Blunt Nose Research Articles

Effect of Lode angle incorporation into a fracture criterion in predicting the ballistic resistance of 2024-T351 aluminum alloy plates struck by cylindrical projectiles with different nose shapes

The aim of the present paper is to evaluate the effect of Lode angle incorporation into a fracture criterion when predicting the ballistic resistance of 2024-T351 aluminum alloy plates struck by cylindrical projectiles with different nose shapes through finite element (FE) simulations. To this end ballistic tests in the sub-ordnance impact velocity range were firstly conducted on 9.94 mm thick 2024-T351 aluminum alloy plates using cylindrical projectiles with blunt, hemispherical and ogival nose shapes, a nominal diameter of 12.66 mm, and a nominal mass of 50 g. Ballistic limit velocities (BLVs) for different projectile-target pairs considered were calculated using initial vs. residual velocity data obtained in the experiments. Subsequently FE simulations of the conducted experiments were performed with SIMULIA Abaqus software. In those simulations Johnson–Cook yield criterion with a modified strain hardening law was accompanied with either the Lode-independent Johnson–Cook (JC) fracture criterion or the Lode-dependent modified Mohr–Coulomb (MMC) fracture criterion to describe target material properties. Finally, the predicted ballistic resistance and fracture patterns of the targets were compared with those observed in the experiments. It was found that in the simulations with the Lode-independent JC fracture criterion the BLVs of the targets were overestimated by 18.0–35.1% depending on the projectile nose shape. In the simulations with MMC fracture criterion the BLVs predictions were substantially improved, so that the difference with the experimental results did not exceed 9.0%. Targets failure mechanisms obtained in the simulations with MMC fracture criterion also better match the experiments than those predicted with JC fracture criterion. In the simulations with MMC fracture criterion the targets impacted by the projectiles with blunt and hemispherical noses failed through shear plugging, whereas ogive-nosed projectiles caused ductile hole enlargement and target fragmentation due to its rear side radial fracture and bending of formed short petals with their eventual shear fracture. At the same time the simulations with JC fracture criterion predicted shear plugging for the blunt-nosed projectiles, very small conical plug ejection and target fragmentation due to discing for the hemispherical-nosed projectiles, and ductile hole enlargement for the ogive-nosed projectiles. A detailed analysis of the failure process revealed that in the case of the ogive-nosed and hemispherical-nosed projectiles, respectively, the ductile hole enlargement occurs during the initial stage of the impact and subsequent through-thickness fracture develops causing either fragmentation of the target or its shear plugging failure. The Lode-independent JC fracture criterion overestimates a material fracture strain under shear, thus leading to poor prediction of 2024-T351 aluminum alloy targets ballistic performance in the numerical simulations.

On the out-of-plane ballistic performances of hexagonal, reentrant, square, triangular and circular honeycomb panels

Ballistic penetration is a serious threat to military vehicles. Honeycomb structure is one of the most focused solution to improve the ballistic performance due to its great lightweight design and high resistant against impact. There are lots of honeycomb structure topologies. However, no comprehensive comparison on out-of-plane ballistic performance between common honeycomb types had been conducted to our knowledge. Therefore, hexagonal, reentrant, square, triangle and two circular (CS and CH types) honeycomb structures with identical mass and thickness are developed and modelled in this paper. Their relative densities are firstly calculated. Their numerical models and the Lambert-Jonas ballistic model are established. The ballistic performances of these six cases impacted by a blunt nose cylinder projectile from out-of-plane direction are detailed compared through numerical method. Their dynamic response, elastic and plastic deformation, failure mechanism and energy absorption capability are researched. Relations between initial velocity and residual velocity of projectile and the ballistic limit velocities of all cases are also compared. The results indicated that reentrant, square and triangular cases perform worse than hexagonal honeycomb. The fascinating deformation mechanism of CS and CH cases results in a long elastic deformation time and high strength after elastic deformation, which leads to a large decrease in residual velocity. Compared with hexagonal honeycomb, ballistic limit velocities of CS and CH type are improved by 15.2% and 25.0%, respectively. This comparison research provides a useful reference for sandwich honeycomb core design.

