Base Drag Research Articles

Performance Prediction of Conical Nozzle Using Navier–Stokes Computation

This paper describes the numerical investigation of convergent conical nozzles done in the AIAA Propulsion Aerodynamic Workshop number one. Axisymmetric, three-dimensional, and unsteady computations were conducted for three main test cases. These test cases were 1) axisymmetric convergent conical nozzles of 15, 25, and 40 deg half-angle as well as a reference nozzle; 2) the 25 deg half-angle nozzle with a splitter plate inserted in its plane of symmetry; and 3) the vortex shedding observed at the base of the splitter plate. The effects of base drag and freestream Mach number on calculated velocity coefficient were analyzed. The discharge coefficients and velocity coefficients determined from the axisymmetric solutions matched the experimental data well. The wall Mach numbers and sonic line positions were also well predicted. In the axisymmetric computations, the shock position was well captured by the fine grid. However, when the nozzle pressure ratio was high, the three-dimensional Reynolds-averaged Navier–Stokes results may have been less reliable. Three-dimensional results with the splitter plate in the nozzle show that the shock wave location is changed by the presence of the splitter plate. The unsteady zonal shear stress transport-detached eddy simulation method was used to simulate the vortex shedding at the base of the splitter plate. The primary shedding frequency of 32.2 KHz was obtained by analyzing the viscous force at the splitter trailing edge.

Numerical study of hot launch of missile inside a tube

The impingement of hot rocket motor plume inside a canister is simulated numerically by solving three-dimensional Reynolds Averaged Navier–Stokes equations using commercial software. The computed methodology is first validated for cold flow jet impingement in a circular tube for different chamber pressure and the simulations captured all the finer aspects of blow-by flow conditions as reported in the literature. A very good comparison is obtained between experimental and numerical surface pressure distribution. The validated methodology is applied to simulate the hot launch of a missile from a canister. It is observed that for low annular gap between missile body and canister the motor plume interaction became intense and gave rise to a very significant base drag which may constrain the motion of the missile inside the canister.

The prediction of axial aerodynamic coefficient reduction using base bleed

To reduce axial aerodynamic force as leading aerodynamic load during projectile flight, one of the possible solutions is to redesign the projectile base, using limited flow as base bleed. Base bleed design needs accurate turbulent supersonic base flow model over a cylindrical afterbody and needs to be developed. Predictions and analysis of the aerodynamic behavior of the base bleed projectile with such a model can reduce development cost and test effort. There are three ways of achieving results of a suitable flow model: by theoretical prediction, by numerical simulation and by restricted experimental measurement. The base bleed effects on the aerodynamic axial coefficient, as increase of the base pressure, and decrease of the eddy component of axial aerodynamic coefficient. Results of the axial aerodynamic coefficient of projectile, without base bleed obtained by experimental and predictional calculation. A validation calculation was performed for several types of projectiles. Computed results show reasonable trends in the base pressure increase – base drag reduction, base corner expansion and downstream wake closure location.

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.

Application of Circulation Controlled Blades for Vertical Axis Wind Turbines

The blades of a vertical axis wind turbine (VAWT) rotor see an inconsistent angle of attack through its rotation. Consequently, VAWT blades generally use symmetrical aerofoils with a lower lift-to-drag ratio than cambered aerofoils tailored to maximise horizontal axis wind turbine rotor performance. This paper considers the feasibility of circulation controlled (CC) VAWT blades, using a tangential air jet to provide lift and therefore power augmentation. However CC blade sections require a higher trailing-edge thickness than conventional sections giving rise to additional base drag. The choice of design parameters is a compromise between lift augmentation, additional base drag as well as the power required to pump the air jet. Although CC technology has been investigated for many years, particularly for aerospace applications, few researchers have considered VAWT applications. This paper considers the feasibility of the technology, using Computational Fluid Dynamics to evaluate a baseline CC aerofoil with different trailing-edge ellipse shapes. Lift and drag increments due to CC are considered within a momentum based turbine model to determine net power production. The study found that for modest momentum coefficients significant net power augmentation can be achieved with a relatively simple aerofoil geometry if blowing is controlled through the blades rotation.

