to those of a single cylinder, while the minimum velocity in the wake of the downstream cylinder dramatically drops between ξ = 0% and 5%. While the flow over the downstream remains attached on the side surfaces, that over the upstream cylinder forms primary and secondary side-surface bubbles on the side surface, and both shrink with increasing ξ. In addition, the leading chamfered corners are accompanied by corner bubbles that play an important role in the flow structure modification. Therefore, the time-mean drag and fluctuating lift of the upstream cylinder remarkably decrease for 0% ≤ ξ ≤ 5% and remain almost unchanged for 5% < ξ ≤ 15%, whereas those on the downstream cylinder diminish in the whole range of 0% ≤ ξ ≤ 15%. The Strouhal number (St) being identical for the two cylinders grows from 0.104 to 0.132 when ξ is increased from 0% to 15%. Overall, the influence of ξ on the wake structure and aerodynamics is remarkable for 0% ≤ ξ ≤ 5% because of the corner bubbles and is less for 5% < ξ ≤ 15%.

Investigation of the coherent structures in flow behind a backward-facing step

Purpose – The purpose of this paper is to investigate the characteristic flow structures behind a backward-facing step. With better understanding of unsteady features, effective control practice with harmonic actuation is illustrated. Design/methodology/approach – The present study employs Improved Delayed Detached Eddy Simulation to resolve flow turbulence with a finite-volume approach on structured grid mesh. The coherent structure is displayed through temporal- and spatial-evolution of pressure fluctuations. Characteristic frequencies in different flow regions are extracted using fast Fourier transform. Dynamic mode decomposition method is applied to uncover the critical dynamic modes. Findings – The time- and spanwise-averaged quantities agree well with experimental data. It is observed that two distinct modes exist: shear layer mode and shedding mode. The former is related to Kelvin-Helmholtz instability mechanism, vortex pairing and step mode with non-dimensional frequency, Sth,st at around 0.2. The latter is of multi-scale, with a typical coherent structure shedding frequency, Sth,st at 0.074. Step mode interacts with shedding mode in the reattachment region, resulting in the low-frequency characteristics. Originality/value – An optimal excitation frequency to reduce recirculation bubble length is obtained at about Sth,st =0.2 with an explanation.

Unsteady laminar mixed convection heat transfer from a horizontal isothermal cylinder in contra-flow: Buoyancy and wall proximity effects on the flow response and wake structure

Particle image velocimetry (PIV) measurements are carried out in an experimental investigation of laminar opposing mixed convection to assess the thermal effects on the wake of an isothermal circular cylinder placed horizontally and confined inside a vertical closed-loop downward rectangular water channel. The buoyancy effect on the flow distributions are revealed for flow conditions with Reynolds number based on cylinder diameter of Re=170, blockage ratio, D/H=0.287, aspect ratio, L/D=6.97 and values of the buoyancy parameter (Richardson number) in the range −1⩽Ri⩽5. In this work, flow distributions are presented in the form of mean and instantaneous contours of velocity and vorticity. To elucidate the effects of the lateral wall proximity effect and cylinder aspect ratio, separation angle, wake structure behind the cylinder, recirculation bubble length, time traces of velocity fluctuation, Strouhal number and vortex shedding modes are obtained as a function of the Richardson number. The results reported herein demonstrate how the flow structure and vortex shedding pattern are significantly modified by the wall confinement and thermal effects. In addition, our measurements show that for assisted buoyancy (Ri=−1), the breakdown of the Kármán vortex street takes place and vortex shedding is completely suppressed.

