Finite-length Square Research Articles

Effects of turbulence integral length scales on aerodynamic characteristics and displacement responses of a square prism

This paper investigates the influence of the inflow turbulence integral length scales on the aerodynamic forces on a surface-mounted finite-length square prism and its displacement responses by computational fluid dynamics simulations. Four turbulent inflow conditions with the same mean wind speed and turbulence intensity but different longitudinal and transverse turbulence integral length scales are generated for the simulations. First, the wind pressures and forces on a rigid square prism model and the shear layer characteristics are simulated by large eddy simulations. The simulation results show that the mean characteristics of the wind pressures and shear layers are not sensitive to the turbulence integral length scales. However, the root mean square (RMS) wind pressures on side faces and RMS across-wind forces are increased with the longitudinal turbulence integral length scale, and the mechanism is analyzed by the proper orthogonal decomposition. Second, the displacement responses at the mean wind speed of vortex-induced resonance are computed based on an aeroelastic square prism model by fluid–solid interaction simulations. The RMS displacements of the model are observed to be more sensitive to the transverse turbulence integral length scale rather than the longitudinal turbulence integral length scale. Finally, the influence of the turbulence integral length scales on the Reynolds stresses around the square prism is presented and discussed.

Numerical study of the boundary layer flow past two wall mounted finite-length square cylinders in staggered arrangement

Flow past two wall-mounted square cylinders in staggered arrangement is simulated through direct numerical simulation to investigate the effects of the degree of staggering on the wake and forces compared to the tandem arrangement. Numerical simulations are conducted for a constant Reynolds number of 500, height to width ratio H = 4, staggered distance to diameter ratios S = 0.5 and 1, and gap to width ratios G in the range from 1 to 8. It is found that moving the downstream cylinder sideway for S = 0.5 and 1 changes flow features and force coefficient significantly. At small gaps of G = 1 to 3, the shear layer from one side of the upstream cylinder is trapped into the gap. In addition, a strong horseshoe vortex in front of the downstream cylinder is found for all the gaps at S = 0.5 and 1, but not found at S = 0. The friction velocity on the plane boundary caused by the horseshoe vortex of the downstream cylinder is greater than that of a single cylinder at S = 1 and G = 1 to 8 and at S = 0.5 and G = 2 to 5. At G = 1 to 3, vortex shedding is caused by the trapped flow into the gap on one side of the downstream cylinder and by the approaching flow on another side. The negative mean drag coefficient at small gaps was found in the tandem arrangement case, but not found at S = 0.5 and 1.

Numerical study of the flow past two wall-mounted finite-length square cylinders in tandem arrangement

Flow past two wall-mounted square cylinders in a tandem arrangement are simulated through direct numerical simulation to investigate the effects of the gap between the two cylinders on the wake. Numerical simulations are conducted for a constant Reynolds number of 500, height to width length ratio H = 4, and gap to width ratios of G = 1 to 8 with an interval of 1. The flow in the wake of the downstream cylinder is found to be significantly affected by the free shear layers from the top and sides of the upstream cylinder. At G = 1 and 2, the free-shear layer generated from the upstream cylinder reattaches the top surface of the downstream cylinder and further develops into a downwash behind the downstream cylinder. At G = 3 to 8, the downwash behind the downstream cylinder disappears because flow separation from the top upstream edge of the downstream cylinder does not occur for G = 3 to 6 and is very weak for G = 7 and 8. The disappearance of downwash in the wake of the downstream cylinder further results in very weak variation of flow along the span of the downstream cylinder. The single, reattachment, and binary wake modes at the mid-span of the cylinder occur at G = (1 and 2), (3 and 4), and (5 and above), respectively.

Effect of inlet shear on turbulent flow past a wall-mounted finite-size square cylinder

The present study analyzes the effect of incoming shear flow on the wake behind a wall-mounted finite-length square cylinder of aspect ratio (AR=7) using large eddy simulation (LES). All the simulations were performed at a Reynolds number (Re) of 10000, based on cylinder width (d) and average inlet velocity (uavg), with different shear parameters, K=0.0,0.025,0.050 and 0.075. The shear-improved Smagorinsky model Lévêque et al. (2007) is used to account for the subgrid scale stress. It is found that the incoming flow shear has a profound effect on the modes of shedding. With increasing shear parameter anti-symmetric modes of shedding tends to dominate along the cylinder height, except close to the free-end where both symmetric and anti-symmetric modes of shedding co-exist. The incoming shear flow has also significant influence on the downwash flow from the free-end, the strength of which decreases with an increase in shear parameter. The resolved turbulent kinetic energy (TKE) associated with the cylinder wake is found to have maximum contribution from the transverse component of normal stress compared to the other components of normal stresses.