Experimental Investigation of Base-Drag Reduction via Boundary-Layer Modification

This investigation identifies and characterizes the changes in the near wake of a bluff body produced by boundary-layer thickening, which, in turn, result in an overall reduction in the base drag. For this study, wedge and flat-plate models with both smooth and rough forebody surfaces were used. The forebody surface roughness on both models was increased to modify/thicken the forebody boundary layer. Hot-wire surveys were carried out to characterize the boundary-layer state ahead of the separation point, and base-pressure measurements were used to quantify base-drag reduction associated with boundary-layer modification. In addition, particle image velocimetry measurements in the near wake were performed for both models, and proper orthogonal decomposition analysis was used to analyze the velocity data. Low-order near-wake velocity reconstructions obtained from proper orthogonal decomposition analysis accurately represented the near-wake behavior for the studied cases. It was observed from these low-order reconstructions that the shed vortices became more organized, convected faster, and decreased in strength when the boundary layer was thickened/modified. These changes in the wake behavior resulted in an increase in base pressure and an overall decrease in the base drag.

1042 AN EXPERIMENTAL STUDY ON THE WAKE OF A RECTANGULAR FOREBODY WITH SUCTION APPLIED OVER THE SURFACES

Previous studies have shown that a modification of the boundary layer of a bluff body can change the wake properties like the vortex shedding frequency and the base drag. In the present study the boundary layer has been modified by applying suction over the top and bottom surfaces of a bluff body. A level of suction above a certain threshold has been shown to give an asymptotic suction boundary layer. For the asymptotic suction boundary layer the boundary layer properties become independent of the free stream velocity and hence the boundary layer Reynolds number. The bluff body studied can be described as a rectangular forebody with a smooth leading edge and a blunt trailing edge. Hot-wire anemometry measurements have been carried out in the boundary layer of the forebody. For four different Reynolds numbers, all within the laminar regime, the suction coefficient has been varied in the range from the neutral case to the case for which the asymptotic suction boundary layer is reached. It is observed that as the suction coefficient approaches the asymptotic suction boundary layer threshold, the Strouhal number based on the shedding frequency approaches a constant value. The base pressure decreases as the boundary layer thickness decreases. Furthermore it is shown that the Strouhal number does not always decrease for an increase of the displacement thickness.

종축 실린더의 항력에 대한 수치 해석적 연구

In this study, the numerical analysis for the flows around an axial cylinder is carried out in order to investigate the basic characteristics of drag of blunt body. A variation of drag and flow separation for the axial cylinder is investigated according to the length-diameter ratio. Also, the flow separation around the head is removed by rounding-off the front edge of the body to analyze the effect of drag reduction. Most of the drag turns out to be a pressure drag component and the variation of drag is caused by the change of pressure and velocity which is affected strongly by the flow separation at the edges of the axial cylinder. Especially, it is found that the pressure drag component acting on the back of axial cylinder, as known as the base drag, mainly changes the drag. As the length-diameter ratio of axial cylinder increases, the drag sharply decreases and the minimum is shown when the length-diameter ratio is about 2.4. Also, as the length-diameter ratio increases further above 2.4, the drag increases at a slower rate. The pressure drag is almost constant when the length-diameter ratio is greater than 8, but the increase of friction drag component is the reason for the increase of the drag. When flow separation is removed completely at the front edge of the axial cylinder, the pressure drag component is reduced to 12~17%, but the total drag is reduced to only 17%~32% due to the friction drag component that increases linearly proportional to the length-diameter ratio.

