Wake Size Research Articles

On the drag reduction of road vehicles with trailing edge-integrated lobed mixers

Trailing edge-integrated lobed-mixing geometries are proposed as a viable method for road vehicle aerodynamic drag reduction. Experiments are conducted on a 1/24th-scale model, representative of a Heavy Goods Vehicle, at a width-based Reynolds number of 2.8 × 105. A broad range of pitches and penetration angle values is examined, with detailed comparisons also made to high-aspect-ratio rear tapering. Changes to mean drag coefficients and wake velocities are evaluated and assessed from both the time-independent and time-dependent perspectives. Results show significant drag reductions for lower pitches at higher penetration angles, where the performance of regular tapering is found substantially degraded. The mechanisms responsible for drag reduction are identified to be reductions in the wake size and a shift in the vertical wake balance. The former is shown to be a result of the enhancement in inboard momentum close to the trailing edges through the generation of pairs of counter-rotating streamwise vortices, with the latter attributed to the downstream evolution of the vortices. Overall, these results identify such geometries to be suitable for improving vehicle drag while minimising the losses in internal space.

The aerodynamics of flying snake airfoils in tandem configuration.

Flying snakes flatten their body to form a roughly triangular cross-sectional shape, enabling lift production and horizontal acceleration. While gliding, they also assume an S-shaped posture, which could promote aerodynamic interactions between the fore and the aft body. Such interactions have been studied experimentally; however, very coarse models of the snake's cross-sectional shape were used, and the effects were measured only for the downstream model. In this study, the aerodynamic interactions resulting from the snake's posture were approximated using two-dimensional anatomically accurate airfoils positioned in tandem to mimic the snake's geometry during flight. Load cells were used to measure the lift and drag forces, and flow field data were obtained using digital particle image velocimetry (DPIV). The results showed a strong dependence of the aerodynamic performance on the tandem arrangement, with the lift coefficients being generally more influenced than the drag coefficients. Flow field data revealed that the tandem arrangement modified the separated flow and the wake size, and enhanced the lift in cases in which the wake vortices formed closer to the models, producing suction on the dorsal surface. The downforce created by the flow separation from the ventral surface of the models at 0deg angle of attack was another significant factor contributing to lift production. A number of cases showing large variations of aerodynamic performance included configurations close to the most probable posture of airborne flying snakes, suggesting that small postural variations could be used to control the glide trajectory.

The Steady Wake of a Wall-Mounted Rectangular Prism with a Large-Depth-Ratio at Low Reynolds Numbers

The wakes of wall-mounted small (square) and large (long) depth-ratio rectangular prisms are numerically studied at Reynolds numbers of 50–250. The large depth-ratio significantly alters the dominance of lateral secondary flow (upwash and downwash) in the wake due to the reattachment of leading-edge separated flow on the surfaces of the prism. This changes the wake topology by varying the entrained flow in the wake region and changing the distribution of vorticity. Thus, the magnitude of vorticity significantly decreases by increasing the prism depth-ratio. Furthermore, the length of the recirculation region and the orientation of near wake flow structures are altered for the larger depth-ratio prism compared to the square prism. Drag and lift coefficients are also affected due to the change of pressure distributions on the rear face of the prism and surface friction force. This behavior is consistently observed for the entire range of Reynolds numbers considered here. The wake size is scaled with Re1/2, whereas drag coefficient scaled with Re−0.3.

The effect of rear cavity modifications on the drag and flow field topology of a Square Back Ahmed Body

This work presents a numerical investigation of turbulent flow over a simplified vehicle model called the Square Back Ahmed Body (SBAB). The simulations are performed at Reynolds number 1 × 105 using the k − [Formula: see text] SSTSAS turbulence model. The numerical results for the standard reference model, that is, SBAB, have been validated against available experimental data and numerical simulations. The performance of a passive flow controller with four variants on the mean wake topology and the resultant drag reduction is evaluated. The four variants are; (i) straight cavity, (ii) straight cavity with rounded edges, (iii) C-shaped cavity, and (iv) tapered cavity. For a straight cavity with a depth equal to 33.33% of the body height, drag is lowered by 5.63%. With the same cavity depth, rounding the straight cavity edges reduces the drag by 10.67% owing to the streamlining and shortening of the recirculation region. For a C-shaped cavity, the amount of drag reduction increased slightly by 1% more than that off straight cavity with rounded edges, due to improvement in the base pressure distribution compared to that of the latter case. Tapering the cavity edges at an angle of 6° gave a significant drag reduction of 22.55% primarily due to a tremendous decrease in wake size. The drag reduction achieved in all the cases results from the modification in the mean wake topology induced by afterbody shape remodeling. The power spectra of the evolution of the lift coefficient over time reveal a noticeable decrement in the flow randomness with the inclusion of a cavity and its modifications, which interprets the mitigation of the chaotic nature in the wake. The cavity presence has also increased the vorticity spreading rate in the mixing layer and produced significant attenuation of Turbulent Kinetic Energy (TKE).

