Boat Tail Research Articles

Alleviation of SWBLI over the payload of a launch vehicle by change of nose shape

Wind tunnel tests on a typical heavy-lift launch vehicle configuration featuring a core vehicle flanked by two strap-on boosters showed very high levels of unsteady pressures over the payload region at transonic Mach numbers when the nose had a cone-cylinder shape. This was due to very high levels of Shock Wave Boundary Layer Interactions (SWBLI) associated with the conical shape, which caused alternating flow featuring the formation of a unsteady λ-shock system over the payload region and a pair of counter-rotating vortices. The phenomenon was found to strongly depend on the proximity of the noses of the strap-on boosters to the boat tail downstream of the payload region. When the conical nose was replaced by an ogive-shaped nose, significant reduction in SWBLI occurred, with dramatic reduction of shock oscillations, pressure fluctuations, replacement of alternating flow by steady flow and disappearance of the vortex-pair. Time-averaged pressure fluctuations indicate reduction in pressure gradients due to a more gradual expansion of flow over the ogive nose compared to conical nose, an observation also supported by CFD simulations. The alleviation of SWBLI with ogive nose was so effective that the presence or absence of strap-on boosters had practically no effect on pressure fluctuations over the payload region. Unlike the conical shape, the ogive nose cone shape seems to meet both the shape and pressure parameter-criteria of NASA to ensure buffet-free environment.

Drag reduction in a spanwise-infinite boat-tailed body through multiple transverse grooves

Appropriately-designed spanwise-extruded small grooves, oriented transverse to the freestream flow, may delay flow separation in adverse pressure gradients by generating local and stable recirculation zones. In this study, we explore the potential of multiple grooves to reduce the drag on a boat-tailed bluff body characterized by vortex shedding. Numerical simulations reveal that the presence of two consecutive transverse grooves results in a maximum boat-tail drag reduction of 23.2%, further decreasing the drag by 15% when compared to the performance of the same boat-tailed bluff body with a single groove. This flow-control mechanism proves effective when the flow reattachment takes place downstream of both grooves throughout the entire vortex-shedding cycle, leading to the formation of two distinct local recirculation zones within the grooves. Lower frictional losses are observed along the streamline bounding the recirculation within the grooves as compared to the friction losses caused by the no-slip boundary conditions along the lateral walls of a plain boat tail (i.e., without grooves). Consequently, the boundary layer exhibits higher momentum downstream of the grooves, resulting in a delayed flow separation and an improved reduction in body drag for the boat tail.

A numerical study of flow over supersonic projectile under heavy rain

This paper presents a computational fluid dynamics study of morphology and structure dynamics of the flow over a supersonic secant-ogive cylinder boat tail projectile under heavy rain. The discrete phase model is employed to approximate the process of droplet–particle collision with the projectile wall and the formation of a liquid film. The simulation results indicate that at certain angles of attack, rain impact decreases lift coefficient of projectile by as much as 14.9%, as the wall pressure distribution is distinctly reformed. Moreover, the rain condition induces the formation of a liquid film on the front end of the projectile, as the angle of attack increases, the stability of this liquid film gradually improves, while its thickness and coverage rise to the peak values and then decrease. On the projectile’s trailing half, however, the liquid film developed a watery pattern. The collision of liquid droplets causes momentum loss to the projectile, while an unstable liquid film tends to exacerbate aerodynamic performance loss, a stable liquid film would mitigate the performance loss.

Experimental study on dynamic mechanical properties of projectile 155mm ERFB BT material

Artillery projectile 155mm high explosive (HE) extended range full bore boat tail (ERFB BT) and its components experience a high pressure loading of the propellant during gun launch. Mechanical properties of an engineering material are measured using quasi-static tensile test at room temperature. The materials of the projectile and its components are expected to survive time dependent propellant pressure loading inside the gun, which lasts for few microseconds. To investigate dynamic mechanical properties of the projectile and its components at medium strain rates and elevated temperature, the experimental analysis is undertaken. This research paper aims to examine the effect of medium strain rates and elevated temperatures on flow properties of the materials used in the projectile. The uniaxial tensile tests at the medium strain rates between 0.01 s-1 to 1s-1 and at elevated temperatures ranging between 150°C to 500°C are performed separately. The Considѐre criterion was applied to evaluate strain hardening exponent of the materials. Values of strain hardening exponents of shell body and boat tail materials are less than that of driving band material. Strain rate sensitivity of the materials is observed to be very low, thus indicating a good resistance to plastic deformation. The yield strength and true yield strength of the materials fall with increase in temperature.
 

