Drag Reduction Devices Research Articles

Fundamental study on the effect of Reynolds number on pipe frictional loss reduction by microbubbles

Microbubbles (MB) injection method is undeniably a promising drag reduction device in water transport. However, the mechanism of drag reduction by MB is complicated and poorly known. This paper presents the effect of Reynolds number on the frictional reduction by MB in a piping system. A fluid friction apparatus and a hydraulic bench were used to execute the experiment. To verify the ability of MB in reducing frictional loss, experiments were carried out in two conditions, without MB and with MB. In order to investigate the effect of turbulent flows, experiments were performed in three different flow rates ranging from 9 × 10−5 to 0.008 m3/s in two types of pipe diameter (20 mm and 33 mm) to yield different Reynolds number. As the results, MB able to reduce frictional loss in pipe up to 20%. However, regarding the Reynolds number effect, no apparent tendency of the frictional reduction efficiency can be observed between Reynolds number 4000-9500. Overall, MB holds potential in reducing pipe frictional loss in turbulent flow. However, further study in wider range of Reynolds number is required to comprehend its tendency.

Drag reduction on a three-dimensional blunt body with different rear cavities under cross-wind conditions

The use of rear cavities at the base of a square-back Ahmed body has been experimentally evaluated as a passive control device under cross-wind conditions with yaw angles β≤10°, by means of pressure, force and velocity measurements. A comparative study has been performed at a Reynolds number Re=105, considering the reference square-back body (i.e. the body without any passive control device), and the same body implementing both straight and curved cavities as add-on devices. It is shown that the performance of a straight cavity, which is widely acknowledged as a robust drag reduction device for car models, is hindered under moderate cross-wind conditions, and does not constitute an efficient control strategy, especially when compared with a curved cavity.In particular, when the freestream is aligned with the body, the curved cavity provides a stronger attenuation of the fluctuating nature and the bi-stable dynamics of the wake (characteristic of the wake behind a square-back Ahmed body) than the straight one. Besides, the reduced size of the near wake, which is provoked by flow re-orientation and the reduced span between the rear edges of the curved cavity, leads to an important base pressure recovery, that translates into relative reductions of the drag of 9.1% in comparison with the reference case (i.e. 2.6%, with respect to the straight cavity). The results are considerably improved under cross-wind conditions, since the increase with the yaw angle of the force is particularly intense for the body with the straight cavity and attenuated for the model with the curved cavity. Thus, the relative reduction of the drag coefficient with respect to the reference body becomes negligible for the straight device at a yaw angle of 10°, while it still represents approximately a 10% for the curved cavity. Furthermore, flow visualizations show that the wake is deflected as the incident flow is increasingly yawed, leading to the formation of a single leeward vortex core that approaches progressively the body, decreasing the base pressure. This phenomenon is minored when a curved cavity is implemented, increasing the low pressure induced at the body base.

Computational study of active flow control drag reduction device for utility vehicle

The study presents a computational study of a drag reduction device based on an active boundary layer control for a generic truck-trailer utility road vehicle. The conceptual device is in accordance with upcoming EU regulations regarding attachable aerodynamic devices for heavy utility vehicles. Design and principles of operation of the conceptual device are presented. The device is intended to increase decrease the trailer’s base drag coefficient by manipulation of the separated flow region behind the vehicle base. Results of a steady state Reynolds averaged analysis and Delayed Detached Eddy Simulation are presented to show the discrepancies of fluid flow patterns between baseline and augmented configuration as well as between mentioned CFD approaches. Results for drag reduction for baseline truck-trailer configuration and aerodynamically augmented configuration are presented.

Microfiber Coating for Drag Reduction by Flocking Technology

The biomimicry of using a hair-like structure is introduced as a drag reduction coating. The hair-like structure consists of an array of microfiber that is introduced as a passive drag reduction device. An effective flow control for a transition delay or a flow attachment is expected via an interaction or counteraction of flexible fibers, compared to the existing passive methods that use a solid or rigid surface device. The effect of the microfiber coating on drag reduction over a bluff-body was experimentally investigated using a circular cylinder in a wind tunnel at Reynolds number of 6.1 × 104. A drag reduction of 32% was obtained when the microfiber coating with a length of 0.012D was located at 40° from the stagnation point. Smoke flow visualization showed that flow separation delay was induced by the microfiber coating when the drag reduction occurred.

