Instability and Transition in Transient Vertical Buoyancy Driven Flows

过去对中等加热条件下瞬态流动的研究已经阐明了流体在保持层流的瞬态流动中的流动和传热响应。其他和最近的实验表明，更剧烈的加热会导致许多不同种类的下游瞬态输运，这取决于施加的加热速率。在发展中的下游响应中，发现了一维和二维瞬态不稳定和过渡。测量结果还表明，非常大的瞬态输运效应在局部出现。传统的暂态失稳分析方法仅限于近似，在某些情况下是不适合的。这里提出的研究包括两个平行的部分。首先是瞬态层流对加热条件的响应。第二种是发展流的下游处理，施加小的和随机的速度扰动。它们将在特定的时间，在特定的当地流量范围内的下游位置引入。它们的演变将在适当的时候或下游进行跟踪。随后的诊断将是由此产生的扰动增长率和光谱分布。中心结果将是扰动增长的数值模型和行为分析来理解这一现象。浮力驱动的瞬态流动出现在许多重要的应用中。这种流动是由静止流体的垂直边界表面的瞬态加热驱动的。这类瞬变出现在许多环境和技术过程中。它们可能源于表面辐射加热的变化。在批量处理操作中，在设备的启动和关闭中也是固有的。它们在大型高密度电子阵列的电瞬变中很常见。在评估电源或泵故障后内部加热元件的冷却情况时，它们也非常重要。本研究旨在了解当瞬态加热导致不稳定流动和转变时的传热和流动。

Past studies of transient-driven flows for moderate heating conditions have clarified the fluid flow and heat transfer responses in transient flows which remain laminar. Other and more recent experiments have shown that more vigorous heating results in many different kinds of downstream transient transport regimes, depending on the imposed heating rate. In the developing downstream response, both one and two dimensional transient instability and transition were found to arise. Measurements also show that very large transient transport effects arise locally. Conventional methods of transient instability analysis are limited to approximations which are not suitable in some of these regimes. The study proposed here includes two parallel components. The first is the transient laminar flow response to the heating condition. The second is the downstream treatment by the developing flow of small imposed and random velocity disturbances. They will be introduced, at specific times, at a downstream location inside the particular local flow regime of interest. Their evolution will be followed in time or downstream, as appropriate. The subsequent diagnostics will be of resulting disturbance growth rates and spectral distributions. The central result will be a numerical model of disturbance growth and behavior analysis to understand this phenomena. Buoyancy driven transient flows arise in many important applications. Such flows are driven by transient heating at a vertical bounding surface in a quiescent body of fluid. These kinds of transients arise in many environmental and technological processes. They may follow from changing radiative heating of a surface. The are also inherent in batch processing operations, in the start up and shutdown of equipment. They are common in electrical transients in large high density electronic arrays. They are also very important in assessing the cooling of internally heated elements after a power or pump failure. This research seeks to understand the heat transfer and flow when transient heating leads to unstable flows and to transition.