Reduction Of Drag Force Research Articles

Effect of near-wake jet on the lock-in of a freely vibrating square cylinder

We present a numerical study of a freely vibrating square cylinder with steady bleeding at its base side. In particular, we focus on the suppression of vortex-induced vibration (VIV) and the reduction of drag force for the elastically mounted square cylinder at a laminar flow condition via near-wake jet. We examine the base bleeding mechanism in the near-wake region of a square cylinder and its influence over the flow dynamics and the wake characteristics for both stationary (nonlock-in) and freely vibrating (lock-in) conditions. We consider the near-wake jet parameter as a function of the bleed coefficient (Cq), which is the ratio of near-wake jet flow velocity to the freestream velocity and depends on the Reynolds number (Re) based on the diameter of the cylinder. Investigations of the hydrodynamic coefficients and the flow features are carried out for the laminar Re range, namely, Re = 40, 60, 100, and 150. A single dominant frequency peak is observed in the lift coefficient spectrum plot for all the Reynolds numbers considered, but two peaks are observed for Re = 150 at Cq = 0.175 and 0.2. Higher Cq values behave like a splitter plate thereby preventing the interaction of alternating shear layers. The variation of the mean drag is associated with the pressure distribution around the cylinder surface and along the streamwise locations. This leads to a thinner wake width, weaker vortices, and higher vortex shedding frequency as observed earlier in the literature. The sharp spikes of pressure coefficient at the base side of the cylinder are observed for Re ∈ [40, 150] due to the near-wake jet, accounting for the fluctuations of drag force coefficient. We demonstrate the formation of multiple vortices at the wake region due to the near-wake jet from our detailed qualitative analysis. We observe counter-rotating pair of recirculating fluids flanking at the near-wake jet location and examine the recovery of base pressure due to the jet flow. We demonstrate the splitting of big circulation fluid bubble into many smaller counter-rotating fluids due to high-velocity jet flow, resulting into the stabilization of flow profiles in the wake region. We extend this investigation to quantify the effect of the near-wake jet on the two-degree-of-freedom cylinder system at the representative Reynolds number Re = 100 and three mass ratios m* = 1, 2, and 3. We demonstrate the reduction of peak transverse VIV amplitude by 90% in comparison to the plain cylinder counterpart.

Theoretical and experimental research on interfacial shear stress and interfacial friction factor of gas-liquid two-phase wavy stratified flow in horizontal pipe

The external flow field over wave interface is reached by the aid of conformal transformation thought, and velocity boundary layer of inner flow field is deduced also, which gives fundamental description of shear stress over wave interface. The viscous drag coefficient is induced with considering the characteristics of interface wave, such as wave height, wave length and ratio between wave lengths of both ascending and descending semi periods, which significantly impact the velocity gradient over the wave interface that accounts for the differences in distribution of local shear stress. The influence of fluid flow is studied also, which gives support that the earlier separating point of the fluid flow over wave interface the stronger depression to the turbulent perturbation is made, and a reduced drag force arrived. There are otherness and commonness between fluid flow over wave interface and interface appearing in air-water wave stratified flow, and a new model is proposed to intrinsically construct the interfacial friction factor encountered in wave stratified flow. The models for viscous drag coefficient and interfacial friction factor are tested, different models are studied, predicted values are compared with experimental data and that computed by solving there-dimension unsteady Navier-Stokes equations, which gives proof that the values predicted by newly proposed models well fit the real values.

Drag force reduction by attached wind deflector on the container of a truck

Drag force reduction by attached wind deflector on the container of a truck

Plants face the flow in V formation: A study of plant patch alignment in streams

AbstractInteractions between biological and physical processes, so‐called bio‐physical feedbacks, are important for landscape evolution. While these feedbacks have been quantified for isolated patches of vegetation in aquatic ecosystems, we still lack knowledge of how the location of one patch affects the occurrence of others. To test for patterns in the spatial distribution of vegetation patches in streams, we first measured the distance between Callitriche platycarpa patches using aerial images. Then, we measured the effects of varying patch separation distance on flow velocity, turbulence, and drag on plants in a field manipulation experiment. Lastly, we investigated whether these patterns of patch alignment developed over time following locations of reduced hydrodynamic forces, using 2‐yr field observations of the temporal patch dynamics of Ranunculus penicillatus in a lowland chalk stream. Our results suggest that vegetation patches in streams organize themselves in V‐like shapes to reduce drag forces, creating an optimal configuration that decreases hydrodynamic forces and may therefore encourage patch growth. Downstream patches are more frequently found at the rear and slightly overlapping the upstream patch, in locations that are partially sheltered by the established upstream vegetation while ensuring exposure to incoming flow (important for nutrient availability). Observations of macrophyte patch dynamics over time indicated that neighboring patches tend to grow in a slightly angled line, producing a spatial pattern resembling the V‐formation in migratory birds. These findings point to the general role of bio‐physical interactions in shaping how organisms align themselves spatially to aerodynamic and hydrodynamic flows at a range of scales.