Laminar to turbulent transition in a finite length square duct subjected to inlet disturbance

Laminar to turbulent flow transition in a finite length square duct has been carried out by imposing novel spatiotemporal finite amplitude inlet disturbance on the laminar flow. The present direct numerical simulation study demonstrates the effect of inlet disturbance on laminar to turbulent transition. A laminar flow in a finite length square duct is considered at bulk Reynolds number Re = 2260 and Re = 1540, to which a novel spatiotemporal disturbance is introduced through a narrow banded region at the inlet of the square duct. The puff (transition) and slug (turbulent) flow dynamics indicate the laminar to turbulent transition in a square duct. Disturbance introduced at Re = 2260 laminar flow propagates downstream, giving puff and slug flow phenomena similar to pipe flows. However, at Re = 1540, inlet disturbance shows only a puff-like structure. The four vortex mean secondary flow is observed in a puff region, while the conventional eight vortex is observed in the slug region. The coherent structures of the transition (puff) flow show the presence of dual-type hairpin structures. The turbulent kinetic energy spectrum indicates conventional −5/3 spectra for slug flow and −2 energy spectra for puff flow. Thus, in this paper, it is shown that the puff and slug characteristics of laminar to turbulent transition in a square duct are similar to that of a circular duct. It is also shown that the novel inlet disturbance through a narrow banded region captures the dynamics of laminar to turbulent transition in a square duct.

Experimental investigation on active control of flow around a finite-length square cylinder using dual synthetic jet

This paper presents an active flow control method for flow around a finite-length square cylinder using dual synthetic jet (DSJ). A piezoelectric dual synthetic jet actuator (DSJA) was installed at the leading edge of the free end for generating the DSJ. The amplitude and frequency of DSJ can be adjusted through the driving voltage and driving frequency. In our research, the pressure measurements and flow visualization are conducted to test the control effectiveness of dual synthetic jet. According to the aerodynamic results, there is a correlation between the control efficiency and the amplitude and frequency of DSJ, which are determined by jet momentum coefficient and driving frequency. After the application of the free-end DSJ with moderate amplitude and frequency, the maximum reduction of total mean drag coefficients, fluctuating drag coefficients and fluctuating lift coefficients respectively reached 4.3%, 18.0% and 30.8%. Moreover, the smoke-wire and PIV results indicate that there is a remarkable suppression of the free-end separated shear flow and a prominent increase in the TKE in the area near the free end. All results point to that the aerodynamic forces and wakes of a surface-mounted finite-length square cylinder can be effectively controlled using dual synthetic jet at free end.

Large Eddy Simulation of the flow around a finite-length square cylinder with free-end slot suction

Large Eddy Simulation (LES) is used to study the effects of steady slot suction on the aerodynamic forces of and flow around a wall-mounted finite-length square cylinder. The aspect ratio H/d of the tested cylinder is 5, where H and d are the cylinder height and width, respectively. The Reynolds number based on free-stream oncoming flow velocity Us/U∞) is is varied as Q = 0, 1 and 3, where Us is the velocity at the entrance of the suction slot. It is found that the free-end steady slot suction can effectively suppress the aerodynamic forces of the model. The maximum reduction of aerodynamic forces occurs at Q = 1, with the time-mean drag, fluctuating drag, and fluctuating lift reduced by 3.75%, 19.08%, 40.91%, respectively. For Q = 3, all aerodynamic forces are still smaller than those for Q = 0 (uncontrolled case), but obviously higher than those for Q = 1. The involved control mechanism is successfully revealed, based on the comparison of the flow around cylinder free end and the near wake for the three tested Q values.

