尖劈对大气边界层试验风场的影响

尖劈是风洞生成边界层风场的主要装置,对下游风场的剖面特征有明显的控制作用,但由于作用机制不明确,在风场调试中大多是依据经验布置尖劈,调试效率低。笔者通过风洞试验,得到了3种常用形状(梯形、三角形及曲边三角形)尖劈下游的风速剖面和湍流度剖面特征,并测试了尖劈数量、高度的影响。结果表明:尖劈形状与生成边界层风场的范围高度、指数率直接相关,尖劈的平均宽度越宽,影响高度越高,指数率越大;增加尖劈数量可提高下游风场的指数率,但对风场范围影响很小;抬高尖劈的高度可增加下游边界层风场的高度,配合使用挡板及粗糙元,可以快速调试出目标风剖面。

在多向锻造过程中温度场对7075铝合金组织的影响

运用Deform 3D软件,研究了7075铝合金多向锻造过程中实际情况难以测量的参数对锻件成型质量的影响。以温度为变量,采用等温锻造和变温锻造对相同尺寸的毛坯进行锻造,初始的锻造温度为450℃,对比研究了两种锻造过程中毛坯所受到的应力、金属流速、金属流线以及晶粒大小等参数的差异。结果表明,7075铝合金变温锻造表面容易产生撕裂现象,等温锻造金属流线更好,组织更加均匀。镦粗和拔长的次数足够多,均可以达到晶粒细化效果,等温锻造晶粒尺寸更小,分布更均匀。

Analyzing the Influence of Aggregate Properties on Stopping Distance on Truck Escape Ramps

Although there are many studies involving influence of runaway truck entry speed and longitudinal grade on stopping distance,focusing on aggregate properties is scarce.This paper investigates the influence of the aggregate properties such as types of aggregate and river gravel radius on stopping distance through numerical analysis of particle flow code in two dimensions(PFC2D).The software is used to generate stopping distance data for two aggregate types and four group gravel radii under various approaching speeds and grades.The generated data are compared with the testing results of full-scale arrester bed.The simulated finding of this paper implies that types of aggregates and river gravel radii have a significant impact on the stopping distance for runaway truck on escape ramps.