Decrease In Drag Coefficient Research Articles

Volume-of-fluid simulations of bubble dynamics in a vertical Hele-Shaw cell

Bubbles in confined geometries serve an important role for industrial operations involving bubble-liquid interactions. However, high Reynolds number bubble dynamics in confined flows are still not well understood due to experimental challenges. In the present paper, combined experimental and numerical methods are used to provide a comprehensive insight into these dynamics. The bubble behaviour in a vertical Hele-Shaw cell is investigated experimentally with a fully wetting liquid for a variety of gap thicknesses. A numerical model is developed using the volume of fluid method coupled with a continuum surface force model and a wall friction model. The developed model successfully simulates the dynamics of a bubble under the present experimental conditions and shows good agreement between experimental and simulation results. It is found that with an increased spacing between the cell walls, the bubble shape changes from oblate ellipsoid and spherical-cap to more complicated shapes, while the bubble path changes from only rectilinear to a combination of oscillating and rectilinear; the bubble drag coefficient decreases and this results in a higher bubble velocity caused by a lower pressure exerted on the bubble; the wake boundary and wake length evolve gradually accompanied by vortex formation and shedding.

Determination of drag coefficient for TOYOTA car model (Using Strain Gauge Method)

The flow field around an automobile is very complex, characterized by a high degree of three dimensionality, flow separation, reattachment and vortex formation. Flow visualization as well as flow simulation are helpful tools during the aerodynamic design of vehicles. The research-undertaken deals with an experimental estimation of CD (drag coefficient) for TOYOTA car,(scale 1/20) using the strain gauge method ( FLA-6-11 type, 120Ω, 2.12 gauge factor, half-bridge connection), which was proved to be practical and reasonably accurate. Experiments were run within a subsonic aspiration wind tunnel, covering an air speed up to 33 m/s (i.e., Reynolds number 5.7 x 10 5 ). Results for drag coefficient were obtained in the range of 1.10 to 0.53. It was noted that the magnitude of CD decreased from 1.10 at 21.18 m/s to 0.53 at 33.00 m/s (i.e., decrease of drag coefficient by about 50%). Comparison of our results with those given by other authors is satisfactory.

U-shaped fairings suppress vortex-induced vibrations for cylinders in cross-flow

We employ three-dimensional direct and large-eddy numerical simulations of the vibrations and flow past cylinders fitted with free-to-rotate U-shaped fairings placed in a cross-flow at Reynolds number $100\leqslant \mathit{Re}\leqslant 10\,000$. Such fairings are nearly neutrally buoyant devices fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIVs). We consider three different geometric configurations: a homogeneous fairing, and two configurations (denoted A and AB) involving a gap between adjacent segments. For the latter two cases, we investigate the effect of the gap on the hydrodynamic force coefficients and the translational and rotational motions of the system. For all configurations, as the Reynolds number increases beyond 500, both the lift and drag coefficients decrease. Compared to a plain cylinder, a homogeneous fairing system (no gaps) can help reduce the drag force coefficient by 15 % for reduced velocity $U^{\ast }=4.65$, while a type A gap system can reduce the drag force coefficient by almost 50 % for reduced velocity $U^{\ast }=3.5,4.65,6$, and, correspondingly, the vibration response of the combined system, as well as the fairing rotation amplitude, are substantially reduced. For a homogeneous fairing, the cross-flow amplitude is reduced by about 80 %, whereas for fairings with a gap longer than half a cylinder diameter, VIVs are completely eliminated, resulting in additional reduction in the drag coefficient. We have related such VIV suppression or elimination to the features of the wake flow structure. We find that a gap causes the generation of strong streamwise vorticity in the gap region that interferes destructively with the vorticity generated by the fairings, hence disorganizing the formation of coherent spanwise cortical patterns. We provide visualization of the incoherent wake flow that leads to total elimination of the vibration and rotation of the fairing–cylinder system. Finally, we investigate the effect of the friction coefficient between cylinder and fairing. The effect overall is small, even when the friction coefficients of adjacent segments are different. In some cases the equilibrium positions of the fairings are rotated by a small angle on either side of the centreline, in a symmetry-breaking bifurcation, which depends strongly on Reynolds number.