Ballistic performance of ceramic and ceramic-metal composite plates with JH1, JH2 and JHB material models

The ceramic and ceramic-metal composite plates upon high velocity impact by a kinetic energy rod may undergo spalling, fragmentation, interface debonding, perforation, bending, and stretching. A challenge in studying these problems is using a material model that realistically simulates the material response at high plastic strains, plastic strain rates and hydrostatic pressures. The choice of the material model and the computational framework influence predictions of deformations of the projectile-target system. Three constitutive relations generally employed to study deformations of a ceramic are due to Johnson and Holmquist, generally abbreviated as JH1, JH2 and JHB. We have implemented these in a computational algorithm using Smoothed Particle Hydrodynamics (SPH) basis functions and a pseudo-spring technique to simulate the initiation and propagation of material failure. After ensuring that the computed results for two flyer plate impact experiments agree well with the test findings, we have studied penetration of monolithic ceramic and ceramic/aluminium targets by kinetic energy rods having blunt, hemispherical and conical nose shapes. It is shown that the present approach can successfully predict spalling, formation of conoid, fragmentation and crack branching in the ceramic, and bending/stretching of metal backing plates. Whereas results from the JH1 and the JHB material models are qualitatively similar, those using the JHB constitutive relation are closest to the test observations. This work suggests that one should use the JHB model for analyzing impact and penetration of a ceramic plate.

Flow Simulation over a Bulbous Heat Shield of a Typical Launch Vehicle

Numerical Simulation of flow field over a sharp and blunt nose cone bulbous heat shield are carried out. Software used for the simulation is ANSYS 19.2 student version. The Simulations are carried out in the Mach no. range 0.8 to 1.2 (transonic regime). Simulation results are analyzed in detail. Besides, Shock Strength on the heat shield for all the simulation results are obtained. Based on the studies, bulbous heat shield which gives least shock strength is recommended for the payload fairing configuration.

Cone nose-tip roughness effect on the transition at M=5, 6, 8

The article presents data on the effect of distributed and isolated roughness on the blunt nose of the cone on the laminar-turbulent transition position. The experiments were performed at M=4.95, 5.95, 7.95, stagnation temperature T0=361–465 K and stagnation pressure P0=2–72 atm. The unit Reynolds number varied in the range Re1=(4.2–69.7)×106 m-1. The study was carried out in the boundary layer of cones with a half-angle of 7º with different zones of applying isolated and distributed roughness of the blunt nose R=5 mm. For all types of roughnesses, the effective values of Reynolds numbers are obtained, at which the transition occurs immediately after the roughness area.

Hypersonic shock-wave/boundary-layer interactions on a cone/flare

Time-resolved surface temperature measurements have been carried out on a 7° half-angle circular cone/flare model in the Air Force Research Laboratory’s Mach-6 Ludwieg Tube using infrared thermography. Measurements were performed at zero angle of attack on a total of 16 different nose bluntness/flare angle geometries at three different freestream unit Reynolds numbers. The boundary layer entering the interaction region was turbulent for the two sharper nosetips and laminar for the two blunter nosetips. Stanton-number contours and profiles were used to investigate the effect of nose bluntness, flare angle, and freestream unit Reynolds number on the length of boundary-layer separation and the peak heating achieved upon reattachment. The classical scaling analyses of Souverein et al.and Hung & Barnett for separation length and peak heating, respectively, have been extended for the use of axisymmetric cone/flare shock-wave/boundary-layer interactions. Scaling the peak heating with the density ratio across the separation shock wave, instead of the pressure ratio, significantly improves/simplifies the analysis.