Effects of position and geometry of curved vortex generators on fin-tube heat-exchanger performance characteristics

The thermal and fluid-flow characteristics of the rectangular-winglet, delta-winglet-upstream (DWU), and delta-winglet-downstream (DWD) curved vortex generators (CVGs) are computationally analyzed in this study. Polar coordinates based on the tube center are considered to define the CVG positions, thereby facilitating a parametric study of the effects of the position angle (α) and radial distance (r) of CVGs. The resulting heat-transfer enhancement, pressure loss, and flow patterns have been analyzed in detail. When CVGs are placed at α=30°, mixed vortices are generated, thereby improving the heat-transfer performance of the fin. In contrast, placing the CVGs near the rear of the tube reduces the wake size and increases heat transfer behind the tube. Furthermore, a secondary flow is induced enhancing the fine heat-transfer performance. However, the most of results obtained in this study reveal that CVGs are not superior to conventional VGs. Further, the realization of optimum heat-transfer performance using CVGs mandates certain position and geometry requirements to be satisfied. For example, as observed in this study, the DWU CVGs (α=105°, r/R=1.25) and DWD CVGs (α=30°, r/R=1.5) exhibit the highest heat-transfer performance improvements of 5.2% and 7.5%, respectively, compared to conventional VGs. However, this enhancement in heat-transfer performance is realized at the cost of a relatively small pressure loss.

Passive flow control on Ahmed body by rear linking tunnels

This paper concentrates on the interaction of a car body with the surrounding airflow, in order to provide small changes for reducing vehicle fuel consumption by reducing drag. There are many passive and active methods for this purpose, the main goal of this project is the development of passive drag reduction devices for the simplified reference model (Ahmed body with 25°, 30°, and 35° slant angle) used to represent bluff body vehicles. Passive flow control is performed by adding linking tunnels at the rear of the bluff body, blowing the flow from the sidewalls (high-pressure zone) to the wake region (low-pressure zone). The movement of a high-pressure flow to the wake zone reduces the wake size, which reduces the pressure drag. A numerical and experimental analysis of this passive method is carried out. The optimal configuration was reached by numerical analysis (showing a drag reduction of up to 5%) and after that, experimental tests were performed in a wind tunnel to check the effect of this passive flow control on an Ahmed body 25°.

An experimental evaluation of blockage effects on the wake of a cross-flow current turbine

While flow confinement, or blockage, is known to affect current turbine performance and wake evolution, there have been limited experimental investigations of the wake evolution of cross-flow turbines under variable confinement. In this study, velocity data were collected in the near wake of a laboratory-scale (0.51 m diameter) cross-flow turbine under two different blockage conditions. To isolate the effects of blockage, other parameters that affect turbine performance, such as the Reynolds number, Froude number, turbine submergence depth, and free-stream turbulence intensity, were held constant. The turbine was operated at the tip-speed ratio corresponding to peak power for each blockage case. Increasing the blockage caused faster streamwise flow speeds through and around the turbine, a decreased overall wake size, higher levels of turbulent kinetic energy in the wake, and an increased viscous dissipation rate. This suggests that higher blockage could increase the power output and reduce the physical footprint of current turbine arrays. However, these benefits must be weighed against the potential for high blockage arrays to reduce a turbine’s “basin efficiency”, which is influenced by how thrust changes with blockage. Furthermore, we observed that decreasing the width of the experimental channel while holding the depth constant decreased the extent of the wake in the lateral direction only. The wake was unaffected in the vertical direction, which suggests that lateral and vertical blockages have independent effects on turbine wakes. Consistent with prior studies, we also observed significant wake mixing in the vertical (i.e., spanwise) direction and negligible wake mixing in the lateral direction for both blockage conditions.