Tomographic Particle Image Velocimetry Measurements in a Bluff-Body Wake Flow

I N the wake of bluff bodies, flow separation occurs along with phenomena like recirculation, increased turbulence levels, periodic shedding processes, and low-pressure regions. For flow bodies, which cannot avoid such geometries, base drag and buffeting effects can vitally reduce performance and efficiency. This holds true for axisymmetric, cylindrical bodies placed parallel to freestream direction, like the generic rocket configuration considered in the current work. Similar geometries were studied extensively for subsonic freestream conditions, with early studies concentrating on static or time-averaged quantities [1,2]. Following the evolution of both experimental and numerical methods, the unsteadiness of the wake turned into focus. Many studies reported a macroscopic vortex shedding mode at a reduced frequency of StD 0:2 [3–5], that was also detected in hot-wire and dynamic base pressure measurements regarding the present case [6]. The instability of the shear layer is assumed to be a key factor in the shedding mechanism and the wake topology [4]. Correspondingly, Pastoor et al. [7] used zero-net-massflux actuation to actively manipulate the shear layer of a bluff body over wide Reynolds ranges. A possible overall drag reduction of 15% was reported. Perret [8] investigated the wake of a quasitwo-dimensional (2-D) cylinder by means of two-component (2C) particle image velocimetry (PIV) for ReD 1:25 10 [8]. Within the shear layer, a vortex population was detected using the swirling strength criterion. The corresponding structures were both small (<5% of the model diameter) and of high intensity compared to the macroscopic Karman vortices, and, thus, connected to the Kelvin– Helmholtz instabilitymechanism. It was also pointed out that for 2-D measurements, only the planar swirling motion can be taken into account and may be affected or biased by the out-of-plane velocity and inclined vortex axes [8]. Fewer results can be found regarding three-dimensional (3-D) test cases. Deck and Garnier [5] performed both detached and large eddy simulations (DES/LES) on the wake flow of a cylindrical, generic launch vehicle model. The bluff base geometry was prolonged by a Presented at the 41st AIAA Fluid Dynamics Conference and Exhibit, Honolulu, Hawaii, June 27–30, 2011; received 27 November 2011; revision received 10 April 2012; accepted for publication 11 April 2012. Copyright © 2012 by Institute of Aeronautics and Astronautics, RWTH Aachen University. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright ClearanceCenter, Inc., 222RosewoodDrive,Danvers,MA01923; include the code 0001-1452/12 and $10.00 in correspondence with the CCC. Research Engineer, Wuellnerstr 7. Member AIAA. Senior Research Engineer, Wuellnerstr 7. AIAA JOURNAL Vol. 50, No. 12, December 2012

Passive Control of High-Speed Separated Flows Using Splitter Plates

An experimental investigation was conducted to study the effects of passive splitter plates placed in the recirculation region behind a blunt-based axisymmetric body in supersonic flow. The goals of this research were to obtain a better understanding of the physical phenomena that govern these massively separated high-speed flows and to determine the flow-control authority of this passive device. Triangular splitter plates dividing the near wake into 1/2, 1/3, and 1/4 cylindrical regions were designed to exploit specific stability characteristics of this flow, to alter the base pressure, and ultimately to affect base drag. Mean static pressure measurements acquired from the base and splitter plate surfaces were the primary metric to assess the influence of these plates. Schlieren imaging, surface flow visualization, and pressure-sensitive paint measurements were also employed to document the near-wake flowfield, surface flow structure, and surface pressure, respectively. The insertion of these splitter plates only slightly affected the base pressure and near-wake flowfield, which suggests that the recirculation region may not be the most sensitive area to apply this flow control methodology.

Spanwise vortex dislocation in the wake of segmented blunt trailing edge

Dislocation of the Karman vortex is forced in the near wake behind a two dimensional blunt trailing edge aerofoil to induce strong three dimensionality to weaken the Karman vortex and inhibit its periodic shedding by segmenting the trailing edge in a novel way, different from a rectangular segmented trailing edge tried in the past. Symmetrical trapezoidal prismatic blocks, with the major and the minor sides being equal to 4 and 2 base heights, respectively, are attached to the base at regular intervals along the span which could be varied in order to render multiple wavelengths of spanwise discontinuity to ensure that at least one of the modes of dislocation is triggered independent of the Reynolds number. Hot-wire measurements confirm effectiveness of the trailing edge configurations with trapezoidal prismatic blocks in creating controlled dislocation along the span which annihilates the Karman vortices and suppresses their periodic shedding completely. Flow visualization in a water tunnel reinforces the hot-wire results and clearly shows that the classical Karman vortex street, seen behind the plain base model, disappears when segmented trailing edge is used. In comparison, the rectangular segmented trailing edge is found to attenuate the Karman vortex strength only partially. However, the base pressure measurements have shown improvement in reducing the associated base drag only by 3–4%. The segmented trailing edges designed for the present study are found to generate strong streamwise vortices that effectively transfer energy from the Karman vortices resulting in suppression of the unsteadiness but, perhaps, lower pressures in their core may be restricting the further rise in the base pressure due to the induced effect.