Nonmodal Growth of Traveling Waves on Blunt Cones at Hypersonic Speeds

The existing database of transition measurements in hypersonic ground facilities has established that, as the nose-tip bluntness is increased, the onset of boundary-layer transition over a circular cone at zero angle of attack shifts downstream. However, this downstream movement in transition slows down and eventually reverses at sufficiently large values of the nose Reynolds number. Because modal amplification is too weak to initiate the measured frustum transition at moderate to large bluntness values, nonmodal growth is investigated as the potential basis for a physics-based model. The present analysis investigates the nonmodal growth of traveling disturbances initiated within the nose-tip vicinity. Results show that both planar and oblique traveling disturbances that peak within the entropy layer experience appreciable energy amplification for successively higher disturbance frequencies as the nose bluntness is increased. For relatively small bluntness values, the initial nonmodal growth is followed by a partial decay that is overcome by an eventual, modal growth as Mack-mode waves. For larger bluntness values, the Mack-mode waves are not amplified anywhere upstream of the experimentally measured transition location, but the traveling waves still undergo a significant amount of nonmodal growth. These findings do not provide a definitive link between optimal growth and the onset of frustum transition on moderately blunt cones, but they are qualitatively consistent with the experimental observations of an initial peak near the nose tip followed by a decay and growth in disturbance energy, as well as the appearance of inclined structures above the boundary-layer edge.

Computational Analysis of Flow over Bluff Bodies using Aero Spikes: Technical Note

This paper presents the computational analysis of the effect of aero spikes on bluff bodies. In the present study, a blunt nose model was designed with and without spike. Two different shaped spikes were chosen for our study. Efforts were taken to obtain the flow pattern and the pressure distribution field over the nose of the blunt body. Flow pattern were observed and recorded for different shaped spike attached in-front of the body. Comparative study was also done to observe which shape of the aero spike provides better zone of pressure field. It is observed that the radius of curvature of the streamline gets reduced by aero spike. Also, there occurs a deflection in the streamline which in turn resulted in creating a narrow zone of positive pressure region. The zone of influence gets reduced by hemispherical shaped spike. The models were designed using Catia V5 software and the analyses were carried out using Ansys software.

Numerical analysis of slotted aerospike for drag reduction

One of the important criteria for designing high speed vehicles is drag reduction and aerodynamic heating. There are plenty of methods available however finding an economical and simple method for drag and heat flux reduction is very challenging. In this paper, the forward facing aerospike for blunt-nosed bodies is introduced at supersonic and hypersonic Mach numbers and tested for drag reduction. Initially, the flow fields are studied around the blunt cone with and without aerospike. In addition, the different shapes and L/D ratios flying at supersonic Mach 2 are computed numerically. The computational simulations is carried out to solve the three-dimensional steady Reynolds-averaged Navier-Stokes equation along with the k-ω turbulence model in computation solver. After comparing the flow properties around the various aerospike models, we have found that the flat aerodisk experiences a drastic pressure reduction on the blunt nose cone but it has relatively less heat flux reduction. Consequently, a slotted aerodisk modification design has been analysed which uses convection flow to reduce heat flux.

Experimental study on cavity dynamics of projectile water entry with different physical parameters

In this paper, we investigate the influences of nose shape, impact velocity (8–14 m/s), and impact angle (60°–90°) on cavity dynamics when a projectile enters water. The Froude number, which characters the kinetic energy against gravitational potential, ranges from 280 to 850. It is found that the cavity diameter changes for different nose shapes, and an elongated cavity is achieved as the impact speed increases. The cavity pinch-off phenomenon is characterized. Experimental data reveal that the nose shape, impact velocity, and impact angle change the pinch-off depth and pinch-off time slightly by changing the occurrence time of the surface seal. For blunt nose shapes, greater impact velocity speeds up the surface seal and then quickens the pinch-off, thus reducing both the pinch-off depth and pinch-off time. Generally, the pinch-off depth follows the Fr1/3 law in our experiments. Cavity ripples were observed after pinch-off, and the wavelength, amplitude, and rippling frequency were measured. The wavelength of a ripple remains constant throughout, and all ripples are fixed with the experimental frame. The rippling frequencies are approximately identical to the Minnaert frequency. The impact velocity significantly changes the rippling frequency by affecting the radius of the air cavity.