Numerical Simulation of Two Different Flexible Bodies Immersed in Moving Flow**Supported by the National Natural Science Foundation of China under Grant Nos 51479007, 51309017, and 11102027, the Natural Science Foundation of Hubei Province under Grant No 2015CFA026, and the Fundamental Research Fund for State Public-Benefic Scientific Institutes of CRSRI under Grant No CKSF2015026/SL.

The coupled motion of two flexible bodies with different lengths immersed in moving fluid is studied numerically. The flapping frequency, flapping amplitude and average drag coefficient of each body are calculated and the influences of the arranging manner and separation distance are analyzed. In our simulation, when placed in the flow individually, the flexible body with a longer length will flap in period and the shorter one will maintain still straightly in the flow direction. The numerical results show that, two different flexible structures near placed in moving flow would strongly interact. When they are placed side by side, the existence of the stable shorter flexible body will restrain the flapping of the longer one while the existence of the longer flexible body may also induce the shorter one to flap synchronously. When placed in tandem with the shorter flexible body in upstream, the flapping of the longer one in downstream will be obviously enhanced. In the situation for the longer flexible body placed in upstream of the shorter one, the coupled flapping amplitude and average drag coefficients increase and decrease periodically with increasing the arranging space, and peak values appear as a result of the mediate of the tail wakes.

Numerical simulation of the effect of relative thickness on aerodynamic performance improvement of asymmetrical blunt trailing-edge modification

In this paper, the aerodynamic performance of wind turbine airfoils with different relative thicknesses and their modifications has been numerically investigated to facilitate a greater understanding of the effects of maximum relative thickness and its position on the aerodynamic performance improvement of asymmetrical blunt trailing-edge modification. The lift and drag coefficients of airfoil NACA4415 are calculated with the k-ω SST turbulence model, and are compared with experimental data to validate the simulation accuracy of the Computational Fluid Dynamics (CFD) approach. The airfoils with different relative thicknesses are modified to be asymmetrical blunt trailing-edge airfoils by means of the software Xfoil. The best trailing-edge thickness distribution ratio is obtained by comparing the aerodynamic performance of the modifications with different distribution ratios. The aerodynamic performance of original airfoils and their asymmetrical modifications with the best thickness distribution ratio being 1:3 is investigated to analyze the increments of lift and drag coefficients and lift-drag ratio. Results indicate that with the increasing of relative thickness, the lift coefficient increment of NACA4418 airfoil is the smallest for the angle of attack more than 9°, and the drag coefficient increment as a whole decreases first and then increases, but the average lift-drag ratio increment of NACA4412 airfoil is the largest, closely followed by NACA4415 airfoil. It is also showed that with the relative thickness position close to the leading-edge, the increments of lift and drag coefficients decrease and increase for the angle of attack more than a certain value, respectively, and the average lift-drag ratio increment of NACA4415 airfoil is positive and larger than those of NACA4415-mod25 and NACA4415-mod20 airfoils. Therefore, the medium thickness airfoil whose relative thickness position is away from the leading-edge is more suited to the asymmetrical blunt trailing-edge modification.

Scaling of Off-Equatorial Jets in Giant Planet Atmospheres

Abstract In the off-equatorial region of Jupiter’s and Saturn’s atmospheres, baroclinic eddies transport angular momentum out of retrograde and into prograde jets. In a statistically steady state, this angular momentum transfer by eddies must be balanced by dissipation, likely produced by magnetohydrodynamic (MHD) drag in the planetary interior. This paper examines systematically how an idealized representation of this drag in a general circulation model (GCM) of the upper atmosphere of giant planets modifies jet characteristics, the angular momentum budget, and the energy budget. In the GCM, Rayleigh drag at an artificial lower boundary (with mean pressure of 3 bar) is used as a simple representation of the MHD drag that the flow on giant planets experiences at depth. As the drag coefficient decreases, the eddy length scale and eddy kinetic energy increase, as they do in studies of two-dimensional turbulence. Off-equatorial jets become wider and stronger, with increased interjet spacing. Coherent vortices also become more prevalent. Generally, the jet width scales with the Rhines scale, which is of similar magnitude as the Rossby radius in the simulations. The jet strength increases primarily through strengthening of the barotropic component, which increases as the drag coefficient decreases because the overall kinetic energy dissipation remains roughly constant. The overall kinetic energy dissipation remains roughly constant presumably because it is controlled by baroclinic conversion of potential to kinetic energy in the upper troposphere, which is mainly determined by the differential solar radiation and is only weakly dependent on bottom drag and barotropic flow variations. For Jupiter and Saturn, these results suggest that the wider and stronger jets on Saturn may arise because the MHD drag on Saturn is weaker than on Jupiter, while the thermodynamic efficiencies of the atmospheres are not sensitive to the drag parameters.

