Effects of Free-Stream Turbulence and Adverse Pressure Gradients on Boundary Layer Transition

Abstract

Boundary layer measurements are presented through transition for six different free-stream turbulence levels and a complete range of adverse pressure gradients for attached laminar flow. Measured intermittency distributions provide an excellent similarity basis for characterizing the transition process under all conditions tested when the Narasimha procedure for determining transition inception is used. This inception location procedure brings consistency to the data. Velocity profiles and integral parameters are influenced by turbulence level and pressure gradient and do not provide a consistent basis. Under strong adverse pressure gradients transition occurs rapidly and the velocity profile has not fully responded before the completion of transition. The starting turbulent layer does not attain an equilibrium velocity profile. A change in pressure gradient from zero to even a modest adverse level is accompanied by a severe reduction in transition length. Under diffusing conditions the physics of the transition process changes and the spot formation rate increases rapidly; instead of the “breakdown in sets” regime experienced in the absence of a pressure gradient, transition under strong adverse pressure gradients is more related to the amplification and subsequent instability of the Tollmien-Schlichting waves. Measurements reveal an exponential decrease in transition length with increasing adverse pressure gradient; a less severe exponential decrease is experienced with increasing turbulence level. Correlations of transition length are provided which facilitate its prediction in the form of suitable length parameters including spot formation rate.

End-Stage Boundary Layer Transition Models for Engineering Calculations

“…The general form is given as where g(m) = 6.91 + 12.75~1 + 6 3 . 6 4~1~ for {rn < 0) (14) and g(m) = 6.91 + 2.48m -1 2 . 2 7~1~ for (rn > 0} (15) Abu-Ghannam and Shaw's correlation, however, was based on data where the start of transition was taken as the position where a rapid increase in the surface velocity, as measured by a hot wire anemometer, was detected.…”

“…Since equations (14) to (16) constitute a general correlation for ROS then the problem of completing equation (18) resolves to one of determining the behaviour of N and how it may be influenced by external flow parameters such as the local pressure gradient and the freestream turbulence levels.…”

On the Calculation of Laminar Separation Bubbles Using Different Transition Models

“…The state of knowledge on the role of transition in gas turbine engines has been well summarized by Mayle (1991) and valuable additional experiments have recentl y been performed b y Malkiel and Mayle (1996) and Walraevens and Cumpst y (1995). Recognizing the lack of information on the effect of pressure gradient and free-stream turbulence on transition Gostelow et al (1994Gostelow et al ( , 1996 have conducted careful attached flow experiments and Solomon et al (19%) have formulated a new prediction model for the transition len gth. However, the difficulties associated with measurements in separation bubbles have prevented the acquisition of similar information in bubbles.…”

Mixed mode transition to turbulence in boundary layers

“…• Discrete instability waves are instigated spontaneously in APG simulations as observed in experiments by Walker and Gostelow (1989); Gostelow et al (1994). The discrete modes could be obtained even without free-stream turbulence being employed at inlet!…”

“…The instability waves are spontaneously excited which have been identified in experiments as well (Walker and Gostelow, 1989). Walker and Gostelow (1989) and Gostelow et al (1994) speculated that these instability waves participate in transition. The breakdown is by an inflectional instability in the wall normal profile of velocity.…”