Recent progresses on hypersonic boundary-layer transition

Boundary-layer transition (BLT) is one of the key problems in developing hypersonic aircrafts. Major aerospace countries all over the world have been heavily supporting relevant researches, and the project funding keeps increasing in recent years. In this paper, wind tunnel experiments, numerical simulations and theoretical analysis are combined to study some hot issues in this area, which include effects of unit Reynolds number, angle of attack and nose bluntness, nonlinear interactions during breakdown, secondary instability of crossflow vortices, etc. Some of our views on understanding hypersonic BLT are summarized. The wind tunnel tests are conducted in the Φ1 m hypersonic wind tunnel of China Aerodynamics Research and Development Center, and the relation between transition location and different unit Reynolds numbers, angles of attack and nose bluntness is obtained. It is well known that hypersonic BLT undergoes an important phenomenon called “nose-bluntness inversal”, but a worldwide accepted reasonable explanation for this phenomenon is still missing. To investigate the “nose-bluntness reversal”, we suggest to consider the influence of front shock waves because free-stream disturbances are changed by shock waves. Meanwhile, we should also consider the following two aspects: blunt-nose receptivity and entropy-layer effect. Some premiliary but encouraging results are presented on these two aspects via direct numerical simulations (DNS) and stablity analysis. During the late stage of transition, a “quiet zone” may appear when the second mode waves reach large amplitude, and this is followed by low-frequency modes. DNS and the dynamic mode decomposition are applied to study this problem. The results indicate that the relation between the second modes and the low-frequency modes is possibly mode transformation. In the end of this paper, a newly discovered secondary unstable mode of saturated crossflow vortices is introduced. This mode has the same level of growth rate as Z mode, but is related to the wall-normal shear stress, thus it is named Y mode.

On the location of the laser energy deposition region in wave drag reduction

A two-dimensional numerical simulation is carried out to determine the optimum location of the concentrated laser energy deposition region for maximum wave drag reduction of a blunt body. A 25.4-mm-diameter semicircle, Mach 3.45, and 50 mJ pulse energy are chosen as the representative blunt body, free-stream Mach number, and magnitude of the deposited laser energy, respectively. The location of the energy deposition region is varied discretely from 21 to 50 mm upstream of the center of the semicircle along its axis of symmetry. The lower limit of the distance is just upstream of the blunt body bow shock along the axis of symmetry. It is found that for the assumed conditions, the maximum wave drag reduction happens at the lower limit of the distances. A time-resolved analysis of the interaction between the laser-induced blast wave and the bow shock is carried out to arrive at the conclusions. It is observed that the blast wave reflects as an expansion wave from the bow shock for the 21 mm case, whereas the reflected wave is a shock wave for all other cases. The time history of the static pressure at the blunt body nose and wave drag per unit depth on the blunt body are also provided for the above locations of the energy deposition region to justify the inferences.

Numerical simulation of shock-disturbances interaction in high-speed compressible inviscid flow over a blunt nose using weighted essentially non-oscillatory scheme

In the present study, shock-disturbances interaction in high-speed compressible inviscid flow is simulated utilizing the weighted essentially non-oscillatory (WENO) scheme by employing the shock-capturing technique. For this aim, the two-dimensional Euler equations in strong conservative form are discretized by using the explicit third-order TVD Runge–Kutta scheme in time and the fifth-order WENO finite difference scheme in space. The main advantage of using the WENO scheme is its capability for properly solving the discontinuities in the domain without needing any artificial viscosity, limiter function or filter. Hence, this scheme is stable, and thus, it is suitable for simulating very high-speed flows with strong shocks. Three numerical tests are presented to demonstrate the remarkable capability of the WENO scheme and the corresponding numerical results are compared with the available analytical and numerical studies in the literature. The final and main test case is the steady and unsteady numerical simulation of the high-speed compressible inviscid flow past a two-dimensional circular cylinder. For this test case, firstly, the steady numerical solution at Mach=8.03 is provided. Then, the disturbances field is added to the upstream flow field and the unsteady numerical solution is performed for the fast and slow acoustic and also entropy waves. A sensitivity study is also performed to check the effects of the grid size and the type of flux splitting method on the solution. Both the steady and unsteady numerical solutions are compared with the existing analytical and numerical solutions and some conclusions are made.