Plastic Deformation of High Explosive Projectile 155 mm during Gun Launch Conditions using Finite Element Method

The structural integrity of artillery projectile 155mm high explosive Extended Range Full Bore (ERFB)boat tail designed for 155mm howitzer guns of 39, 45 and 52 calibre plays a key role inside the gun barrel. Thisprojectile comprises a shell body, a driving band, a boat tail, nubs, an explosive, and a fuze. Plastic deformationof the projectile and stripping of driving band are not permitted, when projectile is fired. An investigational study is necessitated to check the plastic deformation of the projectile subjected to maximum propellant charge pressure.The aim of this study is to check the effective plastic deformation and affirm the structural integrity. A 3-D explicit dynamic structural analysis of 155 mm HE ERFB BT during gun launch conditions is carried out by finite element method using FE code ABAQUS/Explicit. To understand the non-linear mechanical behavior of the projectile, the true stress-strain curves of the materials are considered. The plastic behavior of the projectile subjected to the time-dependent loading is studied by using the von Mises plasticity model. The results reveal that the shell body and boat tail have no plastic deformation and the most stressed component is the driving band. The investigation has affirmed the structural integrity of the projectile during gun launch conditions.

Tomographic imaging of carbon dioxide in the exhaust plume of large commercial aero-engines.

We report here the first implementation of chemically specific imaging in the exhaust plume of a gas turbine typical of those used for propulsion in commercial aircraft. The method used is chemical species tomography (CST) and the target species is CO2, absorbing in the near-infrared at 1999.4 nm. A total of 126 beams propagate transverse to the plume axis, along 7 m paths in a coplanar geometry, to probe a central region of diameter ≈1.5m. The CO2 absorption spectrum is measured using tunable diode laser spectroscopy with wavelength modulation, using the second harmonic to first harmonic (2f/1f) ratio method. The engine is operated over the full range of thrust, while data are recorded in a quasi-simultaneous mode at frame rates of 1.25 and 0.3125 Hz. Various data inversion methodologies are considered and presented for image reconstruction. At all thrust levels a persistent ring structure of high CO2 concentration is observed in the central region of the measurement plane, with a raised region in the middle of the plume assumed to be due to the engine's boat tail. With its potential to target various exhaust species, the CST method outlined here offers a new approach to turbine combustion research, turbine engine development, and aviation fuel research and development.

Aerodynamic Shape Optimization of Payload Fairing Boat Tail for Various Diameter Ratios

Aerodynamic shape optimization of launch vehicle hammerhead payload fairing (PLF) boat tail shapes has been carried out. Multi-objective optimization studies have been carried out to minimize the boat tail length, drag coefficient, the length of separated flow behind the boat tail, and the maximum pressure ratio over the boat tail. The boat tail shape has been parameterized using a B-spline. A novel optimal shape has been obtained, which has a shallow initial slope followed by a rapid increase in the slope near the boat tail end, termed the ramp stepped boat tail (RSBT). The optimization study has been carried out by varying the stage-to-PLF diameter ratios. A novel parameterization strategy has been employed, which convexifies the design space. Computational fluid dynamics (CFD) simulations have been carried out on 950 different boat tail configurations sampled using Latin hypercube sampling (LHS) at Mach number 0.8. Results from CFD have been validated against experimental data from literature. Artificial neural networks have been used as surrogate models for computing the objectives. Multi-objective optimization was carried out using the genetic algorithm for various stage-to-PLF diameter ratios. Pareto-optimal fronts obtained have been analyzed, and optimal shapes derived from them have been validated using CFD. It is found that the optimal RSBT configuration is found to be advantageous over the conventional conical boat tail in reducing drag, separated flow length, boat tail length, shock strength, and the turbulent kinetic energy downstream of the boat tail.

Computation of three - dimensional transonic flow fields over a three - body launch vehicle configuration for low angle of attack conditions

