Low-speed Side Research Articles

Numerical Simulation of Cavitation and Hydrodynamic Characteristics of Supercavitating Projectiles in the Shear Flow

To investigate the cavitation and hydrodynamic characteristics of supercavitating projectiles in the shear flow, the Mixture multiphase and Schnerr-Sauer cavitation models are employed to simulate the underwater projectiles. The inflow average velocity is 600 m/s, and the shear rates range from 0 to 7500 s−1. In the uniform flow, the supercavity enveloping projectiles is vertically symmetrical. The drag is dominated by pressure drag, and the lift coefficient is 0. However, the supercavity is asymmetric in the shear flow, which deviates towards the low-speed side of projectiles. This is because the flow around projectiles runs faster on the high-speed side, and the vortices on the low-speed side entrain more fluid from the high-speed side. Thus, the projectiles suffer from normal shear stress orientating towards the low-speed side, and the lift coefficient turns negative. When the shear rate further increases, the projectile shoulder contacts water on the high-speed side, and the viscosity around projectiles is enhanced, resulting in the significant augmentation of the drag coefficient. As the water pressure is strongly larger than saturated vapor pressure on the low-speed side, the normal component of pressure acts more intensely towards the low-speed side of projectiles, and the lift coefficient is further decreased.

Direct numerical simulation of turbulent structures and asymmetric properties in the supersonic non-isothermal mixing layer

By direct numerical simulations, the non-isothermal effects on turbulent structures and asymmetric properties are investigated in the spatially developing supersonic mixing layers with high convective Mach numbers (Mc > 0.6). Hot air is blown in the high-speed stream, and cold air is added on the low-speed side. Two non-isothermal simulations with different temperature gradients are conducted and compared with the isothermal mixing layer. The self-similar model of the spatially developing supersonic turbulent mixing layer is analyzed to reveal the physical mechanisms for the asymmetry of non-isothermal mixing layers. The supersonic mixing layer is characterized by diverse vortices and unsteady shocklets, which increase in the initial shear layer and then decrease in the self-similar turbulent region. Also, the mixing layer is asymmetric between the high- and low-speed streams, and the shear layer center skews toward the low-speed side with more vortices and less shocklets, which is attributed to the streamwise momentum gradient. The effects of temperature gradients enhance the flow instability and accelerate the growth of vortices and shocklets in the initial mixing layer. Nevertheless, the turbulent structures are attenuated in the fully developed region, as the viscous dissipation is augmented and turbulence decays more strongly. In addition, the streamwise momentum gradient is reduced in the non-isothermal mixing layers. The vortical structures suffer from stronger attenuation on the cold side, while the shocklets are more significantly reduced on the hot side. Thus, the skewness of the shear layer center toward the low-speed side is reduced, and the mixing layer asymmetry is attenuated.

Direct numerical simulation of turbulence anisotropy and growth rate in the supersonic non-isothermal mixing layer

By direct numerical simulations, non-isothermal effects on turbulence anisotropy and growth rate are investigated in three-dimensional spatially developing supersonic mixing layers with high convective Mach numbers (Mc > 0.6). Hot air is blown into the high-speed stream, and cold air is added on the low-speed side. Two non-isothermal simulations with different temperature gradients are conducted and compared with the isothermal mixing layer. The Reynolds stress transport is analyzed to reveal the underlying modulation mechanisms by temperature gradients. The supersonic mixing layer is significantly anisotropic, and the streamwise turbulent intensity is larger than the transverse and spanwise turbulent intensities. The non-isothermal effects enhance the energy transfer from the streamwise Reynolds stress to transverse and spanwise Reynolds stresses in the initial shear layer, and the mixing layer anisotropy is intensely attenuated, which is attributed to the increment of the flow instability. Thus, the shear layer growth is initially accelerated, and the supersonic mixing layer is destabilized. However, the fluid viscosity and the viscous dissipation are enhanced, and the Reynolds stresses decay more strongly in the fully developed region. The transverse and spanwise turbulent intensities decrease more than the streamwise turbulent intensity due to the attenuation of the correlation between pressure and dilatation fluctuations. Consequently, the shear layer turbulence anisotropy is augmented with stronger three-dimensionality. As the Reynolds shear stress is reduced and the entrainment of irrotational fluid from the free streams into the shear layer is attenuated, the mixing layer growth rate is decreased in the self-similar turbulence.

