Turbulence in a transient channel flow with a wall of pyramid roughness
Turbulence in a transient channel flow with a wall of pyramid roughness
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具有金字塔粗糙度壁的瞬态通道流中的湍流
DOI：
10.1017/jfm.2015.488
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发表时间：
2015
影响因子：
3.7
通讯作者：
Seddighi M
中科院分区：
文献类型：
--
作者：
Seddighi M
本文对顶壁光滑、底壁粗糙的密实金字塔形通道内的瞬态流动进行了直接数值模拟研究。将初始稳定的湍流快速加速到一个新的流速，并研究了加速后的瞬态流动特性。初始流和终流的等效粗糙度高度分别为和41.5。在加速结束后，引起的变化立即表现为“plug-flow”方式。在粗糙度峰以上，扰动流引起的附加速度是均匀的；在波峰以下，它在粗糙度元素的底部近似线性地减小到零。扰动流与粗糙壁面的相互作用的特征是一系列类似于在粗糙引起的层流-湍流过渡中观察到的事件。这个过程有两个大的阶段。在第一种情况下，与粗糙度波长相当的大规模漩涡在每个粗糙度单元周围形成，并且沿着单元的脊线形成高速条纹。短时间后，每个漩涡分裂成两个，即(i)在元件前面的站立漩涡和（ii）在元件后面的反向旋转发夹漩涡。前者在很大程度上是不活跃的，而后者则以越来越强的强度向下游平流，随后从壁面升起。这些发夹状的涡旋缠绕在强烈的低速气流上。整个过程的第二阶段是发夹涡分解成许多在空间中随机分布的更小的多尺度涡，最终导致常规湍流状态。在第一阶段开始后不久，由于涡旋结构的作用，速度波动的三个分量的均方根均在近壁区显著增加，其谱具有较强的表面拓扑特征。在第二阶段，该区域的整体湍流能量变化不大，但频谱演变明显，最终接近常规湍流。粗糙度对流动的直接影响仅限于粗糙度峰以上大约三个单元高度的区域。直到壁面附近的过渡基本完成之后，核心区域的湍流才开始增加。在光滑和粗糙的海峡壁上的过渡过程实际上是相互独立的。平滑壁面上的流动遵循层流-湍流过渡，正如以前的工作所知，类似于自由流湍流诱导的边界层旁路过渡。
A direct numerical simulation investigation of a transient flow in a channel with a smooth top wall and a roughened bottom wall made of close-packed pyramids is presented. An initially stationary turbulent flow is accelerated rapidly to a new flow rate and the transient flow behaviour after the acceleration is studied. The equivalent roughness heights of the initial and final flows are and 41.5, respectively. Immediately after the acceleration ends, the induced change behaves in a ‘plug-flow’ manner. Above the roughness crests, the additional velocity due to the perturbation flow is uniform; below the crest, it reduces approximately linearly to zero at the bottom of the roughness elements. The interaction of the perturbation flow with the rough wall is characterised by a series of events that resemble those observed in roughness-induced laminar–turbulent transitions. The process has two broad stages. In the first of these, large-scale vortices, comparable in extent to the roughness wavelength, develop around each roughness element and high-speed streaks form along the ridge lines of the elements. After a short time, each vortex splits into two, namely (i) a standing vortex in front of the element and (ii) a counter-rotating hairpin vortex behind it. The former is largely inactive, but the latter advects downstream with increasing strength, and later lifts away from the wall. These hairpin vortices wrap around strong low-speed streaks. The second stage of the overall process is the breakdown of the hairpin vortices into many smaller multi-scale vortices distributed randomly in space, leading eventually to a state of conventional turbulence. Shortly after the beginning of the first stage, the three components of the r.m.s of the velocity fluctuation all increase significantly in the near-wall region as a result of the vortical structures, and their spectra bear strong signatures of the surface topology. During the second stage, the overall turbulence energy in this region varies only slightly, but the spectrum evolves significantly, eventually approaching that of conventional turbulence. The direct effect of roughness on the flow is confined to a region up to approximately three element heights above the roughness crests. Turbulence in the core region does not begin to increase until after the transition near the wall is largely complete. The processes of transition over the smooth and rough walls of the channel are practically independent of each other. The flow over the smooth wall follows a laminar–turbulent transition and, as known from previous work, resembles a free-stream turbulence-induced boundary layer bypass transition.
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DOI：
--
发表时间：
2009
期刊：
Proceeding of Sixth International Symposium on Turbulence and Shear Flow Phenomena
影响因子：
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影响因子：
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