First instability of the flow of shear-thinning and shear-thickening fluids past a circular cylinder

AbstractThe first bifurcation and the instability mechanisms of shear-thinning and shear-thickening fluids flowing past a circular cylinder are studied using linear theory and numerical simulations. Structural sensitivity analysis based on the idea of a ‘wavemaker’ is performed to identify the core of the instability. The shear-dependent viscosity is modelled by the Carreau model where the rheological parameters, i.e. the power-index and the material time constant, are chosen in the range $0. 4\leq n\leq 1. 75$ and $0. 1\leq \lambda \leq 100$. We show how shear-thinning/shear-thickening effects destabilize/stabilize the flow dramatically when scaling the problem with the reference zero-shear-rate viscosity. These variations are explained by modifications of the steady base flow due to the shear-dependent viscosity; the instability mechanisms are only slightly changed. The characteristics of the base flow, drag coefficient and size of recirculation bubble are presented to assess shear-thinning effects. We demonstrate that at critical conditions the local Reynolds number in the core of the instability is around 50 as for Newtonian fluids. The perturbation kinetic energy budget is also considered to examine the physical mechanism of the instability.

Bluff Body Drag Reduction with Ventilated Base Cavities

Various techniques to reduce the aerodynamic drag of bluff bodies through the mechanism of base pressure recovery have been investigated. These include, for example, boat-tailing, base cavities and base bleed. In this study an Ahmed body in squareback configuration is modified to include a base cavity of variable depth, which can be ventilated by slots. The investigation is conducted in freestream and in ground proximity. It is shown that, with a plain cavity, the overall body drag is reduced for a wide range of cavity depths, but a distinct minimum drag condition is obtained. On adding ventilation slots a comparable drag reduction is achieved but at a greatly reduced cavity depth. Pressure data in the cavity is used to determine the base drag component and shows that the device drag component is significant. Modifications of the slot geometry to reduce this drag component and the effects of slot distribution are investigated. Some flow visualisation using PIV for different cavity configurations is also presented.

Numerical simulation on drag reduction of revolution body through bionic riblet surface

Numerical simulations of flow fields on the bionic riblet and the smooth revolution bodies were performed based on the SST k-ω turbulence model in order to explain the mechanisms of the skin friction drag reduction, base drag reduction on the riblet surface, and flow control behaviors of riblet surface near the wall. The simulation results show that the riblet surface arranged on the rearward of the revolution body can reduce the skin friction drag by 8.27%, the base drag by 9.91% and the total drag by 8.59% at Ma number 0.8. The riblet surface reduces the skin friction drag by reducing the velocity gradient and turbulent intensity, and reduces the base drag by weakening the pumping action on the dead water region which behind the body of revolution caused by the external flow. The flow control behavior on boundary layer shows that the riblet surface can cut the low-speed flow near the wall effectively, and restrain the low-speed flow concentrating in span direction, thus weaken the instability of the low speed steaks produced by turbulent flow bursting.

Supersonic aerodynamics of a projectile with slot cavities

AbstractThis paper presents the results of experimental and computational investigations on the effect of slot cavities on a supersonic projectile. Experimental work was carried out to show the effects of the slots on the drag at Mach numbers M = 1·36, 1·65, 1·83. The computational analysis was done at M = 1·36. A single configuration of the slot pattern was used with two different slot widths (0·5mm and 2·0mm). Flow features were investigated in the slotted area and at the base. The analysis presented includes the pressure distribution, the supersonic cavity flow and the effect of the slots on the overall aerodynamic drag. Unlike the case of slots at transonic speeds, the suction and blowing mechanism is not found at supersonic Mach numbers. Streamwise cavity slots cause a small base drag reduction. The reduction in total drag is modest when the width is 0·5mm. However, the experiments showed that with the wider slots (2mm) the drag actually increases at Mach numbers from 1·36 to 1·83.