Study of the Measurement of Generation 2 Toyota Event Data Recorders in Low-Speed Side Impacts

<div class="section abstract"><div class="htmlview paragraph">Automotive Event Data Recorders (EDRs) are often utilized to determine or validate the severity of vehicle collisions. Several studies have been conducted to determine the accuracy of the longitudinal change in velocity (ΔV) reported by vehicle EDRs. However, little has been published regarding the measurement of EDRs that are capable of reporting lateral ΔVs in low-speed collisions. In this study, two 2007 Toyota Camrys with 04EDR ECU Generation modules (GEN2) were each subjected to several vehicle-to-vehicle lateral impacts. The impact angles ranged from approximately 45 to 135 degrees and the stationary target vehicles were impacted at the frontal, central, and rear aspects of both the driver and passenger sides. The impact locations on the bullet vehicles were the front and rear bumpers and the impact speeds ranged from approximately 7.9 to 16.1 km/h. Instrumentation was mounted at the approximate center of gravity (CG) of the target vehicles, as well as on the front reinforcement bar, rear body panel, airbag ECU, B-pillars, and C-pillars to evaluate the varying lateral ΔV (ΔV<sub>y</sub>) readings from the 3 sensor locations (ECU, B-pillar, C-pillar) represented in the Toyota EDR and to account for rotational (yaw) effects on the data. The lateral ΔVs reported by the Toyota EDRs were then compared to the recorded ΔV<sub>y</sub>s from the reference instrumentation mounted within the test vehicles. A comparison of the data revealed a general under-reporting of impact severity from the Toyota EDRs at the ECU sensor when compared to reference instrumentation at the vehicle CG for impacts near the center of the target vehicle where the vehicle typically rotated about its frontal aspect. There was a general over-reporting of impact severity from the Toyota EDRs at the ECU sensor when compared to reference instrumentation at the vehicle CG for impacts near the front (heavy) axle of the target vehicle where the vehicle rotated about its rear aspect. Rotational data from the reference instrumentation and the distance between the ECU and reference instrumentation at the CG was also utilized to calculate the corresponding ΔV<sub>y</sub> at the ECU location for a direct comparison to the Toyota EDR measurement at the ECU (maximum of 12.0% difference). A comparison of the data recorded at the Toyota ECU sensors and the reference instrumentation mounted to the ECUs revealed accurate measurements by the Toyota ECU sensor (maximum of 7.6% difference). Differences in reported values between the 3 aforementioned Toyota sensor locations, as well as non-recordings and longitudinal ΔV recordings from the Toyota EDRs were also evaluated.</div></div>

Evaluation of Traffic Accessibility Condition and Noise Pollution in Dhaka City of Bangladesh

Sound pollution and traffic jam are two major problems in most of the city roads of developing cities like as Dhaka, Bangladesh. Traffic jam, as well as mixed traffic condition, is the major cause of sound pollution in the city roads. Both of these issues affect mostly on country's public health and development. A lot of new roads have been constructed in Dhaka city every year but the majority of the roads have not been constructed considering the future AADT (Annual Average Daily Traffic) value. The traffic volume has increased a lot as the population of the country increased rapidly and shifting the status of the country from developing county to the developed country increased the demand for privet car and as a result traffic jam is a regular problem in the Dhaka city. Due to increasing traffic volume, the waiting time in the traffic jam has been increased consequently. Drivers bell the horn of their vehicle to attract the concentration of the traffic police so that the traffic police provide the green signal to his/her desired destination road. Also after the starting of the green signal or when the yellow signal is provided to the drivers, the driver bell horn of their vehicle so that low-speed vehicle side to high-speed vehicle. Besides these, bikers continuously press the hydraulic horn to provide them free access throughout their road. In this research work, we have attempted to show how much free access is available to Natun Bazar to Farmgate road, a major traffic-congested road in Dhaka city for both directions of traffic movement and also the present sound pollution condition in this road. On an average of 9.06% and 8.46% of a day the driver gets free access [green signal] on this route based on the traffic movement direction [Natun Bazar to Farmgate route]. At present on average it requires around 2 hr. for traveling 7 km road and this is a hotspot of huge time killing for the city people in their regular life. Besides this passenger has to bear a sound level of around 101 dB including the residents living in this area beside the roadway. This impact is tremendous for the bikers and bus passengers result, they are experiencing significant issues such as headaches, decreased job performance, decreased focus, exhaustion, tension, and tiredness.