Slip Length–Based Boundary Condition for Modeling Drag Reduction Devices

Riblets are one of the most interesting passive drag reduction techniques. They essentially consist of streamwise grooved surfaces. Despite interest in these devices, the literature does not offer numerical simulations of riblets in the presence of pressure gradient or more generally around complex aeronautical configurations because, due to their scale length (microns), direct simulations are unfeasible. In the present paper an accurate and efficient method for the numerical simulation of riblets around complex aeronautical configurations is proposed. Riblets are modeled by a proper wall boundary condition based on the slip length concept. Slip length concept has been used by many authors in different fields; in the present paper a relation between the slip length and riblet height valid also in nonlinear range of drag reduction curve is proposed for the first time. This new boundary condition has been implemented in a standard code solving the Reynolds averaged Navier–Stokes equations. Validation tests in subsonic and transonic flows are performed comparing the results with the available experimental data. The analysis provides an interesting insight into riblets performance in the presence of the pressure gradient. Drag reduction improvement evidenced in some experiments is still debated because it seems to contradict the fact that riblet effect is confined very near to the surface (i.e., it is a local effect) and an explanation was never proposed until now. The present analysis provides a first contribution to understand why riblet performance can improve in the case of airfoil flow.

Comparison of Various Drag Reduction Devices and Their Aerodynamic Effects on the DrivAer Model

Comparison of Various Drag Reduction Devices and Their Aerodynamic Effects on the DrivAer Model

Low-Cost Base Drag Reduction Technique

A simple, low-cost, drag reduction device has been developed for applications in high-speed flows. This low-cost technology is expected to decrease fuel consumption (e.g., for high-speed vehicles such as rockets traveling from a highly over expanded flow at the sea level to a highly under expanded flow in outer space). Somewhere in between the over and the under expansion, the rocket experiences perfectly expanded flows. In this study, only over expanded and perfectly expanded flows have been considered. A single cylinder with a diameter of 2 mm is rotated clockwise inside the recirculation zone (e.g., in high-speed vehicles) to act as a controller. The base pressure in the dead zone and the wall pressure along the square duct length have been measured with and without control. Experiments were carried out for nozzle pressure ratios (NPRs) of 2, 3, 6 and 7.8. When the cylinder was rotated clockwise as an active controller, the base pressure was found to increase by as much as 56% in the perfectly expanded case and up to 17% in over expanded flows. This drastic increase in the base pressure is correlated to an equivalent drag reduction. In addition, adding an active control had no negative impact on the main flow field. This is important as any disturbance in the main flow field at high speeds may lead to increased oscillations and vibrations, which if not checked may cause material failures. Rotating the cylinder in the clockwise direction near the wall was found to be very effective for higher NPRs.

Substantial drag reduction of a tractor-trailer vehicle using gap fairings

Aerodynamic drag reduction of heavy vehicles is one of the main issues in fuel saving and environmental gas emission. In general, 50% of the total aerodynamic drag is induced from the region in the front surface of the vehicle and the gap between the tractor and trailer. Various forebody drag-reducing devices for tractor–trailer vehicles were introduced, however, conventional gap fairings or side extenders have technical limitations in reducing the aerodynamic drag exerting on the cab-roof space and gap between the tractor and trailer effectively. In this study, wind tunnel tests were conducted to investigate the drag reduction effects of the newly proposed gap fairings. The proposed gap fairing reduced drag by 16.4% at maximum as well as changed the gap length and deflecting angle. The aero cab fairing (ACF), which is a combination of cab-roof and gap fairings, and the extended aero cab fairing (EACF) considerably reduced the drag coefficient by approximately 11.1% and 17.5%, respectively. A particle image velocimetry was used to investigate flow characteristics, such as spatial distributions of mean velocity, vorticity, and turbulent kinetic energy around the scaled-down tractor–trailer model (1:17) with and without gap fairings to analyze the drag-reduction mechanism of the proposed gap fairings. The proposed gap fairings including ACF and EACF significantly reduced the mean velocity, vorticity in the gap region between the tractor and trailer, and the turbulent kinetic energy on the trailer's front surface. Results can provide useful information for improving the design of new additive flow control devices in reducing the aerodynamic drag of heavy vehicles.