Incompressible LBGK simulation of flow characteristics in a micro-channel with patterned superhydrophobic surfaces

Characteristics of flow in a micro-channel with patterned superhydrophobic surfaces were simulated by using an incompressible lattice Bhatnagar–Gross–Krook (LBGK) model. By the simulation, it can be seen that the depth-to-width ratio of the cavities between adjacent micro-ridges is an important effect parameter for the flow in the micro-channel. For the larger depth-to-width ratio, it is easy to keep the air in the cavities. Because of the influence of the air, there is no vortex phenomenon downstream of each micro-ridge. Such vortices are a common phenomenon in macro flows. The simulated results coincide with those of experiments. Moreover, keeping the air in the cavities, one can also find that the smaller the solid area fraction, the greater the slipping velocity at the air–water interface. With a decrease of the solid area fractions, the average exit velocity increases and the flow drag coefficient decreases.

Numerical Simulation of Forced Convective Heat Transfer Past a Square Diamond-Shaped Porous Cylinder

Fluid flow and heat transfer around and through a porous cylinder is an important issue in engineering applications. In this paper a numerical study is carried out for simulating the fluid flow and forced convection heat transfer around and through a square diamond-shaped porous cylinder. The flow is two-dimensional, steady, and laminar. Conservation laws of mass, momentum, and heat transport equations are applied in the clear region and Darcy–Brinkman–Forchheimer model for simulating the flow in the porous medium has been used. Equations with the relevant boundary conditions are numerically solved using a finite volume approach. In this study, Reynolds and Darcy numbers are varied within the ranges of $$1<Re<45$$ and $$10^{-6}<Da<10^{- 2}$$ , respectively. The porosity $$(\varepsilon )$$ is 0.5. This paper presents the effect of Reynolds and Darcy numbers on the flow structure and heat transfer characteristics. Finally, these parameters are compared among solid and porous cylinder. It was found that the drag coefficient decreases and flow separation from the cylinder is delayed with increasing Darcy number. Also the size of the thermal plume decreases by decreasing Darcy number.

Numerical Calculation of Effect of Elastic Deformation on Aerodynamic Characteristics of a Rocket

The application and workflow of Computational Fluid Dynamics (CFD)/Computational Structure Dynamics (CSD) on solving the static aeroelastic problem of a slender rocket are introduced. To predict static aeroelastic behavior accurately, two-way coupling and inertia relief methods are used to calculate the static deformations and aerodynamic characteristics of the deformed rocket. The aerodynamic coefficients of rigid rocket are computed firstly and compared with the experimental data, which verified the accuracy of CFD output. The results of the analysis for elastic rocket in the nonspinning and spinning states are compared with the rigid ones. The results highlight that the rocket deformation aspects are decided by the normal force distribution along the rocket length. Rocket deformation becomes larger with increasing the flight angle of attack. Drag and lift force coefficients decrease and pitching moment coefficients increase due to rocket deformations, center of pressure location forwards, and stability of the rockets decreases. Accordingly, the flight trajectory may be affected by the change of these aerodynamic coefficients and stability.