The effects of flexible vortex generator on the wake structures for improving turbulence

Turbulence is a very useful flow characteristic that is used in many engineering devices to facilitate mixing, heat transfer in heat exchanger and improving aerodynamics in propeller. This paper has numerically studied the spatial characteristics of the wake behind a free-oscillating flexible vortex generator (FVG) in order to evaluate its turbulence generation ability. As the wake is playing as the effective turbulence region, the spatial characteristics can reflect the ability of the FVG in turbulence generation when a larger wake size indicates a greater turbulence region. Two VGs are considered in this study; circular and flat plate cantilever. Each VG was submerged individually in a subcritical flow (102 <ReD < 105) and the generated wake was studied. The wake behind the FVG was visualized via the lambda2 vortex identification criterion and it was compared with its respective rigid counterpart. From the comparison, it is found that the FVG has a larger wake compared to its rigid counterpart. The result demonstrates that the FVG can generate a larger turbulence region. The cause of the increasing wake size was found to be the weakening of downwash behind the FVG when it bends. The downwash becomes weaker as the deflection of the FVG increases.

Flow Around Three Side-by-Side Square Cylinders and the Effect of the Cylinder Oscillation

Abstract In this study, the flow field over three square cylinders (SCs) arranged side by side is investigated in a low-speed wind tunnel. The experiments are performed with three similar SCs for Reynolds number (Re) 295. The influences of spacing ratio on the wake size, drag coefficient, and flow interference of the cylinders are reported with the hotwire anemometry, particle image velocimetry (PIV), and the flow visualization techniques. Special attention is paid to the oscillation given to the middle cylinder and its effect on flow structure and related forces. The spacing ratio (s/D) ranges from 1.5 to 3, whereas the forcing frequency ratio ranges from 0.5 to 2 with amplitude of 10% of the cylinder width. It is observed that the spacing influences the flow structure, and the vortex shedding mechanism strongly. A secondary frequency appears in the flow field for spacing ratio s/D = 2 and 3. Depending upon the spacing ratios, the flow pattern is seen to be asymmetric biased, symmetric biased, and weakly interactive. The wake interaction decreases with increase in spacing ratios. With the oscillations, the wake becomes more unstable and complex. Additional wake oscillation frequency appears in the power spectra. With an increase in spacing ratios, the drag coefficient decreases, whereas with oscillations, higher drag force is observed compared to a stationary cylinder. A correlation is developed between the time-averaged drag coefficient with cylinder spacing and Reynolds number.

Tropical Cyclone Cold Wake Size and Its Applications to Power Dissipation and Ocean Heat Uptake Estimates

AbstractMixing of the upper ocean by the wind field associated with tropical cyclones (TCs) creates observable cold wakes in sea surface temperature and may potentially influence ocean heat uptake. The relationship between cold wake size and storm size, however, has yet to be explored. Here we apply two objective methods to observed daily sea surface temperature data to quantify the size of TC‐induced cold wakes. The obtained cold wake sizes agree well with the TC sizes estimated from the QuikSCAT‐R wind field database with a correlation coefficient of 0.51 and 0.59, respectively. Furthermore, our new estimate of the total cooling that incorporates the variations in the cold wake size provides improved estimates of TC power dissipation and TC‐induced ocean heat uptake. This study thus highlights the importance of cold wake size in evaluating the climatological effects of TCs.

Visualization of flow over a thick airfoil with circular cross-sectional riblets at low Reynolds numbers

This study qualitatively investigated the effects of riblets with circular cross section on the aerodynamic performance of a thick airfoil. Smoke lines inside a wind tunnel visualized the flow fields around the airfoils with and without riblets to compare the location of the separation point, the stability of the visualized smoke lines near the wake and the size of the wake region for 0°–20° angles of attack, at a Reynolds number of 2 × 104. Four sizes of riblets with the height-to-chord ratios of 0.002, 0.005, 0.008 and 0.01 were attached to the thick airfoil to prepare the textured models for the visualization tests. The sizes of the wake region were measured at four cross sections behind the models by image processing. Due to the fluctuations of the wake size, many consecutive images were considered for averaging the wake size at each section. The results showed the wake size to be changed as a result of using riblets depending on the size of riblets. The textured model with the height-to-chord ratio of 0.005, compared to the smooth model, significantly decreased the wake size for all the angles of attack.