Comparative investigation on the aerodynamic effects of combined use of underbody drag reduction devices applied to real sedan

To reduce the aerodynamic drag, the performance of the underbody aerodynamic drag reduction devices was evaluated based on the actual shape of a sedan-type vehicle. An undercover, under-fin, and side air dam were used as the underbody aerodynamic drag reduction devices. In addition, the effects of the interactions based on the combination of the aerodynamic drag reduction devices were investigated. A commercial sedan-type vehicle was selected as a reference model and its shape was modeled in detail. Aerodynamic drag was analyzed by computational fluid dynamics at a general driving speed on highway of 120 km/h. The undercover reduced the slipstream area through the attenuation of the longitudinal vortex pair by enhancing the up-wash of underflow, thereby reducing the aerodynamic drag by 8.4 %. The under-fin and side air dam showed no reduction in aerodynamic drag when they were solely attached to the actual complex shape of the underbody. Simple aggregation of the effects of aerodynamic drag reduction by the individual device did not provide the accurate performance of the combined aerodynamic drag reduction devices. An additional aerodynamic drag reduction of 2.1 % on average was obtained compared to the expected drag reduction, which was due to the synergy effect of the combination.

Drag-reducing underbody flow of a heavy vehicle with side skirts

Aerodynamic drag reduction in heavy vehicles is a very interesting research topic that is relevant for both the industries and the environment. The underbody flow induces considerable drag as it passes through the underside of heavy vehicles and interacts with rolling wheels and other underbody structures. Nonetheless, the drag caused by such an underbody flow has received less research attention, compared with those attributed to forebody and base body flows. Side skirts are one of the most effective drag-reduction devices to control the underbody flow. They consist of straight panels curtaining the space between the front and rear wheels. However, the mechanisms of underbody flow modified by side skirts have yet to be fully understood. In this study, the drag reduction underbody flow of a scaled-down 15-ton truck model attached with two different types of side skirts was quantitatively visualized through wind tunnel tests. Results show that the straight-type side skirt and the flap-type side skirt significantly change the flow structures under the vehicle model, reducing drag coefficient by 3.1 and 6.1 %, respectively. Furthermore, flow characteristics in the underbody of the vehicle model with and without side skirts are investigated using a PIV technique to understand the associated drag-reduction mechanism.

Experimental Study of Drag-Reduction Devices on a Flatback Airfoil

Various trailing-edge drag-reduction devices, including a new flap device, were examined experimentally on a flatback airfoil in a wind tunnel. The tests concerned a 30% thick airfoil with 10.6% thick trailing edge. Pressure, hot wire, and stereo particle image velocimetry measurements were performed at a chord Reynolds number of . Results show that the best-performing devices decrease drag, increase the vortex shedding frequency, and reduce flow variation downstream of the wing trailing edge. The best-performing device was a combination of the flap with an offset cavity plate. Further investigation is required for the optimization of the new device to examine its effects on noise reduction, load mitigation, and control.

Reduction of drag in heavy vehicles with two different types of advanced side skirts

Investigating the aerodynamic reduction of drag in heavy vehicles, such as trucks or tractor-trailers, has considerable significance given the strong influence on related industries. The underbody flow that passes through the underside of heavy vehicles induces considerable drag while interacting with rolling wheels and other structures. Nonetheless, the reduction of drag caused by underbody flow has received less attention than that attributed to upper and forebody flows. Side skirts are common underbody drag-reduction devices that consist of straight panels curtaining the underspace between the front and rear wheels to control the underbody flow in the ground clearance. In this study, we propose two different types of side skirts with flaps or additional inclined inner panels to maximize drag reduction. Effects of these devices are quantitatively evaluated by wind tunnel tests and computational fluid dynamics analysis. In wind tunnel tests with 1/8 scaled-down vehicle models, drag coefficient is reduced by more than 5% for both side skirts. Effects of various physical dimensions or angle variations on drag reduction are determined. Large-eddy simulation (LES) estimated similar drag reduction with reduced vortical activities, loss of streamwise momentum, strength of turbulent kinetic energy and global pressure difference, compared to the case without side skirts.