Modelling and simulation of aerodynamic performance of Vortex generators for hatch back type cars

A considerable amount is being invested by the vehicle developers in reducing the power needed for propulsion. The vehicle is optimized such to reduce the aerodynamic drag as the drag force is the leading resistance to the motion of vehicle. The flow separation at the rear end of the vehicle is one of the major causes of this drag. This paper investigates the effect of the delta shaped aerodynamic devices, known as vortex generators, installed at the roof of a hatchback type car to delay the flow separation. Mostly vortex generators are used in aircrafts and faster road vehicles with a notch at rear like the sedans. This experiment, unlike the previous researches, investigates and compares the aerodynamic drag and lift of the hatch back vehicle with and without Vortex Generator. Here it is represented that the improvement in aerodynamic drag force reduction, although of very minimal amount, can be achieved by using VGs on hatch back type cars.

Aerodynamic Design of a Solar Road Vehicle

Solar road vehicles have very specific design requirements. This makes their aerodynamic characteristics quite different from classic sedan vehicles. In the present study, the computational model of a typical solar road vehicle was developed to investigate its aerodynamic forces and flow characteristics. Computations were performed assuming the steady viscous flow and using the Reynolds-averaged Navier Stokes equations along with the k-ω turbulence model. The obtained results indicate some important findings that are commonly not present for classic sedan vehicles. In particular, a contribution of the viscous drag force to the overall drag force is considerably larger (41 %) than it is the case for the standard passenger road vehicles, where the form drag force dominates over the viscous drag force. Surface pressure distribution patterns indicate a favorable aerodynamic design of this vehicle. In particular, larger pressure coefficients on the top of the vehicle body as compared to the bottom surface contribute to increasing a downforce and thus the vehicle traction. The airfoil-shaped crosssection of the designed cockpit canopy has favorable properties with respect to reduction of the aerodynamic drag force.

Reduction of Aerodynamic Drag Force for Reducing Fuel Consumption in Road Vehicle using Basebleed

Reduction of Aerodynamic Drag Force for Reducing Fuel Consumption in Road Vehicle using Basebleed

Reduksi Gaya Drag Silinder Sirkular dengan Penambahan Square Disturbance Body Melalui Simulasi Numerik 2D Unsteady-RANS pada Reynolds Number 34800

Circular cylinder has a strong adverse pressure gradient. When a fluid flows around the circular cylinder, it will produce aerodynamics force. One of them is the drag force which strong enough. Hence, control of flow need to reduce the drag force with placed a disturbance body on the upstream side. The purpose of this study is to compare and complement the experimental research of drag reduction that has been done before. The aerodynamics characteristics of the disturbance body and cylinder are modelled in two dimensions Unsteady-RANS by using fluent with turbulent flow. It is found that the mean drag and the lift fluctuation of the cylinder can be reduced by the upstream disturbance body with the s/D = 0.107 (here s and D are side length of the disturbance body and the diameter of cylinder, respectively). The position of the disturbance body is varied at (α) 20o, 30o, 40o, 50o and 60o with a gap distance (δ = 0.4mm). Reynolds number based on cylinder diameter ReD = 3.48x10 4 . Fluid interaction between circular cylinder with two disturbance body can increase boundary layer transition from laminer to turbulent to produce small drag. It is found that the characteristics of the flow significantly depend on the position of disturbance body. The optimum condition for the drag force reduction is at the angle α = 30o about 53 %.

Numerical investigation of transitions in flow states and variation in aerodynamic forces for flow around square cylinders arranged inline

This study focuses on the transitions in flow states around two-, three- and four-inline square cylinders under the effect of Reynolds numbers at two different gap spacing values using the lattice Boltzmann method. For this purpose, Reynolds number is varied in the range 1–130 while two different values of spacing taken into account are gap spacing = 2 and 5. Before going to actual problem, the code is tested for flow around a single square cylinder by comparing the results with experimental and numerical results of other researchers, and good agreement is found. The current numerical computations yield that for both spacing values and all combinations of cylinders there exist three different sates of flow depending on Reynolds numbers: steady state, transitional state and unsteady state. It is found that the range of Reynolds numbers for these flow states is different for both spacing values. At gap spacing = 2 the range of Reynolds numbers for each flow state decreases by increasing the number of cylinders while at gap spacing = 5 opposite trend is observed. The results also show that at gap spacing = 2 the reduction in drag force is greater than the corresponding reduction at gap spacing = 5. The maximum reduction in drag force is observed at Reynolds numbers = 1 at both spacing values. Similarly, at both spacing values and all Reynolds numbers, the maximum reduction in drag force is observed for the case of four-inline square cylinders.