The present work aims at the prediction of shock wave position on the heat shield region of a three body launch vehicle in transonic region. The launch vehicle consists of a cylindrical core body and two boosters. The core body is made of a spherical nose, followed by a cone and heat shield region which is a flat surface. The heat shield is followed by a boat tail. Simulations are performed with commercial CFD package, ANSYS –FLUENT. Reynolds stresses are computed using the k-ω SST turbulence model. Explicit time integration is used to obtain steady-state solutions. A structured grid over the vehicle is generated using ICEMCFD software. In the near-wall region of the launch vehicle fine mesh is employed to capture the boundary layer. Simulations are performed for angle of attacks (AOA) 0° and 4° for Mach number 0.95. The computed nose stagnation pressure compared with the isentropic relations and deviation found to be .067%. A standing normal shock wave is observed for AOA 0 whereas an oblique shock wave is observed for angle of attack 4°. The predicted flow structure in terms of density distribution is compared with experimental schlieren images. The predictions captured essential flow features such as expansion around the conical part, boundary layer over the heat shield, shock wave on the heat shield, and flow separation in the boat tail region. The predicted flow structure and shock wave position for AOA O matched well with the experiments. However, for AOA 4°, the deviation between flow structure and shock position is noticed. Possible reasons for the deviation are explained.

Numerical Study on the Effect of Boat Tail Shape on Aerodynamic Drag of SUV

Longitudinal vortex formed at the rear area of the vehicle is one of the main sources of vehicle aerodynamic drag. The boat tail shape is applied to reduce the longitudinal vortex strength of the SUV so that the aerodynamic drag is improved. For this study, a commercially available SUV produced by domestic automobile company, Hyundai Motor Company, is selected as a baseline model. The boat tail shape is modeled on the baseline model using the morphing technique. The effects of boat tail shape on the aerodynamic drag has been numerically analyzed for a SUV with or without undercover. A commercial flow solver, ANSYS Fluent, is used for this study. The driving speed of SUV is 140 km/h. Even though the same boat tail shape is applied to the same vehicle, the aerodynamic drag improvement depends on the flow pattern behind the vehicle. It can be improved when the strength of the longitudinal vortex is weakened by the airflow generated by the boat tail shape.

Investigation of Reduction in Base Drag using Experiment and CFD at Subsonic Speed Regimes

Base drag, arising from flow separation at the blunt base of a body can be a sizeable fraction of total drag in the context of projectiles, missiles and after bodies of fighter aircrafts. The base drag is the major contribution of total drag for low speed regimes, flight tests have shown that the base drag may account for up to 50% of the total drag. Computational and experimental investigation for a hemispherical flight vehicle body of length 500mm and diameter 50mm was conducted for the purpose of investigating the base drag. Three case studies were conducted to investigate the properties of the flow field around the flight vehicle at different flow velocities of 20m/s, 30m/s and 50m/s at zero angle of attack (AoA). The three cases were (i) a flight vehicle with flat base configuration, (ii) a flight vehicle with a nozzle at the base and (iii) a flight vehicle configuration with a boat tail, Fig 1. Also, the three configurations were investigated at different AoA of -2ï‚°, 0ï‚° and +2ï‚°. The base drags for three configurations are calculated and the experimental results are compared with the CFD results.

Computational Analysis of Base Drag Reduction for a Subsonic Missile Projectile at Different Flow Velocity Conditions

Base drag is arising from flow separation at blunt base of a body. It can be a sizeable fraction of total drag in context of projectiles, missiles and after bodies of fighter aircrafts. The base drag is the major contribution of total drag for low speed regimes, flight tests have shown that the base drag may account for up to 50% of total drag. In this paper an experimental investigation for simple semi-circular flight vehicle body of length 500mm and diameter 50mm was conducted for the purpose of investigating base drag. The base drags for three configurations are calculated and the results are compared with CFD data. The three configurations used for testing are flat base configuration, closed nozzle configuration and boat tail configuration. The evaluation of base drag for three different flow velocities such as (i) 20m/s, (ii) 35m/s and (iii) 50m/s at different angle of attack such as -2ï‚°, 0ï‚° and 2ï‚° are experimented and compared.

Effectiveness of tail devices for wake control of road heavy vehicles

In the current research, a series of numerical simulations of the flow field around a typical tractor-trailer combination with and without a drag reduction device (i.e., boat tail) have been performed to study the effect of the device on wake structures and flow evolution behind the trailer. The results of the simulations have been validated through comparison of the drag coefficient against available wind-tunnel measurement data. Based on the evolution profiles and flow visualisation behind the trailer, it has been shown that the trailer wake region in proximity of the trailer rear surface is significantly influenced by the trailer boat tail. As such a quicker flow recovery, smaller recirculation bubble and lower level of turbulent kinetic energy is obtained when the boat tail is utilised. This resulted in about 8.6% decrease in the vehicle aerodynamic drag and 4.6% fuel saving based on the travelling speed of 65 mph.

