Flow Control under Constant Power Input

湍流存在于许多自然和技术过程中，其特点是由于流体和壁面之间的摩擦阻力造成的能量损失比层流更大。为了实现能量的有效利用，需要最小化湍流损失，相应的流动控制研究是流体力学的主要课题之一。目前该领域的许多工作都是基于直接数值模拟（DNS）。在初步工作中，开发了一种新的DNS方法，该方法特别适合于研究受控流的能量效率，因为可以规定每时间（功率）的能量输入。现有的DNS方法通过规定固定的体积流速或施加固定的压力梯度来实现通过管道的流动。另一部分是模拟的结果，即两种技术的流量和压力梯度的时间平均值是相同的。如果在任何一种技术中采用减阻流动控制技术，则由流量和压力梯度的乘积给出的流入流的功率（每时间的能量输入）就会发生变化。在第一种情况下，功率输入减少（在降低压力梯度下恒定流量），而在第二种情况下功率输入增加（在恒定压力梯度下增加流量）。因此，控制技术对湍流特性的影响可以用不同的方式来解释。关于能量耗散机制变化的可靠陈述基本上是不可能的。此外，对于主动控制技术，如果要评估被控制流的总能量需求，则必须考虑驱动控制所需的功率输入。在目前的项目中，我们打算进一步开发基于恒定功率输入的新型DNS方法，其中该功率输入包括泵送和控制功率。该方法将用于重新审视选定的现有流动控制技术，并分析其湍流特性，特别关注耗散机制。此外，将对它们的能源效率进行评估，并与现有DNS方法获得的结果进行比较。

Turbulent flows are present in many natural and technological processes and are characterized by larger energy losses due to friction drag between fluid and wall than their laminar counterparts. In order to enable efficient energy usage turbulent losses need to be minimized and the corresponding flow control research is one of the major topics in fluid mechanics. Much work in this field is presently based on direct numerical simulation (DNS). In preliminary work a novel DNS method was developed which is especially suited for investigating controlled flows in respect to their energy efficiency, because the energy input per time (power) can be prescribed.Existing DNS methods realize the flow through a duct by either prescribing a fixed volume flow rate or by imposing a fixed pressure gradient. The other component is a result of the simulation such that the time averaged values for flow rate and pressure gradient are the same for both techniques. If a drag reducing flow control technique is applied to either technique, the power input (energy input per time) into the flow, which is given by the product of flow rate and pressure gradient, changes. In the first case the power input decreases (constant flow rate at reduced pressure gradient) while it increases for the second case (increased flow rate at constant pressure gradient). As a result the influence of control techniques on turbulence properties can be interpreted in different ways. Reliable statements concerning changes in the energy dissipation mechanisms are basically not possible. Furthermore, for active control techniques, the power input required to drive the control has to be considered if the total energy requirement of the controlled flow is to be evaluated.In the present project we intend to further develop the novel DNS method based on a constant power input where this power input includes both, pumping and control power. This method will be used to revisit selected existing flow control techniques and analyze those in respect to their turbulence properties, with special focus on the dissipation mechanisms. In addition, they will be evaluated in respect to their energy efficiency and in comparison with results obtained with existing DNS methods.