Passive control of vortex-induced vibration by spanwise grooves

The objective of this numerical study is to investigate the effect of spanwise grooves on the suppression of vortex-induced vibration (VIV) and the reduction of drag force. For this purpose, we consider a standard configuration of the elastically mounted circular cylinder, which is free to vibrate in both streamwise and transverse directions with identical natural frequency. We introduce a novel staggered groove configuration whose geometry is especially designed by offsetting the cross-sectional portion of the cylinder continuously along the spanwise direction. We assess the characteristic VIV responses of the proposed staggered groove configuration against the helical surface grooves for the identical dimensions and physical conditions. The staggered and helical groove configurations differ only in their arrangement of cross-section geometry along the spanwise direction. Three-dimensional coupled fluid–structure simulations are conducted at low mass and damping values with moderate Reynolds number of Re=4800. The effective width and the depth of surface grooves are determined to characterize the size effects for the assessment of staggered and helical groove configurations. Results show that the staggered groove configuration is effective in suppressing VIV, wherein the net reductions of 37% in the peak transverse amplitude and about 25% in the mean drag coefficient are observed in comparison to the plain cylinder counterpart. Staggered groove configuration produces three dominant effects by introducing a continuous jump in the cross-sectional geometry along the spanwise direction: (i) reduction of the spanwise correlation, (ii) enhancement of the three-dimensional effects in the near-wake flow, and (iii) broadening of the frequency spectra of fluid forces. As a result of these physical effects, the transfer of energy from the surrounding fluid flow to the vibrating grooved-cylinder system is reduced as compared to its plain cylinder counterpart. Owing to the simplicity of mechanical design and the ease of installation, the proposed passive control concept has a potential application to deepwater marine risers and tall structures in a wind environment.

Wavelet analysis on the turbulent flow structure of a T-junction

Experimental measurements in a T-junction with one inlet and two outlets mimicking the airflow in a high-speed train ventilation system were carried out using well-resolved Hot-Wire Anemometry (HWA). Continuous wavelet transform (CWT) is used to analyze the time-frequency contents of the instantaneous streamwise velocity; and the discrete wavelet transform (DWT) is employed to determine the multi-resolution energy characteristics. The measurements and analysis are carried out at three representative streamwise locations, i.e., upstream, mid-center, and downstream of the T-junction. The results show that the normalized time-average velocity at the mid-center of the T-junction is the largest near the wall region. Comparing the CWT data in the region near the wall, it is found that the dominant frequency of the periodic high energy coherent structures increases along the streamwise direction, and the wavelet energy magnitude at mid-center of T-junction decreases with the increase of velocity ratio. The DWT results show that apparent wavelet energy peak appears at the upstream and downstream of the T-junction for different scales from 26 to 210, but not at the mid-center. However, the energy at scale 211 abruptly rises in all flow regions at all the three streamwise locations and this energy decreases with the increase of the velocity ratio. Therefore, a higher velocity ratio is preferred for suppressing the generation of large-scale coherent structures to reduce drag forces and skin frictions for high-speed trains.

Scalable superhydrophobic coating with controllable wettability and investigations of its drag reduction

Superhydrophobic surface (SHS) has always been considered to be favorable for reducing drag force due to the air layer in surface asperities, however, debates over whether such surface could reduce the drag force still remains. In this study, steel surface was sprayed with suspension that contained PMMA and hydrophobic nanoscale silica, and the obtained surface showed variable contact angle from 94° to 159°, sliding angle from 90° to 2°, suggesting the controllable wettability; besides, the facile preparation process showed promising prospect to be scaled up. We studied the drag-reducing property of treated steel via sailing test and rotary disc test, results showed that SHS could efficiently reduce the drag force under low fluidic condition; surprisingly, for the intense turbulent flow, the SHS exhibited a drag-increasing effect, indicating that such surface only functions under mild flow condition.

