Experimental study of free-stream turbulence induced transition in an adverse pressure gradient

2017 ◽  
Vol 84 ◽  
pp. 18-27 ◽  
Author(s):  
Davide Lengani ◽  
Daniele Simoni ◽  
Marina Ubaldi ◽  
Pietro Zunino ◽  
Francesco Bertini
Keyword(s):  
Experimental Study ◽  
Pressure Gradient ◽  
Free Stream ◽  
Adverse Pressure Gradient ◽  
Free Stream Turbulence

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

2001 ◽  
Vol 446 ◽  
pp. 271-308 ◽  
Author(s):  
M. KALTER ◽  
H. H. FERNHOLZ
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Free Stream ◽  
Reverse Flow ◽  
Flow Region ◽  
Adverse Pressure Gradient ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Free Stream Turbulence ◽  
The Mean

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.

Numerical investigation of transition in a boundary layer subjected to favourable and adverse streamwise pressure gradients and elevated free stream turbulence

2015 ◽  
Vol 781 ◽  
pp. 52-86 ◽  
Author(s):  
Joshua R. Brinkerhoff ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Free Stream ◽  
Large Strain ◽  
Adverse Pressure Gradient ◽  
Secondary Instability ◽  
Transition To Turbulence ◽  
Pressure Gradients ◽  
Turbulent Spot ◽  
Free Stream Turbulence

Laminar-to-turbulent transition of a boundary layer subjected to streamwise pressure gradients and elevated free stream turbulence is computed through direct numerical simulation. The streamwise pressure distribution and elevated free stream turbulence levels mimic the conditions present on the suction side of highly-cambered airfoils. Longitudinal streamwise streaks form in the laminar boundary layer through the selective inclusion of low-frequency disturbances from the free stream turbulence. The spanwise spacing normalized by local inner variables indicates stabilization of the streaks occurs by the favourable pressure gradient and prevents the development of secondary streak instability modes until downstream of the suction peak. Two distinct processes are found to trigger transition to turbulence in the adverse pressure gradient region of the flow. One involves the development of varicose secondary instability of individual low-speed streaks that results in their breakdown and the formation and growth of discrete turbulent spots. The other involves a rapid amplification of free stream disturbances in the inflectional boundary layer in the adverse pressure gradient region that results in a largely homogeneous breakdown to turbulence across the span. The effect of high-frequency free stream disturbances on the streak secondary instability and on the nonlinear processes within the growing turbulent spot are analysed through the inviscid transport of instantaneous vorticity. The results suggest that free stream turbulence contributes to the growth of the turbulent spot by generating large strain rates that activate vortex-stretching and tilting processes within the spot.

scholarly journals An Extended Version of an Algebraic Intermittency Model for Prediction of Separation-Induced Transition at Elevated Free-Stream Turbulence Level

2020 ◽  
Vol 5 (4) ◽  
pp. 28
Author(s):  
Slawomir Kubacki ◽  
Daniele Simoni ◽  
Davide Lengani ◽  
Erik Dick
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Free Stream ◽  
Adverse Pressure Gradient ◽  
Bypass Transition ◽  
Turbulence Level ◽  
Boundary Layer Flows ◽  
Free Stream Turbulence ◽  
Eddy Simulation ◽  
Layer Flow

An algebraic intermittency model for boundary layer flow transition from laminar to turbulent state, is extended using an experimental data base on boundary layer flows with various transition types and results by large eddy simulation of transition in a separated boundary layer. The originating algebraic transition model functions well for bypass transition in an attached boundary layer under a moderately high or elevated free-stream turbulence level, and for transition by Kelvin–Helmholtz instability in a separated boundary layer under a low free-stream turbulence level. It also functions well for transition in a separated layer, caused by a very strong adverse pressure gradient under a moderately high or elevated free-stream turbulence level. It is not accurate for transition in a separated layer under a moderately strong adverse pressure gradient, in the presence of a moderately high or elevated free-stream turbulence level. The extension repairs this deficiency. Therefore, a sensor function for detection of the front part of a separated boundary layer activates two terms that express the effect of free-stream turbulence on the breakdown of a separated layer, without changing the functioning of the model in other flow regions. The sensor and the breakdown terms use only local variables.

The influence of background turbulence on Ahmed-body wake bistability

2021 ◽  
Vol 926 ◽  
Author(s):  
D. Burton ◽  
S. Wang ◽  
D. Tudball Smith ◽  
H. N. Scott ◽  
T. N. Crouch ◽  
...  
Keyword(s):  
Experimental Study ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Wind Tunnels ◽  
Experimental Investigations ◽  
Switching Rate ◽  
Free Stream Turbulence ◽  
Background Turbulence ◽  
Ahmed Body

The discovery of wake bistability has generated an upsurge in experimental investigations into the wakes of simplified vehicle geometries. Particular focus has centred on the probabilistic switching between two asymmetrical bistable wake states of a square-back Ahmed body; however, the majority of this research has been undertaken in wind tunnels with turbulence intensities of less than $1\,\%$ , considerably lower than typical atmospheric levels. To better simulate bistability under on-road conditions, in which turbulence intensities can easily reach levels of $10\,\%$ or more, this experimental study investigates the effects of free-stream turbulence on the bistability characteristics of the square-back Ahmed body. Through passive generation and quantification of the free-stream turbulent conditions, a monotonic correlation was found between the switching rate and free-stream turbulence intensity.

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