Experimental investigation on hydrodynamics of floating cylinder in oscillatory and steady flows by forced oscillation test

In this paper the hydrodynamic characteristics of a floating cylinder are investigated via forced oscillation experiments in towing tank. The effects of Keulegan–Carpenter number, Reynolds number, reduced velocity and overtopping on hydrodynamics of the floating cylinder in oscillatory and steady flow are studied. The results show a considerably difference of the hydrodynamic characters between the floating and the fully immerged cylinders due to the influences of free surface. The growth of the reduced velocity, a proven notable effect on hydrodynamics, will lead to the increase of added mass coefficient and the decrease of drag coefficient. Meanwhile the overtopping, a particular phenomenon for the floating cylinder, render the added mass coefficients reach up to 3.6 while for the drag coefficient small influences were made.

A Numerical and Experimental Study of the Aerodynamics and Stability of a Horizontal Parachute

The flow past a parachute with and without a vent hole at the top is studied both experimentally and numerically. The effects of Reynolds number and vent ratio on the flow behaviour as well as on the drag coefficient are examined. The experiments were carried out under free-flow conditions. In the numerical simulations, the flow was considered as unsteady and turbulent and was modelled using the standard - turbulence model. The experimental results reveal good agreement with the numerical ones. In both the experiments and numerical simulations, the Reynolds number was varied from 85539 to 357250 and the vent ratio was increased from zero to 20%. The results show that the drag coefficient decreases by increasing the Reynolds number for all the cases tested. In addition, it was found that at low and high Reynolds numbers, the parachutes, respectively, with 4% vent ratio and without vent are deemed more efficient. One important result of the present work is related to the effect of vent ratio on the stability of the parachute.

Investigation of the turbulent wake flow of generic launcher configurations via a zonal RANS/LES method

A numerical analysis of the turbulent wake flow of a generic space launcher at supersonic freestream conditions (Ma∞ = 6.0 and ReD = 1.7 × 106) is performed using a zonal RANS/LES method. To investigate the influence of various components of a rocket model on the base flow, three supported wind tunnel configurations with the same main body geometry and different aft-body extensions consisting of a blunt base, a nozzle dummy, and a full flowing underexpanded TIC nozzle (Mae = 2.52, pe/p∞ = 100) are considered. Flow topologies for three cases are described in detail including an estimate of the impact of the wind tunnel model support on the flow field. To validate the applied numerical method, the computed flow fields are compared to experimental data from high-speed schlieren measurements provided by DLR Cologne. The influence of the used aft-body extensions on the steady-state and dynamic base flow characteristics is evaluated by a detailed analysis and comparison of the pressure distribution and its spectra along the base and nozzle walls for three investigated configurations. The numerical findings are compared to experimental wall pressure oscillation measurements provided by DLR Cologne. The major results are the non-negligible influence of the model support on the wake even on the strut averted side, the base drag reduction effect of the aft-expanding jet plume consisting of an increase of the base pressure level from \(p/p_\infty\approx0.2-0.25\) (blunt base and nozzle dummy configurations) up to \(p/p_\infty\approx0.7\) leading to a decrease of the base pressure drag coefficient from CDp base = 0.032 to 0.012 correspondingly, and the identified dominant low-frequency modes of the base pressure oscillations at SrD ≈ 0.05, SrD ≈ 0.1, and SrD ≈ 0.2 also detected in the experiments.

Effect of Axis Ratio on Fluid Flow Around an Elliptic Cylinder—A Numerical Study

The bluff body simulations over canonical forms like circular and square cylinders are very well studied and the correlations for bulk parameters like mean drag coefficient and Strouhal numbers for the same are reported widely. In the case of elliptic cylinder, the literature is very sparse, especially for moderate Reynolds number (Re). Hence, in this work, a detailed study about fluid flow characteristics over an elliptic cylinder placed in a free stream is performed. Simulations are carried out for different Re ranging from 50 to 500 with axis ratio (AR) varied between 0.1 to 1.0 in steps of 0.1. Immersed boundary method is used for the solid boundary condition implementation which avoids the grid generation for each AR and a single Cartesian grid is used for all the simulations. The effect of AR for various Reynolds numbers is also focused on using the in-house code. The influence of AR is phenomenal for all the Re and the values of wake length, drag coefficient, and Strouhal number decrease with decreasing AR for a particular Re. The critical ARs, for vortex shedding and wake formation, are identified for various Re. Detailed correlations for wake length, critical ARs for vortex shedding and wake formation, mean drag coefficient and Strouhal number, in terms of AR, are reported in this work.