Simulative investigation of the load propagation in a wind turbine drive train during a power converter fault

Highly dynamic load situations in the drive train of wind turbines can occur during operation as a result of faults in the electrical grid or damage in electrical components. The dynamic loads can lead to damage or increased wear in the drive train components. At the Center for Wind Power Drives a wind turbine research nacelle has been set up in the course of a national project. Extensive experiments with this research nacelle have been conducted with the corresponding 4 MW test rig. Furthermore multi body simulation (MBS) models of the nacelle and the test rig have been developed and validated by experiments with load cases defined in the IEC 61400. Among other experiments, a test with a highly dynamic load situation in case of a power converter fault has been conducted. For that particular load case a power converter short-circuit has been emulated. This test’s results show significant high torque reversals inside the drivetrain. This paper focuses on the implementation of the measured dynamic torque excitation in the existing MBS model in order to investigate the calculated torque load propagation from the high speed side to the low speed side of the drive train. The MBS model delivers further insight into the torque load propagation by providing the torque on the intermediate speed shaft (IMS) which could not be measured on the test bench. It is shown that the highly dynamic torque ripple is filtered out mainly in the spur gear stage from the HSS to the IMS.

Analysis of High Speed Steering Drift of Tracked Vehicle

In order to control the high-speed steering drift of tracked vehicles correctly, the dynamics and kinematics characteristics of tracked vehicles during high-speed steering are analyzed. The expressions of traction force, braking force and steering resistance moment for high-speed and low-speed side tracked tracks are derived. The influence of centrifugal force on the steering of tracked vehicles is analyzed. The steering radius, steering angular velocity and the critical speed and limit speed when the vehicle drifts are given. Finally, the calculation method is used to analyze the safety of high-speed steering vehicle, and an example is given. The conclusion can be used to guide the evaluation of steering performance of tracked vehicle and the correct operation of high-speed steering.

Direct numerical simulation of a three-dimensional spatially evolving compressible mixing layer laden with particles. I. Turbulent structures and asymmetric properties

With the Eulerian–Lagrangian point-source method, the effects of dispersed particles on turbulent structures and asymmetric properties are systematically investigated in a three-dimensional spatially evolving compressible mixing layer with the convective Mach number up to 1.2. Particles interact with the mixing layer through two-way coupling, and three simulations with different particle diameters are conducted and compared with the particle-free flow. The underlying mechanisms responsible for the mixing layer asymmetry are also revealed through analyzing the self-similar equations of the particle-laden spatially evolving compressible mixing layer. The compressible mixing layer is significantly asymmetric on the high- and low-speed sides. The low-speed layer possesses more vortices and less shocklets compared with the high-speed layer in the fully developed region, and the shear layer center tends to skew toward the low-speed stream, which is due to the streamwise momentum gradient. Small particles augment the mixing layer asymmetry with more vortices and shocklets in the low-speed stream, which is attributed to the small inertia and the larger streamwise velocity of particles than fluid across the mixing layer. However, large particles attenuate the asymmetry of the mixing layer where the vortical structures on the low-speed side are further reduced and the shocklets are barely existent in both the layers, which is ascribed to the large inertia and the stronger effect of particle back-reaction on the low-speed fluid than that on the high-speed fluid.