Low-frequency dynamics of the flow around a finite-length square cylinder

Fluctuating pressure on the side faces, lee-ward face and free end of a finite-length square cylinder is measured simultaneously, together with the velocity in its free-end shear flow, to reveal their inherent connection. The width (d) of the tested model is 40 mm, and the aspect ratio is 5. All experiments are conducted in a low-speed wind tunnel at an oncoming flow velocity (U∞) of 10 m/s. The corresponding Reynolds number based on U∞ and d is 27,000. It is found that, the fluctuating pressure on the side faces and free end has remarkable low-frequency instability with the frequency of about 1/10 of the well-known spanwise Karman vortex shedding. Besides, a high-frequency component, corresponding to Karman vortex shedding, is also found in the fluctuating pressure on cylinder side faces. The low-frequency instability is in phase over the entire cylinder span, while the high-frequency component tends to be out of phase, especially at the lower part of the cylinder. The free-end shear flow flaps at the low frequency and strongly correlates with the modes of spanwise vortex shedding. Particularly, when the free-end shear flow flaps to its lower position, alternate spanwise vortex shedding with high pressure fluctuation occurs; when it flaps to its upper position, the occurrence probability of co-shedding with low pressure fluctuation increases significantly especially near the cylinder free end.

Control of the VIV of a cantilevered square cylinder with free-end suction

A steady slot suction near the free-end leading edge of a finite-length square cylinder was used to control its aerodynamic forces and vortex-induced vibration (VIV). The freestream oncoming flow velocity (U) was from 3.8 m/s to 12.8 m/s. The width of the tested cylinder d = 40 mm and aspect ratio H/d = 5, where H was the height of the cylinder. The corresponding Reynolds number was from 10,400 to 35,000. The tested suction ratio Q, defined as the ratio of suction velocity (Us) at the slot over the oncoming flow velocity at which the strongest VIV occurs (Uv), ranged from 0 to 3. It was found that the free-end slot suction can effectively attenuate the VIV of a cantilevered square cylinder. In the experiments, the RMS value of the VIV amplitude reduced quickly with Q increasing from 0 to 1, then kept approximately constant for Q > 1. The maximum reduction of the VIV occurs at Q = 1, with the vibration amplitude reduced by 92% , relative to the uncontrolled case. Moreover, the overall fluctuation lift of the finite-length square cylinder was also suppressed with the maximum reduction of 87%, which occurred at Q = 1. It was interesting to discover that the free-end shear flow was sensitive to the slot suction near the leading edge. The turbulent kinetic energy (TKE) of the flow over the free end was the highest at Q = 1, which may result in the strongest mixing between the high momentum free-end shear flow and the near wake.

Characteristics of the Flow Past a Wall-Mounted Finite-Length Square Cylinder at Low Reynolds Number With Varying Boundary Layer Thickness

Direct numerical simulation (DNS) is performed to investigate the modes of shedding of the wake of a wall-mounted finite-length square cylinder with an aspect ratio (AR) of 7 for six different boundary layer thicknesses (0.0–0.30) at a Reynolds number of 250. For all the cases of wall boundary layer considered in this study, two modes of shedding, namely, anti-symmetric and symmetric modes of shedding, were found to coexist in the cylinder wake with symmetric one occurring intermittently for smaller time duration. The phase-averaged flow field revealed that the symmetric modes of shedding occur only during instances when the near wake experiences the maximum strength of upwash/downwash flow. The boundary layer thickness seems to have a significant effect on the area of dominance of both downwash and upwash flow in instantaneous and time-averaged flow field. It is observed that the near-wake topology and the total drag force acting on the cylinder are significantly affected by the bottom-wall boundary layer thickness. The overall drag coefficient is found to decrease with thickening of the wall boundary layer thickness.

Control of the aerodynamic forces of a finite-length square cylinder with steady slot suction at its free end

A steady slot suction near the free-end leading edge of a finite-length square cylinder was used to control its aerodynamic forces. The aspect ratio (H/d, where H and d are the height and width of the cylinder, respectively) of the tested cylinder was 5. The tested suction ratio Q, which was defined as the ratio of the suction velocity at the slot (Us) to U∞, ranged from 0 to 4. It was found that the overall aerodynamic forces reduce quickly with the increase of Q from 0 to 1, then recovers slightly with Q from 1 to 2, and keep approximately constant with Q ≥ 2. The maximum reduction of the overall mean drag, fluctuating drag and fluctuating lift occurs at Q = 1, which reaches 3.6%, 17.8% and 45.5%, respectively. The steady slot suction reduces the aerodynamic forces not only near the free end, but also over the entire cylinder span. At Q = 1, the shear flow emanating from the free-end leading edge reattaches on the free end forming a recirculation bubble. The enhanced momentum transport between the free-end shear flow and the wake suppresses the spanwise vortex shedding and aerodynamic forces the most effectively.