Aerodynamic Control of Low-Reynolds-Number Airfoil with Leading-Edge Protuberances

This paper presents an experimental investigation of the control of airfoil aerodynamics at a low Reynolds number of within a wide range of attack angle using a leading-edge-protuberance technique. The essence of the technique is to manipulate flow around the airfoil by replacing the straight leading edge of a baseline airfoil with a sinusoidal wavy airfoil. Whereas the lift and drag forces and the lift-to-drag ratio were measured using a three-component force balance, the flow was mainly measured using a particle-image velocimetry. The sinusoidal protuberances effectively suppressed airfoil stall, and the corresponding aerodynamic performance was impaired to some extent. Meanwhile, control significantly improved the airfoil aerodynamics in the poststall region, for example, , leading to a maximum 25.0 and 39.2% increase in lift coefficient and lift-to-drag ratio, respectively, and a maximum 20.0% decrease in drag coefficient. The protuberances may influence control performance in a similar way with low-profile vortex generators. The flow physics behind the observations were discussed in detail.

Propulsion Deceleration Studies Using Planar Laser-Induced Iodine Fluorescence and Computational Fluid Dynamics

Future high-mass spacecraft entering the thin Martian atmosphere will exceed the capabilities of supersonic and subsonic parachutes and will incorporate other means of deceleration. Propulsive deceleration is one technology that is being considered. The interaction of the spacecraft aerodynamics with the propulsion deceleration jets has been shown to cause a decrease in drag coefficient with increasing thrust coefficient, which is not desirable for deceleration. Planar laser-induced iodine fluorescence images for a single-propulsion deceleration jet showed a lifting of the vehicle bow shock away from the aeroshell. Flowfield calculations showed that this lifting provided a shielding effect, preventing the freestream mass and momentum flux from reaching the aeroshell surface, creating a low-pressure region between the jet boundary and the aeroshell. With four peripheral propulsive deceleration jets, planar laser-induced fluorescence images and computational fluid dynamics calculations showed that the vehicle bow shock is maintained between the jets as the thrust coefficient is increased. This bow shock is responsible for greater drag preservation with the peripheral jets. The calculations also showed that high pressure is maintained between the peripheral jets. These results suggest that using a few peripheral propulsion-deceleration jets located near the aeroshell shoulder would provide the greatest drag preservation when using propulsive deceleration.

Effects of underscour depth and horizontal spacing between two bed protection blocks on stability of frontal block

The stability of a river-bed protection block in a flood event is affected by underscouring of the block. The effects of block spacing on block stability were investigated in different underscour conditions. Three blocks were placed at the bottom in a flume, and the underscour depth of the frontal block and distance between the first and second blocks were changed. Drag and lift forces were measured directly in each experimental condition. Particle Image Velocimetry (PIV) was used to visualize the flow and evaluate the streamline pattern. Pressure measurements around the block to understand the pressure distribution show that the pressure distribution on the rear and bottom surfaces of the block is significantly affected by changes in block spacing and underscour depth. The experimental results show that for the same underscour gap, lift, and drag coefficients decreased with increasing space between the two blocks. In addition, if the underscour gap is increased with the same horizontal spacing, the lift coefficient increases and drag coefficient decreases. These results demonstrate that wider spacing enhance stability of the frontal block in the underscour condition. Moreover, analysis of block stability with changing underscouring and horizontal spacing shows that bed protection blocks seem to have more stability when the gap between two bed protection blocks was equal to height of the blocks.