Experimental sensitivity analysis of the global properties of a two-dimensional turbulent wake

AbstractThe sensitivity of the global properties of a two-dimensional turbulent wake produced by the separated flow of a ‘D’-shaped cylinder at$\mathit{Re}= 13\hspace{0.167em} 000$is investigated experimentally using a small circular control cylinder as a local disturbance. The height of the main cylinder is$D= 25~\mathrm{mm} $and control cylinders are of diameters$d= 0. 04D$and$d= 0. 12D$, the former being smaller than the shear layer thickness detaching from the main cylinder, while the latter is larger. In both cases, the control cylinder is able to modify the global frequency, base pressure and spanwise velocity correlation. The results are presented as sensitivity maps. Reynolds stresses spatial structure and the recirculation bubble length are examined in detail when the control cylinder is displaced vertically across the wake at a fixed downstream location. It is found that the increase of the recirculation bubble length is accompanied by a damping of Reynolds stresses with a downstream shift of their spatial structures together with the base pressure increase. The global frequency can be either decreased or increased independently of the bubble length modification. The sensitivity of these global properties is interpreted on the basis of the ability of the control cylinder to change the size of the formation region of the Kármán vortex street by interacting with the primary detached shear layers. The corresponding physical mechanisms are discussed. The impact of a two-dimensional control cylinder on the three-dimensional properties of the wake is examined through spanwise correlation. This is found to be improved whenever the control cylinder is placed inside the recirculation region of the main cylinder wake.

Edge degree-of-sharpness and free-stream turbulence scale effects on the aerodynamics of a bridge deck

This paper discusses the sensitivity of the aerodynamic behavior of a trapezoidal-shaped bridge deck cross-section to its lower corner degree-of-sharpness and to the incoming flow turbulence integral length scale in conjunction with low turbulence intensity. Since these features are hard to set and measure in experimental facilities, the aerodynamic behavior of the body has been investigated through the computational simulation of the flow around it. The results are given in term of force coefficients, Strouhal number, pressure distribution along its surface, and the mean and instantaneous flow patterns. Dramatic changes in the force coefficients and Strouhal number occur following small changes in the parameter values. These changes have been found to be due to significant modifications in the topological structure of the flow. Special emphasis has been given to the analysis of the separation and reattachment points, the recirculation bubble length, the vortex shedding mechanisms and the wake structures. On the basis of the results, four aerodynamic regimes have been pointed out in analogy with the well-known individual Re number regimes. Some of these regimes have also been recognized on the deck section with the considered sharpest and smoothest lower edge treatments, i.e. for the highest and lowest degrees-of-sharpness.

Heat Transfer and Fluid Flow Characteristics of Separated Flows Encountered in a Backward-Facing Step Under the Effect of Suction and Blowing

Abstract Numerical investigation of heat transfer and fluid flow over a backward-facing step (BFS), under the effect of suction and blowing, is presented. Here, suction/blowing is implemented on the bottom wall (adjacent to the step). The finite volume technique is used. The distribution of the modified coefficient of friction and Nusselt number at the top and bottom walls of the BFS are obtained. On the bottom wall, and inside the primary recirculation bubble, suction increases the modified coefficient of friction and blowing reduces it. However, after the point of reattachment, mass augmentation causes an increase in the modified coefficient of friction and mass reduction causes a decrease in modified coefficient of friction. On the top wall, suction decreases the modified coefficient of friction and blowing increases it. Local Nusselt number on the bottom wall is increased by suction and is decreased by blowing, and the contrary occurs on the top wall. The maximum local Nusselt number on the bottom wall coincides with the point of reattachment. High values of average Nusselt number on the bottom wall are identified at high Reynolds numbers and high suction bleed rates. However, the low values correspond to high blowing rates. The reattachment length and the length of the top secondary recirculation bubble are computed under the effect of suction and blowing. The reattachment length is increased by increasing blowing bleed rate and is decreased by increasing suction bleed rate. The spots of high Nusselt number, and low coefficient of friction, are identified by using contour maps.

Simulating Pulsatile Flows Through a Pipe Orifice by an Immersed-Boundary Method

A pulsatile laminar flow of a viscous, incompressible fluid through a pipe with a sudden constriction (an orifice) was simulated by an immersed-boundary method. A fluid is forced to move by an imposed sinusoidally varying pressure difference, Δpt. For a pulsatile flow through a pipe orifice, an oscillating recirculation bubble develops behind the orifice. The induced flow rate, Qt, the recirculation bubble length, Lbt, as well as their phase shift ϕQ,ϕL with respect to the imposed pressure difference were computed for different constriction ratios and the Womersley Ws number.