Direct numerical simulation of particle dispersion in a three-dimensional spatially developing compressible mixing layer

By direct numerical simulations, the particle dispersion is systematically investigated in a three-dimensional spatially developing compressible mixing layer. The convective Mach number is 1.2 and particles interact with fluid through both the one- and two-way coupling. Six simulations are conducted with different particle diameters (Stokes numbers) or particle back-reaction. The compressible mixing layer is characterized by various vortical structures and unsteady shocklets, which both have significant effects on the dispersion of particles. The particles tend to accumulate in the peripheries of the vortical structures with high density, low vorticity, and high strain rate inside the mixing layer, as well as the high-density regions behind the shocklets outside the mixing layer. Due to the sweep and ejection effects, the particles from the high-speed side cluster in the high-speed streaks while those from the low-speed side collect in the low-speed steaks. Also, the particle mixing between the two streams skews towards the low-speed side. Compared with the other cases, medium particles show the strongest preferential concentration in the peripheries of vortices while small particles exhibit the most significant tendency to accumulate behind the shocklets. As the Stokes number decreases, the particle mixing is enhanced with more significant deviation towards the low-speed side. Besides, the particle back-reaction on fluid attenuates the preferential concentration. Outside the mixing layer, the number of the compression regions of the particle dispersion decreases, but the particles have stronger concentration behind the residual shocklets. The particle mixing as well as the skewness towards the low-speed side is also attenuated under the two-way coupling, which is attributed to the reduction of the vortex number and the centrifugal effects on particles.

비접촉 동력 전달을 위한 마그네트 기어 기반 감속기의 속도 제어에 관한 연구

마그네트 기어를 이용하면 기계적인 접촉없이 동력을 전달할 수 있다. 마그네트 기어 기반 감속 시스템에서 종동축은 구동축으로부터 분리되어있기 때문에 시스템은 제한된 공극 강성으로 부하 변화에 대응해야하는 2관성 공진 시스템이다. 종동축 즉, 저속측은 구동축 인가 토크만으로 제어되고 갑작스런 외란에 따라 일반적인 기계식 기어 시스템과 달리 과도한 진동이나 슬립이 발생할 수 있다. 따라서 저속측에 인가되는 부하 등의 외란은 실시간으로 측정되거나 추정되어야 한다. 본 논문에서는 고조파 조절기 일체형 마그네트 기어를 이용한 감속 시스템의 저속측 속도 제어를 위한 전상태 되먹임 제어기를 제안하고 이를 전산 모의 시험과 실험을 통해 검증하였다. 저속측 부하를 추정하기 위해 새로운 상태변수를 도입하여 관측기를 설계하였으며 이를 기반으로 하는 전상태 제어기를 통한 외란에 대한 강건성은 2자유도 PI 속도 제어기와 비교하였다. 상대적으로 짧은 시간안에 극의 슬립이 보정되는 것을 확인하였으며 추정된 변수는 실제 측정 결과와 유사한 경향을 나타내었다. 이러한 결과는 마그네트 기어 감속기의 서보 시스템으로의 응용 가능성을 담보해주는 결과인 것으로 판단된다. Using the magnet gear, it is possible to transmit power without mechanical contact. As the drive shaft in a magnet gear-based speed reducer system is isolated from the drive shaft, the system is a two-inertia resonance system that should cope with an external load with the limited air-gap stiffness. On the other hand, the drive shaft or low-speed side is controlled only by the torque of the drive shaft through an air-gap, and the excessive oscillation or the slip can then be generated because of an abrupt disturbance that is different from the general mechanical gear system. Therefore, the disturbance loaded at the low speed side should be measured or estimated, and considered in the control of the driving shaft. This paper proposes a novel full-state feedback controller with a reduced-order observer for the speed reducer system using a magnet gear with a unified harmonic modulator. The control method was verified by simulation and experiment. To estimate the load at the low speed side, a novel observer was designed, in which the new state variable is introduced and the new state equation is formulated. Using a full-state feedback controller including the observer, the test result against disturbance was compared with two D.O.F PI speed controllers. The pole slip was compensated within relatively a short time, and the simulation result about the estimated variable shows a similar tendency to the test result. The test results showed that the magnet gear-based reducer can be applied to an accurate servo system.