Measurement of frictional drag force on superhydrophobic metallic surface

Material surfaces can be modified to become superhydrophobic using different techniques. Test equipment is necessary to assess the effectiveness of a superhydrophobic surface against frictional drag force. Because the frictional drag force is very small, especially for a sample size to be tested at a laboratory level, a traditional water channel system is not suitable. As a result, a new test setup was designed and fabricated to measure the reduction in the skin frictional drag force on such a superhydrophobic metallic surface. Two different types of sensors were considered. The first one was a spring-based displacement sensor and the other was a spring-less displacement sensor. The developed test setup can have the flow speed with a range of Reynolds numbers up to 70,000 with respect to the test specimen. In this range, the change in the frictional drag force was measured for superhydrophobic surfaces. A microscope was also used to check any change in the air film on the superhydrophobic surface as the flow speed was increased.

Suppression of vortex-induced vibrations using flexible shrouding—An experimental study

A number of offshore compliant structures such as risers, mono-columns, tension leg platforms (TLPs), spars, etc., which are deployed in the offshore field are subjected to vortex-induced vibration (VIV). The harsh environmental conditions often lead to failure of these structures well before their design life period resulting in fatal accidents. To minimize VIV, several investigations were conducted by other researchers. The present study focuses on the experimental investigations of an elastically mounted circular cylinder shrouded with net substructure called as Ventilated Net (VN) to suppress VIV. The VN is an omnidirectional, economical, customizablenet substructure comprising of flexible hollow tubes in a systematic arrangement. This device could be retrofitted to the offshore structures/risers, to attenuate VIV, thereby preventing the structure from fatigue failure. Experimental investigations are conducted to address the reduction of VIV and drag forces acting on the oscillating cylinder shrouded with VN of various configurations. The experiments are performed in towing tank facility in Department of Ocean Engineering, IIT Madras, India. The primary objective of the present investigation is to quantify the VIV amplitudes by employing VN shrouding the cylinder, at high Re regime, ranging from (0.22–2.50) × 105. The effect of spaces between the flexible hollow tubes and shrouding radii of VN around the cylinder are also addressed in this article. From the measurements, it is observed that cylinder with VN of dense mesh at a radial spacing of twice the diameter of the bare cylinder, suppresses VIV by 98% and drag force by 40% at Re of 1.2 × 105.

Mechanism of Electromagnetic Flow Control Enhanced by Electro-Discharge in Water**Supported by the Specialized Research Fund of the Transient Physics Laboratory under Grant No KX21373.

Pulsed discharge utilized to achieve large current density in the electromagnetic flow control is numerically studied. A mathematic discharge model is established to calculate the plasma channel, and an actuator is designed to generate the Lorentz force in the micro plasma channel. During the discharge process, the resistance in the channel decreases rapidly and a large current density appears between the discharge electrodes. After the actuator is applied in the leading edge of a flat plate, the separation region and downstream turbulent boundary layer on the plate disappear. Meanwhile, a skin-friction drag force reduction is achieved.

Control rod drop-time reduction in typical swimming pool research reactors

Control rods play an important role in the safety of nuclear reactors, due to the fact that in the emergency situations, they are to be inserted into the reactor core and scram the reactor secure and fast. As far as safety concerns, drop-time of control rods should be minimized as much as possible. In this paper as a case study, TRR (Tehran Research Reactor) data is chosen as a typical swimming pool reactor and drop-time of control safety rod is to be minimized. Present fork type control rods are more prone to entanglement than former tube type. Methods to ease this problem are considered. Several approaches are studied and proposed in this work aimed to decrease the drop-time of control rods especially in case of emergency. Changing control rod shape into a more aerodynamic form, helps to reduce drag force and thus decreasing drop-time in an aqueous medium. Further improvement is achieved by provision of several holes in the head section. As fluid dynamic computations suggest, drop-time may reduce as much as 6% due to geometry modification. Focusing on electronic part, improved design on magnet (Coil and its core together) could reduce the lag time between scram signal issuance till control rod release. Using magnet with modified air gaps showed that existing drop-time from about 800 ms, could be reduced by as much as 34%. Moreover, by using amorphous core instead of existing one, drop-time is expected to reduce from 800 ms by as much as 30%.Measurements showed that existing electromechanical circuitry of relays, has an inherent 3 ms delay which is not so much compared to 800 ms total scram time. Therefore, working on this part is not a big deal with respect to other modifications and therefore is disregarded.