Performance Assessment of a Transonic Wing–Body Configuration with Riblets Installed

The effectiveness of the riblets, one of the most interesting drag-reduction device, is discussed in this paper. Numerical simulations by the Reynolds-averaged Navier–Stokes equations with the riblets properly taken into account are presented. Riblets are modeled as a singular roughness problem by modifying the classical Wilcox boundary condition for rough walls. The boundary condition is able to predict the flow features in the low roughness range (transitional roughness) where riblets operate. A brief discussion of the simulations performed to validate the model is first presented. Then, a complex wing–body configuration is analyzed, and the overall effect of riblets on the aerodynamic coefficients is evaluated. Calculations of a complete aircraft configuration at transonic conditions show how a proper optimized choice of the riblet height can significantly improve the drag reduction.

Impact of Profile Modifications and Rear View Mirror on Aerodynamics and Fuel Economy of Goods Carrier

This paper focuses on the improvements to the truck trailer aerodynamics this can be achieved by the aerodynamic drag reduction devices. The Devices are used on the truck and trailer undergone aerodynamic analysis, CFD evaluation, fuel economy assessment, these modifications provide combined fuel saving 15% at an average velocity of 30 m/s, the improvement in fuel economy correlates the equivalent drag reduction of 21%. CFD analysis shows that addition of these modification on truck trailer has no negative impact on the stability, the effect of exterior rear view mirror on the truck trailer aerodynamic drag is analysed. Result shows that modification intruck trailer geometry can reduce the drag and fuel consumption.

Aerodynamic Drag Reduction for a Generic Truck Using Geometrically Optimized Rear Cabin Bumps

The continuous surge in gas prices has raised major concerns about vehicle fuel efficiency, and drag reduction devices offer a promising strategy. In this paper, we investigate the mechanisms by which geometrically optimized bumps, placed on the rear end of the cabin roof of a generic truck, reduce aerodynamic drag. The incorporation of these devices requires proper choices of the size, location, and overall geometry. In the following analysis we identify these factors using a novel methodology. The numerical technique combines automatic modeling of the add-ons, computational fluid dynamics and optimization using orthogonal arrays, and probabilistic restarts. Numerical results showed reduction in aerodynamic drag between 6% and 10%.

Analyzing Fuel Savings of an Aerodynamic Drag Reduction Device with the Aid of a Robust Linear Least Squares Method

Analyzing Fuel Savings of an Aerodynamic Drag Reduction Device with the Aid of a Robust Linear Least Squares Method

Performance of an Automotive Under-Body Diffuser Applied to a Sedan and a Wagon Vehicle

Reducing resistance forces all over the vehicle is the most sustainable way to reduce fuel consumption. Aerodynamic drag is the dominating resistance force at highway speeds, and the power required to overcome this force increases by the power three of speed. The exterior body and especially the under-body and rear-end geometry of a passenger car are significant contributors to the overall aerodynamic drag. To reduce the aerodynamic drag it is of great importance to have a good pressure recovery at the rear. Since pressure drag is the dominating aerodynamic drag force for a passenger vehicle, the drag force will be a measure of the difference between the pressure in front and at the rear. There is high stagnation pressure at the front which requires a base pressure as high as possible. The pressure will recover from the sides by a taper angle, from the top by the rear wind screen, and from the bottom, by a diffuser. It is not necessarily the case that an optimized lower part of the rear end for a wagon-type car has the same performance as for a sedan or hatch-back car. This study focused on the function of an under-body diffuser applied to a sedan and wagon car. The diffuser geometry was chosen from a feasibility stand-point of a production vehicle such as a passenger car. The fluid dynamic function and theory of the automotive under-body diffuser working as a drag reduction device is discussed. The flow physics of the under-body and the wake was analyzed to understand the diffuser behaviour in its application to lift and drag forces on a vehicle in ground proximity. This work is mainly a numerical analysis that uses the traditional CFD approach from the automotive industry. Results from this study show a potential to reduce aerodynamic drag of the sedan car approximately 10%, and the wagon car by 2-3 %. The possible gain was much bigger for the sedan vehicle and the optimum occurs at a higher diffuser angle. This was most likely due to the fact that the sedan car in its original shape produced more lift force than the wagon, a wagon usually produces very little lift or even down-force. Lift forces were also reduced with the use of under-body covers with diffuser. The down-force increased, or lift force decreased, linearly with increased diffuser angle, and the trend was the same for both sedan and wagon rear ends. Flow analysis of the wake showed the importance of how the wake is balanced.