Scale interactions in a mixing layer – the role of the large-scale gradients

The interaction between the large and the small scales of turbulence is investigated in a mixing layer, at a Reynolds number based on the Taylor microscale ($Re_{{\it\lambda}}$) of $250$, via direct numerical simulations. The analysis is performed in physical space, and the local vorticity root-mean-square (r.m.s.) is taken as a measure of the small-scale activity. It is found that positive large-scale velocity fluctuations correspond to large vorticity r.m.s. on the low-speed side of the mixing layer, whereas, they correspond to low vorticity r.m.s. on the high-speed side. The relationship between large and small scales thus depends on position if the vorticity r.m.s. is correlated with the large-scale velocity fluctuations. On the contrary, the correlation coefficient is nearly constant throughout the mixing layer and close to unity if the vorticity r.m.s. is correlated with the large-scale velocity gradients. Therefore, the small-scale activity appears closely related to large-scale gradients, while the correlation between the small-scale activity and the large-scale velocity fluctuations is shown to reflect a property of the large scales. Furthermore, the vorticity from unfiltered (small scales) and from low pass filtered (large scales) velocity fields tend to be aligned when examined within vortical tubes. These results provide evidence for the so-called ‘scale invariance’ (Meneveau & Katz, Annu. Rev. Fluid Mech., vol. 32, 2000, pp. 1–32), and suggest that some of the large-scale characteristics are not lost at the small scales, at least at the Reynolds number achieved in the present simulation.

Application of Arnoldi method to boundary layer instability**Project supported by the National Natural Science Foundation of China (Grant Nos. 11202147, 11332007, 11172203, and 91216111) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120032120007).

The Arnoldi method is applied to boundary layer instability, and a finite difference method is employed to avoid the limit of the finite element method. This modus operandi is verified by three comparison cases, i.e., comparison with linear stability theory (LST) for two-dimensional (2D) disturbance on one-dimensional (1D) basic flow, comparison with LST for three-dimensional (3D) disturbance on 1D basic flow, and comparison with Floquet theory for 3D disturbance on 2D basic flow. Then it is applied to secondary instability analysis on the streaky boundary layer under spanwise-localized free-stream turbulence (FST). Three unstable modes are found, i.e., an inner mode at a high-speed center streak, a sinuous type outer mode at a low-speed center streak, and a sinuous type outer mode at low-speed side streaks. All these modes are much more unstable than Tollmien–Schlichting (TS) waves, implying the dominant contribution of secondary instability in bypass transition. The modes at strong center streak are more unstable than those at weak side streaks, so the center streak is ‘dangerous’ in secondary instability.

Modulation of the velocity gradient tensor by concurrent large-scale velocity fluctuations in a turbulent mixing layer

The modulation of small-scale velocity and velocity gradient quantities by concurrent large-scale velocity fluctuations is observed by consideration of the Kullback–Leibler divergence. This is a measure that quantifies the loss of information in modelling a statistical distribution of small-scale quantities conditioned on concurrent positive large-scale fluctuations by that conditioned on negative large-scale fluctuations. It is observed that the small-scale turbulence is appreciably ‘rougher’ when the concurrent large-scale fluctuation is positive in the low-speed side of a fully developed turbulent mixing layer, which gives further evidence to the convective scale modulation argument of Buxton & Ganapathisubramani (Phys. Fluids, vol. 26, 2014, 125106, 1–19). The definition of the small scales is varied, and regardless of whether the small-scale fluctuations are dominated by dissipation or have the characteristic features of inertial range turbulence they are shown to be modulated by the concurrent large-scale fluctuations. The modulation is observed to persist even when there is a large gap in wavenumber space between the small and large scales, although local maxima are observed at intermediate length scales that are significantly larger than the predefined small scales. Finally, it is observed that the modulation of small-scale dissipation is greater than that for enstrophy with the modulation of the vortex stretching term, indicative of the interaction between strain rate and rotation, being intermediate between the two.