Effectiveness of tail devices for wake control of road heavy vehicles

In the current research, a series of numerical simulations of the flow field around a typical tractor-trailer combination with and without a drag reduction device (i.e., boat tail) have been performed to study the effect of the device on wake structures and flow evolution behind the trailer. The results of the simulations have been validated through comparison of the drag coefficient against available wind-tunnel measurement data. Based on the evolution profiles and flow visualisation behind the trailer, it has been shown that the trailer wake region in proximity of the trailer rear surface is significantly influenced by the trailer boat tail. As such a quicker flow recovery, smaller recirculation bubble and lower level of turbulent kinetic energy is obtained when the boat tail is utilised. This resulted in about 8.6% decrease in the vehicle aerodynamic drag and 4.6% fuel saving based on the travelling speed of 65 mph.

Wind tunnel tests on drag reduction of heavy vehicles using sinusoidal boat tails

A sinusoidal boat tail (SBT) is developed and investigated to greatly reduce drag, side force, and yawing moment under crosswind conditions. The drag and side forces of a heavy vehicle model with the SBT are reduced by 15.9 % and 22.6 %, respectively, relative to those of a reference model without the SBT at a yaw angle of 7°. Therefore, the SBT improves the aerodynamic performance and driving stability of the vehicle model. Flow characteristics around the vehicle model are measured using a particle image velocimetry technique. Mean velocity field results show that the SBT effectively suppresses the formation of longitudinal vortices by enhancing the streamwise velocity of the region behind the boat tail. In addition, the streamwise momentum deficit is reduced in the wake region owing to the development of a secondary recirculating flow region.

Large eddy simulation of base drag reduction using jet boat tail passive flow control

This study conducts an implicit large eddy simulation (ILES) of jet boat tail (JBT) flows to investigate its drag reduction mechanism. The concept of JBT passive flow control is to create a circumferential jet around a bluff body toward the center of the base area. It forms a jet cone to have the similar effect of a solid boat tail. The LES is performed for a baseline bluff body and a JBT model modified from the baseline. The LES predicts that the JBT reduces the averaged drag coefficient by 19.3%, a reasonable agreement with the experimental drag reduction of 22.5%. The reduced averaged wake area is also observed for the JBT model, resulting in a decreased drag. In addition, the unsteady flow structures of the baseline and JBT flow are analyzed to study the flow mixing and entrainment mechanism. For the baseline configuration, the coherent vortex structures occur far downstream of the base surface. It hence does not have strong entrainment and energy transfer from freestream to the base area. For the JBT flow, a pulsative jet is induced by the vortex shedding of the shear layer and interacts immediately with the shear layer near the base surface. It generates the small structures that are substantially larger than those of the baseline configuration. The larger vortex structures of JBT enhance the flow entrainment and transfer more energy from the freestream to the base area. It results in higher static pressure in the base area that substantially reduces the pressure drag. Proper orthogonal decomposition of flow field reveals the periodic jet pulsation pattern in the azimuthal direction.

Experimental Investigations Regarding the Behaviour of Composite Panels Based on Polyurea and Kevlar or Dyneema Layers Under Blast and Fragments

This paper presents an experimental study to determine the behaviour of composite panels, made of polyurea sprayed on Kevlar or Dyneema support layers, under blast and fragments produced by an improvised explosive device (IED). The fragments used for tests were steel bearing balls of 8 and 10 mm propelled by a plastic explosive charge and bullets cal. 7.62 mm, type Full Metal Jacketed Armor Piercing (FMJ AP) and Hollow Point Boat Tail (HPBT), with impact velocity greater than 500 m/s. To determine the fragments attenuation, their velocities before and after the impact with the composite panel were measured and compared. In order to assess the blast attenuation, the reflected pressures measured by two face-on sensors, one of which was covered by the composite panel, were compared. Also, to explain the behaviour of composite panels under blast, the shock polars of materials in the panel�s composition were plotted. The results have shown that the composite panels have a low rate of attenuation of fragment velocities but a very good ability to attenuate the pressure and impulse associated with the shock wave.