Evaluation of aerospike for drag reduction on a blunt nose using experimental and numerical modeling

The modification of high-speed flight vehicles by adding a spike attached to the stagnation point yields a complex flow-field full of interesting details. The purpose of this work is to contribute to the following two aspects. First, to analyze the interaction between oblique shock, separation shock and reattachment shock to explain the mechanism of drag reduction caused by the spike. Second, to assess the impact of varying the spike length (L/D = 0.5, 1.0 and 1.5), angle-of-attack (up to 10°) and Mach number (up to 3) on the level of drag reduction. The results reveal that, by comparing with the un-spiked blunt nose, the maximum drag reduction is 42% for L/D = 1.0 at α = 0°. For higher angles-of-attack, the spike is found to have little benefits in reducing the drag. Moreover, the efficiency of the spike in reducing drag and increasing lift depends largely on spike length. However, the spike length does not influence the aerodynamic coefficients when it exceeds the value of the blunt-body diameter ratio (L/D = 1.0).

Control of Aerodynamic Drag and Heating of Nose Cones Through Taper Spikes

The present work numerically and experimentally investigates Mach 5.8 flow past passively modified nose cone configurations such as taper/stepped taper spiked, spherically blunted, and parabolic nose cones at a fineness ratio of 3.6 and zero angle of attack in order to determine the best passively modified geometry that provides the control of both the aerodynamic drag coefficient and surface heat flux. The velocity vectors show the flow directions/recirculation zones and so on at the root of the taper as well as stepped taper spikes, whereas the Mach number contours show the features of shock in front of the taper/stepped taper spikes as well as the nose tip of the taper/stepped taper spiked blunted and parabolic nose cones. The present study reveals that the “stepped taper spiked parabolic nose cone” configuration provides lowest aerodynamic drag coefficient and surface heat flux as compared with all the other nose cone configurations investigated. The modifications in the flow/shock features such as the conversion of strong curved shock into weaker oblique shock waves, flow deceleration, and so on, at the root of the spike due to backward-facing steps in “stepped taper spiked parabolic nose cone” may be the reason for achieving lower aerodynamic drag/heating.

Numerical evaluation of aerodome and cooling jet for aeroheating reduction

Thermal protection is one of the major challenges faced by engineers when designing hypersonic vehicles. Mounting a spike ahead of a vehicle's blunt nose provides at least a partial solution for the longtime aeroheating caused by high-speed flow; however, this solution appears inadequate at high Mach numbers. This study presents three innovative solutions, namely, the double-aerodome concept, the introduction of a cooling jet on the aerodome and the combination of both methods. The effectiveness of these solutions is evaluated by applying of a fluid–thermal interaction procedure. Results indicate that the reduction of heat flux on the blunt body persists when the spike length ratio of the double-aerodome model increases. The reduction then reaches a limit, beyond which an abnormal increase occurs due to the reattachment of streamlines onto the spike. In the case of jet on the single-aerodome model, a higher jet pressure ratio delivers a higher level of heat flux reduction. However, the excessive embedment of the jet into the recirculation zone tends to weaken the effectiveness of thermal protection. In addition, the heat flux can be further reduced by over 45% by introducing a cooling jet on the double-aerodome model. The combination of the two approaches also provides additional jet setting options, depending on the role of hypersonic vehicles. More importantly, the improvement in thermal protection performance is not achieved at the expense of drag reduction performance. The maximum increase in drag coefficient is limited to 8.1%.