Design and Optimization of a Linear Magnetic Field Modulate Gear

Linear magnetic field modulation gear can effectively avoid vibration and noise and lubrication and maintenance problems in mechanical gear, Which can replace mechanical gear in linear motion. According to the working mechanism of traditional magnetic field modulation gear, a linear magnetic gear is designed, whose transmission ratio is 2.67. Its air-gap flux density and static thrust characteristics and dynamic thrust characteristics are calculated by using the finite element analysis method. The calculate results validate the correctness of the design. Based on the above, the influence of ferromagnetic ring's size and the air-gap width to the gear's static thrust is analyzed, and then we have found rules about the influence, which can provide basis in optimizing a linear magnetic gear design. At the last, optimize the relative width of ferromagnetic ring to 0.6 and the thickness to 3 mm, the air-gap width of high-speed side to 1.2 mm and low-speed side to 1 mm.

Detached eddy simulation of shallow mixing layer development between parallel streams

Results of a high resolution Detached Eddy Simulation (DES) are used to characterize the evolution of a shallow mixing layer developing between two parallel streams in a long open channel with a smooth flat bed and medium size dunes. The study discusses the vertical non-uniformity in the mixing layer structure and provides a quantitative characterization of the growth of the large-scale quasi two-dimensional (2D) coherent structures with the distance from the splitter plate. Results show that in streamwise sections situated between 75D (D is the channel depth) and 150D from the splitter plate, the width of the mixing layer close to the free surface is 20–30% more than the width in the near-bed region. This is mostly because of the tilting of the mixing layer interface on the low-speed side toward the low speed stream as the free surface is approached. Power spectra of the horizontal velocity components near the free surface show the presence of a −3 subrange, corresponding to inverse energy cascade in two-dimensional turbulence, at streamwise locations situated more than 10D from the splitter plate, consistent with the presence of large-scale quasi 2D horizontal eddies and the transfer of energy (inverse energy cascade) from the smaller scales toward these eddies. Consistent with visualizations of the mass transport of a passive scalar within the mixing layer, close to the free surface, the estimated streamwise length of the quasi 2D mixing layer eddies is about 2.5–3.0 times larger than the local width of the mixing layer. The presence of large-scale roughness elements in the form of an array of two-dimensional dunes with a maximum height of 0.25D (D is the channel depth) induces a much more rapid and larger shift of the centerline of the mixing layer due to the increased influence of the bottom roughness.

Effect of Unilateral and Bilateral Resistance Training on Muscle Force, Power and Asymmetry of Lower Extremity in Elderly Women

As a training strategy for prevention of falling of the elderly, this study compared effect of unilateral resistance training with bilateral resistance training on muscle force, power and leg asymmetry during leg press movement. Twenty-three untrained elderly women(65-85 years) were participated in this study. They were randomly assigned to 3 groups: unilateral training group(UTG, n=8), bilateral training group(BTG, n=7), control(CG, n=8). Subjects in training groups followed elastic band protocol(10 exercises, 2-3 sets/exercise, 3 day/week) for 12 weeks. UTG exercised additionally more 1-2 sets in weaker leg than in stronger leg. Leg muscle force, power and leg asymmetry were measured at baseline and after training. Muscle force was increased significantly in BTG and UTG compared to CG in weaker side and bilateral side of low speed test. In high speed test, muscle force was increased significantly in BTG and UTG in weaker side. In power, UTG showed significantly increase in weaker side of low speed and stronger side of high speed. Both BTG and UTG significantly showed increase in bilateral side of low speed and weaker side of high speed. Force asymmetry was significantly decreased in UTG and BTG compared to CG in low speed and in only UTG of high speed. In conclusively, unilateral and bilateral resistance training increased muscle force and power in weaker leg. And force asymmetry was improved in both unilateral and bilateral training groups of low speed test and in only unilateral training group of high speed test.