Hydrodynamic and thermal study of a trapezoidal cylinder placed in shear-thinning and shear-thickening non-Newtonian liquid flows

Hydrodynamic and thermal features of two-dimensional, incompressible shear-thinning and shear-thickening non-Newtonian power-law liquids under forced convection across a trapezoidal cylinder are the focus of this study, carried out at Reynolds number Re = 1–40, power-law index n = 0.4–1.8 and Prandtl number Pr = 50 using ANSYS Fluent. In this steady system, the drag coefficient is found to decrease, whereas the wake size and the average Nusselt number increase with the increase in Re. However, with the increase in n value, the average Nusselt number decreases. Similar to a square cylinder, compared to Newtonian liquids, shear-thinning liquids increases heat transfer and shear-thickening reduces it. The maximum increase in heat transfer for shear-thinning liquids compared to Newtonian liquids for a trapezoidal cylinder is approximately 25%, while the average heat transfer is somewhat lower for shear-thickening liquids than for Newtonian. The average Nusselt number is smaller for the trapezoidal cylinder than for a square cylinder in the parameter domain investigated; the largest difference is around 25%. A simple relationship for the average Nusselt number as a function of Re and n has been determined.

Numerical investigation of different flow regimes for multiple staggered rows

In this numerical exploration, the combined effect of Reynolds number (30 ≤ Re ≤ 150) and gap spacing (1 ≤ g ≤ 7) is studied for two dimensional cross flow across multiple staggered rows of square cylinders. Flow is simulated by using lattice Boltzmann method. Outcomes show that for the outset of vortex shedding phenomenon, the critical Re increases as the normalized g increases. At large Re and at g = 7, 6 and 5, the primary vortex shedding frequency controls the flow whereas the secondary frequency almost vanishes. The jets in the gap region have strong influence upon the wake interaction. The nature of the wakes is changed by changing the g and Re which is visualized by the change of wake size behind the cylinders. These g depending on the Re are used to split the flow regimes into chaotic, quasiperiodic-I and quasiperiodic-II flow regimes. Some physical parameters of practical importance are also analysed.

Tropical Cyclone Cold Wake Size and Its Applications to Power Dissipation and Ocean Heat Uptake Estimates

Tropical Cyclone Cold Wake Size and Its Applications to Power Dissipation and Ocean Heat Uptake Estimates

Thermal and hydraulic characteristics of trapezoidal winglet across fin-and-tube heat exchanger (FTHE)

Heat transfer augmentation by using vortex generator (VG) as an enhancement method in plain fin-and-tube heat exchanger (FTHE) has recently captured high attention of many researchers. In the present study, the flow characteristics and thermal performance across the FTHE with and without trapezoidal winglet vortex generator (TWVG) are investigated using dye injection technique by experimental approach and numerical simulation using finite volume method. Two types of TWVG are used, namely flat trapezoidal winglet (FTW) and curved trapezoidal winglet (CTW), either arranged in common flow up (CFU) or in common flow down (CFD) arrangements. In this study, Reynolds number (Re), angle of attack (β) and aspect ratio (Ʌ) in the range of 500–2500, 10–90° and 2–3 were used respectively. From the experimental study, the results show that the heat transfer can be enhanced by using certain TWVG geometry, arrangement, Ʌ and β to reduce the wake region size at the rear portion of the tubes. Meanwhile, from the numerical simulation, augmentation towards the heat transfer rate is normally accompanied with higher pressure drop penalty. It is inferred that FTW in CFU arrangement at Λ = 3 and β = 10° was found to give the best thermal and hydraulic performance across the FTHE channel when both heat transfer enhancement and pressure drop penalty are considered.

Numerical study of thermal and flow characteristics for a fin-and-tube heat exchanger with arc winglet type vortex generators

Vortex generator can effectively enhance the heat transfer performance on the fin surface by introducing the secondary flow and intensifying the mixing of fluid. In this study, four new types of arc-winglet vortex generator (VG) are proposed and the three-dimensional CFD numerical simulation was employed to investigate the fluid flow and heat transfer characteristics. The results indicate that compared with the conventional rectangular-winglet VG, the two types of plane arc VGs provide a significant improvement on the size of secondary flow behind the tubes and all the wake region size of the fins with the four types of arc-winglet VGs decreased to some extent, especially for the two types of curved arc VGs. The heat transfer enhancement of the fin with curved arc VGs is mainly completed by guiding more fluid to flow into the previous wake region and then scour the downstream tubes directly. In addition, compared with the rectangular-winglet VGs, the Nusselt number averagely increases by 11.5% for the equal-perimeter arc VGs fin, 19.9% for the curved equal-area arc VGs fin, and 35.9% for the curved equal-perimeter arc VGs fin. Although the equal-perimeter arc VG and the curved equal-perimeter arc VG have the same geometry size, the Nusselt number of the fin with the former one averagely increases by 22.2% while the pressure drop only increases by 13.5%.