The effect of flow perturbations on the near wake characteristics of a circular cylinder

The mean and fluctuating velocity fields in the near wake of a circular cylinder subjected to an incident mean flow with periodic velocity perturbations superimposed upon it were examined using laser Doppler anemometry. From these measurements the wake was characterized in terms of the recirculation bubble length, vortex formation length, maximum intensity of the velocity fluctuations and the wavelength of the vortex street. The well-known ‘lock-on’ phenomenon was observed for perturbation frequencies around two times the natural vortex shedding frequency. It is shown that the wake structure is modified in a systematic manner within the lock-on range. The forced wake shares many basic characteristics as that of a cylinder oscillating either transversely or in line, relative to the flow direction. These include the shortening of the recirculation bubble and the vortex formation region as well as the variation of the longitudinal vortex spacing with perturbation frequency. Differences but also similarities between forced wakes at low (less than 350) and relatively higher Reynolds numbers (greater than 350) are discussed.

Turbulent characteristics of shear-thinning fluids in recirculating flows

A miniaturised fibre optic Laser-Doppler anemometer was used to carry out a detailed hydrodynamic investigation of the flow downstream of a sudden expansion with 0.1–0.2% by weight shear-thinning aqueous solutions of xanthan gum. Upstream of the sudden expansion the pipe flow was fully-developed and the xanthan gum solutions exhibited drag reduction with corresponding lower radial and tangential normal Reynolds stresses, but higher axial Reynolds stress near the wall and a flatter axial mean velocity profile in comparison with Newtonian flow. The recirculation bubble length was reduced by more than 20% relative to the high Reynolds number Newtonian flow, and this was attributed to the occurrence further upstream of high turbulence for the non-Newtonian solutions, because of advection of turbulence and earlier high turbulence production in the shear layer. Comparisons with the measurements of Escudier and Smith (1999) with similar fluids emphasized the dominating role of inlet turbulence. The present downstream turbulence field was less anisotropic, and had lower maximum axial Reynolds stresses (by 16%) but higher radial turbulence (20%) than theirs. They reported considerably longer recirculating bubble lengths than we do for similar non-Newtonian fluids and Reynolds numbers.

Velocity Fields of Nonpremixed Bluff-Body Stabilized Flames

The velocity fields of nonpremixed circular-disk stabilized flames are measured by a two-component laser Doppler velocimeter. The results are presented in different flow regimes: prepenetration, penetration, and large shear flow. The velocity-vector fields, streamline patterns, corresponding flame appearances, and axial length of recirculation bubble in different flow regimes are illustrated and compared. Mean velocity and turbulent stresses along the central axis are presented and discussed. The combustion characteristics in different characteristic flow regimes are presented through the discussion of entrainment, diffusion, mixing capabilities, flow patterns, and turbulence properties. The detached flames, particularly operating in the transition flow regime, offer a relatively efficient combustion situation. Operating the concentric-jets burner in the regime of high shear flow do not benefit mixing and flame stability through the bluff-body effect. [S0195-0738(00)00102-3]

A general correlation for the local loss coefficient in Newtonian axisymmetric sudden expansions

Results from numerical simulations and guidance from an approximated corrected-theory, developed by Oliveira and Pinho (1997), (Oliveira, P.J. and Pinho, F.T. 1997. Pressure drop coeAcient of laminar Newtonian flow in axisymmetric sudden expansions. Int. J. Heat and Fluid flow 18, 518‐529) have been used to arrive at a correlation expressing the irreversible loss coeAcient for laminar Newtonian flow in axisymmetric sudden expansions. The correlation is valid for the ranges 1.5 < D2/D1 < 4 and 0.5 < Re < 200 with errors of less than 5%, except for 25 < Re < 100 where the error could be as much as 7%. The recirculation bubble length is also presented for the same range of conditions and the pressure recovery coeAcient was calculated for Reynolds numbers above 15. ” 1998 Elsevier Science Inc. All rights reserved.