The convection of large and intermediate scale fluctuations in a turbulent mixing layer

The convection velocity of large and intermediate scale velocity fluctuations in a nominally two-dimensional planar mixing layer, and its dependence upon the length scale, is explored by carrying out particle image velocimetry (PIV) experiments. A “global” convection velocity, containing the convection of all the length scales present in the flow, is produced by examining the autocorrelation functions between velocity fluctuations in successive PIV records across the mixing layer. This “global” convection velocity is found to be similar to the mean flow, although fluctuations on the low speed side of the mixing layer on average convect at speeds greater than the mean and fluctuations on the high speed side of the mixing layer are observed to convect at speeds less than the mean. Scale specific convection velocity profiles are then produced by examining the phase difference between the spectral content specific to one wavenumber in streamwise velocity fluctuation traces in successive PIV records, offset by time τ. Probability density functions (pdfs) are produced of this phase difference, which is subsequently converted into a convection displacement, and these show that the convection of single length scale fluctuations exhibits a significant variance, particularly so for larger scale fluctuations and in the high speed side of the mixing layer. Convection velocity profiles are produced from these pdfs using both the mean convection distance and the modal convection distance. It is observed that the convection velocity is relatively insensitive to the length scale of the fluctuations considered, particularly when the mean convection distance is used. A slight sensitivity to length scale is, however, observed for convection velocities based on the modal convection distance. This dependence is primarily observed in the high speed side of the mixing layer, in which smaller length-scale fluctuations convect more quickly than larger length-scale fluctuations. It is also observed that the magnitude of the fluctuation itself affects the convection velocity with larger magnitude fluctuations convecting less rapidly than lower magnitude ones at the largest length scales investigated with this behaviour being reversed at more intermediate length scales.

Crashworthiness design of vehicle side door beams under low-speed pole side impacts

In low speed collisions, insurance cost is a factor which is taken into account as a design criterion to reduce structure damage and fixing cost. In this study, it is focused on the applied permanent damages of vehicle frontal door caused by pole impacts. In the side impact, the side door beam is responsible to absorb the most possible kinetic energy. Two significant parameters including material and geometry of a side door beam are discussed here to reduce permanent damage of the door. To study the effect of material as the first phase, six different sheets of material have been concerned to investigate the rigidity of the structure. For the second phase, geometry modification of the side door beam has been performed via creating new CAD-reinforcing ribs. Consequently, the strength in elasto-plastic mode, maximum deflection and plastic strain of the equipped system representing the permanent damage is obtained in order to achieve lower losses.

RE-EVALUATING MIXING LENGTH IN TURBULENT MIXING LAYER BY MEANS OF HIGH-ORDER STATISTICS OF VELOCITY FIELD

A turbulent planar mixing layer is composed of two different flow types in its flow field, namely a shear layer in the central region and two free streams in each outer high- and low-speed side. Shear layer is formed after the trailing edge of the splitting plate and develops stream-wisely through successively distinct regions, namely the near field region and the self-preserving region. Two alternative definitions of the mixing lengths (lS and lF) are proposed in terms of the skewness and flatness factors, respectively, which are of third- and fourth-order of turbulence statistics. It is shown that the linear growth rate of the mixing length (either lS or lF) can be, then, used as one of the necessary and sufficient conditions to identify the achievement of the self-preserving state in turbulent mixing layer. Moreover, lF can be taken as the real length scale of the shear layer, which is of shear turbulence, bounded by the two outer high- and low-speed free streams in a given stream-wise station.

PIV measurements of convection velocities in a turbulent mixing layer

Particle image velocimetry (PIV) experiments that resolve a range of scales from 1.5L to 10.4η, where L and η are the integral and Kolmogorov length scales respectively, are performed in the self similar region of a nominally two dimensional planar mixing layer. Data is acquired at 7.25Hz with a field of view that extends to 1.5L thereby permitting spatio-temporal correlations of large and intermediate scale velocity fluctuations to be performed. The overall convection of velocity fluctuations is found to be similar to the mean flow, although fluctuations on the low speed side of the mixing layer on average convect at speeds greater than the mean and fluctuations on the high speed side of the mixing layer are observed to convect at speeds less than the mean. The convection of fluctuations of different length scales in the flow is then observed. It is found that large-scale structures, of the order of the integral scale, convect at speeds that are greater than the mean in the low speed side of the mixing layer and that small scale fluctuations convect at speeds significantly lower than the mean. A crossover wavenumber is also found at which the fluctuations in the low speed side of the mixing layer convect at greater velocities than those on the high speed side of the mixing layer. This wavenumber is found to be Reynolds number dependent.