Analysis of lift, drag and CX polar graph for a 3D segment rigid sail using CFD analysis

ABSTRACTA rigid sail is an alternative source of propulsive power for marine vessels and they have been installed on ocean-going powered ships in the past. To determine the potential power that could be harnessed by a rigid sail its specific lift and drag characteristics must be known. For this purpose, a segment rigid sail was studied utilising computation fluid dynamics analysis and a virtual wind tunnel. This analysis enabled lift and drag force tables to be produced and based on these data a CX (thrust coefficient) polar graph for the sail was derived. Analysis of velocity magnitude and static pressure contours was also undertaken and the aerodynamic characteristics of the sail including airflow separation and wake size studied.

Analysis of laminar flow across a triangular periodic array of heated cylinders

The laminar flow and heat transfer across a triangular periodic array of heated cylinders are simulated computationally and analyzed. The study has been carried out at Reynolds number 10–100 for fluid volume fraction ranging from 0.7 to 0.99 and Prandtl number ranging from 0.7 to 50. The size of the wake region increases continuously with an increase in the Reynolds number for all values of fluid volume fraction. The recirculation bubble from the rear of a cylinder is reaching the front of the next cylinder in the same column of the periodic array for low values of free volume fraction, but this is not the case with the highest free volume fraction, i.e., 0.99. At high Reynolds number, the flow is separating early on the cylinder surfaces. The wake size at higher Reynolds number 75 and 100 for the lowest free volume fraction 0.7 is more in comparison with the wake size at free volume fraction 0.99, which is explained by plotting the location of flow separation against Reynolds number for both the extreme values of free volume fractions, i.e., 0.7 and 0.99. The isovorticity contours are concentrated in the vicinity of the cylinders on increasing the Reynolds number irrespective of free volume fraction and then convected downstream. On increasing free volume fraction, the friction and pressure drags in the array decrease. The increase in Reynolds number also results in the decrease in the values of the individual (friction and pressure drag coefficients) as well as total drag coefficients for all values of free volume fraction. At high values of Reynolds number, the emergence of carbuncle or thermal spike on isotherm near the cylinder’s surface is observed where the value of the local Nusselt number is observed low. The heat transfer improves and the Nusselt number increases as the Reynolds number and/or Prandtl number increases. On the contrary, heat transfer decreases as free volume fraction increases.

Effect of local waviness in confining walls and its amplitude on vortex shedding control of the flow past a circular cylinder

Numerical investigation of vortex shedding control and its suppression due to flow past a circular cylinder has been carried out starting from the Reynolds number 20 and upto the appearance of first instability in the wake. A technique “local waviness” is used in the confining walls for the effective control of shedding. The local waviness with crest near the cylinder wake eliminates vortex shedding with reduced drag (Deepakkumar et al., 2017). Effect of amplitude of local waviness on vortex shedding characteristics are systematically carried out and presented in this article. It is found that local waviness with amplitude A = 0.15D is an optimum value for effective vortex shedding suppression with reduced drag on the cylinder at Re = 200. As the amplitude of waviness increases, the wake size is reduced significantly as compared with plain wall confined cylinder. The presence of crest closer to the cylinder in the downstream generates a favorable pressure gradient and prevents flow separation at Reynolds number 20. The pressure drag is controlled significantly by the trough region in the waviness, whereas the crest region affects both pressure and frictional drag. Also, the appearance of first instability in the flow is identified for different amplitude of waviness.

Flow around a cube for Reynolds numbers between 500 and 55,000

The present work reports incompressible, viscous flow around a rigidly suspended stationary cube placed normal to the flow. The Reynolds number range covered in this work is more than two decades from Re ∈ (500, 55,000). Measurements are conducted using particle image velocimetry and dye visualization is undertaken for better understanding of the flow. The flow is seen to be structureless at higher Reynolds number. Mean and rms velocities at different axial locations in the wake are examined along with the mean vorticity field. Two peaks are observed in the Urms profiles at different axial locations in the wake. Vrms is single peaked at lower Re and has weak double peaks at higher Re. Behaviour of centerline velocity of the wake gives an insight about the recirculation length and shows its non-dependence on Re. Wake size has been determined and trajectory of the maximum vorticity in the wake is discussed. Drag coefficients are evaluated and compared with various correlations for sphere and cube. The modified wake survey method gives coefficients of drag between 0.63 and 0.89, which are close to the values for a cube given by Hölzer and Sommerfeld (2008) within the experimented Reynolds number range. Using the recirculation length, the data shows a dependence on Reynolds number till Re ∼ 1200 beyond which it becomes independent of Re. Overall several similarities in the flow behaviour with respect to sphere is noted from the results. The paper is the first detailed study on a cube over such a wide range of Reynolds number.