MYBP-C Phosphorylation Accelerates Sarcomere Shortening as Thin Filaments Slide through the C-Zone

Myosin Binding Protein-C (MyBP-C) is a 140-150 kDa thick filament protein located in the middle third of each sarcomere, i.e., the C-zone, and is likely a key regulator of contraction. Phosphorylation of MyBP-C occurs at 4 residues within its N-terminus and PKA phosphorylation of MyBP-C in hearts is associated with faster cardiac muscle twitch dynamics (Tong et. al. Circ. Res. 2008). Here we investigated potential mechanisms by which MyBP-C phosphorylation regulates myofilament function. We utilized slow-twitch skeletal muscle fibers since PKA phosphorylates slow skeletal (ss)MyBP-C but not (ss)troponin I. Permeabilized rat slow-twitch fibers were mounted between a force transducer and motor, and calcium activated to elicit ∼30% maximal force. We measured rates of force development and transient force overshoots following a slack re-stretch maneuver and monitored sarcomere shortening during load-clamps. Following PKA, force development rates increased ∼40% and there was a near-doubling in the magnitude of transient force overshoot. We also observed faster shortening velocities at all loads following PKA treatment; this may arise from (i) faster cross-bridge cycling rates, (ii) greater number of cross-bridges during load clamps, and/or (iii) attenuation of internal drag imposed by binding of MyBP-C to actin. To address these mechanisms, we quantified sarcomere length shortening as thin filaments traversed into the C-zone. Before PKA, there tended to be a slowdown as sarcomeres shortened from ∼3.05 to 2.90 µm. After PKA, sarcomeres shortened a considerably greater distance during similar load clamps and, at times, shortening even accelerated as thin filaments slid through the C-zone. Overall these results suggest MyBP-C phosphorylation speeds sarcomere shortening and power output by a combination of faster cross-bridge cycling kinetics, recruitment of cross-bridges, and reduction in drag forces as thin filaments slide through the C-zone.

Mathematical Modeling of Drag Coefficient Reduction in Circular Cylinder Using Two Passive Controls at Re = 1000

One of the many ways to reduce drag force is by adding a small object called passive control. Two passive controls will be placed in front and at the rear of the main object. The main object used is a circular cylinder, and the passive controls used are the Type-I cylinder in the front and an elliptical cylinder at the rear of the main object. The distances between the main object and the passive controls are varied. The Reynolds number used is 1000. The effective distance between the main object and the passive controls is analyzed by using the mathematical model so that the drag coefficient on the main object is compared with the simulated one.

Ecologically Driven Ultrastructural and Hydrodynamic Designs in Stomatopod Cuticles.

Ecological pressures and varied feeding behaviors in a multitude of organisms have necessitated the drive for adaptation. One such change is seen in the feeding appendages of stomatopods, a group of highly predatory marine crustaceans. Stomatopods include "spearers," who ambush and snare soft bodied prey, and "smashers," who bludgeon hard-shelled prey with a heavily mineralized club. The regional substructural complexity of the stomatopod dactyl club from the smashing predator Odontodactylus scyllarus represents a model system in the study of impact tolerant biominerals. The club consists of a highly mineralized impact region, a characteristic Bouligand architecture (common to arthropods), and a unique section of the club, the striated region, composed of highly aligned sheets of mineralized fibers. Detailed ultrastructural investigations of the striated region within O. scyllarus and a related species of spearing stomatopod, Lysiosquillina maculate show consistent organization of mineral and organic, but distinct differences in macro-scale architecture. Evidence is provided for the function and substructural exaptation of the striated region, which facilitated redeployment of a raptorial feeding appendage as a biological hammer. Moreover, given the need to accelerate underwater and "grab" or "smash" their prey, the spearer and smasher appendages are specifically designed with a significantly reduced drag force.

Numerical study of flow characteristics on wing airfoil eppler 562 with whitcomb winglet variations

Winglet is a tool used to improve the efficiency of aircraft and UAV performance by preventing fluid flow jump from lower surface to upper surface at wingtip. The addition of this winglet resulted in improved lift and reduction of drag force from the aircraft wing or UAV. From Whitcomb's research, it was found that the use of winglet on a full size airplane can increase fuel efficiency by 7%. The research led to the idea of conducting research on fluid flow characteristics on the UAV wing with the Eppler 562 airfoil combined with the whitcomb winglet. This numerical study was conducted using the Computational Fluid Dynamics (CFD) method based on the advantages of using this simulation that can review the fluid flow in macroscopic way. This study is provide accurate fluid flow visualization results and can improve the performance of the wings when compared with wings without winglet (plain wing). Wing with the Eppler 562 airfoil combined with the whitcomb winglet results reduction in rotating motion that makes velocity components as opposed to lift.