Experimental investigation of aerodynamic flow over a bluff body in ground proximity with drag reduction devices

An experimental investigation of several drag reduction devices installed in the rear of a bluff body with square back geometry in ground proximity has been conducted. The experiments include drag, base pressure measurements and velocity measurements using particle image velocimetry (PIV). The results of the drag and base pressure measurements show that significant reductions of the total aerodynamic drag (as high as 48%) can be achieved. The results also indicated that models with base cavity have lower drag than their counterparts without it. The base pressure distributions showed a strong effect of the ground, resulting in decrease of pressure towards the lower half of the base. The devices were found to have a strong effect on the underbody flow. For the drag reduction devices a rapid upward deflection of the underbody flow in the near wake was observed. The devices were also found to reduce the turbulent intensities in the near wake region.

The effect of a wake-mounted splitter plate on the flow around a surface-mounted finite-height circular cylinder

The influence of a wake-mounted splitter plate on the flow around a surface-mounted circular cylinder of finite height was investigated experimentally using a low-speed wind tunnel. The experiments were conducted at a Reynolds number of Re=7.4×104 for cylinder aspect ratios of AR=9, 7, 5 and 3. The thickness of the boundary layer on the ground plane relative to the cylinder diameter was δ/D=1.5. The splitter plates were mounted on the wake centreline with negligible gap between the base of the cylinder and the leading edge of the plate. The lengths of the splitter plates, relative to the cylinder diameter, ranged from L/D=1 to 7, and the plate height was always equal to the cylinder height. Measurements of the mean drag force coefficient were obtained with a force balance, and measurements of the vortex shedding frequency were obtained with a single-component hot-wire probe situated in the wake of the cylinder–plate combination. Compared to the well-studied case involving an infinite circular cylinder, the splitter plate was found to be a less effective drag-reduction device for finite circular cylinders. Significant reduction in the mean drag coefficient was realized only for the finite circular cylinder of AR=9 with intermediate-length splitter plates of L/D=1–3. The mean drag coefficients of the other cylinders were almost unchanged. In terms of its effect on vortex shedding, a splitter plate of sufficient length was able to suppress Kármán vortex shedding for all of the finite circular cylinders tested. For AR=9, vortex shedding suppression occurred for L/D≥5, which is similar to the case of the infinite circular cylinder. For the smaller-aspect-ratio cylinders, however, the splitter plate was more effective than what occurs for the infinite circular cylinder: for AR=3, vortex shedding suppression occurred for all of the splitter plates tested (L/D≥1); for AR=5 and 7, vortex shedding suppression occurred for L/D≥1.5.

Unsteady Aerodynamic Flow Investigation Around a Simplified Square-Back Road Vehicle With Drag Reduction Devices

The unsteady flow around a simplified road vehicle model with and without drag reduction devices is investigated. The simulations are carried out using the unsteady RANS in conjunction with the v2-f turbulence model. The corresponding experiments are performed in a small wind tunnel which includes pressure and velocity fields measurements. The devices are add-on geometry parts (a box with a cavity and, boat-tail without a cavity) which are attached to the back of the square-back model to improve the pressure recovery and reduce the flow unsteadiness. The results show that the recirculation regions at the base are shortened and weakened and the base pressure is significantly increased by the devices which lead to lower drag coefficients (up to 30% reduction in drag). Also, the results indicate a reduction of the turbulence intensities in the wake as well as a rapid upward deflection of the underbody flow with the devices in place. A reduction of the unsteadiness is the common element of the devices studied. The baseline configuration (square-back) exhibits strong three-dimensional flapping of the wake. The main shedding frequency captured agrees well with the available experimental data. Comparisons with the measurements show that the simulations agree reasonably well with the experiments in terms of drag and the flow structures. Finally, a blowing system (Coanda jet) is investigated numerically. In this case a drag reduction of up to 50% is realized.