Method of Measuring the Ballistic Coefficient of Bullets

The method, employing universal expressions, was examined to determine the ballistic coefficient of a bullet by the G7 standard (air drag law for the flight of a long boat tail tangent ogive G7 bullet in view of its muzzle velocity and flight trajectory descent within a 100–200-m range). The method permits of measuring the ballistic coefficient of a bullet without a specialized measuring laboratory, i.e., under unequipped shooting-ground conditions. The only metrological equipment is a mobile unit for evaluating the muzzle velocity. The relation is derived based on the experimental design by the central alternate that describes the range of point values for an examined process since the range of nonexistent ones does not permit of employing the uniform alternate. The range of initial bullet velocities is divided into the three subranges to include all points of the central alternate into the range of existent values. The above ballistic coefficient-based approach can be used to compute the bullet velocity at an arbitrary distance in the tests of armored targets under field conditions without bulky equipment. The reliability of describing the examined process is provided by the approved mathematical model of the bullet flight in air as the motion of a solid body based on the system of four differential equations of first order. Adequacy of empirical description of the relation between the ballistic coefficient and muzzle velocity and flight trajectory descent within 100–200 m is verified by determining the standard deviation for the values based on the mathematical model of the bullet flight and the polynomial and their comparison with the accuracy of the ballistic coefficient measurement (no more than 10-3).

Considerable drag reduction and fuel saving of a tractor–trailer using additive aerodynamic devices

The drag reduction of commercial vehicles is the principal challenge to improving fuel saving and decreasing air pollution. Accordingly, several flow control devices, including gap fairing, cab roof fairing, boat tail, and side skirt were introduced to reduce the aerodynamic drag exerted on heavy vehicles. Although such devices have exhibited good drag reduction performance, their aerodynamic performance can be improved further. Moreover, a comprehensive and systematic research is required, including drag force measurement, flow field analysis and real-scale proving ground test, to evaluate the fuel consumption of heavy vehicles. In this study, the drag reduction effect of the aero full package (AFP) of a tractor–trailer, which integrated gap fairing, flap-type side skirt and lower inclined air deflector boat tail, was investigated through wind tunnel experiments and proving ground test. On the basis of the results of drag and PIV measurement, the integrated AFP was found to significantly modify the flow structure around the vehicle, thereby leading to a 26.5% reduction in drag coefficient compared with the reference tractor-trailer model (CD = 0.693). Furthermore, the proving ground test using real tractor–trailer verified that the integrated AFP provides 13.4% fuel saving. The present results will provide practical information for improving the aerodynamic performance and fuel efficiency of heavy vehicles.

Separation delay through contoured transverse grooves on a 2D boat-tailed bluff body: Effects on drag reduction and wake flow features

Abstract The effectiveness of properly contoured transverse grooves in delaying the flow separation occurring on a two-dimensional boat-tailed bluff body is assessed through numerical simulations. The body has a cross-section with a 3:1 elliptical forebody and a rectangular main part followed by a circular-arc boat tail. Three-dimensional Variational Multiscale Large Eddy Simulations are carried out at R e = D u ∞ ∕ ν = 9 . 6 × 1 0 4 , using a mixed finite-volume/finite-element method. The introduction of one contoured groove on each of the boat-tail lateral surfaces produces a significant delay of flow separation and a consequent increase of the base pressure, with a global drag reduction of the order of 9 . 7 % . The wake dynamical structure remains qualitatively similar to the one typical of blunt-based two-dimensional bodies, with quantitative variations that are consistent with the reduction in wake width caused by boat tailing and by the grooves. The introduction of the grooves leads also to a regularization of the vortex shedding downstream of the body, which is more correlated in the spanwise direction. Finally, a few supplementary simulations show that the effect of the grooves is also robust to the variation of the geometrical parameters defining their location and shape.

Modal analysis of passive flow control for the turbulent wake of a generic planar space launcher

The turbulent wake of a generic planar space launcher equipped with two passive flow control devices is simulated using a zonal RANS–LES method and analyzed by dynamic mode decomposition (DMD). In the first approach, the effect of a classical boat tail on the wake is examined. In the second concept, a flow control device consisting of semi-circular lobes integrated at the base shoulder of the main body is used. The objective of the two concepts is to reduce the reattachment length and thus the lever arm of the forces as well as to stabilize the separated shear layer. Using a boat tail, the reattachment length can be reduced by 50%. Furthermore, it is shown that the semi-circular lobes enhance the turbulent mixing and the shear layer growth rate. Hence, they significantly reduce the reattachment length by about 75%. The semi-circular lobes partially reduce undesired low-frequency pressure fluctuations on the nozzle surface. However, this reduction is achieved at the expense of an increase of high-frequency pressure fluctuations due to intensified small turbulent scales. The DMD analysis of the velocity field reveals that the large-scale coherent structures featuring a wave length of two step heights observed in the reference configuration without flow control can be suppressed by the lobes. The spanwise wave length of the coherent structures seems to depend on the geometry of the lobes, since all detected spatial DMD modes show a spanwise periodicity being equal to the distance between two lobes.