Features of the flow over a finite length square prism on a wall at various incidence angles

Wake characteristics of the flow over a finite square prism at different incidence angles were experimentally investigated using an open-loop wind tunnel. A finite square prism with a width D = 15 mm and a height H = 7D was vertically mounted on a horizontal flat plate. The Reynolds number was varied from 6.5X103 to 28.5X103 and the incidence angle a was changed from 0 to 45. The ratio of boundary layer thickness to the prism height was about s/H = 7%. The time-averaged velocity, turbulence intensity and the vortex shedding frequency were obtained through a single-component hotwire probe. Power spectrum of the streamwise velocity fluctuations revealed that the tip and base vortices shed at the same frequency as that of spanwise vortices. Furthermore, the results showed that the critical incidence angle corresponding to the maximum Strouhal number and minimum wake width occurs at acr = 15o which is equal to that reported for an infinite prism. There is a reduction in the size of the wake region along the height of the prism when moving away from the ground plane towards the free end.

Vortex induced vibration of an inclined finite-length square cylinder

This study investigated vortex-induced vibrations of a slender square-section cylinder inclined from the vertical direction by a series of angles experimentally. Both aeroelastic tests and pressure measurements were performed on the cylinder with forward inclinations (inclined to the upwind direction), a vertical attitude, and backward inclinations (inclined to the downwind direction) in a wind tunnel. The cylinder was kept stationary in the pressure measurement tests. The aeroelastic test results show that vortex-induced responses of the cylinder decrease considerably as increasing the forward inclination angle. By contrast, the effect of the backward inclination on the vortex-induced vibration varies. Not all the backward inclined cylinders exhibit crosswind responses smaller than those of the vertical cylinder does. The responses of the cylinder with a small backward inclination are significantly larger, whereas the cylinder with a large backward inclination exhibits lower responses than those of the vertical case. The variation in the vortex-induced vibration response with the inclination is explained by performing power spectral analyses on the pressure data on the side face over the cylinder span, which reveals local vortex shedding characteristics. The findings provide significant guidance to the wind-resistant design of an inclined slender structure.

POD analysis of a finite-length cylinder near wake

The near wake of a wall-mounted finite-length square cylinder with an aspect ratio of 7 is investigated based on the proper orthogonal decomposition (POD) of the PIV data measured in three spanwise planes, i.e., z/d = 6, 3.5 and 1.0, near the cylinder free end, mid-span and fixed end (wall), respectively. The Reynolds number based on free-stream velocity (U∞) and cylinder width (d) is 9,300. A two-dimensional (2D) square cylinder wake is also measured and analyzed at the same Reynolds number for the purpose of comparison. The structures of various POD modes show marked differences between the two flows. While the coefficients, a1 and a2, of the POD modes 1 and 2 occur within an annular area centered at a1 = a2 = 0 in the 2D wake, their counterparts are scattered all over the entire circular plane at z/d = 1.0 and 3.5 of the finite-length cylinder wake. Flow at z/d = 6 is dominated by POD mode 1, which corresponds to symmetrical vortex shedding and accounts for 54.0 % of the total turbulent kinetic energy (TKE). On the other hand, the POD modes 1 and 2, corresponding to anti-symmetrical vortex shedding, are predominant, accounting for about 45.0 % of the total TKE, at z/d = 3.5 and 1. It has been found that the flow structure may be qualitatively and quantitatively characterized by the POD coefficients. For example, at z/d = 6, a larger a1 corresponds to a smaller length of flow reversal zone and a stronger downwash flow. At z/d = 3.5 and 1, two typical flow modes can be identified from a1 and a2. While large a1 and/or a2 correspond to anti-symmetrical vortex shedding, as in a 2D cylinder wake, small a1 and a2 lead to symmetrical vortex shedding. Any values between the large and small a1 and/or a2 correspond to the flow structure between these two typical flow modes. As such, the probability of occurrence of a flow structure may be determined from the distribution of the POD coefficients.