Measurements of Fluctuating Pressures on a Circular Cylinder in Subsonic Cross Flow

A circular cylinder is tested in crossflow over the subsonic speed range. Time-resolved pressure distributions give information on surface pressure fluctuations; the corresponding drag and base drag coefficients are provided. Measured base pressure fluctuations at low Mach numbers are in agreement with the findings of other researchers. Flow changes at higher subsonic velocities and into the transonic range are described. Illustrations are drawn from the observations of other researchers, enabling physical explanations to be given to assist toward a more general modeling of the problem. At Mach numbers above 0.6 the changing strength of the vortices reduces the base drag coefficient up to a Mach number of 0.9, at which the onset of sonic flow increases the drag. Strouhal number variation is compared with the measurements of other authors. The paper concentrates on providing reliable measurements of vortex shedding and base pressure over the subsonic speed range rather than attempting to provide universal correlations.

Computational Investigation of Base Drag Reduction for a Projectile at Different Flight Regimes

A computational investigation for the 155mm artillery shell was conducted for the purpose of reducing the base drag. Three case studies were conducted to investigate the properties of the flow field around the shell for the flight at different Mach numbers at zero angle of attack. The three cases were: a shell with boattail, a shell with base cavity and a shell with base bleed. Also, combinations of these three cases were investigated. The higher drag reduction was demonstrated when using a combination of the three effects. For this latter case it was possible to realize a drag coefficient reduction of ~60% at subsonic regime and ~20 - 30% .at transonic and supersonic regimes. Based on the present methodology, a designoptimization for minimum drag can be applied on similar flying bodies.

遷音速鈍頭2次元物体でのタブによるベース抵抗低減

As base drag accounts for most of the total drag for spacecraft with a large base area, reduction in the base drag improves its aerodynamic performance. In the present study, a method using a pair of tabs normal to the base surface is proposed to reduce the base drag, and its effects on drag reduction are experimentally examined for a two-dimensional, simple bluff body with a large base area, the cross-section of which consists of a circular forebody and a rectangular afterbody, by changing the tab’s position and length. In addition, the flow fields around the model with/without tabs were visualized by the schlieren method with a high speed video camera, and CFD analysis was also made to supplement the experimental results. It was found that the tab method can remarkably increase the base pressure, which leads to a large decrease in the base drag. In the case where a pair of tabs with a non-dimensional length of 0.38 are installed at the base edges, the total drag is reduced by about 20.6% at a Mach number of 0.6 compared with that in the baseline case without tabs. Even when short tabs with a non-dimensional length of 0.07 are employed, about 10.0% reduction in the base drag is achieved. Thus, it was made clear from this study that the tab method proposed here is effective to decrease base drag in the transonic flow.

Optimization of Mass Bleed for Base-Drag Reduction

Efficient mass-bleed conditions to minimize base drag are examined in consideration of various base-to-orifice-exit area ratios for a body of revolution in the Mach 2.47 freestream. Axisymmetric, compressible, mass-averaged Navier-Stokes equations are solved using the standard k-w turbulence model, a fully implicit finite volume scheme, and a second-order upwind scheme. Base flow characteristics are explained regarding the base configuration as well as the injection parameter, which is defined as the mass flow rate of the bleed jet nondimensionalized by the product of the base area and freestream mass flux. The results obtained through the present study show that for a smaller base area, the optimum mass-bleed condition leading to minimum base drag occurs at relatively larger mass bleed, and a larger orifice exit can offer better drag control.

The effect of conical flowfields on the performance of waveriders at Mach 6

The performance of 23 kinds of waveriders, derived from different conical flowfields, is analyzed by the numerical computation under the conditions of fight speed of Mach 6, attack angle of 0° and flight altitude of 30 km. These results indicate that the performance is influenced by the shapes and the width to height ratios (W/H) of generating cones. The geometrical parameter and the lift coefficient are proportional to W/H, while the drag coefficient and the lift to drag ratio (L/D) have extreme values. Considering the base drag and the computation errors, the waverider with the highest L/D is cut from the elliptical cone’s flowfield (W/H = 1.5–1.618), and the configuration with the lowest drag can also be obtained at W/H = 1:1.5. Accordingly, good suggestions are proposed for practical design based on these computational results.