Non-Newtonian power-law flow and heat transfer across a pair of side-by-side circular cylinders

Flow and heat transfer of non-Newtonian power-law fluids across a pair of identical circular cylinders in side-by-side arrangement are investigated numerically by solving the continuity, momentum and energy equations along with the appropriate boundary conditions. The numerical calculations are performed in an unconfined computational domain for the following range of physical parameters: Reynolds number, Re=1–40 and power-law index, n=0.4–1.8 (covering shear-thinning, n<1; Newtonian, n=1 and shear-thickening, n>1 behaviors) for gap ratio, T/D=1.5–4.0 at a constant Prandtl number of 50. The global characteristics such as drag coefficients and average Nusselt number, etc. are calculated and the representative streamline and isotherm contours are presented for the above range of conditions. It has been found that the individual and overall drag coefficients decrease and the average Nusselt number increases with Reynolds number for all T/D and n considered here. The heat transfer is found higher in shear-thinning fluids than Newtonian fluids and followed by shear-thickening fluids for 1.5⩽T/D⩽4.0 and 1⩽Re⩽40.

Test Study on Aerodynamic Performance of Three-Dimensional Stayed Cables in Two-Dimensional Flow Field

Cables, distributed with diversity spatially angle, are rich in wind direction. Circular cross sectional shape is sensitive to the Reynolds number. This paper studed the aerodynamic performance of three-dimentional stayed cable under two-dimensional flow field in Chang'an University wind tunnel by Pitot tube, electronic pressure scanning valve, data conversion and collection systems. The following conclusions are obtained: (1)The stagnation point and the upper and lower airflow separation point clockwise goes on with the wind angle increases. Absolute value of pressure coefficient at stagnation point and separation point are biggest at 0°wind angle.(2)The absolute value of pressure coefficient at upper and lower separation point increase with the increase of wind speed.(3) Pressure stagnation point,and upper and lower airflow separation point don’t changed with the parameters of helix,while pressures near helix changed irregularlly. Drag coefficient decrease with the increase of helix distance and the deduce of helix height. Meaningful references are provided for the further refinement research on aerodynamic performance of cable-stayed with suppressing vibration measures.

Effects of Sideslip on the Aerodynamics of Low-Aspect-Ratio Low-Reynolds-Number Wings

The growing interest in micro aerial vehicles has brought attention to the need for an improved understanding of the aerodynamics of low-aspect-ratio wings at lowReynolds numbers. In this study, flat plate wings with rectangular and tapered planforms were fabricated with aspect ratios of 0.75, 1, 1.5, and 3, and the aerodynamic loading was measured at Reynolds numbers between 5 10 and 1 10. Surface tuft visualization was used to observe the interactions between the tip vortices and the leading-edge vortex. The tests were initially conducted at a sideslip angle of 0 and were then repeated for 10, 20, and 35 with and without winglets. Measurements made with a sixcomponent force balance showed that a decrease in aspect ratio caused an increase in stall and CLmax due to the nonlinear lift induced by the interacting flow on the upper wing surface. In addition, the detachment of tip vortices after stall leads to a sudden decrease in drag coefficient as the magnitude of the induced drag drops significantly. At increasing sideslip angles, the effects of the crossflow still contribute to an increase in lift but significantly reduce the pitching moment about the quarter-chord, thus decreasing the wing’s ability to recover from angle-of-attack perturbations. These results show that, while the effects of tip vortices and the leading-edge vortex complicate the flowfield around a low-aspect-ratio wing, particularly at increased sideslip angles, their impact tends to improve the aerodynamic performance.

Numerical simulation of planar shear flow passing a rotating cylinder at low Reynolds numbers

Two-dimensional shear flow over a rotating circular cylinder is investigated using lattice Boltzmann method. Simulations are performed at a fixed blockage ratio (B = 0.1) while the Reynolds number, nondimensional shear rate (K) and absolute rotational speed range as 80 ≤ Re ≤ 180, 0 ≤ K ≤ 0.2 and −2 ≤ β ≤ 2, respectively. To verify the simulation, the results are compared to previous experimental and numerical data. Quantitative information about the flow variables such as drag and lift coefficients, pressure coefficient and vorticity distributions on the cylinders is highlighted. It is found that, generally, with the increment in |β|, the absolute value of time average lift coefficient increases and time average drag coefficient decreases, and beyond a certain magnitude of β, the vortex shedding vanishes. It is also revealed that the drag coefficient decreases as the Reynolds number increases while the effect of the Reynolds number on lift is almost negligible. At the end, correlations for drag and lift coefficients $${(\overline {C_D } ,\overline {C_L })}$$ are extracted from the numerical data.