Unsteady flow past a finite square cylinder mounted on a wall at low Reynolds number

The flow past a finite length square cylinder resembles flow across a short protrusion or a pin fin in a flow device. Direct numerical simulation (DNS) past a finite length square cylinder has been performed at a Reynolds number of 250. The flow field has been explored by solving three-dimensional unsteady Navier–Stokes equations using second order spatial and temporal discretizations. Four different cylinder height-to-width or aspect ratios (2, 3, 4 and 5) have been used to quantify its effect on the flow field and integral parameters. The cylinder aspect ratio is found to have significant effect on the instantaneous as well as time-averaged flow characteristics. The evolution of various flow structures associated with finite length cylinder such as wakes, tip vortices, base vortices and horse-shoe vortices are discussed. The non-dimensional frequency or Strouhal number and drag coefficient is found to increase with increase in cylinder aspect ratio.

233 有限長の角柱を設置した乱流チャネル内の流動特性(OS3-4 乱流現象の実験と数値シミュレーション(4),OS3 乱流現象の実験と数値シミュレーション)

233 有限長の角柱を設置した乱流チャネル内の流動特性(OS3-4 乱流現象の実験と数値シミュレーション(4),OS3 乱流現象の実験と数値シミュレーション)

0635 有限長の弾性角柱を配置した溝乱流のDNS(OS6-7 噴流,後流およびはく離流れ現象の探求と先端的応用,OS6 噴流,後流およびはく離流れ現象の探求と先端的応用)

0635 有限長の弾性角柱を配置した溝乱流のDNS(OS6-7 噴流,後流およびはく離流れ現象の探求と先端的応用,OS6 噴流,後流およびはく離流れ現象の探求と先端的応用)

Bistable Phenomenon of the Aerodynamic Forces on a Square Prism with the Aspect Ratio of 5

The flow around a finite-length square prism with aspect ratio of 5 is numerical investigated using LES at Red = 3900. The prism is mounted on a flat wall, with one end free. Based on the simulation results, it is found that the near wake is highly three dimensional under the effects of free-end downwash flow. The shear layers from prism side walls and free end form an arch-type structure. There are two typical flow modes presence in the near wake: first, the spanwise vortices are staggered arranged similar to that in 2D cylinder wake; second, the spanwise vortices are quasi-symmetrically arranged. These two modes occur alternately and intermittently. When the first mode occurs, the pressure on the prism side surface fluctuates periodically, corresponding to large values of drag and fluctuating lift coefficients; when the second modes occurs, there is no obvious pressure fluctuation on prism side surfaces, and the correspond drag and fluctuation life coefficients are significantly smaller than those for the first mode.

0124 柔軟構造体を設置した平行平板間乱流の流動特性(OS1 噴流,後流および剥離流れ現象の探求と技術革新,オーガナイズドセッション)

In the present paper in order to investigate the possibility of flow control using elastic body, we conduct a simulation of a finite-length square cylinder placed inside turbulent channel flow. For an elastic cylinder, the rigorous equation of motion for elastic continuum is solved with a finite volume method; the effect of existence of cylinder in the flow computation is taken into account using the immersed boundary method. From time averaged quantities, we demonstrate that the flow pattern around rigid cylinder is in accordance with that of previous reported experimental results and that the elastic cylinder markedly modulates the wake structure, u

The effect of subgrid-scale model on prediction of flow around a surface-mounted finite square cylinder

The present study investigates the effect of the subgrid-scale (SGS) stress model on large eddy simulation of flow over a finite-length square cylinder mounted vertically on a ground plane. The study specifically considers a cylinder of aspect ratio AR = 5 immersed in a thin boundary layer for a Reynolds number of Re = 500 (based on the free stream velocity and cylinder width.) Three different subgrid-scale models are considered: a Smagorinsky model, a dynamic Smagorinsky model and a dynamic nonlinear model. The mean flow structure is similar for all SGS models on both grids, although noticeable differences are observed in the size of the recirculation zone, level of resolved-scale turbulence and vortical structures for different SGS models. The dynamic Smagorinsky model appears to be especially sensitive to the grid resolution.