Stall of airfoils with blunt noses at low to moderately high Reynolds numbers

The onset of leading-edge stall on stationary, smooth, thin, two-dimensional airfoils with blunter than classical nose shapes at low to moderately high Reynolds number flows is studied. Global stall is delayed to higher angles of attack as nose bluntness is increased.

Strakes Effects on Asymmetric Flow Over a Blunt-Nosed Slender Body at a High Angle of Attack

In general speaking, the missiles execute flight at high angles of attack in order to enhance their maneuverability. However, the inevitable side-force, which is caused by the asymmetric flow over these kinds of traditional slender body configurations with blunt nose at a high attack angle, induces the yawing or rolling deviation and the missiles will lose their predicted trajectory consequently. This study examines and diminishes the side-force induced by the inevitable asymmetric flow around this traditional slender body configuration with blunt nose at a high angle of attack (AoA = 50 deg). On one hand, the flow over a fixed blunt-nosed slender body model with strakes mounted at an axial position of x/D = 1.6–2.7 is investigated experimentally at α = 50 deg (D is the diameter of the model). On the other hand, the wingspan of the strakes is varied to investigate its effect on the leeward flow over the model. The Reynolds number is set at ReD = 1.54 × 105 based on D and incoming upstream velocity. The results verify that the formation of asymmetric vortices is hindered by the existence of strakes, and the strake-induced vortices develop symmetrically and contribute to the reduction in side-force of the model. In addition, the increase in strake wingspan reduces asymmetric characteristics of the vortex around the model and causes a significant decrease in side-force in each section measured. The strake with the 0.1D wingspan can reduce the sectional side-force to 25% of that in the condition without strakes.

Modified Collision Energy, a New Chemical Model in the DSMC Algorithm

A new chemical model in the direct simulation Monte Carlo (DSMC) algorithm, entitled modified collision energy (MCE), has been developed for simulation of reactive rarefied flows without some limitations of the conventional macroscopic models. Determination of correct values of the experimental parameters for computing the Arrhenius reaction rate is a serious challenge in some macroscopic chemical reaction models such as total collision energy (TCE) and general collision energy (GCE). A slight variation of these constant parameters in the Arrhenius relation could lead to significant change in the results. On the other hand, these experimental parameters have been extracted empirically only for limit number of gases and so they cannot be used for simulation of chemical reactions in various types of gases. Since some of these constants have been determined experimentally by several studies, they have been reported by different values in different references. The proposed MCE model in the present study is a reliable method to properly determine values of these parameters for all types of gases with the intrinsic properties of the particles and without need of any experimental data. Extraction of the constant parameters has been carried out using the analytical method or numerical quantum kinetics (QK) or modified quantum kinetics (MQK) models. The proposed MCE method has been evaluated in four test cases, including assessment of the reaction rate in equilibrium and nonequilibrium conditions, studying of the rarefied flow along the stagnation line, and investigation of the hypersonic gas flow over the axial symmetry blunt nose. The results show that the proposed method has desirable accuracy without use of any experimental parameters. The MCE method can also be used to calibrate the macroscopic reactive models such as the TCE and GCE.

Experimental study of the influence of small angles of attack and cone nose bluntness on the stabilization of hypersonic boundary layer with passive porous coating

The present work was devoted to an experimental study of small angles of attack and cone nose bluntness on the efficiency of stabilization of high-frequency disturbances in hypersonic boundary layer with a passive porous coating. The experiments were carried out on a cone with an apex half-angle 7° and nose bluntness radii R = 0.03, 0.5 and 1 mm installed under angles of attack α = 0–1° at the freestream Mach number M∞ = 5.8. High-frequency pressure fluctuations on the sides of the cone with the solid and porous surfaces were measured. It is shown that at all angles of attack and cone bluntness, the passive porous coating permits an efficient suppression of disturbances in hypersonic boundary layer both on the windward and leeward sides of the cone.