Management and Control of Turbulent Shear Flows

Fernholz, H. H.

doi:10.1002/zamm.19930731102

Cited by 17 publications

(5 citation statements)

References 40 publications

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“…Instantaneous reverse flow was observed for LFST in a region extending from half a bubble length upstream of mean separation to one bubble length downstream of reattachment (see figure 6). This shows the 'buffeting behaviour' of the flow (see also AF; Simpson, Chew & Shivaprasad 1981;Fernholz 1993;Na & Moin 1998). In agreement with the observations of AF the bubble location for LFST and MFST is well defined in the mean, however, and quite repeatable from day to day.…”

Section: Experimental Results: Free-stream Turbulence and Mean Flow Bsupporting

confidence: 85%

The reduction and elimination of a closed separation region by free-stream turbulence

Kalter

Fernholz

2001

J. Fluid Mech.

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This paper is an extension of an experimental investigation by Alving & Fernholz (1996). In the present experiments the effects of free-stream turbulence were investigated on a boundary layer with an adverse pressure gradient and a closed reverse-flow region. By adding free-stream turbulence the mean reverse-flow region was shortened or completely eliminated and this was used to control the size of the separation bubble. The turbulence intensity was varied between 0.2% and 6% using upstream grids while the turbulence length scale was on the order of the boundary layer thickness. Mean and fluctuating velocities as well as spectra were measured by means of hot-wire and laser-Doppler anemometry and wall shear stress by wall pulsed-wire and wall hot-wire probes.Free-stream turbulence had a small effect on the boundary layer in the mild adverse-pressure-gradient region but in the vicinity of separation and along the reverse-flow region mean velocity profiles, skin friction and turbulence structure were strongly affected. Downstream of the mean or instantaneous reverse-flow regions highly disturbed boundary layers developed in a nominally zero pressure gradient and converged to a similar turbulence structure in all three cases at the end of the test section. This state was, however, still very different from that in a canonical boundary layer.

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Section: Experimental Results: Free-stream Turbulence and Mean Flow Bsupporting

confidence: 85%

The reduction and elimination of a closed separation region by free-stream turbulence

Kalter

Fernholz

2001

J. Fluid Mech.

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“…A historical perspective on many aspects of flow control is documented in the books edited by Lachmann (1961) and written by Chang (1976). More recent surveys on passive and active flow management methods were carried out by Gad el Hak & Bushnell (1991) and by Fernholz (1993). Both articles discuss active and passive management techniques of turbulent shear flows and their respective merits.…”

Section: Introductionmentioning

confidence: 99%

An experimental study of a boundary layer that is maintained on the verge of separation

et al. 2000

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A boundary layer maintained as close as possible to separation over an extended distance was produced, in accordance with the concept of Stratford. The resulting layer was two-dimensional in the mean, had nearly a constant shape factor of 2.5 and approximately linear streamwise growth of its integral length scales. The flow exhibited a definite non-equilibrium character, indicated by the different scales required for collapse of the mean velocity and turbulence intensity profiles. It was also very sensitive to the thickness of the upstream boundary layer. External excitation was imposed for diagnostic purposes and as a tool for delaying separation. The oscillatory momentum level of cμ ≈ 0.1% was tested for its ability to increase the skin friction cf at the prescribed geometry. Various frequencies, corresponding to the Strouhal number 0.008 < fθ0/Uref < 0.064, were used for the free stream reference velocity of Uref = 15 m s−1 and for two different inflow conditions. Notable increase (close to 60%) in cf was observed at higher frequencies that did not undergo maximum amplification. The increase in cf was accompanied by a reduction in the boundary layer thickness and in the shape factor H. The latter decreased in one case from 2.5 to 2.1. The overall turbulence level in the boundary layer decreased due to the addition of plane external perturbations.

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“…worth to know U 0 to evaluate the rates defined below (3),(4) and (5). Equation (2) defines time-averaged SJ orifice velocity [3]: (2) where u SJ (t) is the velocity in SJ actuator orifice, and W is the extrusion stroke duration. The strength of the synthetic jet (relatively to the main jet) can be quantified in terms of the ratios of the velocities, flow rates, and moments, i.e.…”

Section: Methodsmentioning

confidence: 99%

“…Flow control can be carried out using passive techniques like noncircular nozzles or different tabs at the nozzle exit. Active flow control techniques include continuous control jets, acoustic excitations or synthetic jets, Gad-El-Hak [1], Fernholz [2]. Today, active flow control techniques are studied worldwide.…”

Section: Introductionmentioning

confidence: 99%