Flow separation

About: Flow separation is a research topic. Over the lifetime, 16708 publications have been published within this topic receiving 386926 citations.

Boundary-Layer Transition Measurements in Hypervelocity Flows in a Shock Tunnel

Abstract: Experiments to investigate the transition process in hypervelocity boundary layers were performed in the T4 free-piston shock tunnel. An array of thin-film heat-transfer gauges was used to detect the location and extent of the transitional region on a 1500 mm long x 120 turn wide flat plate, which formed one of the walls of a duct. The experiments were performed in a Mach 6 flow of air with 6- and 12-MJ/kg nozzle-supply enthalpies at unit Reynolds numbers ranging from 1.6 x 10(6) to 4.9 x 10(6) m(-1). The results show that the characteristics typical of transition taking place through the initiation, growth, and merger of turbulent spots are evident in the heat-transfer signals. A 2-mm-high excrescence located 440 turn from the leading edge was found to be capable of generating a turbulent wedge within an otherwise laminar boundary layer at a unit Reynolds number of 2.6 x 10(6) m(-1) at the 6-MJ/kg condition. A tripping strip, located 100 mm from the leading edge and consisting of a line 37 teeth of 2 rum height equally spaced and spanning the test surface, was also found to be capable of advancing the transition location at the same condition and at the higher enthalpy condition.

Further space-time correlations of velocity in a turbulent boundary layer

Abstract: This paper describes the results of further experimental investigation of the turbulent boundary layer with zero pressure gradient. Measurements of autocorrelation and of space-time double correlation have been made respectively with single hot-wires and with two hot-wires with the separation vector in any direction. Space-time correlations reach a maximum for some optimum delay. In the case of two points set on a line orthogonal to the plate, the optimum delay Ti is not zero. In the general case it is equal to the corresponding delay Ti, increased by compensating delay for translation with the mean flow. Taylor's hypothesis may be applied to the boundary layer at distances from the wall greater than 3% of the layer thickness. Space-time isocorrelation surfaces obtained with optimum delay have a large aspect ratio in the mean flow direction, even if they are relative to a point close to the wall (0·03δ); the correlations along the mean flow then retain high values on account of the large scale of the turbulence.

Spatial development of particle-laden turbulent pipe flow

Abstract: The inhomogeneity of turbulence in wall bounded flows induces the phenomenology called turbophoresis whereby inertial particles of suitable mass accumulate at the solid wall. Particles injected near the axis of a fully turbulent pipe flow, after an initial spreading phase, undergo a segregation process which eventually leads to a pseudoequilibrium distribution sufficiently downstream. Wall densities up to thousand times the reference value can be easily achieved. The process is discussed here by analyzing the direct numerical simulation (DNS) data of a spatially developing particle laden pipe flow under the assumption of dilute suspension. Development phase and asymptotic state are addressed in quantitative terms. A Shannon-like entropy is introduced to quantify the level of spreading/segregation achieved by the particle distributions along the pipe. This allows to define on a physically sound basis the length of the developing region and to summarize in a single indicator the accumulation level as a function of the particle response time. By conditional statistics, it is unequivocally shown that particles approach the wall dragged by relatively fast yet comparatively rare events where highly accumulating particles follow the fluid in-rush toward the wall. On the contrary, the outward particle flux takes place in the form of much more frequent and gentle motions away from the wall. The analysis of DNS data and a simple argument highlight the role of the elongated clusters of particles at the wall as essential features responsible for the eventual asymptotic equilibrium.

A shear-free turbulent boundary layer

Abstract: A simple wall-turbulence interaction has been studied experimentally. In the idealized model an infinite flat plate is suddenly inserted into a pre-existing field of homogeneous isotropic turbulence, and subsequent changes in the turbulence field examined. The experiment involved passing grid-produced turbulence over a wall moving at the mean speed. Mean velocity gradients vanish in both the model and experiment, and hence production of new turbulence is absent. This allowed the inhibiting effects of the wall to be studied separately. The growth of the ‘inhomogeneity layer’ into the impressed turbulence field and other statistical features of the turbulence were measured.